DUCKWEED CULTURE FOR WASTE-WATER RENOVATION AND BIOMASS PRODUCTION

Outdoor experiments were conducted in shallow mini-ponds (20 and 30 cm deep) for evaluating the performance of the duckweed species Lemna gibba as a purifier of domestic wastewater. It was found that under adequate operational conditions, the quality of secondary effluents meets irrigation reuse criteria. The annual yield (dry matter) of duckweed, harvested two to three times a week, is about 55 ton/ha, with a protein content of 30%. Hence, by cultivating duckweeds the ammonia in ponds for domestic wastewater treatment is converted into valuable protein rich biomass which subsequently can be used for animal feed or agricultural fertilization. The economic benefit of the additional by-product of the biomass reduces wastewater expenditures in the range of 0.020 to US$0.050 per each treated m3 of wastewater.