Fate of Indicator and Pathogenic Organisms during Anaerobic Sludge Digestion

The survival and fate of Fecal coliform (FC), Fecal streptococci (FS), and Salmonella were monitored during operation of laboratory anaerobic digester at temperatures of 25°C and 35°C, at hydraulic detention times of 7, 15, 30 and 45 days and organic loadings of 0.9, 1.8, and 2.7 kg VS/ m3-d. Municipal wastewater sludge containing a mixture of primary solids and excess activated sludge was the feed to the bench-scale anaerobic digesters. The observed results indicate that under these conditions the density of Fecal coliform (MPN/g TS) was below the limit established by the EPA Part 503 requirements for Class B biosolids and the reduction on volatile solids met the minimum 38 percent established EPA Part 503 vector attraction reduction requirements except at a detention time of 7 days. The minimum detention times at which the FC densities and VS reduction met the requirements for a Class B biosolids were 25°C and detention time of 30 days, and at 35°C and a detention time of 15 days. Temperature and time are the principal variables that affect the log reduction in the density of FC in mesophilic anaerobic digestion. ANOVA indicate that the individual effects of time, temperature, and organic loading on the FC densities were all statistically significant at the 5% significance level; however, the main effects of time and temperature were more significant than organic loading. The individual effects of time and temperature on FS were statistically significant but temperature was more significant than detention time. Temperature is the principal variable that affects the reduction of FS in anaerobic digestion. The densities of salmonella in the lab-scale digestion at 35°C and detention times of 30 and 45 days were below the limits established by the Part 503 for Class “A” biosolids. The main effects and two-factor interactions between timetemperature and between time-organic loading were statistically significant on the density of Salmonella in sludge after anaerobic digestion. The interaction between time and organic loading was more significant than the interaction between time-temperature.