Lowering pH enables duckweed (Lemna minor L.) growth on toxic concentrations of high-nutrient agricultural wastewater

The use of duckweed species to remediate nutrient-rich wastewater has grown as a field of research and in industry; however, the need to dilute wastewater to the low ammoniacal-N concentrations tolerated by duckweed represents a barrier to commercially implementing these systems in agriculture. This study investigated the potential for acidifying anaerobically digested cattle slurry (digestate), shifting the NH₄⁺:NH₃ equilibrium towards the less toxic ionised form, thus allowing the growth of Lemna minor on less dilute wastewater. First, a study was conducted to identify the ammoniacal-N concentrations tolerated by L. minor and to confirm the positive effect of lower pH on growth in high nutrient solutions using modified Hutner's solutions at two pH levels (8.2 and 6.5). In Hutner's solution at a pH of 8.2, L. minor growth was highest at the lowest ammoniacal-N concentration of 10 mg L⁻¹ and decreased with increasing concentrations. At a pH of 6.5, L. minor growth remained unaffected with increasing ammoniacal-N up to a concentration of 250 mg L⁻¹. L. minor was then grown in digestate concentrations ranging from 5% (65 mg L⁻¹ ammoniacal-N) to 30% (350 mg L⁻¹ ammoniacal-N), based on its growth in Hutner's solutions. It was hypothesised, that growth would decrease as the digestate concentration increased at pH 8.2, and that acidifying digestate to pH 6.5 would alleviate this effect. On unamended digestate (pH 8.2), L. minor growth was prevented even in the most dilute treatment (5%); however, on acidified digestate (pH 6.5), growth rates remained positive and significantly higher than the unamended controls up to the 20% dilution (239.3 mg L⁻¹ ammoniacal-N). Higher growth rates in the Hutner's solutions compared to digestate, particularly at pH 8.2 where no growth was recorded in digestate, suggest the presence of additional inhibitory factors in complex, high-nutrient wastewaters, and potentially sub-optimal concentrations of some of the nutrients provided in Hutner's solution. Nevertheless, correlation matrix analysis of digestate chemical properties highlighted the importance of acidification, with a strong negative correlation between pH and L. minor growth rate. For the first time, we demonstrate that by lowering pH, L. minor could be grown on dilutions of nutrient-rich agricultural wastewater that were otherwise toxic, and which make it feasible as a nutrient removal method. These findings could have important implications for implementing duckweed-based remediation systems in agriculture, increasing water- and land use efficiency, and thus, their commercial viability.