Pathogens

    Pathogens, such as bacteria, viruses, protozoa, and worms, are always present in sewage and raw sludge.  Treatments such as anaerobic digestion, mesophilic aerobic digestion, and air drying are termed PSRP's or Processes that Significantly Reduce Pathogens.  To virtually eliminate pathogens treatments called PFRP's or Processes that Further Reduce Pathogens must be used.  An example of a PFRP is thermophilic aerobic digestion.
    The most common bacterial pathogens and the diseases they cause are: Salmonella (Salmonellosis), Campybacter (Gastroenteritis), and Shigella (Dysentery).  Escheria Coli is often used as a measure of the effectiveness of treatment.  E. Coli belongs to the Shigella spp., but it is nonpathogenic and has a very short generation time which allows quick incubation of cultures.
    Viruses in sludge cause diseases ranging from meningitis to infectious hepatitis.  Up to 90% of all the viruses present in raw sewage strongly sorb to solid particles and settle out with the sludge.  These viruses can remain sorbed to the particles throughout treatment and stay active, but to infect they must release from the particles.
    Helminth eggs (intestinal worms) also settle out with sludge in primary clarifier.  These eggs are very resistant to treatment.  They also can infect humans and animals in very small numbers.
 

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