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Disinfection Of Spill Flows From Combined Sewer Overflows – Does Ultra-Violet Light Work?

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The Problem

Traditionally CSO problems has been dealt with by building sufficient storage tank capacity to store the effluent until it can be returned to the WwTW for treatment. An alternative approach is to treat the storm effluent to a desired standard and discharge it direct to the receiving water.

Pilot Plant Study

The Company has conducted extensive pilot trials at Millom WwTW over the period 2002/03 to assess the suitability of a range of technologies for this application. The main objective of the trials was to identify process technology capable of treating storm effluent in a robust and cost effective manner. The key requirement is to achieve a 5.4 log removal, to meet Bathing Water standards, of bacteria from the crude sewage concentration to the concentration at the compliance point in the coastal receiving water. Based on the observed performance of the clarification pre-treatment stage reported previously, the UV disinfection process will need to be able to deliver at least a 2.5 log reduction. Concerns have been expressed by the Environment Agency in the United Kingdom that residual iron levels post clarification may have a detrimental effect on the ability of UV light to destroy pathogens, resulting in a lower log reduction than predicted by accepted UV models. The mechanisms here could include iron deposition onto the sleeves of the UV bulbs and absorption of the UV light reducing the exposure intensity.

Results

The trials showed that the UV process can deliver 2.5 log reduction of faecal coliforms as a 5 percentile using iron coagulation and clarification as the pre-treatment stage for a Measured Applied Dose of 50 mj/cm2. A 95 percentile residual iron concentration of 1.8mg/l was found in the clarified storm effluent where typical influent levels were 1.5 mg/l as a 95 percentile, showing a 20% increase as a result of chemical dosing. However, the results obtained from the trial show that the UV plant can still obtain the required log removals while using iron coagulants in the pre-treatment stage.

Conclusion

Pre-treatment using iron coagulation and clarification combined with UV disinfection has been shown to be able to deliver the necessary log reduction in micro-organisms to allow treatment and discharge of intermittent storm wastewater overflows from combined sewers to coastal receiving waters.
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Document Type: Research Article

Publication date: 2004-01-01

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