Nutrient removal also extracts pharmaceuticals
Sewage treatment plants that operate with longer solids retention times remove more pharmaceuticals and personal care products.
Traditional sewage treatment plants are seen as a primary source of trace concentrations of household and industrial chemicals in rivers and streams. That's hardly a surprise, because conventional treatment plants weren't designed to remove drugs, cosmetics, soaps, deodorants, insect repellents, and other exotic contaminants from effluent discharged into waterways. But tightening up current methods on nutrient pollution may be all that's needed. Research now shows that actions taken at facilities to reduce nutrients, such as nitrogen and phosphorus, are also removing a good portion of pharmaceuticals and personal care products (PPCPs).
In one of the first U.S. studies to look at how longer solids retention times (SRTs) affect PPCP removal, researchers from MWH, an environmental engineering consulting firm, characterized the passage of various PPCPs through six treatment plants of varying sizes in California and New Mexico. The SRT is how long a facility holds on to sludge to allow processing by microbes. The researchers found that SRTs of 5–15 days were sufficient to remove many of the 20 compounds they were targeting that are commonly detected in wastewaters and waterways.
Still, some resisted biodegradation. The most problematic compounds were fragrances, such as musk ketone and galaxolide, and trichloroethyl phosphate, a flame retardant, says Joan Oppenheimer, an environmental scientist with MWH. Oppenheimer presented her findings in October at the Water Environment Federation's conference in Dallas, Texas.
The new research validates data from Europe, where similar studies have been conducted at the bench scale or at small full-scale treatment plants. The MWH study, by contrast, sampled large full-scale plants, which operate in major metropolitan areas, with capacities ranging from 5 to greater than 300 million gallons per day. All were conventional activated-sludge plants, the industry standard, operating at SRTs ranging between 0.5 and 30 days.
Oppenheimer and her colleagues also looked at PPCP removal through subsequent filters and disinfectants as well as newer treatment processes, such as membrane bioreactors, but for the most part they found no additional removal. Reverse osmosis after regular filtration, however, did reduce all of the compounds to below detection limits.
What's encouraging about these findings is that a push by the U.S. EPA and states "to go to increased nutrient reduction also helps this problem" of PPCPs, Oppenheimer says. "Even if we don't know what all these compounds are, as we go to longer SRTs, we've got a more diverse community of microorganisms, and we seem to be able to reduce more of them." No federal standards exist on nutrients, just criteria guidelines that EPA issued in 2001. State regulatory agencies are supposed to either implement EPA's criteria or develop their own, but progress has been very slow.
The significance of the MWH study is that "it was done in the U.S. with our style of operation and our contaminants and that it confirms some of the same results that have been seen in Europe," says Rhodes Trussell, head of Trussell Technologies, Inc.
Rolf Halden, an assistant professor of environmental health sciences at the Johns Hopkins Bloomberg School of Public Health, agrees. "This study provides additional evidence for the notion that SRT is an important operational parameter influencing PPCP removal during biological wastewater treatment," he says.
Many sewage treatment plants in the U.S. commonly operate with very low SRTs, Trussell notes. "If we make a decision as a nation that we want to maximize removal of these compounds, and there are a number of them, the science is showing that longer SRTs will be necessary," he points out.
Treated wastewater is also the source water for indirect potable reuse projects, Oppenheimer adds. Better understanding of the fate of these compounds in treatment plants helps "us figure out what we can do about conventional treatment processes to enhance the quality of this source," she says.
