Enhancing nutrient bioavailability in distillery wastewater through electrochemical oxidation for microalgal growth: Insights on biomass yield, nutrient utilisation, and VFA-assisted carbon capture

Abstract

Two-stage treatment of distillery wastewater (DWW) via electrochemical oxidation (EO) using Ti-RuO₂ anodes (35 cm² area) followed by mixotrophic microalgal treatment was investigated. In the first-stage, EO of DWW has improved the bioavailability of nitrogen and phosphorus at 3.95–5.14 mg/Ah and 0.43–1.02 mg/Ah, respectively, which had strong correlation with current density. EO also reduced ∼30 % TOC, 53 % COD and ∼44 % TN. In the second-stage, the ability of a novel microalgae, Asterarsys quadricellulare to mitigate the toxicity of electrochemically oxidised DWW (EO-DWW) while utilising the nutrients effectively was investigated. The mixotrophic algal growth effectively utilised 85 % phosphate and 91 % nitrate present in EO-DWW at a corresponding growth rate of 0.73 d⁻¹. The algal biomass was found to have ∼15 % carbohydrates, ∼12 % lipids and ∼33 % proteins. Subsequently, a bench-scale bubble column photobioreactor investigation was carried out to understand the carbon dynamics during the growth of Asterarsys quadricellulare. The metabolic uptake of monocarboxylic volatile fatty acids (VFA) and nitrate were found to release OH⁻ ions, which eventually helped in dissolving CO₂ in the reactor through a diffusion-limited process. The total energy spent in bench-scale EO system was 840 kWh (3024 kJ) per L of DWW, and the energy recovery potential of second-stage algal reactor was ∼8.7 %. The microtoxicity experiments with Alivibrio fischeri revealed that two-stage treated DWW was found to be safe for reuse as the microalgal growth has abated the toxicity of EO-DWW.