In biological treatment systems, microorganisms are used to degrade the biodegradable organics in wastewater.19 Biological treatment systems can be "suspended growth systems" (e.g., activated sludge systems) or "attached growth systems" [e.g., rotating biological contactors (RBCs) and trickling filters] according to the growth type of the microorganisms.19,20 Considering the type of oxygen utilization, biological treatment systems can be classified as aerobic, anaerobic, and facultative. In the aerobic system the organic is decomposed to carbon dioxide, water, and a series of simple compounds, whereas in the anaerobic system the final products are carbon dioxide and methane.
Typically, the aerobic biological process achieves better quality in treated effluent, easy operation, and shorter solid retention time. However, the cost of aeration is high in the aerobic system and excess sludge is produced. When it comes to the high-load influent treatment (COD > 4000 mg/L), aerobic biological treatment becomes less economical than anaerobic treatment. In most of the cases, the anaerobic biological process is used as a treatment process prior to the aerobic treatment. Selecting the appropriate treatment process largely depends on the wastewater characteristics and the required level of treatment.19
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