Outdoor production of Scenedesmus sp. in thin-layer and raceway reactors using centrate from anaerobic digestion as the sole nutrient source

In this paper, we have studied the outdoor production of the freshwater microalgae Scenedesmus sp. in two open reactors (32 m2): thin-layer (1.2 m3) and raceway (4.4 m3), using centrate from anaerobic digestion as the sole nutrient source. The aim was to recover valuable nutrients (nitrogen and phosphorus) from effluents in order to maximize biomass productivity. Experiments were performed in semicontinuous mode, modifying the centrate percentage within the culture medium. The optimal centrate percentage was 30% - above this value the culture's performance reduced, probably due to ammonium excess (above 122 mg l− 1). Using the raceway reactor, biomass productivity was 24 g m− 2 day− 1, whereas using the thin-layer reactor, it increased up to 42 g m− 2 day− 1. Nitrogen and phosphorus removal was demonstrated to be proportional to biomass productivity, with maximal values up to 38 mgN l− 1 day− 1 and 3.9 mgP l− 1 day− 1 being removed, respectively. Ammonium stripping was only relevant in the raceway reactor, due to its lower biomass productivity, with more than 40% of the inlet nitrogen being lost to the air. The thin-layer reactor also proved to be more photosynthetically efficient, with maximal values of 4.7% being measured. An economic analysis demonstrated that the thin-layer reactor allowed a reduction in the biomass production cost, in addition to utilizing waste as the nutrient source, with a minimum production cost of 0.9 €/kgbiomass being estimated. In conclusion, it is possible to use centrate from anaerobic digestion as the sole nutrient source for the large-scale production of Scenedesmus sp. biomass thus reducing the biomass production cost by avoiding the use of expensive and non-sustainable fertilizers; while also obtaining returns from the treatment of this type of residue. Such a combination helps to increase the possibility of producing commodities, or biofuels, from microalgae by coupling their production to treatment processes.