Nitrogen Dynamics in Natural Systems

Recently states have been required by the US EPA to develop programs to reduce the Total Maximum Daily Load (TMDL) for Nitrogen to a watershed. Soil based onsite treatment and dispersal systems are identified as a minor impact percentage wise and categorized as “non point” pollution source of Nitrogen. Although the ground water quality standard is still enforced, the emphasis has expanded possible impacts on surface waters resulting in a mass balanced approach and requiring a reduction in the pounds of Nitrogen per year discharged.

Non point pollution sources are typically addressed by a series of Best Management Practices (“BMP’s) that are deemed to comply for a percentage of total reduction of the constituent of concern when applied. BMP’s for non point pollution sources are developed utilizing and maximizing natural process based on fundamental understanding of the known variety of natural interactions.

There is a significant difference between the classical wastewater nitrification / denitrification process applied in wastewater treatment and the more complex nitrogen cycle in nature (wetlands and soil).

The classical sewage treatment process are efficient / specific in process / time utilizing particular fast growth microorganisms. The traditional model is not entirely applicable to the heterogeneous natural environment. Soil based treatment have fundamental differences in microbial ecology.

In the soil based biological reactor many additional processes are occurring transforming nitrogen to various forms. There is additional potential, effluent quality and site dependent, for immobilization due to microbial action and vegetation uptake.

 In wetlands, soils, and sediments, the microbial ecology is considerably different. The mean cell residence time is considerably longer. A variety of other denitrification processes, not considered in or applicable to the classical sewage treatment model, provide considerable additional pathways of denitrification as well as conservation (recycling) within the natural system. Many of these processes, “non-classical pathways” are have been recently identified including heterotropic nitifers, certain pathways of nitrogen loss from ammonia, and aerobic denitrification.

At this point the environmental model as applied in the regulatory arena in regard to soil based on site wastewater systems is theoretical and assumes all effluent and its accompanying constituents that enters the soil travels to the directly to the water resource. There is no real “credit” for further attenuation in the natural environment.

Several papers and studies will be reviewed regarding the Nitrogen effects of on site sewage treatment and dispersal systems and possible BMP’s for nitrogen reduction discussed.