Good open source software to help design municipal WW treatment plants? Why so many over designs?

A few months ago I was approached to help design a wastewater treatment plant for municipal wastewater (a draft version for a proposal for activated sludge system). I was not confident about the models I was using for detailed design and calculations and thought that to develop our own would be like reinventing the wheel. So I searched online for some software that could help me to check the results and give confidence not to over or under-design the system.  I found some software but I am not sure if the people in the industry are using the same.  As per my research I found the software below, all of them are paid versions.   1. Audesk (water and wastewater ) 3. Various softwares by Can someone point me towards an open source software that I might be able to use ?  Additionally, I would like to know experts candid view on an observation.   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.