For FREE software your choice ...

For FREE software your choice is limited. There is STOAT - see links above. if you wish to restrict yourself to activated sludge only then you can add the Modelica Wastewater treatment library, which will require that you learn to use Modelica; there is SSSP, which should be downloadable from Clemson University's website; and there is a simple steady-state program called STEADY, from another US university - not sure of the name. I see references above to TEKLA and WATPLANT, which I have not heard of, and BIOWINE, which I think is Biowin - which is not free. You could rent at c. $2,000 for a year. SASSPRO is also not free. A quick web search suggests that TEKLA is intended to assist with the CAD stage of the design, while WATPLANT all I could find was a training course held in 2009. With the data that you indicate there is NO software that will assist you. For designing sewage treatment you should be looking at BOD as a minimum; ammonia and TKN, if you need to remove nitrogen; and phosphorus, if you wish to remove phosphorus. You will also need flow and temperature. Your national design standards should suggest suitable values to use for water flow, BOD, ammonia, etc., given the population that you are treating. You then need to decide what technology you wish to model. If you wish to look at reed beds then there are no models readily implemented, BUT reed beds are commonly designed on rules of thumb. I don't know the values for ?India?; I can give you values for the UK, but this is a colder climate, and you probably need less. If you wish to model activated sludge or trickling filters give STOAT a try. If you want UASBs then you may have some success with STOAT - one user reported problems, but I don't have the details yet to work out what went wrong. With these processes you have additional operational complexity (maybe not for trickling filters - but I don't know what is the malarial risk involved), and so you may be looking at a staffed sewage works. if you go to the IWA Wiki you will find that they have made free to download various books on sewage treatment in the tropical environment. If you have limited data you may find that this is the best approach - the traditional hand calculation design procedures.