Page 59 - Industrial Wastewater Treatment, Recycling and Reuse

42 Industrial Wastewater Treatment, Recycling, and Reuse

1.4.8.2 Anaerobic Treatment
The anaerobic treatment of industrial wastewater, as shown schematically in
Figure 1.16, has become increasingly important in recent years as a result of
environmental protection legislation, rising energy costs, and problems with
the disposal of excess sludge formed in aerobic treatment processes. In anaer-
obic fermentation, the formation of methane, alcohols, ketones, and organic
acids is important. Biogas as a byproduct is utilized to meet the energy
requirements. This gas contains methane and carbon dioxide in a 1:1 to
3:1 ratio, together with hydrogen sulfide to an extent that depends on
the sulfate content of the substrate, as well as traces of nitrogen and hydro-
gen. The growth of anaerobic microorganisms is a function of numerous
factors, including residence time, temperature, redox potential, pH, and
nutrient composition. In the process of anaerobic methane generation in
the wastewater treatment, at least three groups of microorganisms are
involved in the degradation of complex organic molecules—acidogenic
bacteria, acetogenic bacteria, and methanogenic bacteria. First, biopolymers
are hydrolytically degraded by the process of acidogenesis to give soluble
monomers. Acidogenesis is followed by acid formation, with the simulta-
neous generation of hydrogen and carbon dioxide, a step referred to as acet-
ogenesis. It is at this point that methane formation can commence, and the
reaction of hydrogen with carbon dioxide occurs to produce methane. The
complete degradation chain is in reality much more complicated and
requires balanced cooperation among various microorganisms. The kinetics
of the individual steps is inadequately understood even today because sub-
strate mixtures are complex in most cases and collecting reliable kinetic data
is time consuming and difficult.
Given the same organic load, excess sludge formation is lowered by
a factor of 10 in an anaerobic process. The fundamental advantages of
anaerobic wastewater treatment are the following:

Micro-
Untreated Primary Waste + organisms Treated
wastewater treatment wastewater
Anaerobic
+CH
CO 2
4
Sludge for
disposal
Figure 1.16 Schematic of anaerobic biological wastewater treatment process.