A CHEMICAL FREE STRATEGY FOR STAGE 2 DISINFECTION BYPRODUCT RULE COMPLIANCE

摘要

When the Mashantucket Pequot Tribal Nation set out to establish a strategy for compliance with the Stage 2rnDisinfectants and Disinfection Byproduct Rule for its water system in southeastern Connecticut, it soughtrnalternatives that would avoid the introduction of any new chemicals and specifically explore chemical freernalternatives first. The primary water supply for the Mashantucket Pequot Tribal Nation is a groundwaterrnwellfield. The water quality characteristics of this wellfield have changed since it was first placed in service inrn1993, with TOC levels increasing gradually to the current level of 2.5 – 4 mg/l with significant levels of iron andrnmanganese. The existing treatment system, which includes pH adjustment with potassium hydroxide, aeration,rnsettling, and membrane filtration is removing very little of the natural organic matter. Consequently, the finishedrnwater has a high chlorine demand, elevated levels of disinfection byproducts, and localized areas with lowrnchlorine residuals.rnThis presentation will review the alternatives considered, the bench scale testing results used in the evaluation andrnthe plans for the full scale piloting scheduled to be completed using the recommended technology. Twornalternatives were evaluated at the centralized water treatment plant. The first alternative evaluated the use of prechlorinationrnand aeration within the treatment plant process to pre-form trihalomethanes with subsequentrnvolatilization before distribution. An analysis of the plant operating data and supplemental testing to trace therndevelopment rate of disinfection byproduct formation though the plant processes concluded that there is notrnenough hydraulic retention time in the plant to form the disinfection byproducts prior to aeration. The secondrnoption involved the addition of a granular activated carbon contact chamber after membrane filtration and prior torndisinfection. This alternative although technically feasible is anticipated to be high in both capital and operatingrncost and operationally intensive.rnA third alternative remote from the centralized water treatment plant was considered. The study concluded thatrnmost of the water passes through two primary water storage tanks and that trihalomethane concentrations in therndistribution system could be reduced to the target level of 64 ppb using an innovative aeration system installed inrnthe water storage tanks. The installation of localized treatment at the two primary water storage tanks isrnprojected to achieve disinfection byproduct goals throughout most, but not all, of the water distribution system. Arnsmall remote residential area was still projected to have high disinfection byproduct levels and low chlorinernresidual. A small granular activated carbon contactor and rechlorination is proposed for this area.