Principles of Biological Nutrient Removal (1)

1/Biological Nitrogen removal

• Biological nitrogen removal requires a two-step process. In the first step ammonia is oxidized to nitrate (nitrification) and various process configurations are then employed to provide the nitrate as an electron acceptor for biological respiration so that it can be reduced to molecular nitrogen (denitrification). Fundamental considerations will be presented first for the nitrification step in nitrogen removal. Included in this review are the microbiology, basic biochemical reactions, biological kinetics, and the factors that affect the kinetics and performance of biological nitrification and denitrification reactions

2/BIOLOGICAL NITRIFICATION

2.1.Microbiology

• Early work by Schloesing and Muntz (1877) investigating the percolation of sewage through a sand column found that the conversion of ammonia to nitrate and nitrite was due to living organisms, since the reaction could be stopped by introducing chloroform vapor into the column. Classical experiments by Winogradsky (1890) led to the isolation of autotrophic nitrifying bacteria, Nitrosomonas and Nitrobacter, which oxidize ammonia sequentially to nitrite and then to nitrate, respectively. Painter (1970) has listed other autotrophic bacteria genera capable of obtaining energy from the oxidation of ammonia, Nitrosococcus, Nitrospira, Nitrosocyctis and Nitrosoglea, and nitrite, Nitrocystis, but the nitrification in soil or wastewater treatment processes is attributed primarily to Nitrosomonas and Nitrobacter.

• Painter (1970) summarizes other literature sources that show a large number or heterotrophic bacteria capable of forming nitrite or nitrate. In one study by Eylar and Schmidt (1959) of about 1000 heterotrophic organisms isolated from soil, only fifteen wrere found to be able to nitrify. A review by Focht and Chang (1975) indicated that heterotrophic nitrification is possible with diverse genera of bacteria, fungi, and actinomycetes. However, it is doubtful that significant quantities of nitrate are generated by heterotrophic organisms, since autorophic nitrification rates are about ten times greater. Heterotrophic nitrification may be more prominent in atypical environments with either very alkaline or acidic pH conditions.

• Bock et al. (1988) reported on the ability of Nitrobacter to grow and reduced nitrate while using acetate, formate, pyruvate, or glycerine as organic substrates. They further showed in an aerobic/anoxic conditions. This observation has not been studied in wastewater treatment.

2.2.Oxidation and Synthesis relationships

The energy-yielding two-step oxidation of ammonia to nitrate is generally accepted to be as follows:

• Nitrosomonas

2NH4+ + 3O2 → 2NO2- + 4H+ +2H2O

• Nitrobacter

2NO2- + O2 → 2NO3-

• Total reaction

NH4+ + 2O2 → NO3- + 2H+ + H2O

• The standard free energy release associated with ammonia oxidation has been estimated to be 66 to 84 kcal/mole of ammonia, and for nitrite oxiation it has been estimated to be 17.5 kcal/mole of nitrite (Painter, 1970). If the amount of cell prodcution is proportional to the energy released, a greater amount of biomass should be formed from the oxidation of ammonia to nitrite than from the oxidation of nitrite to nitrate.

• Based on the above, the oxygen required for complete oxidation of ammonia is 4.57 g/g N oxidized with 3.43 g/g used for nitrite production and 1.14 g/g used for nitrate production. The amount of oxygen required is less than 4.57 g/g N when synthesis is considered in addition to oxidation due to oxygen obtained from fixation of carbon dioxide and nitrogen into cell mass. Wezernak and Gannon (1967) found that the actual total oxygen consumption was 4.33 g/g N with 3.22 g/g N used for ammonia oxidation and 1.11 g/g N used for nitrite oxidation. The following equation, presented in the U.S.EPA Nitrogen Control Manual (1975), accounts for both synthesis and oxidation and shows an oxygen requirement of 4.2 g/g N. The equation was developed using cell yield coefficients of 0.15 g/g NH4-N oxidized and 0.02 g/g NO2-N oxidized

NH4+ + 1.83O2 + 1.98HCO3- → 0.21C5H7O2N + 0.98NO3-+1.041H2O + 1.88H2CO3

• For activated sludge designs employing nitrification, a single yield coefficient that includes the growth of both Nitrosomonas and Nitrobacter is more convenient and some of the more recently used values.