Page 56 - Industrial Wastewater Treatment, Recycling and Reuse

Industrial Wastewater Treatment, Recycling, and Reuse: An Overview 39

1.4.8 Biological Method of Treatment
1.4.8.1 Aerobic Treatment
A commonly practiced biological treatment methodology involves aerobic
treatment, which is simple in its concept and operation. A high degree of
substrate conversion is the primary aim in aerobic wastewater treatment
(e.g., the activated-sludge process, Environmental Protection Agency,
1997). A major disadvantage of aerobic processes is large amounts of bio-
mass (clarification sludge) that is formed simultaneously because of the aer-
obic nature of the phenomenon. The fundamental advantage of an aerobic
treatment is that oxidative degradation of the carbon substrates provides the
energy required for propagation of the microorganisms that act as the bio-
catalyst. The organisms involved in biological wastewater treatment are
invisible to the naked eye. Depending upon their structures and cellular
components, they can be subdivided into bacteria, fungi, plants, with
viruses representing a special group of their own. The bacterial cells can
be said to represent “biochemical reactors,” wherein heterotrophic organ-
isms oxidize organic compounds with the aid of oxygen. The microorgan-
isms also perform the role of sorbents that bind both organic substances and
heavy metals, subsequently aiding their decomposition and/or removal.
For efficient operation of aerobic processes, the following aspects are to
be carefully evaluated:
• The concentration of nutrients in the wastewater should not be too low.
• As many suitable organisms as possible should be present within the bulk
(bacterial retention) to ensure stable biocenosis.
• The bacteria must be provided with an adequate supply of oxygen to
support aerobic metabolism (aeration).
• The three reaction partners—bacteria, substrate, and dissolved oxygen—
must be brought together using appropriate design.
• The growth rates for the different bacteria must be well matched
(sludge age).
• The optimal environmental conditions (pH, temperature) for biological
degradation must be established.
• The C:N:P ratio required for the growth of cell substance must be
assured, and all required trace elements must be present.
The commonest form of aerobic process is the activated sludge process,
which is now approximately 100 years old. It involves the following steps:
1. Wastewater aeration in the presence of a microbial suspension.
2. Solid-liquid separation following aeration.
3. Discharge of clarified effluent.