Comparing continuous and perfusion cultivation of microalgae on recirculating aquaculture system effluent water

Effluent water from recirculating aquaculture systems (RAS) contains nutrients from fish excrements and leftover feed. This study investigated the nutrient remediation potential from RAS effluent water through microalgae cultivation in 25 L tubular reactors. We compared nutrient uptake and biomass productivity in continuous and perfusion cultivation modes for freshwater, brackish water and saltwater. Stable high biomass densities were achieved with additional nitrate during continuous cultivation (up to 3.88 g L^-1) or by membrane filtration during perfusion cultivation (up to 3.59 g L^-1). A life cycle assessment (LCA) compared the two different cultivation modes in terms of environmental sustainability on a 1 ha scale. The LCA and preliminary economic assessment showed that perfusion cultivation appears to have a lower environmental impact for relatively low nutrient concentrations, but additional equipment and higher energy demands are leading to increased operational (+6 %) and capital expenses (up to +60 %).