Pathogen Characterization of Fresh and Stored Biosolids and Implications of a Screening Level Microbial Risk Assessment

Abstract

A limited study of enteric pathogens from different stages of mesophilic anaerobic digestion (MAD), i.e., primary sludge, and liquid and dewatered (cake) biosolids, was conducted in six wastewater treatment plants (WWTP) in Ontario, Canada. In addition, bacterial pathogens were re-analysed for regrowth after 2 to 3 days storage in a laboratory at 30°C. Pathogen datasets were used in a preliminary screening level quantitative microbial risk assessment (QMRA). The QMRA assumed conservative human health exposure scenarios for ingestion of soil containing freshly surface-applied or incorporated sewage biosolids, and also considered the aerosol pathway. Culture methods (Most Probable Number or plate count) were used to analyse for bacterial pathogens, including: Campylobacter spp., Salmonella spp., Listeria monocytogenes and Yersinia enterocolitica; as well as fecal indicators: fecal coliforms, E. coli, enterococci and Clostridium perfringens. Two additional pathogens, Cryptosporidium parvum and Giardia lamblia, were quantified in a select few samples by polymerase chain reaction coupled with Most Probable Number (PCR-MPN). Salmonella and Listeria were found frequently in primary sludge and liquid digested biosolids (70-100% of samples) but less so in dewatered cake (50-60%). Yersinia was found less frequently (in 20-30% of samples) across all treatment stages. Giardia was found in 80% of cake biosolids while Cryptosporidium was found in 20%. E. coli reduction during MAD was more than 2-log (at 4 of the 6 plants), while reduction of bacterial pathogens was variable, as shown in other literature. Increases in bacteria levels immediately upon dewatering (i.e., "reactivation"), were generally not significant. Salmonella and E. coli densities increased 1 to 3-log after 2-3 days storage at 30°C, signifying potential for regrowth. Significant correlations between E. coli and Salmonella were observed after the regrowth period (r >0.55, p<0.02). Pathogen datasets from dewatered biosolids were used in a preliminary screening level QMRA, using Monte Carlo analyses. For QMRA simulations pathogen data were fit to log normal distributions using censored regression, which statistically accounts for the non-detects in pathogen data. QMRA results must be interpreted with caution given the limited datasets and conservative nature of the assumptions applied; QMRA results were intended solely for research, to identify data gaps and inform priorities. QMRA revealed that median risks of enteric infection and illness from indirect ingestion of aerosolized particulates were orders of magnitude lower than those from direct ingestion. Median risks of gastrointestinal illness from Salmonella were very low, even after regrowth (e.g., 10 -16 ). Giardia and Cryptosporidium posed higher risks compared to Salmonella, but estimates for these protozoan pathogens were based on very few samples (n=9) which were highly censored. In addition these data were derived from molecular detection methods that did not distinguish viable and/or infectious organis...

The Effect of Digestion and Dewatering on Sudden Increases and Regrowth of Indicator Bacteria after Dewatering

“…The results suggest that sudden increase was less likely to occur with mesophilic processes. Similar observations were reported by Flemming et al (2009), who reported that three out of five mesophilic plants with centrifuge dewatering did not experience increased densities of E. coli or fecal coliform immediately after dewatering. The two plants in that study that did measure greater densities showed an increase of less than one order of magnitude.…”

“…These results agree with the literature. For example, Flemming et al (2009) reported that five out of five mesophilic digestion processes with centrifuge dewatering experienced increased densities of fecal coliform and E. coli during cake storage compared with one mesophilic process with BFP dewatering, which did not experience regrowth.…”

Pathogen Characterization of Fresh and Stored Mesophilic Anaerobically Digested Biosolids

“…Wong et al (2010) also reported much higher levels of Salmonella in a dewatered biosolids sample than the liquid digested materials (by more than 3 log 10 units). Quantitative microbial risk assessments by Gerba et al (2008) and Flemming et al (2009) have suggested that the potential human health risk posed by direct contact of Salmonella in land applied biosolids was low; however, risks were potentially elevated after regrowth, based on high Salmonella numbers observed in rare cases. Those potential increased risks of infection after regrowth were computed to be ,10 -4 per exposure event after biosolids incorporation into soil (Gerba et al, 2008).…”

Do Alternate Bacterial Indicators and Pathogens Increase after Centrifuge Dewatering of Anaerobically Digested Biosolids?

“…This agrees well with other research. For example, Flemming et al (2009) reported that Salmonella densities in raw sludge ranged from below detection to 3.53 log 10 MPN/g DS, while fecal coliform ranged between 6.3 and 7.9 log 10 MPN/g DS. The difference in the initial densities can be a factor, because first-order kinetics typically are used to describe inactivation rates of bacteria to thermal treatment.…”