-

Met .O ' r I .

ids of

New techniques are b e i q wed along with traditional

methods for cleaning up hazardous waste silex

by Yogesh E. Patel, REM, Mahabel K. Shah, REM and Paul N. Cheremisinoff, REM, Engineering Editor

58 PI DLLV I ENGINEERING I

Aboveground bioremediation of soils separated into cells can reduce high concentrations of contaminants.

environment. In accordance with this legislation, EPA has established a proc- ess for discovering releases, evaluating remedies, determining the appropriate extent of response, and ensuring that remedies selected are cost-effective.

The remedial investigation/feasibil- i ty study (RI/FS) process is outlined in section F of the revised NCP.

For every f q p t p d 92: remcdja! response action under Section 104 of CERCLA, the NCP requires a detailed RI/FS. The remedial investigation em- phasizes data collection and site chardc- terization. Its purpose is to define the nature and extent of contamination at a site to the extent necessary to evalu- ate, select and design a cost-effective remedial action.

The feasibility study emphasizes data analysis and decision making and uses the data from the KI to develop objec- tives and alternative rcmedial re- sponses. The alternatives are then

evaluated in terms of their engineering feasibility, public health protection, en- vironmental impact and costs.

The remedial investigation and feasi- bility study are interdependent proc- esses and are generally performed con- currently rather than sequentially.

Superfund cleanup technologies Some emerging technologies that

have been investigated by EPA include: . LiltruJiltrurion Tn combination with water soluble, macromolecular com- pounds to selectively remove metal ioiis hm ayueaus wasre suiutions. The application OJ'elrctric and nc'ous- tic fields to treat soil contaminated with fuel oils, hazardous organic com- pounds and heavy metals. Sorption proc"ss based on the affin- ity of algae cell walls for heavy metal ions. Immobilized algae cells in a sil- ica gel polymer re used 3s i ~ i i ex- change resins. Yllorochcvnicd o..\-idolion o f aro in at i cs to non-toxic chemical species. A laser beam is used to contact and oxidize waste particulates filtered and washed from groundwater.

i

Conmined rt"w-y of oily wusirs us-

ods with in situ biodegradation to re-

i

\ ing conventional oil recovery meth-

mediate hazardous oily accumula- tions. I

hitially the presumption was that f

sites could be cleaned up with conven- tional technologies. The high cost esti- mates for such cleanups along with pub- iic objections resulted in a system for developing new cleanup methods, the Superfund Innovative Technology Evaluation (SITE) program.

1

Soli was;Piiigjextraction Soil washing is any technique thai re-

moves gasoline or other constituents from the soil matrix by actively leach- ing the contaminants from the soil into a dissolving medium. The extracted constituents can then be removed from the washing fluid by further treatment methods. Soil washing is accomplished either in situ as a water flushing system or processed through a countercurrent extractor system.

The slurry of soil and washing fluid can be dewatered by conventional tech- niques such as sedimentation, f;!!ra-

tion, evaporation, dissolved air flota- tion or drying beds. The treated soils then can be put back into the original excavation or sent to a sanitary landfill.

This technology has been developed extensively in the mining and oil recov- ery industries to both remove and con- centrate gasoline contaminants. The leachate collected from the extraction process can be treated conventionally and recycled in a closed system.

Contaminated solvents are separated by physical separation techniques such as distillation, evaporation or centrifu- gation. Treated effluent can be re- injected into the ground. This method requires site controls, such as above-or below-ground bamers.

Limitations of soil washing or flush- ing are associated with soil characteris- tics that impede the soil-liquid separa- tion after the washing phase. This may result from a high percentage of silt or clay in the soil. In situ soil flushing can result in decreased permeability with the use of surfactants or other additives.

Whether in situ or excavation sys- tems are used, laboratory and pilot test- ing is necessary to determine feasibility. Contaminant removal rates may not be adequate to reduce soil contamination below required action levels.

Stabilization and solidification Stabilization and solidification treat-

ment processes transform liquids or semi-solids into environmentally safer forms by immobilizing contaminants.

Stabilization limits the solubility or mobility of the contaminants with or without change or improvement in the physical characteristics of the waste. Stabilization usually involves adding materials that ensure the hazardous constituents are aiaintained in their least mobile or toxic form.

Solidification implies the beneficial results of treatment are obtained through the production of a solid block of i i M & rriaieriai with hi@ srmcrurai integrity, a product often referred to as a monolith. The contaminants do not necessarily interact chemically with re- agents, but are mechanically locked within the solidified matrix, which is called microencapsulation.

Contaminant loss is limited largely hy decresir?g ?he scrfgce 2x3 expsed to the environment and/or isolating the contaminants from environmental in- fluences. Wastes also can be macro- encapsulated, bonded to or surrounded by an impervious covering. For major stabilization and solidification Fioc-

esses commonly used for hazardous wastes, waste compatibility plays a key role in selecting a particular treatment process.

Waste stabilization and solidification systems include lime fly ash pozzolan processes, Portland cement systems, thermoplastic microencapsulation and macroencapsulation.

Lime fly ash pozzolanic processes use a finely divided, non-crystalline silica in fly ash and the calcium in lime to produce low strength cementation. Wastes are contained by entrapment in the pozzolan concrete matrix (microen- capsulation).

Portland cement systems use Port- land cement and possibly other pozzol- anic materials to produce a stronger waste-concrete composite. Waste is contained by microencapsulation in the concrete matrix. Soluble silicates can be added to accelerate hardening and contaminant containment.

Thermoplastic microencapsulation involves blending fine particulate waste with mc!ted aspha!t or ather plastic! polymer matrix. Liquid and volatile phases associated with the wastes are driven off, and the wastes are isolated in a mass of cooled, hardened asphalt. The material can be buried with or without a container.

Thermoplastic microencapsulation has been successfully used in nuclear waste disposal and can be adapted to special industrial wastes. The technique for isolating the waste involves drying and dispersing it through a heated, plas-

tic matrix. The mixture is then permit- ted to cool to form a rigid but deform- able solid. In most cases it is necessary to use a container such as a fiber or metal drum to give the material a con- venient shape for transport.

The most common materials, such as polyethylene, polypropylene, wax or elemental sulfur can be used for specific wastes where complete containment is important and cost is not a limiting fac- tor. The major advantage that thermo- plastic (asphalt) encapsulation offers is the ability to solidify very soluble, toxic materials.

Macroencapsulation systems contain potential pollutants by bonding an inert coating or jacket around a mass of ce- mented waste or by sealing them in polyethylene-lined drums or containers. This type of waste stabilization is often effective when others are not because the jacket or coating of the outside of the waste block completely isolates the waste from its surroundings. The waste can be stabilized, microencapsulated, iifidh: solidified k fo ic “icapsii- lation so the external jacket becomes a barrier designed to overcome the short- comings of available treatment systems.

A macroencapsulation system pro- posed for use with hazardous wastes in- volves drying the wastes and bonding the dried material into a compressed block using polybutadiene. The block is placed in a mold and surrounded with powdered polyethylene. The p l y - ethylene is then fused into a solid jacket using heat and pressure.

A well monitoring system measures the deprh to liquid phase petroleum products and water in monitoring wells.

NOVEMBER 1990 POI.I.UTION ENGINEERING 61

Soil-bentonite slurry walls act as barriers to lateral flow of groundwater and water-borne pollutants.

Deep soil mixing An effective soil improvement tech-

nique, deep soil mixing can be used to increase the strength, reduce permeabil- ity, remediate the soil and improve other soil characteristics without exca- vation or soil removal. The system makes use of a crane-supported set of leads that guides a series of hydrauli- cally driven mixing paddles and augers. As the ground is penetrated, stabilizing agents or other fluids are fed through the center of each shaft. The auger flights break the soil loose and lift i t to the mixing paddles, which blend the stabilizing agents with the soil.

As the augers advance to a greater depth, the soil and agent are remixed by the additional mixing paddles on each shaft. When the desired depth is reached, the augers are withdrawn. Left behind is a stabilized continuous wall or a treated block of soil.

This system's overlapping augers also enable installation of a stabilized, con- tinuous vertical barrier underground to depths of up to 120 feet when it is nec- essary to prevent groundwater migra- tion from contaminated sites.

Stabilizatiodsolidification. Waste neutralization. Steam stripping of volatile organics. Oxygen enrichment of soil to er?hance bacterial activity or biological degra-

dation.

Functions of this system are:

Shallow soil mixing In situ solidification and stabilization

of hazardous waste sludges and soils re-

quire a more complex approach as the concentrations of volatile organics or the need for thorough mixing increases. Conventional methods of in situ mixing of soils and sludges have either pro- duced non-uniform mixtures of waste and treatment chemicals or emission of volatile organics during treatment, causing air pollution and job shutdown problems.

Shallow soil mixing is a system used to uniformly mix hazardous sludges/ soiis with treatment chemicals and cap- ture emanating vapors and ducts that may be produced.

Soil bentonite slurry walls Soil-bentonite slurry walls are subsur-

face non-structural walls that act as bar- riers to the lateral flow of groundwater and water borne pollutants. Soil-ben- tonite cutoff walls are constructed using the slurry trench technique and are composed primarily of soil and ben- tonite, a natural clay mineral. The prin- cipal advantage of slurry bentonite cut-

the wall and its general suitability for both new and remedial applications.

Slurry walls are now cost competitive on projects where compacted clay cut- offs, leachate collectors, sheeting, or well points would previously have been used. Tjrpicai applications include:

Seal dams and dikes. Contain sanitary and hazardous waste

Dewater structyral excavations. Hydraulically isolate lagoons and

off wzlls are the !ow pmneabilitj; of

landfills.

holding ponds.

Mobile treatment systems gel equipment to a contaminated site quick&. 62 POLLUTiON ENGINEERING NOVEMBER I990

Enclose oil and chemical tank forms. Intercept seepage from slopes. Contain oil spills. Slurry walls are particularly well

suited for remedial applications. Usu- ally slurry walls can be constructed without disturbing the function or op eration of existing facilities.

Thermal desorption Contaminated soil is excavated and

fed to a device that applies sufficient heat to volatilize and drive off the con- tamination. The contaminants stripped or driven off the heated soils are subse- quently either burned in a secondary combustion chamber, or separated from the gas stream by condensation, quenching or adsorption. At present, low-temperature. 200°F to 800°F ther- mal desorption technology is commer- cially available and has been success- fully used to remove VOCs, semi- volatiles, PCBs and arsenic from soil.

Soil feeding and handling capabilities vary among vendors, but typically, crushing and shredding of soil lumps and screening of large obstacles such as scrap metal and rocks is first per- formed. Since this step can result in the release of volatile contaminants and particulates to the atmosphere, provi- sions for containment and control of emissions within the excavation and soil handling areas are usually needed.

Incineration Incineration involves excavating the

soil and then transporting it to the in- cinerator. Soil preparation operations include crushing, grinding, screening and drying. Both on-site and off-site in- cineration can be used. An on-site in- cinerator can be installed on a perma- nent basis; however, mobile units are typically used due to the short-term na- ture of most remediation projects.

Incinerators most commonly used for soils are rotary kilns and ciw.c2u!lting beds. Rotary-kiln incinerators are cylin- drical, refractory-lined vessels mounted with the axis at a slight incline. Operat- ing temperatures typically range from 1600°F to 3 W F , with residence times ranging from seconds for gases to hours for solids.

While most solids, liquids and gase- ous organic wastes can be treated with this technology, wastes with a high heat- ing value are well suited. The efficiency of treating wastes with a high inorganic salt or metal content is limited.

Incinerators require ash handling sys- tems to remove the deconramixated

soils and tluc gas scruhhcrs to rcniovc acid gases. Wastewater rrom on-site in- cinerators usually has a low flow ratc and can be treated along with wastcwa- tcr from other site activities. such as groundwater remediation.

Metals in the soil will most likely be oxidized and possibly volatilized by the high incineration temperature. Oxi- dized metals are often leachable. and even if organics have been removed, soil can remain hazardous after incin- eration when metals are present.

Water treatment In general, remediation projects deal

with contaminated water either as groundwater or as water impoundment, such as lagoons. In either case, mobile on-site methods are available. Typically they are based on physical and chemical processes commonly used for wastewa- ter treatment.

Several dif'ferent chemical precipita- tion processes can remove metals and other dissolved inorganic compounds from aqueous solution. Inorganic ani- ons, including phosphates and arse- nates. can be precipitated by coagula- tion and flocculation treatment meth- ods. Alumination and iron sulphate salts are typically used as coagulants, and are effective over a pH range of 5 to 8. The rate of precipi!ation can be increased significantly by adding an or- ganic polymer flocculant.

Most dissolved metals can be con- verted to corresponding insoluble salts by direct pH adjustment with lime (cal- cium hydroxide), sodium hydroxide, sodium sulphide or a combination of these. Hydroxide precipitation is inex- pensive, reliable and easy to control. Most metal hydroxides have limited solubilities at pH between 9 and I I .

The biggest disadvantages are that large quantities of sludge are generated and many metal hydroxides will resolu- bilize at neutral or slightly acidic pH

Sodium sulfide, in combination with sodium hydroxide, has been demon- strated effective for removing metals and is one of the most commonly used precipitation agents. By using sodium sulfide, metal sulfides will precipitate at lower pH ranges and are much more stable than corresponding meta! L- " J

droxides. The sodium hydroxide serves to maintain slightly alkaline conditions to prevent the formation of hydrogen sulfide. Many other chemicals, tnclud- ing polymers, are effective precipitating agents for metals.

-,.-A:.: --" L U I I U I L I U I I J .

Monitoring is integral lo remediation of contaminated soils and groundwater.

Aeration or air stripping In the aeration or air stripping proc-

ess, volatile compounds that are dis- solved in aqueous solution are trans- ferred to the vapor phase, that is, air. At any given temperature, a compound in solution exerts an equilibrium vapor pressure proportional to its concentra- tion in the aqueous phase.

A proportionality constant specific for each compound, the Henry's Law Constant (HLC), is a relative indicator of a compound's combined volatility and soluSili!y. and its ability to be stripped out of an aqueous solution. Compounds with relatively high HLCs, greater than 0.003, are amenable to air stripping, while those with HLCs less than 0.003 are dificult to remove.

To be effective, air stripping requires the air !X injecizd aiid iwii dkpetsed through the water. Regardless of the aeration method, overall performance depends o n several factors, including the nature and quality o f contaminants present and the constant time and area provided for mass transfer between the

air and water. Removal efficiencies can be significantly increased by preheating the air stream or otherwise elevating the system temperature.

A packed tower is the most common and efficient aeration method presently used. Water is sprayed into the top of the air stripping tower, which is par- tially filled with inert packing. The water flows down through the packing and exits out of the tower bottom. Air IS blown up through the tower. counter- current to the water flow. The packing provides a large surface area for suiface contact between the air and water.

The air stream leaving the tower may be further treated to remove or destroy contaminants collected from the aque- ous phase. This is typically done by in- stalling a vapor phase carbon adsorp- lion unir or an incinerator on the ex- haust air outlet.

The most frequently encountered problem with the air stripping tower is scaling. Scaling is caused by buildup of insoluble metal salts (particularly iron and mangarnese), suspended wlrds, or

j

POLL UTlGN ENGINE E f3 I “2

Some dissolved metals und inorganic species show potential for being adsorbed b-v carbon.

A portable vapor oxidation system is used for remediation projects where high concentrations of vapor-phase hydrocarbons are present.

biological growth on the packing media. Scaling adversely impacts phase trans- fer, pressure drop, flow rate and mass transfer rate.

Air stripping equipment costs are moderate compared with other tech- nologies. Operating costs are associated with power requirements, off-gas treat- ment and packing replacement.

Packed tower aeration is a well devel- oped, proven technology for removing volatile components from an aqueous stream. It is less effective when applied to semi-volatiles and does not remove metals. Development efforts are now focused on improving efficiencies for semi-volatile organics.

Steam stripping Steam stripping is similar to air strip-

ping except that steam, instead of air, is used to remove contaminants. The advantage of using steam is that Com- pounds are more volatile at higher tem-

out of the aqueous phase. A rule of thumb is that volatiles with

boiling points less than 150°C are good candidates for steam stripping. In the steam stripping operation, steam leav- ing the stripping tower enters a con- denser and condensate collection tank. The condensate volume is much less than the original Wastewater volume, and contaminants are much more con- centrated. in some cases, contaminants can be separated and recovered from the condensate; in others, the conden- sate may require fiurther on-site or OK-

FTE!ST~S, 2nd GGic c~~sil? siiippxd

site treatment or disposal. This technology oflers certair? advan-

tages over air stripping if any of the following conditions apply:

Steam is readily available at the site. The contaminants can be recovered. Contaminants are not easily removed by air stripping, but have boiling points below 150°C. On-site steam generation may be- eco-

nomically justified if other unit opera- tions also use steam, such as an acti- vated carbon system using steam for on- site carbon regeneration.

Carbon adsarptian Carbon adsorption is a physical proc-

ess in which a contaminant is trans- ferred from an aqueous or vapor phase to the surface of the solid carbon, where it accumulates for subsequent extrac- tion or destruction.

The phase transfer occurs primarily as a result of a contaminant’s low affm- ity for the iiquii! or vapor phase or its high afinity for the solid-phas, me- dium, or both.

Activated carbon, usually produced by crushing and heat activating selected grades of bituminous coal, is the most common adsorbent used in water and wastewater treatment. The internal

vides a large surface area for adsorption of different organic compounds.

The cost and effectiveness of adsorp- tion depends on many factors, includ- ing the molecular weight and structure, solubility and polarity d i k e compound

pCTe St!7lC!Wt3 Of aCtiVStCd Carhi3 Pro-

being removed, as well as the pH and temperature of any aqueous phase. I n general, a compound’s adsorbability is favored by increasing molecular size and aromaticity, and decreasing solu- bility, polarity and carbon chain branching.

Some dissolved metals and inorganic species, including arsenic, mercury, sil- ver, chromium and chlorine, also have shown good potential for being ad- sorbed by carbon. Although carbon ad- sorption rates for many individual spe- cies in aqueous and vapor phases can be estimated from existing experimen- tal data or empirical equations, column studies are required to evaluate an over- all adsorption rate if several species are present and competing for the activated carbon’s adsorption sites. Once an over- all adsorption rate is measured, equip- ment can be sized and designed.

Activated carbon is supplied either in granular or powdered form. Granular activated carbon (GAC) is more com- monly used for wastewater treatment. A ?ypia! adsorption operation consists of two or more fixed beds in series or parallel, partially filled with activated carbon. Water to be treated enters the top of the units, flows down through the carbon and exits at the bottom. A vapor phase carbon system is similarly constructed, but the contaminated gas enters the bottom of the units and exits at the top.

Because carbon capacities decrease above certain temperatures, steam is commonly used to regenerate an acti- vated carbon bed. Carbon can be regen- erated in-place with steam, or shipped off-site for regeneration by other meth- ods such as thermal regeneration.

Whether the carbon is replaced or re- generated, further treatment or proper disposal of the spent carbon is required at a considerable cost. For this reason, GAC is preferable where contamination levels are relatively low, and is most efkctive as a secondary or poiishing treatment system.

Landfilling Even though about 25 percent of to-

day’s Superfund sites are old landfills, landfilling still remains an important part of waste management projects. T’nere are many wastes that have been treated, fixed or stabilized, or that can- not otherwise be further treated, that will require permanent burial in an en- vironmentally acceptable landfill.

For such wastes, it is important to select a final disposal site that meets

64 POLUJTION ENGINEERING NOVEMBER I990

POLLUTION ENGINEERING

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Siting, designing and permitting a landjll is a lenglhy and costly process.

applicable disposal standards for the specific waste materials. Since most state-of-the-art landfills for hazardous waste are in remote locations, and since it is imperative wastes be delivered to the facility in a proper manner, it is equally important that responsible and reputable waste haulers are employed to transport the wastes.

Many siting, environmental and eco- nomic factors enter into the decision to construct such a landfill. The pri- mary advantages of an on-site landfill are related to the ability to control costs, maintain possession of the wastes and avoid liabilities incurred through mixing with wastes belonging to others.

However, the siting, design and per- mitting process is lengthy, costly and requires a persistent effort in order to address local concerns. There also is the mandatory process for obtaining a RCRA Part B landfill permit. Given the proper circumstances, such an op tion is often attractive.

Mobile treatment systems Mobile treatment systems consist of

modular equipment that can be used on a number of different sites over the life of the equipment. Mobile technob g k combine process technology with mobility. The primary objective of mo- bile technology, while treating the waste on-site, is phase separation of solids from liquids and, if possible, reduction of the toxic characteristics of both.

Mobile systems show considerable promise for remedial activities at Su- perfund sites. The technologies can pro- vide a permanent solution with many advantages over alternatives involving off-site transport and disposal.

Factors affecting mobile treatment systems include:

Generally higher costs and longer pe- riods for development and operation. Development nature of some tech- nologies.

Limitations of capacity, materials handling or process characteristics that prevent the mobile concept from being a total solution. Waste characteristics. Site constraints. Potential environmental impacts. Technology support reguire”ts.

h! i i ~ ~ t i t i i t i ~ i ~ ! Gani;ei 11s.

Bioremedia tion Bioremediation uses naturally occur-

ring microorganisms to decompose toxic substances. This process has proved an effective remedy for hazard-

ous waste site cleanup in certain situ- ations. Bioremediation works because many organic compounds found in haz- ardous waste mixtures can be used as food by microorganisms. These micro- organisms break complex organisms into simpler compounds, namely car- bon dioxide and water.

Microbial activity can be classified into three main categories:

Aerobic respiration (oxygen accep- tor). Anaerobic respiration (acceptor through sulfate and nitrate).

Microorganisms, like all

living organisms, require

specific inorganic

nutrients to survive.

66 POLLUTION ENGINEERING NOVEMBER I990

* Fermentation (election acceptor through organic compounds). Environmental factors that affect mi-

crobial activity and population size will determine the rate and extent of biode- gradation. These factors include:

Appropriate levels of organic and in- organic trace elements. Oxygen concentration. Redox potential. pH.

* Degree of water saturation. Hydraulic conductivity of the soil. Osmotic potential. Temperature. Competition, including the presence of toxins and growth.

* Predators. Typeiconcentration of contaminants. Hydrogeology will affect not only mi-

crobial activity, but also the feasibility of in situ treatment.

Microorganisms, like all living organ- isms, require specific inorganic nutri- ents S U C ~ a~ fiitiogea, phcjsphoiris ilid trace metals and a carbon and energy source to survive. Many organic con- taminants provide the carbon and en- ergy and thus serve as primary sub- strates if the organic compound that is the taige: of the bioreclrrmation is only

degraded metabolically. Aerobes need oxygen, nitrate respires

need nitrate, and sulfate respires need sulfate. Various anaerobic populations require specific reducing conditions. Optimum microbial activity for biore- clamation purposes occurs within a pH range of 6 to 8, with slightly alkaline conditions being more favorable.

The optimal temperature for organ- ism growth in aerobic biological waste- water treatment processes ranges from 20°C to 37°C. Although microbial populations in colder waters are adapted to lower temperatures, biode- gradation rates can be expected to be much slower than at higher tempera- tures. Additionally, concentrations of inorganic and/or organic contaminants could be high enough to be toxic to the microbial populations.

It is feasible to manipulate some of these factors in situ to optimize envi- ronmental conditions. Nutrients and oxygen can be added to the subsurface. It may be feasible in some cases to en- ham:: i&aiCiiig c ~ ~ d i i i ~ i ~ ~ , thereby low- ering redox potential. The pH can be adjusted with the addition of dilute ac- ids or bases. Water could be pumped into an arid zone.

There are some factors that cannot be corrected, such as the presence of predators, competition between the mi- crobial populations, or the salinity of groundwater. Even if substantial micro- bial activity is present, the wasies are biodegradable only if the hydrogeology of the site is favorable. The hydraulic conductivity must be sufficient enough and the residence time short enough so added substances, such as oxygen and nutrients, are not depleted. Sandy and other highly permeable sites will be far easier to treat than clay soils.

Yogesh B. Patel, REM, is a senior stafl engineer with Test Well Craig Inc. in Fairfield, N.J. Mahabal K. Shah, REM. is a projeci engineer for DSI-USA Inc. in Lincoln Park, N.J. Paul N. Cheremis- inofl REM, is a professor of civil and environmental engineering at the New Jersey Institute of Technology in New ark, N.J. He also serves as POLLU- TION ENGINEERING S engineering editor.