Bm Qm 0dm1820a

where Bm = carbon bed (kg), Qm = influent rate (m3/min), t = contact time (min), and dm = carbon density (kg/m3),


where, B = carbon bed (lb), Q = influent rate (gal/min), t = contact time (min), and d = carbon density (lb/ft3),

where Ccm = actual carbon dosage (kg/m3), T = on-stream cycle time (d), and Qm = influent rate (m3/min),

where Cc = actual carbon dosage (lb/1000 gal), T = on-stream cycle time (d), Q = influent rate (gal/min), and d = 25 (lb/ft3), and

where Pdm = pressure drop (mmHg), ^ = dynamic viscosity (centipoise), Qm = influent flow rate (m3/min), Bhm = carbon bed depth (m), Dp = mean carbon particle diameter (mm), Dcm = carbon column diameter (cm), and Kc = carbon adsorption coefficient.

Dp2Dc where Pd = pressure drop (in.Hg), ^ = dynamic viscosity (centipoise), Q = influent flow rate (gal/min), Bh = carbon bed depth (ft), Dp = mean carbon particle diameter (mm), Dc = carbon column diameter (in.), and Kc = carbon adsorption coefficient.

Gravity flow in downflow carbon beds is usually controlled at hydraulic loadings less than 9.78 m3/h/m2 (4 gal/min/ft2). Upflow carbon beds with bed expansion should be considered when headloss is expected. It should be noted that TSS will break through an upflow carbon bed at about 10% bed expansion. Air Stripping and Activated Carbon Combination

Activated carbon is more suitable for an influent with low VOC concentration, and air stripping is more suitable for treating high VOC concentrations, but yields a relatively higher effluent concentration in comparison to GAC treatment. The cost of air stripping may be doubled if one tries to yield an effluent concentration to be as low as in activated carbon treatment. This is because it would require a taller tower, a higher air/water ratio, or higher air pressure. The combination of air stripping and activated carbon can complement each other and avoid such a high cost.

Many cases have demonstrated that the combination of activated carbon adsorption and air stripping is one of the most common methods for the removal of dissolved gasoline compounds in groundwater. In this case, air stripping lowers the high concentration in the influent, and the GAC further polishes the effluent to a very low concentration. Generally, this method also reduces operation and maintenance costs. O'Brien and Stenzel64 reported that, when using air stripping, a wastewater containing 1000 pg/L TEC was reduced to 200 pg/L. This 80% removal of TEC resulted in 58% reduction in the consumption of activated carbon.

Another process involving the use of both air stripping and activated carbon adsorption has been developed by Wang and colleagues.29 This process purifies and recycles the emitted gas, thus not creating an air pollution problem. Also, the spent GAC can be automatically regenerated for reuse. Integrated Vapor Extraction and Steam Vacuum Stripping

Integrated vapor extraction and steam vacuum stripping can simultaneously treat groundwater and soil contaminated with VOCs. The system developed by AWD Technologies consists of two basic processes: a vacuum stripping tower that uses low-pressure steam to treat contaminated ground-water; and a soil gas vapor extraction/reinjection process to treat contaminated soil. The two processes form a closed-loop system that provides simultaneous in situ remediation of contaminated groundwater and soil with no air emission.

The vacuum stripping tower is a high-efficiency countercurrent stripping technology. A single-stage unit typically reduces VOCs in water by up to 99.99%. The soil vacuum extraction system uses vacuum to treat a VOC-contaminated soil mass, with a flow of air through the soil that removes vapor-phase VOCs with the extracted soil gas. The soil gas is then treated by carbon beds to remove the VOCs. The two systems share a single GAC unit. Noncondensable vapor from the stripping system is combined with the vapor from the soil vacuum system and decontaminated by the GAC unit. Byproducts of the system are a free-phase recyclable product and the treated water. The granulated carbon will have to be replaced and the used carbon disposed of every three years. Ex Situ Biological Treatment for Groundwater Decontamination

The processes of ex situ biological treatment for pumped contaminated groundwater is similar to the processes used in biological wastewater treatment plants. These include activated sludge, waste stabilization ponds and lagoons, trickling filters, rotating biological contactors, and land application.44,45

The immobilized cell bioreactor system developed by Allied Signal is an aerobic fixed-film bioreactor system (Figure 18.13). The system offers improved treatment efficiency through the use of a unique proprietary reactor that maximizes the biological activity, and a proprietary design that maximizes contact between the biofilm and the contaminants. The advantages include a fast and complete degradation of target contaminants to carbon dioxide, water, and biomass; high treatment capacity; compact system design; and reduced operation and maintenance cost resulting from simplified operation and slow sludge production.65

After further polishing, such as clarifying and filtering, if necessary, the biologically treated groundwater may be reinjected into the aquifer in an operation similar to deep well injection.66

The advantage of ex situ biological treatment is the ability to control the effluent quality. The use of air for aerobic treatment is easier to control and costs less. Nutrient can be added more effectively and the temperature can be controlled.

inl inl discharge

Groundwater or

process water




Nutrient addition

Nutrient addition


FIGURE 18.13 Allied Signal Immobilized Cell Bioreactor (ICB).

The disadvantages of ex situ biological treatment in comparison to in situ biological treatment are as follows:

1. After shutting off the system, biological treatment cannot continue in the contaminated site.

2. The contaminants in places where they are strongly adsorbed or where permeability is locally low, or where microcracks are developed in rocks, cannot be efficiently drawn out with water using the pumping method.

3. The emitted gas containing VOCs may cause air pollution problems.

A biological process developed by Wang and colleagues57 does not cause air pollution problems and is highly efficient for the biodegradation of organics present in water. Oxidation

Oxidation is a means of decontamination. There are several methods that can facilitate oxidation to treat contaminated groundwater. In the following we describe two examples of such technologies.

The perox-pure system developed by Peroxidation Systems is designed to destroy dissolved organic contaminants in groundwater or wastewater through an advanced chemical oxidation process using ultraviolet (UV) radiation and hydrogen peroxide. Hydrogen peroxide is added to the contaminated water, and the mixture is then fed into the treatment system. The treatment system contains four or more compartments in the oxidation chamber. Each compartment contains one high-intensity lamp mounted in a quartz sleeve. The contaminated water flows in the space between the chamber wall and the quartz tube in which each UV lamp is mounted. UV light catalyzes the chemical oxidation of the organic contaminants in water by its combined effect upon the organics and its reaction with hydrogen peroxide. This technology can treat water contaminated with chlorinated solvents, pesticides, polychlorinated biphenyls, phenolics, fuel hydrocarbons, and other toxic compounds at concentrations ranging from a few thousand mg/L to 1 |^g/L. For higher organic concentrations, UV light combined with other processes such as air stripping, steam stripping, biological treatment, or air flotation may be more cost effective.29,58,59

Chemical Waste Management have developed a technique using evaporation and catalytic oxidation to treat contaminated water.65 Contaminated water is concentrated in an evaporator by boiling off most of the water and the 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. Suitable wastes include leachates, contaminated groundwater, and process waters. This technique can also be used to treat complex wastewaters that contain volatile and nonvolatile organic compounds, salts, metals, and vo1atile inorganic compounds. Solvent Extraction

Solvent extraction uses an organic solvent to extract toxic substances from contaminated liquid or solid.67 Examples can be found in the section dealing with the treatment of contaminated soil. Dissolved Air Flotation (DAF)

Perhaps the most efficient but least recognized process for groundwater decontamination is dissolved gas flotation, also known as dissolved air flotation (DAF), in which air is used for the generation of extremely fine air bubbles having diameters less than 80 pm.

DAF is used to remove suspended solids by decreasing their apparent density; they then rise and float on the water surface. DAF is also used to remove soluble iron, VOCs, oils, and surface active agents by oxidation, air stripping, and surface adsorption. The flotation technology is becoming one of the most important technologies for groundwater decontamination, industrial effluent treatment, and water purification.58-61,70

A typical DAF process consists of saturating a portion or all of the influent feed, or a portion of recycled effluent with air at a pressure of 1.76 to 6.33 kg/cm2 (25 to 90 psi). The pressurized influent is held at this pressure for 0.2 to 3 min in a pressure vessel and then released to atmospheric pressure in a flotation chamber. A controlled reduction in pressure results in the release of microscopic air bubbles, which oxidize the soluble ferrous iron (Fe2+) to form insoluble ferric iron (Fe3+) and attach themselves to VOCs, surfactants, oil, and suspended particles in the influent water in the flotation chamber. This results in agglomeration, air stripping, and surface adsorption due to the generated air bubbles. The VOCs are removed by air stripping and discharged to a gas-phase GAC adsorber for purification. The floated material (oil, surfactants, TSS) rises to the surface with vertical rise rates ranging between 0.15 and 0.6 m/min (0.5 to 2.0 ft/min) and forms a floating scum layer. Specially designed sludge scoops, flight scrapers, and other skimming devices continuously remove the floating scum. The clarified effluent water that is almost free of suspended solids and oil is discharged near the bottom of the flotation chamber. The retention time in the flotation chamber used to be about 20 to 60 min but has been reduced to 3 to 15 min by innovative design.

The effectiveness of DAF depends upon efficient air oxidation and the attachment of bubbles to the oil, VOCs, surfactants, and other particles that are to be removed from the influent water stream. Flotation can be induced in at least three ways:

1. Air bubbles adhering to the insoluble solids by electrical attraction

2. Air bubbles becoming physically trapped in the insoluble solids original or flocculated structure

3. Air bubbles being chemically adsorbed to the insoluble solids in their original form or their flocculated structure

The attraction between the air bubble and contaminants is believed to be primarily a result of particle surface charges and bubble size distribution. The more uniform the distribution of water and microbubbles, the shallower the flotation chamber can be. Generally, the depth of effective flotation chambers is between 0.9 and 2.7 m (3 and 9 ft). Flotation units can be round, square, or rectangular. Gases other than air can be used. The petroleum industry has used nitrogen, with closed vessels, to reduce the possibilities of fire. Ozone can be fed through with air for more efficient reduction of soluble iron, VOCs, and so on.57 Ozone-UV flotation is another alternative for groundwater decontamination.

Several high-rate air flotation clarifiers (both DAF and dispersed air flotation) with less than 15 min of detention times have been developed for groundwater decontamination, industrial effluent treatment, resources recovery, and water reclamation. Both insoluble and soluble impurities such as

VOCs, activated sludge, fibers, free oil and grease, emulsified oil, lignin, protein, humic acid, tannin, algae, BOD, TOC, iron ions, manganese ions, hardness, titanium dioxide, phosphate, and heavy metals can be separated from a target water stream. Addition of flotation aids to a flotation clarifier is required. Flotation aids include, but are not limited to, aluminum sulfate, ferric chloride, organic polymer, poly aluminum chloride, calcium chloride, ferrous sulfate, calcium hydroxide, ferric sulfate, powdered activated carbon, sodium aluminate, surfactants, and pH adjustment chemicals. Design equations and examples of high-rate DAF clarifiers can be found in the literature.58,59,69,71

Toxic organic compounds commonly found in groundwater are presented in Table 18.4. Other toxic organic compounds (representing 1% of cases) include PCBs (polychlorinated biphenyls), 2,4-D, 2,4,5-TP (silvex), toxaphene, methoxychlor, lindane, and endrin, of which 2,4-D and silvex are commonly used for killing aquatic and land weeds. Inorganic toxic substances commonly found in

TABLE 18.4

Toxic Organic Compounds Commonly Found in U.S. Groundwater

Organic Compounds in Groundwater Percent of Occurrences Concentration Range

TABLE 18.4

Toxic Organic Compounds Commonly Found in U.S. Groundwater

Organic Compounds in Groundwater Percent of Occurrences Concentration Range

Carbon tetrachloride


130 |g/L-10 mg/L



20 |g/L-3.4 mg/L



2.5 mg/L



1 | g/L



1 | g/L



4 |g/L



5 | g/L-4 mg/L



450 | g/L

Diisopropyl ether

20-34 | g/L

Tertiary methyl butylether


33 | g/L

Diisopropyl methyl phosphonate


1250 |g/L



10 |g/L

Dichloroethyl ether


1.1 mg/L

Dichlorosopropyl ether


0.8 mg/L


0.4 |g/L-4.11 mg/L



10-100 |g/L

Ethyl acrylate


200 mg/L



6 mg/L

Methylene chloride


1.21 mg/L



63 mg/L



100 mg/L



5 |g/L-70 mg/L



5 |g/L-16 mg/L



60 | g/L-25 mg/L

Vinylidiene chloride


5 |g/L-4 mg/L



5.7 mg/L



0.2-10 mg/L



10 |g/L



Not available

Source: Wang, L.K. and Wang, M.H.S., Decontamination of groundwater and hazardous industrial effluents by high-rate air flotation process, Proc. Great Lakes Conf., Hazardous Materials Control Research Institute, Silver Springs, MD, September 1990. With permission. DDD, Dichlorodiphenyl dichloroethane; DDE, dichlorodiphenyl dichloroethylene; DDT, dichlorodiphenyl trichloroethane; EDB, ethylene dibromide.

Source: Wang, L.K. and Wang, M.H.S., Decontamination of groundwater and hazardous industrial effluents by high-rate air flotation process, Proc. Great Lakes Conf., Hazardous Materials Control Research Institute, Silver Springs, MD, September 1990. With permission. DDD, Dichlorodiphenyl dichloroethane; DDE, dichlorodiphenyl dichloroethylene; DDT, dichlorodiphenyl trichloroethane; EDB, ethylene dibromide.

groundwater include lead, arsenic, copper, cadmium, barium, chromium, mercury, selenium, silver, and nitrate. In a typical groundwater decontamination project, additional industries that are nontoxic but require pretreatment for their removal include iron, manganese, total dissolved solids, and color.

Innovative air flotation technologies have been developed for more cost-effective groundwater decontamination in comparison with the state-of-the-art technologies.68,69 DAF is very efficient and cost-effective for decontamination of groundwater in which heavy metals, color, TDS, iron, manganese, coliforms, and hardness can all be significantly removed, aiming at not only the decontamination of groundwater but also elimination of biological and chemical fouling for subsequent processes. Furthermore, many VOCs can also be removed by DAF. Table 18.5 represents the U.S. EPA's removal data for DAF processes. The capability of DAF for the treatment of various liquid streams has been well established.58,59,69 However, its application for the decontamination of groundwater is comparatively new.

Special chemicals may be required for the groundwater decontamination process. For instance, PAC may be dosed into a DAF system for enhancement of contaminant removal efficiency. In such a case, the process is called adsorption flotation (PAC-DAF process). In a pilot plant study, a system consisting of adsorption flotation and sand filtration has proved to be feasible for groundwater decontamination.70 PAC was added as an adsorbent for the removal of color, odor, EDB (ethylenedi-bromide), TTHM (total trihalomethane), and other toxic substances from groundwater. Next, the spent PAC was flocculated by coagulants and floated to the water surface by DAF. Finally, the flotation clarified water was polished using the automatic backwash filtration (ABF) process. The results of both bench-scale and pilot plant studies have indicated that using 250 mg/L of PAC at 15 min of detention time can remove color by 100% (from 25 CU [color units]), iron by 100% (from 25 ^g/L), humic acid by 98% (from 3200 ng/L), EDB by 100% (from 1.2 ng/L), TTHM by 98% (from 1265 |ag/L), odor by 99.6% (from 500 TON [threshold odor numbers]), mercaptans by 100% (from 730 |ag/L S), lead by 100% (from 6 ^g/L), and arsenic by 100% (from 1000 |^g/L). The plant was operated at 40 L/min (10.6 gal/min) for the separation of 250 mg/L of spent PAC. Nearly 100% of spent PAC (from 250 mg/L) and total coliform (from 3/100 mL) and over 95% of turbidity (from 4.5 NTU [nephelometric turbidity units]) were removed by the addition of 1.5 mg/L of anionic polymer and 2.5 mg/L of coagulant. The process was operated at 30% recycle flow rate and 0.014 m3/h (0.5 ft3/h) air flow. The sand filter consisted of 28 cm (11 in.) of quartz sand (E = 0.36 mm, U = 1.65) and operated at 102 L/min/m2 (2.5 gal/min/ft2).

A DAF-GAC system involving the use of DAF and GAC has also proved to be equally effective for complete groundwater decontamination for the same influent water mentioned above.

For the treatment of a contaminated groundwater source containing a high concentration of hardness, DAF filtration is also an excellent pretreatment process system for the reduction of scale formation in subsequent processes. In a study, groundwater having 12 units of color, 13 NTU of turbidity, and 417 mg/L of carbon hardness as CaCO3 was successfully treated by a continuous DAF filtration plant consisting of hydraulic flocculation, a DAF clarifier, a recarbonation facility, and three sand filters. The added chemicals were 42 mg/L of magnesium carbonate as a coagulant and a small amount of lime for pH adjustment (to pH 11.3). The plant's treatment efficiency in terms of removal had the following values: color, 100%; turbidity, 98%; total hardness, 62%. Recarbonation with CO2 maintained the effluent pH at 7.2. This plant's operational conditions included a floccula-tion detention time of 5.6 min, DAF detention time of 3.0 min, flotation clarification rate of 102 L/min/m2 (2.5 gal/min/ft2 ), sand depth of 28 cm (11 in.), influent water flow rate of 45.5 L/min (12 gal/min), recycle water flow rate of 11.4 L/min (3 gal/min), air flow rate of 0.028 m3/h (1 ft3/h) at 6.33 kg/cm2 (90 psig) pressure. Soda ash (Na2CO3) may be needed only if permanent hardness (CaSO4) is present. The chemical reactions are as follows:

TABLE 18.5

Control Technology Summary for Dissolved Air Flotation

Effluent Concentration


Classical Pollutants (mg/L)

Total phosphorus

Total phenols

Oil & grease

Toxic Pollutants (pg/L)














Bis (2-ethylhexyl)phthalate Butyl benzyl phthalate Carbon tetrachloride Chloroform

Dichlorobromomethane Di-n-butyl phthalate Diethyl phthalate Di-n-octyl phthalate N-nitrosodiphenylamine 2,4-Dimethylphenol Pentachlorophenol Phenol

Dichlorobenzene Ethylbenzene Toluene Naphthalene


























ND-28 5-30 9-2400 18-260 ND-970 ND-2100 ND-840 0.2-600


20 <10 200 BDL 200 180 54

70 BDL

41 2

19 200 100 ND

36 9

20 ND

11 620 14 13

71 140

44 580 96 10

% Removal

68 66 88 98 12 79

76 45

98 52 75 10 98 75 73

75 58

76 65 39

77 81

Source: Wang, L.K. and Wang, M.H.S., Decontamination of groundwater and hazardous industrial effluents by high-rate air flotation process, Proc. Great Lakes Conf., Hazardous Materials Control Research Institute, Silver Springs, MD, September 1990. With permission. ND, non-detectable; BDL, below detection limit; NM, not measured.

MgCO3 4

- Ca(OH)2

= Mg(OH)2

+ CaCO3


4 MgCO3

= CaCO3 +


MgSO4 4


= Mg(OH)2

+ CaSO4


+ Na2CO3

= CaCO3 +


CO2 4


= CaCO3 +


CO2 +


= MgCO3 +

H2O (coagulant regeneration)

DAF is controlled under laminar hydraulic flow conditions using a very small volume of air flow amounting to about 1 to 3% of the influent groundwater flow. DAF only requires 3 to 5 min of detention time; therefore it is a low-cost process for the decontamination of groundwater. Dispersed or Induced Air Flotation (IAF)

Another innovative process, induced air flotation (IAF), operates under turbulent hydraulic flow conditions by using a large volume of air flow amounting to 400% of the influent groundwater flow. The air bubbles are coarse and large, similar to the air bubbles used in an activated sludge aeration basin. IAF requires only 4 to 10 min of detention time, so it is also a very cost-effective process.58,59 Unlike DAF, IAF is not an effective pretreatment process for the removal of heavy metals, color, turbidity, TDS, hardness, and coliforms, but it is as efficient as conventional air-sparging and air-stripping processes for the removal of iron, manganese, surfactants, and VOCs.

IAF itself is an aeration process, so soluble iron and manganese ions may be oxidized to form insoluble suspended particles that can be separated easily from the liquid phase. The aeration efficiency of IAF is higher than that of DAF. If groundwater's soluble ferrous iron content is 8 mg/L or below, DAF alone using conventional coagulants will be able to remove the soluble iron.69 When groundwater's soluble ferrous iron is higher than 8 mg/L, either IAF or an oxidizing agent (ozone, hydrogen peroxide, oxygen, potassium permanganate, and so on) will be required for iron removal.

In the conventional air-stripping process, groundwater is introduced into a gas phase for stripping VOCs; in IAF, air bubbles are injected into the groundwater. An air-stripping tower is over 3 m (10 ft) tall, and an IAF cell can be as shallow as 1 m (3 ft). An important feature of an enclosed IAF cell for VOC reduction is its capability of recycling and reusing its purified air streaming, thus eliminating any possibility of air pollution.29,30

In summation, both DAF and IAF are good innovative processes for more efficient and more cost-effective groundwater decontamination.

18.8.4 Removal of Gasoline from Contaminated Soil In Situ Soil Vapor Extraction

The technologies for in situ treatment for groundwater can usually be applied to in situ soil remediation, although some of the technologies may have varying suitability for soil. As soil contamination involves a more contaminated phase, the vapor phase, thus vapor extraction is uniquely developed for soil vapor remediation. The decreasing of soil vapor pressure by extraction would cause the free gasoline product to vaporize, so the vapor extraction method also plays a role in the remediation of the liquid phase of VOCs. Based on these observations, the technologies presented in the following discussion will focus mainly on the SVE systems, although other technologies, such as in situ soil flushing and in situ biological treatment, will also be addressed.

SVE has been an effective technique for removing VOCs such as TCE and some petroleum compounds from the vadose zone of contaminated soil.72 The following presents some of the newly developed technologies.

Vacuum extraction

The vacuum extraction process involves using vapor extraction wells alone or in combination with air injection wells. Vacuum blowers are used to create the movement of air through the soil. The air flow strips the VOCs from the soil and carries them to the surface. Figure 18.14 shows the flow diagram for such a process. During extraction, water may also be extracted along with vapor. The mixture should be sent to a liquid-vapor separator. The separation process results in both liquid and vapor residuals that require further treatment. Carbon adsorption is used to treat the vapor and water streams, leaving clean water and air for release, and spent GAC for reuse or disposal. Air emissions from the system are typically controlled by adsorption of the volatiles onto activated carbon, by thermal destruction, or by condensation.

The vacuum extraction method has been effectively applied to removing VOCs with low organic carbon content from well-drained soil, although it may also be effective for finer and wetter soils, but with comparatively slower removal rates. There are generally significant differences in the air permeability of various strata, which can influence process performance. Contaminants with low vapor pressure or high water solubilities are difficult to remove.

Soil vacuum extraction is cost-effective if the volume of contaminated soil exceeds 382 m3 (500 yd3), and if the contaminated area is more than 6 m (20 ft) deep; otherwise, soil excavation and

FIGURE 18.14 In situ soil vapor extraction.

treatment may be more cost-effective. The level of the groundwater is also important. Rising of the water table that occurs as a result of vacuum extraction wells has to be controlled to avoid water entering the contaminated vadose zone. The water infiltration rate can be controlled by placing an impermeable cap over the site, and a pump may be required to draw the water table down and allow efficient vapor venting. Usually, soil washing follows vapor extraction of volatile contaminants.

In an example, as a result of an impending property transfer that necessitated rapid remediation of diesel-affected soil at a former service station, thermal-enhanced SVE (TESVE) was used to accelerate remediation. The recovery system included a network of TESVE units and injection wells that were separately connected to two regenerative blowers. The recovered vapors were treated in a thermal incinerator, an oxidizing unit to destroy the recovered hydrocarbons. Treated air at a temperature of 1800°F was passed through a heat exchanger and ambient air was simultaneously pumped through the heat exchanger, which increased the temperature of the ambient air to 350°F. The heated ambient air was then injected into the affected soil through a network of four carbon steel injection wells using a regenerative blower.73

Soil venting

Soil venting is a technique that removes contaminant vapors from unsaturated soil without excavation. A vacuum extraction system usually consists of gravel packs extending to the soil surface, and a slotted or unslotted well casing that allows gases to move out of the soil. Passive systems consist of vents that are open to the atmosphere and do not require energy for extraction of the gases. Active systems use pressure or vacuum pumps to accelerate the removal of gasoline vapors from the soil. With venting, the vapors are either discharged to the atmosphere or treated before discharge depending on vapor concentrations and regulatory requirements.

Enhanced volatilization

The enhanced volatilization process is operated by putting contaminated soil in contact with clean air in order to transfer the contaminants from the soil into an air stream. The air stream is further treated through the use of carbon canisters, water scrubbers or afterburners to reduce air emission impacts. Four methods are available that can achieve this effect19:

1. Mechanical rototilling

2. An enclosed mechanical aeration system

3. A low-temperature thermal stripping system

4. A pneumatic conveyer system

The mechanical rototilling method involves turning over soils to a depth of about 0.30 m (1 ft) below the surface to increase the rate of volatilization. Following treatment, the topsoil is moved to a nearby pile and rototilling is performed on the next 0.30 m (1 ft) of soil. The effectiveness of this mechanical rototilling method is highly dependent on weather conditions. High-speed rototillers and soil shredders can enhance the rate of volatilization.

For effective volatilization using an enclosed mechanical aeration system, contaminated soil is mixed in a pug mill or rotary drum. The gasoline components are released from the soil matrix by the churning action of the air/soil contact. The induced airflow within the chamber captures the gasoline emissions and passes them through an air pollution control device (e.g., a water scrubber or vapor-phase carbon adsorption system) before they are discharged through a properly sized stack.

The configuration of a low-temperature thermal stripping system is similar to the enclosed mechanical aeration system except that additional heat transfer surfaces allow the soil to heat by coming into contact with a screw auger device or rotary drum system. The induced airflow conveys the desorbed volatile organics/air mixture through an afterburner where organic contaminants are destroyed. The over air stream is then discharged through a properly sized stack.

A pneumatic conveyer system consists of a long tube or duct to carry air at high velocities, an induced draft fan to propel the air, a suitable feeder for addition and dispersion of particulate solids into the air stream, and a cyclone collector or other separation equipment for final recovery of the solids from the gas stream. Several such units heat the inlet air to 300°F to induce volatilization of organic contaminants. Pneumatic conveyers are primarily used in the manufacturing industry for drying solids with up to 90% initial moisture content.

Of the four enhanced volatilization methods described above, documentation exists to support the contention that the low-temperature thermal stripping system has the greatest ability to successfully remove contaminants that are similar to gasoline constituents (i.e., compounds with high vapor pressures) from soil. The limitations of some enhanced volatization techniques can be attributed to the following:

1. Associated soil characteristics that inhibit the mobility of gasoline vapors from the soil to the air

2. Contaminant concentrations that may cause an explosion or fire

3. The need to control dust and organic vapor emissions

Some integrated techniques may be more economical if they can be used simultaneously for soil and groundwater treatment, such as integrated vapor extraction and steam vacuum stripping. In Situ Soil Flushing

Soil flushing treatment is a technique that removes gasoline constituents from the soil matrix by actively leaching the contaminants from the soil into a leaching medium. The most common washing medium is water, which may contain additives such as acids, alkalis, and detergents. The washing fluid can also be composed of pure organic solvents such as hexane and triethylamine.

The washing media are recharged into soil using a spray recharge system or injection wells. Withdraw wells convey the after-washing liquid to an aboveground treatment facility. The after-washing liquid is treated using biological treatment or physical-chemical methods such as air stripping.

Surfactants have been widely used to reduce the interfacial tension between oil and soil, thus enhancing the efficiency of rinsing oil from soil. Numerous environmentally safe and relatively inexpensive surfactants are commercially available. Table 18.6 lists some surfactants and their chemical properties.74 The data in Table 18.6 are based on laboratory experimentation; therefore, before selection, further field testing on their performance is recommended. The Texas Research Institute75 demonstrated that a mixture of anionic and nonionic surfactants resulted in contaminant recovery of up to 40%. A laboratory study showed that crude oil recovery was increased from less than 1% to 86%, and PCB recovery was increased from less than 1% to 68% when soil columns were flushed with an aqueous surfactant solution.7476

Contained recovery of an oily waste process has been developed by the Western Research Institute.65 It uses steam and hot water (through injection wells) to displace oily waste from the soil, which is then conveyed (by production wells) aboveground for treatment (Figure 18.15). Low-quality steam is injected below the deepest penetration of organic fluids. The steam condenses, causing rising hot water to dislodge and sweep the buoyant organic fluid upward into more permeable soil regions. Hot water is injected above the impermeable soil regions to heat and mobilize the oily waste accumulations, which are recovered by hot water displacement. When oily wastes are displaced, the organic fluid saturation in the subsurface pore space increases, forming an oil bank. The oil saturation is reduced to an immobile residual saturation in the subsurface pore space. The produced oil and water are treated for reuse or discharge. In the process, contaminants are contained laterally by groundwater isolation, and vertically by organic fluid flotation.

The contained recovery method is claimed to have the following advantages:

1. It removes large portions of oily waste accumulations.

2. It stops the downward migration of organic contaminants.

3. It immobilizes any residual saturation of oily wastes.

TABLE 18.6

Surfactants Characteristics



Anionic Carboxylic acid salts

Sulfuric acid ester salts Phosphoric and polyphosphoric acid esters Perfluorinated anionics Sulfonic acid salts

Cationic Long-chain amines

Diamines and polyamines Quaternary ammonium salts Polyoxyethylenated long-chain amines

Nonionic Polyoxyethylenated alkyl-phenols, alkylphenol ethoxylates Polyoxyethylenated straight-chain alcohols and alcohol ethoxylates Polyoxyethylenated poly-oxypropylene glycols Polyoxyethylenated mercaptans Long-chain carboxylic acid esters Alkylolamine condensates, alkanolamides Tertiary acetylenic glycols

Amphoterics pH-sensitive pH-insensitive

Selected Properties and Uses

Good detergency

Good wetting agents Strong surface tension reducers

Good oil in water emulsifiers Emulsifying agents

Corrosion inhibitor

Emulsifying agents


Wetting agents

Dispersents Foam control

Solublizing agents Wetting agents


Generally water-soluble

Soluble in polar organics

Low or varying water solubility Water-soluble

Generally water-soluble



Electrolyte-sensitive Resistant to biodegradation

High chemical stability Resistant to acid and alkaline hydrolysis Acid stable

Surface adsorption to silicaeous materials

Good chemical stability

Water insoluble Resistant to biodegradation formulations

Relatively nontoxic

Subject to acid and alkaline hydrolysis

Varied Nontoxic

(pH-dependent) Electrolyte-tolerant

Adsorption to negatively charged surfaces

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

4. It reduces the volume, mobility, and toxicity of oily wastes.

5. It can be used for both shallow and deep contaminated areas.

6. It uses the same mobile equipment required for conventional petroleum production technology. In Situ Biological Soil Treatment

The technology for in situ biological treatment for soil is similar to that for in situ biological groundwater treatment. The following sections present three newly developed techniques.

Deep in situ bioremediation process

This technique was developed by In-Situ Fixation Company for increasing the efficiency and rate of biodegradation in deep contaminated soils using a dual-auger system. Mixtures of microorganism and required nutrients are injected into the contaminated soils without any excavation. The injection and mixing effectively break down fluid and soil strata barriers and eliminate pockets of

Injection well Production well

Injection well Production well

FIGURE 18.15 CROWTM subsurface development.

contaminated soil that would otherwise remain untreated. The drilling is carried out in an overlapping manner to ensure complete treatment of all contaminated soil. The mixing action is continued as the augers are withdrawn. The treatment depth may exceed 30 m (100 ft).65

In situ geolock and biodrain treatment platform

This system consists of an in situ polyethylene tank, an application system, and a bottom water recovery system.65 An underlying, permeable, water-bearing zone facilitates the creation of ingradient water flow conditions. The tank defines the treatment area, minimizes the potential for release of bacterial cultures to the aquifer, and maintains contaminant concentration levels that facilitate treatment. The ingradient conditions facilitate reverse leaching or soil washing and minimize the potential for outmigration of contaminants.

The application system, called the biodrain, is installed within the treatment area. The biodrain aerates the soil column and any standing water. This cerates an aerobic environment in the pore spaces of the soil. Other gas mixtures can also be introduced to the soil column, such as the air/ methane mixtures used in the biodegradation of chlorinated organics. The treatment platforms can be placed in very dense configurations. International Environmental Technology claims that the cost of installation is low.

The bottom water recovery system uses existing wells or new wells to create the water recovery system for removal of the water used to wash the contaminated soil. Reverse leaching or soil washing can be conducted by controlling the water levels within the tank. This design minimizes the volume of clean ex situ water entering the system for treatment. Extremely dense clays may be difficult to treat with this technology.

In situ bioventing technology

Bioventing technology was developed by the U.S. EPA Risk Reduction Engineering Laboratory to treat soil contaminated by numerous industrial wastes, which is subjected to aerobic microbial degradation, especially to promote the degradation of polycyclic aromatic hydrocarbons.65 It uses a series of air injection probes, each of which is attached to a low-pressure air pump. The air pump operates at extremely low pressures to allow the inflow of oxygen without volatilization of contaminants. Additional additives such as ozone or nutrients may also be supplied to stimulate microbial growth.77 Ex Situ Soil Treatment

All ex situ soil treatment methods involve a two-step approach: soil excavation and aboveground treatment of the excavated soil. The differences in the various ex situ excavation/treatment methods for soil remediation lie only in the methods of soil treatment aboveground, such as soil washing plus extraction, and slurry biodegradation.

Soil washing technology

The excavated soil is removed from the site and screened to remove large solid objects. The screened soil is washed and the washing water is treated.78 Clearly, the washing media used in in situ soil-flushing treatment can be used here. The most common washing medium is water. Surfactants are used to reduce the affinity of contaminants to the soil.

Several unit processes can be used in the washing process. The soil is mixed with washing agents and extraction agents that remove the contaminants from the soil and transfer them to the extraction fluid. The soil and washwater are then separated. The soil can be further rinsed with clean water. The soil is removed as clean product, ready to put back into the original excavation, and the washwater is ready to be treated by conventional wastewater treatment processes as addressed in the next subsection.

The big difference in application from the in situ flushing method is that this ex situ method can apply to soils with lower permeability, because soil is excavated and can be sufficiently washed. The following presents two ex situ soil washing processes for organic contaminants: the BioGenesis soil cleaning process and the BioTrol soil washing system.

The BioTrol soil washing system developed by BioTrol, Inc., is shown in Figure 18.16. After debris is removed, the excavated soil mixed with water and is subjected to various unit operations common to the mineral processing industry. Process steps include mixing units, pug mills, vibrating screens, froth flotation or induced air flotation (IAF) cells, scrubbing machines, hydrocyclones, screw classifiers, and various dewatering operations. The core of the system is a multistage, countercurrent, intensive scrubbing circuit with interstage classification. The scrubbing action disintegrates soil aggregates, freeing contaminated fine particles from the coarser sand and gravel. In addition, superficial contamination is removed from the coarse fraction by the abrasive scouring action of the particles themselves. Contaminants may also be solubilized, as dictated by solubility characteristics or partition coefficients. This technology is a water-based volume reduction process for treating excavated soil. Soil washing may be applied to contaminants concentrated in the fine-size fraction of soil (silt, clay, and soil organic matter) and the superficial contamination associated with the coarse soil fraction. This technology can be applied to soils contaminated with PAH (polycyclic aromatic hydrocarbons) and PCP (pentachlorophenol), PCB (polychlorinated biphenyl), petroleum hydrocarbons, and pesticides.65,78

The BioGenesis soil cleaning process developed by BioVersal USA, Inc., uses a specialized truck, water, and a complex surfactant (a light alkaline mixture of natural and organic materials containing no hazardous ingredients) to clean contaminated soil. Ancillary equipment includes gravity oil/water separators, coalescing filters, and a bioreactor. Figure 18.17 shows the soil washing procedure. After washing, the extracted oil is reclaimed, the wash water is recycled or treated, and the soil is dumped for refill. Hazardous organics are extracted in the same manner and then further treated. It was shown that the clean rate is ca. 25 t/h for 5000 mg/L oil contamination and lower rates for more contaminated soils. One single wash removes 95 to 99% of hydrocarbon contamination levels up to 15,000 mg/L. The main advantages of the process are as follows65:

1. Treatment is applicable to soils containing both volatile and nonvolatile oils.

2. Soil containing clay may be treated.

3. The process rate is high.

4. Contaminants are transformed into reusable oil, treatable water, and soil suitable for on-site treatment.

5. There is no air pollution, except during excavation.

Soil Washing Process
FIGURE 18.16 BioTrol soil washing system process diagram.

Oil for Oil for

Contaminated fedamatrnn fedamatrnn

Oil for Oil for

Contaminated fedamatrnn fedamatrnn

Air BioVersal Water BioVersal Air cleaner degrader

FIGURE 18.17 Soil washing procedure.

A solvent extraction technology developed by CF System Corporation uses liquefied gases as solvent to extract organics (such as PCB, dioxins, PCP, petroleum wastes) from sludges, contaminated soils, and wastewater.65,79 Propane is the solvent most typically used for contaminated soils, and carbon dioxide is used for wastewater streams. The system is available as either a continuous flow unit for liquid wastes or a batch system for soils. Contaminated soils, slurries, or wastewaters are fed into an extractor along with the solvent. Typically, more than 99% of organics are extracted from the feed. Following phase separation of the solvent and organics, treated water is removed from the extractor while the mixture of solvent and organics passes to the solvent recovery system. In the solvent recovery system, the solvent is vaporized and recycled as a fresh solvent. The organics are drawn out and either reused or disposed of.

Treatment technologies for washing water

Washing fluid can be separated from soil by conventional techniques such as sedimentation, flotation, and filtration.69 Slurry of soil can be dewatered. The treated soils can then be returned into the original excavation or sent to a sanitary landfill. Treatment of washing water is similar to the treatment of pumped contaminated groundwater, including air stripping of the volatile organics or biological treatment. Ex Situ Biological Treatment on Excavated Soil by Slurry Biodegradation

The procedure for slurry biodegradation is not different from conventional biological treatment. The first step is cleaning the soil and separating it from the washing liquid, which is followed by separate biological treatment for the liquid and the soil slurry. The treated soil is then separated from the slurry. Figure 18.18 shows the slurry biodegradation steps in processing the soil.80

Waste preparation for slurry biodegradation

Several preparation steps after soil excavation are required to achieve the optimum inlet feed characteristics for maximum contaminant removal:

1. Screening of the soil to remove large objects

2. Size reduction for large particles

3. Water addition

4. pH and temperature adjustments


FIGURE 18.18 Slurry biodegradation process.


FIGURE 18.18 Slurry biodegradation process.

Treatment by slurry biodegradation

The pretreated soil is mixed with water in a tank to form a slurry. Sufficient mixing is necessary to ensure contact between the contaminants and the microorganisms to facilitate mass transfer from the contaminants to the microorganisms. The well-mixed slurry is conveyed to a bioreactor or a lined lagoon where the aerobic process takes place. Aeration is provided by either floating or submerged aerators. Once the biodegradation of the contaminants is completed, the treated slurry is sent to a dewatering system to separate the soil phase from the aqueous phase of the slurry. Figure 18.19 shows the process for a slurry bioreactor developed by ECOVA Corporation.65

U.S. EPA has shown that 90% of process water can be recycled to the front end of the system for slurry preparation, and the rest must be treated on site or transported to an off-site facility.80 During the aerobic process, some contaminated air may be formed and emitted from the reactor. Depending on the air characteristics, a compatible air pollution control device may be used, such as activated carbon. Slurry biodegradation has been shown to be successful in treating soils contaminated with soluble organics, PAHs, and petroleum waste. The process has been most effective with contaminant concentrations ranging from 2500 mg/kg to 250,000 mg/kg.

The slurry bioreactor developed by ECOVA Corporation65 showed a 93.4% reduction in PAHs over a 12-week treatment period with an initial 89.3% reduction in the first two weeks. Ex Situ Soil Desorption

In situ SVE methods can be used for desorption of VOC from excavated soils. The excavated soil has the advantage that assist technologies may be applied to enhance vaporization, for example, through venting and heating.

One of the desorption technologies, the anaerobic thermal process, is a thermal desorption process. In this process, heating and mixing of the contaminated soils, sludges, and liquids take place in a special rotary kiln that uses indirect heat for processing.65 The SoilTech anaerobic thermal process is designed to both desorb and treat organic contaminants in soil. The kiln portion of the system contains four separate internal thermal zones: preheat, retort, combustion, and cooling. From the preheat zone, the hot granular solids and unvaporized hydrocarbons pass through a sand seal to the retort zone. Heavy soils vaporize in the retort zone and thermal cracking of the

Soil from mixing process

Soil from mixing process

Nutrient solution

Nutrient solution

Ambient air





Stirred batch reactor

Stirred batch reactor

Sample tap

Air discharge

FIGURE 18.19 Process flow diagram.

hydrocarbons forms coke and low-molecular-weight gases. The vaporized contaminants are removed by vacuum to a retort gas-handling system. After cyclones remove the dust from the gases, the gases are cooled, and condensed, and the oil is separated into its various fractions. The coke (with the coked soil) is burned, and the hot soil is either recycled back to the retort zone or sent to the cooling zone. Flue gases from the combustion zone are treated prior to discharge in a cyclone and a baghouse for particle removal, wet scrubber for removal of acid gases, and carbon adsorption bed for removal of trace organic compounds.

The unit desorbs, collects, and recondenses hydrocarbons from the solids. The unit can also be used in conjunction with a dehalogenation process to destroy halogenated hydrocarbons through a thermal chemical process. The technology can be used for oil recovery from tar sands and shales, dechlorination of PCBs and chlorinated pesticides in soil and sludges, separation of oils and water from refinery wastes and spills, and general removal of hazardous organic compound from soils and sludges. Ex Situ Thermal Destruction


Incineration can effectively eliminate gasoline from soils by complete oxidation. Rotary kilns, fluidized beds, and other systems, either fixed or mobile types, may achieve 99.99% removal. One of the limitations of ex situ thermal destruction is associated with soil excavation. Mobile units may be further limited by the permitting process. Costs for incineration vary significantly depending on the particular characteristics of the soil and waste material. Soil containing higher gasoline waste is more economical to treat than a soil with lower gasoline waste, especially when compared with other treatment methods.

Infrared thermal destruction

Infrared thermal destruction technology is a thermal processing system that uses electrically powered silicon carbide rods to heat organic wastes to combustible temperatures. Any remaining combustibles are incinerated in an afterburner. One configuration made by ECOVA Corporation consists of four components65:

1. An electric-powered infrared primary chamber

2. A gas-fired secondary combustion chamber

3. An emissions control system

4. A control center

Waste is fed into the primary chamber and exposed to infrared radiant heat up to 1010°C (1850°F) provided by silicon carbide rods above the belt. A blower delivers air to selected locations along the belt to control the oxidation rate of the waste feed. The ash material in the primary chamber is quenched by using the scrubber water effluent. The ash is then conveyed to an ash hopper, where it is removed to a holding area and analyzed for organic contaminants, such as PCB content. Volatile gases from the primary chamber flow into a secondary chamber, which uses higher temperatures, greater residence time, turbulence, and supplemental energy (if required) to destroy these gases. Gases from the secondary chamber are passed through the emissions control system.

This technology is suitable for soils or sediments with organic contaminants. The optimal waste characteristics are as follows:

2. Moisture content, up to 50% by weight

4. Heating value, up to 5556 kg-cal/kg, or 5556 cal-g/g (10,000 Btu/lb)

5. Chlorine content, up to 5% by weight

6. Sulfur content, up to 5% by weight

7. Phosphorus, 0 to 300 mg/L

9. Alkali metals, up to 1% by weight Plasma arc vitrification

Plasma arc vitrification, developed by Retech, uses a plasma centrifugal furnace, where heat from transferred arc plasma creates a molten bath that detoxifies the feed material. Organic contaminants vaporize and react at temperatures between 2000 and 2500°F to form innocuous products. Solids are melted and vitrified in the molten bath at 2800 to 3000°F. When metals are cooled, they are rendered to a nonleachable, glassy residue that meets the toxicity characteristic leachate procedure (TCLP) criteria.

This technique can treat soils contaminated with organic compounds and is also suitable for treating liquids and solids containing organic compounds and metals.


Besides petroleum products, other hazardous substances (see Tables 18.7-18.9) are also stored in USTs. Among them, a common and important group is the dense nonaqueous phase liquids (DNAPLs). This group has some different physical properties from petroleum (especially gasoline) that make them behave differently in the way they move underground. This section presents the important factors associated with the cleanup of DNAPLs.

The relative vapor density (RVD) values in Table 18.9 have been calculated as the density of dry air saturated with the compound of interest at 20°C. This represents the weighted mean molecular weight of the compound-saturated air relative to the mean molecular weight of dry air, which is 29 g/mol. The RVD value may be calculated from Equation 18.23:

where RVD = relative vapor density (dimensionless), MW = molecular weight of the compound of interest, and Po = vapor pressure (torr or mmHg).

18.9.1 Chemical and Physical Properties of DNAPLs

DNAPLs are mainly liquid hydrocarbons such as chlorinated solvents, wood preservatives, coal tar wastes, and pesticides. Table 18.7 lists some common such chemicals.81

DNAPLs have higher densities than water, most between 1 and 2 g/mL, some are near 3 g/mL, for example, bromoform, which has a density of 2.89 g/mL. They have limited water solubilities, and are usually found as the free-phase immiscible with water or as residuals trapped by soil. Most DNAPLs are volatile or semivolati1e; Pankow82 has listed information on their physical and chemical properties, such as molecular weight, density, boiling points, solubility in water, vapor pressure, sediment/water partition coefficient, viscosity, Henry's law constant, and so on (see Tables 18.8 and 18.9).

18.9.2 Fate of DNAPL Release Underground

Similar to gasoline, the properties of DNAPLs such as immiscibility with water, volatility, and solubility of some of its components cause the presence of multiphase (pure product, solute, gas, and adsorbate) products and movement that is typical of the phenomena associated with DNAPL release. The theory associated with the interaction of gasoline with soil is applicable to DNAPLs. However,

TABLE 18.7

DNAPL-Related Chemicals

Halogenated Volatiles

Chlorobenzene 1,2-Dichloropropane

1.1-Dichloroethane 1,1 Dichloroethylene

1.2-Dichloroethane 7ra«s-1,2-dichloroethylene Cis-1,2-dichloroethylene

1.1.1-Trichloroethane Methylene chloride

1.1.2-Trichloroethane Trichloroethylene Chloroform Carbon tetrachloride 1,1,2,2-Tetrachloroethane Tetrachloroethylene Ethylene dibromide Halogenated Semivolatiles

1,4-Dichlorobenzene 1,2-Dichlorobenzene

Nonhalogenated Semivolatiles

2-Methyl naphthalene o-Cresol p-Cresol

2,4-Dimethylphenol m-Cresol



Benzo(a) anthracene




Dibenzo (a,h) anthracene






Coal tar Creosote

Aroclor 1242, 1254, 1260 Dieldrin

2,3,4,6-Tetrachlorophenol Pentachlorophenol

Source: U.S. EPA, Estimating Potential for Occurrence of DNAPL at Superfund Sites, EPA Publication: 9355.4-O7FS, U.S. EPA, Washington, January 1992. Many of these chemicals are found mixed with other chemicals or carrier oils.

the gas phase may not be detected as significantly as in the case of gasoline, because the main part of the DNAPL plume sinks below the water table. Therefore the vapor phase does not exist in equilibrium with the free DNAPL phase. Figure 18.20 illustrates such a

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