Encyclopedia of Environmental Science and Engineering, Volume I and II

728 MUNICIPAL WASTEWATER

Water can hold about 10 mg/l dissolved oxygen. Since BOD

values can vary from less than 1 mg/l for a clean stream to

many thousands of milligrams per liter for some industrial

wastes, it is necessary to dilute stronger wastes.

The dissolved oxygen concentration of a seeded nutrient

dilution water is determined. A measured quantity of waste

is then added to a known volume of dilution water. Total

sample volume is usually about three hundred milliliters.

The mixture is then incubated at 20°C for five days. At the

end of this period the dissolved oxygen concentration of the

mixture is again measured. BOD is then equal to the reduc-

tion in dissolved oxygen divided by the dilution factor.

Figure 1 shows the exertion of BOD over an extended

period (more than 50 days). The trajectory displayed is one

which might be followed when waste is added to a rela-

tively clean stream and the watercourse acts as an incuba-

tion bottle. Such a complex reaction is of little utility for day

to day control. In order to make the concept of BOD more

useful, a simplified model is developed. The path followed

in the idealized model is displayed in Figure 2.

The lag phase is eliminated by use of seeded dilution water.

In this phase, microorganisms which utilize the waste material

for food are becoming acclimatized and assuming dominance

in the system. In the carbonaceous phase the dominant organ-

isms are feeding upon the more easily degraded compounds.

The nitrogenous phase, in which the character of the food and

the microbial population change, does not usually begin for at

least ten days. This portion is of interest in research on kinet-

ics but can be ignored in the following. It is hypothesized that

a simple monomolecular rate constant can describe the car-

bonaceous phase and the reaction is asymptotic to a limiting

value L at infinite time. L is known as the ultimate BOD and

is a mathematical artifact. The rate of reaction is assumed pro-

portional to the BOD still remaining,

d

d^1

y

k

t

 ().Ly

Integration and taking of proper limits gives

yL e L


 
()( ).1110kt^11 kt

Phelps’ relation for decolorization of methylene blue with

time

y

L

 1 0.794t

is equivalent to the model equation when k 1 equals

0.10/day.

In practice, the 5 day 20°C BOD is used to describe

the strength of a waste. Simple enumeration of the BOD

value tells nothing about the path by which the number was

reached since L and k 1 are not specified. Sewers and treat-

ment plants are not isothermal entities but it is necessary that

a common basis be established. One of the reasons advanced

for the choice of the five day period is the fact that almost all

rivers in England are within five days flow of the sea. Waste

once discharged to the infinite ocean is no longer of interest.

Times have changed and there is great concerns for effects of

waste on the “infinite ocean.”

In the literature reference to BOD means 5 day 20°C BOD

(BOD 5 ) unless specifically stated otherwise. Recently, some

popular writers have erroneously used the term “ biological

oxygen demand.” There is no such parameter.

The reaction rate constant k 1 actually describes a series of

complex microbiological reactions. In the initial stages of

biological stabilization of sewage, carbonaceous material is

oxidized to carbon dioxide and nitrogenous material is oxi-

dized first to nitrite and then nitrate. One of the measures of

degree of stabilization of organic matter in an effluent is the

nitrate concentration. However, recent theories concerning

eutrophication have raised serious questions concerning the

desirability of high effluent nitrate concentrations.

time

lag phase

carbonaceous

phase

B.O.D

mg/l

nitrogenous

phase

FIGURE 1

B.O.D.

mg/l

L

y

L – y

y

time

FIGURE 2

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