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Increasingly, nitrogen management is becoming an issue for the wastewater treatment community.  Nitrogen concerns mainly fall into two categories: environmental health, and public health.  In regard to the former, wastewater may contribute to eutrophication of lakes and streams.  (Eutrophication is a condition characterized by high biological productivity, leading to algae blooms, oxygen depletion, fish kills, and other disturbances of the surface water ecology.)  In regard to the latter, nitrate contamination of groundwater is often a concern because of claims linking it to blue-baby syndrome (infant methemoglobinemia), increased risk of cancer, miscarriage, and diabetes.  Doubt about the validity of those health concerns has been growing, due to the lack of substantive and reproducible supporting evidence.  Nevertheless, regulations increasingly impose highly restrictive discharge standards for treated wastewater, often requiring that treatment systems meet a limit of less than 10 mg/L total nitrogen at end-of-pipe.  The federal drinking water standard for nitrate is usually cited as the basis for restrictive discharge standards, though by law that standard applies only to public water supplies, not to private wells or to discharged waste.


Whether the concern is environmental health or public health, and regardless of the validity of public health concerns, it is important that the decentralized wastewater treatment industry be prepared to meet stringent discharge standards where required.  The industry is actively responding to this need.  But it's important to not that there is a correlation between cost and level of treatment.  Cost-effective advanced treatment systems are available that - when properly designed, built, installed, operated, and maintained - reliably produce effluent with nitrogen concentrations less than 20 mg/L (a level that represents approximately 69% nitrogen reduction, assuming typical residential wastewater strength).


A few manufacturers also provide systems designed to consistently and reliably meet a more stringent 10 mg/L target (representing further reduction of only 16%).  However, this relatively small gain in nitrogen removal comes at a disproportionately high cost.  Such systems require supplemental treatment steps and devices that add substantially to costs.  For example, devices to deliver supplemental carbon and/or alkalinity may be needed; and additional tanks or compartments may be needed to increase hydraulic residence time and provide anoxic conditions that favor denitrification.  These features require more intensive operation and maintenance, and regulators may demand more frequent monitoring and inspections, raising costs even more.


Moreover, it should be kept in mind that onsite systems are not the only source of nitrogen inputs to ground or surface waters, nor are they typically the source that has the greatest impact on water quality.  Nitrogen from agricultural or residential fertilizers, atmospheric decomposition, and animal feeding operations can be major contributors.  For example, for the Chesapeake Bay area it has been estimated that only about 4% of nitrogen loading to the Bay derives from septic system waste, whereas 31% is from agricultural fertilizers; 26% is from atmospheric decomposition; 20% from municipal and industrial wastewater; 10% from non-agricultural fertilizers; and 8% from animal feeding operations.*  (Anderson, DL, 2006.  A Review of Nitrogen Loading and Treatment Performance Recommendations for Onsite Wastewater Treatment Systems
<OWTS> in the Wekiva Study Area. Figure 6.  February 5, 2007 from Final Wekiva Paper 2 14 06.pdf.


Where nitrogen loading is a concern, it is critical that all of the contributors be identified, and that the burden of nitrogen management be assigned proportionally.  Practical, cost-effective management solutions should be required of all sectors, especially those sectors contributing the greatest share of the load.


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18 Nov 2013



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