EBR Selenium Removal: Example Projects
Various comparisons have been made between the Electro-Biochemical Reactor (EBR) systems and Conventional Bioreactors (CBR). These comparisons span in-house bench, pilot, and full-scale implementations and testing comparisons conducted through impartial third parties against most major water treatment technologies. Overall, the EBR technology has shown better efficiencies and kinetics and was the only technology to meet discharge criteria in a third party comparison of eight water treatment technologies. Presented table gives examples of past EBR projects, corresponding hydraulic retention times (HRT), average water temperatures, and corresponding selenium and nitrate removal efficiencies.
Coal Mining: EBR Selenium Removal Case Studies
|
Four wastewaters, obtained from four different British Columbia (B.C.) coal mine drainages (Waters A through D), were treated using bench-scale EBRs. Total Se concentrations of the waters ranged from 35 to 531 μg/L. Discharge targets for the mine operations range from 5-10 μg/L.
Every water tested produced effluent total Se concentrations well below the required discharge target, with average values of 1.2-1.4 μg/L. The figure to the right shows total vs. dissolved selenium concentrations during tests on Water D (average effluent concentration of 1.0 μg/L). Both total and dissolved Se follow a similar trend, indicating that particulate selenium (elemental Se associated on/in the microbial cells or precipitated within the reactor microbial support matrix materials) is trapped within the bioreactor matrix. Total suspended solids (TSS) generated during both bench and pilot scale tests ranged between <3 to 18 mg/L. In this regard, the EBR system removes selenium without the need for extensive post-treatment for the removal and management of biosolids that contain precipitated or microbial cell-attached Se.
|
|
|
For pilot scale testing, the tested coal mining seepage waters contained on average 105 μg/L total selenium with site discharge target of 10 μg/L. Selenium was removed by the EBR system to an average effluent value of 0.5 μg/L for both total and dissolved species. Selenium concentration was monitored throughout the EBR system to establish the required HRT; the discharge target of 10 μg/L was consistently exceeded in the first sampling port equivalent to a 6-hr HRT. Data extrapolation indicates that a 3- to 4-hour EBR HRT would reliably meet the site's Se discharge criteria for this effluent.
|
|
Hardrock Mining: EBR Se Removal Case Study
A confidential, now-abandoned gold mine site performed small-scale mercury-based gold extraction and later cyanide leaching on a large-scale open pit. After closure, the mining company performed reclamation activities. The chemical precipitation water treatment plant, constructed in 1995 for the sole purpose of treating the pit waters during dewatering and slated to be decommissioned shortly thereafter, has been used to treat the seepage waters ever since. The treatment targets metals that precipitate at elevated pH. Other metals and inorganic species present as oxyanions, such as selenate/selenite or nitrate/nitrite, are not directly targeted.
|
Inotec was contracted for a selenium removal assessment from the mine waters. EBR tests show a consistent selenium removal trend over the 150-day test period. Total selenium in the effluent was well below the 5-μg/L discharge goal within the first-stage of the EBR system throughout the entire testing period (4-hr HRT). Selenium removal kinetics indicates that the selenium discharge goal of 5 μg/L was met with hydraulic retention times (HRT) of 2 hours throughout the testing. Stress testing did not result in degraded performance. There were no treatment issues identified that would influence the pilot- or full-scale design.
|
|
Cold Temperature EBR Selenium Removal Case Study
|
Several pilot tests on mining and FGD waters have been conducted with data collection until system or influent lines freeze up. All test data show very similar trends for selenium removal with temperature; effective selenium removal at low temperatures. The figure to the right shows data from an on-site pilot test conducted at a British Columbia coal mine until system freeze up. Average selenium influent values were about 110 μg/L; influent nitrate-N levels averaged 50 mg/L and were removed to below discharge criteria. As observed with EBR testing, and in bench and pilot-scale comparisons with conventional bioreactors, the added ‘free’ electrons in the EBR systems allow the microbes to perform contaminant transformations better than conventional bioreactors at lower temperatures.
|
|
RO Concentrate: EBR Selenium Removal Case Study
A confidential mine site has been subject to gold and copper mining dating back to the Roman era. Current site water management includes ACTIFLO arsenic (As) removal and clarification, followed by reverse osmosis (RO) membrane treatment. The company contracted with Inotec to evaluate the EBR technology’s potential to treat selenium from the mine waters and to meet the strict discharge limit of 1 μg/L.
|
The permeate from the RO treatment meets discharge regulations, when mixed with Se-free water from the mine’s dewatering system. However, the reject (brine) stream from the RO treatment contains a spectrum of contaminants and cannot be discharged to the environment without further treatment. Because of a complex nature of RO brine water chemistry, treatment of these streams is often difficult and costly.
During bench-scale tests, selenium was removed to the discharge limit within 18 to 24 hours. Average influent total selenium concentrations were 216 µg/L, while the EBR treatment generated on average 0.79 µg/L, after a 7-day system startup period. In addition to removing selenium to below the discharge goal, the bench-scale EBR system also removed nitrate-N from an average of 12.1 mg/L to below the detection limit of 0.06 mg/L. Besides selenium, other metals, such as arsenic, copper, nickel, and zinc, were present above the discharge criteria in the RO brine and all were reduced with the EBR treatment to well below the compliance levels, in a consistent manner throughout testing regimes. |
|
Flotation-Influenced Waters: EBR Se Removal Case Study
An underground mining operation producing copper, lead, zinc, silver and gold uses three floatation circuits for metal recovery. Suppressants (e.g., cyanides) and collectors (e.g., xanthates) are added at each step to recover only the desired products. Water and solids from the floatation circuit are discharged to a tailing pond, where solids are allowed to settle and water is re-circulated to the circuits. Constant water reuse practice without treatment has resulted in a steady degradation in water quality, which cannot be discharged to the environment and has significantly reduced floatation recoveries. The concentration of metals (i.e., selenium, antimony, cadmium, copper, lead, molybdenum, silver, and zinc) and chemical reagents, such as collectors and suppressants, is increasing over time. High concentrations of complex organic compounds and other contaminants (over 700 mg/L COD; TDS of over 4,000 mg/L; selenium 2.0-3.5 mg/L) renders this wastewater difficult to treat using conventional biological, chemical and/or physical processes.
|
Selenium was the targeted contaminant in flotation-influenced waters containing a suite of other metals and inorganics that exceeded the site discharge standards. Chemical treatment methods did not meet the selenium discharge goal of 0.02 mg/L. Side-by-side comparisons of the EBR and a fluidized bed bioreactor technology showed that the EBR was the only method able to meet discharge criteria.
After a 7-day system start-up, and throughout the entire duration of the pilot trial, the EBR systems effluent contained an average of 0.002 mg/L total selenium, representing a significant reduction from average influent selenium concentrations of 2.71 mg/L and meeting the discharge limit of 0.02 mg/L. |
|
FGD Waters: EBR Selenium Removal Case Studies
|
The EBR technology was a part of a three-year testing comparison program conducted through impartial third parties against most major water treatment technologies. The EBR technology demonstrated better efficiencies and kinetics (shorter retention times).
Selenium was the targeted contaminant and was removed to below discharge criteria (Se discharge criteria varied from 7.5 to 10 mg/L at the different test sites). The figure to the right shows results from one coal-fired power plant test; average influent total selenium of 350 µg/L was removed by a two-stage EBR system to an average of 4.3 µg/L. The EBR technology is best suited for FGD waters with chloride levels below ~8,000 mg/L. If total hardness of the FGD water exceeds 10,000 mg/L, as CaCO3, a water softening pretreatment step is needed. |
|
EBR Case Studies
|
It's not only Selenium!The EBR technology is well suited for removal of variety of oxyanions, such as nitrates, sulfates, and dissolved metals.
|
|
