Utilization of centrate for the outdoor production of marine microalgae at the pilot-scale in raceway photobioreactors

In this study, the outdoor production of marine microalgae in raceway photobioreactors was investigated, modifying the centrate percentage in the culture medium (20, 30, 40 and 50%) and using two different dilution rates (0.2 day^-1 and 0.3 day^-1). The data obtained showed that microalgae are capable of producing biomass in addition to recovering the nutrients contained in the centrate. The best results for biomass productivity and light efficiency were obtained when the centrate was set at 20% with a dilution rate of 0.3 day^-1. The biomass productivity was 32.42 g m^-2·day^-1 while the photosynthetic efficiency was 0.74 g_biomass·E^-1 (3.66%). Regarding the nutrients, nitrogen (the majority being in the form of ammonium [NH₄⁺]) and phosphorus were only fixed into biomass when optimal conditions were set; if this was not the case, they were lost to stripping or precipitation. The maximal nutrient removal capacities under the optimal conditions were 28.72 mg_N·l^-1·day^-1 and 3.99 mg_P·l^-1·day^-1. Population changes were determined by the dilution rate set whilst the centrate percentage had little effect. Four strains were present in the culture, Nannochloropsis g. being the main one. Biochemical changes did not vary greatly between the conditions set for the culture, with a composition rich in proteins and carbohydrates being observed. One can conclude that to produce marine microalgal biomass for a range of potential commodities such as feed, biofertilizers and biofuels, it is possible to use centrate from anaerobic digestion as the sole nutrient source, as a way of reducing costs.