Anaerobically Digested Distillery Wastewater Treatment Using Coagulation–Flocculation Followed by Microalgal Treatment: Optimization of Coagulant Dosage, Dilution Ratio, and Photoperiod

Abstract The treatment of anaerobically digested distillery wastewater (ADWW) was investigated via coagulation–flocculation (C–F), followed by a microalgal system. As a first step, C–F was carried out using FeCl3 and polyaluminum chloride (PAC). Around 99.3%, 73.6%, and 58.3% of turbidity, color, and chemical oxygen demand (COD) removals, respectively, were observed under optimized C–F conditions (FeCl3 approximately 3 g/L, PAC approximately 6 mg/L, pH approximately 5, and dilution ratio approximately 4D). Subsequently, C–F-treated ADWW (CADWW) was subjected to microalgal treatment under various dilutions (1D–30D, with and without PO 4 3 − addition) and photoperiods. CADWW treatment was found to be higher at 10D in the microalgal system. However, PO 4 3 − addition has not shown any significant improvement in treatment efficiency. A photoperiod of a 16 h:8 h light:dark cycle was optimal for CADWW treatment, which had shown 85% melanoidin, 22.9% total organic carbon (TOC), and 75% total nitrogen (TN) removal rates. However, phenolics removal was found to be the highest (75.7%) under a 12 h:12 h light:dark cycle, and a phenolic content of 1.74 mg of GAE/g dry weight of algae was observed. The overall removal rates of COD, TOC, TN, melanoidin, and phenolics by this sequential process (C–F followed by algal treatment) under optimum conditions were 68%, 51%, 78%, 93%, and 90%, respectively. The algae harvested in the microalgal system was found to have a higher heating value of 28.66 MJ/kg and a lipid content of 24.2%. Overall, sequential C–F and microalgal treatment could be an effective strategy for internal energy recovery during ADWW treatment. Practical Applications Coagulation–flocculation (C–F) is widely used in wastewater treatment to remove suspended particles and turbidity. The process involves adding coagulants such as alum or ferric chloride to destabilize the suspended particles, causing them to clump into larger particles called flocs. The flocs formed can also remove dissolved organic compounds by sorption onto their surface, thereby removing color and chemical oxygen demand. Algae are known to be very efficient in removing nutrients and organic compounds from water and can be used to enhance the biological treatment of wastewater. In this process, algae consume the organic matter present in wastewater and convert it into biomass, which can be harvested and used for further applications such as biofuels and animal feed. Additionally, the productio