Gravel Bed Reactors (GBR™): A water treatment technology for mining and industrial applications Silvia Mancini, Ph.D., P.Geo. Evan Cox, M.Sc., Len deVlaming, P.Eng.
Gravel Bed Reactors (GBR™): A water treatment technology for mining and industrial applications
Silvia Mancini, Ph.D., P.Geo.Evan Cox, M.Sc., Len deVlaming, P.Eng.
Outline
Water treatment challenges
Gravel Bed Reactors (GBR™)
Treatment of metals and inorganics
Case Studies
Technical, design and regulatory considerations
Water Treatment Challenges
• Metals and inorganics (e.g. selenium and nitrate)
• Highly engineered approaches are a challenge for mining and industrial applications
• Gravel Bed Reactors (GBRs) offer a middle ground and lower cost
Highly engineered Higher cost
Passive systems Lower cost
Water treatment plant
GBR
Engineered wetland
What is a GBR and How Does it Work?
• An engineered system that treats a variety of water quality issues
• Fixed-film bioreactor containing uniform, inexpensive media
• Simple & relatively low cost physical structure; can be in-ground or on-ground (bermed)
• Bioreactors use organic substrate for biofilm growth and reduction of constituents
Tracer, Other AmendmentsFeed:
Impacted Surface Water
Liner
Crushed waste rock
Insulating Layer
Baffles
Monitoring Well
Monitoring Well
Monitoring Well Effluent
Well
Aeration System
Sampling Port
Pump Pump
Not to scale
Sampling Port
Discharge
Electron Donor Solution
Main Components & Design
Constituents Treated
• Metals - As, Cd, Co, Cr, Cu, Pb, U, Zn• Non-metals and metalloids (i.e. Se)• Nitrate, nitrite• Phosphate• Sulfate• Perchlorate, chlorate• Explosives nitrogen residues• Acidity
ORP
O2
NO3‐/NO2
‐
Se (VI)/Se(IV)Mn (IV)Fe (III)U(VI)
SO42‐
TOC/CO2
Oxic
Suboxic
Sulfidic/Methanogenic
Metal ReductionDenitrification
Organic CarbonE.g. CH2OH
Bacteria
Organic CarbonE.g. CH2OH
CO2
Soluble MetalE.g. Se(VI), Cr(VI),
U(VI) Insoluble MetalsE.g. Se(0), Cr(III), U(IV)
e‐
Bacteria
Organic CarbonE.g. CH2OH
CO2
NitrateNitrogen gas
CO2
e‐
Nitrite
e‐Bacteria
Treatment Chemistry
Selenium Transport and Immobilization
Bacterial Dissimilatory ReductionErwinia, Pseudomonas, Arthrobacter
Selenite(Se+4)
Elemental Selenium
(Se0) Precipitation& immobilization
Selenate(Se+6)
• Oxidation state of selenium in surface and groundwater
• Bioavailable and toxic to aquatic environment
Case Studies
GBR Examples
California – Perchlorate
Nevada – Perchlorate, Chlorate, NO3‐ Urban Stream, California – Se & NO3
‐
Coal Mine, West Virginia – Se & NO3‐
Case Study 1 – Urban Stream (California)
Characteristics Summary
Dimensions (m) 60 x 12 x 3
Total Volume of Reactor Cell
2,300 m3
Media ¾ inch gravel
Design Flow 885 m3/day
Influent Selenium
30 to 50 ppb
Influent Nitrate 20 to 50 ppm
GBR Design & Construction
GBR Performance and Testing
• Flow rate ranging from 885 to 1,500 m3/d• Monitored at 7 sampling ports
Flow Direction
GBR Performance and Testing
Operational phases:• start-up; • steady-state operation
and performance evaluation;
• upset testing; and• commercial operation
(8 years)
Summary
Successful treatment of ~40 ppb selenium to <5 ppb (i.e. freshwater water quality criterion)System operated effectively for ~8 yearsRequired a small footprint (2 acres) compared to alternativesSubsurface design to allow for property redevelopment on land above
Case Study 2 - Coal Mine (West Virginia)
Valley Fill Material
Site PlanSite Plan
GBR Construction & Design
Characteristics Summary
Dimensions (m) 27 x 8 x 1.5
Total Volume of Reactor Cell
380 m3
Media ¾ inch gravel
Design Flow 330 m3/day
Influent Selenium 20 to 30 ppb
Influent Nitrate ~30 ppm
GBR Construction & Operation
Control Shed GBR beneath parking lot
GBR Discharge
Discharge to Pond
ORP Probe at Outlet
Seep Collection
Pipe
GBR Performance
• 6 month pilot results• 2 month stabilization
period• Total selenium
treatment to consistently <5 ppb
• Operate in zone at suboxic to transition of sulfate reducing conditions 5 ppb limit
Summary
Successful treatment of ~30 ppb selenium to <5 ppbRequired a small footprint compared to alternativesSubsurface design allowed for property use on land above (parking lot)
Technical, design and regulatory considerations
Evolution of GBRs
2003 2005 2008
Considerations
• Technical & Design:– Temperature of water– Concentrations of nitrate and selenium– Recirculation mode
• Regulatory: – Buffer pond at the effluent (aeration, effluent testing of secondary constituents)– Long term management (e.g. closure)
• Operation & Maintenance:– Ability for periodic water heating, as required
– Biofouling management
– Calcite precipitation management
Summary
• GBRs offer simpler engineered system, small footprint
• Easy installation near source water in challenging locations
• Potential to reduce concentration loading of metals and inorganics, and/or reduce requirements for conventional treatment
• Potentially offer an alternate, cost effective treatment technology
Thank You!
Desirable Characteristics of GBRs
• Can be placed proximal to sources, fit to available space, operated remotely
• Size is scaled to flow rate • Uniform media provides predictable, improved
control of flow and residence time• Simplified monitoring infrastructure, can be
automated and centralized• Ability to easily integrate biofouling controls and
rehabilitation measures• Potential for in-place closure - isolated from
surrounding environment
Effective Treatment
• Requires careful control of geochemistry in the suboxic zone (i.e. REDOX)
• Regulation of the electron donor dosing rate balanced to electron acceptor in influent water
Where: CDO: dissolved oxygen (mg/L); CNO3-N: dissolved nitrate (mg/L as N); SF: safety factor.
• If needed, water quality can be further improved by post-treatment processes (<5 ppb selenium)
SFCCL
mgDoseMethanol NNODO ×⎟⎠⎞
⎜⎝⎛ += −
525.05.1)( 3