A dual concern with respect to nutrients is that their control is necessary to avoid adverse health or environmental effects but the same nutrients are essential for the performance of the natural biological treatment systems discussed in this book. The nutrients of major importance for both purposes are nitrogen, phosphorus, and potassium. Nitrogen is the controlling parameter for the design of many land treatment and sludge application systems, and those aspects are discussed in detail in Chapter 6, Chapter 7, and Chapter 9. This section covers the potential for nutrient removal using the other treatment concepts and the nutrient requirements of the various system components.
Nitrogen is limited in drinking water to protect the health of infants and may be limited in surface waters to protect fish life or to avoid eutrophication. As described in Chapter 8, land treatment systems are typically designed to meet the
10-mg/L nitrate drinking-water standard for any percolate or groundwater leaving the project boundary. In some cases, nitrogen removal may also be necessary prior to discharge to surface waters. More often, it is necessary to oxidize or otherwise remove the ammonia form of nitrogen, as this is toxic to many fish and can also represent a significant oxygen demand on the stream.
Nitrogen is present in wastewaters in a variety of forms because of the various oxidation states represented, and it can readily change from one state to another depending on the physical and biochemical conditions present. The total nitrogen concentration in typical municipal wastewaters ranges from about 15 to over 50 mg/L. About 60% of this is in ammonia form, and the remainder is in organic form.
Ammonia can be present as molecular ammonia (NH3) or as ammonium ions (NH4+). The equilibrium between these two forms in water is strongly dependent on pH and temperature. At pH 7 essentially only ammonium ions are present, while at pH 12 only dissolved ammonia gas is present. This relationship is the basis for air-stripping operations in advanced wastewater treatment plants and for a significant portion of the nitrogen removal that occurs in wastewater treatment ponds.
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