Evaluation of organic matters removal and key factors in the clogging process for a hydrid natural pond treating municipal wastewater in Kigali

Abstract

The rapid population growth, in many countries including both developed and developing countries such as Rwanda, has increased not only the freshwater demand on its limited supplies but also the discharged volume of wastewater into ecosystems has consistently contributed to environmental pollution. For this purpose, the present study was carried out to evaluate organic matters and total suspended solids removal and level of clogging process of Nyarutarama natural hybrid ponds treating municipal wastewater from neighboring estate in Kigali but also the key informant interview was conducted to assess the key triggering factors in clogging process of that treatment system. Three series sets of constructed ponds; consisting of one Primary Facultative Pond, one water Lettuce Covered Pond, and one Free Water Surface Flow Constructed Wetland Pond were built in Kigali. The efficacy of the constructed ponds was assessed through the analysis of biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), oils and grease and sulphate components using standard methods for water and wastewater examination. The results of this study indicated that the Nyarutarama natural hybrid ponds system removed efficiently different pollutants including TSS (71%), BOD5 (55%), COD (52%), oils and grease (58%) and sulphate (64%), but it showed a decrease in performance compared to the results of previous studies. This shortcoming resulted from the lack of management plan of the system leading to its clogging and destruction where the assessment indicated the clogging level of 78%, 60%, and 75% for primary facultative pond, water lettuce covered pond and free water surface flow constructed wetland, respectively. The results of the conducted key informant interview indicated that the ponds system clogging was due to the accumulation of solids materials that resulted from the lack of operational, monitoring and maintenance plans. Therefore, the maintenance strategies including safety operations (availability of permanent skilled staff), water management, permanent desludging, maintenance of structures and embarkments, wetland vegetation control as well as fencing, mapping and designing the buffering zones into an attractive touristic area for recreation and field study activities should be applied to all natural systems treating wastewater highly urban region including the constructed wetland in Excella high school.