Encyclopedia of Environmental Science and Engineering, Volume I and II

BIOLOGICAL TREATMENT OF WASTEWATER 141

continued aertion under these conditions results in auto-

oxidation of biomass. Although the rate of metabolism is

relatively low at high M/F ratio, settling characteristics of

biomass are good and BOD removal efficiency is high.

Goodman and Englande^17 have suggested that the total

mass concentration of solids, X T , in a biological reactor is

composed of an inert fraction, X i , and a volatile fraction, X v ,

which can be further broken down into an active fraction, X,

and non-biodegradable residue fraction, X n , resulting from

endogenous respiration, i.e.:

X T = X i + X v = X i + X + X n. (17)

The total mass concentration of solids in wastewater treat-

ment is called suspended solids, whereas its volatile fraction

is called volatile suspended solids, X. In a biological reac-

tor, volatile suspended solids, X, is assumed to represent the

mass of active microorganisms present per unit volume.

3. TOXICITY

Toxicity has been defined as the property of reaction of a

substance, or a combination of substances reacting with each

other, to deter or inhibit the metabolic process of cells without

completely altering or destroying a particular species, under a

given set of physical and biological environmental conditions

for a specified concentration and time of exposure.^18 Thus, the

toxicity is a function of the nature of the substance, its concen-

tration, time of exposure and environmental conditions.

Many substances exert a toxic effect on biological oxida-

tion processes and partial or complete inhibition may occur

depending on their nature and concentration. Inhibition may

result from interference with the osmotic balance or with

the enzyme system. In some cases, the microorganisms

become more tolerant and are considered to have acclima-

tized or adapted to an inhibitory concentration level of a

toxic substance. This adaptive response or acclimation may

result from a neutralization of the toxic material produced by

the biological activity of the microorganisms or a selective

CELL

CELL MEMBRANE

LIQUID FILM

LIQUID FILM

BY-PRODUCT

OXYGEN

SUBSTRATE

BIOCHEM. REACTIONRD

R 1 R 2

R 2

R 2

R 2

REACTOR

DISSOLVED

DISSOLVED

OXYGEN

SUBSTRATE

SUBSTRATE

C C

PRODUCTS ∆r

CO 2

O 2

BIOCCELLH

EMICAL

REACTION

WASTE

PRODUCTS

SUBSTRATE

TRACE

ELEMENTS

FLOC PARTICLE

O 2

O 2 O 2 O 2

R 2

R 2

∆r

FIGURE 5 Mass transfer in biofloc.

0 2 4 8 12 16 20 24

Aeration time, hr

Oxidized

Net adsorbed and synthesized

0

10

20

40

50

60

70

80

90

100

30

Reduction of total carbonaceous oxygen demand, (%)

Total BOD

FIGURE 6 Removal of organic inbalance by biomass in a batch

operation.

(^0) 0.2 0.5 1 235 10 20
0.5
1.0
UNUSED
BOD ASSIMILATIVE
RESPIRATION
INITIAL SYNTHESIS ENDOGENOUS
RESPIRATION
NET BIOMASS INCREASE
SHORT-TERM
AERATION
CONVEN-
TIONAL
EXTENDED
AERATION
RELATIVE ORGANISM WEIGHT (M/F)
DISPOSITION OF ASSIMILATED BOD
FIGURE 7 Metabolic reactions for the complete spectrum.
growth; also, the BOD removal efficiency is poor as the
excess unused organic matter in solution escapes with the
effluent. On the other hand, high M/F ratio means the opera-
tion is in the endogenous phase. Competition for a small
amount of food available to a large mass of micro organisms
results in starvation conditions within a short duration and
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