By Jeff Herr, P.E., D.WRE, national stormwater leader, Brown and Caldwell
Looking beyond the wastewater treatment plant for sustainable, holistic solutions to nutrient loading
Surface waters including rivers, lakes, and estuaries have the natural ability to assimilate nutrients (phosphorus and nitrogen). In an undeveloped watershed, hydrologic and nutrient loads are commonly low, and surface water quality is very good. Some nutrients in a natural system are desirable and necessary to sustain aquatic life. As nutrient loads increase due to development and land use, surface waters become eutrophic (an increase in algal productivity, causing poorer water quality) and are commonly listed as “impaired” on state 303(d) lists. A surface water segment is listed as impaired when the measured water quality exceeds the state standard. The nutrient producing the impairment may be phosphorus, nitrogen, or both. The primary nutrient causing water quality impairment in a given water body is termed the “limiting” nutrient. Freshwaters are commonly phosphorus-limited, while brackish and salt waters are commonly nitrogenlimited. Certain types of algae present in surface waters can actually fix, or capture, nitrogen from the atmosphere.
Identifying The Issue
Eutrophic surface waters have a variety of undesirable aesthetic, chemical, biological, and human health characteristics. Impairment generally leads to the development of total maximum daily loads (TMDLs) and required nutrient load reductions by the regulatory community to improve surface water quality. There are currently almost 7,000 surface water segments impaired for nutrients in the U.S. with more than 5,700 completed TMDLs. States are currently in the process of transitioning surface water quality standards from “narrative” to “numeric” nutrient criteria, which will lead to additional nutrient TMDLs. In most cases, only the limiting nutrient needs to be reduced to improve surface water quality. In these economic times, it is essential to find cost-effective and sustainable solutions to reduce nutrient loads to nutrient-impaired waters.
There are commonly many potential sources of nutrients in a watershed, including municipal and industrial wastewater discharges, agricultural discharges, snow melt, stormwater runoff, septic systems, dry weather baseflow, groundwater seepage, internal recycling from surface water bottom sediments, atmospheric deposition (both wet and dry), and pets and wildlife. While most of these nutrient sources are direct sources, bottom sediments can release stored phosphorus into the water column under anoxic (low dissolved oxygen) conditions. Low dissolved oxygen is common at the sediment/water interface in eutrophic surface waters. In some cases, the less-recognized sources, such as bottom sediments or waterfowl, are the primary source of nutrients.
When attempting to improve surface water quality, it is extremely important to identify and quantify all sources and magnitudes of the nutrients of concern in a watershed. The primary sources of nutrients can then be identified, and cost-effective and sustainable solutions can be planned and implemented. If all nutrient loads are not properly quantified, millions of dollars may be spent to remove nutrient loads from an unimportant source, resulting in minimal water quality improvement. For example, in a phosphorus-limited lake, if bottom sediments account for 70 percent of the annual phosphorus load, minimal water quality improvement would be realized by treating stormwater runoff inflows. Conversely, there would be little benefit from removing bottom sediments if stormwater runoff was the primary source of nutrients.
An integrated watershed approach to nutrient management is needed to develop and implement cost-effective and sustainable solutions. Step one, arguably the most important, involves quantifying all primary sources and magnitudes of nutrients in the watershed. This step requires a commitment of time and funding to perform the required field monitoring of nutrient sources. Field monitoring should be completed during different seasons throughout the year for stormwater runoff, dry weather baseflow, groundwater seepage, recycling from bottom sediment, and other sources. For nutrient sources with a hydrologic component, such as stormwater runoff and dry weather baseflow, both water quantity and water quality must be monitored to calculate nutrient loads. Monitoring of surface water quality is also recommended during this time to correlate watershed nutrient loads to receiving water quality. Vertical profiling of in situ parameters is also recommended throughout the water column depth in the monitoring period to assess stratification and internal processes. Although the monitoring cost can be substantial, it is most often very small compared to the cost of implementing nutrient reduction projects or the cost of implementing an ineffective solution.
Although modeling is often necessary to quantify watershed nutrient loads, models without field monitoring and calibration often overestimate nutrient loads by up to an order of magnitude. This can be a function of underestimated depressional storage and/or natural nutrient attenuation that occurs in the watershed and surface water system. Overestimation of nutrient loads leads to overestimating the required nutrient reduction and corresponding cost to achieve compliance and water quality improvement.
Step two involves evaluating potential point source solutions. This is only applicable if there are point source discharges containing nutrients in the watershed. Information about the point source discharge nutrient composition can be found in the National Pollutant Discharge Elimination System (NPDES) permit limits or monitoring reports. Examples of point source solutions to reduce effluent nutrient concentrations include biological and/or chemical treatment unit processes, membranes, and wetland treatment. Water volume reduction using infiltration basins or reuse may also be possible. Typical evaluation factors for all potential load reduction alternatives include nutrient load reduction, life cycle cost (capital plus ongoing operation and maintenance costs), life cycle cost per mass of nutrient removed, and greenhouse gas emissions. There may be other prioritization factors identified by the local entity for consideration, such as educational and recreational use, wildlife habitat, fisheries, and aesthetics.
Step three includes evaluating potential nonpoint source solutions. Examples of nonpoint source solutions include nonstructural practices (e.g., street sweeping, inlet inserts, end-of-pipe treatment for gross solids and sediment, and traditional treatment practices), wet ponds, dry basins, coagulant treatment, wetland treatment, and green stormwater infrastructure practices to reduce runoff volume (infiltration and reuse). Coagulant treatment, which involves precipitating phosphorus in an offline settling pond, is often the most cost-effective solution for phosphorus load reduction. Coagulant treatment also uses substantially less land than traditional wet ponds or dry basins. As an example, a 3-acre wet settling basin is used to treat stormwater runoff from a 1,200-acre urban watershed and achieve an 85 percent annual mass phosphorus reduction.
Step four involves evaluating in-water solutions to determine if they are feasible based on the type of water and primary nutrient sources. For lakes, examples include sediment removal, sediment phosphorus inactivation using a coagulant, recirculation treatment system, aeration/destratification, hypolimnetic oxygenation, and the treatment of surface water inflows using practices identified in step three for nonpoint sources. If bottom sediments are a primary source of phosphorus, and the existing water depth is acceptable, it is normally faster, much less disruptive, and much less expensive to complete a coagulant surface treatment rather than dredge and dispose of sediments off-site. For rivers and streams, example solutions are different and may include restoring creek natural hydrology, reconnecting creeks to wetlands/floodplains, improving creek riparian buffers, repairing or restoring degraded creek/tributary segments, removing sediment, aerating in-stream, containing or cleaning up point waste sources, and treating surface water inflows from tributaries.
The task of reducing the primary source of nutrients in a watershed is often complicated by regulatory authority and/or jurisdiction. As an example, assume the state environmental agency only has regulatory authority over point sources (wastewater and industrial discharges) in a nutrient TMDL watershed through the NPDES wastewater discharge permit. Although point source discharges may only be a minor source of nutrients, costly reductions to very low effluent concentrations may be required to meet the TMDL. Stormwater runoff may be the primary nutrient source, but it may not be regulated. This happens frequently and can be overcome using nutrient offsets or trading. Through the use of offsets and trading, the primary sources of nutrients in a watershed can be reduced in a more cost-effective manner.
In Boise, ID, the city was required by the U.S. EPA to reduce the concentration of phosphorus in its wastewater facility effluent to 0.07 milligrams per liter (mg/L) to meet a phosphorus TMDL on the downstream Snake River. The wastewater facility discharges into the Boise River approximately 30 miles upstream of the TMDL water segment. Water from the Boise River is used for agricultural irrigation and returned to the Boise River. For this reason, much of the benefit of the upstream phosphorus reduction to a very low concentration would be lost through agricultural use.
The city elected to partially reduce the phosphorus concentration in its wastewater effluent. The remaining required phosphorus load reduction (equivalent to an effluent of 0.07 mg/L phosphorus) will be achieved through a nutrient offset project. An offset project is typically owned and operated by the same entity responsible for NPDES permit compliance. The city purchased a parcel of land adjacent to an agricultural drain near the downstream end of the Boise River. A coagulant treatment project will be constructed to treat agricultural discharges from 40,000 acres of land. The capital cost for the coagulant treatment project is substantially less than the capital cost to achieve a phosphorus effluent concentration of 0.07 mg/L at the wastewater facilities. This treatment project will address a large agricultural drain, one of the primary sources of phosphorus in the watershed. In the absence of an offset project, this source would not have been treated in the watershed. In addition, the phosphorus reduction at the downstream end of the Boise River will result in lower phosphorus loads and additional environmental benefit for the Snake River. Requirements for both treatment facilities are included in the city’s NPDES wastewater discharge permit issued by the EPA.
In north Georgia, the Coosa River drains into Lake Weiss in eastern Alabama. A phosphorus TMDL for Lake Weiss requires a 40 percent reduction of total phosphorus at the state line. There are a number of municipal and industrial discharges with NPDES wastewater discharge permits in the Coosa River Basin. There are no NPDES MS4 permits. For this reason, the state of Georgia only has the ability to regulate phosphorus discharges through the NPDES wastewater discharge permits. The state is requiring these point sources to reduce their effluent phosphorus concentrations. Chicken litter is generated in the watershed and commonly applied to agricultural fields in north Georgia for fertilizer. Primarily, the nitrogen is needed for its fertilizer value, but the phosphorus is not. The option of not using and exporting chicken litter from the basin is being considered in lieu of modifications at one or more of the wastewater facilities. This may provide an opportunity to achieve the required phosphorus load reduction at a lower cost. The details of this phosphorus trade would be incorporated into the NPDES wastewater discharge permit. In other locations in the U.S., formal nutrient trading programs have been established to assist local entities with satisfying TMDL nutrient reduction requirements. These include both point-to-point source and point-to-nonpoint source trading programs.
The overall objective of the watershed approach to nutrient management is to identify and implement cost-effective solutions that also maximize environmental benefits. This requires taking the time to develop a thorough understanding of the primary sources and magnitudes of nutrients in a watershed. Without this step, substantial funds may be spent with little or no environmental improvement. Various options to cost-effectively reduce nutrient loads should be evaluated, including point source solutions, nonpoint source solutions, in-water solutions, and nutrient offsets and trading. These options should focus on the primary sources of the nutrients of concern. The best solutions can then be planned and implemented based on total nutrient load reduction, cost-effectiveness, environmental benefit, and other factors established by the local entity. It is very important for local entities responsible for nutrient reductions to have a variety of options available to achieve compliance. Having options will lower the overall cost of nutrient solutions, increase total nutrient reductions, and result in greater surface water quality improvement.
Jeff Herr, P.E., D.WRE, is the national stormwater leader for Brown and Caldwell. He received B.S.E. and M.S.E. degrees in environmental engineering and has more than 30 years of professional experience in the areas of surface water quality assessment and restoration and watershed and stormwater management, working throughout the U.S.