Reduction of bacterial indicators and bacteriophages infecting faecal bacteria in primary and secondary wastewater treatments.

AIMS: To compare the suitability of various bacterial and viral indicators to assess the removal of faecal micro-organisms by primary and secondary wastewater treatment processes. METHODS AND
RESULTS: The numbers of several bacterial indicators [faecal coliforms (FC), enterococci (ENT) and sulphite-reducing clostridia (SRC)] and bacteriophages (somatic coliphages, F-specific RNA phages and bacteriophages infecting Bacteroides fragilis strain RYC2056) were determined in incoming raw sewage and effluents from various primary and secondary wastewater treatment processes in several geographical areas. Reductions in the numbers of indicators were calculated as log10 reductions. Processes based on removal and mild disinfection, showed no significant differences in the elimination of any of the indicators tested or between geographical areas. In contrast, treatment processes that include strong microbial inactivation, such as lime-aided flocculation and lagooning, showed significant differences between the log10 reductions of the various micro-organisms studied, FC showing the highest reduction and spores of SRC and phages infecting B. fragilis the lowest.
CONCLUSIONS: The microbial elimination performance of treatment processes based principally on removal and mild disinfection can be evaluated with a single indicator. In contrast, processes with additional disinfecting capabilities require more than one indicator for accurate evaluation of the treatment; bacteriophages are good candidates for use as second indicators. SIGNIFICANCE AND IMPACT OF THE STUDY: Bacteriophages provide additional information for the evaluation of microbial elimination in some treatment plants. The easy, fast and cheap methods available for phage determination are feasible both in industrialized and developing countries.