Nitrogen can exist in variety of oxidation states from minus three in organic compounds to plus five in the nitrate form. The biological transformation of nitrogen among these forms is called the nitrogen cycle: atmospheric nitrogen is the reservoir of nitrogen for the earth ecosystem. For a schematic of the nitrogen cycle click here. Raw sewage is rich in ammonia and organic nitrogen. At neutral pH virtually all ammonia will be in the ionic form, NH4+ . The biological conversion of organic nitrogen to ammonia is called ammonification. Since this process is rapid, our discussion begins with ammonia.
When raw sewage enters a body of water, dissolved oxygen is lost first to the biological oxidation of organic carbon to carbon dioxide and then to the oxidation of ammonia. That is, oxygen demand can be thought of in two stages: a carbonaceous oxygen demand and a nitrogenous oxygen demand. The uptake of oxygen in a body of water is expressed graphically as the classic DO sag curve.
All waters have the ability to replenish a some or all of this oxygen through reaeration created by turbulence and to a lesser extent by aquatic plant life photosynthesis. In high population densities, however, the organic and nitrogen loadings can quickly exceed the capacity of a receiving water to replenish the consumed dissolved oxygen. If all the oxygen is consumed, the water will become septic -- a condition which is not only unpleasant but also uninhabitable to the aquatic life associated with the receiving water. If the dissolved oxygen levels are reduced by even a few parts per million, the species which normally inhabits the steam will be severely impacted. In wastewater treatment, nitrification by secondary treatment (e.g. activated sludge, trickling filters, rotating biological contactors) protects the steam.
Most wastewater treatment plants release nitrogen as nitrate. For many receiving waters this presents no problem. Either there is sufficient dilution of the nitrate (e.g. release into the ocean) or there is sufficient flow (a river with strong currents) to prevent accumulation of the nitrates. For lakes and particularly for estuaries, however, the nitrate acts as a food source for algae, creating an ecological imbalance.
In an estuary the benthic (bottom dwelling) organism such as clams and scallops, require the oxygen released during photosynthesis by aquatic plant life. Let's follow what occurs when excessive nitrates are loaded into a bay. Nitrates are the limiting nutrient (for a WWW site explaining limiting nutrients, click here) in the growth cycle of algae. Given excess nitrates, the algae bloom creating a mat. This mat absorbs the sunlight required for photosynthesis carried out by other aquatic plant life. Since no oxygen is released by photosynthesis where the benthic crustaceans and bivalves live, these creatures quickly die. Algae blooms have been known to completely destroy the productivity of an estuary. As the algae dies, a secondary BOD develops associated with the decomposition of the algae, and this BOD also has negative ecological impacts.
Excessive algae blooms in a lake lead to eutrophication, a process in which a lake prematurely silts up. Algae blooms in lakes are limited by two nutrients: nitrogen and phosphorus. These nutrients tend to be recycled within the lake and to accumulate. Whether nitrogen or phosphorus or both should be controlled prior to discharge into a lake depends on the nature of water. Often, but not always, denitrification plays a critical role in maintaining the ecological integrity of a lake.
Human health is also impacted by the consumption of nitrates in water. Even relatively low levels of nitrates in drinking water can interfere with the uptake of oxygen by a fetus. The condition is called "blue baby syndrome," and the fetus will not survive. The usual source of nitrates in drinking water supplies is runoff of nitrogen rich fertilizers in agricultural area. Watersheds that are sole source aquifers, however, risk nitrate infiltration from wastewater effluent that is used to recharge aquifers. Sole source aquifer regions such as Long Island have invested millions of dollars in upgrading wastewater treatment plants to include denitrification.
There is worldwide trend to rely on recycling of wastewater for drinking water supplies. It should be obvious that this type of water resource management relies heavily on denitrification to protect public health.
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