CITY OF ATLANTA CSO SYSTEM EVALUATION
There are seven permitted combined sewer overflow (CSO) points within the City (19 mi2 drainage area). The City constructed CSO control facilities in the 1980s to 1990s to provide a level of treatment that was approved by the Georgia Department of Natural Resources, Environmental
Protection Division (EPD) at that time. However, the treated discharge does not meet current Federal and State water quality standards, as determined by the presiding judge in a federal lawsuit and the EPD.
To comply with a consent decree, the City must bring its CSOs into compliance by
2007. The consent decree also required several interim steps, including a CSO Control System Evaluation that encompassed an analysis of the combined sewer area and the City's CSO control facilities. Information obtained through this evaluation assisted the City in formulating recommendations
for long-term improvements. This paper presents a synopsis of the 6-volume report that documented the 15-month system evaluation task.
Based on the results, the main pollutants of concern for meeting water quality standards are copper (Cu), zinc (Zn), and bacteria (fecal coliform). These
two metals exceeded the baseline dissolved criteria frequently enough to cause the evaluation of remedial measures to focus on a high level of control for these metals. However, there are also site-specific factors that need to be examined before establishing appropriate metals criteria. Toxicity
testing demonstrated that there was only acute toxicity resulting from residual chlorine, even when metal concentrations were high. The City is improving their existing disinfection facilities, and installing chlorination facilities where there were none previously. Future control improvements
will need to eliminate the toxicity from residual chlorine.
Supplemental storm water runoff sampling from parking lots and parks demonstrated high levels of metals (Cu and Zn) and bacteria too. While concentrations were not quite as high, the frequency of exceedences were similar to that
of CSO; thus, there are similar water quality compliance issues with storm water runoff even if the sewers were fully separated.
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