Annual production of microalgae in wastewater using pilot-scale thin-layer cascade photobioreactors

Microalgae based wastewater treatment has been suggested as an alternative to polluting and energy-consuming conventional processes. The main advantage of this strategy is the dual role of microalgae: they recover nutrients from waste and simultaneously produce biomass with varied industrial applications. In the current study, biomass of Scenedesmus sp. was produced using primary wastewater in two pilot-scale thin-layer cascade photobioreactors (63 and 126 m2). The wastewater used for microalgal growth was not subjected to any conventional treatment process, besides removal of solids, and contained a variable N-NH4+ content of 83.0-210.6 mg·L-1. Biomass productivity values were comparable to those obtained when operating using freshwater and commercial chemicals as nutrient sources. When operating at a dilution rate of 0.3 day-1, the average annual productivity was 24.8 g·m-2·day-1 (82.0 t·ha-2·year1) with a maximum of 32.8 g·m-2·day-1 in summer. Inorganic nitrogen and phosphorus removal rates varied between 695.4-2383.4 and 70.4-111.8 mg·m-2·day-1 respectively. Production of Scenedesmus sp. using wastewater would allow not only to process large volumes of water that could be reused for agricultural irrigation or safely disposed into water streams, but also reduce production costs by 0.44 €·kg-1, based on a preliminary economic analysis. Overall, results demonstrate that thin-layer cascade reactors can be used to effectively remove nutrients from wastewater while simultaneously produce valuable biomass with potential applications in agriculture or animal feed production