An evaluation of duckweed-based pond systems as an alternative option for decentralised treatment and reuse of wastewater in Zimbabwe
Nhapi, I.; Dalu, J.; Ndamba, J.; Siebel, M.A.; Gijzen, H.J.
Water Science and Technology a Journal of the International Association on Water Pollution Research 48(2): 323-330
2003
ISSN/ISBN: 0273-1223 PMID: 14510227 Document Number: 562549
A study was carried out in Zimbabwe to evaluate the performance of duckweed (Lemnaceae) ponds as an option for treating and reusing waste water in small, decentralized communities. The study focused on nitrogen and phosphorus removal, operational problems, and duckweed application. Two full-scale trial plants at Nemanwa and Gutu-Mupandawana growth points were used. Sewage samples were collected and analysed monthly from September 2000 to August 2001 for NO3, NH4-, total Kjeldahl N, total phosphorus (TP), chemical oxygen demand (COD), and other field measurements. The duckweed was harvested daily and fed to chickens. The Nemanwa plant had high nutrient levels due to nil outflows caused by water rationing in the area. The Gutu effluent had averages of 38.7+or-23.1 mg TN/litre and 7.5+or-2.4 mg TP/litre, which are above the respective Zimbabwean standards of 10 mg TN/litre and 1 mg TP/litre. COD removal efficiency at Gutu was poor at 45%. The performance of Gutu and Nemanwa plants suffered from inappropriate design especially pond depth and short-circuiting. The duckweed died off in the November-January period, this being attributed to excessive levels of ammonia. It is concluded that the duckweed pond systems could offer a good alternative for managing and reusing waste water at community level provided due regard is paid to appropriate design criteria.