Environmental Engineering FOURTH EDITION

88 ENVIRONMENTALENGINEERING

BOD concentration might be low. Engineers often talk of “pounds of BOD,” a value
calculated by multiplying the concentration by the flow rate, with a conversion factor,
so that

The BOD of most domestic sewage is about 250 ma, while many industrial wastes
run as high as 30,OoOmg/L. The potential detrimental effect of untreated dairy waste
that might have a BOD of 20,000 mg/L is quite obvious.
As discussed in Chapter 4, the BOD curve can be modeled using Eq. (4.8):

BOD(t) = Lo(1 - e-kjr),

where

BOD(t) = amount of oxygen required by the microorganisms at any time t

(ma),

LO = ultimate carbonaceous oxygen demand (mg/L),

k’, = deoxygenation rate constant (days-’), and

t = time (days).

When it is necessary to know both k’, and LO, as when modeling the dissolved oxygen

profile in a stream, both are measured using laboratory BOD tests.

There are a number of techniques for calculating k; and h. One of the simplest is

a method devised by Thomas (1950). Rewriting Eq. (4.8) using common logarithms

results in

BOD(t) = Lo(1 - 10-k;f),

which can be rearranged to read

(&) ‘I3 = (2.3 k’, + (5.10)

This equation is in the form of a straight line

x=a+bt,

where x is (r~~oD(t)>‘/~, the intercept (a) is (2.3k’,Lo)-’I3, and the slope (b) is

k; 213/(3 .43L:l3).

By plotting BOD versus r, the slope (b) and intercept (a) can be used to solve for

ki andh:

k’, = 2.61 (Ha)

Lo = 1/(2.3k’,a3).