Microbial safety of air-dried and rewetted biosolids

Abstract

To assess microbial safety of treated sewage sludge (biosolids), we examined the inactivation of microbial indicators for potential bacterial, viral and protozoan pathogens. The levels of indicators were determined throughout the air-drying and storage phases of anaerobically digested sewage sludge. Samples were collected from two wastewater treatment plants (WWTPs) in Victoria, Australia. Established methods were applied for analysis of bacteria and coliphages, based on membrane filtration and layered plates, respectively. In the pan drying phase, the prevalence of Escherichia coli was reduced by .5 log 10 compared with sludge entering the pan. Thus, after pan drying of 8-11 months at WWTP A and 15 months at WWTP B, the numbers of E. coli were reduced to below 10 2 cfu/g dry solids (DS). This level is acceptable for unrestricted use in agriculture in Australia (P1 treatment grade), the UK (enhanced treatment status) and the USA (Class A pathogen reduction).Coliphage numbers also decreased substantially during the air-drying phase, indicating that enteric viruses are also likely to be destroyed during this phase. Clostridium perfringens appeared to be an overly conservative indicator. Survival, but not regrowth, of E. coli or Salmonella was observed in rewetted biosolids (15-20% moisture content), after being seeded with these species, indicating a degree of safety of stored biosolids upon rewetting by rain.

Comparison of methodologies for enumerating and detecting the viability of Ascaris eggs in sewage sludge by standard incubation-microscopy, the BacLight Live/Dead viability assay and other vital dyes

“…This is achieved by the treatment of sludge using effective processes to reduce or eliminate the pathogen content, supplemented by cropping and harvesting restrictions preventing use on crops that are in contact with the soil and may be consumed without cooking, and to permit the natural attenuation of pathogens in the soil (USEPA, 1994;DoE UK, 1996;ADAS, 2001;NRMMC, 2004;EPA Victoria, 2004;WHO, 2006). We have observed substantial decay of pathogen indicators during solar drying treatment of sludge, in line with regulation limits for applying sludge in Australia (Rouch et al, 2011b). Ascaris lumbricoides is an important human pathogen, which infects over a billion people worldwide, with most infections occurring in developing countries (Crompton, 2001;Bethony et al, 2006).…”

Persistence of Clostridium difficile in wastewater treatment-derived biosolids during land application or windrow composting

“…Although biosolids can harbour human pathogens, such as shiga toxin Escherichia coli and enteric viruses, these typically die-off within a 3-month period (Lang et al 2003;Pourcher et al 2007). Yet, clostridia, such as Clostridium perfringens, can persist over years by virtue of being in the dormant, resistant, endospore form (Apedaile 2001; Desmarais et al 2002;Rouch et al 2011).…”

Influence of Innate Sludge Factors and Ambient Environmental Parameters in Biosolids Storage on Indicator Bacteria Survival: A Review

“…Subsequently this higher concentration decreased as the biosolids dried to 80% DS in the final study weeks [27]. Re-wetting of biosolids by rainfall events was examined by Rouch et al [28] in air-dry storage of anaerobically digested biosolids. Rouch et al [28] found an inverse relationship to the DS contents of the biosolids with regards to the survival of E. coli, enterococci and coliphages.…”

“…Re-wetting of biosolids by rainfall events was examined by Rouch et al [28] in air-dry storage of anaerobically digested biosolids. Rouch et al [28] found an inverse relationship to the DS contents of the biosolids with regards to the survival of E. coli, enterococci and coliphages. Due to the growth of dormant or small residual populations of bacterial in the biosolids, bacterial regrowth could occur as cells become active upon rewetting [28].…”