Wastewater treatment occurs in a treatment plant in several stages depending on the degree of treatment desired. In the first stage, the preliminary treatment processes prepare the influent wastewater for treatment in subsequent processes. Bar screens, grit chamber, and flow equalization tank are some of the processes included in the preliminary treatment. There is no significant removal of biodegradable organic matter expressed in terms of 5-day biochemical oxygen demand (BOD) or suspended solids by these processes. The next stage is the primary treatment process where settleable (and floatable) solids present in the wastewater are removed by gravity sedimentation. In some rare instances, the flotation process can be used instead of gravity sedimentation for the removal of settleable solids. The primary treatment process can remove up to 40% of the incoming BOD and 50-70% of the suspended solids.1 The subsequent stage is the secondary treatment process, which is needed to remove the remaining soluble and colloidal organic matter from the wastewater that was not removed during the primary treatment processes. The secondary processes invariably use aerobic biological treatment processes to remove the soluble and colloidal organic matter from the wastewater. The biological treatment process converts the soluble and colloidal organic matter into settleable solids and micro-organisms (sludge), which are removed in the secondary settling tank leaving a clearer supernatant effluent for discharge. Thus, the settling tank following the aeration tank is an integral part of the process. In this entry, the secondary tank details are not included. These processes in combination with the primary process can remove 90+% BOD (carbonaceous BOD) and suspended solids. Thus, the secondary wastewater treatment processes can meet the current US Environmental Protection Agency mandated effluent requirements of 30 mg/L of BOD and 30 mg/L of suspended solids for municipal wastewater treatment.1

There are two types of secondary aerobic biological treatment processes: suspended growth processes and attached growth processes. In the suspended growth process, the micro-organisms responsible for the biochemical conversion of organic matter are kept in suspension by aeration or agitation in a tank where the wastewater is introduced. The micro-organisms assimilate the organic compounds for synthesis of new cells (biomass) and for respiration, which provides the energy for the synthesis and other cellular processes. Activated sludge process and its modifications are suspended growth processes. In the attached growth process, the micro-organisms are present in an attached form (biofilm) on a medium, either stone, treated wood, or synthetic plastic materials. The wastewater comes in contact with these attached micro-organisms, and the same biochemical processes as in the suspended growth process take place, namely, cell synthesis and respiration. Trickling filters and rotating biological contactors are the two most common attached growth secondary biological treatment processes used.1

In this article, activated sludge process and some of its modifications are discussed at some length. Only the details of carbonaceous BOD removal from wastewater are included.