The data you have is for the ...

The data you have is for the potable water being used. for designing sewage treatment plant, I believe you will need data related to the sewage that you want to treat. The applicable effluent discharge standards will determine the parameters for which the sewage should be analyzed. At least, you would require flow, BOD5, COD, pH, alkalinity. You will also need TN, NO3 and NH4 etc if nitrogen removal is also required. You may use the data from the available sewage characteristics from similar existing communities if you do not have resources to perform the analysis. I would suggest your team member who has knowledge of sewage treatment processes can develop your own design calculation sheet on Excel using the procedure given in the literature such as Metcalf & Eddy book because the flow is not so big and the treatment process should not be very complex. You may be also able to download some activated sludge system design calculations sheet from the internet by Google search. Biowin and Stoat can be used provided that you wish to boy the software. You might also need training how to use them.