Inactivation of Salmonella during dry co-digestion of food waste and pig manure.

Extremely high volatile fatty acids (VFAs) and ammonia concentrations can accumulate during dry co-digestion of organic wastes, which may inactivate pathogenic microorganisms. In this study, inactivation of Salmonella during dry co-digestion of pig manure (PM) and food waste (FW), which are both reservoirs of zoonotic pathogens, was examined. The effects of pH, VFAs, ammonia and their interactions were assessed on three inoculated Salmonella serotypes. The results show that dry co-digestion significantly decreased the Salmonella inactivation time from several months (in wet digestion) to as short as 6-7 days. A modified Weibull distribution was proposed to simulate Salmonella reduction and to calculate or predict the minimum inhibitory concentrations (MIC) of VFAs and ammonia. Statistical analysis showed that all the factors (pH, VFA type, VFA/ammonia concentration and Salmonella serotype) significantly impacted Salmonella inactivation (P < 0.01). The inhibitory effect sequence was pH > VFA concentration > VFA type > Salmonella serotype in VFA MIC tests, and ammonia concentration > pH > Salmonella serotype in ammonia MIC tests. The toxicity of VFAs was much greater than that of ammonia, and an antagonistic effect was found between VFAs and ammonia on Salmonella inactivation. Apart from the toxicity of free VFAs and free ammonia, the inhibitory effects of pH alone, ionized VFAs and ammonium were also observed.