The membrane bioreactor (MBR) process is typically designed and operated to produce a fully nitrified, high quality effluent (permeate) that makes it possible to use free chlorine for disinfection. In California, regulations (Title 22) require a minimum CT value of 450 mg-min/L
or 5-log virus inactivation when chlorine is used to produce “disinfected tertiary effluent.” In this study, bench-scale experiments were conducted to determine the disinfection requirements for MBR permeate when free chlorine is used. Experiments were conducted, under normal and
simulated membrane failure conditions, to determine (1) the inactivation of two surrogate viruses, MS2 coliphage and poliovirus; (2) total coliform inactivation; and (3) the formation of trihalomethanes (THMs). The results from the study show that free chlorine is very effective for inactivation
of MS2 and poliovirus at very low CT values under both normal and simulated membrane failure conditions. A minimum 5-log inactivation was achieved for both viruses at CT values as low as less than 1 mg-min/L. This suggests that when free chlorine is used to disinfect MBR permeate, 5-log
virus inactivation can be achieved at a CT value (based on free chlorine residual) that is much lower than the currently prescribed 450 mg-min/L, even when a fraction of the membranes are broken. In contrast to free chlorine, chloramines are relatively ineffective against the same microorganisms.
Free chlorine is also effective for controlling bacteria breakthrough in case of a membrane failure. This is evidenced by the increasing degree of total coliform inactivation, under simulated membrane failure conditions, with increasing CT values. Increasing CT values, however, result in more
THM formation. Although total THM levels did not exceeded the U.S. EPA and California Department of Health Services drinking water standard of 80 μg/L under the conditions tested in this study, the results suggest that significant THM formation can be avoided by adopting a CT value
that is protective of human health, yet is much lower than the current Title 22 specification when free chlorine is used for MBR permeate disinfection.
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