ABSTRACT
There are two main categories of biological treatment, namely aerobic treatment or treatment in the presence of oxygen or air, and anaerobic treatment or treatment in the absence of oxygen. Anaerobic treatment should be considered whenever a high strength waste is encountered. It is very cost-effective in some situations. Anaerobic treatment is usually used as a first stage biological unit to reduce biological oxygen demand from 600 or more to the 200 to 300 parts per million range. Aerobic treatment is usually the method of choice for secondary biological treatment as almost all organic compounds are biologically oxidizable by aerobic treatment. There are two types of biological reactors, suspended growth reactors and fixed film reactors. Suspended growth reactors rely on mixing or hydraulic gradients to keep microorganisms in suspension. Fixed film processes rely on the microorganisms attaching themselves of a solid medium, and the wastewater is trickled or passed over the medium as a film.
