The role of indigenous microorganisms in suppression of Salmonella regrowth in composted biosolids

Composting is commonly used as an effective means of stabilizing wastewater biosolids and reducing pathogens to very low concentrations. However, it h as been shown that under certain conditions Salmonella can regrow in previo usly composted biosolids. Growth of seeded Salmonella typhimurium in compos ted biosolids ranging from two weeks to two years maturity was monitored. R esults from sterile and non-sterile composted biosolids were compared. Seed ed S. typhimurium colonized rapidly in sterilized biosolids reaching a maxi mum population density of more than 10(8) g(-1). Growth of seeded S. typhim urium was suppressed in non-sterilized compost with a maximum population de nsity of less than 10(3) g(-1). There was a significant decline in the grow th rate of seeded Salmonella in sterilized compost when the compost was sto red, suggesting that bio-available nutrients declined with storage. However , in non-sterilized compost this was not the case. This suggests that the i ndigenous microflora play a significant role in suppression of Salmonella r egrowth in composted biosolids. There was a strong negative correlation (-0 .85) between the Salmonella inactivation rate and the maturity of compost i n non-sterilized compost. The Salmonella inactivation rate was seven times higher in biosolids composting for two weeks as compared to compost stored for two years. This suggests that the antagonistic effect of indigenous mic roorganisms towards Salmonella declined with compost storage. It was conclu ded that all composted biosolids had a Salmonella regrowth potential. Howev er, the indigenous microflora significantly reduced this regrowth potential , Long-term storage of compost is not recommended as this map increase the pathogen regrowth potential. (C) 2001 Elsevier Science Ltd. All rights rese rved.