Effective Aerobic Wetland Design for Metals Polishing in Mine Water

Treatment

Brad Shultz, E.I.T., HydrologistU.S. Office of Surface Mining Reclamation and Enforcement

Presented at the 2015 PA AMR Conference, State College, PAJune 26, 2015

Purpose of Aerobic Wetlands• Passive form of treatment for net alkaline mine water

– Raw water– Following alkaline treatment of net acidic water

• Provide vegetation in a relatively shallow water environment capable of removing iron and manganese precipitates

• Natural mechanism for treatment• Aesthetically regarded, provides habitat• Provides reliable and proven form of metals removal with

minimal maintenance, potential for use at tail end of active treatment facilities (e.g., H2O2)

• Reduced need for polymer?

Example Aerobic Wetland Systems

• PA DEP has had recent involvement in the design and/or update of several passive treatment systems involving aerobic wetlands

• Some examples of systems with functioning wetlands evaluated as part of OSMRE technical assistance were:– Melcroft Passive Treatment System– Flight 93 Memorial Site Water Treatment System– Lion Mining Water Treatment System

Melcroft Passive Treatment SystemFayette County

• Passive treatment of Melcroft #3 underground mine pool using two VFPs followed by settling pond, two aerobic wetlands (in series), and manganese removal bed

• No deep water forebay• Flow distribution using perforated pipe into first

wetland• Rock baffles (check dams) used in each wetland• Fabricated stop log system installed in the spillway

as the outfall for each wetland

Melcroft System Design Plan

Melcroft Passive System

Influent to Wetland 1:Total Fe = 2.15 mg/LFe2+ = 0.86 mg/LTotal Mn = 2.95 mg/LpH = 7.10Typical Flow = 87.3 gpmDesign Flow = 125 gpm

Two 0.75 acre Aerobic Wetlands

Melcroft Mine Drainage Passive

Treatment System:

Polishing Wetlands

Melcroft Wetland Performance:Total Iron Removal

Melcroft Wetland Performance:Total Manganese Removal

Flight 93 Memorial Site Water Treatment SystemSomerset County

• Approx. 1.25 acre Aerobic wetland added to the existing treatment system to improve iron removal (2013)

• Four large settling ponds in series first treat the water pumped from mine pool prior to entering wetland, followed by additional ponds

• Design flow = 775 gpm• Influent Total Fe = 1 – 2 mg/L• Influent Total Mn = 6 – 9 mg/L• Influent pH = 8.0

Flight 93 Memorial Site Treatment SystemPre-Wetland

Future Wetland

Flight 93 Overall Improvements Site Plan

Wetland Area1.25 acres

Flight 93 Wetland Design Plan

Flight 93 Wetland Performance:Total Iron Removal

Flight 93 Wetland Performance:Total Manganese Removal

Flight 93 Memorial Site Aerobic Wetland

Flight 93 Memorial Site Aerobic Wetland

Lion Mining Water Treatment SystemSomerset County

• Passive treatment system constructed ~ 2010• Multiple settling ponds with two venturi type

aeration systems in parallel at beginning for aeration• Approx. 0.75 – 1.0 acre Aerobic Wetland

constructed at end of system for final polishing of any remaining Fe and Mn

• Level lip spreaders (grouted rock-lined spillway) used to distribute flow into wetland

• Forebay and deep water outlet zones

Lion Mining Treatment System WetlandInfluent to Wetland:Total Fe = 0.86 mg/LTotal Mn = 0.75 mg/LpH = 7.50Typical Flow = 650 gpm

Lion Mining Passive Treatment System

0.75-1.0 acre Aerobic Wetland

Lion Wetland Performance:Total Iron Removal

Lion Wetland Performance:Total Manganese Removal

LTV Clyde Mine Water Treatment Facility

• Constructed pebble quicklime (CaO) treatment plant in 1998 for the flooded underground mine

• Mine water was initially net acidic, but eventually became net alkaline once target mine pool elevation was maintained

• Typical operating flows range from 1,000 – 2,000 gpm

• Main parameters of concern for treatment:– Ferrous Iron (Fe2+)

– TDS

Clyde Project Location

Clyde

Existing Clyde Mine Water Treatment Facility

Proposed Wetland Location ~ 2 acres

LTV Clyde Mine Water Treatment Facility (cont.)

• In 2014, PA DEP and OSMRE personnel conducted on-site pilot testing using 50% hydrogen peroxide (H2O2) in place of CaO

• Tests revealed H2O2 capable of decreasing Fe2+

concentrations similar to CaO treatment• Trade-off = Less Mn was removed with H2O2

• Considerable cost savings using 50% H2O2 in place of CaO

LTV Clyde Mine Water Treatment Facility (cont.)

• Upon conversion to H2O2 for treatment, the operator, PA DEP, and OSMRE personnel discussed idea of constructing a polishing (aerobic) wetland following the clarifier to reduce or eliminate the need to add polymer

• Technical assistance request from PA DEP to OSMRE – Develop engineering design of Aerobic Wetland for final outfall from clarifier

• Small amounts of iron particles and manganese targeted for removal, typically less than 3.0 mg/L

Critical Design Features of Aerobic Wetlands

• Evaluated several existing Aerobic Wetlands in PA• Determined several critical factors that affect

efficiency of wetlands specifically for iron precipitate removal

• Iron precipitates created through aeration/oxidation do not readily floc, need polymer or ‘surfaces’ for removal– Particularly true for last few mg/L of iron

Flow Introduction• Diffuse laminar flow introduction• No concentrated flow outlet (end of pipe)• Level lip spreaders, troughs, deeper water

forebay, etc• Minimize potential for channelization• Bypass available to divert flow• Need to provide additional aeration (dissolved

Fe and Mn still present in the influent)

Forebay• Deeper pool at upstream end of wetland (>2’)• Retention time of concern?• Encourage spreading of the incoming water

across width of wetland, reduce velocity of water and help with precipitate settling, increased storage for sludge

• Baffles needed?• Length of forebay importance

Rock Baffles/Spreaders/Check Dams• Provide a means of helping to distribute flow• Allow for maintenance access• Stone size• CaCO3 content important?• Height of baffle vs water level• Important locations:– Transition zones: Deep to shallow and shallow to

deep– Throughout wetland (Melcroft & Flight 93)

Water Depth in Vegetated Zones

• Is 6 to 18 inches still ideal range for wetland vegetation and function?

• Initial period at shallow depth (<6”) to allow for wetland vegetation proliferation

• Importance of homogeneous vegetation growth throughout wetland area to minimize channelizing flow (short-circuiting)

• Wider and dense wetland = Low velocities

Outfall Structure• Most important factor: Encourage distribution/draw

from entire width of wetland• Deep pool similar to forebay• Large perforated pipe along downstream bottom end

(pipe size and perforations based on design flow)• Ease of access for maintenance• Hydraulically connected to allow water level control

of entire wetland• Ability to measure flow and water quality

Clyde Mine Water Treatment Facility Aerobic Wetland Design

• Design Flow = 2,000 gpm• Typical Flow = 1,000 gpm• Influent Maximum Total Fe & Mn = 3.5 mg/L• Approximate 2.0 acre area available between

treatment facility and South Branch Tenmile Creek• Challenges– Sewer line, process water line, existing outfall pipe– Potential for encountering spoil material during

excavation

Clyde Mine Water Treatment Facility Aerobic Wetland Design

• Flow distribution trough at inflow to forebay• Forebay• Rock baffle/spreader at transition from forebay to

shallow wetland• Three rock baffles/spreaders equally spaced within

the vegetated zone• Rock baffle/spreader at transition from vegetated

zone to deep pool at outlet structure• Perforated pipe along downstream bottom end

connected to a water level control structure

View from Clarifier of Area for Wetland

Existing Final Outlet Location

Clyde Mine Water Treatment Facility Aerobic Wetland Preliminary Design

Acknowledgments

• PA DEP – Rich Beam and Malcolm Crittenden• AMD Industries – Don Charleton• Mountain Watershed Association