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Rx For Medical Waste PUBLIC ACCESS

Microwaves, Steam, Hot Air, and Scrubbers Clean Up the Treatment of Biomedical Refuse.

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Mechanical Engineering 122(09), 52-56 (Sep 01, 2000) (5 pages) doi:10.1115/1.2000-SEP-1

This article reviews many hospitals and medical centers have found it more economical to replace their on-site incinerators with alternative waste treatment technologies, primarily microwave systems or steam autoclaves, or send waste to treatment companies that are equipped with disinfection technologies. Sanitec International Holdings of West Caldwell, NJ, illustrates the in roads that alternatives are making to medical waste incineration. The entire Sanitec disinfection system is enclosed in all-weather steel housing, and is connected to the hospital’s electrical and water systems. Hospital workers bring collected waste in carts to the automated lift and load system, which raises the cart and empties it into the infeed hopper. The MediWaste system at Laredo is designed to treat up to 200 pounds of material per hour, which is more than sufficient to treat the 700 to 800 pounds of waste generated per day. Although incineration alternatives appear to be gaining popularity, combustion is still used to disinfect and reduce much clinical waste.

Hospitals and clinics in the United States generate 600,000 to one million tons of waste each year, and as much as 15 percent of it poses a potential infection hazard, according to the Environmental Protection Agency.

For many years, most hospitals either incinerated contaminated syringes, needles, paper, plastic, glass, fabric, and human tissues on-site, or sent them to incinerators off their grounds to ensure that all pathogens were destroyed.

Regulations for medical waste incinerator emissions that were required by the Clean Air Act of 1990—but passed only three years ago—have changed the economics of this practice. U.S. hospitals must now retrofit their incinerators with costly scrubbers that remove or neutralize dioxins, furans, hydrogen chloride, sulfur dioxide, nitrogen oxide, and the heavy metals lead, cadmium, and mercury to meet those regulations.

Many hospitals and medical centers have found it more economical to replace their on-site incinerators with alternative waste treatment technologies, primarily microwave systems or steam autoclaves, or send waste to treatment companies that are equipped with disinfection technologies.

Sanitec International Holdings of West Caldwell, N.J., illustrates the inroads that alternatives are making to medical waste incineration. “We sell half our microwave disinfection systems directly to hospitals and half to waste treatment firms,” according to Mark Taitz, director of business development. The company was originally part of Asea Brown Boveri in Portland, Maine, until it split off in 1995.

The entire Sanitec disinfection system is enclosed in an all-weather steel housing, and is connected to the hospital’s electrical and water systems. Hospital workers bring collected waste in carts to the automated lift and load system, which raises the cart and empties it into the infeed hopper. The hopper is sealed and the shredder is activated. Shredding reduces the waste’s volume by 80 percent and, just as important, creates a more even waste stream that can be effectively treated at lower temperatures, minimizing the system’s overall energy consumption as well as the potential for releasing potentially harmful air emissions.

An automated lift and load system empties carts filled with medical waste into the steel-enclosed Sanitec microwave disinfection system, where it will be shredded and sanitized.

Grahic Jump LocationAn automated lift and load system empties carts filled with medical waste into the steel-enclosed Sanitec microwave disinfection system, where it will be shredded and sanitized.

Designing a shredding mechanism for medical waste is more challenging than a mechanism that shreds tires or tree stumps, because medical waste is, by definition, a heterogeneous mixture. “The Sanitec system has to shred soft fabric drapes, gowns and bandages, brittle glass, plastic syringes, and hard steel needles, knives, and clamps,” Taitz explained. “We used to rely on shredders made by other manufacturers, but in the past year, we introduced a proprietary shredder. It consists of two rotating shafts with teeth that grind all types of regulated hospital wastes to the proper size, so that it falls through a close tolerance screen to the next stage.”

A fan draws air from the infeed hopper through a series of filters. A high-efficiency particulate air, or HEPA, filter and a carbon filter control odors and prevent harmful emissions from escaping during processing.

A stainless steel screw conveyor moves the shredded waste through the output of an electric steam generator that uses about 8 gallons of water per hour to add moisture to the waste. The moistened waste then passes through a series of a half-dozen 1,400-watt microwave units made by Alter of Reggio Emilia, Italy. The microwaves excite the water molecules on the waste particles, creating friction and raising the temperature of the waste to 205 to 212°F for 25 minutes.

The combination of high temperature and residence time is sufficient to ensure the destruction of pathogens, a process that is validated by regular spot checks using a technique already proven in verifying hospital disinfection systems. “We place small envelopes containing vials of Bacillus subtilis bacterial spores, made by companies including 3M, into the Sanitec unit through a feed port that is downstream of the shredder, so that the envelopes will pass through the steam and microwave stages,” said Taitz. ‘We remove the envelopes and check the vials for bacterial growth. Because bacterial spores are harder to kill than pathogens such as hepatitis and tuberculosis, if we find no bacterial growth, it indicates complete pathogen destruction.”

A secondary screw conveyor removes the treated waste from the Sanitec unit to a standard waste compactor or dumpster prior to its final disposal into a municipal solid waste program. An optional granulator enables the hospital to further reduce waste volume.

The entire Sanitec process is overseen by an Allen- Bradley microprocessor equipped with a computer program that monitors the residence time and temperature parameters to ensure that disinfection is complete before the waste is discharged.

A group of four hospitals in Madison, Wis.—the University of Wisconsin Hospital and Clinics, Meriter Hospital, Methodist Hospital, and St. Mary’s Medical Center—joined forces in 1986 to create a shared medical waste processing facility to reduce costs. The hospitals formed a nonprofit corporation called Madison Energy Recovery Inc., or MERI, to operate the plant, which was originally equipped with a state-of-the- art incinerator.

By 1994, tighter environmental regulations meant that the incinerator would have to be retrofitted with new pollution control equipment at a cost likely to exceed $500,000. After reviewing options, the MERI board selected the Sanitec disinfection system.

“We find the Sanitec system to be quiet, clean, and very efficient in disinfecting waste,” said John Crha, general manager of MERI. Many health care facilities agree, and today the MERI Sanitec system processes more than 1.5 million pounds of regulated medical waste per year generated from 12 additional hospitals and clinics throughout the state, including Mercy Health Systems in Janesville and St. Agnes Hospital in Fond du Lac.

Each day, a specially designated MERI truck picks up plastic carts filled with waste packaged in red bag or plastic sharps containers from 250 locations. After the carts are emptied into the Sanitec system, they are washed, cleaned, and disinfected before being trucked back to their hospitals. Each cart is tracked by signed manifests, which are also used to bill the participating hospitals. Treated wastes are sent to municipal solid waste landfills.

The Sanitec system shreds hospital waste into particles, adds water, and subjects it to microwaves to raise its temperature to 205 to 212°F for 25 minutes, killing all pathogens, a process validated by regular spot checks.

Grahic Jump LocationThe Sanitec system shreds hospital waste into particles, adds water, and subjects it to microwaves to raise its temperature to 205 to 212°F for 25 minutes, killing all pathogens, a process validated by regular spot checks.

Rather than transporting waste to a disinfection site, Safe Waste Inc. of Charlotte, N.C., brings the Sanitec process to hospitals in its home state and Virginia on four truck-mounted, mobile units. Safe Waste’s Sanitec trucks pick up the waste from almost 40 hospitals, including Carolina Medical Center in Charlotte, and Fairfax Hospital in Fairfax, Va., and treat it on-site, using each hospital’s water and power connections. The company uses smaller vans to treat waste from more than 400 smaller medical facilities, including doctor’s offices, rural clinics, laboratories, and veterinary establishments. In all, Safe Waste processes nearly 10 million pounds of potentially hazardous material annually.

Sanitec has set its sights beyond its traditional practice of selling its microwave disinfection systems to hospitals and waste treatment companies. “We are now concentrating on creating our own service companies by forming joint ventures such as Sanitec of Kentucky, in Florence, Ky., and Sanitec of Hawaii in Honolulu,” Taitz said. “We supply the equipment to the joint venture and participate in the revenues, thus making our sterilization equipment more accessible to the end user. Hopefully, we can eventually create a nationwide treatment for all medical waste generators.”

Taitz sees bright prospects for the Sanitec system outside the United States as well. “Our biggest sales growth is in offshore markets, including Brazil, Japan, Korea, Saudi Arabia, the United Kingdom, the Philippines, and Kuwait,” Taitz noted.

The problem of treating hospital waste knows no borders. Some 3,400 French hospitals and clinics generate 700,000 metric tons of medical waste each year, according to Didier Gabarda Oliva, an engineer in charge of the medical waste department at the French Agency for the Environment and Energy Control based in Valbonne.

Approximately 140,000 metric tons of contaminated hospital waste in France is incinerated and, as in the United States, there are environmental concerns that the heavy metal particles this generates are a health hazard in their own right. Incinerating biomedical waste is further complicated at French hospitals because incineration facilities are often remote. In the entire country, only about 50 hospitals operate incineration plants on-site, and an additional 24 off-site facilities are authorized to burn potentially infectious medical waste. Entire regions such as Burgundy, Franche-Comté, Picardy, and Poitou-Charentes have to ship their waste a considerable distance to be burned.

For these reasons, French companies are developing specific, non-incineration techniques for treating biomedical wastes. “It is a question of reducing the microbial contamination of waste, and also of changing its appearance for psychological reasons and safety aspects,” Oliva explained. The treated waste is disposed of in existing landfills and incineration systems that treat household waste.

The Sanitec system operated by Madison Energy Recovery Inc. converts more than 1.5 million pounds of regulated medical waste generated yearly by Wisconsin hospitals and clinics into a form that is sent to landfills.

Grahic Jump LocationThe Sanitec system operated by Madison Energy Recovery Inc. converts more than 1.5 million pounds of regulated medical waste generated yearly by Wisconsin hospitals and clinics into a form that is sent to landfills.

The French Ministries of Health and the Environment have approved several non-incineration processes to treat potentially infectious wastes, including a steam system developed by Ecodas, headquartered in Roubaix. According to Oliva, this company is the leading provider of alternative biomedical waste treatments.

Ecodas drew upon its 20 years of experience manufacturing steam pressure autoclaves for the textile industry to design a medical waste treatment system. “The innovation lies in combining a high-strength grinder with a particularly powerful sterilizer,” said Jaafar Squali, managing director of Ecodas.

The first stage of the Ecodas treatment involves loading contaminated waste into a hermetically sealed chamber that feeds a grinder with 20 rotating blades. These blades are fashioned from an alloy strong enough to shred stainless steel surgical instruments that are sometimes mistakenly disposed of with other clinical wastes. The grinder reverses its rotation at regular intervals to prevent jamming.

Loads of waste are emptied into a loading chamber that feeds the autoclave. Inside the autoclave, the waste is subjected to steam heated to 280°F and pressurized to 55 pounds per square inch for 10 minutes, which sterilizes the waste. A temperature probe in the center of the autoclave embedded in the waste sends signals to the computer control system to regulate temperature.

When disinfection is complete, operators open the lower lid of the autoclave to release the processed waste into a container. The entire process takes about one hour to treat a single load.

Ecodas designed three different versions of its waste treatment machines to accommodate a range of waste volumes and space available for installation. The TDS 300 is 10 feet tall and treats 35 to 55 pounds of waste per hour; the TDS 1000 treats 110 pounds per hour, and the TDS 2000 treats up to 132 pounds per hour.

In France, public hospitals in Ajaccio, Aurillac, Nevers, and Roubaix disinfect their wastes with Ecodas autoclaves. So do hospitals in Odense, Denmark; Majorca, Spain, and Budapest, Hungary. Among the waste processing companies that use the Ecodas system are Cosmolys and Teemed in France, Tecsan in Argentina, Manned in Brazil, and Tremesa in Mexico.

One of the newest clinical waste treatment technologies uses hot air to disinfect shredded hospital waste streams. This technology was developed and is being marketed by KC MediWaste of Dallas. The first MediWaste system was installed at Sisters of Mercy Health System in Laredo, Texas, last summer. KC MediWaste combines a dry sterilization process invented by the company’s president, Keith Cox, with licensed fluidized bed technology from Torftech Ltd. of Reading, U.K.

The Mercy Health System installation was one of several advanced, electricity-based technologies built into the Laredo hospital as part of a joint project sponsored by the local utility, Central and South West Services; its subsidiary, Central Power & Light, and the Healthcare Initiative of Electric Power Research Institute in Palo Alto, Calif. These technologies are designed to help hospitals cut costs, improve operating efficiency, and enhance patient services.

“The greatest challenge in designing the first MediWaste system was making waste hot enough to sterilize it, but cool enough to prevent volatile organic compounds from being released from the plastic waste,” said Sue Herbert, a mechanical engineer and project engineer for the Laredo installation. “We collected samples of everything that ends up in a hospital waste stream, and worked with plastic companies to study the flash points of the different plastic compounds in order to find the optimum heating temperature.”

Mercy Health System workers use covered carts to deliver their waste materials to the MediWaste unit. A hydraulic lifting system empties each cart into the systems feed hopper. Internal exhaust fans create negative pressure within the MediWaste system to control odors.

Inside the unit is a shredder consisting of four shafts covered with closely interlocking teeth, made of a heat- treated stainless steel. The shredder grinds the waste before it is sent to a processor. Air heated to about 320°F by electrical resistance heaters is injected into the processor at high velocity through a fixed blade ring. The blades are angled to direct the air in a manner that optimizes turbulence within the processor. As the ground waste enters the processor, the turbulent air creates a fluidized bed that provides cyclonic mixing action, and high rates of heat and mass transfer.

The waste remains in the fluidized bed for five minutes before a dump door opens so that the material is propelled into a compactor unit that reduces its volume by 80 percent. The Laredo hospital sends its treated waste to a conventional municipal waste landfill.

The processed air that exits the MediWaste system passes through three stages of filtration before entering the atmosphere. First, two fabric prefilters remove gross particulates before the high-efficiency particulate air filter— a membrane contained in a metal frame—removes smaller particulates. Charcoal filters eliminate odors from the airstream.

The MediWaste system at Laredo is designed to treat up to 200 pounds of material per hour, which is more than sufficient to treat the 700 to 800 pounds of waste generated per day. “We are currently developing a unit capable of disinfecting up to 1,000 pounds of material per hour,” said Herbert.

A programmable logic controller regulates the injection of sodium hydroxide into the quench tanks of the Crawford incinerator used at the V.A. Medical Center in West Palm Beach to neutralize acid waste.

Grahic Jump LocationA programmable logic controller regulates the injection of sodium hydroxide into the quench tanks of the Crawford incinerator used at the V.A. Medical Center in West Palm Beach to neutralize acid waste.

Although incineration alternatives appear to be gaining popularity, combustion is still used to disinfect and reduce much clinical waste. Crawford Equipment and Engineering Co. of Orlando, Fla., designs and markets a range of medical incinerators that can process from 20 to 3,000 pounds of biohazardous waste per hour. These units are designed for connection to scrubbers that enable them to meet the provisions of the Clean Air Act.

The Crawford Equipment incinerators are typically natural gas fired, but can also burn propane or fuel oil if they are more readily available or economical. Each incinerator contains a primary and secondary chamber, both refractory-lined to withstand the intense heat of combustion. Hospital workers load waste either manually or hydraulically in red bag or plastic sharps containers through the primary chamber door. They close the door and activate the incineration process.

First, the burners in the secondary chamber, located in a series or below the primary chamber, ignite. Heat then irradiates through the refractory material in order to raise the temperature of the primary chamber, thus making it increasingly energy efficient. When the primary chamber achieves the minimum temperature of 1,800°F, a sensor will activate the primary chamber’s burners to incinerate the waste.

“The 1,800° temperature kills pathogens, and oxidizes all the organic wastes, converting them into carbon dioxide and water,” said Luis Llorens, a chemical engineer and director of solid and liquid waste disposal systems at Crawford Equipment. “All the smoke and odors generated by combustion are vented into the secondary chamber, and remain there for one or two seconds so the 1,800° heat will destroy them.”

Air from the secondary chamber is routed through a customized breech to a standard pollution control system that will remove acids and heavy metals, such as lead, cadmium, and mercury. The system uses wet scrubbers that spray a mist of water and reagent, such as caustic solution, in order to react with the flue gases and remove acid gas emissions.

In addition to reducing the volume of medical waste by more than 90 percent of its original bulk, the Crawford incinerators reduce its weight by 95 to 97 percent, something that microwave and steam autoclave systems cannot do, noted Llorens.

The incineration chambers’ walls consist of brick, insulation, a steel shell, and a second steel outer shell. “We run air through the sidewalls by fan to keep the outer wall of the incinerator cool to prevent injuries,” said Llorens. In addition, Crawford mounted a fan to induce a draft into the incinerators’ refractory-lined stack. This helps the incinerator run cleaner, and keeps the gases in the secondary chamber at a lower flow, increasing their retention times to ensure that they will burn.

“There are other good medical waste treatment technologies, such as microwave, but incineration is still the best option under the right conditions,” Llorens said. “Hospitals’ choice depends on their communities and their needs.”

For example, the Department of Veterans Affairs’ Medical Center in West Palm Beach, Fla., has been using a Crawford incinerator since 1995 to process its wastes, as well as occasional loads of illicit drugs and weapons seized by local and federal law enforcers.

“We selected the Crawford incinerator because it quietly and efficiently destroys all materials, producing an ash that weighs 5 to 10 percent of the pretreated waste, and can be landfilled,” said Wally Thompson, a mechanical engineer and chief of facilities management at the V.A. Medical Center in West Palm Beach.

The key to the success of the Crawford unit hinges on its scrubber. V.A. representatives in West Palm Beach worked with Emcotek of Visalia, Calif., to design a scrubber for the incinerator that processes 500 pounds of waste per hour. Hot gases, leaving the incinerator at 1,900 to 2,100°F, enter the Emcotek scrubber’s primary quench tank. Spray nozzles apply water and sodium hydroxide to cool the gases to approximately 200°F and neutralize the hydrochloric acid generated during incineration. The gas then enters a secondary quench tank where the spray process is repeated, cooling the gases to 120 to 140°F and buffering acids with more sodium hydroxide.

The V.A. directed Emcotek to add a titanium steam coil in their incinerator's scrubber stack to reheat the cool gas stream, removing an unsightly, but otherwise harmless, plume cloud that it generated.

Grahic Jump LocationThe V.A. directed Emcotek to add a titanium steam coil in their incinerator's scrubber stack to reheat the cool gas stream, removing an unsightly, but otherwise harmless, plume cloud that it generated.

The piping system contains pH probes connected to a' programmable logic controller. The PLC controls two positive displacement pumps that inject the amount of sodium hydroxide needed to neutralize the acid waste.

The quenched gases enter a rotary atomizer chamber where a gearbox pumps water into the center of a rotating disc that creates a radial curtain of water. This curtain provides a high-energy wet scrubbing action that reduces particulates to approximately 0.015 gram per dry standard cubic foot of air or better.

A bank of demister filters removes excess water droplets, which can carry various heavy metals and particulate matter, before the gas stream is exhausted through the stack. The Emcotek scrubber removes 95 to 99 percent of acids, heavy metals, dioxin, and various organic compounds from the gas stream. The scrubber’s performance is monitored by various sampling probes installed in the discharge stack.

Because of the emissions standards for Palm Beach County, the V.A. requested that Emcotek add a titanium heat exchanger to lower the temperature of the water that feeds the rotary atomizer to 80 or 85°F to optimize the removal of heavy metals.

As is the case with many incinerators that meet environmental specifications, the West Palm Beach facility gave consideration to aesthetics. “We also had Emcotek add a titanium steam coil in the scrubber stack to reheat the cool gas stream, which is saturated, to remove an unsightly, but otherwise harmless, plume cloud,” Thompson said.

Copyright © 2000 by ASME
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