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Reduction of Bacterial Pathogens in a Single-Stage Steel Biodigester Co-Digesting Saw Dust and Pig Manure at Psychrophilic Temperature

The experiment was conducted using a single-stage steel biodigester of 75 L working volume, charged with 75% pig manure and 25% pine wood sawdust and operated under batch mode at a psychrophilic temperature range (13.16–24.69 °C). The total viable count was determined via the spread plate method on selective microbiological media to determine viable numbers of the selected bacterial pathogens in samples collected from the biodigesting chamber every seven or fourteen days over the duration of study. Multiple linear regression models including the log bacterial counts (response) and number of days, pH, and average daily temperature as predictors were developed using Matlab for each bacterium. The reduction (90–99.9%) in numbers of isolates of E. coli, Salmonella, Yersinia, Campylobacter, and Listeria varied with time (days) from their initial respective counts of 2 × 106, 7 × 104, 3 × 105, 9 × 105, and 1 × 104 cfu/g to concentrations lower than the detection limit (DL = 102 cfu/g substrate). E. coli demonstrated the least resistance to the environmental conditions in the biodigester and survived only for 77 days, unlike L. monocytogenes that lasted for 175 days and was the most resistant bacterium. From the models, the number of days and temperature were directly and inversely related to log Listeria counts, respectively, contrary to the others. The predictors, number of days, pH, and average daily temperature, were described as either primary or secondary factors based on the bacteria via the reliefF test.