Life-Cycle Case Study Comparison of Permeable Reactive Barrier versus Pump-and-Treat Remediation
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Life-Cycle Case Study Comparison ofPermeable Reactive Barrier versusPump-and-Treat RemediationM O N I C A R . H I G G I N S A N DT E R E S E M . O L S O N *
Department of Civil and Environmental Engineering,University of Michigan, Ann Arbor, Michigan 48109
Received May 27, 2009. Revised manuscript receivedOctober 16, 2009. Accepted October 27, 2009.
A permeable reactive barrier (PRB) is a passive remediationtechnology, which over decades of use, may reduce lifetimeenvironmental impactswhencomparedwithaconventionalpump-and-treatsystem(PTS).Greatermaterialproductionrequirementsto install PRBs may offset the expected reductions inoperational phase impacts and the trade-offs canbe investigatedin a life-cycle assessment (LCA). The life-cycle environmentalimpacts of a zerovalent iron (ZVI) containing PRB with afunnel and gate configuration and a PTS were compared in acase study. Potential impacts of the model PRB are drivenby the ZVI reactive medium and the energy usage duringconstruction,while for the PTS they are driven by the operationalenergy demand. Medium longevity governed the magnitudeof the potential PRB impacts and the extent to which it wasoptimal relative to the PTS. Even at conservatively low estimatesof longevity, the PRB offers significant environmentaladvantages in impact categories of human health and ozonedepletion. The minimum ZVI longevity for PRB benefit over thePTS system in all impact categories was 10 years. SuggestedPRBdesign innovations to reduceenvironmental impacts includethe development of alternative reactivemedia and constructionmethods.
Groundwater resources are critical to meeting current andfuture global water needs, but are threatened by extensivecontamination, as illustrated by the more than 900 sites ontheU.S.National Priorities List (1), with chlorinated solventsoccurringmost frequently at industrial sites (2). Selection ofremediation technologies to restore groundwater dependsonsite-specificconditionsaswell as technologyperformance,cost, and environmental impacts. One technology oftenconsidered is apump-and-treat system(PTS),which removesthe contaminated groundwater by pumping and use ofaboveground treatment facilities. APTSprovidesquick initialreductions in contaminant concentrations, but often resultsin a slow, steady reduction for the long-term (3). If conditionsare suitable for PTS, remediation goals can be achieved inreasonable time scales (4). However, a 2001 summary ofexperiences at groundwater remediation sites found that ofthe 32 sites surveyed only two had met remediation goalswith an average capital cost of $4.9 million and $26 perthousand gallons treated (5).
A permeable reactive barrier (PRB)wasfirst tested in 1991as an alternative for remediation (6). PRBs are installed insitu, allowing groundwater toflowunder thenatural gradientthrough a reactive cell where a reactivemediumdegrades orcaptures contaminants (5). A variety of PRB configurationshave been employed. The two most common designs arecontinuous trench configurations, in which the reactivemedium is continuously placed in an excavated trench, orfunnelandgatearrangements,where impermeable surfacesdirect flow through smaller cells of reactive material (7).According to aU.S. Environmental ProtectionAgency survey,approximately30%ofPRB installationsuse the formerdesign,30% the latter, with the remainder consisting of several lesscommonconfigurations (5). PRB installationshave alsobeendesigned with several types of reactive media, although themost common reactive medium has been zerovalent iron(ZVI). Approximately 55% of the PRB installations surveyedin 2002 relied on ZVI to effect treatment (5). These surveysindicate that the length of time over which the reactivemedium remains effective, the longevity, is amajor factor inthe long term success of the technology. Though some field-scale barriers have been in operation for more than 10 years(8), theabsolute longevityofZVIand the factorswhichcontrollongevity at PRB installations are relatively unknown (9).
Due to its minimal material and energy requirementsduring operation, a PRB system offers potential economicand environmental advantages over a PTS (7, 8, 10, 11).However, a thoroughevaluationofenvironmental advantagesmust be made with respect to all relevant life-cycle stages.Life-cycle assessment (LCA) is used to quantify and compareenvironmental impactsofproductsor systemsover theentirelife cycle (12). Applications of LCA to site remediation,including remediation of contaminated soil and/or ground-water, have been investigated in generic applications andthrough case studies (13-17). A conceptual framework forthe application of LCA to site remediation technologies wasdeveloped by Diamond et al. (13), which was subsequentlyapplied to a case study involving excavation and disposal oflead-contaminated soil (14). Suer and colleagues reviewedthe methods and results of eight case studies on theapplication of LCA to site remediation (15) and found thatenergy consumption was a major cause for environmentalimpact. However, of the eight case studies examined, onlytwo of the assessments included technologies for ground-water remediation and neither considered PRB or otherpassive technologies among the alternatives (15). In the solepublished LCA comparison of a PRB and a PTS system (16),a relatively atypical reactive medium, activated carbon, wasconsidered for the remediationof acenaphthene, apolycyclicaromatic hydrocarbon (PAH).
Although ZVI is one of the most common reactive mediaemployed in PRBs, no LCA comparisons involving this typeof PRB have been reported. In this study, an LCA of a ZVI-type PRB was compared to a PTS for a case study sitecontaminatedwith chlorinated solvents. Theassessmentwasdesigned to examine the impact ofmedium longevity on thelife-cycle impacts of a PRB, and thereby quantify the designlife at which the two remediation approaches are equivalentfromanLCAperspective. TheLCAcomparisonwas alsousedto identify specific components of PRB design which, ifimproved,would result in thegreatest environmental benefit.
Materials and MethodsCaseStudyDescription.Thecase studywas conductedusingpublicly available design documents for two remediationstrategies designedbyBattelle forDoverAir ForceBase (AFB)
* Corresponding authorphone: (734) 647-1747fax: (734) 763-2275;e-mail: email@example.com.
Environ. Sci. Technol. 2009 43, 94329438
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in Dover, DE (18, 19). Contaminants on site include severalvolatile organic compounds (VOCs) including 1,2-dichloro-ethylene (DCE), 1,2-dichloroethane (DCA), trichloroethylene(TCE), perchloroethylene (PCE), and vinyl chloride (VC) (18).Though thegeochemical conditionsonsite are representativeof many contaminated sites, some hydraulic conditions(specifically low hydraulic gradient, 0.0018, and high depthtoaquitard, 11m)are somewhat atypical forPRBapplications(20). Since these characteristicsmake the site amoredifficultPRB application, the life-cycle assessment of the PRBsenvironmental impactsmay be less favorable than siteswithshallower water tables and greater hydraulic gradients.
Pilot scale testing of both PTS (18) and PRB (19)technologies was carried out on-site. Although these tech-nologies were never installed on-site at full scale, full-scaledesigns of both systems were developed by Battelle. Thesecompleted designs have served as the basis for engineeringandeconomiccomparisonsof the two technologies in severalpublications (19-21). Acknowledging that there is generaluncertainty in the validity of design assumptions, especiallywith respect to the design life of a PRB, the effect of designlife on its life-cycle environmental impacts was examined inthis study.
The full-scale PRB was designed as a funnel-and-gateconfigurationwith a 36.6m length of funnel and four- 2.4mdiameter cylindrical gates. The funnel was to be constructedfrom prefabricated steel sheet piling sealed together withcementitious grout. The gates were to be constructed byexcavating within a 2.4 m diameter steel caisson, installinga 1.2 m by 1.2 m column of ZVI, and backfilling the outerpretreatment and exit zones with sand. The ZVI used in thepilot-scale unit and recommended for use in the full-scalePRB was commercially available, high quality granular iron.The design of the full-scale PRBwas similar to the pilot-scalePRB unit tested; the most significant differences were thesize (the full-scale PRBwas twice as large), andmodificationsto the pretreatment zones (the full-scale PRB used only sandwhile the pilot-scale unit employed sand/iron mineralmixtures).
The full-scale PTS was designed to remove groundwaterfrom three pumping wells using electric pumps. It includeda packed-tower air-stripping unit that was housed above-ground in a building. Air emissions from the tower were tobe treated using catalytic oxidation and the effluent waterstream was further polished using GAC adsorption beforereinjection to the aquifer. The pilot PTS facility evaluatedtwoair-stripper tower configurations at 190Lmin-1 (50 gpm)each, and four different catalytic oxidation units. In the fullscale design the assumed process flow rate for the selectedconfiguration was 76 L min-1 (20 gpm).
PermeableReactiveBarrier SystemModel.ThePRBwasmodeled as three subsystems: funnel, gate, and reactivemedium. The model PRB funnel was constructed using avibratory hammer mounted on a 100-ton crane (nominalcapacity of 835 kW at 5.6 m2/hour) and sealed together withcement. Model PRB gates were constructed using the samehammer (at 0.6 m/hour) to drive the caissons into position,then excavated using an auger (435 kW at 0.3 m/hour). Thegateswere then backfilledwith sand, ZVI, and soil before thecaisson was removed with the vibratory hammer. Thoughdesigned as part of the gate, the reactive medium wasconsidered a separate subsystem to investigate the effect ofmedia longevity. ZVI production was modeled as theproductionof high-iron content cast iron,without additionalprocessing. The exclusion of additional processing mayreduce the energyburdenassociatedwith theZVI subsystem,however, the additional processing energy was assumed tobe small when compared with the energy demand of thematerial. The ZVI longevity was assumed to be 10 years forthe base model case. Only in the investigation of media
longevity effects on potential impacts was the longevityallowed tovary.Followingconstruction, thePRBwasassumedto operate for the duration of the medium lifetime withoutadditional inputs. Upon exhaustion of themedium, the gatewas to be removed with an auger before major materialcomponents were generated, transported, and constructedinto a replacement gate. It was assumed that the funnel doesnot require repair during the 30 year study period.
Pump and Treat System Model. The PTS was modeledwith five subsystems: extraction wells, air-stripping unit(ASU), catalytic oxidation unit (COU), granular activatedcarbon (GAC)unit, and treatment facilities.Model extractionwells were constructed using an eight inch auger (80 kW at5 m/hour) and were composed of PVC well pipe, filter pack,grout, and a 0.75 kW (1 hp) well pump. The model ASU wascomposed of an aluminum tower, packedwith polyethylenepall ring packing, and a 0.75 kW (1 hp) blower. The modelCOU was modeled as a fixed bed reactor made out ofaluminum and steel, with catalyst, and electric heaters. Themodel GAC unit was two steel drums each containing 180kg (400 lb) of GAC.Model treatment facilities included a 37.1m2, 0.15m thick structural slabpoured fromconcretemixingtruck (260 kWat 0.14m3/hour), 61mof 0.05mdiameter PVCpiping, miscellaneous PVC fittings and valves, and a steelshed. Following construction and assembly, the systemoperated using electricity obtained from the U.S. grid. Theonlymaintenance activity considered for themodel PTSwasthe replacement of GAC filter units every 10 years.
Life-Cycle Assessment. LCA methods were based oncurrent industrial standards (12), governmentguidelines (22),and previously published work (13, 16). The LCA case studywas conducted using SimaPro 7.1 LCA software and associ-ated inventory databases and impact assessment methods(23). Unit processes with inputs or emissions that were notincluded in the databases were estimated from availableliterature, calculated using fundamental principles, or omit-ted. The impact assessment was conducted with character-ization factors within the Tool for the Reduction andAssessment of Chemical and other environmental Impacts(TRACI) method (24) version 2.0. The following environ-mental impact categories were considered: global warming,acidification,humanhealth, eutrophication,ozonedepletion,and smog formation. The determination of uncertainty wasconducted using Monte Carlo simulations with set stopfactors, available within SimaPro software, to generate 95%confidence intervals. System input data was given anuncertainty value based onour perceived quality of the data.Additional information on assumptions, omitted processes,and uncertainty values are available in the SupportingInformation (SI).
Thegoal of theLCAwas tomodel theDoverAFB treatmentsystems inorder todetermine theenvironmentallypreferableoptionand to investigate strategies thatwould reduce impactswithin each system.Theassessmentwasbasedonacommonfunctional unit: the system-specific requirements (energy,materials) needed to provide effective capture of the con-taminant plume and treatment for 30 years. According todesign documents, the PRB captures the plume and treats38 L min-1 (10 gpm), while the PTS is designed to operateat a flow rate of 76 L min-1 (20 gpm) to meet the same goal(19, 21). Specifications for both systems incorporated factorsof safety into the designs, whichwere roughly 1.5 for the PRBand 2 for the PTS (18, 19), and while the two systems do nothave identical safety factors, they are similar andboth reflectthe need to over design groundwater treatment systems.
The system boundaries, which define the scope of thestudy and illustrate the processes included, were inclusiveof raw materials acquisition, materials production, and usephases. System boundaries for the PRB and PTS system areshown in Figures 1 and 2, respectively. Notable omissions
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include monitoring and end-of-life processes. Monitoringschemes for the two full-scale systemswere similar in designdocuments andanticipated to cost similar amounts annually
(19, 21) so the processes were omitted from the life-cyclecomparison. While the inclusion of monitoring processeswould change themagnitude of PRB impacts, only a relative
FIGURE 1. Schematic of permeable reactive barrier remediation activities. ZVI, zero-valent iron.
FIGURE 2. Schematic of pump-and-treat remediation activities. Abbreviations: Al, aluminum; PVC, polyvinylchloride; GAC, granularactivated carbon; HD...