In developing countries, water pollution caused by discharging sewage without sufficient treatment is a serious problem. Since activated sludge processes are not easily applied for developing countries, cost-effective and easy-maintenance sewage treatment processes such as down-flow hanging sponge (DHS) process have been developed. Nevertheless, removal of nitrogen and phosphorus is a remaining issue for the DHS process. To overcome this issue, we focused on duckweed by which nitrogen and phosphorus can be removed during their growth. Duckweed is a fast-growing plant and is attracting attention as biomass for energy production and livestock feed in recent years since it contains a large amount of protein and starch. In this study, we investigated a novel sewage treatment system, primary sedimentation + DHS + duckweed pond + anaerobic digestion. Treatment efficiency of duckweed pond supplied with DHS-treated sewage was evaluated in terms of nitrogen and phosphorus removal as well as biomass production rate. As a result, certain amount of nitrogen and phosphorus were removed from DHS-treated sewage and highly treated water was obtained with the production of duckweed. Stable Methane fermentation of a mixture of primary sludge and duckweed (TS rate 20%) was achieved for more than 3 months.
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