Effects of coagulation/flocculation followed by dissolved air flotation on anaerobic digestion of coffee processing wastewater

This study aimed to optimize the conditions of chemically assisted physical treatment (CAPT) using coagulation/flocculation followed by dissolved air flotation (C/F + DAF) for solid removal, assess the impact of primary treatment on anaerobic digestion using a biochemical methane potential (BMP) test, and evaluate kinetic models’ fit to biogas production results for coffee processing wastewater (CPW). In Phase 1, CPW underwent characterization, and C/F + DAF conditions were optimized using ferric chloride in a rotational central composite design. pH and coagulant were independent variables, while removed turbidity and residual phenolic compounds (PC) were response variables. PC, associated with coffee fruits, can inhibit microbial activity, affecting anaerobic digestion efficiency. Phase 2 evaluated the primary treatment’s impact on anaerobic digestion via biomethane potential tests (BMP). CPW exhibited an acidic pH of 4.20 and high organic matter levels, with a total organic carbon (TOC) concentration of 2950 mg L⁻¹. Additionally, it contained phenolic compounds (PC) at 87.2 mg L⁻¹ and turbidity of 592.7 NTU, indicating a significant pollution load. In the CAPT optimization, turbidity removal ranged from 4 to 90% NTU, and PC removal ranged from 2.4 to 61% mg L⁻¹ as C₇₆H₅₂O₄₆ in the optimization experiment. Substantial reductions in organic load (35%), turbidity (68.23%), and PC (21.11% reduction) were observed after DAF in the absence of a coagulant. Untreated CPW (CPW_u) and CPW treated with dissolved air flotation (CPW_f) underwent a BMP test. The accumulated methane production was 379.1 mL ± 4.67 g VS^⁻1 for CPW_u and 388.6 mL ± 5.23 g VS^-1 for CPW_f. The first-order kinetic model described methane production best, with rate constant (k) values of 0.51 and 0.48 d⁻¹ for CPW_u and CPW_f, respectively. DAF effectively removed turbidity and PC without hampering biogas production. The results of this research add to sustainable wastewater management in the coffee industry and have implications for biogas and energy recovery, underlining the potential for generating renewable energy through anaerobic digestion.