Effectiveness of Anaerobic Pretreatment of Silver-Recovery Photographic Wastewater with Readily Biodegradable Supplementation: From Laboratory Screening to Semi-Technical-Scale Verification

Silver-recovery photographic wastewater (SR-PW) constitutes a challenging wastewater stream owing to its high chemical oxygen demand (COD), acidic pH, and the presence of oxidizable, inhibitory, and toxic compounds. This study evaluated the feasibility of its anaerobic pretreatment with skimmed milk powder (SMP) supplementation. SMP was added to improve the unfavorable C/N balance, replenish nutrient deficiencies, and supply readily assimilable nutrients to support the activity of anaerobic microorganisms. Raw SR-PW was characterized by a pH of 5.09 ± 0.32, total organic carbon (TOC) of 7638 ± 824 mg C/L, COD of 60,540 ± 10,086 mg O2/L, an imbalanced COD/TOC ratio of 7.9 ± 1.6, and an unfavorable C/N ratio of 385 ± 197. During the laboratory stage (S1), SMP supplementation improved TOC removal, CH4 production, and the development of methanogenic biomass. The most favorable results were obtained at doses of 7–8 g SMP/L SR-PW, with TOC removal of 66.9 ± 3.2–68.4 ± 3.0%, COD removal of 42.2 ± 3.6–43.5 ± 3.5%, and CH4 yields of 87.0 ± 5.4–91.9 ± 7.2 mL CH4/g TOC. The CH4 content in biogas increased from 7.5 ± 0.6% in the control variant to 18.5 ± 0.9–18.9 ± 0.8% in V5–V6. Stage 2 (S2) used a flow-through reactor whose design increased biomass retention and improved process performance. Variant V6 achieved TOC and COD removal of 82.7 ± 1.5% and 52.7 ± 1.7%, respectively, with a CH4 yield of 115.5 ± 6.2 mL CH4/g TOC. The CH4 content in biogas increased to 20.4 ± 0.8%, and the biofilm and granular sludge favored higher proportions of Archaea and Methanosarcinaceae in the anaerobic microbial biomass. The results showed that anaerobic digestion can be considered only a pretreatment technology for SR-PW, rather than a method for complete treatment with energy recovery.