Comparative Performance Studies of Water Lettuce, Duckweed and Algal Stabilization Ponds Using Low Strength Sewage

A bench scale continuous flow wastewater treatment system comprising of 3 parallel lines using water lettuce ( Pistia stratiotes ), duckweed ( Spirodela polyrhiza ) and algae (natural colonization) as treatment agents was set up to determine environmental conditions, faecal coliform profiles and general treatment performance. Each treatment system consisted of 4 ponds connected in series and was fed with diluted sewage. Influent and effluent parameters measured included environmental conditions, turbidity, biochemical oxygen demand (BOD), chemical oxygen demand (COD), nitrate, nitrite, ammonia, total phosphorous, faecal coliforms, mosquito larvae and sludge accumulation. Environmental conditions and faecal coliform profiles were determined at the surface (0.10m), middle (0.35m) and bottom part (0.63m), of each pond. Acidic conditions were observed in the water lettuce ponds, neutral conditions in duckweed ponds and alkaline conditions in algal ponds. Faecal coliform log removal of 3, 4 and 6 were observed in water lettuce, duckweed and algal pond systems respectively in the final effluents, with log removal rate per pond of 1.6, 2.0 and 2.7 respectively. Sedimentation accounted for over 99% faecal coliforms removal in most of the water lettuce and algal pond systems. BOD removal was least in the algal system, followed by water lettuce and duckweed at 25%, 93% and 95% respectively. COD removal was 59% and 65% respectively for water lettuce and duckweed whiles it increased in algal ponds by 56%. Total phosphorus removal was 9% for duckweed systems and 33% for water lettuce pond system while it increased by 19% in the algal pond system. Ammonia removal was 93% for the algal and 95% in both water lettuce and duckweed pond systems. Removal of total dissolved solids (TDS) were 9%, 15% and 70% respectively for algae, duckweed and water lettuce. Mosquito populations of 96/m 2 , 3,516/m 2 and 11,175/m 2 , and were found in duckweed, algal and water lettuce ponds respectively.