Effect of volatile fatty acids on Shigella dysenteriae during anaerobic digestion of human night soil

The in vitro toxic effect of different volatile fatty acids (VFA) on Shigella dysenteriae was studied in pure culture. Volatile fatty acids viz., acetate, propionate, butyrate, valerate, caproate and heptanoate, exerted pH dependent toxic effect on the pathogen, with minimum inhibitory concentration in the range of 10-3000 mg l(-1). The effect of high levels of VFA on S. dysenteriae was studied during anaerobic digestion of human night soil in an experimental digester with VFA level congruent to 9000 mg l(-1) and pH congruent to 6.5. Another digester, with VFA level congruent to 700 mg l(-1) and pH 7.4, served as the control. In the experimental digester, S. dysenteriae was completely eliminated within 18 days. In the control digester, a four-log reduction in pathogen count was achieved however the pathogen was not completely eliminated. T-90 values for the experimental and control digesters were 2.2 and 3.7 days respectively.