Ammonification is the conversion of soluble organic nitrogen into ammonia-N that occurs as bacteria consume soluble organic matter containing nitrogen. Actually, the true rate of ammonification is difficult to measure because ammonia-N is being consumed by the bacteria as they grow, and the only measurable event is the net accumulation or loss of ammonia in the medium. If the amount of nitrogen available in the organic substrate is just sufficient to meet the biosynthetic needs of the new biomass, there will be no net change in the ammonia-N concentration in the medium. On the other hand, if that amount exceeds the need, the ammonia concentration in the medium will increase, whereas if that amount is less than the need, the ammonia concentration will decrease. It should be recognized, however, that whether organic nitrogen is incorporated directly into new biomass depends on its form. The nitrogen in simple compounds like amino acids may be incorporated directly as the amino acids are used for protein synthesis, while nitrogen in complex synthetic organic chemicals may be released to the medium as ammonia.
In an effort to make this complex situation mathematically tractable, most modelers assume that all nitrogen goes through the medium before being used. Thus, ammonification is assumed to release all organic nitrogen to the medium as ammonia, and nitrogen utilizing reactions are assumed to obtain their ammonia from the medium. Whether ammonia accumulates or is removed depends on the relative rates of its production and utilization.
Because ammonification occurs as heterotrophic biomass destroys nitrogen containing soluble organic matter, it is likely that its rate is proportional to the rate of soluble substrate removal. Relatively little work has been done to investigate the rate of ammonification in complex substrates in which only a part of the soluble organic matter contains nitrogen, and it is uncertain whether a direct proportionality can be assumed between soluble substrate removal and ammonification. Consequently, ammonification has been represented as a reaction that is first order with respect to both the heterotrophic biomass concentration and the concentration of soluble, biodegradable organic nitrogenr 54
where k, is the ammonification rate coefficient (L/(mg biomass CODhr)). Very little information is available about its value. As discussed above, the assumed stoichi-ometry of ammonification is such that all nitrogen removed from nitrogen containing soluble organic matter is released as ammonia, although some may ultimately be used for biomass synthesis. Thus, the rate of production of ammonia nitrogen (SNH) through ammonification is:
Ammonia is removed from solution by two reactions. First, it is used in the synthesis of new biomass as seen in Eq. 3.6 and others presented in Section 3.2. Second, it is used as a substrate by autotrophic biomass. The rate expression for the second use is the same as any other substrate, as discussed in Section 3.2.6. The rate expression for the first use can be determined from the generalized rate expression and the stoichiometry of growth. Since iN XH is the mass of nitrogen per unit of biomass COD, the rate of ammonia removal through biomass growth is simply:
Equation 3.81 is true for both heterotrophic and autotrophic growth, and thus the general symbol for biomass, XH, has been used in it.
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