Influence of Seaweed Extract in Enhancing Phytoremediation of Bio-Digester Effluent by Duckweed

Accumulation of nitrogen and phosphorus in surface water bodies renders aquatic ecosystem non-functional. Duckweed may be used to remove excess nutrients from wastewater, while it requires large surface areas. However, under limited space, nutrient uptake by duckweed may be enhanced by applying seaweed extract (SWE) to the wastewater. The effectiveness of SWE to enhance nutrient uptake under the hot-humid conditions of coastal Kenya has not been assessed. A study was therefore conducted to determine the optimum application rate and a number of applications of SWE to enhance maximum N and P uptake by duckweed from the bio-digester effluent. Four application rates (0, 3, 6, 9, and 12 µL SWE per litre of effluent) and a number of applications of SWE (none, one, two, three and four) were evaluated. A Randomised Complete Block Design with a factorial arrangement of treatment was used. The experiment was replicated for three times. The results showed a significant negative relationships between effluent N content and duckweed biomass (r2 = 0.982, P = 0.013) and effluent P content and duckweed biomass (r2 = 0.908, P = 0.04). Three applications of SWE reduced the effluent N content by 5.4%. An application rate of 9 µL SWE per litre of effluent reduced the N and P contents of effluent by 9-10% and 20-23% respectively, and increased the duckweed biomass by 22-62%. It is therefore recommended that SWE can be applied at least three times at the rate of 9 µL SWE per litre of effluent.