OVERVIEW

Secondary, or biological, treatment technologies comprise those unit operations that remove biodegradable compounds from wastewater. One variety, activated sludge treatment, removes most of the BOD treated in municipal and industrial wastewater treatment plants. Secondary treatment technologies are generally preceded by one or more of the pretreatment operations described earlier in the previous chapter.

Many types of biological treatment systems are in use today. Their defining feature is the use of biomass i.e., bacteria, protozoans, rotifers, and other microorganisms to remove BOD by degrading organic pollutants via biochemical metabolic processes. The tremendous power and versatility of biological systems derives from the fact that virtually every organic substance, no matter how toxic to many or most organisms, is attractive food to some species of microorganisms, in appropriate concentrations and under the right conditions.

Biological reactors, accordingly, are carefully controlled environments in which conditions are maintained for optimum biomass synthesis (i.e., growth) and organic degradation, based on the organics being treated. Biodegradable wastewaters are contacted with (or fed to) microorganisms acclimated to degrade soluble and suspended organic compounds.

Biological systems are operated as aerobic, facultative, anaerobic, and anoxic reactors. Aerobic processes occur in the presence of dissolved oxygen. Obligate aerobes (microorganisms requiring oxygen to survive) are the predominant microorganisms in aerobic reactors. The oxygen may be supplied through mechanical means, by the vaporization of liquid oxygen, or by natural absorption of oxygen in solution by gas/liquid transfer from the atmosphere and/or by photosynthesis.

Facultative processes...