Fig. 2

Daily biogas production in digesters F1, F2, and F3 (a) and sulfide-amended digesters F4, F5, and F6 (b) during continuous feeding of primary and activated sewage sludge. c Theoretical biogas potential of oleate, and differences in daily biogas production between oleate-amended digesters (F2 and F4) and control digesters (F1 and F6, respectively). Oleate was added to F2 and F4 from day 133 onward. d Theoretical biogas potential of stearate, and differences in daily biogas production between stearate-amended digesters (F3 and F5) and control digesters (F1 and F6, respectively). Stearate was added to F3 and F5 from day 133 onward. Theoretical biogas production values were calculated based on the daily amount of long-chain fatty acids added, using the Buswell equation [51]. The arrow marks the time when sulfide was introduced to F4, F5, and F6 (day 76)