It depends entirely on the ...

It depends entirely on the resources available to you. Many of the suggestions below are great - if you have the funds and access to warehousing to store plant for possible deployment etc. In a 3rd world environment these resources may not be readily available.

Slow sand filtration is a proven technology that can be implemented with minimum resources. A system can be constructed by local people and does not require sophisticated operator capability.

Combine a sedimentation tank, if necessary dosed with some sort of chitosan based flocculant, a slow sand filter, and disinfection dosing using sodium or calcium hypochlorite, to provide a simple, robust, and scalable water treatment system.