BOD load in Kg/day = Inlet BOD in mg/l x Flow in m³/day /1000 =800 x 300 / 1000=240 Kg/day. Theoretical oxygen requirement = BOD load x Kg of O₂/Kg of BOD= 240 x 1.8 = 432 Kg of O₂/day. Standard O₂ Requirement = Theoretical O₂ requirement/ Alpha x [Beta x Csw – D.O.] x Theta^(Tww-20�C)/ C_ss= 432/24/[0.7 x [0.95 x 8.64 ? 2] x 1.024^(30-20)]/9.86=38.29 Kg of O₂/hr. Air requirement = Actual Oxygen requirement/ (Density of air x % of O₂ in air by weight x SOTE%) = 38.29 /( 1.201 x 0.232 x 0.1599)= 696 m³/hr. (Rounded off to 700 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).

After this kind of treatment that is after physical removal of solids and biological removal of solids, wastewater may be called secondary-treated wastewater.

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).

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.

Inert particulate COD derives from inert particulate COD present in influent wastewater and entering plant. Autotrophic biomass is made up of nitrifying bacteria.

BOD load in Kg/day = Inlet BOD in mg/l x Flow in m³/day /1000 =800 x 200 / 1000=160 Kg/day. Theoretical oxygen requirement = BOD load x Kg of O₂/Kg of BOD= 160 x 1.8 = 288 Kg of O₂/day. Standard O₂ Requirement = Theoretical O₂ requirement/ Alpha x [Beta x Csw – D.O.] x Theta^(Tww-20�C)/ C_ss= 288/24/[0.7 x [0.95 x 8.64 ? 2] x 1.024^(30-20)]/9.86=23.05 Kg of O₂/hr.

When filling/ aeration phase is complete, aeration system shuts off and basin begins to function as a secondary clarifier. Suspended solids and micro-organisms settle to bottom of tank and clarified water is left near surface.

Various standard conditions are mentioned for aeration process. The standard temperature is considered to be 20?C along with 1 atmospheric pressure, zero salinity and zero dissolved oxygen.

For development of flocculent biomass for treating domestic water through Activated Sludge Process (ASP), solid retention time considered is 1-3 days. In case it is industrial waste water, retention time considered would be different. The difference is due to difference in concentration of BOD in influent.

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