Optional pond colonized by water lettuce

The common hippopotamus (Hippopotamus amphibius) is one of the most imposing terrestrial mammals. It can weigh between 1300 and 1500 kg. As its name suggests, it leads a semi-aquatic life and lives in rivers, marshes and mangroves in Africa. For several years, at least two hippopotamuses are established in Bujumbura wastewater treatment plant’s ponds, mainly in the optional ponds and maturation ponds.These ponds provide an adequate habitat for this herbivore that grazes peacefully on the ponds’ banks and edges.

Hippos are not the only ones who have established themselves in the treatment basins.They are joined by the water lettuce (Pistia stratiotes), a floating aquatic plant, present in shallow rivers, ponds, rice fields, wetlands, etc. Water lettuce has a remarkable ability to develop and reproduce in hyper-eutrophic environments such as the treatment plant’s optional lagoons, which are therefore almost entirely colonized by water lettuce. The complete coverage of the lagoons by water lettuce prevents light from entering the water and developing algae and reduces the purification efficiency based on biodegradation of organic matter under aerobic conditions. The solar disinfection capacity radiation is severely compromised.

Only periodic mechanical harvesting of the water lettuce’s biomass covering nearly 16 ha and its composting would restore the two optional lagoons’ purification efficiency. But beneath the water lettuce, the hippos have taken up residence! A new challenge for the Lake Tanganyika Water Management project and the wastewater treatment plant manager (OBUHA): move the hippopotamus, protected species, into a natural area; develop the mechanical means of harvesting water lettuce and ensure its composting for agricultural use. In Bujumbura, Latawama project focuses on improving the operation of the Buterere wastewater treatment plant. In addition to the actions already undertaken to rehabilitate the drying beds allowing the regular removal of sludge, the project intends studying solution to deal with this new challenge.

Lake Water Tanganyika Management project is funded by the EU, implemented by Enabel, Belgian Development Agency, in partnership with Lake Tanganyika Authority.