Removal of pathogenic and indicator microorganisms by a full-scale water reclamation facility

Monitoring data and dose-response models were used to evaluate risks associated with viruses and protozoa at a full-scale reclamation facility. Indigenous indicator and pathogenic microorganisms were monitored at different stages of treatment for 1 yr at a full-scale water reclamation facility that produced reclaimed water for residential irrigation. The combination of biological treatment, sand filtration, and chlorination employed at the plant reduced total and fecal coliforms by > 7 log10 and coliphages and enteroviruses by > 5 log10. Protozoa (Cryptosporidium and Giardia spp.) were reduced by > 3 log10. Viruses and protozoa were detected in the reclaimed water at levels between 0.01 and 5 per 100 l. The addition of large numbers of bacteriophage as a tracer permitted an assessment of inactivation and/or removal by the filtration and chlorination units, which were shown to provide 1.6 and 1.5 log10 reductions, respectively. Analysis of the risk associated with exposure to the reclaimed water shows that the probability of infection following a single exposure to 100 ml of the water was between 10−6 (1 in a million) and 10−8 (1 in 100 million) for landscape irrigation. In general, anaerobic treatment of residual solids produced during the wastewater treatment process reduced the levels of indicator and pathogenic microorganisms by approximately 1 log10. The levels of viruses detected in the final residual would not meet acceptable levels for land application for class A residuals according to EPA regulations.