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Pathogen regrowth in composted biosolids

Sidhu, Jatinder (2000) Pathogen regrowth in composted biosolids. PhD thesis, Murdoch University.

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Composting is commonly used as an effective means of stabilising wastewater biosolids and reducing pathogen concentrations to very low levels. However, under certain conditions enteric bacteria such as Salmonella and E. coli can regrow in previously composted biosolids and compost based soil amendments. Regrowth of Salmonella in composted biosolids can pose a potential threat to public health.

Pathogen regrowth in composted biosolids is affected by a number of factors, such as moisture content, bio-available nutrients, temperature and indigenous microorganisms. However, assessment of the bio-safety of composted biosolids on the basis of these parameters is very difficult and unreliable. Moreover regrowth of
pathogens in composted biosolids is unpredictable. The main aim of this study was to find out whether regrowth of pathogens in composted biosolids could be prevented or controlled. To accomplish this goal, the effect of composting and storage on survival and regrowth of Salmonella typhimurium was investigated. The role of bio-available nutrients and indigenous microorganisms in suppression of Salmonella regrowth was investigated in detail.

The results of this study suggest that a full-scale windrow composting process carried out in accordance with recommended guidelines (ARMCANZ) is effective in reducing Salmonella concentrations to below detection limit. However, Salmonella regrowth in stored biosolids after 26 weeks, coinciding with a rainfall after a dry spell was observed. This suggests that Salmonella can survive the composting process in low numbers and regrowth can take place in the presence of favourable growth conditions.

A pathogen regrowth potential test using antibiotic-resistant S. typhimurium was developed to evaluate the pathogen regrowth potential of composted biosolids. The regrowth potential test was found to be a very useful tool for laboratory investigation. However, further validation of the pathogen regrowth potential test is required, prior to its routine use for monitoring composted biosolids.

The antagonistic activity of indigenous microorganisms was found to be the most significant factor in suppression of S. typhimuriwn growth in composted biosolids. Rapid growth of seeded S. typhimurium, with a maximum population density of more than 108 MPN g-1, was observed in sterilised biosolids. Conversely, growth of S.typhimurium was suppressed in non-sterilised compost with a maximum population density of less than 103 MPN g-1. The inactivation rate of Salmonella was also found to be significantly greater in non-sterilised compost as compared to sterilised compost. Maximum inhibition of Salmonella growth was observed in biosolids that had been composting for two weeks.

The specific growth rate of Salmonella was found to have a strong negative correlation (-0.85) with the maturity of the compost. However, a decline in bioavailable nutrients was not sufficient to prevent regrowth of Salmonella in composted biosolids stored for two years. The role of bio-available nutrients (age. of compost) was non significant (P<_ 0.05) as compared to the role of indigenous microorganisms in inhibition of Salmonella regrowth.

The antagonistic effect of indigenous microorganisms towards Salmonella declined with the storage of compost. A strong negative correlation (-0.85) between the Salmonella inactivation rate and age of compost was observed. Salmonella inactivation rate was also found to be seven times higher in biosolids composting for two weeks as compared to compost stored for two years. Consequently, it can be concluded that all composted biosolids had a Salmonella regrowth potential. However, the presence of biologically active indigenous microflora significantly reduced this regrowth potential. As a result of a decline in the antagonistic activity of indigenous microflora with storage, a longer Salmonella survival time could be expected in stored compost as compared to freshly composted biosolids. Consequently, long term storage of compost is not recommended as this may lead to an increased pathogen regrowth potential and longer survival time.

The dilution-plate procedure adopted in this study showed that bacterial concentrations in compost declined by two log10 during storage for two years, whereas population of actinomycetes and fungi increased during the same period. Indigenous bacteria and actinomycetes isolated from composted biosolids of different maturity were screened for their role in the suppression of Salmonella regrowth. Some of the indigenous bacteria were found to suppress Salmonella growth by one to two log10 when Salmonella was seeded into stationary phase culture of indigenous bacteria. None of the isolated indigenous microorganisms produced secondary metabolites active against Salmonella.

Somatic Salmonella (SS) phages were found to survive in composted biosolids for up to two years. Growth of Salmonella was suppressed by one to two log10 in the presence of SS phages. It is possible that Salmonella growth suppression in compost is due to an intense competition for limited nutrients in the presence of biologically active indigenous microorganisms, with some anti-Salmonella activity from SS phages.

The results of this study suggest that prevention of pathogen regrowth in composted biosolids is difficult due to the availability of nutrients and limited control over environmental factors which influence the antagonistic activity of indigenous microorganisms. However, regrowth of pathogens in composted biosolids can be prevented if a biologically active population of indigenous microorganisms is maintained. It is possible that by preventing rapid drying of compost during maturation biological activity of indigenous microorganisms can be maintained. Covering of compost piles during maturation can preserve moisture and the effect of preserving moisture on the antagonistic activity of indigenous microorganisms should be investigated. Research should be carried out further to identify the indigenous microorganisms which suppress Salmonella regrowth. Additional research work should also be carried out to determine the mechanism of growth suppression. Resolving this issue could provide a better understanding of the antagonistic effect of indigenous microflora towards pathogenic bacteria in composted biosolids.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Gibbs, Robyn and Ho, Goen
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