Tetracycline removal-methanogenesis trade-off in anaerobic digestion: Divergent roles of granular activated carbon and electrical stimulation.

Tetracycline (TC) in wastewater impairs methanogenesis during anaerobic digestion. Although granular activated carbon (GAC) and electrical stimulation alone have been shown to mitigate TC inhibition, their combined effects remain unclear. Four up-flow anaerobic sludge blanket (UASB) reactors were operated at 5 mg/L TC, designated UASBCON (control), UASBGAC (5 g/L GAC dosed), UASBELE (0.6 V voltage applied), and UASBGAC/ELE (5 g/L GAC dosed and 0.6 V applied). Voltage was applied (Stage Ⅰ), terminated (Stage Ⅱ), and resumed (Stage Ⅲ) in UASBELE and UASBGAC/ELE. During the stable period of Stage I, UASBGAC enhanced methane production by 24.8% relative to UASBCON (p < 0.001), while UASBELE remained comparable to UASBCON. TC removal efficiency during Stage I followed the order UASBGAC (64.0%) > UASBGAC/ELE (57.5%) > UASBCON (34.5%) > UASBELE (30.1%). Neither single-amendment reactor showed a TC removal-methanogenesis trade-off. In contrast, UASBGAC/ELE suppressed methane production by 71.0% relative to UASBCON (p < 0.001), with a persistent trade-off (R2= 0.492, p < 0.001). In UASBGAC, GAC adsorption initially reduced bioavailable TC and sustained methane production. Meanwhile, UASBELE exhibited a zone-dependent community structure: TC-degrading genera were enriched at the electrode-proximal zone (cathode and anode biomass combined; 3.4 times that of bulk sludge), while methanogens predominantly resided in the bulk sludge. However, this zone-dependent pattern was attenuated in UASBGAC/ELE, where the electrode-to-bulk ratio of TC degraders declined to 2.2. GAC and electrical stimulation thus acted antagonistically rather than synergistically under TC stress. Mechanistic compatibility should guide hybrid amendment design in anaerobic systems treating antibiotic-containing wastewater.