Phosphorus removal: how low can we go?
Faced with over-enriched waterways, sewage treatment plants across the U.S. must meet ever-lower discharge limits.
Until recently, water pollution control experts thought that the limits of technology had been reached for nitrogen and phosphorus removal from sewage treatment plants. But as urban populations grow, the operators of these plants must find ways to remove higher levels of nutrients to keep waterways from becoming choked with algae.
Research now shows that treatment plants may be able to remove 99–99.9% of nutrients, particularly phosphorus. A major question about the need for these removal levels arose during the Water Environment Federation's conference in Dallas, Texas: does a push to 100% removal make economic sense, given that other big sources, including agricultural and urban runoff, remain virtually unregulated?
Frequently, state regulators establish total maximum daily loads, or pollution budgets for waterways, that call for values as low as 10 micrograms per liter (µg/L) of total phosphorus in the water column. The Spokane River in Washington is a prime example. To achieve levels closer to 10 μg/L phosphorus, regulators there set an interim discharge limit for treatment plants of 50 μg/L, opting not to go lower because of concerns related to cost and technological reliability.
"Everybody agrees we can do better than 50 [ μg/L], but the question is how much better," says Richard Koch, a senior environmental engineer with Washington's Department of Ecology. Accommodating future population growth in the region will clearly require treatment plants to achieve ever-lower discharge limits, because as effluent concentrations go down, plants can discharge higher volumes. The 50 μg/L limit is merely "buying them time to do more research and develop better technologies," Koch says.
Some plants around the country have achieved levels just under 10 μg/L, but those levels aren't always met, notes Glen Daigger with CH2M Hill, an environmental engineering firm. Others with similar treatment technologies haven't, and it's not obvious why.
"This is still evolving science, and it gets down to better understanding the mechanisms of phosphorus removal at these very low levels," Daigger says. Research is starting to suggest that there might not be a scientific limit to removal in terms of effluent concentration, he adds.
Few plants have to remove phosphorus to such low levels, but it is possible and "people want to position themselves" in case the need to reduce more phosphorus arises, says James Barnard with Black & Veatch, an environmental engineering firm. The U.S. EPA issued guidance in 2001–2002 to help states set nutrient criteria for different ecoregions and water body types. The phosphorus levels vary by region and water body. EPA recently completed a national survey of treatment plants that can remove phosphorus to extremely low levels, and officials plan to release the findings this month, says Dave Ragsdale with EPA's Region 10 office.
One utility at the cutting edge of phosphorus removal is Clark County Water Reclamation District, a 110 million gallon per day plant serving Las Vegas, Nev. Using biological treatment with chemicals applied at the end of the treatment cycle, operators have achieved an average of less than 80 μg/L total phosphorus and less than 20 μg/L orthophosphorus over a year, says Doug Drury, deputy manager of operations. The plant can get very low phosphorus levels using fewer chemicals than any other plant.
The facility's current permit lists a waste load allocation of 171 pounds per day of total phosphorus, which translates to about 200 μg/L. However, with Las Vegas's population increasing by about 5% annually over the past several years and showing no signs of slowing, plant operators are already making adjustments. By replacing sand filters with reverse-osmosis membranes in any treatment-plant expansion, they expect to achieve annual averages of 20 μg/L total phosphorus.
The plant discharges into Lake Mead, Nev., the drinking water source for a multistate area from Las Vegas to Los Angeles. Because of an ongoing drought, lake levels have declined, and regulators say that if the lake goes down even more, they're going to change the permit limits, Drury notes. "All of these things combined are telling us we're going to have to do better than we are today."
Because it doesn't rain much in the Las Vegas area, runoff from lawns and streets is scarce, so most of the water flowing into that arm of Lake Mead is wastewater, Barnard says. Consequently, treatment plants "can really make a difference by going down to these extremely low levels."
Along parts of the wetter East Coast, it's a different story. Maryland, for example, recently set a maximum of 300 μg/L, which is considered the limit of technology when economics are factored in, Daigger notes. Generally, "it'll take both point-source and nonpoint-source controls to reach these very low ambient concentrations" that EPA's nutrient criteria are calling for, he adds.
