EOSi can help petroleum refineries develop compliance strategies in anticipation of regulatory changes expected to greatly reduce the amount of selenium that can be discharged to the environment. We have also helped refineries during their annual or periodic turnaround maintenance. During these times, their biological wastewater treatment systems need BOD supplementation to keep their microbial populations alive and our MicroC® products have been used successfully at various facilities in North America.
Selenium is a naturally-occurring constituent of crude oil that is transferred to wastewater streams throughout the refining process. Some refineries currently have discharge limits for selenium in their NPDES permits. However, regulatory momentum appears to be building for more consistent and stringent selenium standards that will affect petroleum refineries. USEPA is currently updating the national recommended water quality criterion for selenium, and is currently performing a detailed study of the petroleum refining point-source category. It is likely that petroleum refineries will need to meet wastewater discharge limits for selenium in the low parts per billion range within the next few years.
Refineries that remove selenium from wastewater often find it advantageous to segregate high-selenium streams and treat them separately. Iron co-precipitation is an established treatment process, but is expensive due to high chemical costs and large volumes of sludge that has to be managed as a hazardous waste. In addition, iron co-precipitation is highly effective at removing the selenite form of selenium, but not very effective at removing selenate, another form of selenium commonly found in wastewater.
Biological treatment is a less expensive and more effective technology for removing selenium from refinery wastewater. Under anoxic conditions, microbes will reduce the forms of selenium commonly found in wastewater (primarily selenate and selenite) to elemental selenium, which either precipitates or is trapped in the biomass. However, the microbes will only reduce selenium in the absence of preferred electron acceptors such as nitrate, another contaminant present in refinery wastewater. Therefore, anoxic bioreactors for selenium treatment need to be designed to achieve denitrification prior to selenium reduction.
How EOSi can help
MicroC® supplemental carbon sources enable denitrification and selenium reduction by providing a readily-available food supply to support the biomass while it carries out the desired removal processes. MicroC® is also used for BOD augmentation during periods of low wastewater flows or loadings due to turnarounds or other shut-downs, and for establishing an acclimated biomass during startup of new or upgraded biological treatment systems. EOSi can conduct nutrient deficiency analyses and, if necessary, design a custom blend that includes essential microbial nutrients not present in the refinery wastewater.
If refineries need to install new or upgraded biological treatment systems for selenium removal, EOSi offers services such as bench-scale analyses, modeling and simulations, and equipment selection and procurement for the supplemental carbon. EOSi also delivers solutions through our Objective Based Programs such as ongoing technical and application support, remote process monitoring, troubleshooting and optimization programs to our MicroC® customers.