By Kar Munirathinam, CH2M HILL Principal Technologist

Dr. Munirathinam will be presenting the paper, “Selenium Treatment of Mine Water Effluent in a Fluidized Bed Reactor” on Monday, October 17, 2:00pm-2:30 pm, in room 402B as part of Technical session 004 during WEFTEC. See a full list of CH2M HILL’s WEFTEC events and participation.  

Due to its ubiquity and associated aquatic toxicity, even at low concentrations, selenium (Se) treatment in wastewater has become a major challenge for a wide range of industries, including oil and gas, agriculture and mining. Mining operations are a major anthropogenic source of Se release in the environment. Since Se is naturally present in most soils, mining operations, including excavation, can result in the release of Se in waste rocks and tailings. Surface runoff due to precipitation, groundwater intrusion, or mine operations transfers the Se from rock to the water matrix and introduces it to the aquatic environment. As a result, surface coal mines, especially in the eastern U.S., are coming under increased scrutiny to control release of Se into aquatic streams.

CH2M HILL conducted a study of activities undertaken at a confidential Appalachian surface coal mine to address some of the issues associated with meeting compliance for Se. The facility treats both the surface runoff water and groundwater contacting the overburden fill areas (e.g., valley fills) discharged from these operations to sedimentation ponds. Effluent from the treatment systems are discharged to a surface water of the state as permitted by National Pollutant Discharge Elimination System (NPDES) permits issued by the state Department of Environmental Protection (DEP). The facility has exceeded the total recoverable Se limits included in its permit, and is evaluating options to ensure future compliance with these limits.

We conducted a conceptual treatment alternatives evaluation to compare Se treatment alternatives for the site’s NPDES outfalls. The technologies evaluated were: chemical treatment – zero-valent iron; physical treatment – ion exchange, reverse osmosis, evaporator, and crystallizer; and biological treatment – biological fluidized bed reactor (FBR), subsurface flow wetlands, and ABMet®. Technologies were compared on the basis of treatment effectiveness, capital and O&M (operations and maintenance) costs, footprint, and availability. Our evaluation identified the FBR to be the most cost-effective technology for Se removal from the mine water. The primary advantage of the FBR is the low hydraulic retention time resulting in low capital cost and small footprint. FBR technology is well proven for nitrate and perchlorate removal; however it was not specifically demonstrated for Se removal. For this reason, we recommended that FBR pilot testing be conducted to confirm Se treatment at this mine site.

The FBR pilot study demonstrated consistent filtered Se reduction to less than the required limit of 4.7 µg/L for 6 weeks, from start of steady-state operations to the end of the study. However, the average total Se concentration was 11.6 µg/L, which is above the discharge limit, making tertiary treatment of the FBR effluent necessary to remove the Se-containing suspended solids to achieve compliance for total Se. The FBR pilot equipment operated for about 3 months without major process or mechanical issues, and the design parameters for sizing a full-scale system were also developed from the pilot study results. The design parameters include hydraulic retention time, electron donor dosage, nutrient requirements, and sludge production. Full-scale design also would require influent heating to maintain the minimum reactor temperature in the range of 50-55 °F during winter months and a tertiary treatment step to reduce the residual excess soluble effluent biochemical oxygen demand to acceptable discharge levels.

Overall, the FBR pilot study for Se treatment of mine water effluent was successful, and the facility is now implementing a full-scale system to ensure compliance with all NPDES limits. I look forward to sharing the details of the pilot study with those of you who are attending WEFTEC.

Dr. Munirathinam has more than 25 years of experience in treatability studies, process designs, and coordination of detailed design of biological and physical/chemical systems for treating waters and wastewaters laden with conventional pollutants and priority pollutants. Before joining CH2M HILL, Dr. Munirathinam served as a technical director for a water treatment solutions firm, with responsibility for industrial and municipal wastewater treatability studies, conceptual design of wastewater treatment projects, coordination of detailed design, industrial and municipal wastewater treatment plant operations consulting, plant capacity evaluations, and existing treatment plant modifications/upgrades.