Hi Sara,
in my mind the most important factor is the reuse of the wastewater. That is, centralised plants tend to discharge to water bodies, which is the design "behaviour" that must change and therefore be influenced by the scale of the plant. Land disposal should be a prerequisite, with design at the optimum scale to make that happen and to retain the nutrients for the application e.g. crops, forestry depending on treatment level and land use.
cheers
Dean

Dear SuSanA,

I am researching to find a methodology to determine the best combination between centralised and decentralised systems in a rural area of ca. 500 km2 in India that comprises more than 150 small villages. The questions in these cases are: how many WWTP should be installed? How the villages should be grouped (or clustered) to connect to one WWTP? Would it be better that each of the villages has its WWTP?

Nowadays is known that the extremes of fully centralised or fully decentralised are not always the best solution, but a flexible combination according to local conditions. 

Therefore, I would appreciate having your opinion on which factors should be essential or important to define the clusters or groups to be connected to a common WWTP. Until now, I have considered the following, but I am pretty sure more factors need to be considered. 

- Location in the same river basin, which favours the transport of water by gravity, reducing pumping costs
- Similar characterisation of the wastewater produced
- Proximity between villages
- Cost differences between centralised and decentralised systems

Thanks a lot for your contributions. 

Sara Ramos
Master Student
Technical University Dresden