Comparison of performance of full scale duckweed and algae stabilization ponds.

The aim of this study was to compare the performance of a duckweed pond and an algae pond treating effluent of sewage fed UASB reactor under similar environmental and operational conditions. The real scale experimental system was composed of two continuous flow channels. One operated as an algae pond and the other as a duckweed pond ( Spirodela polyrrhiza and Lemna minor). The volume of each channel was 225 m 3 , an average surface area of 322 m 2 , L/W ratio= 13.1 and depth of 0.7. The wastewater flow was 19.7 m 3 d -1 , for each system and the theoretical hydraulic retention time was 11.5 days. The ponds were monitored for the following parameter: Organic matter (BOD 5 ), total suspended solids (TSS), ammonium nitrogen (NH 4 + -N), total Kjeldahl nitrogen (TKN), nitrite nitrogen (NO 2 -N), nitrate nitrogen (NO 3 -N), total phosphorus (TP) and faecal coliform (FC). The duckweed pond developed different environmental conditions in terms of pH, temperature and oxygen, compared to the algae pond. These differences are likely to affect treatment efficiency for organic matter and nitrogen. The duckweed pond was more efficient in removing organic matter and the algae pond was more efficient in nitrogen removal. Denitrification accounted for most of the nitrogen removal in the algae and duckweed ponds. The second most important mechanism for nitrogen removal was ammonia volatilization for the algae pond and plant up-take for the duckweed pond. In the design of duckweed pond systems special attention should be paid to the reactor configuration, flow pattern, in order to obtain good contact between water column and the duckweed cover and to reduce the presence of short circuiting and dead zones.