AWWA ACE93179

A classic chicken-egg problem exists for many distribution system managers. On one hand, inadequate chlorine residual anywhere in the system can create the potential for serious public health and regulatory compliance problems. On the other, increasing clearwell residual to adequately meet end-of-distribution-system requirements risks bringing about a myriad of problems associated with chlorine overfeed. By adopting an unconventional strategy which involves identifying areas of chlorine residual deficiency and treating these areas to achieve and maintain specific Redox (Oxidation Reduction Potential or ORP) parameters, adequate chlorine residual can be restored without chlorine overfeed. This is based on Lund's Law, which states that the log decrease in microorganism activity is proportional to the Redox maintained multiplied by the contact time. Unlike the CT values, ET values are independent of the various chlorine species which invariably form in actual water treatment conditions. Through redox control of the rechlorination process, adequate chlorine residual can be maintained. Because Redox is the direct measurement of chlorine work value, it can be effectively used to feed the required amount of chlorine needed to meet the everchanging oxidant demand in the system without overfeed. Since most rechlorination sites are remote, the ability to operate unattended for extend periods makes Redox control particularly well suited for this application.