title: 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.
authors.affiliation: Biological and Environmental Engineering, Cornell University, 214 Riley-Robb Hall, Ithaca, NY 14853, USA. Electronic address: jgu3@cornell.edu.
authors.forename: J G
authors.initials: JG
authors.lastname: Usack
authors.affiliation: Biological and Environmental Engineering, Cornell University, 214 Riley-Robb Hall, Ithaca, NY 14853, USA. Electronic address: la249@cornell.edu.
authors.forename: L T
authors.initials: LT
authors.lastname: Angenent
mesh_terms: D000818:Animals; D001673:Biodegradation, Environmental; D018533:Biomass; D019149:Bioreactors; D002417:Cattle; D003612:Dairying; D005735:Garbage; D005990:Glycerol / Q000378:metabolism; D008372:Manure / Q000032:analysis*; D008697:Methane / Q000032:analysis*; D064307:Microbiota
chemical_list: D008372:Manure; D008697:Methane; D005990:Glycerol
keywords: Ammonia; Biogas; Co-digestion; Dairy manure; Food waste; Long-chain fatty acids