1990 Thermal Remediation Industry Contractor Survey

The treatment of soil contaminated with organics and inorganics is becoming a major industry in the United States and Europe. The soil cleanup bill for the United States could run as high as $200 to $300 billion over the next 30 to 40 years. European soil cleanup costs could run as high as $130 billion.¹

The types of sites in the United States that will require soil treatment can be broken down into the following categories: ? CERCLA (Superfund) Actions

? RCRA Corrective Actions

? RCRA Closures

? Underground Storage Tanks

? Real Estate Transfers

? Spill Clean-ups.

The cleanup of sites in each of these categories, with the exception of the Real Estate Transfer category, is being driven by different sets of Federal regulations. Real Estate Transfer type regulations were first instituted in New Jersey and have now been promulgated in a number of other states.

The eventual cleanup cost for the Superfund sites will be close to $200 billion. Estimated costs for the industrial sector Superfund are $25 to $50 billion and the estimated cost for the Department of Energy sites is over $150 billion.2 An early RCRA Corrective Action cleanup estimate is $25 billion.3 This estimate may well be low, however, since the permitting, cleanup and delisting criteria are still not clearly defined. The EPA’s RCRA Corrective Action cost estimate is $7.4 billion. However, the Office of Management and Budget feels that this estimate is low.⁴

The potential magnitude of the cleanup costs has resulted in the development and implementation of many technologies for the decontamination of soils. Of the available remedial technologies, thermal treatment has perhaps had the most field testing. The purpose of this paper is to focus on the full scale site remediations which have been or are being conducted using thermal processing equipment. Projects which have been completed, are on-going, or have been contracted for, through January of 1990 are described.     

The Use of Electrokinetics for Hazardous Waste Site Remediation

The Superfund Innovative Technology Evaluation (SITE) program was authorized as part of the 1986 amendments to the Superfund legislation. It represents a joint effort between U.S. EPA’s Office of Research and Development and Office of Solid Waste and Emergency Response. The program is designed to assist and encourage the development of waste treatment technologies that would contribute to more solutions to our hazardous waste problems.

Recently, EPA, through the SITE program, issued a work assignment to assess the “stateof- the-art” of electroklnetically enhanced contaminant removal from soils. Prior research efforts, both laboratory and field, have demonstrated that electroosmosis has the potential to be effective In facilitating the removal of certain types of hazardous wastes from soils. Particularly encouraging results have been achieved with inorganics in fine-grained soils where more traditional removal alternatives are less effective.

Although the results of various studies suggest that electrokinetics is a promising technology, further testing Is needed at both the laboratory and field levels to fully develop this technology for site remediation. A conceptual test program Is presented based on best available data which incorporates system design and operating parameters used in previous applications of this technology In the use of electrokinetics treatment as a remediation technique at hazardous waste sites.     

SITE demonstration of the CF systems organic extraction process

The CF Systems Organic Extraction Process was used to remove PCBs from contaminated sediment dredged from the New Bedford Harbor. This work was done as part of a field demonstration under EPA's Superfund Innovative Technology Evaluation (SITE) program. The purpose of the SITE program is to provide an independent and objective evaluation of innovative waste remediation processes. The purpose of this paper is to present the results of the SITE demonstration of this technology. Results of the demonstration tests show that the system, which uses liquefied propane, successfully removed PCBs from contaminated sediments in New Bedford Harbor. Removal efficiencies for all test runs exceeded 70 percent. Some operational problems occurred during the demonstration which may have affected the efficiency with which PCBs were removed from the dredged sediment. Large amounts of residues were generated from this demonstration project. Costs for using this process are estimated to be between $150/ton and $450/ton.     

Pilot-Scale Evaluation of an Incinerability Ranking System for Hazardous Organic Compounds

The U.S. Environmental Protection Agency’s (EPA) hazardous waste incinerator performance standards specify a minimum destruction and removal efficiency (DRE) for principal organic hazardous constituents (POHCs) designated in the incinerator waste feed. In the past, selection of appropriate POHCs for incinerator trial burns has been based largely on their heats of combustion. Attempting to improve upon this approach, the University of Dayton Research Institute (UDRI), under contract to the EPA Risk Reduction Engineering Laboratory, has developed a thermal stability-based ranking of compound "incinerability". The subject study was conducted to evaluate the laboratory-developed ranking system in a pilot-scale incinerator.

Mixtures of POHCs, spanning the ranking scale from most- to least-difficult to destroy (Class 1 to Class 7, respectively), were prepared and combined with a clay-based sorbent matrix. These mixtures were then fed into the rotary kiln incineration system at the U.S. EPA Incineration Research Facility (IRF). In a series of five tests, the following conditions were evaluated: baseline/ typical operation; thermal failure (quenching); mixing failure (overcharging); matrix failure (low feed H/CI ratio); and a worst-case combination of the three failure modes.

Under baseline conditions, mixing failure, and matrix failure, kiln-exit DREs for each compound were comparable from test to test. Operating conditions in these 3 modes appeared to be sufficient to effect considerable destruction (greater than 99.99 percent DRE) of all compounds. As a result, separation of the highest-ranked POHCs from the lowest-ranked POHCs according to observed DRE was not possible; a correlation between POHC ranking and DRE could not be confirmed.

A correlation between predicted and observed incinerability was more evident for the thermal failure and worst-case conditions. Kiln-exit DREs for the four POHCs predicted to be most stable (those in Classes 1 and 2) ranged from 99% to 99.99% under these conditions, and were generally lower than DREs for the POHCs predicted to be more easily destroyed. Statistically significant correlations above the 99 percent and 93 percent confidence intervals were identified for the thermal-failure and worst-case tests, respectively.     

Mercury Emissions from a Hazardous Waste Incinerator Equipped with a State-of-the-Art Wet Scrubber

Abstract

Over a six-week period, eleven tests were performed at the U.S. EPA Incineration Research Facility (IRF) in Jefferson, Arkansas to evaluate the fate of trace metals fed to a rotary kiln incinerator equipped with a Calvert Flux-Forcer/Condensation Scrubber pilot plant as the primary air pollution control system (APCS). Test variables were kiln temperature, ranging from 538 °C to 927 °C; waste feed chlorine content, ranging from 0% to 3.4%; and scrubber pressure drop, ranging from 8.2 kPa to 16.9 kPa. Mercury was among the six hazardous constituent trace metals fed to the IRF’s pilotscale rotary kiln incineration system as part of a synthetic waste feed. This paper focuses on the test results solely with respect to mercury.

As expected, mercury behaved as a very volatile metal throughout the tests; it was not detected in any kiln ash samples. Scrubber collection efficiency for mercury ranged from 67% to >99%, averaging 87%; this was somewhat lower than expected and may be attributable to low scrubber loadings.

The ability to collect and analyze representative scrubber water samples appears to have been affected by the waste feed chlorine content; detection of mercury at higher concentrations during high waste-chlorine-content tests is thought to be largely the result of the formation of mercuric chloride, a more water-soluble species, during those tests. As a result, no firm conclusions may be drawn regarding the true impact of waste feed chlorine content on mercury partitioning to the scrubber water. As expected, no significant relationship was observed between kiln exit-gas temperature and mercury partitioning, nor was there a significant relationship with scrubber pressure drop.     

PCDD/PCDF reduction by the co-combustion process

A novel process, termed the co-combustion process, has been developed and designed to utilise the thermal treatment of municipal solid waste (MSW) in cement clinker production and reduce PCDD/PCDF emissions. To test the conceptual design; detailed engineering design of the process and equipment was performed and a pilot plant was constructed to treat up to 40 tonnes MSW per day. The novel process features included several units external to the main traditional cement rotary kiln: an external calcinations unit in which the hot gas calcined the limestone thus making significant energy savings for this chemical reaction; the lime generated was used in a second chamber to act as a giant acid gas scrubber to remove SOx and particularly HCl (a source of chloride); an external rotary kiln and secondary combustion unit capable of producing a hot gas at 1200 degrees C; a gas cooler to simulate a boiler turbogenerator set for electricity generation; the incorporation of some of the bottom ash, calcined lime and dust collector solids into the cement clinker. A PCDD/PCDF inventory has been completed for the entire process and measured PCDD/PCDF emissions were 0.001 ng I-TEQ/Nm(3) on average which is 1% of the best practical means [Hong Kong Environmental Protection Department, 2001. A guidance note on the best practicable means for incinerators (municipal waste incineration), BPM12/1] MSW incineration emission limit values.     

SITE Demonstration of the CF Systems Organic Extraction Process

The CF Systems Organic Extraction Process was used to remove PCBs from contaminated sediment dredged from the New Bedford Harbor. This work was done as part of a field demonstration under EPA’s Superfund Innovative Technology Evaluation (SITE) program. The purpose of the SITE program is to provide an independent and objective evaluation of innovative waste remediation processes. The purpose of this paper is to present the results of the SITE demonstration of this technology. Results of the demonstration tests show that the system, which uses liquefied propane, successfully removed PCBs from contaminated sediments in New Bedford Harbor. Removal efficiencies for all test runs exceeded 70 percent. Some operational problems occurred during the demonstration which may have affected the efficiency with which PCBs were removed from the dredged sediment. Large amounts of residues were generated from this demonstration project. Costs for using this process are estimated to be between $150/ton and $450/ton.     

Removal of PCBs from a Contaminated Soil Using CF-Systems® Solvent Extraction Process

ABSTRACT

The U.S. Environmental Protection Agency’s (EPA) Superfund Technical Assistance Response Team (START) in cooperation with EPA’s Superfund Innovative Technology Evaluation (SITE) program evaluated a pilot scale solvent extraction process developed by CF-Systems. This process uses liquefied propane to extract organic contaminants from soils, sludges, and sediments. A pilot-scale evaluation was conducted in Golden, CO at Hazen Research, Inc., using CF-Systems’ trailer-mounted organics extraction unit. Approximately 1,000 pounds of soil, with an average poly-chlorinated biphenyl (PCB) concentration of 260 mg/kg, was obtained from a remote Superfund site. Six 100-pound batches of the contaminated soil were extracted using multiple extraction sequences. Three of the six batch runs were subjected to three extraction sequences each, so that process variability could be evaluated. Results showed that PCB removal efficiencies varied between 91.4 and 99.4%, with the propane-extracted soils retaining low concentrations of PCBs (19.0–1.8 mg/kg). Removal efficiencies of oil and grease (O&G) were found to be 96.0 to 99.6% with propane-extracted soils retaining O&G concentrations from 279 to <20 mg/kg. Overall extraction efficiency was found to be dependant upon the numberof extraction cycles used.     

Incineration at Bayou Bonfouca Remediation Project

The Bayou Bonfouca hazardous waste site is located in Slidell, Louisiana, approximately 96 kilometers (60 miles) northeast of New Orleans. This site is ranked number 1,006 on the National Priorities List of Superfund sites. The U.S. Environmental Protection Agency (EPA) conducted a remedial investigation in 1986 and determined the primary potential exposure sources to be groundwater, surface waste piles, and contaminated sediment in Bayou Bonfouca. Based on the results of investigations, EPA and the Louisiana Department of Environmental Quality chose a remedy that involves dredging contaminated sediment from the bayou, excavating contaminated waste piles and soil, and incinerating the solid wastes in a transportable incinerator. The site remedy, which included incineration, was specified in the Record of Decision signed in March 1987.

Of the total 142,000 megagrams (Mg) (157,000 tons) of waste to be incinerated, approximately 119,000 Mg (132,000 tons) consist of hazardous sediment from the bayou; 22,600 Mg (25,000 tons) consist of lightly contaminated soils and waste piles, cellulosic materials, and other miscellaneous wastes on the ground. The solid wastes are primarily low heat content sediment and soils and cellulosic materials with polyaromatic hydrocarbon (PAH) concentrations from milligrams per kilogram (parts per million) levels up to two percent. The dredged bayou sediment will be dewatered in six, 115-cubiometer (150-cubic-yard) plate and frame filter presses before processing in the incinerator. A rotary-kiln-based single train incinerator is deployed at Bayou Bonfouca to process the solid waste feed.

On-site pilot studies indicated that the PAHs in groundwater could be removed by on-site pumping, treatment, and discharge of treated effluent to the bayou. The groundwater treatment plant went on-stream in June 1991. Treatment involves oil/water separation, filtration, carbon bed adsorption, and aeration.

IT Corporation-OH Materials, a joint venture, was awarded a contract in May 1991 and a notice to proceed in February 1992 to remediate and restore the Bayou Bonfouca site. The remediation project includes air quality monitoring and controls, site preparation, dredging and excavation, bayou bank stabilization and monitoring, equipment mobilization and erection, the trial burn, incineration, demobilization, and site closure. The project completed a successful trial burn in November 1993, and the commercial operation began in December 1993. The expected duration of the project is 40 months from mobilization to site closure.     

Waste Analysis Plan Review Advisor: An Intelligent Database to Assist RCRA Permit Reviewers

The Waste Analysis Plan Review Advisor (WAPRA) system assists in the review of the Waste Analysis Plan Section of RCRA Part B facility permit applications. Specifically, this program automates two functions of the waste analysis plan review. First, the system checks all wastes which will be processed in the same treatment or disposal unit to identify possible chemical interactions which might pose a health threat to unit operators or degrade the performance of the waste processing unit. Second, sampling methods are analyzed and judged for appropriateness based on guidelines in EPA Reference SW-846. WAPRA is best classified as an intelligent database. It contains the expertise of a chemist in a menu-driven user-friendly interface.     

Thermal Encapsulation of Metals in Superfund Soils

Abstract

Superfund sites frequently contain both heavy metals and organic hazardous waste. If not properly controlled, the metals may be changed to a more leachable form and may also be emitted to the atmosphere via the exhaust stack. This paper documents a batch kiln R&D test program to solve these metal-related problems. It was performed under the U.S. EPA’s SITE (Superfund Innovative Technology Evaluation) Emerging Technology Program. Allis Mineral Systems has developed the Thermal Encapsulation Process. Metals with limits set by EPA’s TCLP (Toxicity Characteristic Leaching Procedure) test and BIF (boiler and industrial furnace) stack emission regulations, such as cadmium, chromium, and lead, are the initial target of this process. This process, while unproven in these areas, may also apply to mixed waste (EPA hazardous waste/low-level radioactive wastes) and may also benefit commercial hazardous waste or Superfund thermal treatment systems. The results of the SITE tests were positive: strong, durable nodules were produced with excellent crush strength and improved resistance to leaching. Feed preparation, particularly control of moisture content, was found to be a key element in initiation of agglomeration. A good correlation was found between decreasing TCLP metals leachate levels and increasing crush strength.     

Alternating Current Electrocoagulation for Superfund Site Remediation

Abstract

The Superfund Innovative Technology Evaluation (SITE) Emerging Technology (ET) Program, authorized under the Superfund Amendments and Reauthorization Act (SARA) of 1986, implements the goal of the SITE Program to promote, accelerate the development of, and make commercially available alternative innovative treatment technologies for use at Superfund sites.

Under this program, the technical and economical feasibility of alternating current electrocoagulation (ACE) developed by Electro-Pure Systems, Inc., was evaluated for a two-year period. ACE is an electrochemical technology where highly-charged aluminum polyhydroxide species are introduced into aqueous media for the removal of suspended solids, oil droplets and soluble ionic pollutants. ACE can break stable aqueous colloidal suspensions of up to 10 percent total solids and stable emulsions containing up to 5 percent oil.

Major operating parameters have been defined for different classes of effluents based on experimental results using complex synthetic soil slurries and metals. Test results indicate that ACE produces aqueous and solid separations comparable to those produced by chemical flocculent additions, but with reduced filtration times and sludge volumes. The technology has application where removal of soluble and suspended pollutants from effluents is required, and in the recovery of fine-grained products from process streams. The technology, however, has not yet been demonstrated at full-scale for Superfund site remediation. The principal results of the SITE research program, and results of ACE treatment on some different classes of industrial effluents not part of the SITE Program, are summarized.     

Treatment of Halogenated Organic Vent Streams for the Reduction of Air Emissions

Abstract

This work presents a three-stage treatment system to process halogenated organic vent streams for compliance with the Hazardous Organic NESHAP (HON) Rule. The three stages are incineration, energy recovery, and wet scrubbing. In particular, this work concentrates on the design of the scrubber, which the HON Rule states must remove at least 99% of any halogens or hydrogen halides generated during the combustion step. Computerized process simulation was found to be ineffective in designing a scrubber for this application, so laboratory data on the partial pressure of the hydrogen halide species over aqueous solutions was employed. The number of overall gas transfer units required for 99% removal was found to be slightly less than 5, and the overall gas transfer unit height was found to be approximately 0.5 m.     

EPA SITE Demonstration of the BioTrol Soil Washing Process

A pilot-scale soil washing process, patented by BioTrol, Inc., was demonstrated on soil contaminated by wood treating waste, primarily pentachlorophenol (PCP) and creosote-derived polynuclear aromatic hydrocarbons (PAHs). Although soil washing was the main object of this demonstration, the treatment train that was evaluated included two other BioTrol technologies for treatment of waste streams from the soil washer. The three technologies were: ? The BioTrol Soil Washer (BSW)—a volume reduction process, which uses water to separate contaminated soil fractions from the bulk of the soil.

? The BioTrol Aqueous Treatment System (BATS)—a biological water treatment process.

? The Slurry Bioreactor (SBR)—a BioTrol biological slurry treatment process conducted in an EIMCO BIOLIFT^tm reactor.

The sandy soil at the site, consisting of less than 10 percent of fines, was well suited for treatment by soil washing. The soil washer was evaluated in two tests on soil samples containing 130 ppm and 680 ppm of PCP, respectively.

The BSW successfully separated the feed soil (dry weight basis) into 83 percent of washed soil, 10 percent of woody residues, and 7 percent of fines. The washed soil retained about 10 percent of the feed soil contamination while 90 percent of the feed soil contamination was contained within the woody residues, fines, and process water.

The soil washer achieved up to 89 percent removal of PCP and 88 percent of total PAHs, based on the difference between their levels in the as-is (wet) feed soil and the washed soil. PCP concentrations of 14 ppm and 87ppm in the washed soil were achieved from PCP concentrations of 130 ppm and 680ppm in the feed soil. Concentrations of total PAHs were reduced from 247 ppm to 42 ppm and 404 ppm to 48 ppm, respectively, in the two tests.

The BATS degraded up to 94 percent of PCP in the process water from soil washing. PAH removal could not be determined due to low influent concentrations.

The SBR achieved over 90 percent removals of PCP and 70 to 90 percent removals of PAHs from the slurry of contaminated fines from soil washing. However, steady state operation was not achieved during the single test and the results were variable.

Cost of a commercial-scale soil washing, assuming use of all three technologies, was estimated to be $168 per ton of soil treated. Incineration of woody material accounts for 76 percent of the cost.     

A Comparison of Two Techniques for the Collection and Analysis of Polynuclear Aromatic Compounds in Ambient Air

A demonstration of the GHEMFIX solidification/stabilization process was conducted under the United States Environmental Protection Agency’s (EPA) Superfund Innovative Technology Evaluation (SITE) program. The demonstration was conducted in March 1989, at the Portable Equipment Salvage Company (PESC) uncontrolled hazardous waste site in Clackamas, Oregon. Waste containing lead, copper, and polychlorinated biphenyls (PCBs) from four different areas of the site were treated. Results showed substantial reduction of leachable lead and copper between the untreated waste and treated waste utilizing the EPA Toxicity Characteristics Leaching Procedure (TCLP) test. The effectiveness of this process for immobilizing PCBs could not be determined since the raw waste did not leach PCBs at high concentrations, utilizing the TCLP test. Data from other leaching tests for lead and copper would need to be utilized as input into a site specific groundwater model to determine whether solidification/stabilization would be an acceptable remedy for the site. Physical testing results indicated durability in exposed conditions.     

Recovery of an Aqueous Iron Phosphating/ Degreasing Bath by Ultrafiltration

Aqueous cleaners are emerging as safe and effective alternatives to solvent degreasers, but switching to water-based cleaners may create a new waste which is high in oil and grease and potentially RCRA hazardous. In the case study summarized here, one metal fabricator replaced a trichloroethylene degreaser with an aqueous iron phosphating/degreasing bath to clean and precondition steel parts. The aqueous bath typically lasted three to four months, until the buildup of oil in the tank began to sacrifice product quality and raise oil and grease levels in the rinse water discharge. Hauling away and replacing the spent cleaner resulted in more than 15,000 gallon/year of hazardous waste.

Ultrafiltration was selected as the most promising technology to recycle the aqueous cleaner and thereby reduce hazardous waste generation. Following pilot-scale testing at the Illinois Hazardous Waste Research and Information Center, on-site full-scale testing integrated the new waste reduction scheme directly into the facility’s production process. Ultrafiltration continuously filtered and returned clean process solution back to the iron phosphating/degreasing bath during normal plant operation, substantially reducing and maintaining oil concentrations at acceptable operating levels. The new process design successfully reduced hazardous waste generation 99.8 percent with a payback period of only 6.9 months.     

Performance Audit Results for Volatile POHC Measurements

Audit materials containing principal organic hazardous constituents (POHCs) have been developed by EPA for use by federal, state, and local agencies or their contractors to assess the accuracy of measurement methods used during RCRA trial burn tests. Audit materials are currently available for 27gaseous organics in five, six, seven, and nine-component mixtures at parts-per-billion levels (7 to 10,000 ppb) in compressed gas cylinders in a balance gas of nitrogen. The criteria used for the selection of 27 gaseous organic compounds is described.

Stability studies indicate that all of the organics tested (with the exception of ethylene oxide and propylene oxide below 10 ppb levels) are stable enough to be used as reliable audit materials.

Subsequent to completion of the stability studies, 89 performance audits have been conducted with the audit materials to assess the accuracy of the Volatile Organic Sampling Train (VOST) and bag measurement methods during or prior to RCRA trial burn tests. A summary of the audits conducted for each POHC and the measurement system audited is shown in this paper. The audit results obtained with audit gases during RCRA trial burn tests are generally within ±50 percent of the audit concentrations.     

Relationship of Particulate Control,SO2 Removal,and Waste Management

Results with the EPRI 2.5 MW(e) Integrated Environmental Control Pilot Plant (IECPP) indicate the interrelationship of particulate penetration, SO₂ scrubber operation, waste production, and waste properties. Tests compared a fabric filter/wet scrubber and ESP/wet scrubber, the latter operated to simulate 1979 New Source Performance Standards (NSPS), 1971 NSPS, and pre-NSPS ESP units. Tests were conducted with low-sulfur coal producing a flue gas concentration of400ppm; flue gas spiking could be used to increase SO₂ to 2000 ppm. Scrubber waste was dewatered in a thickener and vacuum belt filter (to 55 percent solids content), and mixed with fly ash. The pilot SO₂ scrubber—when preceded by an ESP and forced to operate in zero-discharge—captured less SO₂ than when preceded by a fabric filter. Also, scrubber operation with the ESP produced a greater quantity of waste with difficult handling characteristics, as compared to operation with the fabric filter. These difficulties occurred with particulate penetration above 0.10 lb/MBtu, which could reduce reagent utilization to 80percent. These results are attributable to inhibited limestone dissolution due to accumulation of an aluminum/fluoride compound. For both lowsulfur and simulated high-sulfur test conditions, allowing wastewater discharge to purge aluminum/fluoride content restored performance to design levels. Particulate control efficiency also affected solid waste physical properties. The fabric filter/wet scrubber produced the lowest solid waste permeability (10^?8 cm/s). ESP operation at 1979 NSPS and pre-1971 NSPS ESPs increased solid waste permeability to 10^?7 and 10^?6 cm/s, respectively. These results are meaningful for SO₂ scrubbers both for new plants and for retrofit to units with pre-NSPS ESPs, and could become significant with the increasing trend to restricted water discharge.     

Performance of Gas-Atomized Spray Scrubbers at High Pressure

A large number of pressurized coal gasification processes being developed propose to use venturi scrubbers for particulate removal at high pressures. Theoretical predictions based on venturi scrubber performance models indicate that particle collection efficiency will decrease severely in these high gas pressure applications.

An exploratory theoretical and experimental program was performed to study the effect of gas pressure on venturi scrubber performance. Experiments were done on a 0.47 m³/s (1000 acfm) pilot scale venturi scrubber. Particle collection performance was determined as a function of scrubber pressure drop for venturi scrubbers operating In the range of 1-10 atm total pressure. Experimental results confirmed that the particle collection efficiency of venturi scrubbers decreases for a given scrubber pressure drop as total gas pressure Is increased. To achieve the same particle collection efficiency, the pressure drop across a venturi scrubber operated at 10 atm Is about 10 times that of the same scrubber operated at 1 atm pressure.     

Lime/Limestone Scrubber Operation and Control

This paper examines operating and control experience obtained at full-scale lime-limestone scrubber systems both in the eastern and western United States. Methods of resolving typical problems in pH control of reagent feed, water balance closure, and slurry solids de-watering are discussed and described. Control problems can be severely aggravated by scale formation inside scrubbing equipment, especially on control sensors. If the sensor reliability problems can be resolved in a properly-designed scrubbing system, then the scrubber chemistry can generally be controlled and scale formation can usually be prevented. Current regulatory trends towards zero blowdown requirements increase scrubber operating and control problems, both from a process control standpoint (due to system complexity) and from a sensor reliability standpoint (due to an increased likelihood for chemical scale formation). These factors highlight the need for continuing development of improved scrubbing systems.