Optimization of co-valorisation techniques for dairy and paper pulp wastewater in the cultivation of Chlorococcum sp. with a focus on mixture design, microwave-assisted pretreatment, and bioethanol production

This study optimized biomass and lipid accumulation using mixed dairy and paper-pulp wastewater for the cultivation of <i>Chlorococcum</i> sp. The obtained microalgal biomass was thereafter subjected to microwave-assisted pretreatment for optimal fermentable sugar release. Microwave power (100-700 W), pretreatment time (1-7 min), and acid-liquid ratio (1-5 %) were the input parameters for the pretreatment optimization study. The wastewater mixture ratios (25:75, 50:50, 100:0) of dairy and paper-pulp wastewater (DWW and PWW respectively) were achieved using simplex lattice mixture design to obtain high biomass and lipid accumulation in <i>Chlorococcum</i> sp cultivation. The model recommended a mixture of 64.69 % DWW and 35.31 % PWW for optimal biomass concentration, and a ratio of 34.21 % DWW and 65.79 % PWW for maximum lipid accumulation, predicting biomass concentration of 1.17 g/L and lipid accumulation of 0.39 g/g. Experimental validation resulted in biomass concentration and lipid accumulation 0.94 g/L and 0.39 g/g, respectively. Moreover, the experimental confirmation of the predicted fermentable sugar (11.14 g/L) yielded 15.67 g/L with pretreatment set points of 2.52 % HCl for 4.06 min at 700 W. Additionally, the prospect of the optimized pretreated microalgal biomass for bioethanol production (7.85 g/L) was achieved. Findings from this study could facilitate the implementation of DWW and PWW wastewaters utilization that could significantly lower the use of scarce potable water in keeping with portable water, energy, and environmental sustainability nexus towards the realisation of a circular bioeconomy.