Development of a pilot scale anaerobic and aerobic/anoxic sequencing batch reactors and a constructed wetland system to treat abattoir wastewater.

Abstract

Over the past 27 years, Kampala, the Capital City of Uganda has witnessed a very high rate of population growth, urbanization and industrialization. Due to these developments, most wetlands in the environs of Kampala, especially Nakivubo wetland have been tremendously reduced in size and their ecological capacity to cleanse domestic, municipal and industrial effluents severely diminished. Most of the industries in Kampala are located in the precincts of Nakivubo wetland and discharge untreated effluents to Nakivubo Channel, which drains into Lake Victoria at the Inner Murchison Bay. The City Abattoir is one of the key sources of pollutants into the bay, which is the source of potable water for Kampala. This study was undertaken to develop and evaluate the performance of bioprocesses to enhance the removal of pollutants from the City Abattoir wastewater and eventually protect the ecology of Murchison Bay. A pilot scale, consisting of integrated anaerobic and aerobic/anoxic sequencing batch reactors and a constructed wetland system was developed at the City Abattoir and its performance evaluated for the removal of organic carbon, nitrogen, phosphorus, turbidity, and faecal coliforms. Furthermore, the potential of four macrophyte species, namely Cyperus papyrus, Typha domingensis, Phragmites mauritianus and Miscanthidium violaceum to polish pre-treated abattoir wastewater was evaluated in experimental mesocosms. Overall, C. papyrus achieved the highest removal rate of nutrients from abattoir wastewater, attributable to its fast growth rate, high biomass and extensive root network which provide suitable conditions for physical and biochemical removal mechanisms. The integrated anaerobic and aerobic/anoxic sequencing batch reactors (SBRs) and constructed wetland system achieved high removal rates of pollutants and contaminants from abattoir wastewater. The anaerobic SBR was most important for the removal of faecal coliforms (98%), chemical oxygen demand (93%) and turbidity (82%). On the other hand, the aerobic/anoxic SBR was most instrumental in the removal of total nitrogen (80%) and total phosphorus (21%). The constructed wetland retained all faecal coliforms, considerably reduced turbidity (76%), and total phosphorus (63%). Each unit in the integrated bioprocess provided unique environmental conditions that enhanced the degradation and removal of specific components of wastewater. The City Abattoir management should use the results of this study to implement a full scale treatment system to treat all process wastewater before discharge.