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The Uptown Park Combined Sewer Overflow (CSO) facility in Columbus, Georgia has served as a full-scale national demonstration project to examine the performance of various wet weather treatment technologies for pollutant removal and disinfection. The demonstration facility was sponsored by a Congressional appropriation and EPA grant. Several chemical disinfectants and UV disinfection have been examined at this facility to quantify the performance, operations and costs of treating CSO's. A subsequent collaborative project between the Columbus Water Works, the Georgia Institute of Technology and BioLab, Inc. is employing the demonstration facility to examine the capabilities of bromine disinfection.

The difficulties of CSO disinfection lie in the variable quality conditions typical of a CSO. Variations in ammonia and other immediate chemical demands, pH and temperature pose performance and operational constraints on the chemical disinfection of CSO's. Findings show that dose can be normalized by quality parameters such as ammonia or COD and correlated to effluent bacteria. Findings suggest that in order to achieve performance objectives of low bacteria counts, dose would be at breakpoint levels for chlorine. Chlorine dioxide and peracetic acid required similar dose as chlorine suggesting that disinfection performance is primarily controlled by the chemical demand of the CSO.

Automatic disinfectant feed control can be implemented to replicate the temporal chemical demand of the typical CSO in order to insure proper bacteria levels throughout the event. A control algorithm can be developed from bench-scale demand tests using flow and time and adjusted from full-scale operational experience. CSO temporal quality can vary from the norm with frequency of occurrence and climatic spatial conditions in the watershed. This requires the control algorithm to envelope such variations. The result will provide excess chlorine residual at times. Dechlorination will likely be required depending upon the sensitivity of the receiving water. Chlorine residuals may also be minimized using flow through feedback control from ORP measurement.

Disinfection capabilities of BCDMH (1-bromo, 3-chloro, 5,5-dimethylhydantoin), in a suspension form manufactured by BioLab, Inc were examined on a bench-scale level along side of other chemical disinfectants to quantify disinfection performance at different CSO strengths. BCDMH dosing system set-up, tests and calibrations were performed. Bench scale performance and the long-term quality data were used to define an algorithm for controlling the facility for optimum feed of the chemical disinfectants to match demands.

BCDMH has several advantages over other chemicals for CSO disinfection. This chemical disassociates into greater proportions of the active hypohalous acid at CSO pH levels (typically between 6.5 and 7.5). When combined with ammonia, BCDMH appears to be 3 times more effective oxidant and germicide than the other disinfectants tested and has the advantage of being less stable and quickly decomposes leaving little or no residual.

The EPA CSO Policy requires cities with these systems to examine controls such that discharges meet water quality standards and associated wasteload allocations. Most of these systems at a minimum will need to provide disinfection to meet bacteria standards. Urban stormwater is an order of magnitude lower than CSO on the bacteria scale but is still an order of magnitude or more higher than water quality criteria. The enormity of this problem guided by the regulatory framework with the nations objectives to make all waters fishable and swimable will demand a safe and effective disinfection process.

Bromine with its abilities to be more effective under the quality variations imposed by CSO and stormwater runoff and its decomposable nature to rapidly dissipate may play an important role in the solution to wet weather disinfection and achieve other pollutant load reductions.
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Document Type: Research Article

Publication date: 2002-01-01

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