Virus numbers in wastewaters vary from plant to plant with the numbers influenced by (i) the inputs to sewage (whether domestic, industrial wastes or a mixture of both), (ii) the incidence of disease within the community (most enteric viruses are seasonal), (iii) the treatment processes used at the treatment plant and (iv) the end point use of the water (this determines the level of treatment applied to the wastewater). In the literature, this variation is also influenced by the concentration, recovery and detection methods that are used for viruses. Few studies have investigated the removal of viruses across the full treatment train with the inclusion of a large range of pathogens and indicators in the assessment. The numbers of viruses in primary wastewaters varies considerably and ranges have reported in the AGWR, with enteroviruses present at 102-106 / L, adenovirus at 101-104 / L, norovirus at 101-104 / L, rotavirus at 102-105 / L, somatic coliphage at 106-109 / L and F-RNA coliphage at 105-107 / L.
Estimated log10 removals of viruses and bacteriophage for individual processes are provided in Table 6. The virus log10 removals are process dependant and reductions are dependent on specific features of the process, including detention times, pore size, filter depths and disinfectant type. When under consideration by a state Department of Health, a system or process will be attributed the default minimum log10 credit listed in Table 7 unless it has been proven otherwise that greater removal of inactivation is achievable. The default values are accumulated across the treatment train processes for a total log10 removal achievable by the system with a maximum of 4 log10 credit attributed to any single process. The level of removal required is dependent on the quality required in the product water whether it be Class A, Class B or other.
Table 7. Indicative log10 removals of enteric viruses and indicator organisms. Indicative log10 removals
a
Treatment Enteric Viruses Phage E. coli
Primary treatment 0-0.1 N/A 0-0.5
Secondary treatment 0.5-2.0 0.5-2.5 1.0-3.0
Dual media filtration with coagulation 0.5-3.0 1.0-4.0 0-1.0
Membrane filtration 2.5->6.0 3.0->6.0 3.5->6.0 Reverse osmosis >6.0 >6.0 >6.0 Lagoon storage 1.0-4.0 1.0-4.0 1.0-5.0 Chlorination 1.0-3.0 0-2.5 2.0-6.0 Ozonation 3.0-6.0 2.0-6.0 2.0-6.0 UV light >1.0 adenovirus
>3.0 enterovirus, hepatitis A virus
3.0-6.0 2.0->4.0 a
Reductions depend on specific features of the process, including detention times, pore size, filter depths, disinfectant. Adapted from AGWR (2006).
Bacterial Indicators
2.7.2
Enterococci/faecal streptococci have potential to be used for source tracking and do not appear to multiply in municipal wastewater. Some reports question whether these are a good indicators for pathogenic enteric bacteria and more information is required. Based on the limited information from available studies, total coliforms appear to be worthwhile along with E. coli as a surrogate/indicator for other enteric bacteria. Few of the microbiological indicators evaluated proved to be good predictors of pathogen removal for full-scale treatment for wastewater. Coliforms generally correlated with E. coli removal, but in most wastewaters E. coli is the dominant coliform so there is little advantage to test for both (Keegan et al. 2009).
Particle profiling using a Coulter Counter Multisizer II was used to determine the particle profile of water samples (Chavez et al. 2004). A good correlation was found between the removal of particles <8 microns and the removal of faecal coliforms and Salmonella spp (R2 = 0.8217 and 0.7148 respectively). Using a LISST-100 Particle Size Analyser Type C (Sequoia, Redmond, WA, USA), Keegan et al., (2009) reported no correlation between pathogen removal and the removal of particles of any particular size category, but the data suggested a direct correlation between the volume of particles removed and the numbers of pathogens removed across ASP.
Protozoan Indicators
2.7.3
In the absence of a better indicator, sulphite-reducing clostridia (SRC) are often used as an indicator for protozoa. There are conflicting reports on their suitability as an indicator as well as some variation in methodologies. A study investigating pathogen removal across activated sludge treatment for different sites showed that SRCs and particle profiling generally provided a conservative measure of removal comparable with Cryptosporidium (within the same order of magnitude), while Giardia cysts were better removed at most plants (Keegan et al. 2009). However, more detailed comparison of removal at a single site demonstrated a lack of correlation for Cryptosporidium and SRC, although the correlation between SRC or particle removal with Giardia removal was good (Keegan et al. 2009). The removal of SRC reported by Keegan et al (2009) was higher than that reported by Rose et al. (2004), although the two studies reported similar removals for Cryptosporidium and Giardia. A strong correlation has been reported between particle volume (2 – 14 micron size class) and
Cryptosporidium and Giardia numbers (Keegan et al. 2009).
Fluorescent microspheres are spherical micro particles having similar size and density characteristics to microorganisms. They are available in a size range of 0.08–3,300 μm and are generally made from latex or polystyrene. Fluorescent microspheres have successfully been used in the laboratory as a surrogate for Cryptosporidium parvum oocysts in assessing disinfection employing ozone followed by chlorine (Baeza and Ducoste 2004). They have also been used to successfully measure UV fluence distribution to complement biodosimetry validation of a pilot scale UV reactor (Bohrerova et al. 2005). They can provide a conservative estimate of C. parvum oocyst removal in hydrophilic negatively charged filter media (e.g. sand) but are insufficiently conservative for hydrophobic media such as GAC (Dai and Hozalski 2003). Although not present naturally in wastewater, they may be added to treatment processes making them a possible model indicator. The cost of employing them in full-scale processes, however, detracts from their use.