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The increased interest in producing biosolids that have reduced pathogen concentrations has resulted in the development of advanced digestion systems. A variety of advanced digestion systems are now available and include temperature phased, two phase and three phase configurations. Traditionally temperature has been considered to be the principal agent responsible for pathogen reduction in anaerobic digestion. However, it is known that the presence of other toxic substances such as volatile fatty acids can also have a disinfecting effect. Volatile fatty acids may play a role in the inactivation of pathogens for systems that employ an acid phase reactor. While the impacts of volatile fatty acids on pathogens have been documented for mesophilic temperatures there is less information on the impact of volatile fatty acids on pathogens at higher temperatures. The objective of this study was to assess the impact of volatile fatty acids on the inactivation of pathogens over a range of digestion temperatures.

In this study digesters were operated at temperatures that spanned the range from 35-49°C and had a solids residence time of 15 days. Samples collected from the effluent of the digesters were dosed with solutions containing acetic, propionic and butyric acids alone and in mixtures and the dosed effluents were analyzed for SALMONELLA SPP. over time. In the first round of testing the digester effluents were dosed with individual organic acids and also a mixture containing all three VFA's over range of concentration from 750-6000 mg/L while the pH of the samples was fixed at a value of 5.5. In the second round of testing the sample sludges were spiked with a fixed amount of organic acid mixture, while the pH was varied from 4.5 to 7.5.

The reduction of SALMONELLA SPP. in digester effluents, when dosed with volatile organic acids, was found to depend on pH, temperature, the chain length of the acids and the concentration and composition of the acids present. A mixture of organic acids was found to be more inhibitory than the individual organic acids. It was found that the inhibitory effects of the organic acids decreased as their chain length increased. Increases in temperature appeared to increase the inhibitory effects of the volatile organic acids. An interaction between temperature and pH on inhibition of SALMONELLA SPP. was observed. At mesophilic temperatures acidic pH's resulted in a greater inhibition of SALMONELLA SPP. while at higher temperatures neutral pH's were found to be more inhibitory.

The results suggest that acid phase digesters that operate at elevated temperatures and low pH can achieve substantial reduction of pathogen such as SALMONELLA SPP. Process modifications that can accentuate acid generation and pH reduction will enhance pathogen reduction.
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Document Type: Research Article

Publication date: 01 January 2004

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