Industry Finds New Ways To Process Hazardous Waste

Cathy Dombrowski

Nine years after Congress banned hazardous waste from landfills, U.S. industries are adopting new thermal, biological and chemical treatment schemes to manage modern society's byproducts.

Along with the development of economical and efficient methods to manage wastes and eliminate associated liabilities, innovative approaches to detoxify and minimize waste are being commercialized.

The waste treatment market has evolved, according to Eliot Cooper, vice president of environmental affairs for Waste-Tech Services Inc., Golden, Colo.

Prior to the ban, Cooper notes, there was no economic incentive to treat hazardous wastes. Since land disposal restrictions came into effect, companies have concerned themselves with compliance.

The Incineration Alternative Incinerators were the initial alternative to landfills, but public concerns about air emissions made new facilities difficult to site. After years of permitting efforts, two commercial units have come on line.

Still engulfed in public opposition, the Waste Technologies Inc. unit in Liverpool, Ohio, has started to burn wastes under certain restrictions and the careful scrutiny of the federal and state Environmental Protection Agencies (EPA). Equipped with a rotary kiln and heat recovery boiler, the incinerator employs a spray dryer, electrostatic precipitator and wet scrubber to reduce contaminants in the exhaust.

An enhanced carbon injection system has been added so the facility can meet the 30 nanogram per dry standard cubic meter requirement established for dioxins and furans by EPA's new combustion policy. This system injects carbon into the flue gas stream at two locations in addition to the spray dryer. The additional injection points provide for increased absorption of organics onto the carbon particles, which will be removed from the flue gas the same as flyash.

In recent tests, total dioxin/furan emissions averaged 13 ng/m[Superscript]3 at seven percent oxygen, while particulate emissions averaged 0.0016 grains per dry standard cubic foot at seven percent oxygen. The new standard for particulates, which helps in the control of metals, is 0.15 gr/dscf at seven percent oxygen.

The first commercial fluidized bed system to receive an RCRA permit has been built by Waste-Tech in Kimball, Neb. and is expected to begin accepting wastes next month. The facility will handle 45,000 tons per year of solids, liquids and sludges. A trial burn is scheduled for early next year

In this system, the primary combustion chamber contains three to four feet of ground refractory brick. When these particles are heated and suspended by an upward flow of pressurized combustion air, incoming waste is caught in the flow. The particles provide high heat transfer to the waste, resulting in high thermal reaction rates.

The result is almost instantaneous waste destruction, Cooper said. The turbulence also causes the ash to be sheered from partially burned waste particles, leaving their cores exposed for further combustion. The primary combustion chamber operates at about 1,450Degrees F. Off-gases pass to a secondary chamber, which provides additional residence time to complete waste destruction. The air pollution control system includes a spray dry absorber for acid gas control and a bag house.

The new dioxin standard will not require additional treatment because the system's good combustion efficiency and low operating temperatures work against dioxin formation, Cooper said. The small amount that forms will condense on the particulates and be collected in the bag house.

A 1987 trial burn conducted on a similar but smaller incinerator in Louisiana operated by Waste-Tech found that the system produced about 0.3 ng/dscm of dioxins, he said. The flyash from the unit has been determined by Nebraska state officials to meet delisting criteria and can be handled as non-hazardous waste. An on-site monofill, designed to meet Subtitle C standards, will be used to bury the material.

Waste-Tech has no plans for additional incinerators, Cooper said, noting that there is now sufficient capacity. However, he points out, if cement companies decide not to go through the RCRA permitting process for their kilns, the hazardous waste incineration market will become very attractive.

Recovering Oil and Water Waste-Tech has developed another thermal technology to recover used oil and water from refinery wastes. The company will build a commercial facility in the Gulf Coast region which will begin operations next year. A desorption and recovery unit (DRU) that evolved from a distillation process will treat 20,000 tons per year of material.

The DRU indirectly heats the oily wastes to 400-500Degress F to volatilize water and light hydrocarbons. The vapors are then cooled and condensed and the oil and water fractions are separated in a gravity settler. Meanwhile, the dry, partially de-oiled solids are heated in a second stage to a maximum of 950Degrees F to volatilize the remaining hydrocarbons.

The volatilized oil is partially condensed by direct contact with a recirculating oil stream in a quench tower. Uncondensed water and lower-boiling point hydrocarbons are further cooled in a glycol/water-cooled heat exchanger. Uncondensed vapors are scrubbed to remove sulfur compounds and then mixed with propane and thermally oxidized to provide heat to the process. The solid residues are cooled and ready for final disposal.

During a pilot demonstration at a Texas refinery, the process produced solids that met TCLP and BDAT standards for metals, cyanide and volatile and semi-volatile compounds, said George Rasmussen, executive director of process development. The recovered oil could be recycled to the refinery or used as fuel. The unit met exemption requirements for recycling refinery wastes so an RCRA Part B permit was not necessary, he said.

Waste-Tech estimates that a DRU processing 10,000-20,000 tons per year of hydrocarbon-contaminated sludge and soil would produce 80 barrels of oil, 25 tons of solids and 8,500 gallons of water each day. The latter is sent to a wastewater treatment facility. Earlier this year, the technology was awarded for management excellence.

Wastewater Innovations Because wastewater makes up more than 90 percent of hazardous waste, it is a prime target of innovators. Clean Harbors, Baltimore, has installed a system that can remove organics and heavy metals from wastewater using carbon dioxide as a solvent. As much as 99.999 percent of organics and heavy metals can be eliminated from the treated effluent by the company's patented critical extraction system (CES), which evolved from a critical fluid extraction process.

The wastewater is fed into the top of a pressurized chamber and flows down through a stack of perforated plates as liquefied carbon dioxide flows up from the bottom. The two mix and contaminants dissolve into the carbon dioxide, while clean water flows from the bottom of the reactor. The contaminated carbon dioxide is sent to a separator where the pressure is decreased, which causes the carbon dioxide to vaporize and leaves behind a concentrated liquid waste that can be recovered or sent for disposal, depending on the contents. The carbon dioxide vapor is recompressed for reuse. Waste-water containing up to 30 percent organics can be cleaned. Chelating agents are added to the waste to inhibit precipitation of heavy metals and maintain them as organometallic complexes that can be extracted by the CES.

A membrane technology that can operate at high turbidity and Silt Density Index levels has been introduced to the United States from Europe by Rochem Separation Systems, Torrance, Calif. The Rochem system's open channel plate and frame design received the British Seatrade Award for excellence in engineering. The modules consist of octagonal membrane cushions produced from flat membranes with internal fabric fleece. The cushions are stacked alternately with hydraulic discs on a tension rod, providing a packing density of 200 m[superscript]2/m[superscript]3, said David LaMonica, president.

The Rochem system pumps water through the module at a pressure of between 30 and 140 bar. As water flows over the membrane cushions, cross-flow filtration occurs. A variety of membranes, including microfiltration, reverse osmosis, nanofiltration and ultrafiltration, can be used. Because the module is designed to eliminate concentration polarization and physical flow impediments, scaling and fouling are greatly reduced, LaMonica said.

Used to treat landfill leachate in Schoenberg, Germany, Rochem reports that a two-stage system has reduced cyanide of 2.35 mg/l and arsenic of 0.25 mg/l to non-detectable levels; chloride from 3,091 to 2.7 mg/l; magnesium from 97 to 0.3 mg/l; ammonium from 380 to 0.4 mg/l; sodium from 3,255 to 2.4 mg/l; and calcium from 192 to 0.9 mg/l. Rochem systems throughout the world are producing more than one million gallons per day of pure water from landfill leachate, LaMonica said. Costs range from one dollar per 1,000 gallons for treating wastewater to three cents per gallon for concentrated hazardous waste.

Biological treatment has become a popular method to treat waste because of its low cost and ability to degrade chemicals into non-toxic compenents.

A biological system that uses horse manure as a digestive filter medium has been developed by Advanced Bio-Gest Inc., Willits, Calif. The manure contains a diverse microbe population of bacteria, nematodes, rotifers and paramecia. As the wastewater is sprayed on top and filters down, heated air is blown up through the manure, resulting in a moist, warm environment that fosters the microbes needed to degrade contaminants.

While the land ban has dictated a shift away from landfilling hazardous waste, some residues remain which cannot be destructed or detoxified to meet delisting standards. To handle these byproducts, Envirosafe Services of Ohio Inc. has opened a new 2.3 million cubic yard landfill cell in Toledo, Ohio. Cell M can accept and stabilize up to 235,000 tons of hazardous waste per year to ensure compliance with land disposal restrictions.

Provisions have been made for perpetual post-closure monitoring. A fund set up under the facility's permit currently contains $7.2 million and is required to have $11.5 million by the time the permit expires. About $1.5 million is dedicated to closure costs, and the remainder will go for perpetual care.

Medical Waste Disposal While states remain the primary regulators of medical waste disposal, the EPA will develop air emissions standards for med-waste incinerators by mid-1995. To develop those standards, the agency has conducted tests at seven medical waste incinerators (MWIs) to determine uncontrolled emissions and the results of changing combustion conditions and using add-on air pollution control devices.

Of the technologies currently employed on MWIs, dry sorbent injection, followed by a fabric filter (DI/FF) and coupled with carbon injection, provides the best overall reduction of pollutants, according to EPA findings. The DI/FF system reduced particulates by 97 percent (down to 0.01 gr/dscf), metal emissions by 99 percent and HCl by 95 percent.

Another option, lime injection coupled with fabric filters, achieved little to no control of dioxin or mercury emissions; but with the addition of carbon injection, the removal rate increased to >80 percent reduction in mercury and >95 percent removal of dioxins.

A spray dryer/fabric filter achieved 83 percent control of dioxins and 24 percent control of mercury without the addition of carbon. Combustion conditions providing a minimum secondary chamber temperature of 1,800Degrees F and residence time of two seconds produced a 94 percent reduction in dioxin, 99 percent lowering of carbon monoxide and 78 percent reduction in particulates.

The EPA estimates that approximately 5,000 existing MWIs and 700 new MWIs installed over the next five years may be subject to the emission guidelines. The average size of on-site units is about 300 pounds per hour, while average commercial units handle 1,200 pounds per hour. The DI/FF annual costs are about 28 to 120 percent higher than wet scrubbing systems, and carbon injection adds another four to 14 percent to annual outlays.

While the EPA debates the air standards, vendors are continuing their efforts to improve the technology. Interel Environmental Technologies Inc., Englewood, Colo., has a dry lime injection/fabric filter system that can be used in conjunction with activated carbon injection. Compliance testing of an installation at a New York facility found average emissions from the two-incinerator system to be 0.0006 gr/dscf at 7 percent oxygen for particulates, 99.97 percent removal of HCl and 99.99 percent removal of dioxin furans as 2,3,7,8-TCDD toxicity equivalents, said E. B. Mull, vice president. A comparison of ambient ground level concentrations of various pollutants attributable to the stack and federal and state standards found that dioxins were 3.8 percent of the standards and mercury was 4.5 percent of the standards.

In the dry additive process (DAP), flue gas leaving the waste heat recovery boiler is injected with dry powdered hydrated lime at a rate of 40 pounds per hour prior to entering a flat-tube heat exchanger designed to bring the temperature below 300Degrees F. An internal cleaning mechanism is incorporated into the unit to prevent deposit build-up on the exchanger surfaces, Mull said. Activated carbon is injected at a feed rate of three pounds per hour into the ductwork between the heat exchanger and a scrubber reactor.

The reactor is built with a vertical, two-pass design. A drum, made of perforated steel and partially filled with ceramic grinding balls, rotates in the bottom. The flue gas enters the bottom of the reactor and passes radially through the perforated drum walls.

Some of the dust from the baghouse is fed into the gas inlet side of the reactor and is then distributed by the movement of the balls and re-entrained in the flue gas, he noted. The drum also promotes agglomeration of submicron, adhesive dust particles with the recirculated dust, which facilitates removal in the subsequent fabric filter. The latter is a top-entry, multiple compartment of horizontal bags that are pulse-jet cleaned.

Compliance tests conducted early this year at a New Jersey hospital found that the Rotary Atomizing Scrubber of Emcotek Corp., Visalia, Calif., was controlling particulates in flue gas from a medical waste incinerator to a level of 0.009 gr/dscf at 7 percent oxygen, according to Hank Marschall of Emcotek, who notes that the system can remove particulates in the submicron range. Hydrogen chloride emissions routinely were below five parts per million, and often less than one ppm, he said.

The Emcotek scrubber uses a high-speed rotating disc to generate high-energy atomized water droplets. These travel only a short distance before hitting the surrounding cylindrical duct, thereby creating a dense liquid curtain equivalent to a filter pad. The result is a wet scrubbing system that acts like a liquid baghouse, Marschall said.

Particulates passing through the constantly renewed, dense spray curtain of atomized droplets are removed by impaction, while gases are collected by absorption. As a result, the scrubber's efficiency is independent of flue gas flow rates and, unlike a baghouse, the liquid spray curtain does not plug up and gas side back pressure does not occur. Because the energy for the scrubber is separate from the draft control, the system has lower pressure drop than other systems, Marschall said.

A special three-stage demister is used to dry the flue gas. Each stage of the demister has a unique combination of mesh pads to coalesce and remove the droplets.

Alternatives to incineration are being explored by other companies. Synthetica Technologies Inc., Richmond, Calif., is developing a system that uses superheated steam. Originally devised for hazardous waste treatment, the Synthetica Detoxifier is a two-step process.

The initial stage employs superheated (900Degrees F) steam to vaporize organic liquids and gasify the organic solids. As a result, the waste is sterilized and reduced in volume by about 50 times, according to Dr. Terry Galloway, vice president of technology. The vapors are mixed with excess superheated steam and passed to the detoxifier, which is electrically heated to as much as 2,800Degrees F and operated under negative pressure.

Steam reforming chemistry converts the vaporized organics to carbon monoxide, carbon dioxide, water, hydrogen gas and a small amount of CH4, Galloway said. Absorber beds, operated at 350DegreesF, remove trace organics and metals, including halogens. A small vent stream of the off-gas is oxidized in a CO converter, while the remainder of the hot steam-reformed gas is recirculated to the evaporator.

Tests conducted with a California grant using simulated medical waste found that one cubic yard of solid material could be reduced from 14.2 pounds to 4.2 pounds in less than two hours and that all the solids except glass, metal and the interior of large thermal masses, were steam gasified at around 600Degrees F. To ensure sterilization of large masses, Synthetica also is developing a heated shredder.

Plasma Energy Applied Technology (PEAT), Huntsville, Ala., has developed a plasma energy system which will be installed at San Diego's Zion Medical Center. The system will treat 1,000 pounds per hour of medical wastes. The PEAT technology employs an electric plasma torch to create temperatures of 2,000-3,000Degrees F in the reaction chamber.

As a result, organics are pyrolyzed to a gas composed mainly of diatomic hydrogen and carbon dioxide without forming new complex organics, said PEAT's Marlin Springer. The off-gas, which is about 10 percent of the volume produced by an incinerator, is cooled rapidly and passed through a scrubber to produce a stream that is primarily hydrogen (40 to 50 percent) and carbon monoxide (30 to 35 percent). Inorganic compounds melt into a silica-based slag that chemically bonds the heavy metals present in the waste, said Springer.

At Zion Medical Center, the system is being installed as a demonstration program that is sponsored by Kaiser Permanente, San Diego Gas and Electric, Southern California Edison and the Electric Power Research Institute. The hospital is expected to generate 1,800 pounds of red bag waste and 7,000 pounds of clear bag waste per day, enabling the system to process all the material in one eight-hour shift at off-peak power rates. The life cycle operating cost, which includes capital recovery as well as maintenance, labor and utilities, is projected at less than 20 cents per pound.

Amid public opposition to the WTE incinerator in Ohio - centered largely on fears about exposure to dioxin and metal emissions - the Environmental Protection Agency revised its combustion policy for hazardous waste facilities last spring, tightening standards to restrict both types of contaminants.

But even as Administrator Carol Browner set more stringent controls, the agency is re-evaluating the health risk of dioxins. Since dioxin toxicity occurs at higher doses than its cancer effects, the current standards have been lowered to a level designed to prevent cancer.

But the contribution of dioxin to cancer remains unclear, according to some scientists. A panel organized by Environ Corp., Fairfax, Va., for the American Forest and Paper Association, has conducted a review of the epidemiologic literature and "determined that the evidence was inadequate to conclude that dioxin causes human cancer," said Suresh Moolgavkar of the Fred Hutchinson Cancer Research Center.

Four major studies of large groups of people exposed to dioxin are not consistent and suffer from design or methodological shortcomings, the group said to the EPA. "Many of these studies were among heavily exposed individuals either because they were chemical workers or because of industrial accidents, and even here the weight of evidence was not persuasive," Moolgavkar said.

The panel also pointed out that: * A few studies suggest an association between dioxin and soft tissue sarcoma, but many other, equally well-designed studies have not found an association. On balance, the weight of evidence is inadequate to support a causal association.

* The level of exposure of individuals to dioxin often was not determined in the studies.

* Workers were exposed to many other known carcinogens, as well as dioxin, so it was difficult to isolate the effect of dioxin alone.