Wastewater Treatment by a Natural Wetland: The Nakivubo Swamp, Uganda
Frank Kansiime, Maimuna Nalubega
Abstract
The Nakivubo swamp is located in Uganda, near its capital Kampala, and has been receiving wastewater from Kampala for over 30 years. This swamp consists of a floating root mat co-dominated by the sedges Cyperus papyrus and Miscanthidium violaceum. Tbe partially treated wastewater mostly flows beneath the floating mat into Lake Victoria via the Murchison Bay. Papyrus has a loose floating root mat which facilitates vertical mixing bebfl/een the interstitial mat water and the free water column beneath the mat. This leads to a less steep gradient of nutrients over the vertical profile and facilitates nutrient uptake from the water column by papyrus vegetation. In comparison, Miscanthidium has a thick compact floating root mat. This mat structure prevents free vertical and lateral mixing of the mat water with the water column beneath. This leads to reduced interactions of theplants with wastewater in these zones, and therefore less nutrient abstraction by plants from the wastewater. Differences in the morphological, hydraulic, physico-chemical, floristic and overall wastewater treatment performance between areas covered by the two major vegetation types were elucidated. Water flow is highly channelised and hydraulic retention times in the swamp during the rainy periods may be as low as 18 hours. Nutrient uptake and removal efficiency of nutrients and faecal coliforms is higher in zones dominated by papyrus compared to those of Miscanthidium. The potential of this swamp to remove nutrients and pathogens from wastewater in a sustainable way (with emphasis on the description and quantification of their pathways), while maintaining ecological quality and biodiversity, was investigated.