Comparing the inhibitory thresholds of dairy manure co-digesters after prolonged acclimation periods: Part 1 – Performance and operating limits

Abstract

Co-digestion has been used to improve biogas yields and the long-term stability of anaerobic digesters compared to mono-digestion; however, less is known about the ultimate inhibition from co-substrates at their maximum loading rates and mixing ratios because these limits cannot be practically tested by existing facilities. Here, we performed a controlled experiment with long operating periods to ensure sufficient acclimation with the goal to observe ultimate inhibition and the full benefit that can be gained from co-digestion. The three substrates: 1) food waste (FW); 2) alkaline hydrolysate (AH); and 3) crude glycerol (GY) were individually co-digested with dairy manure (MN) for more than 900 days using continuously stirred anaerobic reactors at mesophilic temperatures. Food waste caused no reduction in performance or stability when co-digested with manure up to a total organic loading rate (OLR) of 3.9 g volatile solids (VS)·L−1·Day−1 (MN:FW = 51:49; VS basis), resulting in a specific methane yield (SMY) of 297 ± 3 mL CH4·g VS−1 for the combined wastes. Alkaline hydrolysate was co-digested with manure up to a total OLR of 2.7 g VS·L−1·Day−1 (MN:AH = 75:25) with a corresponding SMY of 299 ± 6 mL CH4·g VS−1. However, the free ammonia concentration reached levels previously reported as inhibitory, and may have led to the observed accumulation of volatile fatty acids at higher loading rates. Crude glycerol co-digestion resulted in an optimum SMY of 549 ± 25 mL CH4·g VS−1 at a total OLR of 3.2 g VS·L−1·Day−1 (MN:GY = 62:38). Stable digestion beyond this level was prohibited by an accumulation of long-chain fatty acids and foaming. These results can be used to implement effective co-digestion strategies. Co-substrates that possess similar inhibiting characteristics should be monitored to prevent severe instability at high loading rates and mixing ratios.