Reduction in water consumption during the production of microalgae using diluted pig slurry in thin-layer cascade photobioreactors

Microalgal biomass was produced outdoors over a 16-month period using 30 m 2 thin-layer cascade reactors. Diluted pig slurry was utilised as the nutrient source. The present work aimed to minimise the water requirements during microalgae production by (i) recirculating the outlet effluents back into the reactor and (ii) simultaneously reusing the effluents while covering the reactors with plastic covers. Overall, water recirculation negatively affected biomass productivity throughout the year; in summer, the biomass productivity decreased from 22.3 ± 0.9 g·m -2 ·day -1 (when there was no water recirculation) to 16.5 ± 0.8 g·m -2 ·day -1 (when using a recirculation percentage of 75%). The increase in the turbidity and colour of the diluted pig slurry makes the recirculation of outlet effluents back into the system a challenge. However, recirculating the water in the uncovered reactors reduced the water requirements for biomass production from 900-1,000 L·kg -1 to 600-700 L·kg -1 , depending on the season. The experiments combining water recirculation and plastic covers were efficient at reducing water evaporation losses; however, the biomass productivity was negatively affected when using covers while the amount of water needed to produce 1 kg of biomass was, in general, not positively affected. The biomass productivity decreased on average by 16.6% when the reactors were covered. Operating the reactors with a recirculation percentage of 75% (the largest percentage studied) and plastic covers led to a reduction of over 50% in the amount of water required to produce 1 kg of biomass, from 900.8 L·kg -1 to 444.8 L·kg -1 (in winter).