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

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 ${\mathrm{PO}}_4^{3-}$ addition) and photoperiods. CADWW treatment was found to be higher at 10D in the microalgal system. However, ${\mathrm{PO}}_4^{3-}$ 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.

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 production of algae biomass offers an economic benefit by providing a potential source of revenue and helping to offset the costs of wastewater treatment. Moreover, refractory and recalcitrant compounds such as melanoidin and phenolics are removed effectively by sequential coagulation–flocculation and microalgal treatment of anaerobically digested distillery wastewater. Coagulation–flocculation, followed by microalgal treatment, represents a promising approach for sustainable and cost-effective wastewater treatment. Some research has been previously done for treating anaerobically digested distillery wastewater using coagulation–flocculation and algal treatment separately. However, sequential treatment with coagulation–flocculation and microalgal treatment has not been carried out. The sequential process of coagulation–flocculation and microalgal degradation can also be applied to other types of wastewater.