The Past, Present and Future for Selenium Treatment

14th Annual BC MEND WorkshopNovember 29, 2007Kevin W. Conroy, P.E.

Where is it found?

Paint, pigments, dye formulatingElectronicsGlass manufacturingInsecticide productionPulp and paperAsh piles, FGD blowdown,Coal/oil combustion Agricultural waterPetroleum processing Mining operations

Why is it a problem?

Aquatic life hazard1983 – Kesterson National Wildlife Refuge - CaliforniaBirth defects/death of birds, small animals, fish

Selenium cycle not well understood

Uncertainty on bioavailabilityEven if bioavailable – what is toxic?

Often times low concentration, high volume - makes treatment expensive

How is it regulated?

5 µg/L Freshwater aquatic life 50 µg/L Primary DWS MCLU.S. Fish and Wildlife Service has recommended 2 µg/L to protect fish, waterfowl and endangered aquatic species

Chemistry

3 primary oxidation states-2 selenide+4 selenite (HSeO3

- and SeO3-)

+6 selenate (SeO4-2)

Chemical equilibrium principals don’t really applyDriven by

Redox conditionsBiological activitySorption processes

Past and Present - EPA BAT

Ferric coagulation/filtrationTypically pH <7Coprecipitation effectEffective removal requires reduction of selenate to seleniteProblem if arsenic present

Lime softeningReverse osmosis

Non-preferential processPretreatment due to other typical mine water issues may be required

Past and Present - EPA BAT

ElectrodialysisAlumina

Selenite adsorbed at pH range of 3 – 8Silica can interfere at pH >4Selenate adsorption is poor

Ion exchangeNeed oxidized divalent selenateCompeting ion effects can hinder effectivenessSome specialty resins tested

Past and Present - EPA BDAT

Ferrihydrite precipitation with concurrent adsorption of selenium on the ferrihydrite surfaceFor adsorption – need ferric ion (Fe+3) presentMost effective removal at pH 4-6Somewhat effective up to pH 8Phosphate, silicate, arsenic, carbonate can interfere

Past and Present – EPA/DOE MWTP

Selenium Treatment/Removal Alternatives DemonstrationReport issued in 2001Three technologies tested in field

Ferrihydrite Adsorption (baseline)Catalyzed CementationBiological Reduction

One technology tested on bench scale

Enzymatic Reduction

Past and Present – EPA/DOE MWTP

Objective – treat to <50 µg/LWork done in 1999-2001Basis – KUCC Garfield Wetlands-Kessler Springs site<50 to >10,000 µg/L Se 95%+ selenateTDS 1,000 - 5,000 mg/L

Past and Present – EPA/DOE MWTP

FerrihydriteDid not work on a consistent basisVarious iron types, concentrations and ratios usedCould achieve objective but at prohibitive reagent consumptionQuestions on TCLP stability

Past and Present – EPA/DOE MWTP

Catalyzed cementationDeveloped for arsenic, selenium, thallium removalRemoves metals by cementation on the surface of iron particlesBelieved to work on both selenite and selenateProprietary catalysts usedBench test work had shown favorable resultsDid not work on a consistent basis

Past and Present – EPA/DOE MWTP

Biological Reduction (BSeR™)Used anaerobic solids bed reactors Selenium reduced to elemental selenium by biofilms and proprietary microorganismsMolasses used as carbon sourceWas able to consistently meet objectiveOver 70% of samples less than detection (2 µg/L)

Past and Present – EPA/DOE MWTP

Economics

BDAT Cementation BSeR™Capital $1.0M $1.1M $0.6MO&M $2.1M $1.2M $0.14MNPV $17M $9.5M $1.1M$/1,000 gal $13.90 $8.17 $1.32

Based on 300 gpm plant, 2 mg/L selenium2001 dollars

Past and Present - Nanofiltration

Some test work by USGS in 1996Agricultural drainage

Selenate removal better than selenite

Not surprising – designed for divalent not monovalent ions

95+ % removal at Se<1,000 µg/LMembrane scaling is an issue in high SO4

-2 water

Problems With the Past and Present

Non-selective processesLarge amounts of secondary wasteMultiple reagentsOkay for bulk selenium removal with other metalsCan’t consistently get to <10 µg/L

Biological Reduction – General

Studied for decadesMicrobes degrade/transform contaminant because

Energy sourceDetoxification mechanismResembles another ionCombination of the above

Anaerobic reactorsReduction to elemental seleniumNitrate/sulfate interference?

90%+ removal reported

Biological Reduction – Ponds/Wetlands

Panoche Drainage District – San Joaquin Valley

74-1,400 µg/L due to Se rich soilPrimarily selenate formNumerous bioremediation studiesAlgal-Bacterial Selenium Removal (ASBR)Anoxic ponds – reduce selenate to selenite to elemental and settleGenerally about 80% maximum removal

Biological Reduction – Ponds/Wetlands

Additional California work - 2005Constructed wetlands9 plant species tested63% - 71% removal~20 µg/L influent, 3 – 6 µg/L effluent

Problem with ponds/wetlandsHRT’s in days

Biological Reduction – Advances

More selective microbes isolatedAdvances in fixed film/biofilm mediaBetter understanding of operating conditionsResult – retention times have been reduced from days to hours for active systemsAdvances also made in passive technology

Biological Reduction – Advances

ABMet®Offered by GE Water and Process TechnologiesSame as BSeR™ processSeveral FGD projects at commercial scale3,000 – 5,000 µg/L seleniumUp to 20,000 mg/L chloride98% – 99% removal projectedEffluent as low as 10 µg/L

Biological Reduction – Advances

Passive Selenium Reducing BioreactorTested on Colorado Western SlopeBureau of Reclamation Science and Technology ProgramInfluent typically ~20 µg/LSpike to 70 µg/L1,000-2,000 mg/L SO4

-2 background

Biological Reduction – Advances

Passive Selenium Reducing BioreactorFour reactors with different substrate compositionsOrganic substrate composed of wood chips, hay, manureZVI incorporated; no advantage12 hour detention time adequate, optimization possibleOperated for 20 weeksEffluent typically <2 µg/LUp to 98% removal

Biological Reduction – Advances

Active Anaerobic Bioreactor SystemWaste rock seepage250 gpm capacityFixed film bioreactor with high surface area mediaMolasses used as carbon sourcePhosphate/urea addedReverse osmosis system used during high flow – 700 gpmBioreactor feed switched to RO brine

Biological Reduction – Advances

Active Anaerobic Bioreactor System18 hour retention timeLow flow (raw seepage)

Se ~30 µg/LSO4

-2 ~6,000 mg/LHigh flow (RO brine)

Se ~70 µg/LSO4

-2 ~13,500 mg/L

Biological Reduction – Advances

Creek

Feed Pond

Low Flow Operations

High Flow Operations

Pre-Filtration

HeatRecovery

AnaerobicBioreactor

AnaerobicBioreactor

AerobicBioreactor

Sewer

Pre-Filtration

HeatRecovery

ReverseOsmosis

AnaerobicBioreactor

AnaerobicBioreactor

AerobicBioreactor

Sewer

Permeate

Permeate

Brine

Brine

Biological Reduction – Advances

Biological Reduction – Advances

Active Anaerobic Bioreactor SystemEffluent goal is 10 µg/LPilot plant operated for 7 monthsHigh sulfide was an issue

Discharge qualitySolids fouling

Full scale system designed and constructedOperating for about 18 months Compliant water being produced

Future?

Selenium with other metalsConventional lime-iron based processes for bulk removalBiological polishing process

Low concentration seleniumBiological reductionBoth active and passive options

Combinations with other processes – i.e. membranesReduce costs to <$5.00/1,000 gal? Maybe down to $1.00/1,000 gal?

Biological Reduction – Application Keys

Nutrients are vital in establishing microbial populationUnderstanding of site chemistry and environmental interactionsAnalytical methods

Can get discrepancies in total and dissolvedPossibly related to digestionVolatile selenide

Need for aerobic post-treatmentHigh COD, P, N

Conclusions

There is no silver bullet for selenium removal to low levelsAll sites must be evaluated individuallyPaper designs risky –development work always recommendedSelenium can be removed to <10 µg/LCost of selenium reduction to low levels is decreasing

Questions?