Help needed in choosing a software for designing a sewage treatment plant

We are planning to design a sewage treatment plant for a small community.We need help in choosing a software for designing this plant. If you know any best suited free software for deigning sewage tretmant plant with below details please let us know.

Details for estimating the plant capacity: Population: 1500, water distibuted daily from the community tank : 420000 liters, No industries arround only househlod and shops. We have data like water acidity (at different soursesc), turbidity, PH, alkalinity, chloride content etc. We are looking for FREE softwares which can desigh a STP depending on this much of data only. Thanks in advance.