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South Florida wastewater utilities treat and dispose of about one-half billion gallons per day of secondary effluent. Over 400 million gallons per day of the effluent is discharged to the Atlantic Ocean along the southeast coast. One focal point for regulatory concerns has been potential toxicity of ocean outfall discharges as a result of residual chlorine. Regulations address disinfection through monitoring and compliance for maximum fecal coliform levels as well as minimum and maximum levels for total residual chlorine (TRC). Hence, there are opposing concerns for ensuring a minimum TRC level for disinfection while limiting the maximum level of TRC to address potential toxicity to aquatic life. While significant ocean outfall research studies have been conducted in the past, including the Southeast Florida Outfalls Experiment (SEFLOE) I and II, as well as mathematical modeling of outfall discharges, research that is specific to potentially toxic effects of residual chlorine has been limited, particularly relative to acute toxicity. The potential impacts related to these concerns could include significant process modifications for major South Florida utilities (as well as all ocean discharge utilities).

To address the concern with potential toxicity associated with TRC, a number of approaches can be considered as follows:

Dechlorination Facilities

Oceanographic Studies

Mathematical Modeling / Mixing Zones

Chlorine Speciation

Bioassay Tests

TRC Decay Studies

Each of the alternatives named above have been considered and evaluated to some degree in the South Florida community. Based upon the significant potential costs for related research and for the potential additional facilities, a practical approach to address TRC concerns is being pursued in general terms. The approach consists of the use of portions of results from each of the abovenamed alternatives and focuses on TRC decay measurements and analysis.

The results of the analyses performed at these facilities in general indicates that specific effluent minimum and maximum TRC levels can be determined for each WWTP that comply with TRC level at the point of ocean outfall discharge, while still maintaining a minimum level of TRC for a minimum contact period (minimum disinfection requirements and regulatory limit of 0.01 mg/l at the edge of a mixing zone are met).
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Document Type: Research Article

Publication date: 2004-01-01

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