Free water surface constructed wetlands for polishing sugar factory effluent in western Kenya - macrophyte phosporus recovery and treatment results

• Phosphorus removal and uptake by two macrophyte species receiving two wastewater loading rates was studied in a free water surface constructed wetland system (FWS CW) in Kenya. Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Inlet and outlet water samples were analyzed for selected water quality parameters. The first macrophyte harvest for determination of biomass, P and N content was done after 21 months of operation, followed by two harvests with 7 months intervals. Mass reductions of TP and TDP were higher in the low-load CWs than in the high-load ones. Each macrophyte species had similar tissue P content independent of mass load suggesting excess available phosphorus in all CWs (low N:P ratios). During a 7 month period, the amount of P stored daily in the green biomass of the macrophytes represented 18-29% and 25-100% of the daily removal of TP and TDP, respectively. Still, the CW system was not functioning at optimal conditions for simultaneous wastewater treatment and P recovery. Frequent harvesting along with an enlargement of the CW area would yield higher relative nutrient removal rates, and increase the biomass that could be used by local communities for animal fodder and building material.

Nyckelord

Constructed wetland; Phosphorus removal; N uptake; P uptake; Cyperus papyrus ; Echinochloa pyramidalis

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