Table 1. Most frequent values of fecal indicators in faeces

5.1.2 Raw sewage (untreated municipal wastewater)

Values in raw sewage depend on the concentration of fecal remains in it, which depends mainly on the characteristics of the sewer system, rainfall of the area in the hours previous to sampling, per capita water consumption in the area and the time of day of sample collection. Values of microbial indicators are steadier than those of feces, and what is more important when tested in the same sample, most of the comparative values show the same trend. Fecal coliforms and E. coli are the most abundant with numbers 5–10 times higher than those for somatic coliphages, 5–20 times higher than enterococci, 25-100 times higher than values for SSRC and 20-80 times higher than F-specific RNA bacteriophages (Mandilara et al, 2006; Yahya et al., 2015; Blanch et al., 2006; Lucena et al., 2003; Lucena et al., 2004; Contreras-Coll et al., 2002; Lodder and de Roda Husaman, 2005).

Table 2. Most frequent values of fecal indicators in raw sewage

Important observations regarding the concentrations in raw sewage of the fecal indicators displayed in the tables are that they are neither seasonal nor show appreciable differences in different areas of the planet, regardless of the level of development of the area.

The raw sewage with the microbial indicator load shown in table 2 is released into the environment either untreated or treated by conventional wastewater treatment plants, mostly those based on activated sludge.

5.1.3 Conventional wastewater treatment plant effluents

Table 3. Most frequent values of fecal indicators in conventional wastewater water treatment plant effluents

Effluent water from conventional (activated sludge digestion) wastewater treatment plants all around the world that is released into water bodies still contains important amounts of microbial indicators. The precise concentrations will depend on the performance of the treatment plant, but the relative proportions of the different indicators remain similar to those of raw sewage (Mandilara et al., 2006; Yahya et al., 2015; Lucena et al., 2004; Costan-Longares et al., 2008; Gomila et al., 2008)

Other treatments that remove similar amounts of all indicators, regardless of the extent of removal, are: primary sedimentation, flocculation-aided sedimentation, activated sludge digestion plus precipitation and trickling filters, depth filtration and precipitation/flocculation plus filtration (Harwood et al., 2005; Rose et al., 2005; Nieuwstad et al., 1988; Otosson et al., 2006).

In contrast, advanced (tertiary) treatments remove the various indicators with different efficiencies.

5.1.2 Untreated sludge

Conventional wastewater treatment plants produce huge amounts of sludge that constitutes an important source of microbial indicators. Theoretically, the sludges generated in conventional plants cannot be released untreated into the environment, but in many areas of the world it still is.

The results available in the scientific literature for untreated sludge are not easy to interpret because of the differences when reporting the concentration (wet versus dry matter, different amounts of sample reported, etc.), and a degree of uncertainty regarding the tested sludge characteristics (primary, activates, mixed, dewatered, etc.).

Table 4. Fecal contamination indicators in untreated sludge from municipal wastewater

However, instead of the variability of results reported in the scientific literature, untreated sewage sludge contains great amounts of fecal indicators. In fact, they are concentrated regarding their numbers in raw sewage. As for the proportions between different indicators, they remain similar to those of raw sewage (Mandilara et al., 2006; Guzman et al., 2007).

The effect of the most frequent treatment of sludges, anaerobic mesophilic digestion, has a very modest impact on the reduction in the numbers of indicators without significant differences between them (Mandilara et al., 2006; Guzman et al., 2007). In contrast, further treatments such as pasteurization, composting, etc., bring about important and differential reductions (Guzman et al., 2007; Astals et al., 2012).

5.2 Animal sources

In the previous section we looked at the numbers of the indicators for wastewater originated by human populations.

Regarding animal sources we take into account wastewater from slaughterhouses and slurries generated in farms.

If we take a look at the numbers, we can observe that in the case of animal sources the amount of bacterial and viral indicators appear to be more variable than those from a human population (animal sources often present higher maximums and lower minimums for every indicator). This higher variability may be due to the higher variability of the sources studied.

Table 5. Presence of indicators from slaughterhouses and slurry from farms of different locations. Concentrations in CFU or PFU 100 mL (Data extracted from Blanch et al, 2006)

*Ga17 is a strain infecting Bacteroides (Payan et al., 2005 )

 **Municipal raw sewage

As shown in table 5, the relative abundances of indicators of fecal contamination are very similar for both animal and human contamination sources (Table 2).

In light of the results, the only conclusion we can come to is that there are no significant differences between animal fecal pollution and human fecal pollution in terms of the concentrations of fecal bacteria indicators and coliphages, with the exception of phages infecting strain GA17 of Bacteroides that is a marker of fecal contamination of human origin.

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