Dan Porath

Ammonia stripping by duckweed and its feasibility in circulating aquaculture

The duckweed Lemna gibba L. was evaluated for its potential as a biological ammonia stripper which thus improves protein production in a combined circulating aquaculture of fish. Ammonia uptake was compared with respect to varying conditions of circulating water, temperature, pH, and nitrate concentrations. Results indicate that uptake is an active process with preference for ammonia over nitrate. In an axenic culture of 0.1–0.3% duckweed biomass, Lemna gibba stripped 50% of the ammonia present at levels 10−4M NH3har2; NH4+ in 5 h, while the nitrate level (10−2 M NO3−) remained constant. Circulation of fish effluent under a duckweed mat promoted an uptake of 80% ammonia in less than 48 h. The role of duckweed in direct conversion of ammonia into plant protein as an abridgement of the nitrogen cycle is discussed.

Nitrogen removal and conversion by duckweed grown on waste-water

Abstract Duckweed was grown in outdoor ponds for the purpose of domestic waste-water treatment, recycling of the nutrient and effluent renovation. Growth of the plant was examined at two pond depths and various hydraulic retention times. Temperature stratification in the ponds probably affected ammonia uptake. Dry yield of the duckweed approached 15 g m−2 day−1 with a protein content of about 30% in the short retention-time treatments. Effluent quality was at the secondary level and met criteria for re-use for agricultural irrigation.