As remedial actions

Immediate response for release is required, including release reporting, immediate containment, monitoring of explosive hazards, performing a site check to evaluate the extent of release, determining the presence of free product on the water table, and remedying hazards posed by excavated soils. Further corrective actions may be required such as removing the released free product, soil gas, and contaminated groundwater and soils, as well as removal and replacement of tanks. Detailed correction action plans are required if such further corrective actions are needed.3637

An underground storage system that is found to leak or likely to leak should be abandoned, repaired, or replaced. Removal and cleaning of the tank are usually carried out before repair.

18.6.1 Tank Removal

Removal of a leaking storage tank can limit liability and environmental damage. The following steps may be followed38,39:

1. Analyzing the tank content according to the U.S. EPA hazardous waste characterization process to determine the proper disposal procedure for the contents

2. Emptying the tank

3. Cleaning the tank interior with high-pressure water, steam, or solvent

4. Purging vapors from the tank using air, carbon dioxide, or nitrogen

5. Removing the tank from the ground

6. Rendering the tank to ensure it will not be reused any further, then disposing of it

7. Examining soil around the excavation for contamination

8. Removing and disposing of obviously contaminated soil (note that groundwater analyses are usually not required when a tank is removed)

9. Obtaining soil samples in the cleaned area for analysis, and documenting the effectiveness of the cleanup effort

10. Backfilling the excavation

11. Documenting the removal and disposal of the tank and soils; filing a report with the controlling government agencies and with the tank's owner, if any spills occurred during the work

Tanks should be removed only by contractors familiar with pertinent government regulations and knowledgeable about the safeguards necessary to prevent environmental harm so as to limit potential liability to the owner of the storage system.

18.6.2 Tank Repair

Some tanks, after repair, may stay in service to store gasoline. Most steel tank repairs are done by lining the interior of the tank with epoxy-based resins or some other coating that is compatible with fuel products.5 Before the tank can be repaired, all free products must be emptied, and all vapors must be removed completely. The tank should be cleaned thoroughly to ensure the lining material adheres to the interior surface of the tank. Before putting the tank back into service, the tank should be tested and examined to be sure that all leaks are repaired, and whether or not additional work needs to be done. For example, recoating the tank, reinforcing the tank area, and lining or relining can all extend a tank's life.

18.6.3 Tank Replacement

There are cases in which tanks should be replaced rather than repaired. For instance, the American Petroleum Institute (API) does not recommend the lining of a tank that has open seams more than 3 in. long, perforations larger than about 1.5 in. in diameter, more than five perforations per square foot of surface area, or more than 20 perforations per 500 square feet of surface area.40 Some localities have certain restrictions on repairing tanks.5 It is also recommended to replace an unsecured underground storage system with a new one.

Compared to earlier tanks, current underground storage systems have two advantages:

1. Minimization of leaks

2. Leak monitoring devices

Leaking is minimized in new tank systems by including corrosion protection and using a doublewalled tank construction. Corrosion protection is achieved by coating, by using cathodic protection, or by using fiberglass-enforced plastic tanks. In double-walled construction, the outer wall protects the erosion of the inner wall and contains any leakage that may occur.

New tank systems are also equipped with leak monitoring devices that take advantage of the double-walled construction. Leakage can be reported in real time and more accurately using these detection devices, which include water- or product-sensitive probes, or pressure detection devices if the space between the two walls is designed to remain under vacuum.

18.6.4 Alternatives for Tank Abandonment and Replacement

There are two alternatives to tank abandonment and replacement:

1. Abandonment in place

2. Installation of an aboveground storage tank

Although it is more desirable to remove unreliable underground tanks, a tank may be abandoned in place, for example, when it is indoors, under a building, beneath a foundation, or barricaded with other constructions.

Before a tank is abandoned in place, the following measures should be taken into consideration38,39:

1. Assessment of the tank's integrity, knowing that a tank may be abandoned in place only if it has never leaked; otherwise, a broader remediation effort might be required if it has leaked and contaminated the soil and groundwater

2. Removal of all liquids

3. Removal and disposal of sludge and residues

4. Cleaning of the tank and disposal of the cleaning residue

5. Filling of the tank with inert material such as sand, gravel, or concrete

6. Disconnection of piping, and plugging it with concrete or nonshrinkable grout or removing all piping.

The second alternative is to construct an aboveground tank, whenever it is feasible, in order to avoid the liability of uncontrolled USTs. This alternative is being chosen by many tank owners for new storage. However, the aboveground storage system has the following disadvantages compared with underground storage:

1. There are more strict fire regulations.

2. Space is needed for installation.

3. It is more likely to be exposed to accidental damage.

4. It is more exposed to local building codes, which usually do not favor aboveground tank systems.

18.7 CONTROL OF CONTAMINANTS MIGRATION AS REMEDIAL ACTIONS

18.7.1 Gas Control

Gas control is required, because the vapor phase of gasoline components in an unsaturated zone can pose a significant health and safety threat. The gas control and safety concern are discussed in another chapter. Some of the remedial technologies presented in subsequent sections of this chapter can also act as gas control measures.

18.7.2 Control of Plume Migration

Migration of the gasoline free product and the contaminated groundwater plumes should be controlled. The containment of a plume prevents its further migration and the enlargement of contaminated areas. The most effective method is to pump so as to cause a depression of the water

Plantillas Unicornio Para Imprimir
FIGURE 18.5 Using overlapping cones of influence to contain a gasoline plume.

table, which modifies and controls the flow direction of groundwater (Figure 18.5). The trench method can also intercept the plume and prevent it from further migration.

These two methods, which are used as an emergency action, can also be utilized for the cleaning of plumes. Containment methods can often be extended to plume treatment by using the trench or well pumping to recover the free product.

There are other methods for containment such as slurry walls and piling sheets, which are only used as methods for containment but not for treatment.

18.8 REMOVAL OF CONTAMINANTS AS REMEDIAL ACTIONS 18.8.1 Removal and Recovery of Free Product

Recovering free product comprises the following major steps41:

1. Establishing gasoline plume containment

2. Gathering and extracting (associated with gasoline/water separators) the contained plume from underground

3. Recovering gasoline

18.8.1.1 Gasoline Plume Containment and Extraction

Trench method

The trench method uses an excavator to dig a trench down to the water table to intercept the flow of the floating gasoline. The trench should be dug deep enough to pond groundwater and the floating gasoline. Pumping out the water in the trench can increase the hydraulic gradient and increase the movement of gasoline to the trench.

Groundwater flow direction should be predetermined. An impermeable membrane is placed on the downgradient side of the trench to ensure that the gasoline in the trench does not escape back into the soil. A better practice is to install an upgradient membrane that can allow more gasoline but less water to enter the trench, and a downgradient membrane to prevent gasoline from moving into the soil while allowing water to pass out to the soil on the downgradient side.

The ponded gasoline in the trench is removed separately from the water for recovery. Special equipment has been used for this purpose, including skimmers and filter separators that are automatically activated when gasoline is present in the trench to separate and remove the gasoline from the water.19 It is inevitable that a gasoline and water mixture will be pumped out. Gasoline will then be recovered using methods (detailed later) for the treatment of pumped contaminated groundwater.

The trench method is applicable only when the water table is relatively shallow, less than 10 to 15 ft below the ground surface. For a deeper water table, the cost of the trench method becomes more expensive than other methods such as pump systems. Another limitation of the trench method is the soil structure. The soil above the water table has to be firm and well aggregated to allow for the trench to be self-supporting. Otherwise, embankment enforcement or screening would be needed. A third limitation is that continuous pumping and skimming is required to maintain a flow gradient towards the trench. Otherwise, the free product will move back and reenter the soil.

Pumping well method

The pumping well method is more suitable for a water table that is too deep for the trench method. Pumps draw water, forming a cone of depression in the water table to control the movement of floating gasoline. The gasoline is then pumped out. The pumps can be either single- or a dual-pump systems.

Groundwater models and other analytic techniques are available to assist in proper pump siting, choosing pump capacities, and calculating the movement of the contaminant plume. The characteristics of the aquifer, the flow of groundwater, and the size of the plume should be known.

In the single-pump system both gasoline and water are recovered through a single pipeline to aboveground storage tanks or oil/water separators (Figure 18.6a). There are two problems encountered with this single-pump system:

1. During pumping, gasoline and water are mixed, which complicates aboveground separation.

2. Large volumes of contaminated water must be stored, treated, and disposed of.

Therefore, the single-pump method is commonly used only for smaller spills when the gasoline-water recovery rates are relatively low (e.g., less than 1892 L/h or 500 gal/h).

The dual-pump system is used when a large amount of gasoline is to be recovered. Separate gasoline and water pumps are used. The dual-pump system significantly reduces the amount of water that must be treated. Water pumps are placed at a depth lower than the water table to be able to establish a cone of depression, and the gasoline pumps draw out the gasoline that floats into the depression on the top of distorted water table for product recovery (Figure 18.6b).

Dual-pump systems are better able to control a constant cone of depression than the singlepump system. It is important to maintain a nearly constant cone of depression to prevent the migration of the gasoline plume. If a constant cone of depression is not maintained, the water table and

Oil-water separator

Oil-water separator

FIGURE 18.6 (a) Single-pump and (b) dual-pump gasoline recovery systems.

the gasoline plume will rise, and gasoline droplets may adhere to soil particles and consequently remain in the soil.

The cone of depression in a dual-pump system is controlled by a detection probe. Initially, the probe is set in the well at the depth of the proposed cone of depression of the gasoline-water interface. The water pump draws the water table down, reaching the pump probe. The water pump ceases when the pump probe detects gasoline. The depressed water table will rise slightly. As soon as the probe detects water again, the water pump resumes, thus maintaining a constant cone of depression. Gasoline will accumulate in the depression. The product pump, both inlet and probe of which are placed a few inches above the water probe, draws gasoline aboveground.

Installation of the pumping well is more time consuming than digging a trench. There is a lag period between the start of pumping, the formation of the depression cone, and containment of the plume. This limits its use as a rapid containment measure. The water table depression must be kept constant; otherwise, if the water table is allowed to fluctuate, gasoline droplets may adhere to soil particles and get trapped below the water table, especially when the depth of the cone of depression gets lower.

The pumped free product is usually accompanied by water. Hence, it is necessary to separate water from the oil, which is usually performed aboveground, although recently a subsurface recovery system has been developed.

18.8.1.2 Subsurface Gasoline Recovery

Subsurface gasoline recovery is analogous to in situ oil-water separation. The main advantage of this technique is that the pumped gasoline, which moves with the groundwater gradient, can be intercepted and recovered with minimum energy input.19 The plume is trapped and directed to the separator influent nozzle. Other advantages are that it reduces the likelihood of water being frozen in the separator in cold weather, it eliminates the evaporation of potentially dangerous volatile organic compounds, and it saves aboveground space for other uses. The disadvantages of subsurface gasoline recovery are as follows:

1. It is difficult to excavate a hole large enough and deep enough to install the separator at the water table.

2. Installation is time consuming and may not be completed quickly enough to contain the migration of a rapidly moving plume.

3. The separator effluent usually contains a residual dissolved gasoline concentration of 15 mg/L.

4. Treatment of separated gasoline is also needed if the reuse of gasoline is desired. In such a case, an aboveground advanced gasoline-water separator is needed.

18.8.1.3 Aboveground Gasoline Recovery

Aboveground separators are typically large tanks whose function is to slow down the flow of the incoming water; this allows gravity separation of the less dense gasoline emulsions.19,41 Separators are composed of two or more chambers. The first chamber is used for the deposition of settleable solids, and the second is used for the separation of liquids of dissimilar specific gravities and the removal of the lighter liquid.

In the preseparation chamber, the less dense oil droplets rise, collide, and fuse with adjacent droplets. According to Stoke's law, the larger the diameter of a particle, the faster is its rate of rise. Thus, as small droplets coalesce to form larger droplets, their upward vertical velocity increases. Coalescing tubes or plates are designed to enhance the separation of oil-water emulsions. The emulsion free water is directed away from the tubes or plates and enters the separation section. Some separators are built with an outlet zone for the discharge of clarified water.

Under optimum conditions, an oil-water separator can reduce the hydrocarbon emulsion in water down to 15 mg/L. The separator is most effective when the gasoline plume is relatively small and the rate of water flow is slow enough to allow for complete separation.

If it is desirable to reuse the oil, then more efficient oil-water separators utilizing heating and nebulization techniques will be needed. U.S. patents issued to Weber and colleagues42 and Wang and colleagues43 make use of such techniques.

18.8.1.4 Recovered Gasoline

Recovered gasoline can either be disposed of by incineration or reused. If the gasoline is to be reused, it must be refined or mixed with other gasoline as it gets degraded while in the soil. There are three processes that affect the degradation of gasoline:

1. Aromatic hydrocarbons such as benzene, toluene, and xylene become oxidized in the presence of oxygen.

2. Gasoline constituents are metabolized by soil microbes.

3. Water particles may coalesce with the hydrocarbons.

18.8.1.5 Recovered Water

Recovered water that contains a small amount of floating free product and dissolved constituent is usually passed through an oleophilic-hydrophobic adsorbent filter to remove the remaining free product.19

If the remedial action involves the treatment of contaminated water (such as pump-treatment for groundwater recovery or soil-washing for soil recovery, which will be discussed in Section 18.8.2), then the preliminarily recovered water can be combined with a treatment stream for further treatment.

There are many options for the disposal of the filter-treated water and dissolved hydrocarbons:

1. The aquifer may be recharged with the recovered water in order to flush out the remaining pockets of free gasoline. A drawback to this technique is that the recharging water contains dissolved constituents.

2. The water may be discharged to a natural water course where dilution and exposure to oxygen will reduce the hazards of its dissolved gasoline constituents. In such a case, a National Pollutant Discharge Elimination System permit and a State Pollutant Discharge Elimination System permit must be obtained.

3. The water may be sent through a wastewater treatment plant where the remaining dissolved constituents can be removed.

4. The water may be treated with on-site air strippers and carbon adsorption filtration systems.

18.8.2 In Situ Biological Treatment of Groundwater Decontamination

Several methods are available to remove gasoline constituents from water, such as air stripping, biorestoration, activated carbon adsorption, reverse osmosis, ozonation, oxidation, resin adsorption, oxidation with hydrogen peroxide, ultraviolet irradiation, flotation, and land treatment.

Biological in situ treatment is based on the concept of stimulating microflora to decompose the contaminants in place, resulting in the breakdown and detoxification of those contaminants. Biological degradation or biological remediation is generally considered a cost-effective method for the removal of organic compounds, although it is site-specific for in situ biological degradation. For removing volatile organic compounds (VOCs), on the other hand, cost-efficiency may be achieved by using the technologies involving volatization (such as air-stripping), as well as other technologies. In fact, about 95% of cases that involve removing a gasoline plume dissolved in groundwater use air stripping and filtration through GAC.19 Biological treatment is not widely applied in the field, although it is cost-effective and promising for coarse-grained soils.

18.8.2.1 Classification of Biological Treatment

Bacteria can grow in two main environments, aerobic and anaerobic. In aerobic treatment, aerobic and facultative bacteria use molecular oxygen as their terminal electron acceptor. The treatment occurs in the presence of a molecular oxygen supply. In anaerobic treatment, anaerobic and facultative bacteria use some other compound as their terminal electron acceptor, for example, carbon dioxide, sulfate, or nitrate, in the absence of molecular oxygen. In fact, there is another type of biological treatment called the fermentative and methanogenic process, which is carried out by what is referred to as a methanogenic consortium.44,45

So far, only aerobic processes have proved to be effective for in situ removal of organic waste in groundwater and soil.

18.8.2.2 Characteristics and Factors Affecting Aerobic Biological Treatment

In the aerobic process, organic contaminants such as gasoline releases are broken down by bacteria to produce new biomass (bacteria) and other byproducts:

Bacteria + organics + oxygen + nutrients (N, P) —> more bacteria + byproducts (18.7)

The organics contaminants, whose concentration is usually expressed in terms of biochemical oxygen demand (BOD), are utilized as food for the bacteria. Besides oxygen, nutrients (nitrogen and phosphorus) are also needed by the bacteria for its metabolism. The concentrations of oxygen, bacteria, organic contaminants, and nutrients, as well as other factors, have an affect on the biological treatment rate.

Dissolved oxygen (DO) in a bioreactor should be maintained above a critical concentration in order to maintain good aerobic biological activity. The minimum required DO concentration ranges between 0.2 and 2.0 mg/L with 0.5 mg/L being the most reported value.

Significant and active microbial populations are usually found in the subsurface soil and groundwater. However, if there is a lack of required microorganisms, then bacteria can be injected in situ. An optimum food/microorganisms (F/M) ratio should be maintained for effective removal of organic contaminants.

An equally important factor is the biomass/oxygen ratio. If oxygen is deficient, then the biomass cannot be sustained under aerobic conditions. Thus, control of the oxygen supply becomes important. In fact, in bioremediation the most important part of the design is the provision of an appropriate level of oxygen supply to maintain an efficient process.

Another important factor is the food/nutrient ratio. Many of the necessary nutrients may already be present in the aquifer, such as K, Mg, Ca, S, Na, Mn, Fe, and trace elements; however, N and P may be deficient and need to be added. The optimum ratio of BOD : N : P is 100 : 5 : 1. It is not a good practice to inject a large quantity of nutrients in the aquifer at one go. They should be fed at the required usage rate throughout the cleanup process. Both the organic contaminants and the nutrients should be completely exhausted by the end of the in situ remediation of an aquifer.

pH should be maintained near neutral, between 6 and 8. Generally, the optimal value is slightly higher than 7.

The optimal temperature for bacterial growth is between 20 and 37°C. For every 10°C decrease in temperature, bacterial activity is approximately halved. Temperature in deep groundwater is rather constant. However, for shallow soil and water, in cold weather the rate of biodegradation becomes depressed compared to in warmer weather, and therefore warm water may need to be injected into the subsurface.

Other factors affecting performance include the presence of toxic material, the redox potential, salinity of the groundwater, light intensity, hydraulic conductivity of the soil, and osmotic potential. The rate of biological treatment is higher for more permeable soils or aquifers. Bioremediation is not applicable to soils with very low permeability, because it would take a long time for the cleanup process unless many more wells were installed, thus raising the cost.

Clogging of aquifers by the growth of biomass is an operational problem. The permeability of an aquifer could be reduced due to the precipitation of biomass sludges and chemicals, or due to clay dispersion.

18.8.2.3 Design of an In Situ Bioremediation System

The concentration of biomass is important for the degradation of organic contaminants. Designers can utilize the available microbial population in the soil and groundwater. However, the biomass grows slowly, and remediation requires an accelerated growth rate. This can be realized by a delivery and recovery system. The delivery directs oxygen and nutrients to the underground formations; the recovery stage recovers the spent treatment solution. Circulation of groundwater is very important. A complete delivery and recovery system will do the following:

1. Deliver a high concentration of oxygen and supply additional nutrients or commercially available bacteria if bacteria and nutrients are deficient

2. Provide adequate contact between the biomass and contaminants

3. Prevent the clogging of the soil voids to ensure a sufficient groundwater flow

4. Flush the groundwater

5. Provide hydrologic control of treatment agents and contaminants to prevent their migration beyond the treatment area

6. Provide for complete recovery of the spent treatment solution or contaminants where necessary

As bioremediation proceeds, the bacterial population increases due to the growth of the biomass. Thus, although bacteria may be deficient at the beginning they do not usually need to be added after the startup.

The following design example of an injection and extraction system (Figure 18.7) illustrates the bioremediation process. Both the soil and groundwater are contaminated. Groundwater is extracted downgradient and reinjected upgradient of the zone of contamination. Water is also injected to flush the soil.

There are two methods for the injection of oxygen: in situ and in line. In an in situ oxygen supply, oxygen is supplied directly from the aeration well to the contaminated plume. A mechanical aeration unit produces sufficient mixing of oxygen and bacteria with the leachate plumes. In an in-line oxygen supply, oxygen is added together with nutrients or bacteria to the mixing tanks (Figure 18.7b).

The most common sources as oxygen supply are air, pure oxygen, hydrogen peroxide, or possibly ozone. Table 18.3 summarizes the advantages and disadvantages of these oxygen supply alternatives.

Using air is economical. However, an in-line method using air may not provide adequate oxygen supply because the maximum oxygen supply is approximately 10 mg/L O2, which is sufficient for the degradation of only about 5 mg/L of hydrocarbons. Even when using pressurized air or pure oxygen, an in-line supply of oxygen can only degrade low levels of contaminants, less than 5 to 25 mg/L of hydrocarbons.

Pure oxygen can also be used. The injection method can be the same as for air injection. The advantage of using pure oxygen over conventional aeration is that higher oxygen transfer to the biomass can be attained. The in-line injection of pure oxygen will provide sufficient dissolved oxygen to degrade 20 to 30 mg/L of organic material.

Using hydrogen peroxide (H2O2) has the following advantages:

1. Greater oxygen concentrations can be delivered to the subsurface.

2. Less equipment is required.

3. Hydrogen peroxide can be added in-line along with the nutrient solution, and aeration wells are not necessary.

4. Hydrogen peroxide keeps the well free of heavy biological growth, thus reducing clogging problems.

Subsurface aeration wells

Injection well

Subsurface aeration wells

Injection well

Extraction well

Direction of flow

Nutrients cr In-line rz: oxygen source

Soil flushing

In situ „ aeration

Aeration well bank

Injection well

Soil flushing

Aeration well bank

Injection well

Simplified view of bioreclamation of soil and groundwater

Aeration zone Direction of groundwater flow

Extraction well

FIGURE 18.7 Simplified view of groundwater bioreclamation.

Simplified view of bioreclamation of soil and groundwater

Aeration zone Direction of groundwater flow

Extraction well

FIGURE 18.7 Simplified view of groundwater bioreclamation.

Ozone is not widely used, because of its high cost and the possibility of some toxicity to bacteria if used at high dosage for low BOD concentrations (higher than 1 mg/L of ozone per mg/L total organic carbon).

In situ oxygen supply requires aeration wells for the injection of oxygen. The criteria are that the aeration well zone must be wide enough to allow the total plume to pass through, and the flow of air must be sufficient to produce a substantial radius of aeration while small enough so as not to create an air barrier to groundwater flow. The required residence time tr for aeration can be calculated from Darcy's law as a function of the groundwater head and hydraulic conductivity:

TABLE 18.3

Oxygen Supply Alternatives

Substance Application Method

Air In-line

Oxygen-enriched air or pure oxygen

Hydrogen peroxide

In situ wells In-line

In situ wells In-line

Ozone

In-line

Advantages

Most economical

Constant supply of oxygen possible Provides considerably higher O2 solubility than does aeration

Constant supply of oxygen possible Moderate cost Intimate mixing with groundwater Greater O2 concentrations can be supplied to the subsurface (100 mg/L) H2O2 provides 50 mg/L O2) Helps to keep wells free of heavy biogrowth Chemical oxidation will occur, rendering compounds more biodegradable

Disadvantages

Not practical except for trace contamination <10 mg/L COD Wells subject to blow out

Not practical except for low levels of contamination <25 mg/L COD Very expensive Wells subject to blow out H2O2 decomposes rapidly upon contact with soil, and oxygen may bubble out prematurely unless properly stabilized

Ozone generation is expensive Toxic to microorganisms except at low concentrations May require additional aeration

Source: U.S. EPA, Cleanups of Releases from Petroleum USTs: Selected Technologies, EPA/530/UST-88/00l, U.S. EPA, Washington, 1988.

where tr = residence time (T), La = length of aerated zone (L), h1 = groundwater elevation at beginning of aerated zone (L), h2 = groundwater elevation at the end of the aerated zone (L), K = hydraulic conductivity (L/T).

The design conditions for the injection and extraction system are as follows:

1. The groundwater injection rate should be determined by a field testing program.

2. All injected groundwater and associated elements are to be kept within the site boundary to prevent the transport of contaminants to adjacent areas.

3. The distance between the injection-pumping wells should be such that approximately six injection-pumping cycles can be completed within a six-month period.

4. Aquifer flow rate should be sufficiently high so that the aquifer is flushed several times over the period of operation.

5. Flow and recycle rates should not be high enough to cause excessive pumping costs or loss of hydraulic containment efficiency due to turbulent conditions, corrosion, flooding, or well blow out.

18.8.2.4 Case History of In Situ Bioremediation

A bioremediation system described by U.S. EPA19 consists of a downgradient dewatering trench and well, two mobile biological activating tanks, two mobile settling tanks, and two upgradient

10 gal/min pump

10 gal/min pump

Gravel

Slotted

Manhole

Excavated soil cover r\

15 mm plastic 10 sheeting

Collection trench

Nine equally spaced aeration wells

Recharge trench

FIGURE 18.8 Flow diagram of Biocraft biorestoration.

Gravel

Slotted

Collection trench

Manhole

Excavated soil cover r\

15 mm plastic 10 sheeting

Nine equally spaced aeration wells

Recharge trench

FIGURE 18.8 Flow diagram of Biocraft biorestoration.

reinjection trenches (Figure 18.8). The system was used to treat between 53,000 and 76,000 L/d (14,000 and 20,000 gal/d) of groundwater that had been contaminated with 114 m3 (30,000 gal) of organics that leaked from USTs. The reduction of contaminant mass ranged from 88 to 98% for methylene chloride, acetone, and n-butyl alcohol, and 64% removal for dimethylaniline. Most of the contaminants in the groundwater (over 95%) had been removed during its operation from 1981 to 1985.46

There are several advantages of using in situ bioremediation47 49:

1. Cost-effectiveness

2. Minimal disturbance to an existing site

3. On-site destruction of contaminants

4. Continuous treatment after shutdown of the project

5. Permanent solution

6. Possibility of simultaneous cleanup for both groundwater and soil

Most contaminations of aquifers are a result of material being released above the saturated zone. The contaminant pumping method is limited to the cleanup of the saturated zone. Contaminants in the unsaturated zone can still be a source of future contamination. In situ bioremediation techniques can also be designed to clean up the unsaturated zone simultaneously.

The limitations of in situ bioremediation are as follows:

1. It is not suitable for short-term projects (it usually needs two to eight weeks of startup period to have the bacteria grown to a sufficient concentration in order to effectively remove the contaminants).

2. It is not suitable for low-permeability and high-salinity areas, as well as areas with extreme pH levels.

3. It is not suitable for the removal of nonbiodegradable organics, toxic material, or material whose concentration is too high and thus toxic to bacteria.

4. It requires continuous operation (a biological treatment system cannot be turned on and off frequently).

18.8.3 Pump-and-Treat Processes for Groundwater Decontamination 18.8.3.1 Air Stripping

Air stripping is an effective and widely used method to remove VOCs from water. It is the most cost-effective option for removal of gasoline from groundwater.19

The basic principle of air stripping is to provide contact between air and water to allow the volatile substances to diffuse from the liquid to the gaseous phase. Mass transfer occurs across the air-water interface. The theory of air stripping is related to Henry's law. At a given temperature, the partition of VOCs in the contacting air and water follows Henry's law:

where Pa = particle vapor pressure of VOC (atm), H = Henry's law constant (atm), and Xa = mole fraction of VOC in water (mol/mol).

The Henry's law constant can be regarded as the partitioning coefficient of VOCs between air and water. Molecules of VOCs can pass freely between gaseous and liquid phases. At equilibrium, the same numbers of molecules move in both directions through a unit area in a unit of time. Departure from equilibrium provides the driving force for mass transfer. This can be affected by a change of temperature or by driving the VOC out of the air phase. Air stripping can be regarded as a "controlled disequilibrium".1950 Removal of a VOC from the contacting air-water system leads to it being at a decreased concentration in the water. The eventual outcome is the removal of the VOC from water.

Types of air stripping facilities

There are many methods to introduce fresh air for air stripping, including diffused aeration, tray aerators, spray basins, and packed-towers methods.

In the air diffusion method, compressed air is injected into the water through diffusers or sparging devices that produce fine air bubbles.51 Mass transfer occurs across the air-water interface of the bubbles. Consequently, contaminants are removed from the wastewater. Mass transfer rates can be improved by producing fine bubbles, increasing the air/water ratio, improving basin geometry, using a turbine to increase turbulence, or increasing the depth of the aeration tanks. Reported removal of organics by air diffusion is between 70 and 90%.52

The tray aeration method is a simple, low-maintenance method of aeration that does not use forced air.19 Water is allowed to cascade through several layers of slat trays to increase the exposed surface area for contact with air (Figure 18.9). Tray aeration is capable of removing 10 to 90% of some VOCs, with a usual efficiency of between 40 and 60%.53 This method cannot be used where low effluent concentrations are required, but could be a cost-effective method for reducing a certain amount of VOC concentration prior to activated carbon treatment.

The spray aeration method comprises a grid network of piping and nozzles over a pond or basin. Contaminated water is simply sprayed through the nozzles and into the air to form droplets. Mass transfer of the contaminant takes place across the air-water surface of the droplets. Mass transfer efficiency can be increased by multiple passing of the water through the nozzles. This method has three disadvantages:

1. A large land area is necessary for the spray pond.

2. Mist is formed, which could be carried into nearby residential areas.

Inlet chamber

Distributor nipples

Staggered slat trays

Air inlet

Inlet chamber

Distributor nipples

Staggered slat trays

Air inlet

Blower

krd) > 1' S c^J f^i 4* k. <db «cr, eb e±=>

FIGURE 18.9 Schematic diagram of Redwood slatted tray aerator.

Blower

Air seal

—icaOO ■^czia fc^s daoacbctcaa ocaocbi

•=> CT3 <~> Csdj C^D O CJ C=3 C^» cbi c^.

^jc^tiiodacistiio CD caOoci:

J tin db Ca tij (inrtt tta cn tia ¿1 a krd) > 1' S c^J f^i 4* k. <db «cr, eb e±=>

Air seal

Air seal

FIGURE 18.9 Schematic diagram of Redwood slatted tray aerator.

3. There is the possibility of ice formation, which lowers the usefulness of the technique in colder climates.

The packed-tower method involves passing water down through a column of packing material while pumping countercurrent air up through the packing (Figure 18.10). The packing material breaks the water into small droplets, causing a large surface area across which mass transfer takes place. The towers are very effective in removing VOCs. Typical removal efficiencies are between 90 and 99%, although 100% (i.e., down to nondetectable levels) removal has been reported. These countercurrent packed towers are the most common of the air-stripping methods. The air emission problems associated with air stripping units have been eliminated from the units developed in the early 1990s by Wang and colleagues29 and Hrycyk and colleagues.30

Henry's law constant

Henry's law constant (H) is usually expressed as follows:

where Patm = pressure (atm) (here 1 atm = 760 mmHg), Mw = weight of water (mol), and Mc = weight of contaminant (mol).

Off-gases

Incoming

- Demister

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