| Abstract | Sewage sludge contains adequate amounts of major nutrients such as nitrogen and phosphorus that are useful for plant growth. However, wastewater entering the municipal wastewater treatment process contains high amounts of pathogens and the pathogens become concentrated in sludge. Hence, the sludge cannot be directly used as plant fertilizer or soil conditioner. It has to be treated before application for agriculture or other purposes. Anaerobic digestion is known to inactivate pathogenic organisms in sludge. This research was conducted to study the destruction of Salmonella spp. by treating the secondary sludge in different conditions of organic concentration, organic acid composition, variation of pH and applied in bench-scale two-phase anaerobic digestion. The results found that at different concentrations of organic acid mixture (acetic, propionic and butyric acid), Salmonella spp. detected was not significantly different. If the composition and concentration of organic acid is kept constant, the survival of Salmonella spp. was found to be different depending on the pH. At pH of 3.6, Salmonella spp. concentration was non-detectable, while at pH 5.5, concentration was reduced from the initial concentration but could be detected until ten days incubation. In sludge, which had no added organic acid, the concentration of Salmonella spp. was reduced and was nondetectable since the eighth day of incubation. To study the effect of pH on the survival of Salmonella spp., pH was varied between 3.5 to 5. 5 with same amount of organic acid concentration and composition. The results indicate that at pH five or lower, the survival of Salmonella spp. is inhibited. From the kinetic study of pathogen destruction, it was found that pH 3. 5 was about four times more effective than pH 4.0, 4.5, and 5.0 and ten times more than pH 5.5 in terms of survival of Salmonella spp. Thus, it can be said that at pH 5.5, the time required to effect non detectable level was ten times longer than pH 3.5. This condition was applied in acid phase of two-phase anaerobic digestion process and pH was varied from five to seven. It was found that at the fifth day of incubation, the generation of VF A was 20,595 mg/L, 11 ,064 mg/Land 5,942 mg/Lin sample of pH 7, pH 6, and pH 5, respectively. Concentration of Salmonella spf" was increased to more than 1.60 x 103 MPN/mL from initial concentration (3.00 x 10 MPN/mL) in pH 7 sample. It decreased to 0.2 MPN/mL in pH 6 and in pH 5 it was non-detectable. It can be concluded for the results that pH affects most to the survival of Salmonella spp. and acid phase of anaerobic digestion process can be applied for stabilization of sewage sludge in term of Salmonella spp. destruction. |