January 2008, Vol. 20, No.1

Small Communities

Nutrient Management

Pio Lombardo

Excessive nitrogen or phosphorus discharges to waterbodies cause water quality decline and ecological and economic losses. In particular, the U.S. East Coast, because of its numerous embayments and estuaries and heavy population density, has many waterbodies impaired by excessive nutrients, notably Chesapeake Bay, Long Island Sound, Cape Cod, and the Florida coast. The Gulf of Mexico “dead zone” and some West Coast embayments, such as Puget Sound, also are nutrient-impaired. The degree of estuarine degradation by nutrients recently was well characterized in the 2007 report Effects of Nutrient Enrichment in the Nation’s Estuaries: A Decade of Change, published by the National Centers for Coastal Ocean Science (NCCOS), a division of the U.S. National Oceanic and Atmospheric Administration. According to NCCOS, numerous freshwater lakes and streams throughout the United States are nutrient-impaired, and many groundwater supplies have nitrogen levels near or above the drinking water standard of 10 mg total nitrogen per liter.

For brackish and saline waters, nitrogen generally is accepted as the limiting nutrient, with levels higher than 0.3 mg/L total nitrogen causing water quality degradation. In fresh waters, phosphorus generally is the limiting nutrient, with levels higher than 0.03 mg/L (0.01 mg/L in the Florida Everglades) causing excessive algae growth and water quality and ecological degradation. Toxins excreted by certain algae that grow in nutrient-enriched fresh surface waters have made water unfit for human consumption without exceptional treatment efforts.

Nutrients typically come from fertilizers, wastewater, atmospheric deposition, stormwater runoff, agricultural runoff, and sediment nutrient release, and the relative contributions vary depending on location, development, and land-use practices within the watershed. Management strategies to achieve water quality objectives must to be tailored to the existing and planned future conditions of the watershed — with the proper understanding of surface and subsurface hydrology, subsurface biogeochemistry, and surface water quality dynamics. Nutrient trading is a relatively new vehicle to economically optimize the overall watershed solutions to achieve total maximum daily load (TMDL) requirements.

Management strategies traditionally have focused on point and nonpoint sources. Point sources are typically wastewater treatment plants, and their discharge limits have been reduced progressively to (and in some cases below) the limits of treatment technology. The generally accepted limits of surface water-discharging wastewater treatment technology are 3 mg/L of total nitrogen and 0.1 mg/L of total phosphorus using biological nutrient removal and 0.01 mg/L using chemical salt addition and polishing, with its associated higher solids production and costs. For nonpoint sources, management strategies (such as stormwater best management practices) are focused on reducing contributions at the source, such as ordinances on fertilizer practices, and on pretreatment nutrient removal requirements for soil-based wastewater treatment systems.

In some large Chesapeake Bay subwatersheds, the current on-lot wastewater nitrogen contributions exceed mass loadings from wastewater treatment plants, in part owing to stringent nutrient requirements for treatment plants. In virtually all Cape Cod watersheds, current on-lot wastewater nutrient contributions are the predominant sources of nitrogen and phosphorus.

Nonpoint source wastewater-derived nutrients require management and high percentages of removal. Commonly, municipalities automatically rush to conventional centralized sewering to solve existing problems. This strategy often is unwarranted technically and economically, as there are distributed decentralized options that are cost-competitive and provide comparable performance to the most sophisticated centralized wastewater treatment system. Additionally, the decentralized approach provides groundwater recharge near the sources, whereas centralized systems may discharge far from those sources, creating interbasin water transfer issues.

Distributed wastewater management solutions, often employing decentralized systems, are effective alternatives that generally are easier and quicker to implement and do not have some of the negative secondary impacts of centralized systems. Distributed systems also can address the microconstituents issue at least as effectively as centralized facilities. Siting decentralized systems can be easier, as they generally are located underground. Also, the use of constructed wetland systems is viewed as an amenity in many areas.

The U.S. Geological Survey has noted in numerous studies that even after nutrient sources are eliminated, it often takes decades for the groundwater to cleanse itself. Therefore, water quality restoration will be delayed (often by several years) even after the source discharges are terminated. With the large investment needed for nutrient removal, such delays represent a significant political challenge for local decision-makers. For example, in some large areas, conventional sewering proposals to upgrade nitrogen removal can take the form of 20-year projects, for situations that will require 10 to 20 years to see measurable water quality improvements, at a cost as high as $45,000 to $60,000 per property. Decentralized systems can be implemented significantly faster and at a lower cost.

One decentralized option that is quick and relatively inexpensive to implement in areas of groundwater nutrient enrichment is permeable reactive barriers (PRBs). In several situations, we have determined that PRBs can be implemented for 15% to 35% of the cost of conventional sewering, when comparing the costs per pound of nutrient removed per day. This is in large part due to salvaging much of the existing septic system infrastructure that provides biochemical oxygen demand, total suspended solids, and bacterial removal, as well as full nitrification, so that only a denitrifying system is needed. Importantly, PRBs can be designed to remove all groundwater nutrients, whether they are from wastewater, fertilizer, or agricultural sources. These PRBs can be located close to the receiving waterbody so that the lag time to demonstrated surface water quality improvement can be shortened dramatically. PRBs also can be a significant tool for nutrient trading. Perchlorate removal, required in many water supply aquifers in the southwestern United States, also can be achieved with PRB nitrate removal, at no additional cost, as both constituents are removed concurrently. PRB systems are below the surface and produce excellent water quality.

As with any technology, proper management of distributed and decentralized systems is necessary to ensure that they perform as designed and for the routine maintenance needs. With simple and inexpensive telemetry systems, the typical minimal management requirements for decentralized systems are cost-competitive with centralized systems.

Today, water quality managers addressing nutrient issues have economically attractive and technically viable options. Progressive water quality and wastewater managers hopefully will find these options exciting and rewarding. Conventional solutions may result in enduring unnecessary costs, longer periods of water quality degradation, and potentially unfavorable public reactions.

Pio Lombardo is president of Lombardo Associates Inc. Environmental Engineers and Consultants (Newton, Mass.) and a member and former chair of the Water Environment Federation (Alexandria, Va.) Small Communities Committee

Pio Lombardo is president of Lombardo Associates Inc. Environmental Engineers and Consultants (Newton, Mass.) and a member and former chair of the Water Environment Federation (Alexandria, Va.) Small Communities Committee