Hi Lara,  The critical ...

Hi Lara, 

The critical determinant - is the expertise of the engineer who should take the sewage water quality samples (preferably on seasonal basis), analyze the results against the constraints of the particular project including discharge/reuse requirements, land availability etc.. There could be a number of different methods/treatment techniques to achieve the outcome required. No need to restrain to one only. I would rather write a functional specification - here is the sewage quality coming in - and here is the water quality that we want - going out. Then go to the market and ask those good treatment companies out there to provide you with a solution.  Otherwise - you are relying on the expertise of one company/engineer against the combined expertise of the market. 

Best of luck with your project!

Regards,

Iouri