4.3.4 Nitrogen

Nitrogen levels in sewage can range from 20 to more than 100 mg/l, depending on in-house water use and diet of the local people and on the treatment of the sewage effluent prior to SAT. Nitrogen is primarily present as organic, ammonium, and nitrate nitrogen. The relative amounts of these nitrogen forms depend on the form of treatment prior to SAT. For secondary effluent, much of the nitrogen will often be in the ammonium form but some processes are designed to achieve nitrification and the effluent will then contain primarily nitrate-nitrogen. Raw sewage has considerable organic nitrogen.

The desirable form and concentration of nitrogen in the renovated sewage water from an SAT system depends on the nitrogen and water requirements of the crops to be irrigated, the need for preventing nitrate pollution of groundwater in the irrigated area due to excess nitrogen application to the crops, and on other possible uses of the water (including fish ponds, for which low concentrations of ammonium are required).

Control of the form and concentration of the nitrogen in renovated water from an SAT system is possible by properly selecting hydraulic loading rates and flooding and drying periods for the infiltration basins. For example, if the nitrogen in the sewage effluent is mostly in the ammonium form, short flooding periods and frequent drying of the infiltration basins (for example, 2 days flooding-5 days drying) will cause essentially complete nitrification of the ammonium in the soil, due to frequent aeration of the soil profile and resulting aerobic conditions. Thus, almost all the nitrogen in the renovated water from the SAT system will then be in the nitrate form and at concentrations about equal to the total nitrogen concentration in the sewage effluent applied to the basin. Long flooding and drying periods (for example, 1 month flooding-1 month drying) would eventually lead to complete breakthrough of ammonium in the renovated water because of anaerobic conditions in the soil and absence of nitrification. If flooding and drying periods are of intermediate length (for example, 1 to 2 weeks flooding-1 to 2 weeks drying), there will be a succession of aerobic and anaerobic conditions in the upper part of the soil profile, which stimulates nitrification and denitrification. The latter is an anaerobic bacterial process that reduces nitrate to free nitrogen gas and oxides of nitrogen that return to the atmosphere. With this process, about 75% of the nitrogen in sewage has been removed in an SAT system in Arizona, USA, with almost all of the remaining nitrogen in the renovated water occurring in the nitrate form.

Denitrification requires the presence of nitrate and organic carbon (an energy source for denitrifying bacteria) under anaerobic conditions. About 1 mg/l of organic carbon is required for each mg of nitrate nitrogen to be denitrified. If the nitrogen in the sewage is already mostly in the nitrate form and the water quite stabilized, organic carbon (as primary effluent, for example) may have to be added to the sewage effluent to achieve sufficient denitrification when the system goes anaerobic. Local experimentation is usually required to find the optimum schedule for flooding and drying, hydraulic loading, and organic carbon addition for stimulating denitrification. More information can be found in Bouwer (1985) and Bouwer and Chaney (1974).