ABSTRACT
After primary treatment, a major portion of the dissolved organic component still persists in the wastewater. This organic matter is removed using the secondary treatment or biological treatment. Microorganisms in biological processes help to degrade the organic matter either in the presence (aerobic process) or absence (anaerobic process) of oxygen. The mechanisms involved in the degradation of organic matter by aerobic and anaerobic processes have been discussed in this chapter. The biological treatment processes may be further classified as suspended growth, attached growth, or a hybrid growth process. In this chapter, the fundamentals, design, and working principles of different aerobic suspended, attached, and hybrid growth processes and anaerobic suspended and attached growth processes have been discussed in detail. Special emphasis has been given to processes, such as activated sludge process, sequential batch reactor, trickling filter, moving bed biofilm reactor, upflow anaerobic sludge blanket reactor, anaerobic fluidized bed reactor, to name a few. The secondary settling of the treated effluent from the biological processes has been discussed along with the requirement of post-aeration. A complete overview of the different biological processes, including their advantages, drawbacks, and issues in operations, is thoroughly covered in this chapter.
