Encyclopedia of Environmental Science and Engineering, Volume I and II

137

B

BIOLOGICAL TREATMENT OF WASTEWATER

1. INTRODUCTION

Biological treatment is the most widely used method for

removal, as well as partial or complete stabilization of bio-

logically degradable substances present in waste-waters.

Most often, the degradable substances are organic in nature

and may be present as suspended, colloidal or dissolved

matter. The fraction of each form depends on the nature of

wastewater. In the operation of biological treatment facili-

ties, the characteristics of wastewater are measured in terms

of its chemical oxygen demand, COD, biochemical oxygen

demand, BOD, total organic carbon, TOC, and volatile sus-

pended solids, VSS; concepts of which have been discussed

elsewhere.^1

Most of the conventional biological wastewater treat-

ment processes are based on naturally occurring biologi-

cal phenomena, but are carried out at accelerated rates.

These processes employ bacteria as the primary organ-

isms; however, certain other microorganisms may also

play an important role. Gates and Ghosh^2 have presented

the biological component system existing in the BOD pro-

cess and it is shown in Figure 1. The degradation and sta-

bilization of organic matter is accomplished by their use

as food by bacteria and other microorganisms to produce

protoplasm for new cells during the growth process. When

a small number of microorganisms are inoculated into a

bacteriological culture medium, growth of bacteria with

time follows a definite pattern as depicted in Figure 2 by

plotting viable count and mass of bacteria against time.^3

The population dynamics of bacteria in biological treat-

ment processes depends upon various environmental fac-

tors including pH, temperature, type and concentration

of substrate, hydrogen acceptor, availability and concen-

tration of essential nutrients like nitrogen, phosphorous,

sulfur, etc., and essential minerals, osmotic pressure, tox-

icity of media or by-products, and degree of mixing.^4 In

recent years, cultures have been developed for biological

treatment of many hard-to-degrade organic wastes.

2. METABOLIC REACTIONS

The metabolic reactions occurring within a biological treat-

ment reactor can be divided into three phases: oxidation, syn-

thesis and endogenous respiration. Oxidation–reduction may

proceed either in the presence of free oxygen, aerobically,

or in its absence, anaerobically. While the overall reactions

SUBSTRATE

ORGANICS

OXYGEN

GROWTH

FACTORS

LYSISED

PRODUCTS

OXYGEN

BACTERIA

(PRIMARY FEEDERS)

DEAD

BIOMASS

AUTO-

DESTRUCTION

OXYGEN

GROWTH

FACTORS

CO 2

CO 2

H 2 O

H 2 O

ENERGY

ENERGY

OTHER PRODUCTS

OTHER

PRODUCTS

CO 2

H 2 O

ENERGY

OTHER

PRODUCTS

P R O T O Z O A

B A C T E R I A

PROTOZOA

(SECONDARY FEEDERS)

FIGURE 1 Biological component system existing in BOD process.

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