Harnessing spent granular activated carbon from point-of-use water treatment for anaerobic digestion enhancement: characterization and pretreatment effects

As an alternative to environmentally unfriendly disposal of spent activated carbon (SAC) from point-of-use water treatment systems, this study harnessed SAC as a co-substrate in anaerobic digestion for biogas production. The study investigated how SAC pre-treatment methods, namely, washing, particle size (milled vs. granular), and dosing levels (2.5%, 5%, and 7.5%), influenced biogas yield, methane content, and retention time using cow dung as the primary substrate. SAC from chlorinated water purification (KC) and the other from borehole water treatment (KZ) were investigated in this study. KZ had a higher %TOC (5.4955%) compared to < LOD for KC, indicating it retained more usable carbon for microbial activity. However, KC had a higher surface area (110.58 m²/g) than KZ (78.41 m²/g). suggesting better microbial support. Digesters dosed with 2.5% and 5% SAC generally maintained the most stable retention time, sustaining active digestion across the full 30-day period. Overall, unwashed granular KZ dosages of 2.5%, 5.0%, and 7.5% respectively, supported the highest and most stable biogas output at 5047 mL, 2605 mL, and 1685 mL, respectively. A 2.5% KZ dosage outperformed the control (pure cow dung), which produced 2624 mL. Milled washed KC dosages of 2.5%, 5.0%, and 7.5% generally showed lower biogas outputs, yielding 217.8 mL, 540.8 mL, and 300 mL, respectively. Their relatively low performance suggests that even after washing, the milled coconut husk SAC, at these concentrations, did not significantly enhance microbial activity, or perhaps the fine particle size led to aggregation or hindered mass transfer. KZ-dosed digesters' highest methane composition range was 14.90-37.70% whereas for KZ digesters it was 47.73-52.53% compared to the control's 33.70%. These findings underscore the complex interplay between particle size, washing, and dosage of SAC for enhancing anaerobic digestion, necessitating optimization.