Complex problem will need ...

Complex problem will need complex solutions. I suggest we segregate the problems you mentioned to a more solvable scopes:

1. Water supply. I concur with the comment of John Turner below. the water source is like a bank where you withdraw and deposit your asset (this time the water resource). I suggest we consider sustainability because the state-of-the-art asset investment will not be utilized once the water supply runs out or the raw water quality becomes more polluted. I am assuming that the designed treatment will be specific for the current water quality of the water source.

2. Water demand. the 15000 population and 50-100 l/d are good inputs to know how much water we need to produce for the community. Set a target on how much water availability (e.g. x hours per day). This can be easily augmented by building additional reservoirs or over head tanks (so distribution will be less costly).

3. Distribution. our experience for poor communities is usage of public faucets. Strategically located to the community. Doing so will make the water pipes more manageable. your target here is lowering Non Revenue Water (NRW) or water loss along the pipes. Failure to manage the  water loss will entail more production cost and less benefit to the community. Industry standard is 20% of production volume/actual volume received by the community. Lowering NRW further will save operating cost and raw water.

4. Tariff. This should shoulder the operating cost of the system. If the community will not be paying, it means that the operational efficiency targets should depend heavily on the capacity of your financer. It will kill the project of the operating cost will be more than your operating budget.

5. Waste Water. the water provided to the community will eventually return to the bodies of water as waste water. If not considered, waste water will end up polluting adjacent water bodies of the community.