Effects of moisture content on long-term survival and regrowth of bacteria in wastewater sludge

Abstract

The effects of moisture content on the survival and regrowth of seeded and indigenous enteric bacteria in raw sludge were determined. Cultures of six strains of fecally associated bacteria grown in sterilized, liquid sludge (5% solids) were all quite stable at this moisture level for over 90 days at 21 degrees C. When the moisture content of the sludge containing these organisms was reduced by evaporation and the samples were stored at 21 degrees C for extended periods, bacterial inactivation rates were generally proportional to the moisture losses of the samples. A dramatic reversal in this effect was observed in samples containing more than 90% solids. In this dried sludge, every bacterial species studied except Proteus mirabilis was found to be extremely stable. Bacteria indigenous to sludge were also found to survive for long periods in dried sludge. Regrowth of bacterial isolates in sterilized raw sludge was found to occur readily at 37 degrees C in samples containing less than or equal to 75% solids, but no growth was observed in samples with greater than or equal to 85% solids. Some growth, but to less than saturation densities, occurred with 80% solids. Growth of seeded Salmonella typhimurium was also found to occur in the presence of indigenous organisms in both liquid and dewatered raw sludges. However, the population density attained was well below that found in sterilized samples of the same sludges. In addition, the number of salmonellae dropped below detectable limits within a few days in sludges containing viable indigenous organisms, whereas little decrease occurred during this time with salmonellae grown in previously sterilized sludges.

Survival ofEscherichia coliandSalmonellaspp. after application of sewage sludge to aPinus radiataforest

“…There was a significant relationship between moisture content of the sludge and E. coli numbers, which decreased rapidly and reached background after only 3 weeks. Numerous studies have demonstrated a direct relationship between bacterial die-off and moisture loss from sewage sludge (Yeager and Ward 1981;Vasseur et al 1996;Zaleski et al 2005). It has also been suggested that as soil temperature rises, possible increases in the metabolic activity of soil micro-organisms (which compete with the introduced pathogen) may also reduce pathogen survival (Hon 2003), however further research is required to fully validate this hypothesis.…”

Mobility and survival of Salmonella Typhimurium and human adenovirus from spiked sewage sludge applied to soil columns

“…1). Numerous studies have demonstrated that increased soil moisture increases survival rates of pathogens in soil (Yeager and Ward 1981;Vasseur et al 1996;Zaleski et al 2005). It has also been shown that certain conditions, such as increased moisture and carbon availability, can cause re-growth of Salmonella ssp.…”

“…Factors controlling survival, transport and regrowth include soil type (Mubiru et al 2000;Aislabie et al 2001;Cools et al 2001;Trevisan et al 2002), agricultural practice (Hutchinson et al 2004), method of pathogen delivery (i.e. type of waste) (Avery et al , 2005Sun et al 2006) and environmental factors such as rainfall, temperature and humidity (Yeager and Ward 1981;Palacios et al 2001;Hepburn et al 2002;Jenkins et al 2002;Zaleski et al 2005). It has been shown that if transported below the top layers of the soil, pathogens can survive for extended periods of time after the initial application (Gagliardi and Karns 2000).…”

Assessment of survival ofListeria monocytogenes,SalmonellaInfantis andEnterococcus faecalisartificially inoculated into experimental waste or compost

“…In general, studies on pathogen survival in compost were conducted with strains not isolated from compost (i.e. laboratory strains) (Yeager and Ward 1981;Hussong et al 1985;Sidhu et al 1999Sidhu et al , 2001Lemunier et al 2005). For example, Hussong et al (1985) used two laboratory strains of Salmonella (S. typhimurium and S. Newport) inoculated into compost.…”