Take the pool floats that consist of long inflatable parallel tubes and super engineer them. Embed a fiber net into the plastic, make the tubes very long and add tubes to make the float very wide. This can then be used to cover open sewers and collect the solar energy from the sun in the sewage, heating
the sewage to pasteurize it.
A border would be added to the edges of the float to anchor the embedded fiber net and allow the float to be tied to other stuff or staked out.
By making a special "float" with one less tube you can nestle this float between two of the previously described floats. The border would be made wider on this float so that it could be tied with the rest of the sandwich. The bottom side of this float would be blackened to absorb solar radiation and the float filled with sewage. The ends of this float would be made such that a manifold could be attached to allow the floats to be emptied, filled or extended to greater length.
At a refuge camp, organizers could transport these in a deflated state to the site. The lateens could then be added to the end of a set of tubes such that the wastes fill the tube during the day. Every morning before sun up the middle tube would be emptied at the other end. This would allow the wastes to heat during the day and continue to disinfect themselves during the night. The number of tubes would be sized to meet the needs of the camp.
The drainage field can be furrowed by the camp residents before use. There are seeded varieties of bananas, and the drainage field can be densely planted with banana plants. The UN could run a breeding program to find banana varieties that produce a strong fiber. (There is always a need for strong cord.) And banana leaves make useful disposable liners for plates bowls and garbage containers.
Where there is already a polluted stream, the flow of the stream could be diverted into such a sandwich and the sandwich extended in length down the natural drainage of the stream. The length of the extension would be chosen to give the flow sufficient heat absorption to reach pasteurization temperature.
By building a small pond from native materials at the outlet the time-at-temperature could be extended without too many more resources. Once so pasteurized this water should meet requirements for bioremediation or could be used for agricultural irrigation of pasturage.
The bioremediation can be initiated by injecting thermopile algae species into the stream head. This algae could grow in the sunlight and absorb nitrate pollutants while creating oxygen to meet the BOD.
Putting a CLEAN solar sandwich upstream can produce disinfected water for bathing and drinking. Or they could be drained and refilled every morning with the water retained throughout the day to heat and disinfect. A camp could be surrounded by a large number of these lying on the ground. Some could be used to produce drinking water, others to disinfect used bathing water if the supply of water was restricted.