Selenium Removal from Refinery Wastewater
By Avijit Dey, Ph.D., and Rajendra Kulkarni
Selenium can be found in waste streams from copper refining, acid coal mine drainage, coal-fired power plants, and petroleum refining. The source of selenium in the refinery wastewater stream is the stripped sour water (SSW). Selenium enters a refinery in crude oil and is transferred to water along with hydrogen sulfide in cracking and coking processes. Sulfide-laden water, called sour water, is treated in steam stripping columns to remove hydrogen sulfide. Selenium is generally not removed overhead, so it leaves the column in the residual water. This water, also called SSW, is discharged to the process sewers for treatment along with other refinery wastewaters. Although selenium is an essential trace nutrient, it is a poison at concentrations exceeding 4 milligrams per liter (mg/L). To prevent human disease, the U.S. Environmental Protection Agency (EPA) has established a maximum contaminant level (MCL) for selenium in drinking water at 0.05 mg/L (50 parts per billion [ppb]). With the studies showing adverse effects to wildlife such as fish and birds from extremely low levels of bio-accumulated selenium, the EPA has stepped to set new limits of 5 ppb for discharge. None of the conventional treatment technologies can reliably meet this discharge standard. Thus, the technological limitations are the crux of the industryﾒs problem of meeting revised permits limits for selenium.
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