Just 25 years ago there was almost no discussion of excess nutrients in our nation’s waters. But as our understanding has grown about the effects of too much nitrogen and phosphorus in our water, so have the wastewater industry’s efforts to remove them.
For example, in 1988, scientists studying Long Island Sound discovered that nitrogen was the probable cause of low dissolved oxygen concentrations that led to severe fish kills. To help prevent this, personnel at the 76,000-m3/d (20-mgd) City of Stamford (Conn.) Wastewater Treatment Facility, which discharged into the sound, looked at simple process changes to voluntarily reduce the amount of nitrogen discharged.
With no capital investment and no changes in plant equipment or structures, the plant was able to begin denitrifying. The only process changes were to increase mean cell residence time to 8 to10 days (it had been 6 to 8 days) and operate the aerator at low speed in the first of the plant’s four aeration basins to create an anoxic zone. The changes led to nitrogen removal in the 65% to 83% range. The average total nitrogen concentration in the effluent was 7.3 mg/L.
Good, but not enough. In December 2001, Stamford began a project to upgrade and expand the treatment plant at a cost of $105 million, of which approximately $50 million was devoted to nitrogen removal to meet a new permit limit of 4.5 mg/L at a design flow of 90,840 m3/d (24 mgd). Following this project, which included adding new tanks with a volume of 37.9 million L (10 million gal), 16 mixers, a methanol storage and delivery system, and instrumentation, the plant’s total nitrogen discharge averaged 3.7 mg/L during 2010.
Many wastewater plants nationwide share stories like this one, especially near sensitive waters such as Chesapeake Bay and Puget Sound. These plants use nutrient strategies that involve specialized design, construction, and operation to meet mandated limits.
While this could sound like the end of the story, in fact, it’s just the beginning. According to the U.S. Environmental Protection Agency (EPA) and the U.S. Geological Survey (USGS), excess nutrient concentrations in our nation’s water are still a growing problem.
A growing problem
In 2004, EPA released its National Water Quality Assessment Report. This report lists nutrients among the top three impairments for rivers, streams, and lakes. EPA reported 163,250 km (101,461 mi) of rivers and streams are threatened or impaired due to nutrients, as well as 993,295 ha (2,454,459 ac) of lakes, reservoirs, and ponds.
In September 2010, USGS added another layer to this issue when it released USGS Circular 1350: Nutrients in the Nation’s Streams and Groundwater, which is a comprehensive national analysis of nutrients in streams and groundwater from 1992 through 2004 by the National Water–Quality Assessment (NAWQA) Program. The USGS findings show that widespread concentrations of nitrogen and phosphorus remain 2 to 10 times greater than levels recommended by EPA to protect aquatic life.
Most often, these elevated levels were found in agricultural and urban streams. Continued reductions in nutrient sources and implementation of land-management strategies for reducing nutrient delivery to streams are needed to meet EPA’s recommended levels in most regions, according to a USGS press release.
Commenting on this document, USGS Associate Director for Water Matthew C. Larsen said, “Despite major federal, state, and local efforts and expenditures to control sources and movement of nutrients within our nation’s watersheds, national-scale progress was not evident in this assessment, which is based on thousands of measurements and hundreds of studies across the country from the 1990s and early 2000s.”
Expanding the discussion
The message here is that this problem cannot be solved by one industry alone. Even if every wastewater treatment plant in the country reduced its nitrogen and phosphorus discharges to the limits of technology, water quality would still suffer from excess nutrients from nonpoint sources.
It makes sense that the discussion about removing excess nutrients started with us. Our facilities are designed and built specifically to remove pollutants. Our engineers and operators dedicate their skills to protecting water quality. But in most cases, point sources are only part of the issue. The USGS study says various sources can contribute nutrients to surface and groundwater, such as wastewater and industrial discharges, fertilizer and manure applications to agricultural land, runoff from urban areas, and atmospheric sources.
Variations in cause
One of the challenges of achieving reductions most cost-effectively is figuring out what is needed in different areas of the country. The contribution of nutrient loading from point versus nonpoint sources can vary widely by watershed.
For example, nutrient loadings in the Chesapeake Bay watershed are dominated by agriculture, a nonpoint source, which represents approximately 45% of the watershed’s total loadings. Wastewater point sources account for only about 20% of the nutrient loading.
On the other end of the spectrum, nutrient loadings to Long Island Sound are dominated by wastewater discharge. About 65% of the total nitrogen loadings in the watershed come from point sources; only 4% come from agriculture.
Luckily, the USGS study includes data on how natural features, land use, and human activities throughout the nation affect nutrient loading rates. This information can help “decision-makers anticipate where watersheds are most vulnerable to contamination and set priorities and management actions in different geographic regions of the country,” according to Neil Dubrovsky, the USGS hydrologist who led the study.
Using every tool available
Just as important as targeting what sources to address is selecting the right tool for the job. When the scope of work is so great and the conditions so variable, there is no “one-size-fits all” answer.
For instance, the National Resources Defense Council (New York) has petitioned EPA to redefine secondary wastewater treatment to include nutrient removal and require total phosphorus limits of 1.0 mg/L and total nitrogen limits of 8.0 mg/L. To know whether such a change makes sense requires answering the fundamental question as to whether every receiving water would benefit from both nitrogen and phosphorus removal from point sources.
It’s complicated, because while we strive for the cleanest water possible, we understand this objective must be balanced against the ecology of the receiving water, most effective use of environmental resources, and diverse considerations that affect water quality at the regional and local levels.
For some plants the solution may be advanced nutrient removal technologies; for others it may be simple process change, and for others still it may be using nutrient trading programs to reach a net reduction in nutrient loading to a watershed. We sit at a point where the path of using technology to address what is discharged from the effluent pipe has reached a nexus with reducing the amount of pollutants we add to the system.
As the protectors of our waters, it is our responsibility to lead others in expanding the discussion to include all stakeholders in our watersheds to address point and nonpoint sources alike. We need to ensure that everyone understands that we must approach water holistically — as “one water” — no matter what its source or use. We need to make sure our users, ratepayers, and elected officials understand where excess nutrients originate and how we can work together with everyone involved to get the most improvement for the least expense.
is the executive director of the Stamford (Conn.) Water Pollution Control Authority, an adjunct professor of environmental engineering at Manhattan State College (Riverdale, N.Y.), and president of the Water Environment Federation (Alexandria, Va.).
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