By Pio Lombardo, P.E.
Excessive nitrogen levels pose a threat to the ecological health of water resources. Nitrogen has been identified in the nutrient-related declines of shellfish and aquatic plant life in many US sites, including the Chesapeake Bay, Florida coastal areas, and Cape Cod, the hypoxic "dead zone" in the Gulf of Mexico, and Long Island Sound.
Nitrogen loads to watershed result predominantly from inadequate wastewater treatment, agricultural or domestic fertilisers and atmospheric deposition, with the relative contributions varying among watersheds.
In many watersheds, nitrogen from wastewater contributes 40% or more of total nitrogen loads. Due to concerns about eutrophication and public health limits for nitrate in drinking water, many US states, including Connecticut, Massachusetts, Indiana, Arizona, and Oregon, require wastewater nitrogen removal in water supply and ecologically sensitive areas. Further, wastewater treatment plants in the Chesapeake Bay will be required to reduce their total nitrogen effluents from 8 to 3 milligrams per litre (mg/l) to meet water quality goals, while Connecticut wastewater treatment plants that discharge into Long Island Sound will need to reduce their total nitrogen (across all treatment plants) by 64%, or, on average, to 3 mg/l.
Figure 1: Total nitrogen in septic tank and Nitrex� effluent measured during two-year operation of a pilot project at LaPine, Oregon
Centralised wastewater treatments plants can achieve total nitrogen concentrations of 3 mg/l N, which is considered the current limit of technology. Conventional septic systems, however, produce effluent with 10 to 20 times more nitrogen, which causes a significant problem given that septic tanks contribute up to 40% to 50% of the nitrogen in some watersheds. Moreover, septic system effluent will be increasingly subject to stringent nitrogen limits as total maximum daily load (TMDL) studies are completed and corrective actions required. Further, states will be adopting their own or US Environmental Protection Agency (US EPA) eco-regional nutrient criteria as water quality standards by 2004 (EPA 2000).
When faced with the need to remove nitrogen from wastewater, communities with septic systems have two options: construct sewers and centralised treatment facilities or implement reliable technologies for removing nitrogen from septic tank effluent on an individual or cluster basis.
Many communities currently are debating whether sewers are necessary to manage nitrogen and protect water resources. Sewers enable economies of scale to be achieved along with the use of sophisticated technologies at a central treatment plant. However, sewers are expensive, can be politically undesirable, unacceptably disruptive to the community, and can cause environmental degradation because of water dislocation across watersheds. Therefore, more communities are pursuing non-sewering options and regulators are requiring treated effluents to be discharged locally and/or near the source from which the water was extracted.
More importantly, wide-scale sewer installation can be expensive. The range of capital (including construction and engineering, etc.) costs for a complete sewerage system for currently unsewered coastal areas in the Northeast – Middle Atlantic region typically is US$18,000 to US$25,000-plus per connection. Naturally, this cost will be dependent upon density, depth of groundwater, depth of bedrock and technology selection. Annual operation and maintenance costs can reach approximately US$400 or more.
Nitrogen is one of the major pollutants in runoff from fertilised fields. Non-point sources contribute the majority of nitrogen pollution to many watersheds, including the Mississippi River. An increasing number of TMDLs are under development to address concerns associated with excess nitrogen coming from non-point sources, such as fertilised field runoff. Of the more than 9,000 TMDLs prepared by states and approved by the EPA since 1996, nutrients (both nitrogen and phosphorus) account for more than 10%. Thus, nutrient impairment of water bodies, of which approximately half is due to nitrogen, is the third-leading reason for TMDL development.
Table 1: Total nitrogen in effluent from technologies evaluated at LaPine, Oregon
Many alternative technologies are available to meet the challenge of removing nitrogen from septic tank effluent or fertiliser-containing runoff. Seventeen of these alternative wastewater technologies were recently tested during the LaPine, Oregon National Decentralized Wastewater Treatment and Disposal Demonstration Project funded by the US EPA and managed by the Oregon Department of Environmental Quality, the US Geological Survey and the Deschutes County Environmental Heath Division (Table 1).
The results suggest that there are a number of technologies (i.e., technologies 1 through 6) that are capable of removing approximately 50% of total wastewater nitrogen (i.e., 50 mg/l total nitrogen in septic tank effluent). Other nitrogen removal technologies have been able to achieve 14 to 20 mg/l of effluent total nitrogen and are being evaluated in the nitrogen-sensitive New Jersey Pinelands.
The results from the LaPine project also indicate that one technology shows particular promise for removing nitrogen from septic tank effluent: the Nitrex™ Filter. The Nitrex system removed more than 90% of total nitrogen from septic tank effluent during the LaPine project (Figure 1). During additional testing at other independent installations, Nitrex systems have consistently produced effluents with 3 to 4 mg/l, even in a cold climate. Thus, the Nitrex Filter can perform as well as the nitrogen removal technologies for centralised wastewater treatment facilities. Moreover, recent studies suggest that Nitrex can handle intermittent wastewater flows, while still removing more than 90% of total wastewater nitrogen.
The ability to handle intermittent flows is critical and has been a cause of non-compliance for other technologies. The Nitrex Filter is suitable for new construction and retrofits to conventional septic systems in residential applications. Nitrex Filters can also be used for commercial and institutional applications, in addition to cluster wastewater collection and treatment systems and centralised systems.
Nitrex is also suitable for treating agricultural and golf course runoff. A Nitrex Filter treating farm field drainage in Ontario at a design flow rate of up to 5,000 gpd, reduced total influent nitrogen concentrations from an average of 4 mg/l to usually less than 1 mg/l during 18 months of operation. These data, in addition to data from two additional Nitrex filters treating fertiliser-laden runoff, show that virtually complete nitrate removal occurs when the appropriate hydraulic retention time is provided.
Cost and maintenance requirements are both important considerations in selecting a technology for removing nitrogen from septic tank effluent. Using the Nitrex Filter as an example, total costs would be approximately US$15,000 for a retrofit to a conventional septic system. Economies of scale are possible for larger wastewater flows Nitrex filter applications with costs of US$10,000 per property achievable. Little ongoing operational or maintenance costs are necessary for the Nitrex. The longevity of the Nitrex filter is estimated at 20 years.
Author's Note
Pio Lombardo, P. E. of Lombardo Associates is based in Newton, Massachusetts, USA. For more information, visit the company website: http://www.lombardoassociates.com.
