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A small (<50,000 bbl/day) petroleum refinery was marginally complying with the 200 ppb total cyanide limit in its NPDES permit. However, the refinery was facing a NPDES permit renewal, in which the new total cyanide limit was expected to be 1.0 ppb, the water quality standard (WQS) for cyanide in marine waters. To address the looming compliance issue, the refinery took a multi-faceted approach, including consideration of source treatment, supplemental stripped sour water treatment, end-of-pipe wastewater treatment optimization, and discharge permit modification. This paper addresses the strategies that were used to develop a modified NPDES permit with favorable discharge limits.

Although the refinery was implementing process changes to substantially reduce cyanide levels in the plant effluent, achieving the 1 ppb standard was considered to be difficult and uncertain. Since cyanide was not believed to be present in the receiving waterbody, obtaining a mixing zone for cyanide was considered to be a viable strategy for effectively increasing the end-of-pipe discharge limit. However, using EPA analytical methods, the method detection limit for cyanide in marine waters was higher than the water quality standard. Even though there was no indication that cyanide was present in the receiving water, the responsible environmental agency's policy was to assume that the concentration in the receiving stream was at the detection limit, i.e., above the WQS. Under this scenario, no mixing zone would be available.

To address this detection limit issue, ENSR developed an analytical method for total cyanide in seawater with a method detection limit of 0.1 ppb. Using this method, cyanide concentration in the receiving water was shown to be below the detection limit. As a result, it was demonstrated that dilution in the receiving water was viable, and an application for a mixing zone was submitted. Using this approach, a discharge limitation of 48 ppb (average) and 97 ppb (maximum) was requested. The agency approved the mixing zone, and a NPDES permit with a maximum limit of 72 ppb was issued
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Document Type: Research Article

Publication date: 2002-01-01

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