comparing groundwater remediation options

Comparing Groundwater Remediation Options

Paul N. Cheremisinoff Glenn Goessmann

Although many conventional physical remediation methods are viewed aspnwen, they often only relocate wastes to othersites orinto the air. How do the emetging biologkal and chemical in situ methodsperfonn in

N. c-ff is a pqfessor of ctd and

anglnecrtng at tbe N e w Jersey Irrrtitute of Tscbrdogy in Newark andisamemberof Reuudiation’s Editorial AdzdsqBoard Glenn Coessnrarr is a cbemlcdand envfronmen~ engineer.

the same applications? mis article rmrlezus their results (much of it in the laboratory) as well as theirpromise of more complete neutralization of hazardous wastes, lower capUa1 CW, and longer-duration cleanup processes. The optimal method may be a combination of chemical and biological in situ techniques with physical pump-and-mat methods.

The single most important aspect of groundwater is its value as a potable water supply. More than 50 percent of the US. population may use groundwater as its sole source of potable water. Contaminated groundwater is very difficult to clean up because of the mechanism of groundwater flow, which is slow, taking a seemingly endless time to flush contaminated water through an aquifer. Contaminants can sorb onto the soil and slowly leach into the groundwater, or they can move quickly into the groundwater. The contaminants can float on the top, sink to the bottom, and mix with the groundwater.

Individual wells and well fields are increasingly being contaminated by toxic compounds. The contamination is generally attributable to nearby industrial discharges, agricultural operation, and landfill leachate. The organic contaminants of concern include organic solvents, volatile organic compounds (VOCs), petroleum products, pesticides, and nitrosamines. The inorganics include heavy metals, arsenic, nitrates, and total dissolved solids C’IDS). Although the inorganic compounds are in groundwater less frequently and in lower concentrations, they still leave the water unusable.

EPA has identified more than one hundred priority pollutants and has stated that low levels of these compounds can have long-term health effects (e.g., carcinogenic, mutagenic, and teratogenic). Many of the organic priority pollutants are VWsolvents that are subject to rigorous regulation by EPA and tolerated only in amounts approaching the limits of detectability.

Most contaminants identified in groundwater have already been encountered in wastewater. For decades, physical, chemical and biological treatment has been used on wastewater; groundwater remediation has been applied only during the past decade. The physical methods include air stripping, carbon adsorption, ion exchange, and membrane separation.

REMBDIATION/SPRING 1992 153

L

Chemical methods include precipitation and oxidationheduction. All methods are limited.

Federal and state legislation and increased public awareness of environmental problems are spurring the search for new treatment technologies. For example, biological principles applied to groundwater remediation show promise. Bioremediation, which uses microorganisms to detoxify, degrade, or destroy toxic compounds, is particularly attractive because it has the potential to remove contaminants at lower capital and operating costs than do conventional technologies.

The physical methods work exclusively aboveground; the chemical and biological methods are applicable aboveground or in sib. Because no single unit operation or process can treat every contaminant in groundwater, two or more unit operations are often combined into an overall treatment train.

F e d e d and s t a t e kgislation and incretued public

envimnwwntal probkmsare 8pumhg the 8e-h for mew tmatment

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PERCOLATION, AQUII;ERS, AND IMPENETRABLE LAYERS Surface water slowly percolates down through the soil and rock until

it reaches a layer it cannot penetrate. Water accumulates above thii impermeable layer, and the soil matrix becomes saturated with water. This saturated zone marks the groundwater table. The zone of soil layers that contain both liquid and vapor is the vadose (unsaturated) zone.

An aquifer is a geological formation (e.g., soil, sand, gravel, rock) that contains groundwater and can deliver it in sufficient, usable quantities. An aquifer can be close to the surface, situated in an area of highly permeable layers, constituting an unconfined aquifer. Seepage from the surface into the groundwater is largely unimpeded. A confined aquifer is bounded above by a relatively impermeable confining layer. The groundwater flows through the void spaces (pores) in the soil matrix of the aquifer. A measure of the resistance to this fluid flow within the aquifer is permeability, which is influenced by hydraulic conductivity, which defines the volume of water that passes through a unit cross-sectional area in a unit time.

Groundwater significantly influences the engineering properties of soils and rocks. The relation between groundwater flow and geologic conditions is the basis of the study of hydrogeology. Groundwater flows under the influence of the gradient between discharge and recharge. Manipulation of the flow of groundwater can be achieved by creating a hydraulic gradient through drawdown and recharge pumping. A zone of influence is created around a well where the natural water level and flow are modified As water is withdrawn, a cone of depression is formed. As water is injected, a recharge mound is formed.

The flow of groundwater is not solely a function of the aquifer orientation. The surface topography strongly influences the groundwater flow. The regional hydrologic and hydrogeologic systems are related components of the overall groundwater flow system.

When considering the problem of groundwater contamination, a thorough understanding of the hydrogeology of the surrounding area is essential to predict the fate of the contaminants in the environment. A detailed understanding of the groundwater flow system is needed to

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154 RGMBDIATXON/SPIUNG 1992

Table 1. Data Needed for a Site-Specific Evaluation.

Contaminant Characteristics Solubility Volatility Degradability Toxiatyhdth risks Number of competing compounds

Contaminant Source Characteristics Pointhon-point SpilVlong-term release M e x t e n t

Soils Properties Type (e.g., day, sand, loam) Heterogeneity Formation porosity and permeability Hydraulic condudvity Ionchemisvy

Plume Delineation Physical and chemical character of the contaminants Fate of contaminant Contaminant transport mechanism and pathways Areal extent, depth, and amount of contaminants

Soil profde (physical and chemical properties) Regional hydrologic flow system Aquifer: confined and unconfined Groundwater level and fluctuation Groundwater flow pattern Gradient and recharge areas

Hydrogeological Characterization

determine the direction, rate, and areal extent of contaminant migration in the groundwater. Equally important is an understanding of the position of contaminant sources relative to the groundwater flow system.

WHAT IS "REATABLE? Before contaminated groundwater can be cleaned up, a treatability

study can help determine what combination of physical, chemical, and biological treatment will most efficiently remove the contaminants and collect enough data for a full-scale process design. Site-specific characteristics affect the removal of the contaminants. The parameters that define a contamination site are related to the distribution of contaminants and the factors that control their transport. Characterizing a site involves an analysis of past behavior to predict future conditions (see Table 1).

The extent of the contamination must be assessed. Groundwater quality, specific contaminants, and subsurface soil characteristics are determined by taking subsurface samples. Monitoring wells can help

155

PAUL N. Cmmwasmom GLENN GOBSSMA"

characterize the rate and extent of contaminant plume migration in the subsurface environment.

When the specific contaminants are identified and if physical or chemical treatment appears to be most appropriate, parameters of the particular unit operation must be defined further. For example, air stripping packed tower loading tests or adsorption isotherm tests might be conducted. If biological techniques are indicated, the related data must be collected.

A biotreatability study provides data on the degree of achievable biological treatment, including organic removal rates, oxygen requirements, oxygen transfer characteristics, and biokmetic constants. The biodegradation rate of target compounds is the main concern.

Analysis includes evaluation of the pH condition, dissolved oxygen level, subsurface microorganisms, and nutrient availability. A geochemical evaluation of the site can determine which chemicals, if any, are deficient, helping determine whether indigenous microbes can sufficiently degrade the contaminants. Techniques for enhancing natural biodegradation must also be investigated, including addition of supplemental microbes, nutrients, cosubstrate, and oxygen, as well as pH adjustment.

Techniques for ew.ng narud biodegradation must also be investigated.. .

COMPARING C 0 " I I O W T R E A " T S

are discussed in the following sections. The five principal conventional treatment methods and technologies

StriPPhg Stripping is a unit operation of mass transfer that enhances the

separation of volatile compounds from solution, exploiting the difference between the actual concentration and the equilibrium concentration of the dissolved compounds (e.g., gases, vapors) in water. A stripping process provides a liquid-vapor interface, the point at which phase change occurs. The volatile compounds leave the liquid phase at the liquid surface. The rate of mass transfer depends on two factors:

Transfer area, or interface surface area Driving force, or the concentration differential between liquid and =Par

Maximizing the separation can be achieved by increasing the interface surface area, increasing the mass flow rate through the system, or increasing the driving force.

In the stripping process, temperature is an important variable; as the temperature increases, so does the volatility along with the rate of mass transfer of the dissolved target compounds (usually VOCs). Although high temperature or steam stripping can be used to capture this benefit, operating costs of heating are greatly increased.

For air stripping, equipment can be as simple as a spray n o d e that releases a stream of water into the air. The packed tower is the most popular technology. Air stripping can also, however, release volatile

156 RBMGDIATION/SPRING 1992

COMphRING GROUNDWATER REMEDIATION oPl3ONS

Ion exchange is the process of removing unwanted ionic species fiom solution by replacing them with a different species of ion.

compounds into the atmosphere, merely relocating the pollutants from the water into the air.

Adsorption Adsorption is the process of capturing the molecules of dissolved

solids, liquids, or gases on the surface of active solids, physically or chemically at the molecular level. The advantage of physical adsorption is related to the surface tension of the active solid (the adsorbent); for chemical adsorption, chemical bonding occurs at the surface of the adsorbent.

Adsorption on activated carbon follows the physical model. The great surface area that activated carbon has available for the adsorption process is attributable to its porous internal structure; activated carbon has a matrix of micropores, yielding a surface area of up to 1,400 m2 pergram of material.

Although much tabulated data is available on the adsorption capacity of activated carbon for many compounds, environmental engineers who are designing a system for a particular situation should perform a column pilot test to support the operation. Many compounds in solution compete for adsorption sites; activated carbon collects and holds compounds of lower volatility relatively easily.

Adsorptive capacity increases as solubility (of the target compound) decreases and increases as temperature decreases.

Carbon adsorption has been used for decades, traditionally to remove taste and odor from drinking water and to remove organic and inorganic compounds from water, although the process is most common to organics removal. Carbon adsorption is also used together with other treatments as a polishing step.

Ion Exchange Ion exchange is the process of removing unwanted ionic species from

solution by replacing them with a different species of ion. This process can displace or remove all ions including nitrites, nitrates, heavy metals, alkali metals, sulfites, sulfates, chlorides, fluorides, and bicarbonates. An ion’s removed charge is specific to the exchange resin; there are applications for positive-charge cations and negative-charge anions. For complete removal, a mixed-bed unit or a multistep system is used. In the mixed-bed exchanger, both cation and anion resins are contained in a single unit. A multistep system alternately arranges cation and anion exchangers in series flow. One disadvantage associated with ion exchange is that, for waters with high concentrations of dissolved ionic solids, the exchange capacity is rapidly exceeded, requiring frequent regeneration or replacement of the resin. Pretreatment may be required to avoid plugging or fouling of the resin bed by suspended solids; certain residual organic compounds can deteriorate the resins.

Reverse Osmosis This separation process removes dissolved compounds from solution

by filtration through a semipermeable membrane at high pressure. The product stream is clear permeate; the waste stream is a concentrate of

RBIKBDIATION/SPRING 1992 157

PAUL N. -PP GLENN GOESSMA"

mostly dissolved metal salts. Depending on the specific membrane composition, the dissolved solids rejection rate can range from 50 percent to more than 99 percent. Although some organic compounds are removed, the rate is low. Reverse osmosis is generally not acceptable For organics removal.

Chemical Treatment Like biological treatment, chemical treatment is applicable aboveground

or in situ. Aboveground chemical treatment systems are the same as conventional wastewater treatment systems; the groundwater is extracted and treated, then discharged on the surface or recharged to the aquifer. Treatments include oxidation and precipitation.

Chemical oxidationheduction is the loss or gain of electrons by atoms. Applied to organics, oxidation is the chemical process of degrading the compounds to carbon dioxide and water. The two chemical oxidizing agents commonly used in groundwater treatment are hydrogen peroxide and ozone. Chemical precipitation is often used to remove metals and other inorganics from wastewater; the metals are generally removed by precipitation as metal hydroxides, carbonates, or sulfides. Following the chemical reaction that forms the insoluble solids, these precipitates must be removed by a physical separation, usually filtration or sedimentation. (The chemical methods are described in the section below titled "In Situ Chemical Treatment. ")

Aboveground chemical treatment ryrtenrr are the same QI conventional wastewater trecrfmenf syateme. . . r-

COMPARING EMERGING TECHNOLOGIES

discussed in the following sections. The three principal emerging treatment methods and technologies are

Biodegradation For about ten years, the effort to harness natural biological processes

for the destruction of hazardous wastes has resulted in much experimental data on many organic compounds, and compounds previously viewed as resistant to biodegradation (refractory) have proven amenable to biological treatment. Bioremediation began decades ago with the application of microbes originally developed for the production of enzymes used in detergents. Since then, strains of bacteria have been modified to resist the toxic effects of many organics on EPA's list of priority pollutants. Basically, any indigenous microorganism is viewed as adaptable to and a degradation agent for any synthetic organic compound. Selective development involves identifying a microorganism known to have some activity in the presence OF the desired toxic compound, adapting it to progressively higher concentrations, selecting the most active colonies, and preserving them for later application.

Most organics with toxic properties are still biodegradable and can serve as a carbon and energy source for microbial growth under select environmental conditions. Priority pollutants can generally be biodegraded under lab conditions through specially acclimated microbes that must be evaluated for biological processes that treat specific inhibitory compounds.

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158 REMEDIATION/SPRING 1992

CoMpARINO GROUNDWATER REMBDIATION ~ O N S

Microbes metabolize the organic compounds, producing energy and new cellular material, by w y of three mechunisms. . .

To maintain a contaminant-degrading microorganism population, a minimum level of the target compound must be maintained in the bioreactor's influent. Very low concentrations ( G O ppm) of inhibitory organic compounds may be insufficient to stimulate production of the specific enzymes needed to degrade these compounds.

Microbes metabolize the organic compounds, producing energy and new cellular material, by way of three mechanisms:

Fermentation Anaerobic respiration Aerobic respiration

Fermentation is the process of metabolism through a series of enzyme reactions that do not involve an electron transport chain. Anaerobic respiration is a metabolic process that involves an electron transport chain; the terminal electron acceptor is nitrate, sulfate, or carbon dioxide. Aerobic respiration is a metabolic process that involves an electron transport chain in which the terminal electron acceptor is oxygen. Respiration is the more efficient mechanism; aerobic respiration is the most popular.

A KieticaUy Limited Process Biodegradation is a kinetically limited biochemical process whose

biokinetic characteristics of inhibitory compounds must be investigated when designing a system. Substrate inhibition occurs when the growth- supporting substrate becomes inhibitory at high concentrations; although the cell growth rate increases with increasing substrate concentration for low concentrations, the growth rate decreases with increasing substrate concentration for higher concentrations. Unlike noninhibitory substrates, when degrading inhibitory compounds, cell growth rates usually achieve a peak value and then decrease.

Contaminated groundwater is usually characterized by multiple organics. There is a great range in metabolic responses of microbes to mixtures of organic compounds when more than one substrate can serve as carbon and energy sources that are still inhibitory. These responses range from diauxic growth to concurrent use of several substrates.

The organics should be the limiting factor to the specific growth rate. In groundwater environments, however, the dissolved oxygen level is identified as the growth rate limiting factor; oxygen's ability to dissolve in water is limited. When dissolved compounds other than oxygen are in the water, the rate of oxygen transfer is usually decreased, as is the overall saturation value. Mechanical aerating devices for oxygen transfer limits the dissolved oxygen level to approximately 10 mgA. Such limitations of oxygen transfer can be avoided through chemical oxidizing agents (e.g., hydrogen peroxide). The concentration of the oxidizing agent needs to be carefully determined-high enough to maximize the microbial growth rate yet not so high as to chemically oxidize the biomass.

There are seven principal environmental parameters that influence microbial growth and metabolism:

RBMBDIATIoN/SPKXNG 1992 I59

PAUL N. CWBIIHWISZNOPP GLBNN GOBSS~~A"

Temperature Pressure PH Salinity Inorganic nutrients Oxygen The absence of high concentrations of toxic or inhibitory compounds

Biotreatment systems must support acceptable levels of these parameters as required by the microbial population for a given substrate. Inorganic nutrients, including nitrogen, phosphorus, sulfur, potassium, magnesium, calcium, iron, and manganese, substantially reduce the time needed for acclimation by microorganisms whose natural transition period would otherwise be several months.

Bloaugtnentation Bioaugmentation is the process of adding nonindigenous microbial

supplements to a bioreactor to artificially increase the microbial population diversity and activity. This, in turn, should enhance the treatment rate or reduce acclimation time. Such supplements often fail to accomplish these desired results, however, because of difficulty metabolizing target compounds.

Some organics are so resistant that microbes cannot directly degrade them. Cometabolism is the process of degrading compounds without metabolizing them. Microorganisms produce enzymes for use by specific substrates, and to initiate the oxidation of a range of nonspecific organic compounds; the microbes do not consume these nonspecific compounds, however, as a source of energy or carbon. An example of this process is the cometabolism of halogenated aliphatics (e.g., trichloroethane (TCA), using methane as the primary substrate. Methanotrophic bacteria cometabolically oxidize the halogenated compounds. Heterotrophic bacteria continue the degradation to stable end products.

Anaerobic biodegradation is slower and used less often but is sometimes the most suitable remediation approach. Some organic compounds require the strong reducing conditions of anaerobic degradation. Anaerobic reductive dehalogenation is viewed as the mechanism in the direct metabolism of halogenated aliphatics (e.g., TCA, TCE, PCE).

Some so resistant that microbes cannot directly degrade them.

Process Design Before actually treating the contaminated groundwater, the source of

contamination must be removed, and the contamination plume and movement of the contaminated groundwater must be controlled. That control usually means pumping. The zone of influence of the drawdown wells must encompass the contaminant plume, which is usually an elongated body of fluid that moves with the prevailing groundwater flow. The desired cone of depression around the well may require high water removal rates.

For aboveground treatment systems, the processes are analogous to

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160 R~MTJ)IATION/SPRING 1992

A biological treatment syatem can be aerobic or anaerobic, although aerobic oystemr seem to be more common.

the treatment of wastewater. In situ methods effectively provide a reaction vessel in the ground. Pumpand-treat aboveground systems pump contaminated groundwater to the surface and feed it into the treatment equipment (e.g., an air stripping tower, a carbon adsorption column, and chemical or biological reactors). The treated effluent is either recharged to the aquifer or discharged at the surface.

A biological treatment system can be aerobic or anaerobic, although aerobic systems seem to be more common. These systems can be designed as suspended growth (e.g., the activated sludge process) or fured film (e.g., trickling filter or RBC).

In suspended growth systems, biomass is suspended in the liquid phase in the reactor. The most common type is an activated sludge process. Although such a process would theoretically produce the highest quality effluent, control is relatively complex. In fured film systems, the biomass is attached to an immobile carrier medium within the reactor. They are particularly suited to degradation of inhibitory contaminants and have a large biomass volume and a high solids retention time, allowing the microbes to become acclimated to the inhibitory compounds in the groundwater while minimizing the hydraulic detention time. This approach is fairly easy to control but is associated with four major disadvantages:

Clogging the media with biomass Greater sensitivity to temperature Greater cast for capital equipment Less operating flexibility than activated sludge process

Introducing External Agents In situ treatment is an emerging technology for treating contaminated

groundwater, relying on the introduction of external agents into the subsurface environment of an aquifer. They are introduced through wells and circulated through the treatment zone by pumping one or more drawdown wells. The subsurface soil must have sufficient porosity to allow for mixing and transport of the external agents, which interact with the contaminants and detoxify them without transfer of the contaminated groundwater to the surface. Complete mixing of treatment reagents with the groundwater is a necessary aspect of in situ treatment; reagents must be delivered to the target compounds at a sufficient rate to prevent decomposition or reaction with nontarget compounds. The treatment zone must be completely contained so that contaminants cannot spread beyond its limits.

Treatment reagents in solution can be applied on the surface by means of flooding ponds or trenches that are partially filled with gravel. Shallow subsurface drains are gravity flow delivery systems that may be suitable for treatment of shallow groundwater contamination .

Gravity flow methods are generally suited to disposal of extracted and treated groundwater. By design, these methods can be used to create a fluid flow loop that flushes contaminants from the soil in the vadose zone into the groundwater for subsequent treatment, an arrangement that can

RBMBDIATIoN/SPRING 1992 161

PAUL N. cHBaBMIsINopp GLENN GOBSS~IA”

The pow rate of groundwater through the treatment zone should be great enough to provide for multiple flushing cycles over the designed treatment period.. .

be used to quickly modify the flow pattern of shallow water table aquifers. The most commonly used delivery scheme is forced injection-a

pressurized well through which the chemical reagents are pumped into the subsurface. For deep or confined groundwater formations, forced injection is the only possible method.

In situ methods must meet two criteria:

They must be operated within the natural environmental conditions

They must permit monitoring of the treatment progress. (e.g., pH, temperature, precipitation).

In situ remediation involves systems with less equipment and lower cost than pump-and-treat systems, but it is not as time-tested as the more conventional technologies. Furthermore, injection of the external agents may be considered the unlawful application of hazardous waste; injection wells may be clogged with biomass or suspended sediments; and subsurface conditions can cause chemical precipitation of external agents.

In Situ Bioti-eatment Of the three types of microbial metabolism, aerobic respiration is the

most applicable for in situ biological treatment, prompting the greatest related research efforts. These aerobes degrade organics faster and more completely than anaerobes. In situ biological treatment manipulates subsurface environmental conditions to nurture the naturally occurring microbes that degrade or destroy organic contaminants. Thorough understanding of the subsurface environment and how it interacts with the distribution and growth of the microbes is needed before effective biological treatment techniques can be developed. This treatment method allows natural selection to control the population of microorganisms, limiting external control to manipulating such parameters as loading rates, nutrient addition, and supplemental microbial cultures.

Microbial activity relies on the intimate contact of microbes, nutrients, and contaminants. The aquifer permeability must be suficient to provide for a relatively short residence time of the chemical supplements, ensuring complete mixing throughout the entire biologically active treatment zone. Because complete mixing is not automatic, it must be considered in the design of the chemical supplement delivery system.

The flow rate of groundwater through the treatment zone should be great enough to provide for multiple flushing cycles over the designed treatment period, perhaps requiring pumping to manipulate the natural flow. Because in situ treatment requires approximately two to six recirculations of water through the treatment zone of the aquifer, groundwater flow must be manipulated to flow through the aquifer As fast as possible.

A passive in situ treatment method involves the application of a solution of active microorganisms and nutrients through surface trenches, where they percolatedown from thesurface into thesubsurface environment.

In situ treatment requires a minimum amount of aboveground equipment to supply nutrients, oxygen, and microorganisms, and is often


COMPARING GROUNDWATER REMEDIATION ~ P ~ O N S

. . . pretreatment by chemical oxidution enhances the ability of microbes to biodegrade refractory organics.

REUFDIATION/SPRING 1992

combined with pump-and-&eat operations. A central drawdown well is installed at the center of the contaminant plume with recharge wells at the periphery of the central well zone of influence. The contaminated groundwater is withdrawn, aerated, and mixed with supplements, and the pH and temperature are adjusted for optimal growth conditions. The microbes consume the organics, increasing the biomass. The nutrient and dissolved oxygen levels are monitored and maintained so that the recharge water has sufficient levels of each to continue the residual cleanup in the biologically active subsurface treatment zone.

SUPPlvtng OjEYg- Biological treatment only partially degrades refractory compounds, but

pretreatment by chemical oxidation enhances the ability of microbes to biodegrade refractory organics.

Oxygen’s solubility in water is approximately 8 to 10 mg/l, depending on the temperature, under critical conditions. Because the water in the aquifer can supply only a limited amount of oxygen, water rich in oxygen must be recirculated quickly through the treatment zone. Oxygen can also be supplied by:

Bubbling and sparging compressed air into wells; Diffusing pure oxygen into wells; or Introducing a more reactive oxidizing agent.

At concentrations in groundwater of 100 ppm or less, the cytotoxic properties of hydrogen peroxide cannot destroy the active microbial cultures.

Although the simplest active method of supplying oxygen and chemical supplements to the treatment zone consists of pumping a low continuous dose, the nutrients and oxygen can create a zone of high activity around the injection well. A single large dose could cause the biomass in the immediate vicinity to deplete the dissolved oxygen, however, upsetting the overall treatment process. Alternatives include multiple point injection and pulsed injection. The pulsed injection process alternately pumps nutrients and oxygen for specific time periods. The reagents gradually mix in the subsurface so that growth of the biomass is less dense around the well. Both methods reduce biofouling, supporting a more uniform distribution of biomass growth throughout the treatment zone.

In Situ Chemical Treatment In situ chemical treatment involves the inuoduction of specific

chemical agents into the subsurface to degrade, immobilize, or increase the mobility of the contaminants. The fluid flow is similar to biological in situ treatment. External agents are introduced through injection wells and circulated through the contamination treatment zone through drawdown pumping. Many of the chemical in situ methods are based on studies and laboratory data; field data is scarce, and practical limitations are imposed

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PAUL N. GLH" GOBSSMA"

Chemical aridation can d e certcrin organicn mom amenable to biodegradation.

164

by incomplete mixing of contaminated groundwater with treatment reagents.

Chemical treatments include precipitation, oxidation-reduction, polymerization, and neutralization. Precipitation is designed to immobilize inorganic heavy metals, many of which can be treated by precipitation. All divalent metal cations are subject to precipitation by sulfide, phosphate, hydroxide, or carbonate. Addition of the reagent (e.g., caustic, lime) raises the pH above 7, at which point precipitation of metals starts. Not all metals precipitate at the same pH, however, and some have an optimum point above which their solubilities increase. Some metals must be in a particular valence state to precipitate, and others require the presence of a tertiary component. The agents are sodium sulfide, calcium carbonate, sodium hydroxide, calcium oxide/hydroxide, or sodium phosphates.

The metal sulfides are the most stable over a wide pH range and generally have lower solubilities than hydroxides or carbonates. Although sulfide treatment has greater removal capability, the difficulties of chemical and sludge handling negate any benefits. Metal phosphate precipitation may also be subject to interference from naturally occumng calcium in the subsurface.

The metal hydroxides and carbonates have an optimum pH range for precipitation which is narrow and specific to the particular metal. Outside of this range, solubility increases. The most commonly used process is hydroxide.

Although removal of heavy metals (through chelating agents) is primarily applied to contaminated soils, the chelating agent increases the mobility of the heavy metal in solution, resulting in a stable metal complex that resists degradation. This treatment is used in combination with aboveground processes for removal of heavy metals.

Oxihtios of w a n & and Inorganic Compounds Oxidation of organic compounds includes a range of organics (e.g.,

aromatics, aldehydes, ketones, halogenated aromatics) that are affected by ozone or hydrogen peroxide. The contaminants are either detoxified or their toxicity is reduced. Chemical oxidation can make certain organics more amenable to biodegradation.

Oxidizers can be used with catalysts (e.g., ferrous ions, or W light for aboveground) to increase the reaction rate. Without catalysts, partial oxidation usually occurs. Three oxidizing agents can be used for in situ treatment of organics: ozone, hydrogen peroxide, or hypochlorites. Hypochlorites have limited application because there is the possibility of the formation of chlorinated hydrocarbon compounds. These chlorinated organics can be more hazardous andor difficult to treat than the original compounds.

Because ozone, which has greater but limited application, is highly reactive and relatively unstable, it must be generated as needed on-site. It rapidly decomposes upon application to groundwater.

Hydrogen peroxide, which seems the best suited for a range of treatment systems, is more stable and easier to handle than ozone. It does

RGMBDIAnoN/sPluNG 1992

COMPARING GROUNDWATER REMEDIATION OPTIONS

Oxidation of inorganics focuses on arsenic, iron, and manganese.

not react to yield chlorinated organic products, and is commercially available in stabilized aqueous solutions of varying concentrations. & the oxidizing agent, it also has the benefit of providing dissolved oxygen for the subsequent biodegradation. One drawback is that it tends to increase the sorptive capacity of soils, possibly adversely affecting contaminant removal.

Oxidation of inorganics focuses on arsenic, iron, and manganese. The oxidizer of choice is potassium pemanganate. The result is the precipitation of metal oxides.

Reduction of organics is applicable to nitro aromatics, chlorinated aromatics, and aliphatics. The agents are catalyzed metal powders, and the result is removal of the nitro or halo side chain or saturation of the aromatic structure, detoxifying the compound for subsequent biological treatment.

Reduction of inorganics is limited to hexavalent chromium and selenium. The reducing agent is ferrous sulfate, which acts to reduce these metals to the trivalent species. Neutralization is a straightforward chemical reaction, balancing the pH to 7, a step that is often integrated into other chemical treatment operations.

Polymerization focuses on a few specific organic compounds. Con- tamination by monomers of vinyl chloride, isoprene, or acrylonitrile can be converted to the corresponding polymers by applying a suitable catalyst agent, immobilizing the contaminants into a gel-like mass. Widespread application of polymerization is questionable, because many injection wells are required to introduce the catalyst, assuring sufficient mixing between the monomer and the catalyst is difficult, and the catalyst can be considered a hazardous substance whose application to the groundwater is restricted.

Hydrolysis focuses on five specific types of organics:

Esters Amides Carbamates

Certain pesticides ’ Organophosphorus esters

The reaction is base-catalyzed and the reagent is either caustic soda or lime. This process is purely theoretical; there is no field data.

Although data on chemical in situ systems is limited, there is one proven process: the Vyredox method, which is designed to remove iron and manganese by injecting highly aerated water in a zone around the supply well. Iron is precipitated first, and the solids are retained in the strata farthest from the supply well. Closer to the well, the conditions are maintained as favorable for bacteria that preferentially oxidize (i.e., precipitate) manganese. The water that enters the well is free of iron and manganese.

SUMMARY Remediating contaminated groundwater begins with a preliminary

assessment. The site’s characteristics must then be investigated; field

REhlBDIATION/SPRXNG 1992 165

PAUL N. cwaraMLSpr0~~ GLBNN GOBSSM~N

Pump-and-hat ryttenm that use phynical treatment meflux& am &It luedbymoet nmedicrtion pr&?Ct8..

samples are collected if necessary. Specific treatment alternatives are developed that involve suitable technologies that are site- and waste- specific. Because contaminated soil frequently accompanies groundwater contamination, an integrated strategy must be developed for concurrent remediation. Pilot plant studies are often conducted. The final treatment process design must also account for project cost and time limitations. A pumpand-treat air stripper with carbon adsorption may be the quickest treatment, but the cost may be excessive. Biological in situ treatment may be an effective technology, but the time required may be excessive.

hunpand-treat systems that use physical treatment methods are still used by most remediation projects, including those used to contain contaminants at most Superfund sites where hopes for complete remediation have been abandoned.

Inorganics are generally treated aboveground by conventional physical and chemid methods. Heavy metals, in particular, are effectively removed by chemical precipitation. Volatile and semivolatile organics are generally treated by air stripping or carbon adsorption. Biological methods are Winning adherents for treatment of organics.

Although biological insitu treatment has been successfully demonstrated, field data is limited. Chemical in situ treatment, which has received much less research attention, is still largely experimental. In situ techniques are frequently combined with aboveground systems as part of an overall treatment train.

Most remediation projects that use biological methods use aerobic systems. In one case, groundwater was pumped to the surface and fed along with nutrients into an aerated submerged growth bioreactor; the clarified effluent was discharged to percolation trenches.

Anaerobic systems are being pilot-tested. In the subsurface groundwater environment contaminated by toxic compounds, a unique indigenous microorganism population will develop, resulting from the biochemical adaptation to the characteristics of the inhibitory compounds. In one case of contamination by TCA, the contaminated groundwater was pumped to the surface, chemical supplements added, and fed into an anaerobic bioreactor. The effluent, carrying an enhanced population of anaerobes, was recharged into the subsurface; some of the effluent was applied at the surface as a barrier to oxygen, thus protecting the subsurface anaerobic zone. Biological treatment reduces the amount of organic contamination to below the desired limit.

CONCLUSION A variety of remediation technologies is available for treatment of

contaminated groundwater. Bioremediation is emerging as a cost-effective alternative to treat contaminated groundwater while maintaining relatively low capital costs. In addition, it minimizes site disruption and reduces or eliminates the costs associated with transportation, handling, and disposal of the recovered contaminants. Among the biological methods, a trade-off exists. The in situ approach is generally less costly but does not offer stringent biological process control. When subsurface conditions do not

166 REMBDIATION/SPRING 1992

COMPARING G R O U N D W A ~ REMEDIATION OPTIONS

~ ~ _ _ ~ ~ ~~

Table 2. Summary of Treatment Technologies.

T E C € i N O ~ Y APPLICATION PRO<=ESS DESQupnoN

Pbysfcal Stripping VOCs, ammonia

Adsorption High M.W. organics

Ion exchange Ionic compounds (typically inorganics)

Reverse osmosis Metals, heavy metals

Cbemical Precipitation Metals, heavy metals

Oxidatiodreduction Inorganics

Riological

Organics

Organics

Pump & treat system removes compounds from water into air.

Pump & treat system captures compounds on activated carbon.

Pump 8t treat system captures charged ions on specific resins.

Pump & treat system filters water to produce dean water and a concentrated waste stream.

Reagents react with contaminants forming insoluble compounds which fall out of solution. Applicable in situ but has been used largely above ground.

Reagents react with compounds changing their oxidation state. Resulting compounds are removed by subsequent means. Selectively applicable in situ. Generally used above ground.

Reagents convert the organics to less toxic compounds. Typically used as a pretreat to biological treatment. Applicable in situ and above ground.

Naturally occuring microbes are stimulated to biochemically degrade the organic compounds. Applicable in situ and above ground.

provide a conducive environment for the indigenous microbes, treatment in place may not be efficient or even possible. The pump and treat systems, although capital intensive, are able to be directly controlled. (See Table 2)

As environmental awareness increases and governmental regulations become more strict, alternatives to bioremediation will be limited. Many physical and chemical methods do not destroy the hazardous compounds, concentrating and relocating them instead. In many cases, the biological methods can really convert these hazardous compounds into safe and acceptable products. Because the concentrations of toxics that can be degraded are relatively weak, however, bioremediation is slow. BE3

RECOMMENDED READING

HYdtoseology Brassington, R. FkkdHydrogeology. New York: John Wiley & Sons, Inc., Halsted Press, 1988.

REMEDIATION/SPIUNG 1992 167

PAUL N. C ~ ~ ~ a ~ ~ s p r o p p GLENN GoBSSMA"

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Taylor, S.W. and P. Jaffe. 'Enhanced In-Situ Biodegradation and Permeability Reduction.' A S C E J d o/Envirrmmental Engineering, vol. 117 @n./Feb. 1991):25.

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Graves, Wton, McKinley, &Luke. 'Study ofthe Potential for Groundwater Contamination by Alcohol Blended Gasolines.' h e e d i n g s of the 1986 Spcia@ Cmfmce on Em'- rontr#nkd Engineering. N e w York: ASCE (19s6>:147.

Nyer, E.K. Gnncndwlter Treatment Tec&nology. New York: Van Nostrand Reinhdd, 1985.

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System procesS Design

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Chung and Dyksen. 'Design Considerations and Experience with VOC Treatment Technologies.' Fmadings of the l ~ S p e & L t y C o n f m c e on Environmental Engineer- ing. New York: ASCE (1986):72.

Hale and Nyer. "Removal of Phenol from a Brine Aquifer.' Rvcaedings ofthe4lsr Indurhial Waste Cmfmce, Purdue University, West Lafayette, IN. Chelsea, MI: Lewis Publishers (198n415.

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Skladany, Thomas, Fisher, and Ramachandran. T h e Design, Economics, and Operation of a Biological Treatment System for Ketone Contaminated Ground and Solvent Recovery Pr?xess Waters.' Ruceedings cftbe42nd Industrial War& Conference, Purdue University, West Lafayette, IN. Chelsea, MI: Lewis Publishers (1988):53.

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Chemical In Situ Treatment

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Cowan, R.M., K. Shanahan, and A. Weber. T h e Effect of Interspecies Competition on Bacterial Supplementation Effiacy." plocaedings of tbe 45tb fndushial Waste Cmfice, Purdue University, West kfayette, IN. Chelse?, MI: Lewis Publishers (195m261.

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Semprini, Roberts, Hopkins, and McCarty. 'Field Evaluation of Aquifer Restoration by Enhanced Biotransformation.' International Conference on Physiochemical and Biological Detoxification of Hazardous Wastes (1988):955.

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vol. 60, no. 10 (Oct. 1988)1&13.

RBMEDI~ON/SPRXNG 1992 169

comparing groundwater remediation options

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