Influence of microalgae wastewater treatment culturing conditions on forward osmosis concentration process

Abstract

Forward osmosis is envisioned as a technology for microalgae concentration but fouling propensity during dewatering is currently a limiting factor that requires better understanding. The purpose of this study is to define the impact of microalgae culturing conditions on the downstream forward osmosis (FO) separation process—water recovery and microalgae harvesting. Chlorella vulgaris was cultivated in an outdoor lab-scale reactor fed with synthetic wastewater mimicking primary settled municipal influent under changing environmental conditions (temperature, solar radiation, nutrient balance) with varying hydraulic retention time. High efficiency of nutrient removal was achieved under all tested conditions but microalgae autoflocculation and lower rate of pollutant removal were observed with batches where culturing temperature (6.5–21 ^°C), solar irradiation rate (181 W/m²), and nitrogen/phosphorous ratio (2.9) were below the optimal range. Regarding FO concentration, high initial water fluxes (in the range of 18.2 to 19.5 L·m²·h⁻¹) and water extraction rate (60.1–83.9%) were observed in all subsequent FO concentration tests. Significant membrane fouling (microalgae deposition on surface) associated with poor biomass recovery from the FO cell was found to be dependent on exopolymeric substance accumulation, which was a response to non-optimal environmental culturing conditions.

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