The combined effect of cefazolin and oxytertracycline on biogas production from thermophilic anaerobic digestion of dairy manure

► The effect of cefazolin (CFZ) and oxytetracycline (OTC) on anaerobic digestion was tested. ► Concentrations of 30, 60 and 90 mg L−1 were compared with no-antibiotic control. ► CFZ concentrations did not significantly affect (P > 0.05) methane production. ► Individual OTC and combined CFZ and OTC treatments showed a significant reduction. ► No noticeable effects on process stability from CFZ and OTC concentrations tested.