Floc and Sink Removal of Cyanobacteria From Koka Reservoir (Ethiopia): Using Natural Coagulants

Abstract

Cyanobacteria bloom in freshwater sources such as reservoirs produce harmful toxins that can adversely affect water quality, aquatic environment and public safety. These harmful organisms can be removed from water sources using flock and sink techniques. In this study, we examined the efficacy of a low dose coagulants (Moringa stenopetala seeds (MSS), Aloe barbadensis miller leaf (AVL) and Opuntia ficus-indica leaf (OFIL)) both alone and in combination with local red soil (LRS) as a ballast material to remove the cyanobacterial biomass from Koka Reservoir's water (Ethiopia). The plant extracts obtained from dried biomass treated with NaCl. The results of the experiments determined the lowest dose of coagulants and ballast that effectively coagulate and sediment cyanobacterial biomass from the sample. The removal efficiency was estimated using phycocyanin pigment as surrogate of biomass. The mixed effect model was conducted to assess the effects of coagulant dose, LRS dose, and replication on the measured responses. The results indicated that treatment dose significantly affected the phycocyanin concentrations both at the top and bottom of the test-tubes, whereas the effect of replicates was negligible. MSS extracts proved to be more effective alone (87.64%) or combined with LRS than other treatments. The coagulants, either alone or in combination with natural ballast, provide prominent sustainable alternative for cyanobacterial removal in freshwaters in tropical systems particularly for low-income countries.