The decrease of water resources values as a result of eutrophication is a major problem in numerous lakes around the world, including the Great African Lakes. One of the most important tasks to limit this eutrophication, is to decrease the nutrient input from land-based activities. In the catchment area of Lake Victoria, industries such as sugar factories have been identified as being responsible for a large part of the increased nutrient flow to rivers and lakes. Constructed wetlands (CW) have been suggested as a low-cost method to improve the current wastewater treatment, commonly consisting of stabilization ponds. The aim of this thesis was to explore and quantify the effect of mass loading rates and two macrophyte species on treatment performance of free water surface (FWS) CWs polishing sugar factory effluent, with a special focus on phosphorus (P) removal. Also, another objective was to evaluate the amount of P that could be removed by harvesting the emergent macrophytes. The studies were performed in a pilot-scale FWS CW system in western Kenya. Four CWs were operated at a low loading rate and four at a high one, and half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Methods included field and laboratory measurements, and mass balance calculations. Water fow measurements and a rough chloride balance indicated relatively large water leakages through the banks and/or to the groundwater. Various estimates of the evapotranspiration (ET) were evaluated, and finally ET was estimated from pan evaporation data using a coefficient of 1.4, resulting in a mean ET of 8 mm day-1 for the whole study period.
The mass removal (g m-2 day-1) of total P (TP), total particulate P (TPP), ammoniumnitrogen (NH4+-N) and total suspended solids (TSS), was significantly and positively correlated with the respective mass loads to the CWs. Also, season had a significant effect on the mass removal of TDP, TSS, and NH4+-N, with increasing removal as the study progressed for the two former and a decreasing removal for the latter parameter. Results also showed that the removal of TP (0. 15-0.23 g m-2 day-1) by the studied CW systems were in the higher range of P removal rates for temperate FWS wetland systems, indicating a high potential of high-load tropical FWS CW to remove P from wastewater. The relative reduction of mass load varied between 20 to 57% for TP, 1.4 to 70% for total dissolved solids (TDP), 24 to 86% for NH4+-N and 32 to 73% for TSS, and was generally lower at higher mass loads. The only significant difference between the two macrophyte species was associated with area-specific mass removal of NH4+-N, with C. papyrus CWs having twice as high removal as those with E. pyramidalis. However, when the ET for the CWs was calculated in a different way, this relationship became insignificant. These results showed that conclusions about treatment performance of tropical CWs are strongly influenced by the accuracy of the water balance.
High relative reduction of TSS mass load, but low reduction of both TDP and NH4+-N were observed when the macrophytes had been growing for at least 5 months, whilethe opposite was observed when the macrophyte shoots were around I month old.This suggested that macrophytes were important for the dynamics of TDP, NH4+-N and TSS removal, possibly through uptake of dissolved ions, and influences onsedimentation/resuspension processes.
The P recovered in the above-ground green parts of the harvested macrophytes (0.03-0.06 g m-2 day-1) was in the higher range of values reported from CWs in temperate regions, suggesting a higher potential for nutrient recovery by macrophytes in tropical wetlands than in temperate ones. Also, P stored in the harvested green biomass of the macrophytes represented roughly 20-28% and 25-100% of the daily mass removal of TP and TDP by the CWs, respectively, indicating the important function of plants as nutrient traps. Similar tissue P concentrations were found in each macrophyte species independent of mass load, suggesting excess available P in the CWs. However, the biomass of C. papyrus, and the standing stock of P, responded positively to an increase in nutrient load. This was not observed for E. pyramidalis, suggesting that species specific harvest strategies should be used in order to attain high nutrient storage and a crop of good fodder quality.
General management strategies that could be beneficial not only from an optimal plant quality perspective, but also from a water quality point of view, include more frequent harvests (suggested < 4 months intervals), and leaving part of the wetland intact at each harvest event. Shorter harvest intervals would also minimize macrophyte tissue losses to senescence and decomposition. This would result in better conservation of biomass resources for the local communities.
Linköping: Linköpings universitet , 2006. , 47 p.