Nitrogen and phosphorus removal from anaerobic baffled reactor effluent using Lemna minor and fertiliser value of the biomass for ryegrass production

Effluents from decentralised wastewater systems, using an anaerobic baffled reactor (ABR), have high concentrations of nitrogen (N) unsuitable for safe disposal. The study investigated the effects of duckweed (Lemna minor) density and effluent dilution on N removal and biomass accumulation, over 14 days. The duckweed biomass was tested as a fertiliser for ryegrass (Lolium perenne) at (i) 200 kg N ha^-1 (DWN), (ii) 80 kg P ha^-1 (DWP), (iii) DWN with mineral P to 80 kg P ha^-1 (DWN + P), compared with (iv) inorganic N fertiliser and two negative controls with (v) P and potassium (K), and (vi) K only. Biomass and N (11-56 mg L^-1) removal increased with effluent dilution and higher density. The 1:3 dilution (effluent: water) and chemical fertiliser (CF) had higher biomass than other treatments, except 1:1 at 800 g m^-2. The treatments had similar percentage N removal (> 79%), except the 3:1 with 400 g m^-2 duckweed (73%). Duckweed treatments had higher ryegrass dry matter, and N and P uptake, than the controls without N. The DWP and inorganic fertiliser with N had similar ryegrass dry matter and N uptake, which were higher than for DWN and DWN + P. The findings showed that duckweed, cultured at 600-800 g m^-2, could efficiently remove N from diluted ABR effluent for safe disposal, and that duckweed biomass increases ryegrass N uptake and dry matter to similar levels as inorganic N fertiliser, especially when applied to meet P requirements.