Available information from the literature was reviewed to determine pathogen removal efficiency of activated sludge treatment plants (Appendix A). This literature review aims to explore recent research on pathogen removal in activated sludge treatment plants along with detection methods used that may assist the current research. The importance of the different pathogen types is dependent on the geographical location of the wastewater systems and the social and economic standards of the local population. As such, the presence of helminths and various bacterial pathogens (Vibrio cholerae and Shigella dysenteriae) are important in regions with lower socio-economic standards but not high socio-economic regions such as Australia. Climatic conditions as found in different geographical regions of Australia (and elsewhere around the world) are known to affect the removal efficiencies for biological treatment systems. This has been highlighted in a number of studies below.
2.5.1 Pathogen removal
Recently a small study was undertaken at a WWTP in Adelaide over six weeks to investigate virus and protozoa removal. The results indicated a minimum of 1 Log10 reduction of viruses and 0.5
Log10 reductions of protozoa for a well operated and maintained activated sludge plant. This has
resulted in South Australia’s Department of Health and Aging (DHA) revising their default virus reduction values (Keegan et al., 2013).
Abreu-Acosta and Vera (2011) investigated the occurrence and removal of pathogenic microorganisms in two wastewater reclamation systems and found that both demonstrated efficient reduction of faecal contamination indicators in the wastewater (2 Log10 removal). Both Cryptosporidium and Giardia (oo)cysts were found to be efficiently removed. The authors acknowledged, however, that the prevalence of Giardia in the local human communities and the presence of cysts in the effluents suggested that Giardia should be used as indicators for quality control in reclaimed effluent. This study suggested also using E. coli and indicators such as
Clostridium perfringens and somatic coliphages due to a range of factors including having a direct relationship to Giardia, higher resistance than other bacterial indicators, simple and economical determination or a short 4 hours turnaround time for results.
Simmons and Xagoraraki (2011) showed an average 4.2 Log10 reduction of infectious viruses
occurred through the wastewater treatment process in a study involving 5 full-scale WWTPs. These results are comparable to other previous studies which also reported up to 4.0 Log10
reductions of infectious viruses (Petrinca et al., 2009; Sedmak et al., 2005).
Castro-Hermida et al. (2008), investigated the ability of Spanish WWTPs to remove
Cryptosporidium and Giardia (oo)cysts. They found that the (oo)cysts were present in both the influent and effluent of all the samples from the WWTPs with minimal removal of (oo)cysts by the treatment process. In fact, it was noted that the number of (oo)cysts in the effluent from several of the WWTPs were actually greater than detected in the influent. The residence time of the wastewater in the treatment plants was taken into consideration when sampling. No specific information was given on actual log removal capacity of any of the treatment plants but the averaged results suggest that the LRV was less than 2 Log10.
Castro-Hermida et al. (2008) studied the contribution of treated wastewater to the contamination of recreational river areas with Cryptosporidium spp. and Giardia duodenalis. Both Cryptosporidium spp and Giardia duodenalis were found in the influent and final effluent samples of 12 WWTPs in Spain (Castro-Hermida et al., 2008). The numbers of Giardia in the influent were found to be significantly higher than Cryptosporidium. Numbers in the final effluent ranged from 2-390 and 79-2469/L for Cryptosporidium and Giardia, respectively. The highest numbers of parasites were observed at all WWTPs in spring and summer. Da Silva et al. (2007) examined the removal of noroviruses in French WWTPs with different treatment types (stabilization ponds, a small and large activated sludge systems and membrane bioreactors). They observed that the removal efficiency varied from 1 Log10 to as high as 3 Log10 depending
on the WWTP. None of the WWTPs tested completely removed noroviruses all of the time. The small activated sludge and membrane bioreactor systems were found to give the highest removal efficiencies.
19 | Development of Validation Protocols for Activated Sludge Process in Water Recycling
Human adenoviruses have been reported to be 10 times higher in numbers than enterovirus in wastewater (Reynolds, 2004), and are known to survive better than enterovirus during wastewater treatment (Bofill-Mas et al., 2006). High numbers (105 gene copy numbers/L) of adenoviruses have been reported for both sewage and primary sludge (Albinana-Gimenez et al., 2006; He and Jiang, 2005). However, the cell culture technique has resulted in reports of lower numbers (102 pfu/L) (He and Jiang, 2005). The discrepancy is possibly due to overestimation of infectious viral numbers by qPCR, while cell cultures assays tend to underestimate virus numbers (He and Jiang, 2005) indicating that it is importance of a standardised detection method.
Adenoviruses are known to be more UV and thermos-stable compared to other enteric viruses and can survive in the environment for a long time (Enriquez et al., 1995; Gerba et al., 2002). Several studies have investigated the removal and correlations between bacterial indicator micro-organisms and viruses (Keegan et al., 2013; Muela et al., 2011; Petrinca et al., 2009). Petrinca et al. (2009) found that high removal rates of bacteria were contrasted with limited removal of viruses determining that the bacteria were not predictive of enteric virus presence. In another study, adenovirus was reported to have a minimal correlation between adenovirus and indicator microorganisms (sulphite-reducing clostridia and F-RNA bacteriophage) (Keegan et al., 2013). This led the authors to conclude that where possible, pathogens should be used for assessing plant pathogen reduction performance.
Seasonal variations in pathogens have also been suggested to be important. However, an eight month study found no significant seasonal differences in adenovirus numbers in the effluent from four WWTPs in the USA (Kuo et al., 2010). The presence of infectious human viruses in non- disinfected effluent regardless of the treatment method used has been reported with adenovirus, norovirus and enterovirus found to be always present in primary influent in small, medium and large WWTPs (Hewitt et al., 2011). Norovirus infection were found to be associated with peaks in the winter season in Japan, Norway and The Netherlands (Mounts et al., 2000). In contrast, no clear seasonal peak for norovirus infection was found in New Zealand (Hewitt et al., 2011). Reported norovirus reductions in wastewater treatment plants ranged from 0.0 to 3.6 Log10 (Kuo et al., 2010; Lodder and de Roda Husman, 2005; Nordgren et al., 2009; Ottoson et al., 2006a; van den Berg et al., 2005).
Van den Berg et al. (2005) reported high numbers (105/L) of noroviruses in raw sewage with limited inactivation during wastewater treatment (103/L). Similar limited inactivation of noroviruses has also been reported elsewhere (Hewitt et al., 2011; Laverick et al., 2004). Norovirus cannot be cultured, therefore comparison of data on noroviruses with other enteric virus numbers and inactivation in wastewater determined using culture based methods is theoretically not possible. In one study, removal rate of norovirus was compared with other viruses during wastewater treatment using PCR as a detection method for all the viruses, a similar removal for norovirus (0.2-2.1 Log10), reovirus (0.9-1.4 Log10) and enterovirus (0.7-1.8 Log10) but a greater removal rate than rotaviruses (0.003-1.1 Log10) was reported (Lodder and
de Roda Husman, 2005).
In a literature review, enterovirus numbers were reported to range from 102 to 104 /gm dry weight in raw sludge and 300/gm in anaerobically digested sludge (Straub et al., 1993). Monpoeho et al.
(2004) reported that enterovirus numbers in raw sludge varied between 37-288 cytopathic units/gm (cell culture), whereas qPCR based quantification resulted in higher detection of 105 copy numbers/gm. Similarly enteric viruses were found in raw wastewater samples at concentrations between 102 to 104 MPN/100 L in a study in Brazil (Hachich et al., 2013).
Data from a study in the US has found that Cryptosporidium oocysts were present in low numbers in many wastewaters as well as in the effluents being discharged from the studied WWTP (McCuin and Clancy, 2006). However, the assays used in this study were unable to differentiate between live and dead oocysts and therefore further research would be required to determine the infectivity of the detected oocysts and thereby assess the associated health risks. Cheng et al. (2012) reported positive correlations between the abundance of enterococci and E. coli and the abundance of Cryptosporidium spp. and Giardia spp. This study also noted a strong correlation between Giardia and Cryptosporidium. Fu et al. (2010) compared faecal coliforms and somatic coliphages numbers with Cryptosporidium and Giardia in untreated wastewater and secondary treated effluent and found that the somatic coliphages correlated better than the faecal coliforms with the protozoa. It was noted however, that the concentrations of pathogenic protozoa could not be determined by detecting concentrations of somatic coliphages.