ABSTRACT
Biological treatment is the most widely used method for removal, as well as partial or complete stabilization of biologically 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 facilities, the characteristics of wastewater are measured in terms of its chemical oxygen demand, COD, biochemical oxygen demand, BOD, total organic carbon, TOC, and volatile suspended solids, VSS; concepts of which have been discussed elsewhere. 1
Most of the conventional biological wastewater treatment processes are based on naturally occurring biological phenomena, but are carried out at accelerated rates. These processes employ bacteria as the primary organisms; however, certain other microorganisms may also play an important role. Gates and Ghosh 2 have presented the biological component system existing in the BOD process and it is shown in Figure 1. The degradation and stabilization 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 treatment processes depends upon various environmental factors including pH, temperature, type and concentration of substrate, hydrogen acceptor, availability and concentration of essential nutrients like nitrogen, phosphorous, sulfur, etc., and essential minerals, osmotic pressure, toxicity 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.
