After treatment, BOD demand decreases. BOD stands for biological oxygen demand and COD stands for chemical oxygen demand.

Centrifuge is used for dewatering sludge. This is also used for thickening sludge. The or methods for dewatering sludge are done by a filter press, lagoons, sludge drying bed, belt filter press and Reed bed.

The organisms in reactor multiply by feeding of organic solids present in reactor. The reactor consists of wastewater containing organic solids.

H₂S is highly soluble in water. Hydrogen sulphide poisoning is one of leading causes of accidents. If its concentration is above 5 ppm moderate odour is present. If its concentration increases above 300 ppm, it leads to unconsciousness and death.

A sewage treatment plant providing both primary and secondary treatment is expected to remove at least 85 percent of BOD and suspended solids from domestic sewage.

BOD load in Kg/day = Inlet BOD in mg/l x Flow in m³/day /1000 =1000 x 300 / 1000=300 Kg/day. Theoretical oxygen requirement = BOD load x Kg of O₂/Kg of BOD= 300 x 1.8 = 540 Kg of O₂/day.
Standard O₂ Requirement = Theoretical O₂ requirement/ Alpha x [Beta x Csw – D.O.] x Theta^(Tww-20°C)/ C_ss= 540/24/[0.7 x [0.95 x 8.64 ? 2] x 1.024^(30-20)]/9.86=48 Kg of O₂/hr. Air requirement = Actual Oxygen requirement/ (Density of air x % of O₂ in air by weight x SOTE%) = 48/( 1.201 x 0.232 x 0.1599)= 875 m³/hr. Factors assumed: SOTE=17.27%; Alpha=0.7; Beta=0.95; Theta=1.024; Surface saturation C=7.97 (Calculated); Site basin saturation C_sw=8.64 (Calculated); Standard basin saturation C_ss=9.86 (Calculated).
Number of fine diffusers required = 875/6=146. (flow rate considered for fine diffusers is 6 m³/hr).

For complete removal of Nitrogen through ASP, solid retention time considered is 3-18 days. However, this depends on temperature. In case temperature is very low, n solid retention time considered would be around 15-18 days. For higher temperatures, lower solid retention time is considered.

SBR operates as a batch process. Because sequential batch reactors operate as a batch process, level of basin starts out low and basin fills over several hours as influent enters.

The sludge constituting biosolids can be treated anaerobically also. While carrying out anaerobic treatment, solid retention time assumed is 15 days. The temperature is maintained at 15-20 degree celsius.

Clarifier (settling tank) is next reactor where solids (mostly cells, called sludge at this stage) are separated from water. The system is commonly operated in continuous mode (as opposed to batch mode).