Removing phosphate from wastewater: evaluation of the performance of duckweed (Lemna minor) operating under cool temperate conditions

Reserves of rock phosphate are expiring, increasing the pressure on global agriculture already under stress from growing populations and unsustainable water supplies. The UK imports phosphate for agriculture and primary industries, but subsequently returns much of what it has imported after use as wastes to watercourses through diffuse discharges from agricultural runoff and point discharges from wastewater treatment works. It proves costly to control phosphorus to the low discharge concentrations required in order to avoid eutrophication in water courses.
Duckweed is a free floating macrophyte that has been shown to remove large quantities of nutrients from wastewater under tropical and sub-tropical conditions, but its potential for wastewater remediation and nutrient recovery in cool temperate countries is largely unknown. This thesis explores that potential, by exposing the duckweed Lemna minor to simulated and real wastewater treatment conditions of a cool temperate climate, while observing the influence of process variables controlling biomass growth and phosphorus uptake from both growth solution and wastewater.
Under controlled microcosm experiments, it was found that the most influential variables controlling biomass growth and phosphorus uptake were photoperiod and acclimation to phosphorus respectively. When duckweed was acclimated to low phosphorus concentrations, cellular phosphate reduced, causing subsequent periods of rapid phosphate uptake when the inoculum was resupplied with higher phosphorus concentrations. As a result, phosphate in solution was removed from 15 mg P L-1 to <0.1 mg L-1 in four days while under simulated UK summer and winter conditions. In mesocosm experiments conducted under continual flow conditions with a hydraulic retention time of two days, it was found that two duckweed tanks in series were able to remove phosphate from wastewater at 10 mg P L-1 to concentrations similar to that achieved by large wastewater treatment works in the UK (< 0.2 mg P L-1). Preliminary results using an outdoor pilot-scale system helped to better understand the challenges of operating within more realistic conditions, as the entire process is profoundly affected by changes in wastewater characteristics feeding the system; however, time constraints prevented a full study to assess the magnitude of such impacts on phosphorus uptake and biomass growth.