Comparative analysis of pathogen occurrence in wastewater : management strategies for barrier function and microbial control

Abstract: This project was initiated to fill knowledge gaps on the occurrence of pathogens in different streams of wastewater, e.g. greywater and domestic wastewater. The aims were also to measure the removal of pathogens in different treatment processes, conventional and innovative, and correlate the removal to that of common microbial process indicators, such as faecal coliforms, enterococci, Cl. perfringens spores and bacteriophages. One study also assessed the correlation between the removal of microorganisms and some commonly measured physico-chemical process indicators. The results can be applied in microbial risk assessments (MRAs) of urban wastewater systems. Indicators and parasitic (oo)cysts were enumerated with standard methods and viruses with rtPCR. High levels of Giardia cysts and enteroviruses were found in untreated wastewater (103.2 and 104.2 L-1 respectively) indicating high incidences in the society. Noroviruses were also often found in high numbers (103.3 L-1) during winter, but less frequent and in lower numbers (102.3 L-1) during the rest of the year. This temporal variation correlated to the clinical laboratory reporting of noroviruses. A temporal variation was also shown for Giardia with significantly lower cyst counts in untreated wastewater during spring. Cryptosporidium oocysts were not as numerous in untreated wastewater (5 L-1) reflecting a lower incidence in the society than for the other pathogens during the time of the study. Since temporal variation had a larger impact than spatial, site-specific measurements may not be necessary to perform screening level MRAs of wastewater discharge and reuse. Good data can be found in the literature and corrected for by recovery of the detection method, flow and incidence in the society. Removal of microorganisms in wastewater treatments varied from 0 to >5.8 log due to process combination and organism in question. Treatment in integrated hydroponics removed microorganism more efficiently than did secondary conventional treatment, though having longer hydraulic retention time. Tertiary treatment and treatment in a membrane bioreactor (MBR) showed better removal potential than treatment in upflow anaerobic sludge blankets (UASB) in a pilot plant. Human virus genomes were less removed and Giardia cysts more removed than all of the studied indicators. Enumeration with PCR, however, may underestimate infectious virion removal. Spores of sulphite-reducing anaerobes and somatic coliphages were significantly less removed than E. coli and enterococci in all the studied processes. Bacterial indicator and spore removals correlated to enterovirus genome removal (p<0.05), but the predictive values were low (R<0.4). Removals between microbial indicators and NH4-N, Kjeldal-N, COD and TOC correlated stronger (10-18 To manage the risk with reuse and discharge of wastewater, treatment performance targets have been calculated as a step in a hazard analysis and critical control point (HACCP) approach. These targets varied from 0 to 10.4 log removal due to water (grey or wastewater), organism (rotavirus, Campylobacter or parasitic (oo)cysts) and exposure (drinking water, surface water, aerosols, irrigation of crops or public parks). Faecal contamination in greywater was measured by coprostanol and was shown to be 980 times lower than in wastewater, corresponding to 2.9 log removal in treatment. Somatic coliphages were suggested to function as an index of virus removal in wastewater treatment processes as well as to be included in the monitoring of bathing water. The guideline level was suggested to be 300 PFU 100 mL-1 based on MRA of enteroviruses. This level in a water sample would equal a probability of infection of 0.3% (95th percentile 4%). The risk is overestimated if animal sources dominate the faecal pollution. Development in methods to track sources of faecal pollution showed that if somatic coliphages are enumerated together with phages infecting Bacteroides strain GA17, discriminating human from animal faecal pollution is possible based on the ratio between the phages