"While considering the ...

"While considering the municipal wastewater treatment plant design for SBR or AS, we assume a value of around BOD 150-250 mg/L and 250-500 mg/L at the influent. I saw many WWTP in Thailand are designed based around these values. But in reality one of my friends (operator for municipal wastewater treatment) and I have personally seen that the influent is generally not over 30-60 mg/L BOD and a COD 45-100 mg/L but had a lot of SS. I suspect this is due to storm water is being mixed with wastewater. So my second question is: Is it overkill to design a wastewater treatment plant with high influent BOD and COD ? While the actual situation barely meets the criteria to keep the MLSS alive ? Then why do so many consultants design wastewater treatment plants with such influent values? Maybe a safety factor but so much energy and capital is lost due to this safety factor. Any opinions or personal experience or case studies or standard methodology regarding this matter would be really nice." You may have low BOD/COD. Many sewage works are designed to discharge an effluent that shall not exceed 30 mg/l BOD, 125 mg/l COD. You may also have taken your samples at an inappropriate time. There is a diurnal pattern to sewage flow and concentration. Typically the peak flow/load entering the sewage works is around 7 – 9 am – may be different for your location – linked in with the morning rush. How long this takes to reach the sewage works then depends upon the design and layout of the sewers. There is a second peak, for the evening. If you are mostly industrial, with people living away from the area, your peak may be around lunch but with a larger baseline from the industrial contribution. So – if you sampled (say) 11 am you may have taken a minimum concentration rather than a maximum; and either way you really need to consider taking hourly samples for (say) Mon, Weds, Fri to pick up the weekday pattern, and Sat/Sun for the weekend. If you have ‘a lot of SS’ with low BOD/COD you may be looking at some industrial component discharging a lot of inert solids. It is worth doing a volatile solids test. If the volatile solids is around 75% - say, 65 – 80% - then most of your waste is probably of domestic origin. Lower than 65% and a lot of grit is entering the system from somewhere. Higher than 80% and you may have someone disposing a lot of degradable waste – brewery? The values that you see quoted as the common values are those that are usually found with domestic sewage and a moderate level of industrial contribution, and look more like US than European values. The values are affected by how much people eat, how much water they use, how much infiltration takes place, how much the sewers are double up a storm sewers. There has been much analysis from the Thai universities, but usually in Thai, so I cannot comment on what the theses contain.