Control of Hazardous Gas Emission

According to U.S. EPA, the techniques that are used to control air pollution include the following15:

1. Covering surface impoundments

2. Passive perimeter gas control systems

3. Active perimeter gas control systems

4. Active interior gas collection/recovery systems

Covering surface impoundments

Covering surface impoundments is important for the control of hazardous gases emission. A common covering method places a barrier at the water-air interface to reduce gaseous emissions. The technology available includes lagoon covers, floating immiscible liquids, and floating (polypropylene) spheres.

Covers provide temporary methods for reducing volatile emission from surface impoundment. Floating lagoon covers function as both a surface water control mechanism and a mechanism for controlling gaseous emissions. They are suitable in situations where more than a year will elapse before final closure of a lagoon. They are not suitable for lagoons with weak berms or for lagoons located in areas that cannot support heavy construction equipment.

Floating immiscible liquids are suitable for controlling emissions of water-soluble organics. However, the effectiveness is temporary, estimated to be between one and two weeks. Some chemicals in water may prevent the formation of a monolayer, and wave action can destroy the monolayer effectiveness.

Floating polyethylene spheres are capable of reducing volatile emissions by up to 90%. Polyethylene spheres are compatible with a broad range of compounds including inorganic acids and bases and most aromatic and aliphatic organic compounds.16

Passive perimeter gas control systems

Passive gas control systems control gas movement by altering the paths of flow without the use of mechanical components. There are generally two types, high-permeability and low-permeability.

High-permeability passive perimeter gas control systems entail the installation of highly permeable (relative to the surrounding soil) trenches or wells between the hazardous waste site and the area to be protected (Figure 16.6). The permeable material offers conditions more conductive to gas flow than the surrounding soil, and provides paths of flow to the points of release. High-permeability systems usually take the form of trenches or wells excavated outside the site, then backfilled with a highly permeable medium such as coarse crushed stone.

Low-permeability passive perimeter gas control systems (Figure 16.7) effectively block gas flow into the areas of concern by using barriers (such as synthetic membranes or natural clays) between the contaminated site and the area to be protected. In the low-permeability system, gases are not collected and therefore cannot be conveyed to a point of controlled release or treatment. The low-permeability system can also alter the paths of convective flow.

High-permeability and low-permeability passive perimeter gas control systems are often combined to provide controlled venting of gases and blockage of available paths for gas migration.15 The applications and limitations of passive gas control systems must also be understood. They can be used at virtually any site where there is the capability to trench or drill and excavate to at least the same depth as the landfill. Limiting factors could include the presence of a perched water table or rock strata. Passive vents should generally be expected to be less effective in areas of high rainfall or prolonged freezing temperatures.

Plan view

Plan view

Area to be protected

Drainage swale around landfill

Paved drainage crossing if required

Area to be protected

Drainage swale around landfill

4" PVC, vent pipe* (space @ 50 ± O.C.) 4" PVC perforated collector (continuous)

Paved drainage crossing if required

Drainage swale

Monitoring probe

4" PVC, vent pipe* (space @ 50 ± O.C.) 4" PVC perforated collector (continuous)

Drainage swale

Monitoring probe

For applications where venting of gases to atmosphere is acceptable.

** Collector can be used to convey gases to a treatment system.

For applications where venting of gases to atmosphere is acceptable.

** Collector can be used to convey gases to a treatment system.

FIGURE 16.6 Passive gas control using a permeable trench.

The cost of passive gas control systems is low. The "passive" concept has virtually no operating or maintenance costs. However, it is recommended that periodic inspections be made and that the surface gas be periodically monitored in the area being protected to ensure that the systems are performing their intended functions.

Active perimeter gas control systems

Active perimeter gas control systems control off-site gas migration with the use of an active control system to alter pressure gradients and paths of gas movement by mechanical means. Three or four major components are required in active perimeter gas control systems:

1. Gas extraction wells

2. Gas collection headers e

Section A-A

Any convenient width

FIGURE 16.7 Passive gas control synthetic membrane.

Any convenient width

FIGURE 16.7 Passive gas control synthetic membrane.

3. Vacuum blowers or compressors

4. Gas treatment or utilization systems

Figure 16.8 shows an active perimeter gas extraction system. Active systems can be used at virtually any site where there is the capability to drill and excavate through the materials in the action area to the required depth. Limiting factors of active systems include the presence of freestanding leachate (i.e., saturation) or impenetrable materials. Active perimeter gas control systems are not sensitive to freezing or saturation of the surface or cover soils.

Centrifugal blowers create a vacuum through the collection headers and wells to the wastes and ground surrounding the wells. A pressure gradient is thereby established, inducing flow from the landfill (which is normally under positive pressure) to the blower (creating a negative, or vacuum, pressure). Subsurface gases flow in the direction of decreasing pressure gradient (through the wells, the header, and the blower) and are released directly to the atmosphere, treated and released to the atmosphere, or recovered for use as fuel.15

Plan view

Blower/burner facility

Plan view

Blower/burner facility

Area to be protected

Section A-A

Area to be protected

Section A-A

Gas extraction well Control valve

Monitoring probe

Gas collection header

Gas extraction well Control valve

Gas collection header

FIGURE 16.8 Active gas extraction.

Active interior gas collection/recovery system

Similar to the active perimeter gas control system, an active interior gas collection/recovery system consists of gas extraction wells, gas collection headers, vacuum blowers or compressors, and a treatment system. However, it is used to directly remove the hazardous gases from the site (beneath a landfill), instead of off-site removal. Figure 16.9 shows a schematic view of such a system.

Applications and limitations of the active interior gas collection/recovery system are similar to those of the active parameter gas control system. The active interior gas collection/recovery systems can be used at virtually any site where there is the capability to drill and excavate through landfilled material to the required depth. Limiting factors of the active interior gas collection/ recovery systems include the presence of free-standing leachate or impenetrable materials within the landfill.

16.7.1.2 Control of Fugitive Dusts

Fugitive dusts are caused by wind erosion on waste sites, by vehicular traffic, and by excavation of waste during remedial action. The most commonly used control methods include the following:

1. Dust suppressants

2. Wind fences/screens

3. Water spraying

The dust suppressant method uses chemicals to (temporarily) strengthen bonds between soil particles and reduce fugitive dust emissions from inactive waste piles. Dust suppressant is expected to be 100% effective for a period of one to four weeks if the use of chemical is appropriate and undisturbed. Dust suppressants can also be used to control dust from work areas; however, it is less effective and requires frequent reapplications.

The wind fences/screens method uses screens, which take up or deflect a sufficient amount of wind so that the wind velocity is lowered below the threshold required for initiation of soil movement. The maximum reduction of wind velocity is expected for a distance of one to five fence heights downstream. Tests have shown that wind screens can achieve up to 60% efficiency in controlling inhalable particulates and 75% of total suspended particulates at wind speeds of about 10 to 13 mi/h.

The water spraying method is most commonly used to reduce fugitive dusts emission by spraying water onto the exposed surface area, for example, along active travel paths, excavation areas, and truck boxes loaded with soils.

16.7.1.3 Treatment of Emitted Gases

The gaseous phase of organic and inorganic contaminants that are collected from gaseous waste-streams can be treated. The most common methods are carbon adsorption and scrubbing with water or chemicals.

A mobile gaseous waste treatment unit developed by QUAD Environmental Technologies Corporation17 utilizes atomizing nozzles within the scrubber chamber to disperse droplets of a controlled chemical solution, resulting in 85 to 100% removal (for benzene, toluene, phenol, and so on). Very small droplet sizes (less than 10 ^m) and long retention times allow the use of a "once-through system" that generates low volumes of liquid residuals. This technology is best suited for

VOCs, although it is claimed to treat gaseous wastestreams containing a wide variety of organic or inorganic contaminants.

16.7.1.4 Surface Water Control through Control of Run-On and Runoff

Surface water control is necessary to minimize contamination of surface waters, to prevent surface water infiltration, and to prevent off-site transport of surface waters that have been contaminated. Control of run-on and runoff will accomplish the following:

1. It will prevent surface runoff, which carries contaminants to rivers and to places where the contaminants will infiltrate and percolate into soil and groundwater.

2. It will prevent surface water runoff from entering contaminated areas and in turn migrate into the contaminated plume. The methodology used involves dikes, terraces, diversion channels, floodwalls, grading, and revegetation, for example, using bench, terrace, or grading to divert or intercept surface water.

16.7.1.5 Surface Water Control through Prevention of Infiltration

Capping

Capping is a process used to cover buried waste materials to prevent them coming into contact with the land surface. Hence, capping on landfill can prevent infiltration of surface water to ensure minimum liquid migration through the waste. The materials used for capping usually have a permeability lower than or equal to the underlying liner system or natural soils, and high resistance to damage by settling or subsidence. Capping requires low cover maintenance and increases the efficiency of site drainage.

Capping is necessary whenever contaminated materials are to be buried or left in place at a site. Capping is often performed together with groundwater extraction or containment technologies to reduce further plume development, thus reducing the time needed to complete groundwater cleanup operations. In addition, groundwater monitoring wells are often used to detect any unexpected migration of capped wastes. A gas collection system should always be incorporated into a cap when wastes may generate gases. Capping is also associated with other surface water control technologies as discussed later. The main disadvantages of capping are the need for long-term maintenance and uncertain design life. A final cap should be inspected on a regular basis for signs of erosion, settlement, ponding liquid, invasion of deep-rooted vegetation, or subsidence, especially in the first six months when problems are most likely to appear. However, the long-term maintenance requirements are usually considerably more economical than excavation and removal of the wastes. Another disadvantage is the high cost of proper soil and drainage materials in certain areas of the country.

Caps can be single-layered or multilayered depending on the cap materials used. For construction and implementation considerations the reader can refer to U.S. EPA15 and Matrecon, Inc.18

Grading

Grading is the technique used to reshape the surface in order to minimize infiltration by maximizing the amount of water thath will run off without causing significant erosion. Grading is often performed in conjunction with surface sealing practices and revegetation as part of an integrated landfill closure plan.

Grading is a relatively inexpensive remedial action component when suitable cover materials are available on site or close to the disposal site. Surface grading serves several functions:

1. It reduces ponding, which minimizes infiltration and reduces subsequent differential settling

2. It reduces runoff velocities and do reduces soil erosion

3. It roughens and loosens soils in preparation for revegetation

4. It is a factor in reducing or eliminating leaching of wastes

It is important upon completion of grading to establish vegetation cover as quickly as possible. This cover is essential to help prevent drying and erosion.

Revegetation

Revegetation is a cost-effective method to stabilize the surface of hazardous waste disposal sites, especially when preceded by capping and grading. Revegetation decreases erosion by wind and water and contributes to the development of a naturally fertile and stable surface environment. It may be part of a long-term site reclamation project, or it may be used on a temporary or seasonal basis to stabilize intermediate cover surfaces at waste disposal sites. A systematic revegetation plan includes the following steps:

1. Selection of suitable plant species

2. Seedbed preparation

3. Seeding/planting

4. Mulching and/or chemical stabilization

5. Fertilization

6. Maintenance

Revegetation may not be feasible at disposal sites with high cover soil concentrations of phyto-toxic chemicals, unless these sites are properly sealed and vented and then recovered with suitable topsoil. In some cases, clays or synthetic barriers below supporting topsoil in poorly drained areas may cause swamping of the cover soil and subsequent anaerobic conditions. A cover soil that is too thin may dry excessively in arid seasons and irrigation may be necessary. Improperly vented gases and soluble phytotoxic waste components may kill or damage vegetation. The roots of shrubs or trees may penetrate the waste cover and cause leaks of water infiltration and gas exfiltration. Also, periodic maintenance of revegetation areas (liming, fertilizing, mowing, replanting, or regarding eroded slopes) will add to the costs associated with this remedial technique.

Although vegetation cover requires frequent maintenance, it prevents the more costly maintenance that would result from erosion of surface soils.

16.7.1.6 Surface Water Control through Control of Erosion

Control of erosion is usually implemented through reducing slope length (using interception dikes, diversion channels, and terraces), slope steepness (using proper grading), or improving soil management, as well as controlling infiltration or erosion (using grading and revegetation). Most of these technologies have been addressed above (e.g., grading and revegetation) or will be addressed later (e.g., dikes and channels).

16.7.1.7 Surface Water Control through Collection and Transfer of Water

The purpose of the collection and transfer of water is to collect water that has been diverted away from the site or been prevented from infiltrating, and discharging or transferring the collected water to storage or treatment.15 Surface water control can be carried out using dikes and berms, channels, chutes, and downpipes.

Dikes and berms are well-compacted earthen ridges or ledges located immediately upslope from or along the perimeter of a disturbed area (e.g., disposal sites). They can prevent excessive erosion of newly constructed slopes until more permanent drainage structures are installed or until the slope is stabilized with vegetation, and are widely used to provide temporary isolation of wastes until they can be removed or effectively contained, particularly during excavation and removal operations, to prevent runoff and mixing of incompatible wastes. For cost estimates of various technologies used to prevent infiltration one can refer to the U.S. EPA publication "Remedial Action at Waste Disposal Sites."15

Dikes and berms usually provide short-term protection of critical areas by intercepting storm runoff and diverting the flow to natural or manmade drainage ways, to stabilized outlets, or to sediment traps. These can only handle relatively small amounts of runoff and are not recommended for drainage areas larger than five acres.19 Channels are wide and shallow excavated ditches used to intercept or divert water as well as collect and transfer the diverted water elsewhere. Chutes (or flumes) and downpipes are used to carry surface runoff from one level to a lower level without erosive damage and to enable the transfer of water away from diversion structures. They provide temporary erosion control while slopes are being stabilized with vegetative growth. Chutes are limited to head-drops of about 5.5 m (18 ft) or less, and downpipes are limited to drainage areas five acres in size.

16.7.1.8 Surface Water Control through Protection from Flooding

Flood control dikes (or embankment), levees, and floodwalls are the most common flood protection structures. They are used in areas subject to inundation from tidal flow or riverine flooding, but not for areas directly within open floodways. Levees create a barrier to confine floodwaters to a flood-way and to protect structures behind the barrier. Floodwalls perform much the same function as levees, but are constructed from concrete.

16.7.1.9 Surface Water Control through Storage and Discharge of Water

Sedimentation basins can be used to collect and store surface water flow and to settle suspended solid particles. Seepage basins and ditches can be used to discharge uncontaminated or treated water downgradient of the site. It is important to separate clean surface runoff from contaminated water and store and treat them separately. Table 16.4 summarizes the surface water control methods.

16.7.1.10 Control of Waste Movement at Roads and Residential Areas

Site control at roads and residential areas will include at least the following activities:

1. Clearing the road, or, alternatively, building a detour route

2. Establishing signs at dangerous areas

TABLE 16.4

Primary Functions of Various Surface Water Control

Technology

Capping Lagoon covers Grading Revegetation Dikes and berms Channels and waterways Terraces and benches Chutes and downpipes Seepage basins and ditches Sedimentation basins and ponds Levees and floodwalls

Prevent or Intercept Run-on/Runoff

Prevent or Minimize Infiltration x x

Reduce Erosion

Collect and Transfer Water

Protection from Flooding

Discharge Water

Source: U.S. EPA, Remedial Action at Waste Disposal Sites, EPA/625/6-85/006, U.S. EPA, Washington, DC, 1985.

3. Preventing fire associated with low ignition point volatile organics

4. Evacuating residents and protecting the area, or providing a facility for treatment of drinking water and cleanup of the site

5. Providing subsurface control of migration of contaminants

16.7.2 Subsurface Site Control of Waste Movement 16.7.2.1 Controls of Groundwater

The purpose of groundwater control includes the following aspects:

1. To contain a plume

2. To prevent migration of contaminated groundwater that may enlarge the size of the contaminated area and lead to the contamination of clean groundwater

3. To prevent clean groundwater from moving into the contaminated site, which may cause further migration and enlargement of the contaminated area

4. To prevent leachate formation by lowering the water table beneath a source of contamination or by preventing infiltration

5. To pump out the contaminated groundwater or perform in situ treatment to halt the source of contamination

Groundwater pumping

Groundwater pumping can remove the contaminated plume directly, or reconfigure the migration of groundwater through the cone of depression, which can either prevent further migration of contaminants or prevent movement into clean groundwater, or lower the water table. Extraction wells or a combination of extraction and injection wells can be used for this purpose. Figure 16.10a shows how an extraction well controls the movement of groundwater through the cone of depression, thus ensuring that clean water will be withdrawn from the domestic well. Figure 16.l0b shows the use of a line of extraction wells to protect a domestic well.

The cone of depression can be evaluated based on an expression that relates to the measured saturated thickness of the aquifer, the height of water at the well from the bottom of the aquifer, pumping rate, hydraulic conductivity, and the radius of the observation wells (Jacob and Theis methods). Note that the formulae for calculation of the cone of depression are different for different confining conditions, for example, unconfined, artesian, and leaking confined aquifers. Various computer models have been established for groundwater flow, or associated with particle transfer or with chemical reactions (such as MODFLOW, MODPATH, and MOC, developed by the U.S. Geological Survey). Graphical or computer-aided calculations are usually used for composite drawdowns by multiwells (extraction or injection).

Subsurface drains

Subsurface drains include any type of buried conduits that convey and collect aqueous discharges by gravity flow (Figure 16.11). Water collected in a storage tank or a collection sump is then pumped for further treatment. Filters are usually needed in drain systems to prevent fine particles from causing clogging.

Subsurface drains function like an infinite line of extraction wells, and can be used to contain and remove a plume or to lower the groundwater table (Figure 16.12). They are more cost-effective than pumping for shallow contamination problems at depths of less than 12 m (40 ft). Depths may be increased if the site is stable, if the soil has a low permeability, and if no rock excavations are encountered.

Subsurface barriers

Subsurface barriers, low-permeability cutoff walls or diversions below ground are used to contain, capture, or redirect groundwater flow. The most common method uses bentonite slurry

(a) Cross-sectional view

To treatment

Domestic

To treatment

Domestic

(a) Cross-sectional view

Plan v

Extraction wells with radius of influences

Domestic well

FIGURE 16.10 Containment using extraction wells: (a) cross-sectional view; (b) plan view.

walls. Less common are other types of slurry walls (such as concrete), grouted barriers, and sheet piling cutoffs. The limiting factor for slurry walls is site topography, which may cause increasing engineering effort and cost. Also, slurry walls may not maintain good performance over a long period of time.

Grouted barriers use a variety of fluids injected into a rock or soil mass, which is set in place to reduce water flow and strengthen the formation. Grouted barriers are seldom used for containing groundwater flow in unconsolidated materials around hazardous waste sites because they cost more and have lower permeability than bentonite slurry walls. Nevertheless, they are suited to sealing voids in rock for waste sites remediation.

Sheet piling uses wood, precast concrete, or steel to form barriers for groundwater. They are seldom used because of high costs and unpredictable wall integrity, except for temporary dewater-ing for other construction or as erosion protection for other barriers. Bottom sealing is the technique used to place a horizontal barrier beneath an existing site to act as a floor and prevent downward migration of contaminants.

FIGURE 16.11 Subsurface drainage system components. Control of sediments

Various technologies such as dikes, covers, and in situ grouting can be used for the control of migration of contaminants from contaminated sediments or for prevention of contamination of clean sediments.

FIGURE 16.11 Subsurface drainage system components. Control of sediments

Various technologies such as dikes, covers, and in situ grouting can be used for the control of migration of contaminants from contaminated sediments or for prevention of contamination of clean sediments.

16.7.3 In Situ Groundwater Remediation

In situ groundwater treatment is an alternative to the conventional pump-and-treat methods. In situ treatment uses biological or chemical agents or physical manipulations that degrade, remove, or immobilize contaminants. In situ treatment technologies can usually treat both contaminated groundwater and soil. In many instances a combination of in situ and aboveground treatment will achieve the most cost-effective treatment at an uncontrolled waste site.

resulting in larger collection and treatment.

FIGURE 16.12 Use of a one-sided subsurface drain for reducing flow from uncontaminated sources.

resulting in larger collection and treatment.

FIGURE 16.12 Use of a one-sided subsurface drain for reducing flow from uncontaminated sources.

16.7.3.1 Biological Treatment

Bioremediation is a technique for treating zones of contamination by microbial degradation, which involves altering the environmental conditions to enhance microbial catabolism or cometabolism of organic contaminants, resulting in the breakdown and detoxification of those contaminants.15 According to microbial metabolic activity, bioremediation can be classified into three categories20,21:

1. Aerobic respiration, in which oxygen is required as a terminal electron acceptor

2. Anaerobic respiration, in which sulfate or nitrate serves as the terminal electron acceptor

3. Fermentation, in which the microorganism rids itself of excess electrons by exuding reduced organic compounds

The in situ biological treatment technique for organic contaminants is fully discussed in the Chapter 17. An example of a cost estimate for bioremediation is shown in Table 16.5. The data is based on a U.S. EPA study15 of a project performed by Biocraft Laboratories, Waldwick, New Jersey.

16.7.3.2 Chemical Treatment

Chemical treatment of groundwater uses chemicals to immobilize or detoxify the organic or inorganic contaminants. Appropriate chemicals should be selected and pH or Eh are generally controlled. For example, for in situ treatment of inorganics, the most commonly used chemicals are sulfide, carbonate/hydroxide, and phosphate, which lead to the oxidation or reduction of contaminants or cause the precipitation of target materials;22-24 for in situ treatment of organics, the methods of chemical oxidation and hydrolysis are used for detoxification, and polymerization is used to reduce the mobility of the contaminants. Generally, it is easier to control chemical processes in pumped

TABLE 16.5

Example—Summary of Project Costsa (Biocraft Laboratories, Waldwick, NJ)

Task

Hydrogeological study: problem definition In-house process development (R&D) Groundwater collection/injection system total Design Installation

Biostimulation plant design and construction total Engineering design Masonry construction Equipment and miscellaneous installation Capital and R&D total Operation and maintenance (O&M) Utilities

Electricity: 26.4 kW (24 h/d) Steam: 72 lb (33 kg)/d & 90 psi Maintenance (see text) O&M total Total water treated

Actual Expenditure

$73,948

$446,280

$184,243

$193,187

$926,158

$47.40/d ($195.25/d) ($61.92/d) $159.93/d $226.53/d 13,680 gal/d (51,779 L/d)

Unit Cost

Period of Performance

1976-1978 1978-1981

1980-1981

1981

1981

1981

1981

1983 rates 1981

Source: U.S. EPA, Remedial Action at Waste Disposal Sites, EPA/625/6-85/006, U.S. EPA, Washington, DC, 1985. a U.S. ACE (Cost Index for Utilities) may be used to convert costs into current USD.

TABLE 16.6 Chemical Costsa

Category

Acids

Bases

Chelating agents

Fertilizers

(microbial nutrients)

Liming material

Oxidizing agents

Reducing agents Precipitating agents

Surfactant Anionic

Nonionic

Chemical

Hydrochloric acid, 20° Baume tanks Nitric acid 36° to 42° Baume tanks Sulfuric acid Virgin, 100% Smelter, 100% Caustic soda, liquid 50%, low iron Ammonium chloride Citric acid

Ammonia, anhydrous, fertilizer

Ammonium chloride

Ammonium sulfate

Sodium monophosphate

Sodium diphosphate

Phosphoric acid

75%, commercial grade

52-54% a.p.a., agricultural grade

Potassium-muriate, 60 to 62%, minimum

Potassium chloride

Potassium-magnesium sulfate

Agricultural limestone (dolomite)

Lime

Hydrated lime Hydrogen peroxide, 35% Potassium permanganate Caustic soda, liquid 50%, low iron Ferrous sulfate Heptahydrate Monohydrate

Witconate 605A

Witconate P-1020BV (calcium sulfonates) Adsee 799

Cost/Unit

$61-95.9/t $48-65/t $255-285/t $18/1GG lb $G.81-$1.19/lb $14G-$215/t $18/1GG lb $73-79/t $55.75/1GG lb $54.5G/1GG lb

$3.1G/unit-tonb

$255-285/t

Source: U.S. EPA, Remedial Action at Waste Disposal Sites, EPA/625/6-85/006, U.S. EPA, Washington, DC, 1985.

a Use Appendix (USACE, Cost Index for Utilities) to convert costs into current USD.

b Unit-ton: 1% of 2000 lb of the basic constituent or other standard of the material. The percentage figure of the basic constituent multiplied by the unit-ton price gives the price of 2000 lb of the material.

groundwater than in in situ groundwater. Costs of chemicals are listed in Table 16.6. Oxidizing agents, such as hydrogen peroxide, are commonly used for in situ groundwater remediation.23 24 72

16.7.3.3 Permeable Reactive Barrier

A permeable reactive barrier (PRB) is defined as an in situ method for remediating contaminated groundwater that combines a passive chemical or biological treatment zone with subsurface fluid flow management. Treatment media may include zero-valent iron, chelators, sorbents, and microbes to address a wide variety of groundwater contaminants, such as chlorinated solvents, other organics, metals, inorganics, and radionuclides. The contaminants are concentrated and either degraded or retained in the barrier material, which may need to be replaced periodically. There are approximately 100 PRBs operating in the U.S. and at least 25 internationally.

PRBs can be installed as permanent or semipermanent units. The most commonly used PRB configuration is that of a continuous trench in which the treatment material is backfilled. The trench is perpendicular to and intersects the groundwater plume. Another frequently used configuration is the funnel and gate, in which low-permeability walls (the funnel) direct the groundwater plume toward a permeable treatment zone (the gate). Some gates are in situ reactors, which are readily accessible so as to facilitate the removal and replacement of reactive media. These PRBs use collection trenches, funnels, or complete containment to capture the plume and pass the groundwater, by gravity or hydraulic head, through a vessel containing either a single treatment medium or sequential media. In circumstances where in situ treatment is found to be impracticable, reactive vessels have been located above ground.

Zero-valent iron has performed so successfully in PRB technology that it is now being applied directly for source zone treatment. Alhough this measure is not considered a PRB, examples of the technology will be included under the heading PRB because the reactive media and treatment mechanism are related. Pneumatic fracturing and injection, hydraulic fracturing, and injection via direct push rigs have been used successfully to introduce the reactive media to the groundwater or soil source area.74-76

16.7.3.4 Circulating Wells and In-Well Air Stripping Technologies

Circulating wells (CWs) provide a technique for subsurface remediation by creating a three-dimensional circulation pattern of the groundwater. Groundwater is drawn into a well through one screened section and is pumped through the well to a second screened section where it is reintroduced to the aquifer. The flow direction through the well can be specified as either upward or downward to accommodate site-specific conditions. Because groundwater is not pumped above ground, pumping costs and permitting issues are reduced and eliminated, respectively. Also, the problems associated with storage and discharge are removed. In addition to groundwater treatment, CW systems can provide simultaneous vadose zone treatment in the form of bioventing or soil vapor extraction.

CW systems can provide treatment inside the well, in the aquifer, or a combination of both. For effective in-well treatment, the contaminants must be adequately soluble and mobile so they can be transported by the circulating groundwater. Because CW systems provide a wide range of treatment options, they provide some degree of flexibility to a remediation effort.

In-well vapor stripping technology involves the creation of a groundwater circulation pattern and simultaneous aeration within the stripping well to volatilize VOCs from the circulating groundwater. Air-lift pumping is used to lift groundwater and strip it of contaminants. Contaminated vapors may be drawn off for aboveground treatment or released to the vadose zone for biodegradation. Partially treated groundwater is forced out of the well into the vadose zone, where it reinfiltrates to the water table. Untreated groundwater enters the well at its base, replacing the water lifted through pumping. Eventually, the partially treated water is cycled back through this process until contaminant concentration goals are met.

16.7.3.5 Air Sparging in Aquifers

Air sparging involves the injection of air or oxygen through a contaminated aquifer. Injected air traverses horizontally and vertically in channels through the soil column, creating an underground stripper that removes volatile and semivolatile organic contaminants by volatilization. The injected air helps to flush the contaminants into the unsaturated zone. Soil vapor extraction (SVE) is usually implemented in conjunction with air sparging to remove the generated vapor-phase contamination from the vadose zone. Oxygen added to the contaminated groundwater and vadose-zone soils can also enhance biodegradation of contaminants below and above the water table.77

16.7.3.6 Multiphase Extraction

Multiphase extraction uses a vacuum system to remove various combinations of contaminated groundwater, separate-phase petroleum product, and vapors from the subsurface. The system lowers the water table around the well, exposing more of the formation. Contaminants in the newly exposed vadose zone are then accessible to vapor extraction. Once above ground, the extracted vapors or liquid-phase organics and groundwater are separated and treated.

16.7.4 Pump-and-Treat Groundwater Remediation

The pump-and-treat methodology is effective for groundwater remediation. It is also an effective way to prevent the further extension of a contaminated area. The cleanup involves two steps:

1. Pumping the contaminated groundwater out from the site

2. Treating the pumped contaminated water on ground so that it can be returned to the system

In order to effectively pump all contaminated water out of an aquifer (or soil) pore space, water injection is usually needed, and sometimes a chemical flushing agent.

The pump-and-treat method is comparable to soil flushing. In fact, the pump-and-treat method can treat both groundwater and aquifer soil at the same time, and can also be directly applied to unsatu-rated soil zones. The soil flushing method is mainly considered as a treatment in unsaturated zones.

16.7.4.1 Pumping Systems

There are three common methods for groundwater collection using pumping systems: a well point system, a gravity drain system, and in combination with injection wells.

Well point system

A well point system consists of several individual well points spaced at 0.6 m to 1.8 m intervals along a specified alignment. A well point is a well screen (length 0.5 to 1.0 m) with a conical steel drive point at bottom. Individual well points are attached to a riser pipe (diameter 2.5 to 7.5 cm) and connected to a header pipe (diameter 15 to 20 cm). At the midpoint, the header pipe is connected to a centrifugal pump. As yield at different well points may vary, a valve at the top of each riser pipe is used to control the drawdown so that the screen bottom is exposed. The pump provides 6 to 7.5 m of suction, but friction losses reduce the effective suction to 4.5 to 5.4 m.

The well point system is the most economical method of groundwater collection where the water table is less than 3 m and the contaminant is less 9 m below the surface.

Gravity drain system

A trench is excavated perpendicular to the flow of groundwater to a depth below the water table. A perforated pipe is placed in the trench and the remainder of the trench is backfilled with gravel. Groundwater is collected in a main collector pipe and flows to a sump, from which it is pumped to the surface for treatment.

The gravity drain system is most effective when circumstances are suitable to gravity flow, the water table is less than 3 m and the contaminant is less than 9 m below the surface.

Combination with injection wells

The main purpose of recharging water into an aquifer is to elevate the hydraulic gradient to promote the movement of groundwater towards the collection system, thus enhance the efficiency of pumping.

There are two general recharge systems, recharge basins and injection wells. The recharge of treated groundwater into the system provides a method for the discharge of treated groundwater. The recharging of water can also have other purposes, such as creating a hydraulic barrier to restrict the migration of a contaminant plume, and providing a method for introducing flushing solutions into the groundwater to flush the pollutants out of soil.

16.7.4.2 Treatment of Pumped Water

Gravity liquid separation

Gravity liquid separation uses gravity force to separate the liquid-phase contaminant from water (immiscible with the contaminant) by the force of gravity.

Gravity separators can take many shapes and arrangement, depending in part on the characteristics of the waste. Typical design configurations include horizontal cylindrical decanters, vertical cylindrical decanters, and cone-bottomed settlers.

Sedimentation

Sedimentation is commonly applied to the treatment of pumped groundwater containing high concentrations of suspended solids.25 It can also be used to remove the suspended solids from collected surface runoff, leachate or landfill toe seepage, and dredge slurries as a pretreatment step for biological treatment or many chemical processes, including precipitation, carbon adsorption, ion exchange, stripping, reverse osmosis, and filtration.22-24

Chemical precipitation/coagulation, flocculation, and clarification

Chemical precipitation/coagulation methods transfer the target substances (mainly metals) in solution into a solid phase. Many heavy metal hydroxides and sulfides have very low solubility (within a certain pH range) and are therefore insoluble. The metal sulfides have significantly lower solubility than their hydroxide counterparts over a broad range of pH.26 Precipitation/coagulation is also applicable for removing certain anionic species such as phosphate, sulfate, and fluoride.

Lime and sodium sulfide are the most common chemical agents added to contaminated water in a rapid mixing tank. Generally, flocculating agents (such as alum, lime, or iron salts) are added along with the precipitating agents.27 Agglomerated particles are separated from the liquid phase by settling in a sedimentation clarifier, by floating in a dissolved air flotation (DAF) clarifier,28,29,71-73 or by other physical processes such as filtration.22 Figure 16.13 is a typical configuration for precipitation, flocculation, and sedimentation clarification,15 in which the sedimentation clarifier may also be replaced by a DAF cell28-30,71-73 for cost and space saving.

Certain physical or chemical characteristics of the wastestream may limit the application of precipitation. For example, some organic compounds (as well as cyanide or other ions) may form organometallic complexes with metals, decreasing the precipitation potential.

Wang and colleagues71-73,100 have developed a physical-chemical sedimentation sequencing batch reactor (PCS-SBR) process and a physical-chemical flotation sequencing batch reactor (PCF-SBR) process for the treatment of contaminated groundwater, potable water, and wastewater. The reactor of a PCS-SBR process is similar to a conventional biological sequencing batch reactor (SBR), except that chemical flocs (instead of biological activated sludges) are used for water and wastewater treatment. A PCF-SBR is another physical-chemical SBR process in which flotation (instead of sedimentation) is used for the separation of chemical flocs from the flocculated water.

Precipitation Precipitating chemicals Flocculating agents

Inlet liquid stream -

Sedimentation

Sedimentation

Rapid mix tank After the addition of precipitating chemicals, the precipitation reaction commences to form very small particles called precipitation nuclei. The flocculating agents allow these particles to agglomerate.

By slow and gentle mixing, the precipitated particles aided by the flocculating agents, collide, agglomerate, and grow into larger settleable particles.

Sedimentation basin The settleable particles produced by the flocculation step are settled, collected and periodically removed.

FIGURE 16.13 Representative configuration using precipitation, flocculation, and sedimentation.

Rapid mix tank After the addition of precipitating chemicals, the precipitation reaction commences to form very small particles called precipitation nuclei. The flocculating agents allow these particles to agglomerate.

By slow and gentle mixing, the precipitated particles aided by the flocculating agents, collide, agglomerate, and grow into larger settleable particles.

Sedimentation basin The settleable particles produced by the flocculation step are settled, collected and periodically removed.

FIGURE 16.13 Representative configuration using precipitation, flocculation, and sedimentation.

Ion exchange

Ion exchange is a reversible interchange of ions between a liquid and a solid phase. The ions (contaminants) in a liquid wastestream and the ions on the surface of an ion-exchange resin are exchanged, purifying the wastestream while concentrating the waste constituent in the resin.22-24 Mixed resins are sometimes effective in removing both cations and anions.

The ion-exchange process is applicable for removing a broad range of ionic species from water containing all metallic elements, inorganic anion such as halides, sulfates, nitrates, cyanides, organic acids such as carboxylics, sulfonics, some phenols at sufficiently alkaline pH conditions, and organic amines at sufficiently acidic conditions.

The upper concentration limit for ion exchange is about 2500 to 4000 mg/L. A high concentration of pollutants can result in the rapid exhaustion of the resin, resulting in high regeneration costs. The feed stream must be free of oxidants. Suspended soil material in the feed stream should be less than 50 mg/L to prevent plugging the resins. Recently, an ion-exchange sequencing batch reactor (IX-SBR) was developed by Wang and colleagues71 for groundwater decontamination and industrial effluent treatment.71,100

Conventional filtration and automatic backwash filtration

Conventional filtration is widely used to remove suspended solids from solution by forcing the fluid through a porous medium. Filter media usually consist of a bed of granular particles, typically sand or sand with anthracite or coal. The filtrates are usually greater than 1 pm in diameter. The filtration is termed conventional, in order to distinguish it from other types of filtration such as membrane filtration (for particles less than 1 pm). As water passes through the filter bed, the particles become trapped on top of and within the filter bed, thus in time reducing the filtration rate. Therefore, backwash is periodically needed and filtration is often preceded by sedimentation31 or flotation28,32,33,71-73 to reduce the suspended solid load on the filter.

Membrane filtration processes

Membrane filtration processes have been successfully applied to the field of environmental engineering for air pollution control,34 potable water purification,22-24 groundwater decontamination,35,36 industrial effluent treatment,37 hazardous leachate treatment,35,36 and site remediation,36 mainly because membrane filtration can remove heavy metals and organics.

There are three major types of membrane processes, each with different physical means of operation: reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF). In addition, electro-dialysis (ED) is also considered to be a membrane process.

In ED, cation-exchange membranes are alternated with anion-exchange membranes in a parallel manner to form compartments 0.5 to 1.0 mm thick. The entire membrane assembly is held between two electrodes. When an electrical potential is applied to the electrodes, all positive ions tend to travel towards the negative electrode, and all negative anions tend to move toward the positive electrode.

In the other three membrane processes, for example, in RO, a membrane is mounted in an apparatus so that a two-section compartment is formed. Contaminated water is pressurized and circulated through the high-pressure-solution compartment. Water permeates to the low-pressure side and is removed. The concentrated brine is removed separately.

The main difference between the UF, MF, and RO arrangements is membrane pore size, which allows different sizes of particles to pass through the membrane. All three processes allow certain solvent molecules to pass through, and impede certain sizes of particles. MF impedes the passage of large colloids and small particles, UF membranes impede the passage of molecules with a molecular weight of 100 or higher, and the membranes used in RO allow the passage of water, but impede the passage of salts and small molecules. of the three membrane filtration processes, RO requires the highest pressure.

The main advantages of membrane processes are their ability to separate impurities from water for recovery, low operation cost, and a requirement for only a small amount of space for installation. Their limitation lies in the possibility of deterioration of the membranes by certain kinds of water streams, for example, water containing certain strong oxidizing compounds or at high temperatures.

Recently, Wang100 introduced a membrane sequencing batch reactor (membrane-SBR) process for groundwater decontamination, water purification, and industrial effluent treatment. A membrane-SBR is similar to conventional SBR except that membrane filtration is used (instead of sedimentation) for the separation of mixed liquor suspended solids (MLSS) from the mixed liquor.

Activated carbon adsorption

Activated carbon has high specific surface area with respect to its volume, and thus has high adsorption capacity. Activated carbon adsorption is considered to be one of the most versatile treatment technologies and can remove classical pollutants such as COD, TOC, BOD, and nitrogen, as well as toxic pollutants such as phenol, refractory organic compounds, VOCs, and soluble heavy metals.38 Activated alumina and peat have also demonstrated similar abilities.

Once the micropore surfaces of activated carbon are saturated with target material, the spent carbon must be replaced or regenerated. Granular activated carbon (GAC) is favored over powder activated carbon (PAC) in most cases, because the former is considered to be capable of regeneration and sustainable to flow, although the costs of both carbons and the cost of regeneration are high.

Activated carbon adsorption is used to remove soluble organics, suspended solids, and refractory organics that cannot be biodegraded in groundwater. Because of its high cost and its ability to result in low pollutant concentration in effluents, activated carbon is usually used following biological treatment or granular media filtration in order to reduce the load on the carbon columns. PAC can be dosed into an SBR for facilitating physical-chemical or biological reactions for groundwater decontamination.71-73

Biological sorption

The biological sorption technique uses biogenetic materials for the adsorption of contaminants. The AlgaSorb sorption process developed by Bio-Recovery Systems, Inc., is designed to remove heavy metal ions from aqueous solution based on the mutual affinity of the cell walls of algae and heavy metal ions. The sorption medium comprises algal cells immobilized in a silica gel polymer. The system functions as a biological ion-exchange resin to bind both metallic cations (positively charged ions) and large metallic anions. Like ion-exchange resins, the algae-silica medium can be recycled. This technology is useful for removing metal ions from groundwater and surface leachate that contain high levels of dissolved solids.23 24

Solvent extraction

Solvent extraction is the separation of constituents from a liquid solution by contact with another immiscible liquid. It is mainly used for the recovery of organics from liquid solutions.39 Specifically, solvent extraction uses an organic solvent as an extractant to separate organic and metal contaminants from soil. The organic solvent is mixed with contaminated soil in an extraction unit. The extracted solution is then passed through a separator, where the contaminants and extractant are separated from the soil. Organically bound metals may be extracted along with the target organic contaminants.78 From a process viewpoint, three steps are involved:

1. Actual extraction of the solvent by forced mixing or countercurrent flow

2. Solute removal from the extracting solvent

3. Solvent and extracted solute recovery

Significant energy consumption and other operating costs are expected. This method of treatment becomes cost-effective when material recovery is significant.

Chemical oxidation

Chemical oxidation typically involves reduction/oxidation (redox) reactions that chemically convert hazardous contaminants to nonhazardous or less toxic compounds that are more stable, less mobile, or inert. Redox reactions involve the transfer of electrons from one compound to another. Specifically, one reactant is oxidized (loses electrons) and one is reduced (gains electrons). The oxidizing agents most commonly used for the treatment of hazardous contaminants in soil are ozone, hydrogen peroxide, hypochlorites, chlorine, chlorine dioxide, potassium permanganate, and Fentons reagent (hydrogen peroxide and iron). Cyanide oxidation and dechlorination are examples of a chemical treatment. This method may be applied in situ or ex situ, to soils, sludges, sediments, and other solids, and may also be applied to the in situ treatment of groundwater.22 24,79,80

Chemical oxidation technology is primarily used for the detoxification of cyanide and other oxi-dizable organics such as aldehydes, mercaptans, phenols, unsaturated acids, and certain pesticides.40

For example, cyanide detoxification involved the following process106:

Oxidation can be an effective way of pretreating waste prior to biological treatment either by detoxification or by rendering refractory compounds to be more amenable to biological treatment.

Chemical Waste Management, Inc., has developed a technique that is a combination of evaporation and catalytic oxidation processes. Contaminated water is concentrated in an evaporator by boiling off most of the water and its volatile contaminants, both organic and inorganic. Air or oxygen is added to the vapor and the mixture is forced through a catalyst bed, where the organic and inorganic compounds are oxidized. This stream, composed mainly of steam, passes through a scrubber, if necessary, to remove any acid gases formed during oxidation. The stream is then condensed or vented to the atmosphere. This technique can be used to treat complex contaminated waters that contain volatile and nonVOCs, salts, soluble heavy metals, and volatile inorganic compounds.

The limitation for chemical oxidation is that oxidation is frequently not completed to the final products CO2 and H2O. This can be due to a number of factors, including oxidant concentration, pH, redox potential, or the formation of stable intermediate toxic oxidation products.

Chemical reduction

Chemical reduction is used to transform a toxic substance with a higher valence to a nontoxic or less-toxic substance with lower valence. The most promising application is the reduction of hexava-lent chromium to trivalent chromium. This method is also applicable to other multivalent metals such as lead and mercury. Commonly used chemical agents for this purpose are sulfite salts, sulfur dioxide, and base metals (e.g., iron and aluminum).22-24

Biological treatment

Biological treatment technology, also known as bioremediation technology, is mainly used to treat organic contaminants (as terminal electron acceptor to bacteria). Bioremediation techniques include the use of two primary respiratory pathways: aerobic and anaerobic.20,21 Each approach has advantages and limitations. To date, aerobic systems using naturally occurring microorganisms are most widely implemented. Aerobic systems tend to be more efficient when degrading petroleum-based organic contaminants such as benzene, toluene, ethylbenzene, xylenes (BTEX), and naphthalene. Research suggests that aerobic systems are not as effective for the treatment of highly chlorinated compounds such as PCBs (polychlorinated biphenyls). However, genetically engineered microbial system (GEMS) are increasingly used in research applications for recalcitrant compounds. Research scientists41 have developed techniques to modify microbial DNA to enable organisms to degrade contaminants that are currently very recalcitrant (i.e., PCBs) or extremely toxic (i.e., dioxin). Some bacteria can use certain inorganics as the terminal electron acceptor, so biological decontamination of inorganic materials is feasible. The following presents an example on biological decontamination of inorganic materials.

Ehrlich42 used biotechnology coupled with physicochemical extraction to remove chromium from contaminated soil including recovery and reuse. Ehrlich's biological treatment is based on an oxygen-insensitive bacterial respiration, with chromate as the terminal electron acceptor using intact cells and cell extracts. The bacterial strain used to reduce chromate is Pseudomonas fluorescens LB300, which has chromate resistance to more than 2000 mg/L of potassium chromate, although very slight resistance to potassium dichromate. In the Ehrlich process, the highly Cr-concentrated solution was recovered through ion exchange, and the low-concentration solution was then treated by reducing Cr(VI) to Cr(III) in a rotating biological contactor (RBC). The Cr(III) slurry was recovered through sedimentation and purification for reuse.

Wang and colleagues43,71,100 have developed a biological flotation process for the treatment of contaminated groundwater. The process has a built-in air emission control device for the removal of toxic organics and inorganics from water without causing air pollution problems. One of the biological processes is a conventional biological SBR equipped with an enclosure on top for air emission control. Another new biological high rate process is the dissolved air flotation sequencing batch reactor (DAF-SBR), which is also equipped with an air pollution control enclosure on top, which is suitable for temporary groundwater decontamination in the field. The DAF-SBR process is similar to a conventional biological SBR process, except that DAF (instead of sedimentation) is used in the reactor for the separation of mixed liquor suspended solids (MLSS) from the mixed liquor.

Air and steam strippings

Stripping methods, including steam stripping and air stripping, are mainly used for the removal of volatile organics from contaminated water. The difference between steam stripping and air stripping is the stripping agent, the former obviously using steam and the latter air. Moreover, steam stripping is more like a distillation process, in which steam is used as both the heating medium and the driving force for removal of the volatile materials. After condensing the steam, the waste compounds are concentrated and separated from the water. Air stripping, on the other hand, is based on the distribution coefficients of volatile organics between the contaminated water and the stripping stream at a certain temperature.

Stripping can be integrated with vapor extraction for a better contamination removal. The stripping technology can also be combined with activated carbon adsorption to result in a higher removal efficiency. The conventional air stripping process can only remove VOCs from contaminated water while its gaseous effluent may pollute the air environment. A new stripping process developed by Wang and colleagues32 and Hrycyk and colleagues44 can remove VOCs, VICs (volatile inorganic compounds), and radioactive radon from water, without the creation of an air pollution problem.

16.7.5 In Situ Soil Treatment 16.7.5.1 In Situ Heating

In situ soil remediation with physical methods includes the in situ heating (in situ thermal treatment), ground-freezing, hydraulic fracturing, immobilization/stabilization, flushing, chemical detoxification, vapor extraction, steam extraction, biodegradation/bioremediation, electroosmosis/ electrokinetic processes, etc.

In situ heating (in situ thermal treatment) uses thermal decomposition, vaporization, and distillation techniques to destroy or remove organic contaminants. The most common in situ heating methodologies include electrical resistance heating, radio frequency heating, hot air/water/steam injection, and thermal vitrification. These different methods or their combinations can be used to apply heat to polluted soil or groundwater in situ. The heat can destroy or volatilize organic chemicals. As the chemicals change into gases, their mobility increases, and the gases can be extracted via collection wells for capture and cleanup in an ex situ treatment unit. Thermal methods can be particularly useful for dense or light nonaqueous phase liquids (DNAPLs or LNAPLs). Heat can be introduced to the subsurface by electrical resistance heating, radio frequency heating, dynamic underground stripping, thermal conduction, or injection of hot water, hot air, or steam.

The main advantage of in situ thermal methods is that they allow soil to be treated without being excavated and transported, resulting in significant cost savings; however, in situ treatment generally requires longer time periods than ex situ treatment, and there is less certainty about the uniformity of treatment because of the variability in soil and aquifer characteristics and because the efficacy of the process is more difficult to verify.

Electrical resistance heating

Electrical resistance heating uses an electrical current to heat less permeable soils such as clays and fine-grained sediments so that water and contaminants trapped in these relatively conductive regions are vaporized and ready for vacuum extraction. Electrodes are placed directly into the less permeable soil matrix and activated so that electrical current passes through the soil, creating a resistance, which then heats the soil. The heat dries out the soil, causing it to fracture. These fractures make the soil more permeable, allowing the use of SVE to remove the contaminants. The heat created by electrical resistance heating also for

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