Pilot-scale annual production of Scenedesmus almeriensis using diluted pig slurry as the nutrient source: Reduction of water losses in thin-layer cascade reactors

Abstract

The microalga Scenedesmus almeriensis was produced using diluted pig slurry as the sole nutrient source during one year using a 30 m2 thin-layer cascade photobioreactor located outdoors. Evaporation contributes significantly to the water requirements of microalgae production; approximately 1000 L of water are required to produce 1 kg of microalgal biomass. The aim of this study was to assess the potential of plastic covers to avoid or minimise water evaporation in open photobioreactors. The N–NH4+ concentrations at the inlets ranged from 40 to 100 mg L−1, depending on the experimental run. These values could not be increased because of the dark colour and turbidity of the pig slurry. The biomass productivity of the system was 17.6, 18.8, 23.4, and 17.3 g m−2·day−1 in winter, spring, summer, and autumn, respectively. Covering the reactors led to a decrease in biomass productivity that ranged from 8.5 to 16.9% depending on the season. However, water evaporation was reduced by 24.9–42.0% depending on the season. In summer, covering the reactors reduced the amount of water evaporated from 6.3 to 3.6 L m−2·day−1. In a theoretical 1 ha production facility, this would represent 2700 L of water saved per day. Overall, the processing of pig slurry using microalgae is challenging because of their variable nutrient concentration, colour and turbidity. Covering the reactors with inexpensive commercial plastics could significantly reduce the water requirements of the system.