Water Quality Management - River Dynamics and Integrated River Management - page 565

and process 1 is the source of the benthic deposits of organic material. In the following sections, processes
(1)-(9) are discussed and simple models for their consideration are presented.

9.2.3 Biochemical Oxygen Demand

9.2.3.1 Nitrogenous Biochemical Oxygen Demand
The progressive exertion of the BOD of freshly polluted water generally breaks down into two stages: a
first stage, in which it is largely the carbonaceous matter that is oxidized; and a second stage, in which
nitrogenous substances are attacked in significant amounts and nitrification takes place (Fair et al., 1971).
This occurs because the growth of nitrifying bacteria lags behind the growth of microorganisms which
perform the carbonaceous reaction. In raw wastewater, the NBOD normally does not begin to exert itself
for at least 5-8 days (Fig. 9.7), but for well treated waste the nitrifying bacteria are present in sufficient
numbers to begin NBOD consumption.

Fig. 9.7 General relation between carbonaceous and nitrogenous biochemical oxygen demand (CBOD and NBOD)
in raw wastewater

Nitrogen is the critical element required for protein synthesis and, thus, is essential for life (Masters,
1991). The nitrogen cycle in water is as follows. When living things die, or excrete waste products, nitrogen
that was tied to complex organic molecules (i.e. organic nitrogen) is converted to ammonium by bacteria
and fungi. Then in aerobic environments Nitrosomonas bacteria (nitroso in relation below) convert
ammonium to nitrite and Nitrobacter bacteria (nitro in relation below) convert nitrite to nitrate.

nitroso nitro

NHDO o NODO o NO

3 2 3

bacteria bacteria

Nitrogen in the form of N 2 is unusable by plants and must first be transformed into either ammonium or
nitrate in the process called nitrogen fixation. Under anaerobic conditions, certain denitrifying bacteria are
capable of reducing NO 3 back into NO 2 and N 2 , completing the nitrogen cycle.
The NBOD can be estimated by multiplying the sum of the organic nitrogen and ammonia nitrogen
concentrations (known as total Kjeldahl nitrogen, TKN) by 4.57. Davis and Masten (2004, p. 281) report
the actual NBOD is slightly less than the theoretical value due to the incorporation of some of the nitrogen
into new bacterial cells, but the difference is only a few percent. The NBOD consumption can then be
approximated using a first-order model in the same way as the CBOD as follows:

N
t LL

N

e

K t

(9.19)

0