According to the Indian guidline, 150 lts/ person of water is consumed in a residence in a day. This entire water reaches the wastewater treatment plant. There could be other extraneous water also resulting from leaking taps, seepage in pipelines, rainwater intrusion, etc. Any design should take these factors in to consideration while designing a wastewater treatment plant. Also typical consideration of BOD is 400 mg/l and BOD is 800 mg/l for design. After this there is the collection tank design should take care of the varying flow, buffer and sludge accumulation and make liberal provision in the collection tank, especially in small community STPs. It is always advisable to keep the collection tank volume as the same size as that of total daily inflow. Aeration is calculated based on oxygen requirement. There is a need for 1200 mg/l oxygen to remove the pollutant. This is present as 20% composition in air, and is diffused in to the wastewater at an effeciency of 2- 7% per meter depth depending on the type of diffusion systems. The retention time in a chamber also determines the cleaning of water. As a rule of thumb, if the aeration chamber and the sedimentation chamber combined is the volume equivalent to the daily inflow, it is an adequate design. The main factors influencing treatment are the capacity of collection, aeration and sedimentation tank. In SBR based plant the aeration and sedimentation happens in a single tank. The other important factors influencing are the choice of pumps, blowers and filtration systems. Automated systems are more reliable that manual plants that have to depend on human operators. Please write to us your specific questions and we will be happy to answer.