Evaluating Free Chlorine Disinfection of Viruses in Recycled Water: Effects of Free Chlorine CT, Dose, Residual, and Contact Time

Historically, water reclamation plants have used chloramines to disinfect their effluents. Disinfection regulations are often based on chloramines, and require minimum CT values, where“T” is the contact time and “C” is the total chlorine residual at that contact time. For example,the California Department of Public Health's Title 22 regulations for disinfected tertiary recycledwater require a CT of 450 mg-min/L for all chlorine-based disinfection processes, with aminimum modal contact time of 90 minutes; alternatively, a disinfection process mustdemonstrate 5-log inactivation of poliovirus or MS2 coliphage. Free chlorine is a much strongerdisinfectant than chloramines, and the required CT values should be much lower than thoserequired for chloramination. The goal of this work was to investigate the effects of free chlorineCT, dose, residual, and contact time on disinfection of MS2 coliphage and poliovirus.Inactivation increased rapidly with increasing CT values; 4-log virus disinfection was alwaysachieved in nitrified secondary effluent at free chlorine residual CT values above 0.5 mg-min/L.Beyond this CT value, inactivation increased more slowly. Data analysis suggests that at CTvalues greater than 1.5 mg-min/L, the lower 75% confidence level of mean disinfection wasgreater than 5-log inactivation. Disinfection was rapid, with most inactivation occurring withinthe first minute. Free chlorine dose needed to be greater than the chlorine demand; the presenceof a reliably measurable free chlorine residual (>0.13 mg/L in these experiments) could be usedas an indicator of adequate dose. At the CT values tested, free chlorine alone did not achieve thetotal coliform levels required by the Title 22 regulations for disinfected tertiary recycled water.However, the virus results, along with published literature on chloramination of total coliform,suggest that a combination of free chlorine and chloramines could meet Title 22 requirements forboth viruses and total coliform at CT values much lower than 450 mg-min/L.