Pathogen indicator regrowth potential as a method to evaluate compost stability

Many different processes have been used to treat sewage sludge. In terms of beneficial reuse, composting is one of the most important, because it stabilizes the organic matter and kills pathogens. There are many different parameters that can be analyzed to verify the progress of the composting process (as well as compost quality), such as solids analysis, C/N ratio, temperature evolution, oxygen uptake rate, and carbon availability. At present, none of these parameters can be used alone to characterize compost stability. Another aspect of compost quality that has received a lot of attention from both regulatory agencies and compost users is the regrowth potential of pathogenic microorganisms during storage and handling of the compost product, which could compromise its quality. This study investigated pathogen regrowth potential in biosolids compost. Regrowth potential was correlated with moisture content and carbon availability, and was evaluated as a parameter to measure compost stability. In addition, the influence of compost indigenous population on regrowth was verified. To accomplish the goals of this study, one sample of a very well stabilized compost, from a static pile compost process was used. This sample was dried and half of the dried sample was sterilized. Tests with dried (D) and sterile and dried (S/D) samples were conducted at 25, 30, 40 and 50% moisture content, with addition of 10 g C/kg ds using milk powder as a carbon source. Controls ofthe experiments were also performed with no additional external carbon source. A series of four different carbon concentrations, 0, 2.5, 10 and 20 g C/kg ds, was used to check the influence of carbon availability on pathogen regrowth potential. The experimental approach also included side experiments evaluating two full-scale composting plants, treating sewage sludge, involving two different composting configurations, one in-vessel and one static pile. To perform the regrowth potential tests, total coliforms and E. coli concentrations were determined in samples incubated at 37$\sp\circ$C over a period of 20 days. It was found that regrowth was inhibited at moisture levels of 30% or less. The amount of regrowth was related to the amount of carbon added in non-sterile samples. The small amount of carbon available in the sterile very well stabilized compost sample, without any addition of external carbon source, was sufficient to support regrowth at high levels, showing the importance of microbial competition on compost stability. The field experiments confirmed the results obtained in the laboratory, showing the extreme importance of the microbial diversity of the system on pathogen regrowth potential and indirectly on compost quality