Lack of Data on Virus Reductions

Despite the importance of viruses as etiologic agents of diarrhea in diverse settings, little research has been done on virus reductions in SSF and the results vary widely. In the US, viruses are responsible for about 80% of enteric disease outbreaks for which the causative agent can be identified (Ryan et al., 2002). In developing countries, pathogenic viruses have been identified in stool samples of over 40% of diarrhea hospitalizations (Ramani and Kang, 2009).

Reductions of waterborne viruses reported in reviews of SSF vary widely [<1-1.5 log (Kool, 1979); <1 to3 log (Rachwal et al., 1996); and 2-4 log (Amy et al., 2006)]. Variability in reductions could be caused by differences in design and operational conditions such as extent of filter maturation and physical-chemical characteristics of challenge viral agents.

2.3.3.1 Mechanisms of Virus Reduction

The effects of design and operational parameters on virus reductions in SSF are listed in Table 2.6. Even less is known about the mechanism of reduction than those to explain reductions of bacteria or protozoa. Parameters that have been linked to an increase in reductions are: decreased filtration rate; increased temperature; increased time-in-use; and increased sand bed depth (DeLoyde, 2007). Increased residence time is another factor although it is simply the quotient of two parameters already mentioned, i.e., sand bed depth/filtration rate. However, there is possibly an independent effect of filtration rate; for example, pore velocity affects collector efficiency (Yao et al., 1971; Qi, 1998).

As noted earlier, removal of the schmutzdecke was shown to have little or no effect on virus reductions (Hijnen et al., 2004; Dullemont et al., 2006; DeLoyde, 2007). Yet, virus reductions increase with time of operation of the SSF. Thus, there is an influence of a maturation process that is not associated with the topmost layer of the filter. Instead, media aging which affects the entire depth is responsible (Poynter and Slade, 1977; Wheeler et al., 1988) (see also Section 2.3.2).

The most probable mechanisms of virus reductions in SSF are summarized in Table 2.7. Adsorption (and desorption) of viruses in SSF has been reported (Dullemont et al., 2006; Hijnen et al., 2004). Direct microbial activity has been implicated through either predation (grazing) or the activity of microbial exoproducts including proteolytic enzymes. Viruses are inactivated by proteolytic enzymes through hydrolysis of the peptide bonds in proteins that comprise the virus capsids.

Table 2.7 Possible mechanisms of virus reduction in slow sand filtration. Removal Mechanism Mechanistic Explanation

Predation/grazing on virus particles

Filter feeding protozoa and bacteria can ingest virus particles (Kim and Unno, 1996; Pinheiro et al, 2007; Cliver and Herrman, 1972)

Biological activity, including microbial exoproducts

Non-sterile conditions, and presence of aerobic microorganisms lead to more rapid reductions of infectious viruses (Hurst et al., 1980; Jansons et al., 1989; Quanrud et al., 2003; Herrmann et al, 1974). More specifically, proteolytic enzymes can inactivate viruses (Cliver and Herrmann, 1972; Deng and Cliver, 1992; Nasser et al., 2002; Ward et al., 1986).

Attachment to biofilms

Viruses can become associated with biofilms in drinking water systems (Skraber et al, 2005; Storey and Ashbolt, 2001 and 2003; Wheeler et al, 1988)

Adsorption/attachment to granular media

Viruses undergo reversible adsorption/attachment; long-term detachment has been observed after seeding stops (Schijven and Hassanizadeh, 2000; Dullemont et al., 2006; Hijnen et al., 2004; Dizer et al., 2004)

Sequestration or straining in schmutzdecke

No effect or little effect of schmutzdecke removal on virus concentration in product water (Hijnen et al, 2004; Dullemont et al, 2006; DeLoyde, 2007)

Adapted from DeLoyde, 2007

Table 2.6 Influence of selected design and operational parameters on virus reductions in SSF.

Parameter Direction of Change in Parameter to Increase Virus Reduction

Filtration rate/pore velocity Lower filtration rate increases virus reductions (Poynter and

Slade, 1977; Windle-Taylor, 1970; DeLoyde, 2007).

Water temperature Warmer temperature (Poynter and Slade, 1977; Dullemont et

al., 2006; DeLoyde, 2007).

Sand depth Deeper bed (DeLoyde, 2007; Poynter and Slade, 1977).

Time-in-use/ biological maturation

Longer time-in-use. Often stated to be increased biological maturity (Wheeler et al, 1988; Windle-Taylor, 1969; Poynter and Slade, 1977; Dizer et al., 2004; DeLoyde, 2007).

Removal of schmutzdecke

Little or no effect. Small decrease reported following removal of schmutzdecke in some filters (Hijnen et al, 2004; Dullemont et al., 2006; McConnell et al., 1984; Poynter and Slade, 1977; Ellis, 1985; DeLoyde, 2007).

A deficiency of most research on virus reductions in SSF is the use of infectivity assays to determine concentrations. Infectivity assays do not provide differentiation between mechanisms of inactivation and mechanisms of sorption or sequestration in the sand bed. The use of 125I radio-labelled reovirus is an exception (McConnell et al., 1984). Labelled reovirus was found throughout the media bed with about five-percent recovered in the product water. However, reductions of infectious reovirus in excess of 4-log were observed. In fact, no infectious viruses were recovered in the sand bed or in the product water. Additionally, there was no apparent difference in 125I-labelled reovirus transport between aged sands and clean sands. By contrast, studies in which infectivity assays were used suggest that aged sands were more effective than clean sands in reducing virus concentrations (Poynter and Slade, 1977; Wheeler et al., 1988; Windle- Taylor, 1970). This may indicate that media aging affects virus reduction primarily through inactivation of viruses rather than retention of virus particles in the sand bed.

The general conclusion reached from review of many studies is that virus reduction in SSF is a deep-bed filtration process. Both biological and physical/chemical processes have been cited to explain the treatment of viruses by SSF, but their relative importance is unresolved. Attachment to sand grains and biofilms has been reported to contribute to virus removal and inactivation. However, elimination of viruses by active biological processes, including predation and proteolytic enzyme activity, may be of greater importance (McConnell et al., 1984; Wheeler et al., 1988; Poynter and Slade, 1977). There is indirect evidence that media aging may affect virus reductions primarily through increased inactivation rather than increased sorption or sequestration in the sand bed (McConnell et al., 1984).