Acidification increases efficiency of Lemna minor N and P recovery from diluted cattle slurry

Livestock slurry is high in N and P and as such is a valuable fertiliser, however, poorly timed or overapplication can result in the eutrophication of surface water. As a result, the agricultural industry is subject to increasingly stringent slurry management regulations, leading to interest in novel methods of recovering nutrients from wastewater. This study investigated the potential for using Lemna minor to recover N and P from slurry over a five-week trial, assessing whether previously reported increased growth on acidified wastewater translates into greater nutrient removal. It was hypothesised that acidifying diluted slurry with H₂SO₄ would lead to greater (i) reductions in total ammonia nitrogen (TAN) and PO₄^3- concentrations and (ii) cumulative N and P uptake by L. minor, relative to unamended diluted slurry (control). Consistent with previous studies, L. minor growth was significantly higher in the acidified treatment relative to the control, with greater reductions in PO₄^3- concentrations also observed in the acidified treatment. Contrary to the first hypothesis, the same was not observed for TAN, where final concentrations were lowest in the control despite poor growth on this medium. Consistent with the second hypothesis, greater cumulative L. minor N uptake over the five-week trial was observed from the acidified treatment, with L. minor uptake accounting for 94.8 % of the reduction in inorganic N concentration, relative to 7.5 % in the control. Cumulative P recovery from the acidified treatment was also significantly higher than in the control, with L. minor uptake accounting for 99.5 % of the reduction in the inorganic P concentration of the acidified diluted slurry, relative to only 18 % in the control. Our findings have important implications for the operation of duckweed growing systems in practice. We show that lowering pH enabled efficient PO₄^3- recovery, to the point where its concentrations may have been limiting to growth. Lowering pH also increased the proportion of N and P removal directly attributable to L. minor uptake, increasing the efficiency of the nutrient recovery process whilst also minimising the amount of N lost via other pathways such as gaseous emissions.