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Polar Micropollutants and Metals in Centrate from Dewatered Sewage Sludge Intended for Reuse in Soilless Horticulture
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Cite this: ACS EST Water 2022, 2, 12, 2548–2557
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https://doi.org/10.1021/acsestwater.2c00345
Published November 16, 2022

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Abstract

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The so-called centrate, the water remaining from digested sludge centrifugation in municipal wastewater treatment, is an untapped resource of both macro- and micronutrients for plant cultivation. However, both organic and inorganic contaminants present in the centrate may affect the quality of the produce if taken up by the plants. Little is known about the micropollutants present in the centrate. The centrate, influent, and effluent of a municipal wastewater treatment plant were analyzed for 27 polar organic micropollutants over a period of 28 days by direct injection liquid chromatography tandem mass spectrometry. The median dissolved concentrations in the centrate exceeded the concentrations in both the influent and effluent for most compounds: highest concentrations were found for benzotriazole (79 μg/L), valsartan (57 μg/L), and ibuprofen (18 μg/L). Micropollutants that are anaerobically degradable were not detected in the centrate. Among the heavy metals analyzed, highest concentrations were detected for Zn (47 μg/L) and Ni (17 μg/L). The increasing turbidity of the centrate led to elevated concentrations of less-polar micropollutants, while higher coagulant dosing increased the concentrations of more polar compounds. This first comprehensive study on polar organic micropollutants and metals in the centrate outlines that nutrient recovery from the centrate has to deal with both organic micropollutants and heavy metals present in the centrate.

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Synopsis

Centrate as a potential nutrient resource for soilless plant cultivation contains both organic and inorganic contaminants making further treatment prior to reuse necessary.

1. Introduction

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Reuse of wastewater contributes significantly to the provision of irrigation water in agriculture, reaching up to 60% during summer months─especially in Mediterranean countries. (1) While the reuse of wastewater in agriculture reduces fresh water withdrawal for irrigation, it can also help to recycle nutrients that are still present in treated wastewater. The recycling of nutrients from wastewater streams is promoted through the EU circular economy policy and has gained additional importance since phosphorous was classified a critical raw material by the EU in 2014. (2) National governments have started to promote phosphorous recovery through legislative measures, for example, with the amendments to the 2017 German Sewage Sludge Ordinance. (3) Concepts for recovering nutrients often focus on sludge, sludge ash, or effluent as streams exiting the wastewater treatment plant (WWTP). Internal streams may, however, also be of interest, such as the water remaining from the dewatering of the digested sludge.
Depending on the dewatering process used, the portion of the liquid phase of the digested sludge that is physically removed from the sludge matrix is referred to as centrate, sludge liquor, or process water. It is usually recirculated back into the influent of the treatment plant where it contributes to only 2% of volumetric flow but up to 25% of the total influent nitrogen load. (4) Compared to treated effluent, which is commonly reused as irrigation water for agricultural purposes in Mediterranean Europe, (5) centrate appears attractive because of its higher nutrient content. In addition to high nitrogen concentrations, centrate also contains high concentrations of micronutrients (e.g., Fe, Mn, Cu, Zn, Ni, Mo, and Cl) essential to plant growth, (6) which are solubilized during anaerobic sludge treatment.
The presence of micronutrients in centrate makes it especially attractive for the use in soilless cultivation, where all essential nutrients have to be present in the nutrient solution due to the absence of soil matter as a source for minerals. For this reason, for example, anaerobic digestate from biogas production has been investigated as a sole nutrient source in soilless horticulture. (7)
In spite of its high nutrient concentrations, the reuse of centrate as a nutrient source has not yet been reported in the literature and it poses some challenges. For instance, plant availability, for example, of nitrogen, which is present mostly as ammonia–nitrogen, has to be improved through nitrification. Also, elevated concentrations of some metals and metalloids present in centrate, acting as micronutrients, may negatively affect plant growth if plant-toxic levels are exceeded. (8) Moreover, especially in food production, inorganic and organic pollutants in centrate pose a risk for the produce, as plants may take up micropollutants dissolved in the recycled streams. (9,10) For organic micropollutants, properties such as a high polarity and low molecular weight have been reported to favor plant uptake. (11,12) Concerning inorganic micropollutants, quality standards are defined for heavy metals in the EU legislation on minimum requirements for wastewater reuse. (13) Here, technology targets are defined for minimum treatment based on the intended use of the produce, minimizing the risk posed by, for example, pathogens. Quality standards are defined for pathogens but also for turbidity and the biological oxygen demand. Additionally, a water reuse risk management plan should consider other substances of emerging concern. (13)
Therefore, a concept to reuse nutrients from centrate for plant cultivation will have to consider dissolved organic micropollutants and inorganic contaminants that are potentially plant available or have adverse effects on plant growth and to include means to remove such compounds from water, if necessary. Thus, more comprehensive knowledge on dissolved micropollutants in centrate is needed to develop a safe reuse scheme.
So far, the presence of heavy metals in centrate has been studied alongside their fate in WWTPs. (14) High particulate concentrations are reported for Cu, Pb, Ni, or Zn. (15) Literature shows that for some heavy metals, that is, Cu (16) in centrate can contribute ≥20% to the total load into the influent of WWTPs. However, insights into the time dynamics of heavy metals focusing especially on the aqueous phase of centrate are lacking.
The same is true for organic micropollutants. Studies on full-scale WWTPs are scarce and focus mostly on micropollutants with low to moderate polarity (log Kow > 2). Mailler et al. showed that rather unpolar phthalates (DEHP and DEP) are present in centrate at concentrations of 1–10 μg/L, while the more polar micropollutants tramadol, acetaminophen, and propranolol were found at <1 μg/L. (17) Also, Yoshida et al. have shown that linear alkyl sulfonates and polycyclic aromatic hydrocarbons are found at high concentrations in centrate. (14) So far, the presence of more polar compounds (log Kow < 2) in centrate has not been investigated thoroughly. However, knowledge on these is crucial prior to reuse, as Dettenmaier et al. have shown, plant uptake is more likely for dissolved highly polar compounds of low molecular weight. (11) Consequently, a study focusing on polar micropollutants in centrate is needed to close this knowledge gap. Moreover, with increasing polarity leading to stronger partitioning to the water phase, (18) the compounds most relevant for plant uptake are expected to be found in the dissolved phase.
Therefore, this study focused on the dissolved phase of centrate, influent, and effluent of a full-scale WWTP and on two major questions: (I) which concentration ranges of micropollutants as well as metals and metalloids can be expected in centrate and how do they compare to an influent or an effluent? (II) How variable are concentrations over the course of time? This study is the first published in-depth study of organic hydrophilic micropollutants as well as metals and metalloids in centrate.

2. Experimental Section

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Analytes for LC–MS target analysis were selected using several qualitative criteria: (i) based on their occurrence in studies on micropollutants in WWTPss and (ii) based on physical–chemical properties that may foster their uptake into plants. For this study, a total of 27 compounds were selected as target analytes (Table S1).
Analysis with ICP equipment targeted most metals and metalloids included in the WHO requirements for safe reuse of wastewater (19) as well as those referenced in the EU regulation for wastewater reuse (13) pointing toward the EU water framework directive, (20) the EU regulation for food stuffs, (21) as well as the EU regulation on application of sewage sludge in agriculture. (22) Additionally, some non-regulated metals and metalloids acting as essential micronutrients to plant growth as well as K and P as macronutrients were part of the analysis.

2.1. Wastewater Treatment Plant

The sampling campaign was carried out at the WWTP Leipzig-Rosental (550.000 P.E., daily influx: 110,000 m3/d). A simplified scheme of the relevant treatment stages is supplied in Figure 1. The primary treatment stage separates the primary sludge from the inflowing water stream by the use of grits, fine screens, a sand trap, and a grease separation. After this, the wastewater is treated in an activated sludge process with a biological nitrogen removal and chemical precipitation of phosphorous. The average hydraulic retention time amounts to 10 h, while the average sludge age of the activated sludge is 8–9 d. While the treated wastewater is released into the receiving water, the excess sludge originating from the activated sludge process and the primary sludge undergo post-treatment. First, excess sludge and primary sludge are pre-thickened statically. Then, both are treated anaerobically in a mesophilic digestion process with a theoretical sludge retention time of 24 d. The digested sludge is then dewatered in belt filter presses and after this in centrifuges. The products of the centrifugation process are biosolids which are disposed of and centrate which is recirculated back into the influent of the treatment plant.

Figure 1

Figure 1. Simplified scheme of the WWTP. The three sampling points at the effluent of the primary clarifier (INF), the effluent of the WWTP (EFF), and the effluent of the centrifuge (CEN) are indicated.

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2.2. Sampling

The sampling campaign was conducted from Jan 13th to Feb 9th 2020 for 28 consecutive days. For this study, the liquid phase of three different streams were investigated: (I) the centrate (CEN), (II) the effluent of the wastewater treatment plant (EFF), and (III) the effluent of the primary clarifier (INF) (Figure 1).
For both influent and effluent samples, flow-proportional 24 h composite samples were collected using a cooled automatic sampler (MAXX GmbH, Rangendingen, Germany). The centrate samples were collected by grab sampling.
To verify the validity of the grab sampling approach in centrate, pre-experiments were carried out in December 2019. Over a period of 24 h, centrate grab samples (n = 5) were obtained in 4 h intervals. Median centrate concentrations showed a mean relative standard deviation for all quantifiable compounds of RSD = ±6% along the 24 h interval (Table S2). Grab sampling was thus considered to be adequate for representative average concentrations for 24 h.

2.3. Sample Preparation

2.3.1. LC–MS Analysis

Samples of centrate, influent, and effluent were filtered (0.45 μm, regenerated cellulose) and stored refrigerated at 4 °C for a maximum of 19 days. Experiments to determine filter recovery showed acceptable recoveries for most compounds (Table S3). Prior to LC–MS/MS analysis, samples were diluted 1:10 in Milli-Q water and spiked with 10 μL/mL of a labeled internal standard mix (Table S4).

2.3.2. ICP-MS/ICP-OES Analysis

Samples for elemental analysis of dissolved concentrations of centrate, influent, and effluent were filtered (0.45 μm, regenerated cellulose) and after filtration acidified with HNO3 (65%) to pH 2–3 to prevent precipitation. The samples were stored refrigerated at 4 °C until analysis. Moreover, for analysis of total concentrations in all samples, unfiltered samples were homogenized and digested using a microwave-assisted digested approach following DIN EN ISO 11885.

2.4. Instrumental Analysis

2.4.1. LC–MS Analysis

For quantification of the micropollutants, samples were analyzed by LC–ESI–MS/MS using an Agilent 1260 Infinity series HPLC system (Agilent Technologies, Waldbronn, Germany), an Atlantis T3 column, 2.1 × 100 mm (Waters Corp., Milford, USA) with a SecurityGuard C18 guard column (4 × 2.0 mm2; Phenomenex), and a QTRAP 6500 (Sciex, Darmstadt, Germany) mass spectrometer. Parameters for chromatographic separation are based on Kahl et al. and are shown in Table S5. (23) Parameters for mass spectrometry are listed in Table S6. Electrospray ionization in the positive and negative modes was used. The analytes were detected using scheduled multiple reaction monitoring and identified based on two mass transitions as well as the retention time. Quantification of 19 analytes was carried out by external calibration in ultrapure water. Another eight analytes were quantified by their isotopically labeled standards added into each sample (Table S4). The matrix effects in centrate, WWTP influent, and effluent were estimated by matrix-matched calibration (Table S7). For most compounds, the matrix effects during the direct injection were in the range of −10 to +1%, except for the highly polar MET and MEL which eluted close to the dead volume of the LC column (Table S6). The matrix effects in centrate exceeded the matrix effects in the influent and effluent, likely due to its high salt content. However, as matrix effects can vary over time, it was not possible to apply a constant correction factor to concentrations determined by external calibration. Thus, they were not corrected for matrix effects.

2.4.2. ICP-MS/ICP-OES Analysis

Elemental analysis was performed to determine both the dissolved and total concentrations of metals and metalloids. For analysis of the dissolved concentrations, filtered samples were analyzed in an iCAP Qs ICP-MS (Thermo Scientific, Waltham, USA) system. For total concentrations, the homogenized samples were analyzed with an ICP-720-ES system (Varian Inc, Palo Alto, USA).

2.5. Chemicals and Analytical Standards

Ultrapure water was produced using a Merck Milli-Q Integral 5 System (Merck, Darmstadt, Germany) and used for dilution of samples as well for the mobile phase A in the LC–MS/MS gradient. As mobile phase B, UPLC grade methanol (Biosolve, Valkenswaard, Netherlands) was used.
For analysis by LC–MS, stock solutions were prepared in methanol or a methanol/water mix and stored at −20 °C until calibration standards were prepared.
JChem for Office (Excel) was used for chemical database access, structure-based property calculation, search, and reporting, JChem for Office 21.1.0.787, 2021, ChemAxon (http://www.chemaxon.com).

2.6. Data Analysis and Calculation

For peak integration, quantification with external calibration, and signal correction using internal standards, the Multiquant software was used. For further processing of concentration data, the software R was used. If analyte’s signals were clearly identifiable (S/N > 3) in Multiquant but still below the respective limit of quantification (as reported in Table S8), the concentrations were automatically adjusted using an in-house R script to their respective LOQ/2.

2.6.1. Cout,aq–Cin,aq Ratio

To estimate concentration dynamics within the aqueous phase during the sludge treatment process (digestion and centrifugation), dissolved input cin,aq and dissolved output cout,aq of the sludge treatment process are compared for every compound i based on eq 1. As only the output (cCEN) was analyzed, a “theoretical cin,aq” is estimated based on measured concentrations in the influent of the treatment plant cINF and effluent of the treatment plant cEFF considering the relative contributions of primary sludge and excess sludge into the sludge treatment. Due to data availability reasons, the calculation is based on two simplifying assumptions: (I) the ratio of the volumetric flows of the liquid portion of both primary and excess sludge is comparable to the total volumetric flow of primary and excess sludge into the sludge treatment process. (II) Concentrations of micropollutants in the aqueous phase of primary sludge/excess sludge are comparable with INF concentrations/EFF concentrations. This appears reasonable based on the relatively high polarity of the analyzed components, which will cause most of them to partition mainly to the liquid phase. Considering the average hydraulic retention time of 24 d in the digester median, concentrations and volumetric flows were calculated for separate time frames for the input and the output of the digester.
(cout,aqcin,aq)i=(c¯CEN,iQ¯PSQ¯Raw·c¯INF,i+Q¯ESQ¯Raw·c¯EFF,i)
(1)
Q¯PS,Q¯ES,Q¯Raw = median volumetric flows of primary sludge (PS), excess sludge (ES), and mixed raw sludge (Raw) for Jan 13th to Jan 15th. cINF,i; cEFF,i = median concentration of compound i in influent/effluent for Jan 13th to Jan 15th. cCEN,i = median concentration of compound i in centrate for Feb 7th to Feb 9th.

3. Results and Discussion

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The centrate of a WWTP as well as its influent and effluent was sampled daily over a period of 28 days, and the dissolved phase was analyzed for 27 polar, potentially plant-available micropollutants (Table S1) as well as 18 elements, mostly metals and metalloids.

3.1. Concentrations in Centrate

3.1.1. Organic Micropollutants

The concentrations span a range of several orders of magnitude, from a median of 0.8 μg/L for the opioid-analgesic lidocaine (LDC) up to a median of 79 μg/L for the corrosion inhibitor benzotriazole (BZTR) (Figure 2).

Figure 2

Figure 2. Concentration of targeted micropollutants in centrate of a municipal WWTP over a period of 3 weeks (n = 21; for IBU and LAMO n = 7) from Jan 20th to Feb 9th. The period from Jan 13th to 19th was excluded from aggregation because disturbance in the centrifuge operation was observed. Ac-SMX, SMX, and ACET were <LOD. BEZA, PROP, BTSA, MET, TRA, BGP, and MEL detected <LOQ. Boxes: 25–75 percentile, whiskers: 95 percentile).

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Unlike influent and effluent, in centrate, concentrations of many compounds such as metoprolol (METO), valsartan (VAL), or lamotrigine (LAMO) show only little variability (relative standard deviation RSD < 10%) over 21 days of grab sampling. Still, for some compounds such as acesulfame (ACE), benzothiazole-sulfonic acid (BTSA), or BZTR, a higher variability (RSD > 30%) was found (Figure 2). Interestingly, even structurally similar compounds such as BZTR and 4/5-methylbenzotriazole (4/5-MBZTR) exhibit dissimilar day-to-day variation in their concentration (Figure 3). For BZTR concentration, changes of up to +86% (Jan 25th to Jan 26th) from day to day can be observed in centrate (Figure 3d), while day-to-day concentration changes of 4/5-MBZTR do not exceed ±14% in the period from Jan 20th to Feb 10th. Compared to the WWTP influent and effluent of the WWTP, the variability of concentrations in centrate is low for all compounds investigated. Moreover, distinct weekly patterns (e.g., in influent, see Figure 3b,e) do not translate into centrate samples─likely due to the high hydraulic retention in the digester.

Figure 3

Figure 3. Concentration of 4/5-MBZTR and BZTR (a,d) in centrate, (b,e) in WWTP influent, and (d,f) WWTP effluent between Jan 13th and Feb 9th. Black: phase 1, high concentration of total suspended solids in centrate, gray: phase 2, low concentration of total suspended solids in centrate.

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In contrast, absolute centrate concentrations of around one-third of all investigated compounds exceed both effluent and influent concentration while 15 of 27 compounds exceed at least the concentration in one of the two (Figure S1).
At concentration levels found in centrate, uptake, for example, of ACE or diclofenac (DCF) into the roots of plants or carbamazepine (CBZ) into the shoots has to be expected for soilless cultivation, as demonstrated by Kreuzig et al. (24)

3.1.2. Metals and Metalloids

The median dissolved concentration of metals and metalloids in centrate ranged between 0.05 μg/L for Cd and 0.3 μg/L for Sn up to 200 μg/L for B (Figure 4). Among the heavy metals, the highest median concentration was found for Zn with 50 μg/L and Ni with 20 μg/L. The median concentration for As reached 10 μg/L.

Figure 4

Figure 4. Concentrations of metals and metalloids in centrate of a municipal WWTP over a period of 3 weeks (n = 21) from Jan 20th to Feb 9th. The period from Jan 13th to 19th was excluded from aggregation because disturbance in the centrifuge operation was observed. The asterisk indicated the recommended maximum concentration in wastewater used for fertilizing plants as defined by WHO. (19)

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The RSD of dissolved concentrations of metals and metalloids varied between 8% for B and 100% for Cu and up to 140% for Cd (Table S9). Similar to organic micropollutants, for B and Mo, the RSD in centrate was 1 order of magnitude below influent and effluent. In contrast, many heavy metals, for example, Cd, Cu, or Fe, showed higher fluctuations in centrate concentrations compared to influent and effluent (Table S9).
As for organic micropollutants, the median concentrations of most metals and metalloids in centrate exceeded the concentrations in the effluent. Influent concentrations were significantly exceeded in centrate only by few heavy metals such as Ni and As and most notably for Co (Figure S2).
As legislation for mineral fertilizers sets limit values in the mg/kg range, centrate concentrations fall well below the limits. (25) Up to this point, no legislation defining heavy metals limit values for liquid fertilizers is in place. However, WHO defines maximum recommended concentrations of metals in wastewater for use in agriculture defined by potential plant toxicity or adverse effects on soil. (19) Median metal concentrations in centrate did not exceed the limit values. However, for Fe concentration levels reach almost the maximum recommended value due to its potential limiting effect on availability of nutrients (e.g., P and Mo) in the soil as well as acidification. (19) Moreover, for Mo, maximum recommended values are in the same order of magnitude as the concentrations in centrate. For others such as Cd, Cr, Pb, and Zn, the median centrate concentrations are at least 1 order of magnitude below the recommended maximum. Furthermore, Lee et al. have shown that during soilless cultivation of lettuce, toxicity associated with heavy metal uptake in plants was insignificant according to the health risk index determined in the edible parts of the plant. (26)

3.2. Concentrations in Sludge Treatment

For a conventional WWTP, primary sludge from the primary treatment and excess sludge from the activated sludge process are the main input streams to the digestion process (Figure 1) and determine the input load of organic micropollutants and metals and metalloids. Especially for the more polar organic micropollutants, the load in the water phase of both primary sludge and excess sludge may be expected to govern the digester input, while the amounts sorbed to the respective particulate phase should be less relevant. Contrastingly, for most metals, it has been shown that the concentrations in primary and excess sludge are much higher in the particulate compared to the dissolved phase. (15)
For the investigated WWTP, the sludge treatment encompassed a digestion process as a biological step, and for sludge dewatering, a filter press and a centrifuge. To better understand the effects of the sludge treatment process on dissolved concentrations, a theoretical input concentration for the dissolved phase (cin,aq) was calculated (eq 1). Considering HRT in the digester, cin,aq was compared to cout,aq (=ccentrate) to derive knowledge on concentration trends during the sludge treatment. It is to be noted that the amount of organic micropollutants introduced with the solid phase of the sludges was not investigated; thus, a closed mass balance could not be achieved.
It has to be noted that during the sampling campaign, a drastic decrease in total suspended solid (TSS) concentration (Figure S3) in centrate was observed, indicating a change in centrifuge operation. This also led to a decrease in dissolved concentrations of a number of organic micropollutants as well as heavy metals, which is discussed below.

3.2.1. Organic Micropollutants

The calculated cout,aq/cin,aq ratios in Figure 5 show that the concentration in the dissolved phase increased during sludge treatment for 10 of the 27 organic micropollutants by a factor of 2.1 (BZTR) to 5.6 (CBZ). For the other half, the concentrations showed no change (e.g., ibuprofen─IBU) or compounds were partially (e.g., BTSA) or completely [e.g., metformin (MET)] removed from the liquid phase of the centrate. MET and sulfamethoxazole (SMX) were removed completely during sludge treatment, according to their reported anaerobic biodegradability. (27,28) BTSA, for which varying removal rates are reported in WWTPs, (29,30) shows a moderate removal during sludge treatment (cout,aq/cin,aq ratio = 0.4). Surprisingly, also ACE was partially removed, although studies have shown its recalcitrance against anaerobic transformation. (31) However, evolving biodegradation under aerobic condition has been observed. (23)

Figure 5

Figure 5. Ratio cout,aq/cin,aq of the median micropollutant concentration in filtrated centrate samples on Feb 7th to 9th and the calculated median input concentration into the digester from Jan 13th to 15th. The inset shows the TSS concentration in centrate. P-values based on the median micropollutant concentrations in centrate compared for phase 1 (black) and phase 2 (gray) (ns = not significant, **** = p < 0.0001, *** = p < 0.001, ** = p < 0.01, and * = p < 0.1).

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Stable concentrations throughout the sludge treatment are expected for compounds that are neither transformed under anaerobic conditions nor removed by adsorption to the sludge matrix. With a cout,aq/cin,aq ratio of close to 1, this was found for IBU or Tramadol (TRA), reported to be stable under anaerobic conditions. (32,33) With medium polarity of log D(pH=8) ≈ 1, adsorption to the sludge matrix seemed to be of minor importance for the removal. However, a cout,aq/cin,aq ratio close to 1 may also be found for compounds that show partial removal by adsorption, as they may be (re-)formed by biological processes. Gonzales-Gil et al. found that reversible biotransformation can take place among transformation products and their respective parent compounds in anaerobic systems. (31)
An increase during sludge treatment was observed for a number of compounds. The highest cout,aq/cin,aq ratios were observed for compounds CBZ, DCF, or VAL. All the three are reported as anaerobically stable. (28,33,34) Moreover, CBZ and DCF have shown to partition to the solid phase by >20% (18) and are likely partially sorbed to the particulate sludge input into the digester. With the amount of sludge organic matter being reduced by about 40 (±10)% during this study and know changes in sludge quality (smaller particles, more negative surface charge, and higher amount of excreted extracellular polymeric substance) during anaerobic treatment, (35) the adsorptive capacity of sludge also changes. During digestion more polar, weaker sorbing compounds are replaced first when sorption capacity becomes limited, while less polar, more strongly sorbing compounds exhibit a higher concentration in the particulate phase and are thus more affected by the loss of particulate phase in the digester. Moreover, polarity of compounds with acidic groups increases during digestion, as they will be deprotonated based on their pKa. With pH changes during the digestion process, from pH 5.5 in the influent to pH 8 in the effluent of the sludge treatment (Figure S4), a number of compounds show drastic changes in log D during the digestion process (Figure S5), that is, DCF and VAL. Both become more hydrophilic during digestion, increasing their tendency to partition to the liquid phase. If initially sorbed to the sludge surface, this charge change explains increased dissolved concentrations after sludge treatment.
3.2.1.1. Influence of TSS
As mentioned above, centrifuge operation was disturbed from Jan 13th to Jan 20th, as indicated by a drastic decrease in TSS concentration in centrate (Figure S3). The TSS decrease is also paralleled by a drop in the concentration of a number of centrate micropollutants. Correlation between TSS and concentrations of rather unpolar compounds has been observed by Mailler et al. (17) As a number of compounds remained seemingly unaffected by the TSS in this study, a correlation with log D was attempted. However, it was not significant (Figure S6). Still, as the group of compounds not affected by high TSS also displays much higher RSD throughout the sampling campaign, relevance of a governing influencing factor connected to TSS affinity can be suspected.

3.2.2. Metals and Metalloids

For most metals, a decrease of the dissolved concentrations during sludge treatment was observed. Likely, initially dissolved metals are affected strongly by the anaerobic environment in the digester. With low redox potential and neutral pH, some heavy metals, for example, Zn, will form insoluble sulfides, leaving the dissolved state. (36) Moreover, with an increasingly negative surface charge during digestion, sludge flocs are likely to assert strong electrostatic attraction toward cationic metals and metalloids promoting adsorption to the sludge surface. During centrifugation as the last step of the sludge treatment, both precipitate and flocs are separated from the centrate.
While the centrifugation thereby removes most heavy metals associated with the particulate phase, El-Aassar et al. have demonstrated that Ni concentrations in the centrate correlate with centrifugation intensity. (37) Ni has been shown to associate to some extent with the colloidal fraction of organic matter, (38) making release during centrifugation plausible, when instable flocs disintegrate.
Interestingly, the cout,aq/cin,aq ratios for metals and metalloids show only a few elements with increased concentrations after sludge treatment (Figure 6). Only As (cout,aq/cin,aq ratio = 4.6), Co (cout,aq/cin,aq ratio = 3.9), and Ni (cout,aq/cin,aq ratio = 1.8) showed an increase, while for all other metals, median concentrations decreased by >60%. The highest increase in concentrations in the sludge treatment was observed for As (cout,aq/cin,aq ratio = 4.6), indicating its solubilization from the sludge matrix. As forms carbonates in aerobic treatment, (39) which may be dissolved due to the increasing pH (from pH 5.5–8) during sludge digestion. (40) Interestingly, K and P as macronutrients also increased considerably during sludge treatment by more than a factor of 4. This is favorable for the intended reuse of the nutrients in centrate.

Figure 6

Figure 6. Ratio cout,aq/cin,aq calculated from the median dissolved concentration of metals and metalloids in centrate samples on Feb 7th to 9th and the calculated median input concentration into the digester from Jan 13th to 15th.

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3.2.2.1. Influence of TSS
While increased TSS concentrations in the centrate correlated well with the increased dissolved concentrations of a number of organic micropollutants (e.g., Figures 3a and 5), the same was not observed for dissolved concentrations of metals and metalloids (Figure S7). Contrastingly, investigating the homogenized samples showed that concentrations of heavy metals with affinity to the solid phase, for example, Zn or Cr, did increase with higher TSS concentrations in centrate. Moreover, even homogenized concentrations of Ni, which is preferentially found in the aqueous phase, seemed to increase with the TSS to a small degree.

4. Conclusions

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One focus of this study were rather hydrophilic (log D(pH=8) < 3) and low molecular weight organic (MW < 300 Da) micropollutants. They were found at concentrations of up to 80 μg/L in centrate. Anaerobically (bio-)degradable compounds were below the detection limit in the centrate. Notably, many organic micropollutants in centrate exceed the concentrations found in the influent or effluent. However, unlike influent or effluent, in centrate, organic micropollutant concentrations for many compounds are stable over the course of the sampling period. Some more hydrophilic compounds showed higher variability over time.
For metals and metalloids, highest concentrations were found for Zn with 50 μg/L. The concentrations of most metals and metalloids decreased during sludge treatment. Only As, Ni, and Co showed increasing concentrations.
While the incoming load of organic micropollutants, metals, and metalloids is determined by the input through the amount and ratio of primary and excess sludge, there are a number of processes affecting concentrations in the centrate during sludge treatment. Among those, adsorption and desorption to/from the sludge matrix during anaerobic digestion, speciation changes for metals and metalloids, and the extent of particle removal from centrate during centrifugation appear to have a strong influence on concentrations in centrate. A better understanding of these processes offers the potential to minimize the content of contaminants in centrate. For heavy metals, concentrations in centrate are low compared to the limit values for mineral fertilizers, and plant uptake does not appear to be critical. However, for the yet unregulated organic micropollutants, plant uptake has to be expected for some of them. Therefore, reduction of heavy metals and especially organic micropollutants is recommended before reuse in soilless cultivation of plants for human consumption.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestwater.2c00345.

  • Analytical method, comprehensive display of the concentrations of organic micropollutants, as well as metals and metalloids and total removal rates of the WWTP observed during sampling, and scatter plots elucidating the analyte’s properties and corresponding concentrations and cout–cin ratio are displayed (PDF)

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Author Information

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  • Corresponding Author
    • Thorsten Reemtsma - Department of Analytical Chemistry, Helmholtz-Centre for Environmental Research─UFZ, Permoserstrasse 15, 04318Leipzig, Germany;  Institute for Analytical Chemistry, University of Leipzig, Linnéstrasse 3, 04103Leipzig, Germany;  Orcidhttps://orcid.org/0000-0003-1606-0764;  Email: [email protected]
  • Author
  • Author Contributions

    P.G. contributed to conceptualization, investigation, data curation, formal analysis, writing─original draft, visualization, and project administration. T.R. contributed to writing─review and editing, supervision, funding acquisition, and resources. CRediT: Paul Genz conceptualization (lead), data curation (lead), formal analysis (lead), investigation (lead), visualization (lead), writing-original draft (lead); Thorsten Reemtsma funding acquisition (lead), project administration (lead), supervision (lead), writing-review & editing (lead).

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This study was financed in part by the German Ministry for Education and Research (BMBF) through the project “Entwicklung eines nachhaltigen Kultivierungssystems für Nahrungsmittel resilienter Metropolregionen─SUSKULT” (FKz: 031B0728M). We are grateful to the head of the WWTP Leipzig-Rosental, Daniel Jentzsch (KWL, Leipzig) for the cooperation and his staff for provision of samples and data.

References

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    The applicability of anaerobic effluent (AE) from an anaerobic membrane bioreactor (AnMBR) treating domestic wastewater as a nutrient medium was evaluated through hydroponic cultivation of lettuce. The growth of lettuce plants on AE media was significantly inhibited to 31-40% in height and 36-48% in no. of leaves compared to that on half-strength Hoagland soln. (HHS) as a control. The primary cause of inhibition was nitrite toxicity as induced by partial nitrification. Therefore, the nitrification of AE as a pre-treatment step was adopted to prevent the toxicity of nitrite. The heights of lettuce grown on nitrified anaerobic effluent (NAE) and nitrified anaerobic effluent with 96 mg/L sulfate (NAES) were in the range of 11.4-11.5 cm and was comparable to that on control soln. (11.4 cm). The potential health risk for heavy metals was insignificant based on health risk index (HRI < 1) and targeted hazardous quotient (THQ < 1). These results show that efficient crop prodn. can be achieved with AE, but suitable pre-treatment steps should be followed.
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    Campbell, A. J. The Behaviour of Pharmaceuticals in Anaerobic Digestor Sludge. Ph.D. Dissertation; University of Portsmouth: Portsmouth, 2013.
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    Carballa, M.; Omil, F.; Ternes, T.; Lema, J. M. Fate of Pharmaceutical and Personal Care Products (PPCPs) during Anaerobic Digestion of Sewage Sludge. Water Res. 2007, 41, 2139– 2150,  DOI: 10.1016/j.watres.2007.02.012
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    Fate of pharmaceutical and personal care products (PPCPs) during anaerobic digestion of sewage sludge
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    Water Research (2007), 41 (10), 2139-2150CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    The behavior of 13 pharmaceutical and personal care products (PPCPs) has been studied during anaerobic digestion of sewage sludge: 2 musks (Galaxolide and Tonalide), one tranquilliser (Diazepam), one anti-epileptic (Carbamazepine), 3 anti-phlogistics (Ibuprofen, Naproxen and Diclofenac), 2 antibiotics (Sulfamethoxazole and Roxithromycin), one x-ray contrast medium (Iopromide) and 3 estrogens (Estrone, 17β-oestradiol and 17α-ethinylestradiol). Two parallel processes were carried out, one in mesophilic range (37°) and the other in thermophilic range (55°). The effect of temp. and sludge retention time (SRT) was analyzed. Among the substances considered, the higher removal efficiencies were achieved for the antibiotics, natural estrogens, musks and Naproxen. For the other compds., the values were 20-60%, except for Carbamazepine, which showed no elimination. The removal of estrogens, Diazepam and Diclofenac occurred after sludge adaptation. In general, no influence of SRT and temp. on PPCPs removal was obsd. Considering the difficulty of obtaining reliable PPCPs concns., esp. those corresponding to the fractions sorbed onto sludge, a methodol. to validate the exptl. data has been developed and successfully applied.
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    De Wever, H.; Verachtert, H. Biodegradation and Toxicity of Benzothiazoles. Water Res. 1997, 31, 2673– 2684,  DOI: 10.1016/S0043-1354(97)00138-3
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    Biodegradation and toxicity of benzothiazoles
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    A review with many refs. Benzothiazoles appear in the environment mainly as a result of their prodn. and use as rubber vulcanization accelerators. This review focuses on the potential environmental hazard these substances pose and their biol. removal from and transformation in aquatic and terrestrial ecosystems. Benzothiazole degrdn. by mixed and pure cultures and the toxic impact on microorganisms are discussed. It is concluded that biol. purifn. of wastewaters from 2-mercaptobenzothiazole (MBT) prodn. can be achieved, provided the MBT influent concn. is 100 mg/L at max. So far, no attempts have been made to study the mol. mechanisms of benzothiazole degrdn., mainly due to the unavailability of axenic cultures capable of benzothiazole breakdown. However, the toxic effects of MBT towards microorganisms have been studied and can be attributed to its metal chelating properties and(or) its interference with membrane-bound (co)enzymes in particular.
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    Kloepfer, A.; Gnirss, R.; Jekel, M.; Reemtsma, T. Occurrence of Benzothiazoles in Municipal Wastewater and Their Fate in Biological Treatment. Water Sci. Technol. 2004, 50, 203– 208,  DOI: 10.2166/wst.2004.0329
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    Occurrence of benzothiazoles in municipal wastewater and their fate in biological treatment
    Kloepfer, A.; Gnirss, R.; Jekel, M.; Reemtsma, T.
    Water Science and Technology (2004), 50 (5, Assessment and Control of Hazardous Substances in Water), 203-208CODEN: WSTED4; ISSN:0273-1223. (IWA Publishing)
    A no. of 2-substituted benzothiazoles that are known to be used as fungicides, corrosion inhibitors, and vulcanization accelerators in industry were analyzed in municipal wastewater and the effluents of activated sludge and membrane bioreactor (MBR) treatment over a three month period. All six analytes were regularly detected in the municipal wastewater by LC-MS and amt. to a total concn. of 3.4 μg/L. Of these compds. benzothiazole-2-sulfonic acid (1,700 ng/L), benzothiazole (850 ng/L) and 2-hydroxybenzothiazole (500 ng/L) were most prominent. The source of the benzothiazole emission is yet unknown. Activated sludge treatment did not reduce total benzothiazole concn. significantly. Removals of the individual compds. ranged from 90% for 2-mercaptobenzothiazole and 70% for hydroxybenzothiazole to 40% for benzothiazole. The concn. of benzothiazole-2-sulfonic acid increased by 20%, whereas 2-methylthiobenzothiazole increased by 160% during activated sludge treatment, likely due to the methylation of mercaptobenzothiazole. Total benzothiazole removal in two parallel operated MBRs was significantly better (43%) than in the conventional activated sludge treatment. Namely benzothiazole and benzothiazole-2-sulfonic acid were more effectively removed. This first systematic study on the occurrence of benzothiazoles in municipal wastewater has shown that this is a relevant class of trace contaminants in municipal wastewater which is only incompletely removed in biol. wastewater treatment. Emission from sewage treatment is dominated by the most polar benzothiazole-2-sulfonic acid. MBR treatment may reduce but cannot avoid this emission.
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    Gonzalez-Gil, L.; Krah, D.; Ghattas, A.-K.; Carballa, M.; Wick, A.; Helmholz, L.; Lema, J. M.; Ternes, T. A. Biotransformation of Organic Micropollutants by Anaerobic Sludge Enzymes. Water Res. 2019, 152, 202– 214,  DOI: 10.1016/j.watres.2018.12.064
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    Biotransformation of organic micropollutants by anaerobic sludge enzymes
    Gonzalez-Gil, Lorena; Krah, Daniel; Ghattas, Ann-Kathrin; Carballa, Marta; Wick, Arne; Helmholz, Lissa; Lema, Juan M.; Ternes, Thomas A.
    Water Research (2019), 152 (), 202-214CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Biotransformation of org. micropollutants (OMPs) in wastewater treatment plants ultimately depends on the enzymic activities developed in each biol. process. However, few research efforts have been made to clarify and identify the role of enzymes on the removal of OMPs, which is an essential knowledge to det. the biotransformation potential of treatment technologies. Therefore, the purpose of the present study was to investigate the enzymic transformation of 35 OMPs under anaerobic conditions, which have been even less studied than aerobic systems. Initially, 13 OMPs were identified to be significantly biotransformed (>20%) by anaerobic sludge obtained from a full-scale anaerobic digester, predestining them as potential targets of anaerobic enzymes. Native enzymes were extd. from this anaerobic sludge to perform transformation assays with the OMPs. In addn., the effect of detergents to recover membrane enzymes, as well as the effects of cofactors and inhibitors to promote and suppress specific enzymic activities were evaluated. In total, it was possible to recover enzymic activities towards 10 out of these 13 target OMPs (acetyl-sulfamethoxazole and its transformation product sulfamethoxazole, acetaminophen, atenolol, clarithromycin, citalopram, climbazole, erythromycin, and terbutryn, venlafaxine) as well as towards 8 non-target OMPs (diclofenac, iopamidol, acyclovir, acesulfame, and 4 different hydroxylated metabolites of carbamazepine). Some enzymic activities likely involved in the anaerobic biotransformation of these OMPs were identified. Thereby, this study is a starting point to unravel the still enigmatic biotransformation of OMPs in wastewater treatment systems.
  32. 32
    Gonzalez-Gil, L.; Papa, M.; Feretti, D.; Ceretti, E.; Mazzoleni, G.; Steimberg, N.; Pedrazzani, R.; Bertanza, G.; Lema, J. M.; Carballa, M. Is Anaerobic Digestion Effective for the Removal of Organic Micropollutants and Biological Activities from Sewage Sludge?. Water Res. 2016, 102, 211– 220,  DOI: 10.1016/j.watres.2016.06.025
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    Is anaerobic digestion effective for the removal of organic micropollutants and biological activities from sewage sludge?
    Gonzalez-Gil, L.; Papa, M.; Feretti, D.; Ceretti, E.; Mazzoleni, G.; Steimberg, N.; Pedrazzani, R.; Bertanza, G.; Lema, J. M.; Carballa, M.
    Water Research (2016), 102 (), 211-220CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    The occurrence of emerging org. micropollutants (OMPs) in sewage sludge has been widely reported; nevertheless, their fate during sludge treatment remains unclear. The objective of this work was to study the fate of OMPs during mesophilic and thermophilic anaerobic digestion (AD), the most common processes used for sludge stabilization, by using raw sewage sludge without spiking OMPs. Moreover, the results of anal. chem. were complemented with biol. assays in order to verify the possible adverse effects (estrogenic and genotoxic) on the environment and human health in view of an agricultural (re)use of digested sludge. Musk fragrances (AHTN, HHCB), ibuprofen (IBP) and triclosan (TCS) were the most abundant compds. detected in sewage sludge. In general, the efficiency of the AD process was not dependent on operational parameters but compd.-specific: some OMPs were highly biotransformed (e.g. sulfamethoxazole and naproxen), while others were only slightly affected (e.g. IBP and TCS) or even unaltered (e.g. AHTN and HHCB). The MCF-7 assay evidenced that estrogenicity removal was driven by temp. The Ames test did not show point mutation in Salmonella typhimurium while the Comet test exhibited a genotoxic effect on human leukocytes attenuated by AD. This study highlights the importance of combining chem. anal. and biol. activities in order to establish appropriate operational strategies for a safer disposal of sewage sludge. Actually, it was demonstrated that temp. has an insignificant effect on the disappearance of the parent compds. while it is crucial to decrease estrogenicity.
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    Psoma, A. K.; Attiti, S.; Arvaniti, O.; Stasinakis, A. S.; Thomaidis, N. S. Fate of Selected Emerging Pollutants and Formation of Transformation Products in Activated Sludge Batch Reactors under Aerobic/Anoxic/Anaerobic Conditions. Proceedings of the 14th International Conference on Environmental Science and Technology; Rhodes, 2015.
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    Lahti, M.; Oikari, A. Microbial Transformation of Pharmaceuticals Naproxen, Bisoprolol, and Diclofenac in Aerobic and Anaerobic Environments. Arch. Environ. Contam. Toxicol. 2011, 61, 202– 210,  DOI: 10.1007/s00244-010-9622-2
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    Microbial transformation of pharmaceuticals naproxen, bisoprolol, and diclofenac in aerobic and anaerobic environments
    Lahti, Marja; Oikari, Aimo
    Archives of Environmental Contamination and Toxicology (2011), 61 (2), 202-210CODEN: AECTCV; ISSN:0090-4341. (Springer)
    Although biotransformation is generally considered to be the main process by which to remove pharmaceuticals, both in sewage treatment plants and in aquatic environments, quant. information on specific compds. is scarce. In this study, the transformations of diclofenac (DCF), naproxen (NPX), and bisoprolol (BSP) were studied under aerobic and anaerobic conditions using inocula taken from activated and digested sludge processes, resp. Whereas concn. decays were monitored by LC-tandem mass spectrometry, oxygen consumption and methane prodn. were used for the evaluation of the performance of overall conditions. DCF was recalcitrant against both aerobic and anaerobic biotransformation. More than one third of the BSP disappeared under aerobic conditions, whereas only 14% was anaerobically biotransformed in 161 days. Under aerobic conditions, complete removal of NPX was evident within 14 days, but anaerobic transformation was also efficient. Formation of 6-O-desmethylnaproxen, a previously reported aerobic metabolite, was also detected under anaerobic conditions and persisted for 161 days.
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    Barret, M.; Carrère, H.; Latrille, E.; Wisniewski, C.; Patureau, D. Micropollutant and Sludge Characterization for Modeling Sorption Equilibria. Environ. Sci. Technol. 2010, 44, 1100– 1106,  DOI: 10.1021/es902575d
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    Micropollutant and Sludge Characterization for Modeling Sorption Equilibria
    Barret, Maialen; Carrere, Helene; Latrille, Eric; Wisniewski, Christelle; Patureau, Dominique
    Environmental Science & Technology (2010), 44 (3), 1100-1106CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The sorption of hydrophobic micropollutants in sludge is one of the major mechanisms which drive their fate within wastewater treatment systems. The objective was to study the effect of both sludge and micropollutant characteristics on the equil. of sorption to particles and to dissolved and colloidal matter (DCM). For this purpose, the equil. consts. were measured for 13 polycyclic arom. hydrocarbons, 5 polychlorobiphenyls and the nonylphenol, and 5 different sludge types encountered in treatment systems: a primary sludge, a secondary sludge, the same secondary sludge after thermal treatment, after anaerobic digestion, and after both treatments. After thermal treatment, no more sorption to DCM was obsd. Anaerobic biol. treatment was shown to enhance micropollutants sorption to particles and to DCM of one logarithmic unit, due to matter transformation. Partial least-squares linear regressions of sorption data as a function of micropollutant and sludge properties revealed that sludge phys. and chem. characteristics were more influential than micropollutant characteristics. Two models were provided to predict the sorption of such micropollutants in any sludge. To our knowledge, this is the 1st time that a 3-compartment approach is used to accurately model micropollutant sorption in sludge and to understand the driving mechanisms.
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    Trace Elements in Anaerobic Biotechnologies; Fermoso, F. G., van Hullebusch, E., Collins, G., Roussel, J., Mucha, A. P., Esposito, G., Eds.; IWA Publishing, 2019.
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    The release of heavy metals from digested sludge by centrifugation
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    Environmental Technology (1998), 19 (5), 537-542CODEN: ENVTEV; ISSN:0959-3330. (Selper Ltd.)
    The effect of centrifugal shear on the quality of the centrates produced from both mesophilic and thermophilic sludges was examd. The sludges were obtained from two single-stage digesters and from a thermophilic/mesophilic dual digestion system. Three centrifugal forces were used; 1000, 2000 and 3000 g. Increasing the magnitude of the centrifugal force had a slight effect on the concn. of ammoniacal-nitrogen in the centrates. It also caused a decrease in the COD concns. and an increase in the metal (Ni, Cu, Zn) concns. Although there were differences between the single-stage and dual digestion systems, the quality of centrates produced from thermophilic sludges was worse than those from mesophilic sludges. The data for metals showed that nickel was different from the other two metals in that it was less tightly bound. It also showed that, although the concns. produced were less than those frequently encountered in municipal sewage, they were of a sufficient magnitude to have the potential for the inhibition of side-stream biooxidn.
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    Hargreaves, A. J.; Vale, P.; Whelan, J.; Constantino, C.; Dotro, G.; Campo, P.; Cartmell, E. Distribution of Trace Metals (Cu, Pb, Ni, Zn) between Particulate, Colloidal and Truly Dissolved Fractions in Wastewater Treatment. Chemosphere 2017, 175, 239– 246,  DOI: 10.1016/j.chemosphere.2017.02.034
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    Distribution of trace metals (Cu, Pb, Ni, Zn) between particulate, colloidal and truly dissolved fractions in wastewater treatment
    Hargreaves, Andrew J.; Vale, Peter; Whelan, Jonathan; Constantino, Carlos; Dotro, Gabriela; Campo, Pablo; Cartmell, Elise
    Chemosphere (2017), 175 (), 239-246CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
    The distribution of Cu, Pb, Ni and Zn between particulate, colloidal and truly dissolved size fractions in wastewater from a trickling filter treatment plant was investigated. Samples of influent, primary effluent, humus effluent, final effluent and sludge holding tank returns were collected and sepd. into particulate (i.e. > 0.45μm), colloidal (i.e. 1 kDa to 0.45μm), and truly dissolved (i.e. < 1 kDa) fractions using membrane filters. In the influent, substantial proportions of Cu (60%), Pb (67%), and Zn (32%) were present in the particulate fraction which was removed in conjunction with suspended particles at the works in subsequent treatment stages. In final effluent, sizeable proportions of Cu (52%), Pb (32%), Ni (44%) and Zn (68%) were found within the colloidal size fraction. Calcd. ratios of sol. metal to org. carbon suggest the metal to be adsorbed to or complexed with non-humic macromols. typically found within the colloidal size range. These findings suggest that technologies capable of removing particles within the colloidal fraction have good potential to enhance metals removal from wastewater.
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    Andrianisa, H. A.; Ito, A.; Sasaki, A.; Aizawa, J.; Umita, T. Biotransformation of Arsenic Species by Activated Sludge and Removal of Bio-Oxidised Arsenate from Wastewater by Coagulation with Ferric Chloride. Water Res. 2008, 42, 4809– 4817,  DOI: 10.1016/j.watres.2008.08.027
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    Biotransformation of arsenic species by activated sludge and removal of bio-oxidized arsenate from wastewater by coagulation with ferric chloride
    Andrianisa, Harinaivo Anderson; Ito, Ayumi; Sasaki, Atsushi; Aizawa, Jiro; Umita, Teruyuki
    Water Research (2008), 42 (19), 4809-4817CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    The potential of activated sludge to catalyze bio-oxidn. of arsenite [As(III)] to arsenate [As(V)] and bio-redn. of As(V) to As(III) was studied. In batch expts. (pH 7, 25°) using activated sludge taken from a treatment plant receiving municipal wastewater non-contaminated with As, As(III) and As(V) were rapidly biotransformed to As(V) under aerobic condition and As(III) under anaerobic one without acclimation, resp. Sub-culture of the activated sludge using a minimal liq. medium contg. 100 mg As(III)/L and no org. C source showed that aerobic As-resistant bacteria were present in the activated sludge and one of the isolated bacteria was able to chemoautotrophically oxidize As(III) to As(V). Anal. of As species in a full-scale oxidn. ditch plant receiving As-contg. wastewater revealed that both As(III) and As(V) were present in the influent, As(III) was almost completely oxidized to As(V) after supply of O by the aerator in the oxidn. ditch, As(V) oxidized was reduced to As(III) in the anaerobic zone in the ditch and in the return sludge pipe, and As(V) was the dominant species in the effluent. Co-pptn. of As(V) bio-oxidized by activated sludge in the plant with ferric hydroxide was assessed by jar tests. It was shown that the addn. of ferric chloride to mixed liquor as well as effluent achieved high removal efficiencies (>95%) of As and could decrease the residual total As concns. in the supernatant from ∼200 to <5 μg/L. It was concluded that a treatment process combining bio-oxidn. with activated sludge and coagulation with ferric chloride could be applied as an alternative technol. to treat As-contg. wastewater.
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  2. Caglar Akay, Nadin Ulrich, Ulisses Rocha, Chang Ding, Lorenz Adrian. Sequential Anaerobic–Aerobic Treatment Enhances Sulfamethoxazole Removal: From Batch Cultures to Observations in a Large-Scale Wastewater Treatment Plant. Environmental Science & Technology 2024, 58 (28) , 12609-12620. https://doi.org/10.1021/acs.est.4c00368
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  4. S. Navajas-Valiente, R. Mompó-Curell, M.J. Luján-Facundo, J.A. Mendoza-Roca, M.A. Bes-Piá. Enrichment of nutrients from anaerobically digested centrate minimizing microplastics content using a combination of membrane processes. Environmental Technology & Innovation 2024, 36 , 103758. https://doi.org/10.1016/j.eti.2024.103758
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  • Abstract

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    Figure 1

    Figure 1. Simplified scheme of the WWTP. The three sampling points at the effluent of the primary clarifier (INF), the effluent of the WWTP (EFF), and the effluent of the centrifuge (CEN) are indicated.

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    Figure 2

    Figure 2. Concentration of targeted micropollutants in centrate of a municipal WWTP over a period of 3 weeks (n = 21; for IBU and LAMO n = 7) from Jan 20th to Feb 9th. The period from Jan 13th to 19th was excluded from aggregation because disturbance in the centrifuge operation was observed. Ac-SMX, SMX, and ACET were <LOD. BEZA, PROP, BTSA, MET, TRA, BGP, and MEL detected <LOQ. Boxes: 25–75 percentile, whiskers: 95 percentile).

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    Figure 3

    Figure 3. Concentration of 4/5-MBZTR and BZTR (a,d) in centrate, (b,e) in WWTP influent, and (d,f) WWTP effluent between Jan 13th and Feb 9th. Black: phase 1, high concentration of total suspended solids in centrate, gray: phase 2, low concentration of total suspended solids in centrate.

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    Figure 4

    Figure 4. Concentrations of metals and metalloids in centrate of a municipal WWTP over a period of 3 weeks (n = 21) from Jan 20th to Feb 9th. The period from Jan 13th to 19th was excluded from aggregation because disturbance in the centrifuge operation was observed. The asterisk indicated the recommended maximum concentration in wastewater used for fertilizing plants as defined by WHO. (19)

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    Figure 5

    Figure 5. Ratio cout,aq/cin,aq of the median micropollutant concentration in filtrated centrate samples on Feb 7th to 9th and the calculated median input concentration into the digester from Jan 13th to 15th. The inset shows the TSS concentration in centrate. P-values based on the median micropollutant concentrations in centrate compared for phase 1 (black) and phase 2 (gray) (ns = not significant, **** = p < 0.0001, *** = p < 0.001, ** = p < 0.01, and * = p < 0.1).

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    Figure 6

    Figure 6. Ratio cout,aq/cin,aq calculated from the median dissolved concentration of metals and metalloids in centrate samples on Feb 7th to 9th and the calculated median input concentration into the digester from Jan 13th to 15th.

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      There is no corresponding record for this reference.
    6. 6
      Broadley, M.; Brown, P.; Cakmak, I.; Rengel, Z.; Zhao, F. Function of Nutrients: Micronutrients. Marschner’s Mineral Nutrition of Higher Plants; Elsevier, 2012; pp 191– 248.
      There is no corresponding record for this reference.
    7. 7
      Weimers, K.; Bergstrand, K.-J.; Hultberg, M.; Asp, H. Liquid Anaerobic Digestate as Sole Nutrient Source in Soilless Horticulture─Or Spiked With Mineral Nutrients for Improved Plant Growth. Front. Plant Sci. 2022, 13, 770179,  DOI: 10.3389/fpls.2022.770179
      7
      Liquid Anaerobic Digestate as Sole Nutrient Source in Soilless Horticulture-Or Spiked With Mineral Nutrients for Improved Plant Growth
      Weimers Kristina; Weimers Kristina; Bergstrand Karl-Johan; Hultberg Malin; Asp Hakan
      Frontiers in plant science (2022), 13 (), 770179 ISSN:1664-462X.
      Digestate from biogas production high in plant-available macro- and micro-nutrients could replace mineral fertilizer in protected (soilless) horticulture. Previous uses of digestate have shown that low concentrations of plant-available phosphorus (P) and sulfur (S) may be limiting factors for growth when using digestate as the sole fertilizer. In this study, digestate collected from a municipal biogas plant in Sweden was nitrified in a moving-bed biofilm reactor prior to its use as fertilizer. A greenhouse pot trial with pak choi grown in peat-based growing medium was established to assess the (i) macro- and micro-nutrient availability in the digestate, with particular focus on P and S and (ii) the effect of amending the digestate solution with nutrients considered to be lacking [P, S, magnesium (Mg), manganese (Mn), boron (B), and molybdenum (Mo)]. The results showed that plants fertilized with raw digestate suffered from S and B deficiency and early P deficiency. Supplementing the digestate with nutrients originating from mineral salts resulted in sufficient plant tissue concentrations of all elements except S. The marketable yield was similar to that achieved using standard mineral fertilizer and the dry matter yield was 17% higher. In the light of the present results, the use of nitrified digestate in soilless plant production seems like a fruitful way forward to recycle organic nutrients from waste streams. In the case where a strict organic protocol is not needed, amendment with inorganic nutrients may be a way to increase the utilization of organically derived nutrients.
    8. 8
      Keren, R.; Bingham, F. T. Boron in Water, Soils, and Plants. In Advances in Soil Science; Stewart, B. A., Ed.; Springer, New York: New York, NY, 1958; Vol. 1, pp 229– 276.
      There is no corresponding record for this reference.
    9. 9
      Fu, Q.; Malchi, T.; Carter, L. J.; Li, H.; Gan, J.; Chefetz, B. Pharmaceutical and Personal Care Products: From Wastewater Treatment into Agro-Food Systems. Environ. Sci. Technol. 2019, 53, 14083– 14090,  DOI: 10.1021/acs.est.9b06206
      9
      Pharmaceutical and Personal Care Products: From Wastewater Treatment into Agro-Food Systems
      Fu, Qiuguo; Malchi, Tomer; Carter, Laura J.; Li, Hui; Gan, Jay; Chefetz, Benny
      Environmental Science & Technology (2019), 53 (24), 14083-14090CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Irrigation with treated wastewater (TWW) and application of biosolids introduce numerous pharmaceutical and personal care products (PPCPs) into agro-food systems. While the use of TWW and biosolids has many societal benefits, introduction of PPCPs in prodn. agriculture poses potential food safety and human health risks. A comprehensive risk assessment and management scheme of PPCPs in agro-food systems is limited by multiple factors, not least the sheer no. of studied compds. and their diverse structures. Here we follow the fate of PPCPs in the H2O-soil-produce continuum by considering processes and variables that influence PPCP transfer and accumulation. By analyzing the steps in the soil-plant-human diet nexus, we propose a tiered framework as a path forward to prioritize PPCPs that could have a high potential for plant accumulation and thus pose greatest risk. This article examines research progress to date and current research challenges, highlighting the potential value of leveraging existing knowledge from decades of research on other chems. such as pesticides. A process-driven scheme is outlined to derive a short list that may be used to refocus the future research efforts on PPCPs and other analogous emerging contaminants in agro-food systems.
    10. 10
      Goldstein, M.; Shenker, M.; Chefetz, B. Insights into the Uptake Processes of Wastewater-Borne Pharmaceuticals by Vegetables. Environ. Sci. Technol. 2014, 48, 5593– 5600,  DOI: 10.1021/es5008615
      10
      Insights into the Uptake Processes of Wastewater-Borne Pharmaceuticals by Vegetables
      Goldstein, Myah; Shenker, Moshe; Chefetz, Benny
      Environmental Science & Technology (2014), 48 (10), 5593-5600CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      An increasing no. of reports on plant uptake of pharmaceutical compds. (PCs) have been recently published, raising concerns of human exposure through dietary intake. In this study, PC uptake and translocation were evaluated in cucumber and tomato plants to elucidate the effects of PC physicochem. properties, soil type, and irrigation-water quality. Nonionic PCs were taken up and accumulated at higher levels in plants grown in soils of lower org. matter and clay content. While the concn. of most PCs in cucumber and tomato leaves were of similar order, their concns. in the tomato fruit were much lower than in the cucumber fruit. This is related to differences in fruit physiol. Our data suggest that irrigation with treated wastewater reduces the bioavailability of acidic PCs for uptake by cucumber plants as compared to fresh water irrigation. This study sheds light on factors affecting the uptake of PCs by crops irrigated with treated wastewater, the governing role of PCs' physicochem. properties along with the physiol. nature of the plant, soil properties and water quality that together det. uptake, translocation, and accumulation within plant organs. Occurrence of metabolites in plant suggests that PC metab. has to be evaluated to reveal the total uptake.
    11. 11
      Dettenmaier, E. M.; Doucette, W. J.; Bugbee, B. Chemical Hydrophobicity and Uptake by Plant Roots. Environ. Sci. Technol. 2009, 43, 324– 329,  DOI: 10.1021/es801751x
      11
      Chemical Hydrophobicity and Uptake by Plant Roots
      Dettenmaier, Erik M.; Doucette, William J.; Bugbee, Bruce
      Environmental Science & Technology (2009), 43 (2), 324-329CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The transpiration stream concn. factor (TSCF), the ratio between a compd.'s concn. in the xylem to that in the soln. adjacent to the roots, is commonly used to describe the relative ability of an org. compd. to be passively transported from root to shoot. Widely cited bell-shaped curves relating TSCF to the octanol/water partition coeff. (log KOW) imply that significant root uptake and transfer into shoot tissues occurs only for compds. falling within an intermediate hydrophobicity range. However, recent lab. and field data for relatively water sol. compds. such as sulfolane, Me tert-Bu ether (MTBE), and 1,4-dioxane suggest that these relationships are not universally applicable, esp. for nonionizable, highly polar, water sol. orgs. To re-evaluate the relationship between root uptake and chem. hydrophobicity, TSCFs were measured for 25 org. chems. ranging in log KOW from -0.8 to 5 using a pressure chamber technique. Using the TSCF values measured in this study, a new empirical relationship between TSCF (0 and 1) log KOW (-0.8 to 5) is presented that indicates that nonionizable, polar, highly water sol. org. compds. are most likely to be taken up by plant roots and translocated to shoot tissue.
    12. 12
      Riemenschneider, C.; Al-Raggad, M.; Moeder, M.; Seiwert, B.; Salameh, E.; Reemtsma, T. Pharmaceuticals, Their Metabolites, and Other Polar Pollutants in Field-Grown Vegetables Irrigated with Treated Municipal Wastewater. J. Agric. Food Chem. 2016, 64, 5784– 5792,  DOI: 10.1021/acs.jafc.6b01696
      12
      Pharmaceuticals, Their Metabolites, and Other Polar Pollutants in Field-Grown Vegetables Irrigated with Treated Municipal Wastewater
      Riemenschneider, Christina; Al-Raggad, Marwan; Moeder, Monika; Seiwert, Bettina; Salameh, Elias; Reemtsma, Thorsten
      Journal of Agricultural and Food Chemistry (2016), 64 (29), 5784-5792CODEN: JAFCAU; ISSN:0021-8561. (American Chemical Society)
      The reuse of treated municipal wastewater for crop irrigation is a necessity in arid and semiarid regions but a potential entrance for emerging contaminants into the food chain. However, little attention has yet been paid to the detection of micropollutants and possible metabolites in vegetables grown under realistic field conditions. In this study, the uptake of 28 micropollutants and carbamazepine metabolites in 10 different field-grown vegetable species (among them carrot, lettuce, potato, and zucchini) from Jordan was studied. A total of 12 micropollutants and six carbamazepine metabolites, four of which have never been analyzed before in plant-uptake studies, could be detected in all of the samples in concns. ranging from 1.7 to 216 ng per g of dry wt. In edible tissues, the total concn. of micropollutants decreased in the order of leafy (247-533) > root (73-126) > fruit-bearing (5-76 ng per g of dry wt.) vegetables. A preliminary health-risk assessment for nine compds. according to the TTC concept shows no risk for seven of the micropollutats; for ciprofloxacin and 10,11-epoxycarbamazepine, however, more-specific toxicity data would be required for a refined risk assessment.
    13. 13
      EU Regulation on Minimum Requirements for Water Reuse; Regulation (EU) 2020/741 on minimum requirements for water reuse. 2020, https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32020R0741 (accessed Oct 12, 2022).
      There is no corresponding record for this reference.
    14. 14
      Yoshida, H.; Christensen, T. H.; Guildal, T.; Scheutz, C. A Comprehensive Substance Flow Analysis of a Municipal Wastewater and Sludge Treatment Plant. Chemosphere 2015, 138, 874– 882,  DOI: 10.1016/j.chemosphere.2013.09.045
      14
      A comprehensive substance flow analysis of a municipal wastewater and sludge treatment plant
      Yoshida, H.; Christensen, T. H.; Guildal, T.; Scheutz, C.
      Chemosphere (2015), 138 (), 874-882CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
      The fate of total org. carbon, 32 elements (Al, Ag, As, Ba, Be, Br, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, N, Na, Ni, P, Pb, S, Sb, Se, Sn, Sr, Ti, V, and Zn) and 4 groups of org. pollutants (linear alkylbenzene sulfonates, bis(2-ethylhexyl)phthalate, polychlorinated biphenyl and polycyclic arom. hydrocarbons) in a conventional wastewater treatment plant were assessed. Mass balances showed reasonable closures for most of the elements. However, gaseous emissions were accompanied by large uncertainties and show the limitation of mass balance based substance flow anal.Based on the assessment, it is evident that both inorg. and org. elements accumulated in the sewage sludge, with the exception of elements that are highly sol. or degradable by wastewater and sludge treatment processes. The majority of metals and metalloids were further accumulated in the incineration ash, while the org. pollutants were effectively destroyed by both biol. and thermal processes. Side streams from the sludge treatment process (dewatering and incineration) back to the wastewater treatment represented less than 1% of the total vol. entering the wastewater treatment processes, but represented significant substance flows. In contrast, the contribution by spent water from the flue gas treatment process was almost negligible. Screening of human and eco-toxicity by applying the consensus-based environmental impact assessment method USEtox addressing 15 inorg. constituents showed that removal of inorg. constituents by the wastewater treatment plant reduced the toxic impact potential by 87-92%.
    15. 15
      Karvelas, M.; Katsoyiannis, A.; Samara, C. Occurrence and Fate of Heavy Metals in the Wastewater Treatment Process. Chemosphere 2003, 53, 1201– 1210,  DOI: 10.1016/S0045-6535(03)00591-5
      15
      Occurrence and fate of heavy metals in the wastewater treatment process
      Karvelas, Michalis; Katsoyiannis, Athanasios; Samara, Constantini
      Chemosphere (2003), 53 (10), 1201-1210CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Science Ltd.)
      The occurrence and the fate of heavy metals (Cd, Pb, Mn, Cu, Zn, Fe and Ni) were studied in the activated sludge wastewater treatment plant of the city of Thessaloniki, Greece. Wastewater and sludge samples were collected from the influent wastewater, the effluent of the primary sedimentation tank, the effluent of the secondary sedimentation tank, sludge from the primary sedimentation tank, activated sludge from the recirculation stream, and the digested/dewatered sludge. The distribution of metals between the aq. and the solid phase of wastewater was studied. Good exponential correlation was found between the metal partition coeff., logKp, and the suspended solids concn. The mass balance of heavy metals in the primary, secondary and the whole treatment process showed good closures for all metal species. The relative distribution of individual heavy metals in the treated effluent and the sludge streams indicated that Mn and Cu are primarily (>70%) accumulated in the sludge, while 47-63% of Cd, Cr, Pb, Fe, Ni and Zn remain in the treated effluent.
    16. 16
      Ziolko, D.; Martin, O. V.; Scrimshaw, M. D.; Lester, J. N. An Evaluation of Metal Removal During Wastewater Treatment: The Potential to Achieve More Stringent Final Effluent Standards. Crit. Rev. Environ. Sci. Technol. 2011, 41, 733– 769,  DOI: 10.1080/10643380903140299
      16
      An Evaluation of Metal Removal During Wastewater Treatment: The Potential to Achieve More Stringent Final Effluent Standards
      Ziolko, D.; Martin, O. V.; Scrimshaw, M. D.; Lester, J. N.
      Critical Reviews in Environmental Science and Technology (2011), 41 (8), 733-769CODEN: CRETEK; ISSN:1064-3389. (Taylor & Francis, Inc.)
      A review. Metals are of particular importance in relation to water quality, and concern regarding the impact of these contaminants on biodiversity is being encapsulated within the latest water-related legislation such as the Water Framework Directive in Europe and criteria revisions to the Clean Water Act in the United States. This review undertakes an evaluation of the potential of 2-stage wastewater treatment consisting of primary sedimentation and biol. treatment in the form of activated sludge processes, to meet more stringent discharge consents that are likely to be introduced as a consequence. The legislation, sources of metals, and mechanisms responsible for their removal are discussed, to elucidate possible pathways by which the performance of conventional processes may be optimized or enhanced. Improvements in effluent quality, achievable by reducing concns. of suspended solids or BOD, may also reduce metal concns. although meeting possible requirements for the removal of copper my be challenging.
    17. 17
      Mailler, R.; Gasperi, J.; Patureau, D.; Vulliet, E.; Delgenes, N.; Danel, A.; Deshayes, S.; Eudes, V.; Guerin, S.; Moilleron, R.; Chebbo, G.; Rocher, V. Fate of Emerging and Priority Micropollutants during the Sewage Sludge Treatment: Case Study of Paris Conurbation. Part 1: Contamination of the Different Types of Sewage Sludge. Waste Manage. 2017, 59, 379– 393,  DOI: 10.1016/j.wasman.2016.11.010
      17
      Fate of emerging and priority micropollutants during the sewage sludge treatment: Case study of Paris conurbation. Part 1: Contamination of the different types of sewage sludge
      Mailler, R.; Gasperi, J.; Patureau, D.; Vulliet, E.; Delgenes, N.; Danel, A.; Deshayes, S.; Eudes, V.; Guerin, S.; Moilleron, R.; Chebbo, G.; Rocher, V.
      Waste Management (Oxford, United Kingdom) (2017), 59 (), 379-393CODEN: WAMAE2; ISSN:0956-053X. (Elsevier Ltd.)
      This article provides data on the contamination of different kinds of sludge (raw, centrifuged, digested, thermally dried sludge and sludge cake) from Paris conurbation by 71 various pollutants including pharmaceutical products (PHPs), hormones, perfluorinated acids (PFAs), linear alkylbenzene sulfonate (LAS), alkylphenols (APs), phthalates (PAEs), polycyclic arom. hydrocarbons (PAHs) and polychlorobiphenyls (PCBs). Very high contents of LAS (0.1-10g/kg dry matter - DM) compared to other compds. were found in all types of sludge followed by DEHP (10-100 mg/kg DM) and fluoroquinolones (1-100 mg/kg DM). APs were measured at intermediary contents in Parisian sludge, lying in the 2-20 mg/kg DM range. Finally, hormones, PAHs, PCBs, PAEs, PFAs and the remaining PHPs were all found at contents lower than 1 mg/kg DM. For most compds. (PHPs, PFOS, DEHP, PAHs), no significant differences in the micropollutant contents were found for similar types of sludge from different WWTP in Paris, highlighting the homogeneity of sludge contamination in downstream Paris catchment. The variability of concn. is rather high (coeff. of variation >100%) for several PHPs, PFAs or PCBs while it is moderate (<100%) or low (<50%) for fluoroquinolones, hormones, PAHs, APs or LAS.
    18. 18
      Narumiya, M.; Nakada, N.; Yamashita, N.; Tanaka, H. Phase Distribution and Removal of Pharmaceuticals and Personal Care Products during Anaerobic Sludge Digestion. J. Hazard. Mater. 2013, 260, 305– 312,  DOI: 10.1016/j.jhazmat.2013.05.032
      18
      Phase distribution and removal of pharmaceuticals and personal care products during anaerobic sludge digestion
      Narumiya, Masanori; Nakada, Norihide; Yamashita, Naoyuki; Tanaka, Hiroaki
      Journal of Hazardous Materials (2013), 260 (), 305-312CODEN: JHMAD9; ISSN:0304-3894. (Elsevier B.V.)
      The fate and removal of 48 pharmaceuticals and personal care products (PPCPs) during anaerobic digestion of sewage sludge were investigated in 4 full-scale sewage treatment plants (STPs). We measured concns. in both the liq. and solid phases of the sludge to compare the distribution ratios (Kd) between phases before and after digestion. The results showed changes in Kd values of PPCPs with carboxyl or amino functional groups, probably due to a shift of dissocn. equil. with the increase in pH. Sulfamethoxazole and trimethoprim were almost completely degraded (>90%); triclosan, triclocarban, and ofloxacin were moderately degraded (around 30-50%); but carbamazepine was not eliminated. To our knowledge, this is the first report that shows (i) the occurrence and removal of several tens of PPCPs by anaerobic sludge digestion in full-scale municipal STPs and (ii) the change of distribution between the liq. and solid phases during digestion.
    19. 19
      WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater. 4: Excreta and Greywater Use in Agriculture; World Health Organization: Geneva, 2006.
      There is no corresponding record for this reference.
    20. 20
      Water Framework Directive. EU Directive 2000/60/EC on establishing a framework for Community action in the field of water policy. 2000, https://eur-lex.europa.eu/resource.html?uri=cellar:5c835afb-2ec6-4577-bdf8-756d3d694eeb.0004.02/DOC_1&format=PDF (accessed Oct 12, 2022).
      There is no corresponding record for this reference.
    21. 21
      EU Regulation on Foodstuffs. EU Regulation (EC) No 1881/2006 on setting maximum levels for certain contaminants in foodstuffs. 2006, https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2006:364:0005:0024:EN:PDF (accessed Oct 12, 2022).
      There is no corresponding record for this reference.
    22. 22
      EU Directive Sewage Sludge in Agriculture. EU Directive 86/278/EEC on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture. 1986, https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:31986L0278 (accessed Oct 12, 2022).
      There is no corresponding record for this reference.
    23. 23
      Kahl, S.; Nivala, J.; van Afferden, M.; Müller, R. A.; Reemtsma, T. Effect of Design and Operational Conditions on the Performance of Subsurface Flow Treatment Wetlands: Emerging Organic Contaminants as Indicators. Water Res. 2017, 125, 490– 500,  DOI: 10.1016/j.watres.2017.09.004
      23
      Effect of design and operational conditions on the performance of subsurface flow treatment wetlands: Emerging organic contaminants as indicators
      Kahl, Stefanie; Nivala, Jaime; van Afferden, Manfred; Mueller, Roland A.; Reemtsma, Thorsten
      Water Research (2017), 125 (), 490-500CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Six pilot-scale subsurface flow treatment wetlands loaded with primary treated municipal wastewater were monitored over one year for classical wastewater parameters and a set of emerging org. compds. (EOCs) serving as process indicators for biodegrdn.: caffeine, ibuprofen, naproxen, benzotriazole, diclofenac, acesulfame, and carbamazepine. The wetland technologies investigated included conventional horizontal flow, unsatd. vertical flow (single and two-stage), horizontal flow with aeration, vertical flow with aeration, and reciprocating. Treatment efficiency for classical wastewater parameters and EOCs generally increased with increasing design complexity and dissolved oxygen concns. The two aerated wetlands and the two-stage vertical flow system showed the highest EOC removal, and the best performance in warm season and most robust performance in the cold season. These three systems performed better than the adjacent conventional WWTP with respect to EOC removal. Acesulfame was obsd. to be removed (>90%) by intensified wetland systems and with use of a tertiary treatment sand filter during the warm season. Elevated temp. and high oxygen content (aerobic conditions) proved beneficial for EOC removal. For EOCs of moderate to low biodegradability, the co-occurrence of aerobic conditions and low content of readily available carbon appears essential for efficient removal. Such conditions occurred in the aerated systems and with use of a tertiary treatment sand filter.
    24. 24
      Kreuzig, R.; Haller-Jans, J.; Bischoff, C.; Leppin, J.; Germer, J.; Mohr, M.; Bliedung, A.; Dockhorn, T. Reclaimed Water Driven Lettuce Cultivation in a Hydroponic System: The Need of Micropollutant Removal by Advanced Wastewater Treatment. Environ. Sci. Pollut. Res. 2021, 28, 50052– 50062,  DOI: 10.1007/s11356-021-14144-6
      24
      Reclaimed water driven lettuce cultivation in a hydroponic system: the need of micropollutant removal by advanced wastewater treatment
      Kreuzig Robert; Haller-Jans Jaqueline; Bischoff Cornelia; Leppin Johannes; Germer Jorn; Mohr Marius; Bliedung Alexa; Dockhorn Thomas
      Environmental science and pollution research international (2021), 28 (36), 50052-50062 ISSN:.
      For a novel approach of resource-efficient water reuse, a municipal wastewater treatment plant was extended at pilot scale for advanced wastewater treatment, i.e., ozonation and biological activated carbon filtration, and a hydroponic system for reclaimed water driven lettuce cultivation. The treatment specific wastewater lines with the corresponding lettuce plants, differentiated into roots and shoots, were monitored for priority wastewater micropollutants, i.e., acesulfame (sweetener), caffeine (stimulant), carbamazepine, diclofenac, ibuprofen, sulfamethoxazole with acetyl-sulfamethoxazole (human pharmaceuticals), 1H-benzotriazole, and 4/5-methylbenzotriazole (industrial chemicals). As clearly demonstrated, conventional tertiary treatment could not efficiently clean up wastewater. Removal efficiencies ranged from 3% for carbamazepine to 100% for ibuprofen. The resulting pollution of the hydroponic water lines led to the accumulation of acesulfame, carbamazepine, and diclofenac in lettuce root systems at 32.0, 69.5, and 135 μg kg(-1) and in the uptake of acesulfame and carbamazepine into lettuce shoots at 23.4 and 120 μg kg(-1) dry weight, respectively. In contrast, both advanced treatment technologies when operating under optimized conditions achieved removal efficiencies of > 90% also for persistent micropollutants. Minimizing the pollution of reclaimed water thus met one relevant need for hydroponic lettuce cultivation.
    25. 25
      Düngemittelverordnung, V. German Ordinance on Fertilizer. 2012, https://www.gesetze-im-internet.de/d_mv_2012/D%C3%BCMV.pdf (accessed Oct 12, 2022).
      There is no corresponding record for this reference.
    26. 26
      Lee, E.; Rout, P. R.; Bae, J. The Applicability of Anaerobically Treated Domestic Wastewater as a Nutrient Medium in Hydroponic Lettuce Cultivation: Nitrogen Toxicity and Health Risk Assessment. Sci. Total Environ. 2021, 780, 146482,  DOI: 10.1016/j.scitotenv.2021.146482
      26
      The applicability of anaerobically treated domestic wastewater as a nutrient medium in hydroponic lettuce cultivation: Nitrogen toxicity and health risk assessment
      Lee, Eunseok; Rout, Prangya Ranjan; Bae, Jaeho
      Science of the Total Environment (2021), 780 (), 146482CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      The applicability of anaerobic effluent (AE) from an anaerobic membrane bioreactor (AnMBR) treating domestic wastewater as a nutrient medium was evaluated through hydroponic cultivation of lettuce. The growth of lettuce plants on AE media was significantly inhibited to 31-40% in height and 36-48% in no. of leaves compared to that on half-strength Hoagland soln. (HHS) as a control. The primary cause of inhibition was nitrite toxicity as induced by partial nitrification. Therefore, the nitrification of AE as a pre-treatment step was adopted to prevent the toxicity of nitrite. The heights of lettuce grown on nitrified anaerobic effluent (NAE) and nitrified anaerobic effluent with 96 mg/L sulfate (NAES) were in the range of 11.4-11.5 cm and was comparable to that on control soln. (11.4 cm). The potential health risk for heavy metals was insignificant based on health risk index (HRI < 1) and targeted hazardous quotient (THQ < 1). These results show that efficient crop prodn. can be achieved with AE, but suitable pre-treatment steps should be followed.
    27. 27
      Campbell, A. J. The Behaviour of Pharmaceuticals in Anaerobic Digestor Sludge. Ph.D. Dissertation; University of Portsmouth: Portsmouth, 2013.
      There is no corresponding record for this reference.
    28. 28
      Carballa, M.; Omil, F.; Ternes, T.; Lema, J. M. Fate of Pharmaceutical and Personal Care Products (PPCPs) during Anaerobic Digestion of Sewage Sludge. Water Res. 2007, 41, 2139– 2150,  DOI: 10.1016/j.watres.2007.02.012
      28
      Fate of pharmaceutical and personal care products (PPCPs) during anaerobic digestion of sewage sludge
      Carballa, Marta; Omil, Francisco; Ternes, Thomas; Lema, Juan M.
      Water Research (2007), 41 (10), 2139-2150CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      The behavior of 13 pharmaceutical and personal care products (PPCPs) has been studied during anaerobic digestion of sewage sludge: 2 musks (Galaxolide and Tonalide), one tranquilliser (Diazepam), one anti-epileptic (Carbamazepine), 3 anti-phlogistics (Ibuprofen, Naproxen and Diclofenac), 2 antibiotics (Sulfamethoxazole and Roxithromycin), one x-ray contrast medium (Iopromide) and 3 estrogens (Estrone, 17β-oestradiol and 17α-ethinylestradiol). Two parallel processes were carried out, one in mesophilic range (37°) and the other in thermophilic range (55°). The effect of temp. and sludge retention time (SRT) was analyzed. Among the substances considered, the higher removal efficiencies were achieved for the antibiotics, natural estrogens, musks and Naproxen. For the other compds., the values were 20-60%, except for Carbamazepine, which showed no elimination. The removal of estrogens, Diazepam and Diclofenac occurred after sludge adaptation. In general, no influence of SRT and temp. on PPCPs removal was obsd. Considering the difficulty of obtaining reliable PPCPs concns., esp. those corresponding to the fractions sorbed onto sludge, a methodol. to validate the exptl. data has been developed and successfully applied.
    29. 29
      De Wever, H.; Verachtert, H. Biodegradation and Toxicity of Benzothiazoles. Water Res. 1997, 31, 2673– 2684,  DOI: 10.1016/S0043-1354(97)00138-3
      29
      Biodegradation and toxicity of benzothiazoles
      De Wever, H.; Verachtert, H.
      Water Research (1997), 31 (11), 2673-2684CODEN: WATRAG; ISSN:0043-1354. (Elsevier)
      A review with many refs. Benzothiazoles appear in the environment mainly as a result of their prodn. and use as rubber vulcanization accelerators. This review focuses on the potential environmental hazard these substances pose and their biol. removal from and transformation in aquatic and terrestrial ecosystems. Benzothiazole degrdn. by mixed and pure cultures and the toxic impact on microorganisms are discussed. It is concluded that biol. purifn. of wastewaters from 2-mercaptobenzothiazole (MBT) prodn. can be achieved, provided the MBT influent concn. is 100 mg/L at max. So far, no attempts have been made to study the mol. mechanisms of benzothiazole degrdn., mainly due to the unavailability of axenic cultures capable of benzothiazole breakdown. However, the toxic effects of MBT towards microorganisms have been studied and can be attributed to its metal chelating properties and(or) its interference with membrane-bound (co)enzymes in particular.
    30. 30
      Kloepfer, A.; Gnirss, R.; Jekel, M.; Reemtsma, T. Occurrence of Benzothiazoles in Municipal Wastewater and Their Fate in Biological Treatment. Water Sci. Technol. 2004, 50, 203– 208,  DOI: 10.2166/wst.2004.0329
      30
      Occurrence of benzothiazoles in municipal wastewater and their fate in biological treatment
      Kloepfer, A.; Gnirss, R.; Jekel, M.; Reemtsma, T.
      Water Science and Technology (2004), 50 (5, Assessment and Control of Hazardous Substances in Water), 203-208CODEN: WSTED4; ISSN:0273-1223. (IWA Publishing)
      A no. of 2-substituted benzothiazoles that are known to be used as fungicides, corrosion inhibitors, and vulcanization accelerators in industry were analyzed in municipal wastewater and the effluents of activated sludge and membrane bioreactor (MBR) treatment over a three month period. All six analytes were regularly detected in the municipal wastewater by LC-MS and amt. to a total concn. of 3.4 μg/L. Of these compds. benzothiazole-2-sulfonic acid (1,700 ng/L), benzothiazole (850 ng/L) and 2-hydroxybenzothiazole (500 ng/L) were most prominent. The source of the benzothiazole emission is yet unknown. Activated sludge treatment did not reduce total benzothiazole concn. significantly. Removals of the individual compds. ranged from 90% for 2-mercaptobenzothiazole and 70% for hydroxybenzothiazole to 40% for benzothiazole. The concn. of benzothiazole-2-sulfonic acid increased by 20%, whereas 2-methylthiobenzothiazole increased by 160% during activated sludge treatment, likely due to the methylation of mercaptobenzothiazole. Total benzothiazole removal in two parallel operated MBRs was significantly better (43%) than in the conventional activated sludge treatment. Namely benzothiazole and benzothiazole-2-sulfonic acid were more effectively removed. This first systematic study on the occurrence of benzothiazoles in municipal wastewater has shown that this is a relevant class of trace contaminants in municipal wastewater which is only incompletely removed in biol. wastewater treatment. Emission from sewage treatment is dominated by the most polar benzothiazole-2-sulfonic acid. MBR treatment may reduce but cannot avoid this emission.
    31. 31
      Gonzalez-Gil, L.; Krah, D.; Ghattas, A.-K.; Carballa, M.; Wick, A.; Helmholz, L.; Lema, J. M.; Ternes, T. A. Biotransformation of Organic Micropollutants by Anaerobic Sludge Enzymes. Water Res. 2019, 152, 202– 214,  DOI: 10.1016/j.watres.2018.12.064
      31
      Biotransformation of organic micropollutants by anaerobic sludge enzymes
      Gonzalez-Gil, Lorena; Krah, Daniel; Ghattas, Ann-Kathrin; Carballa, Marta; Wick, Arne; Helmholz, Lissa; Lema, Juan M.; Ternes, Thomas A.
      Water Research (2019), 152 (), 202-214CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Biotransformation of org. micropollutants (OMPs) in wastewater treatment plants ultimately depends on the enzymic activities developed in each biol. process. However, few research efforts have been made to clarify and identify the role of enzymes on the removal of OMPs, which is an essential knowledge to det. the biotransformation potential of treatment technologies. Therefore, the purpose of the present study was to investigate the enzymic transformation of 35 OMPs under anaerobic conditions, which have been even less studied than aerobic systems. Initially, 13 OMPs were identified to be significantly biotransformed (>20%) by anaerobic sludge obtained from a full-scale anaerobic digester, predestining them as potential targets of anaerobic enzymes. Native enzymes were extd. from this anaerobic sludge to perform transformation assays with the OMPs. In addn., the effect of detergents to recover membrane enzymes, as well as the effects of cofactors and inhibitors to promote and suppress specific enzymic activities were evaluated. In total, it was possible to recover enzymic activities towards 10 out of these 13 target OMPs (acetyl-sulfamethoxazole and its transformation product sulfamethoxazole, acetaminophen, atenolol, clarithromycin, citalopram, climbazole, erythromycin, and terbutryn, venlafaxine) as well as towards 8 non-target OMPs (diclofenac, iopamidol, acyclovir, acesulfame, and 4 different hydroxylated metabolites of carbamazepine). Some enzymic activities likely involved in the anaerobic biotransformation of these OMPs were identified. Thereby, this study is a starting point to unravel the still enigmatic biotransformation of OMPs in wastewater treatment systems.
    32. 32
      Gonzalez-Gil, L.; Papa, M.; Feretti, D.; Ceretti, E.; Mazzoleni, G.; Steimberg, N.; Pedrazzani, R.; Bertanza, G.; Lema, J. M.; Carballa, M. Is Anaerobic Digestion Effective for the Removal of Organic Micropollutants and Biological Activities from Sewage Sludge?. Water Res. 2016, 102, 211– 220,  DOI: 10.1016/j.watres.2016.06.025
      32
      Is anaerobic digestion effective for the removal of organic micropollutants and biological activities from sewage sludge?
      Gonzalez-Gil, L.; Papa, M.; Feretti, D.; Ceretti, E.; Mazzoleni, G.; Steimberg, N.; Pedrazzani, R.; Bertanza, G.; Lema, J. M.; Carballa, M.
      Water Research (2016), 102 (), 211-220CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      The occurrence of emerging org. micropollutants (OMPs) in sewage sludge has been widely reported; nevertheless, their fate during sludge treatment remains unclear. The objective of this work was to study the fate of OMPs during mesophilic and thermophilic anaerobic digestion (AD), the most common processes used for sludge stabilization, by using raw sewage sludge without spiking OMPs. Moreover, the results of anal. chem. were complemented with biol. assays in order to verify the possible adverse effects (estrogenic and genotoxic) on the environment and human health in view of an agricultural (re)use of digested sludge. Musk fragrances (AHTN, HHCB), ibuprofen (IBP) and triclosan (TCS) were the most abundant compds. detected in sewage sludge. In general, the efficiency of the AD process was not dependent on operational parameters but compd.-specific: some OMPs were highly biotransformed (e.g. sulfamethoxazole and naproxen), while others were only slightly affected (e.g. IBP and TCS) or even unaltered (e.g. AHTN and HHCB). The MCF-7 assay evidenced that estrogenicity removal was driven by temp. The Ames test did not show point mutation in Salmonella typhimurium while the Comet test exhibited a genotoxic effect on human leukocytes attenuated by AD. This study highlights the importance of combining chem. anal. and biol. activities in order to establish appropriate operational strategies for a safer disposal of sewage sludge. Actually, it was demonstrated that temp. has an insignificant effect on the disappearance of the parent compds. while it is crucial to decrease estrogenicity.
    33. 33
      Psoma, A. K.; Attiti, S.; Arvaniti, O.; Stasinakis, A. S.; Thomaidis, N. S. Fate of Selected Emerging Pollutants and Formation of Transformation Products in Activated Sludge Batch Reactors under Aerobic/Anoxic/Anaerobic Conditions. Proceedings of the 14th International Conference on Environmental Science and Technology; Rhodes, 2015.
      There is no corresponding record for this reference.
    34. 34
      Lahti, M.; Oikari, A. Microbial Transformation of Pharmaceuticals Naproxen, Bisoprolol, and Diclofenac in Aerobic and Anaerobic Environments. Arch. Environ. Contam. Toxicol. 2011, 61, 202– 210,  DOI: 10.1007/s00244-010-9622-2
      34
      Microbial transformation of pharmaceuticals naproxen, bisoprolol, and diclofenac in aerobic and anaerobic environments
      Lahti, Marja; Oikari, Aimo
      Archives of Environmental Contamination and Toxicology (2011), 61 (2), 202-210CODEN: AECTCV; ISSN:0090-4341. (Springer)
      Although biotransformation is generally considered to be the main process by which to remove pharmaceuticals, both in sewage treatment plants and in aquatic environments, quant. information on specific compds. is scarce. In this study, the transformations of diclofenac (DCF), naproxen (NPX), and bisoprolol (BSP) were studied under aerobic and anaerobic conditions using inocula taken from activated and digested sludge processes, resp. Whereas concn. decays were monitored by LC-tandem mass spectrometry, oxygen consumption and methane prodn. were used for the evaluation of the performance of overall conditions. DCF was recalcitrant against both aerobic and anaerobic biotransformation. More than one third of the BSP disappeared under aerobic conditions, whereas only 14% was anaerobically biotransformed in 161 days. Under aerobic conditions, complete removal of NPX was evident within 14 days, but anaerobic transformation was also efficient. Formation of 6-O-desmethylnaproxen, a previously reported aerobic metabolite, was also detected under anaerobic conditions and persisted for 161 days.
    35. 35
      Barret, M.; Carrère, H.; Latrille, E.; Wisniewski, C.; Patureau, D. Micropollutant and Sludge Characterization for Modeling Sorption Equilibria. Environ. Sci. Technol. 2010, 44, 1100– 1106,  DOI: 10.1021/es902575d
      35
      Micropollutant and Sludge Characterization for Modeling Sorption Equilibria
      Barret, Maialen; Carrere, Helene; Latrille, Eric; Wisniewski, Christelle; Patureau, Dominique
      Environmental Science & Technology (2010), 44 (3), 1100-1106CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The sorption of hydrophobic micropollutants in sludge is one of the major mechanisms which drive their fate within wastewater treatment systems. The objective was to study the effect of both sludge and micropollutant characteristics on the equil. of sorption to particles and to dissolved and colloidal matter (DCM). For this purpose, the equil. consts. were measured for 13 polycyclic arom. hydrocarbons, 5 polychlorobiphenyls and the nonylphenol, and 5 different sludge types encountered in treatment systems: a primary sludge, a secondary sludge, the same secondary sludge after thermal treatment, after anaerobic digestion, and after both treatments. After thermal treatment, no more sorption to DCM was obsd. Anaerobic biol. treatment was shown to enhance micropollutants sorption to particles and to DCM of one logarithmic unit, due to matter transformation. Partial least-squares linear regressions of sorption data as a function of micropollutant and sludge properties revealed that sludge phys. and chem. characteristics were more influential than micropollutant characteristics. Two models were provided to predict the sorption of such micropollutants in any sludge. To our knowledge, this is the 1st time that a 3-compartment approach is used to accurately model micropollutant sorption in sludge and to understand the driving mechanisms.
    36. 36
      Trace Elements in Anaerobic Biotechnologies; Fermoso, F. G., van Hullebusch, E., Collins, G., Roussel, J., Mucha, A. P., Esposito, G., Eds.; IWA Publishing, 2019.
      There is no corresponding record for this reference.
    37. 37
      El-Aassar, A.; Kitcher, D.; Forster, C. F. The Release of Heavy Metals from Digested Sludge by Centrifugation. Environ. Technol. 1998, 19, 537– 542,  DOI: 10.1080/09593331908616710
      37
      The release of heavy metals from digested sludge by centrifugation
      El-Aassar, A.; Kitcher, D.; Forster, C. F.
      Environmental Technology (1998), 19 (5), 537-542CODEN: ENVTEV; ISSN:0959-3330. (Selper Ltd.)
      The effect of centrifugal shear on the quality of the centrates produced from both mesophilic and thermophilic sludges was examd. The sludges were obtained from two single-stage digesters and from a thermophilic/mesophilic dual digestion system. Three centrifugal forces were used; 1000, 2000 and 3000 g. Increasing the magnitude of the centrifugal force had a slight effect on the concn. of ammoniacal-nitrogen in the centrates. It also caused a decrease in the COD concns. and an increase in the metal (Ni, Cu, Zn) concns. Although there were differences between the single-stage and dual digestion systems, the quality of centrates produced from thermophilic sludges was worse than those from mesophilic sludges. The data for metals showed that nickel was different from the other two metals in that it was less tightly bound. It also showed that, although the concns. produced were less than those frequently encountered in municipal sewage, they were of a sufficient magnitude to have the potential for the inhibition of side-stream biooxidn.
    38. 38
      Hargreaves, A. J.; Vale, P.; Whelan, J.; Constantino, C.; Dotro, G.; Campo, P.; Cartmell, E. Distribution of Trace Metals (Cu, Pb, Ni, Zn) between Particulate, Colloidal and Truly Dissolved Fractions in Wastewater Treatment. Chemosphere 2017, 175, 239– 246,  DOI: 10.1016/j.chemosphere.2017.02.034
      38
      Distribution of trace metals (Cu, Pb, Ni, Zn) between particulate, colloidal and truly dissolved fractions in wastewater treatment
      Hargreaves, Andrew J.; Vale, Peter; Whelan, Jonathan; Constantino, Carlos; Dotro, Gabriela; Campo, Pablo; Cartmell, Elise
      Chemosphere (2017), 175 (), 239-246CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
      The distribution of Cu, Pb, Ni and Zn between particulate, colloidal and truly dissolved size fractions in wastewater from a trickling filter treatment plant was investigated. Samples of influent, primary effluent, humus effluent, final effluent and sludge holding tank returns were collected and sepd. into particulate (i.e. > 0.45μm), colloidal (i.e. 1 kDa to 0.45μm), and truly dissolved (i.e. < 1 kDa) fractions using membrane filters. In the influent, substantial proportions of Cu (60%), Pb (67%), and Zn (32%) were present in the particulate fraction which was removed in conjunction with suspended particles at the works in subsequent treatment stages. In final effluent, sizeable proportions of Cu (52%), Pb (32%), Ni (44%) and Zn (68%) were found within the colloidal size fraction. Calcd. ratios of sol. metal to org. carbon suggest the metal to be adsorbed to or complexed with non-humic macromols. typically found within the colloidal size range. These findings suggest that technologies capable of removing particles within the colloidal fraction have good potential to enhance metals removal from wastewater.
    39. 39
      Andrianisa, H. A.; Ito, A.; Sasaki, A.; Aizawa, J.; Umita, T. Biotransformation of Arsenic Species by Activated Sludge and Removal of Bio-Oxidised Arsenate from Wastewater by Coagulation with Ferric Chloride. Water Res. 2008, 42, 4809– 4817,  DOI: 10.1016/j.watres.2008.08.027
      39
      Biotransformation of arsenic species by activated sludge and removal of bio-oxidized arsenate from wastewater by coagulation with ferric chloride
      Andrianisa, Harinaivo Anderson; Ito, Ayumi; Sasaki, Atsushi; Aizawa, Jiro; Umita, Teruyuki
      Water Research (2008), 42 (19), 4809-4817CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      The potential of activated sludge to catalyze bio-oxidn. of arsenite [As(III)] to arsenate [As(V)] and bio-redn. of As(V) to As(III) was studied. In batch expts. (pH 7, 25°) using activated sludge taken from a treatment plant receiving municipal wastewater non-contaminated with As, As(III) and As(V) were rapidly biotransformed to As(V) under aerobic condition and As(III) under anaerobic one without acclimation, resp. Sub-culture of the activated sludge using a minimal liq. medium contg. 100 mg As(III)/L and no org. C source showed that aerobic As-resistant bacteria were present in the activated sludge and one of the isolated bacteria was able to chemoautotrophically oxidize As(III) to As(V). Anal. of As species in a full-scale oxidn. ditch plant receiving As-contg. wastewater revealed that both As(III) and As(V) were present in the influent, As(III) was almost completely oxidized to As(V) after supply of O by the aerator in the oxidn. ditch, As(V) oxidized was reduced to As(III) in the anaerobic zone in the ditch and in the return sludge pipe, and As(V) was the dominant species in the effluent. Co-pptn. of As(V) bio-oxidized by activated sludge in the plant with ferric hydroxide was assessed by jar tests. It was shown that the addn. of ferric chloride to mixed liquor as well as effluent achieved high removal efficiencies (>95%) of As and could decrease the residual total As concns. in the supernatant from ∼200 to <5 μg/L. It was concluded that a treatment process combining bio-oxidn. with activated sludge and coagulation with ferric chloride could be applied as an alternative technol. to treat As-contg. wastewater.
    40. 40
      Yekta, S. S.; Svensson, B. H.; Björn, A.; Skyllberg, U. Thermodynamic Modeling of Iron and Trace Metal Solubility and Speciation under Sulfidic and Ferruginous Conditions in Full Scale Continuous Stirred Tank Biogas Reactors. Appl. Geochem. 2014, 47, 61– 73,  DOI: 10.1016/j.apgeochem.2014.05.001
      There is no corresponding record for this reference.

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