I agree with Mr. Mann and Mr. ...

I agree with Mr. Mann and Mr. Lauenstein. I would recommend a shallow, watertight pool which is populated by duckweed or water hyacint ( Lemnoidae and Eichhornia crassipes -take care, the second is considered to be highly invasive and dangerous, especially in Africa). If possible use local surface or underwater floating  water plant species, the ones growing the fastest. Let the water flow through slowly the pool. The size/volume of the pool, the flow and harvest intensity should be tested first with a pilot.  I would start with 15-20 cm depth, a few square meters, 2 day contact time (flow/volume) and a biweekly harvest of 25%. I would measure effluent NO2 daily, starting from the seventh day,  and I would set the parameters (flow, depth) based on the result. Temperature might influence plantal growth, if the water is cold, they grow slower. If this works: it is cheap but requires labour to harvest the yield intermittently. Also, the pool have to be cleaned regularly  to produce adequate water. Pumping needs at least a windmill wheel, so this one could be installed without electricity.