Persistence in the environment and resistance to water and sludge treatments
Once the bacteriophage indicators (coliphages and bacteriophages infecting bacteroides) are released into the environment by defecation, they have no chance of replicating in almost any environment.
Despite this, phages can persist for variable periods of time. Other than persisting, like animal viruses bacteriophages show some tendency to adsorb to surfaces, including solid particles. Bearing this in mind, the question is: what is the fate of bacteriophage indicators outside the gut where they are submitted to certain natural and anthropogenic stressors?
On the one hand they are removed from water by physical processes depending on size, association or not with solid particles, and surface charge. These characteristics lead them to flocculate, sediment or to be retained by filters, comprising soil columns. These events occur both in water treatment plants and in nature. Although in some situations, such as heavy rain, they can also be suspended from sediments or eluted from filters (soil for example) into water, generally bacteriophages tend to be removed from the water fraction by the processes outlined above.
On the other hand, either suspended in water or immobilized in solids, phages are affected by chemical or physical factors that kill (inactivate) them. Naturally occurring or introduced by humans, temperature, radiation and chemical disinfectants are among the main bacteriophage inactivating agents.
With few exceptions (in developed countries) wastewater reaches water bodies after secondary treatment.
Generally speaking, bacteriophages behave similarly to animal viruses, both regarding their persistence in the environment and resistance to treatments with inactivating agents. And, with the exception of spores of sulfite-reducing clostridia, bacteriophages are more resistant to treatment and persistent in the environment than traditional bacterial indicators. Link a indicators in numbers.
6.1 Resistance to treatments
In contrast to what has been indicated in the previous section regarding conventional treatments (secondary treatments), advanced treatments cause the different indicator concentrations to be reduced to a different extent (Lucena and Jofre, 2010; Agulló-Barceló, 2013; Montemayor et al., 2008; Alcalde et al., 2003; Francy et al., 2012; Mocé-Llivina et al., 2003)
Regarding advanced water (UV irradiation, photo-oxidation, chlorination, ozonation, etc.), and sludge treatments (heat, lime, etc.) other than membrane filtration, we find the following order of susceptibilities:
E. coli/FC > enterococci > somatic coliphages (≥ I ≤) F-specific phages >SSRC
The extent of reduction and the differences depend on the doses applied and the number of treatments included in the full treatment process. Consequently, relative concentrations of bacterial indicators and coliphages in tertiary effluents are different from those in raw sewage and secondary effluents.
As for the resistance of coliphages, some differences can be observed regarding susceptibility to treatments:
1. F-specific are clearly more resistant than somatic coliphages to UV rays and slightly more resistant than somatic coliphages to chlorination
2. In contrast, somatic coliphages are more resistant than F-specific to photo-oxidation in water and to thermal treatments and lime treatment of sludges.
For membrane filtration the situation is as follows. In microfiltration, bacteria are retained but phages are not and ultrafiltration retains some coliphages but does not retain 100 % F-specific viruses (Leviviridae) or somatic coliphages (Microviridae) (Ferrer et al., 2015; Martí et al., 2011; Francy et al., 2012).
6.2 Persistence in the environment
Numerous data available on persistence experiments conducted with laboratory-grown model somatic coliphages and F-specific coliphages show that the phage disappearance rate resembles the one for viruses, and that it is lower than the one observed for bacterial indicators.
Some experiments performed “in situ” in both fresh and marine waters (Sinton et al., 1999; Sinton et al., 2002; Duran et al., 2002; Mocé-Llivina et al., 2005) with fecal indicators (both bacteria and phages) diluted in fresh and marine waters in Spain and in New Zeeland show that at temperatures below 20ºC the order of susceptibility is:
E. coli/FC > enterococci > F-specific phages > somatic coliphages > SSRC
And at temperatures greater than 23ºC it is:
E. coli/FC = F-specific phages > enterococci > somatic coliphages > SSRC
At similar temperatures, inactivation in marine waters is always greater than in freshwater.
6.3 Final outcome
Altogether, adsorption to surfaces, resistance to treatments and persistence in environments lead to the situation that in many water environment set-ups, the proportions between bacterial indicators and coliphages clearly favor coliphages, in contrast to what can be observed in raw sewage (Ibarluzea et al., 2007; Lucena et al., 2003; Contreras-Coll, 2002; Jian et al., 2001; Skraber et al., 2002)
Regarding coliphage proportions, in surface waters somatic coliphages are favored, whereas in groundwater relative concentrations are variable (Locas et al., 2007; Lucena et al., 2006; Abbaszadegan et al., 2003), probably depending on the type of soil.
As an example, data obtained in the Northwestern Mediterranean area (Mocé-Llivina et al., 2005; Contreras Coll, 2002; Rubiano et al., 2012) displayed in table 1 demonstrate the ideas set out above.
Table 1. Concentrations of indicators in raw and treated water, as well as in river and seawater
|Indicator (CFU or PFU per 100 ml of water, or 1g of mussel)||Raw sewage||Llobregat river||Barcelona beaches||Tertiary effluents|
|Somatic coliphages||1.0-5.0x106||1.0x 104||5.1x101||5x102|
|F-specific RNA phages||1.0x105-106||2.2x102||< 0.5||1.6x102|
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