gravel bed reactors (gbr™): a water treatment technology...

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

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