Unlocking the potential of microalgae: Cultivation in algae recycled effluent with domestic wastewater for enhancing biomass, bioenergy production and CO₂ sequestration

The current study aimed to enhance microalgal biomass productivity by re-using microalgae supernatant obtained from an algae cultivation system. In addition, raw and final effluent from a domestic wastewater treatment plant (WWTP) were integrated with the recycled effluent (supernatant). In order to accomplish the aim, the performance of various dilutions of raw wastewater (RW), recycled effluent (RE) and wastewater final effluent (FE) was assessed for resource recovery and biomass production. The 10 different dilutions were 100RW, 25RW + 75RE, 50RW + 50RE, 75RW + 25RE, 100RE, 100FE, 25FE + 75RE, 50FE + 50RE, 75FE + 25RE and RW + RE + FE. Results showed that the maximum biomass production (1.59 g L⁻¹) was observed in 75RW + 25RE followed by RW + RE + FE (1.13 g L⁻¹) diluted wastewater and lowest in 100FE (0.53 g L⁻¹). Similarly, the highest lipid content was in 75RW + 25RE (32.16 %) and lowest in BG11. The highest protein was obtained in BG11 (48.65 %) followed by 33.32 % in 75RW + 25RE. Furthermore, maximum CO₂ fixation rate (0.19 gCO₂ L⁻¹ d⁻¹), theoretical biochemical methane potential (471.54 mL CH4 g⁻¹ VS) and high heating value (21.52 Kg J⁻¹) were observed in 75RW + 25RE. These findings indicated that 75RW + 25RE is a suitable combination that could be used as a potential ratio to achieve optimum biomass production. This may be due to the presence of phytohormones within RE in combination with other nutrients from RW. Moreover, 75RW + 25RE showed high metabolites yield, high CO₂ fixation rate, nutrient removal efficiency and high heating value. The mixture of RW and HE may be a sustainable and feasible strategy for acquiring fast microalgal growth while reducing dependency on nutrient and freshwater.