Decay rate of virus and faecal indicator bacteria (FIB) in seawater and the concentration of FIBs in different wastewater systems

Information about the concentration, growth and decay rates of enteric microorganisms in different wastewater systems and water bodies is crucial for quantitative microbial risk assessment (QMRA). In general, there is little information about this and in particular about the decay rates of faecal bacteria and viruses at different depths of the marine environment where many wastewater treatment plants discharge their effluent. This study was designed to investigate the concentration of faecal indicator bacteria (FIB) in different types of wastewater, the potential for growth of FIB in greywater, and the decay rates of FIB and pathogenic viruses in seawater samples from different depths. The intent of this work was to mimic summer water quality. Average concentrations of total coliforms (TC), Escherichia coli (E. coli) and intestinal enterococci (IE) (log10 per 100 ml) were 7.2, 6.7 and 6.1 in blackwater; 6.7, 6.1 and 3.8 in greywater; and 6.8, 6.6 and 5.2 in municipal wastewater, respectively. The numbers of TC and E. coli increased when greywater was stored for 24 h at 20 °C but decreased after 96 h. No growth of IE was observed in the greywater. The decay rate experiment showed that viruses and FIB were inactivated relatively rapidly at 20 °C in seawater collected from 1 m depth, while a slow inactivation was observed at 4 °C in seawater collected from 60 m depth, with a significant difference in decay rates (p < .05). Our results suggest that temperature and biological activity may be important factors for the self-purification capacity when wastewater containing faecal bacteria and viruses is discharged to coastal water, and these varying decay rates could be important in risk assessments.