SAVAGE WELL « TREATABILITY STUDY Savage Well Site Milf … · SAVAGE WELL TREATABILITY STUDY «......

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SAVAGE WELL TREATABILITY STUDY «... Savage Well Site Milf ord, NH Phase I - Technology Assessment October, 1989 Prepared by: u, o II h m m HMM ASSOCIATES, INC. ENGINEERS, ENVIRONMENTAL CONSULTANTS & PLANNERS

Transcript of SAVAGE WELL « TREATABILITY STUDY Savage Well Site Milf … · SAVAGE WELL TREATABILITY STUDY «......

SAVAGE WELLTREATABILITY STUDY

«. . .

Savage Well SiteMilf ord, NH

Phase I - Technology Assessment

October, 1989

Prepared by:

0»u,o

II

hmm H M M A S S O C I A T E S , I N C .

ENGINEERS, ENVIRONMENTAL CONSULTANTS & PLANNERS

HMM Ref. No. 2176-120/HAZ/2946

November 9, 1989

Richard GoehlertU. S. Environmental Protection AgencyJFK Federal Building, HSN-CAN5Boston, MA 02203-2311

Subject: Rationale for Removal of the Pilot Plant Program from the Savage WellRI/FS Process

Dear Mr. Goehlert:

The purpose of this letter is to provide all parties with both the general and technicalrationale as to why the pilot plant program included in the Administrative Order shouldbe modified. Also included in this letter is the chronology of the events which led to thecurrent decision to remove the air stripping pilot tower from the project.

General

The pilot program was included in the Administrative Order at the request of the PRPGroup. This program was to include the installation of an 8" production well,performance of a 5-day pumping study, and the installation and operation of a 100-gpmair stripping tower to remove volatile organics. The decision to include this program wasbased on 1986 data and knowledge of the site conditions at that time. The primary intentwas to provide early treatment of the groundwater and to limit off-site migration ofcontaminants. The primary change being requested as a part of this correspondence isthe removal of the pilot air stripping tower component of the treatability studies.

Chronology

The remedial investigation field program started on November 23, 1988 with the approvalof the Project Operations Plan which provided for the commencement of a 2-phasegroundwater study in accordance with the work plan. The first phase of this programresulted in the installation of 36 monitoring wells and the collection and analysis of 54groundwater samples for volatile organics. Results were presented to EPA in two reportsdated March 28, 1989. The purpose of these reports was to provide a clear understandingof the existing conditions of the site and to help scope the Phase II program. Based onthese new data, the treatability study tasks commenced concurrent with the Phase IIprogram. This work was based on the VOC analysis available to date which was used as

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input data in conducting computer modeling efforts to assist in the design of the airstripping pilot tower (size, air to water ratios, packing materials, emission rates, andtreated effluent quality for discharge). Based on a high concentration of VOCs and thespecific compounds detected in the Phase I program, both the efficiency of the airstripping tower and the emission rate became issues of concern. Specifically, it becameapparent that a 100-gpm pilot plant would require a tower over 25' high which wouldachieve an average efficiency of 95%, insufficient to meet the current MCL dischargelimitations. Based on these findings, HMM included an assessment of granular activatedcarbon, assessing mass transfer zones, contact times, and multicomponent adsorption.Computer modeling for this effort was conducted for vapor phase carbon to assess airemission limits for the air stripper and liquid phase carbon for effluent polishing to ensureachievement of the MCLs.

Concurrent with the treatability study efforts, the Phase II groundwater monitoringprogram started in May of 1989. This program resulted in installation of 29 monitoringwells. The data were presented to EPA in a report dated August 1, 1989. This wasfollowed up by a Phase II groundwater sampling program which consisted of sampling 65groundwater wells. These data were presented to EPA in a report dated September 29,1989.

The Phase II program focused on the heart of the plume identified in the Phase I program,included the analysis of chemical constituents, specifically VOCs, ABNs and metals, andthe investigation of physical influences of localized production well pumping. Two keyfindings resulted from that effort. First, there is a very high concentration of naturallyoccurring iron in the groundwater. The iron would oxidize out on the air stripper packingmaterial leading to precipitate and biological fouling of the packing material, ultimatelyresulting in degradation of the efficiency of the air stripper and a reduction in effluentquality. The second finding is that the cone of influence inferred from a 320 gpmproduction well is limited to approximately 300'. This would indicate that a 100 gpmpilot study would only impact a very small percentage of the plume and would not provideany significant long-term treatment benefit.

Technical Basis and Recommendation

A meeting was held on September 29, 1989, attended by EPA, HMM, NHDES, and amember of the PRP Group. Key issues discussed during that meeting are as follows.First, based on the results of the Phase II sampling program, there are new data that werenot available to the PRP Group in 1986. This includes the data for the VOCs whichindicate overall higher concentrations of contaminants that increase in concentrationwith depth. Additionally, data show high levels of iron. Both of these findings have adirect impact on the viability and operational efficiencies of an air stripping tower.Furthermore, these data indicate that additional components of treatment would need tobe included as part of the pilot program. This resultant expansion of the pilot studywould include the addition of pretreatment for metal precipitation and filtration in frontof the air stripper and, most likely, the addition of vapor phase carbon for controlling airemissions and liquid phase carbon for polishing of the effluent to ensure compliance withcurrent MCLs. This expansion is well beyond the scope of the original pilot studyproposal. Additionally, data relative to the transmissivity of the aquifer indicates that a100 gpm pumping well would provide only a fraction of the recovery necessary to impedethe flow of contaminants. In other words, the pilot plant program would not achieve theoriginal objectives of providing significant early treatment of the groundwater nor wouldit limit the off-site migration of contamination.

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Additionally, while pilot studies can assist in selecting components of design prior toimplementation of a remedial alternative, pilot testing of a single unit process is notwarranted during the feasibility study. The advent of new proven computer modelingprograms allows rapid and cost-effective evaluation of individual unit processes relativeto efficiency and size based on the chemical constituents in the influent water. Theseindividual treatment units are then combined into an overall process train during thefeasibility study. It is the resultant treatment process train that should be pilot testedduring the design phase. Any treatability/pilot programs conducted at Savage during orafter the feasibility study should also focus on site-specific data for groundwaterrecovery.

Recommendations

The purpose of the treatability study is to supplement a feasibility study process and toaid in the selection of the most implementable, cost-effective remedial alternative. Thedata collected to date and the engineering effort conducted during the initial portion ofthe treatability study will be used in development and selection of the alternatives in thefeasibility study. Based on our current understanding of site conditions and remedialobjectives, it is HMM's recommendation that the air stripping pilot tower be removedfrom the program as it will not substantially improve the selection of alternatives norwill it significantly remediate the groundwater during this interim period and would onlycause an unnecessary delay in the project.

There are two related components of the treatability study that should be carried forwardto improve the database for the feasibility study. First, it is recommended that a 5-daypumping study be conducted to further characterize aquifer hydraulics and to aid in thedevelopment of groundwater recovery systems. This step includes field studies necessaryto assess the hydrodynamics of the aquifer which are important factors in remedialalternative development. Secondly, based on the varying chemical constituents of theplume, it is also recommended that desorption coefficients be developed at two separatelocations within the aquifer near the OK Tool and Hitchiner facilities so that the overalltime frame for remediation of the aquifer can be evaluated. Soil column leaching studiesshould be conducted for each area to determine site-specific desorption coefficients forspecific contaminants in order to assess the method and time frame required to flushcontaminants through the soil column.

If you have any questions, please do not hesitate to call me at (508) 371-4111.

Sincerely,

Richard C. CoteProject Manager

RCC/c

2176-120/HAZ/2946- 11/9/89

TABLE OF CONTENTS

Page

1.0 INTRODUCTION 1-1

1.1 Technology Evaluation 1-1

1.2 Computer Simulation Modeling 1-1

2.0 TREATABILITY STUDIES 2-1

2.1 Influent Sources 2-1

2.2 Air Stripper Evaluation 2-5

2.2.1 Experimental Design 2-5

2.2.2 Results 2-6

2.2.3 Minimum Tower Requirements (MCL) 2-11

2.2.4 Loss of Tower Treatment Efficiency 2-11

2.3 Vapor Phase Activated Carbon Evaluation 2-12

2.3.1 Experimental Design 2-13

2.3.2 Results 2-14

2.3.3 Vapor Phase Carbon Considerations 2-14

2.4 Liquid Phase Activated Carbon Evaluation 2-18

2.4.1 Experimental Design 2-19

2.4.2 Results 2-19

3.0 CONCLUSIONS 3-1

4.0 REFERENCES 4-1

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SAVAGE WELL SITE TREATABILITY STUDY

1.0 INTRODUCTION

The purpose of this treatability study is to evaluate treatment technologies which

have potential for remediating contaminated groundwater at the Savage Well site. The

primary contaminants identified in the groundwater at the Savage Well site are the

volatile organic compounds (VOCs), Tetrachloroethylene (PCE), Trichloroethylene (TCE),

Trans-l,2-Dichloroethylene (DCE) and 1,1,1-Trichloroethane (1,1,1-TCA). The

technologies studied were air stripping, vapor phase granular activated carbon (GAC)

adsorption, and liquid phase GAC adsorption. These technologies are the most

appropriate for removing the specific VOCs from contaminated groundwater on the

Savage Well site.

1.1 Technology Evaluation

This study employed computer simulation models to evaluate the effectiveness of

the various treatment technologies. During the engineering evaluation, thesenewly-developed computer simulation models can be used to develop a wide range of

treatment technology configurations, operating parameters, and influent contamination

concentrations. This approach to assessing process trains allows for a quicker and more

accurate analysis, conducted at lower cost than by pilot study. For instance, a pilot air

stripper installed on a site would normally be able to test one packing medium depth, one

packing material, and several air to water ratios. Additionally, contaminated

groundwater concentrations are generally limited to those extracted from the aquifer in

the vicinity of the air stripper. This limits the ability of the treatability study program

to address a range of parameters such as the various pumping scenarios necessary to

recover the plume; various influent/effluent concentrations (to support cost-effective

in-line unit processes); and the ability of the system to respond to shock loadings.

1.2 Computer Simulation Modeling

Based upon historical design and remedial experience, experimental research, and

application of chemical and environmental engineering theory, it has been possible to

develop treatment technology-based analytical mathematical models. These models have

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the capability of simulating the behavior of various contaminants and concentrations

when the contaminated water is subjected to air stripping or carbon adsorption treatment

processes. These models have been shown to provide good simulation of actual

technology performance during on-site treatment. As with any computer models,

however, there are technical considerations that must be understood in order to utilize

the resultant data. A discussion of these issues is provided below to ensure that they are

taken into account during the evaluation process.

The major considerations in computer simulation modeling are the actual on-site

conditions encountered. The effectiveness and reliability of computer models is

dependent on using data which closely resembles on-site conditions. For instance, VOC

concentrations in groundwater entering the treatment units will probably vary with time,

potentially impacting treatment system performance and effluent concentrations.

Variation in aquifer geochemical parameters include pH, temperature, dissolved

inorganics and suspended materials. These changing or variable parameters can also

significantly affect system performance.

In particular, dissolved iron and manganese may precipitate onto air stripper

packing material, causing a reduction in packing material mass transfer efficiency and

resulting in higher effluent VOC concentrations than anticipated. Also, since

volatilization of organics is temperature dependent, the actual removal efficiencies will

vary depending upon sustained aquifer groundwater temperature and temperature changes

induced by tank storage and ambient climatic conditions. Therefore, the modeling

analysis must vary these parameters to assess the range of system operating efficiencies.

A tower would be designed with a factor of safety by using a mass transfer coefficient in

the simulation run which is lower than what may be used in practice.

Granular activated carbon adsorption of volatile organic compounds is affected by

pH, dissolved inorganics, and suspended material. The GAC adsorption models assumed

optimum values for pH and dissolved inorganics, and that suspended matter would be

removed prior to groundwater treatment by GAC. Less than optimum pH affects the

GAC volatile organics removal chemistry, while precipitating iron and filtered suspended

matter will plug the GAC contactor bed pore space, thus reducing the useful life of the

carbon. Also, for vapor phase carbon adsorption of air stripper offgas, there is a direct

impact on efficiency due to the moisture content of the stripped vapors. A dehumidifier

must operate preceding the vapor phase carbon contactor and this has been included as a

part of the computer model design parameters.

A listing of the computer simulation models used in this treatability study is

presented below. All of these computer models have been used on other sites and have

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been developed by research scientists and engineers who are recognized authorities in the

field of volatile organic removal from contaminated groundwaters.

TREATABILITY STUDY COMPUTER MODELS

"Design of Packed Tower Aeration System" (IBM PC DOS 2.10, Advanced BasicASCII) D. W. Hand and J. C. Crittenden, Michigan Technological University,Houghton, Michigan.

"Air Strip", (IBM PC DOS 2.10 Advanced Basic), Johannes Haarhoff, P.E., Universityof Iowa, Ames, Iowa.

"The Use of Equilibrium Theory to Evaluate Multicomponent Competition in FixedBeds" (IBM PC DOS 2.10, Professional FORTRAN, ASCII) Thomas F. Speth, John C.Crittenden and David W. Hand, Michigan Technology University, Houghton,Michigan.

"Prediction of Multicomponent Adsorption Equilibrium Using Ideal AdsorbedSolution Theory" (IBM PC DOS 2.10, Professional FORTRAN, ASCII) Thomas F.Speth, John C. Crittenden and David W. Hand, Michigan Technology University,Houghton, Michigan.

"User Oriented Solutions to the Homogeneous Surface Diffusion Model for Design ofGas-Phase Fixed-Bed Adsorbers" (IBM PC DOS 2.10, Professional FORTRAN,ASCII) David W. Hand, John C. Crittenden, and Randy C. Cortright, MichiganTechnology University, Houghton, Michigan.

Computer simulation modeling facilitates the process of formulating the treatment

alternatives to be used on this site, and it is an integral part of the feasibility study

process. Upon completion of the feasibility study and selection of the most practical,

site-specific and implementable remedial alternative, the computer modeling program

can be carried through into the preliminary design, pilot testing to scale up each unit in

the system for the final design phases of the project. This is especially important in the

case of GAC where competitive adsorption, varying mixed VOC concentrations and

specific GAC-type mechanical and chemical properties (virgin versus reactivated carbon)

can have a significant impact on results obtained. The result is the implementation of a

pilot testing program consisting of the complete process train, including pretreatment,

primary treatment, and polishing as required. The purpose of the pilot plant is to

simulate the interactive efficiencies of each unit in the process train.

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2.0 TREATABILITY STUDIES

Treatability studies conducted for this site focused on the existing areas ofgroundwater contamination, types of contaminants present and concentrationsdetermined through sampling and laboratory analysis. By evaluating and incorporatingsite-specific data from the Remedial Investigation, this Treatability Study was developedto simulate and model actual conditions expected during remediation.

2.1 Influent Sources

Prior to evaluation of the three treatment technologies, an analysis of remedialinvestigation data was performed to ascertain types, distribution and concentrations ofvolatile organic compounds. This analysis resulted in the identification of three distinctlydifferent areas of groundwater recovery specific to plume contaminant concentrations(Figure 1). These three identified areas are as follows:

1. Southwest of the trailer park, north of Route 101 in the vicinity ofmonitoring well MW-10 and west toward O.K. Tool and Die, "Site #1";

2. East of the trailer park, north of Route 101 in the vicinity of monitoring wellMW-20, "Site #2"; and

3. Northeast of the Old Savage Well, south of the Souhegan River in the vicinityof monitoring well MI-7, "Site #3".

The approximate distribution of contaminant concentrations for the three general areas isshown in Table 1. These concentrations are an estimated average for that area.

Recovery of groundwater for treatment would probably yield periodicconcentrations of contaminants in the groundwater that were higher than those listed inTable 1. In order to better evaluate the technologies' efficiencies in a more highlycontaminated groundwater "worst case" scenario, concentrations of contaminants ingroundwater from monitoring wells MW-10 and MI-7 were also used as influentcontaminant levels. These concentrations are shown in Table 2.

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FIGURE 1

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TABLE 1

VOLATILE ORGANIC COMPOUND DISTRIBUTION

AT THE SAVAGE WELL SITE*

Contaminant Site

#2

Tetrachloroethylene (PCE)Trichloroethylene (TCE)1,2-Trans-Dichloroethylene (DCE)1,1,1-Trichloroethane (TCA)

5,000

500

500

80

1,1007060

100

1,0007020

100

Concentrations in parts per billion (ppb)

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TABLE 2

VOLATILE ORGANIC COMPOUND "WORST CASE"

INFLUENT CONCENTRATIONS*

Contaminant Monitoring Well

MW-10 MI-7

Tetrachloroethylene (PCE)

Trichloroethylene (TCE)1,2-Trans-Dichloroethylene (DCE)

1,1,1-Trichloroethane (TCA)

7,700

380

1,400

90

1,900

85

65

89

* Concentrations in parts per billion (ppb)

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Since each contaminant has a different volatilization constant (Henry's Law) and a

different affinity for granular activated carbon (Freundlich constants), the mix and

concentration of contaminants present in the groundwater is very important. For

instance, while 1,1,1-TCA at all five locations is below the Safe Drinking Water Act

(SOWA) limit of 200 ppb, it does have an affinity for GAC and, as such, will adsorb onto

the carbon, thereby shortening the useful life of any carbon contactor bed. The Henry's

Law constant for trichloroethylene (TCE) is lower than that for tetrachlorethylene

(PCE). While concentrations for TCE are less than those for PCE, TCE may require more

stringent air stripping tower design than for PCE alone in order to achieve effluent which

meets the SDWA standards. Since there is a range of treatment effectiveness for each

contaminant and treatment technology, each source must be evaluated in order todetermine the optimum VOC removal technology or combination of technologies.

2.2 Air Stripper Evaluation

The first focus of this treatability study was to analyze the effectiveness of air

stripping as a technology to remove volatile organic compounds from groundwater. This

first evaluation considered air stripping tower components, influent concentrations and

operating parameters. The apparent need for vapor phase granular activated carbon

usage to control air stripper offgases was evaluated under a separate model. The results

of vapor phase carbon adsorption modeling are discussed in the next section.

2.2.1 Experimental Design

In air stripper tower design the objective is to obtain the best balance between

capital costs (tower volume) and operation and maintenance costs (pressure drop, air to

water ratio, loading). The computer models help to achieve this balance by allowing

analysis of a variety of structural and operating parameters.

The computer modeling evaluation initially considered three different influent

contaminant concentration levels, Sites 1, 2, 3. Three tower packing materials were also

simulated. These packing materials were chosen for their range in cost, pressure drop

and mass transfer efficiencies. The packing materials evaluated were Jaeger Tripacks,

Glitsch Mini Rings and Norton Snow Flakes. Tower packings depths evaluated were based

upon commercially available off the shelf air stripping towers. These towers had packing

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bed depths of approximately 8.5, 13.0 and 17.5 feet. Total tower height in each case is

actually several feet greater than the packing bed depth. The tower diameter was based

upon a liquid loading rate of 100 gallons per minute (GPM). This resulted in a two foot

diameter tower. The liquid loading rate of 100 GPM was selected based upon the flow

rates established in the treatability study work plan. This loading rate was also used in

the evaluation of liquid phase activated carbon for comparison purposes. While it isrecognized that higher liquid flow rates and larger air stripping towers could have been

evaluated, this would have been beyond any pilot scale simulation. Since this is a

treatability study for technology evaluation and not final design engineering, the sizes

and parameters selected were deemed appropriate. The last variable evaluated in the air

stripping computer model was the volumetric air to water ratio. Air to water ratios of

10, 20, 30, 40 and 50:1 were used during various computer runs. Current literature and

engineering practice suggests that in many cases the most economical air to water ratio

is approximately 30:1, therefore, two ratios on either side of this value were evaluated.

2.2.2 Results

The results of the air stripper computer simulation runs are shown in Tables 3

through 6. The increased volatile contaminant removal efficiency by employing more

efficient packing material (Tripacks) and greater tower packing material height is obvious

in all of the tables. Detailed computer printouts for the air stripper evaluation of all

sites and influent contaminant concentrations are available for review. A significant

observation is that none of the tower packing materials nor combinations utilized in a

single tower, with tower heights currently available for use in the pilot program were

able to remove all four volatile compounds to SDWA standards in the effluent. The

effluent concentrations for the 30:1 air to water ratio are shown in Tables 3 through 6.

However, even at the highest air to water ratios modelled (50:1), the effluent from the

17.5 foot packing material tower was still 129 ppb for PCE for Site 1; 28 ppb for Site 2;

and 26 ppb for Site 3. For Site 1, TCE and DCE were also in excess of SDWA limits of 5

and 7 ppb, respectively, at 19 ppb. MW-10 had PCE, TCE and DCE at 238, 15, and 53

ppb, respectively. Based upon these data, readily available, off-the-shelf equipment, an

individual pilot-scale air stripper would most likely not produce effluent which meets the

Maximum Contaminant Levels (MCLs).

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TABLE 3

AIR STRIPPER EFFLUENT* FROM SITE 1

Packing Depth**

Packing

Snowflakes

Mini Rings

Tripacks

VOC

PCE

TCE

DCE

TCA

PCETCE

DCE

TCA

PCE

TCE

DCE

TCA

8.5 feet

1,175

131

129

18

982

105

101

15

892

97

94

14

13.0 feet

560

67

67

8

427

48

47

6

370

43

42

5

17.5 f

269

35

35

8

187

23

22

3

154

19

19

2

* Concentrations are in parts per billion** Air to water ratio: 30/1

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TABLE 4

AIR STRIPPER EFFLUENT* FROM SITE 2

Packing Depth**

Packing

Snowflakes

Mini Rings

Tripacks

VOC

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

8.5 feet

258

18

16

22

216

15

12

19

196

14

12

17

13.0 feet

123

9

8

10

94

7

6

8

81

6

5

7

17.5 f

59

5

4

5

45

3

3

3

34

3

2

3

* Concentrations are in parts per billion

** Air to water ratio: 30/1

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TABLE 5

AIR STRIPPER EFFLUENT* FROM SITE 3

Packing Depth**

Packing

Snowflakes

Mini Rings

Tripacks

VOC

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

8.5 feet

235

18

5

22

196

15

4

19

179

14

4

17

13.0 feet

112

9

3

10

86

7

2

8

74

6

2

7

17.5 f

54

5

2

5

38

3

1

3

31

3

1

3

* Concentrations are in parts per billion

** Air to water ratio: 30/1

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TABLE 6

AIR STRIPPER EFFLUENT* FROM RECOVERY WELLS

Packing Depth**. ***

Packing

MW-10

voc

PCE

TCE

DCE

TCA

8.5 feet

1,374

74

264

15

13.0 feet

570

33

117

6

17.5 feet

238

15

53

3

MI-7 PCE

TCE

DCE

TCA

339

17

12

15

141

7

5

6

59

3

3

2

* Concentrations are in parts per billion

** Air to water ratio: 30/1

*** Two-inch Tripacks

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2.2.3 Minimum Tower Requirements

An analysis was made to determine the depth of packing material required toachieve the proposed SDWA standard of 5 ppb for PCE. This resulted in a packing bed

depth of 38 feet. Overall tower height would be approximately 45 feet. This analysisincluded a high efficiency packing material, same tower diameter and a 30:1 air to water

ratio.

2.2.4 Loss of Tower Treatment Efficiency

In addition to increased height and additional packing there is an associated

increase to pump cost. Several issues must be emphasized in considering the foregoing

findings. The first is that the removal efficiencies calculated assume no loss of mass

transfer efficiency. Remedial investigation data indicate high levels of iron in

groundwater at the Savage Well site. Thus, there will be some loss in air stripper

efficiency due to metals precipitation and/or biofouling even with metals removal.

Second, the groundwater temperature used in these modeling runs was 50°F. Data from

the Savage Well Site aquifer show that, at times, groundwater temperatures are lower

than 50°F. This is probably due partly to recharge from the Souhegan River. This cooler

temperature will result in decreased volatilization, (and subsequent removal), of volatile

organics. Third, the influent groundwater contaminant concentrations can be expected to

vary. While at times concentrations may decrease, there is also a significant possibility

that contaminant concentrations will increase. The net result is that even a tower with

38 feet of packing material may not meet discharge limits in the effluent during periods

of increased contaminant concentrations. Therefore, any tower designed to achieve

MCLs will have to incorporate either a factor of safety; greater height, two towers in

series, or employ granular activated carbon polishing of the air stripper effluent.

The reason for GAC polishing, or a second air stripping tower in series, is that the

concentrations of VOCs in the effluent and the volume of treated groundwater would call

for a level of safety and redundancy to ensure meeting selected effluent criteria over a

range of influent concentrations.

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2.3 Vapor Phase Activated Carbon Evaluation

Air stripping of volatile organics from groundwater results in transfer of these

compounds to the atmosphere. Air stripping volatile organic compounds from the

groundwater in the vicinity of monitoring well MW-10 at 100 gallons per minute would

result in approximately 10 pounds of VOCs being emitted to the atmosphere on a daily

basis at that location. While concentrations of volatile organics at other locations are

lower, a larger volume of groundwater would require treatment which would still result ina larger mass of VOCs being stripped out of the water and into the air on a daily basis.

Since the aquifer is very large, it is probable that 1,000 gallons per minute may be a

design treatment rate required to remediate the aquifer. If, for example, 250 GPM came

from Site # 1 and 375 GPM from each of Sites # 2 and # 3, approximately 29 pounds of

volatile organics would be transferred to the atmosphere on a daily basis.

Site-specific variables including wind speed and direction, downwind receptors, and

tower height are critical to establishing potential risk, it should be noted that allowable

ambient levels on a time-weighted average for the volatile organic compounds in question

would need to be developed. In particular tetrachloroethylene, which is the contaminant

of highest concentration on-site.

Vapor phase carbon adsorption as a technology was evaluated in this treatability

study as an air control measure. Furthermore, during the initial screening of alternatives

and detailed evaluation of alternatives phases of the Feasibility Study, the added cost of

a vapor phase carbon component in the treatment train must be considered when

comparing air stripping to granular activated carbon adsorption. This includes costs for

both capital and operation and maintenance. The computer evaluations conducted as part

of this treatability study will facilitate identification of vapor phase carbon operation and

maintenance costs.

2.3.1 Experimental Design

In order to evaluate vapor phase activated carbon usage for this treatability study,

air stripper data generated during the air stripper evaluation portion of this study was

used in conjunction with a vapor phase granular activated carbon simulation model. Air

stripper data used was selected based on the results of modeling the several source sites

under varying tower configurations and operating conditions. Four air stripper runs were

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chosen. These were all from Site # 1, the area of highest groundwater contamination

concentrations. A range of representative tower packing materials and packing material

bed depths were chosen to allow evaluation of vapor phase carbon usage without

recreating every air stripper computer run. The four tower configurations chosen were:

8.5 Feet of Snow Flakes17.5 Feet of Snow Flakes8.5 Feet of Tripacks

17.5 Feet of Tripacks

Additionally, a vapor phase model run was conducted for air stripper offgas from

groundwater of concentration similar to MW-10. Lastly, a comparison is made between

treating the volume of VOCs removed by air stripping and adsorbed by vapor phasecarbon, with the amount of carbon which would be required to treat the offgas VOCs in

the liquid phase. The carbon data used in this model has characteristics of Type BPLfrom Calgon Carbon. Air flow at an air to water ratio of 30:1 was 401 cubic feet per

minute. The vapor phase carbon adsorber data used was that for Calgon Carbon's 600

cfm unit. It should be noted that the vapor phase carbon evaluation computer model used

does not account for multicomponent competitive adsorption. Because of the

competitive effects of the various organics for carbon particle pore space actual usage

will be less than calculated. Also, this treatability study assumes that the optimum

carbon contactor configuration is employed to make maximum use of the activated

carbon. Actual design is beyond the scope of this treatability study. During actual

operation, vapor phase GAC usage would probably be higher than calculated and

presented here. Lastly, other operating parameters such as humidity will affect vapor

phase GAC usage rates.

2.3.2 Results

The volume of air put through the tower at a rate of 30:1 is approximately 401

cubic feet per minute or 11,353 liters per minute. Influent concentrations vary for each

air stripper configuration based upon the mass of volatiles removed. The results of the

vapor phase carbon modeling for the four selected air stripping tower configurations are

shown in Table 7. The carbon usage values shown in the table below are based upon liters

of contaminated air, containing the particular VOC, per gram of vapor phase carbon.

Table 8 shows the vapor phase carbon usage for an air stripper using contaminated

groundwater from MW-10 as influent. The first configuration is for a tower with 17.5

2176-060/HAZ/2926-11/9/89 2-13

TABLE 7

TowerConfiguration

Tripacks

17.5 feet

Tripacks

8.5 feet

Snowflakes

17.5 feet

Snowflakes

8.5 feet

VAPOR PHASE

AIR

VOC

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

PCE

TCE

DCE

TCA

CARBON USAGE FOR FOUR SEL

STRIPPER CONFIGURATIONS

OFFGAS FROM SITE #1

Liters Air Treatedper Gram Carbon

8,399

14,003

2,098

30,024

9,382

15,368

2,178

32,630

8,761

14,612

2,178

31,169

10,120

16,430

2,288

34,655

,ECTED

Equivalent

280

467

70

1,001

313

512

73

1,088

292

487

73

1,039

338

548

76

1,155

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TABLE 8

VAPOR PHASE CARBON USAGE FOR

AIR STRIPPER CONFIGURATIONS

OFFGAS AT MW-10

Tower Liters Air Treated EquivalentConfiguration VOC per Gram Carbon Water

Tripacks PCE 6,233 208

17.5 feet TCE 16,168 539

DCE 1,684 56

TCA 28,288 943

Tripacks PCE 5,818 194

8.5 feet TCE 15,194 507

DCE 1,641 55

TCA 26,787 893

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TABLE 9

EQUIVALENT LIQUID PHASE CARBON USAGE

FOR CONTAMINATED GROUNDWATER AT SITE #1

Liters of Water TreatedVOC per Gram Carbon

PCE 14

TCE 22

DCE 3

TCA 4

2176-060/HAZ/2926 - 11/9/89 2-16

feet or Tripacks. The second is for a tower which would achieve an effluent quality

which would meet SDWA MCLs. Also shown in Table 9, is an equivalent volume of

contaminated groundwater treated for the Site #1 tower with 17.5 feet of Tripacks.

The data suggests that significantly less vapor phase carbon would be required to

treat air stripper offgases from Site #1 than if carbon were used to treat the groundwater

directly. This ratio is approximately 1:20 vapor phase GAC compared to liquid phase

GAC. Data on liquid phase carbon usage rates is presented in the next section. It is

important to remember, however, that optimum conditions including humidity, etc. were

assumed for these modeling efforts. Under actual operating conditions, this ratio would

probably fall to 1:10.

2.3.3 Vapor Phase Carbon Considerations

Actual employment of vapor phase GAC entails a significant additional capital cost

for venting, fans, dehumidifier, carbon contactor vessel and changes tower design due to

increased pressure drops.

Data obtained from an American Water Works Association Research Report shows

that the mix of technologies used in the overall treatment process train is very

site-specific and warrant detailed study to obtain the optimum cost-effective

treatment. However, at this point in the program, the results of this treatability study

modeling suggests that vapor phase carbon adsorption of air stripper offgas requires

significantly less GAC than liquid phase adsorption and may be the most cost-effectivetreatment at Site #1. At other sites (Sites #2 and #3) the economics may dictate the use

of liquid phase carbon as the primary treatment technology. Liquid phase activatedcarbon adsorption was evaluated during this treatability study and is discussed below. It

must be noted, that any technology used, including combinations of technologies such as

air stripping with liquid phase GAC polishing of effluent, must be evaluated for present

worth cost during the detailed evaluation of alternatives portion of the Feasibility Study.

2.4 Liquid Phase Activated Carbon

The benefits of employing liquid phase activated carbon, air stripping, vapor phase

activated carbon, or a combination of these treatment technologies to remove volatile

organics from groundwater are determined by the distribution of contaminants and their

concentrations. The previously discussed portions of this treatability study have shown

2176-060/HAZ/2926 - 11/9/89 2-17

that while air stripping is an effective method of removing bulk concentrations of volatile

organic compounds from contaminated groundwater, an air stripper designed and

employed alone to reach effluent concentrations below SDWA standards may not be

cost-effective in some areas of the aquifer with high levels of contamination. Theeffluent would require treatment by routing through a second air stripping tower in a

series configuration. One possible more cost-effective approach to be evaluated in the

Feasibility Study is polishing the tower effluent by a liquid phase granular activatedcarbon adsorption system. Often, due to lower capital cost, when the contamination inthe groundwater is relatively low, a more cost-effective method of treating groundwater

could be by using liquid phase activated carbon.

2.4.1 Experimental Design

This treatability study evaluated the effectiveness of liquid phase carbon at the

three site areas as a primary treatment technology to remove VOCs from groundwater.

A multicomponent solute model was employed to account for "worst case" scenario usage

of GAC. The carbon characteristics entered into the model were for Calgon Carbons

Filtersorb 400. Two loading rates were used which resulted in empty bed contact times

(EBCT) of approximately 10 and 15 minutes. In addition to the three site areas, a

multicomponent solute model run was also conducted for effluent from an air stripper

(17.5 feet of Tripacks) at the location of monitoring well MW-10 and for the groundwaterpotentially extracted from monitoring well MI-7. The groundwater from MI-7 was

simulated as being treated directly by liquid phase carbon.

2.4.2 Results

The results of these computer simulations are presented in Tables 10 and 11.

Detailed data and computer printouts are enclosed in the GAC computer modeling data

used with this treatability study report. At 10 and 15 minute empty bed contact times,

there was little difference in the carbon usage rates.

The results show that at the site with higher VOC concentration, it is probably more

cost-effective to remove the bulk of VOCs with an air stripper equipped with vapor phase

carbon adsorption. The amount of vapor phase carbon which would be used to treat air

stripper offgas at Site #1 would be approximately 25 to 50 times less than for liquid phase

alone. A cost analysis must consider other variables, however, before a decision is made.

2176-060/HAZ/2926 - 11/9/89 2-18

TABLE 10

LIQUID PHASE ACTIVATED CARBON USAGE

(SITES #1-3)

voc

PCE

TCE

DCE

TCA

TreatmentCapacity*

91.3

104.1

494.9

663.0

Liters WaterTreated**

11.0

9.6

2.0

1.5

#2 PCE

TCE

DCE

TCA

46.4

51.9

273.2

471.0

21.6

19.3

3.7

2.1

PCE

TCE

DCE

TCA

44.6

51.6

276.1

471.6

22.4

19.4

3.6

2.1

* Milligrams carbon per liter of water

** Per gram of carbon

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TABLE 11

LIQUID PHASE ACTIVATED CARBON USAGE

(MW-10. MI-7)

MonitoringWell

MW-10

voc

PCE

TCE

DCE

TCA

TreatmentCapacity*

23.7

28.0

173.0

178.4

Liters WaterTreated**

42.2

35.7

5.8

5.6

MI-7 PCE

TCE

DCE

TCA

58.9

63.7

265.7

455.5

17.0

15.7

3.8

2.2

* Milligrams carbon per liter of water

** Per gram of carbon

2176-060/HAZ/2926 - 11/9/89 2-20

At Sites #2 and #3, the rate of liquid phase carbon usage drops to one-half the rate of

that at Site #1. At these two locations, the use of liquid phase carbon may be

competitive with air stripping and vapor phase carbon adsorption.

As the concentrations of VOCs decreases, the rate of GAC usage drops

considerably. The effluent from an air stripper at MW-10 in the vicinity of Site #1 is

treated by liquid phase GAC with a usage rate of almost one-quarter that of liquid phase

GAC alone. This is significant since the effluent is from a tower with only 17.5 feet of

Tripacks for packing material. As discussed previously, a tower to remove VOCs toSDWA levels at this site would need at least 38 feet of packing material with an

associated increase in capital and operation and maintenance costs. Also of significance,

as mentioned previously, an air stripping tower alone at this location would probably not

ensure continued effluent standards which meet SDWA MCLs unless a factor of safety

(larger tower or two towers in series) was designed and implemented. Activated carbon

as a component of the treatment train could reduce the present worth cost of the

remediation and also ensure protection of effluent quality by treating any surges of

volatile organic compound concentrations through the system. This is primarily due to

the fact that GAC adsorption performs as a "total trap" for VOCs passing by, provided

that the GAC is not saturated.

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3.0 CONCLUSIONS

This treatability study has been conducted to identify the appropriate treatmenttechnology for the removal of volatile organic compounds from groundwater at theSavage Well Site. The study indicates that the basic technology used should include acombination of air stripping with liquid phase carbon adsorption to polish the effluent inorder to achieve SDWA MCLs. However, the exact configuration of this process trainwill depend directly on the area of plume and net influent concentration to be treated.

The study suggests that the system should include a tower of modest height (15-20feet of packing material), high-efficiency packing material (2-inch Tripacks) and an airto water ratio of 25-31:1. Effluent from the stripper should be treated by liquid phaseGAC contactors with an empty bed contact time of 10-15 minutes.

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4.0 REFERENCES

1. Activated Carbon for Water Treatment. 2d Ed., Sontheimer, Crittenden, Summers,University of Karlsruhe, FRG, 1988, DVGW-Forschungsstelle.

2. AEEP Computer Software Manual. John C. Crittenden, Department of CivilEngineering, Michigan Technological University, June 1986.

3. "An Evaluation of the Technical Feasibility of the Air Stripping Solvent RecoveryProcess", American Water Works Association Research Report, June 1987, Denver,Colorado.

4. "Draft Savage Well Site Remedial Investigation Report", HMM Associates, Inc.(In-Progress).

5. Groundwater Treatment Technology. Evan K. Nyer, Van Nostrand ReinholdCompany, Inc., New York, 1985.

6. "Optimization and Economic Evaluation of Granular Activated Carbon for OrganicRemoval"; American Water Works Association Research Report, June 1989, Denver,Colorado.

7. Perry's Chemical Engineers Handbook. 6th Ed., Robert H. Perry, Don Green,McGraw-Hill Book Company, New York, 1984.

8. "Treatment Processes for the Control of Synthetic Organic Chemicals", ProceedingsAmerican Water Works Association Conference, June 14, 1987, Kansas City,Missouri.

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