Pathogen Reduction

Several studies have demonstrated that liquid lime and dry lime stabilization achieve significant pathogen reduction, provided that a sufficiently high pH is maintained for an adequate period of time. Bacteria levels measured during full-scale studies performed at Lebanon, Ohio indicated that liquid lime stabilization of raw sludge reduced total coliform, fecal coliform, and fecal streptococci concentrations reduced by more than 99.9% (U.S. EPA, 1979). The number of Salmonella and Pseudomonas eruiginosa were reduced below the level of detection. The same studies showed that pathogen concentrations in different types of lime-stabilized sludge ranged from 10 to 1000 times less than those in anaerobically digested sludge from the same plant.

Research by Christensen on the pathogen reduction performance of dry lime stabilization using quicklime at doses of 13 and 40% as Ca(OH)2 indicated that dry lime stabilization can achieve a reduction in fecal coliform and fecal streptococcus pathogens of at least two orders of magnitude (Christensen, 1982). This was as good as and in some cases better than standard liquid lime stabilization and then liquid lime stabilization followed by vacuum filtration. Neither organism had grown by day 7 (Westphal and Christensen, 1983). Westphal and Christensen also reported that liquid lime and dry lime stabilization processes performed as well as or better than mesophilic aerobic digestion, anaerobic digestion, and mesophilic composting in reducing densities of fecal coliform and fecal streptococcus.

There is little information about the amount of virus reduction during lime stabilization, but a few studies suggest rapid destruction at a pH of 12 (U.S. EPA, 1982). Qualitative analysis has shown substantial survival of higher organisms, such as hookworms, and amebic cysts, after 24 hours of contact time at high pH. It is not known whether long-term contact would eventually destroy these organisms. However, helminthes do not survive for a long time without a host. What is more important is that helminth ova can be destroyed in the alkaline stabilization process by heating the sludge to 50°C for two hours, to 60°C for several minutes, or to 70°C for several seconds.

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