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North Shore Sanitary District (NSSD) is a unit of local government that serves the eastern half of Lake County, Illinois. NSSD serves approximately 350,000 people in a rapidly developing service area that encompasses 112 square miles. Three advanced wastewater treatment plants are owned and operated by NSSD, and treat approximately 54 million gallons per day of wastewater on average. On a maximum weekly basis, NSSD generates 187 wet tons of sludge per day. Prior to this project, the sludge was mixed with fly ash and then disposed of in a NSSD owned landfill.

NSSD began researching biosolids disposal alternatives several years ago. The typical disposal alternatives (land application, landfilling, and incineration) were evaluated on their merits. NSSD concluded each of the alternatives had significant shortcomings from an environmental and/or financial prospective.

Following an evaluation of “outside the box” alternatives, NSSD determined the best alternative would be a biosolids drying and vitrification process. This process would yield a glass aggregate product that could be used in our society without any potentially harmful environmental issues associated with it.

NSSD has successfully implemented the most environmentally sound biosolids disposal method ever developed. Each day, the facility could convert up to 200 tons of sludge into 7.5 tons of reusable glass aggregate.

NSSD is the first municipality in the United States to construct a sludge drying and vitrification facility. The cyclone furnace and the fluidized bed dryer are the first municipal installation of its type in the United States.

This project is unique because the sludge is transformed into a reusable, marketable product. The glass aggregate offers no risk of soil or groundwater contamination since the microorganisms, including bacteria and viruses, are destroyed through the drying and vitrification processes. The trace metals and other inorganic materials associated with biosolids are “locked” into the glass matrix. NSSD has a 20-year agreement in place to sell the glass aggregate, which is permitted for use in trench backfill and concrete products. Other potential uses include asphalt paving, roofing shingles, and ceramic floor tile.

This process has tremendous potential for larger municipalities that are land use restricted or want the most environmentally sound disposal method available.

The sludge is transformed through a unique closed loop drying and melting process. The process begins when dewatered sludge is delivered to the facilities via truck. The sludge is stored in one of two receiving bins, and then pumped from the storage silos to the fluidized bed dryer, where it is heated to 482oF. The biosolids leaving the dryer (granulate) contain approximately 5 percent moisture.

From the dryer, the biosolids is fed into the vitrification process, which has a capacity of 35 tons per day. The melter has a three-zone operation comprised of separate but interconnected chambers. In the first, melting zone, dried biosolids is injected along with synthetic air. The organic component of the biosolids is used as fuel, releasing a significant amount of heat energy. Melting zone temperatures are 2,600°F to 3,000°F.

At these temperatures, the inorganic (mineral) component forms a pool of molten glass at the bottom of the zone chamber. In the second zone, the molten glass drains to the bottom and is dropped into a quench tank and cooled into the glass aggregate product. Hot combustion gases within the zone are directed into the third zone, where it is cooled with lower temperature recirculated gases.

No auxiliary fuel is needed to operate the dryer or melter system. During startup, natural gas is used to heat the melter to the required temperature before sludge can be introduced. Approximately 16.5 mmBtuh of energy is recovered from the vitrification loop to provide the heat required to dry the sludge.

Through the vitrification process, the metals contained in the sludge are permanently stabilized within the glass matrix and does not leach into the environment. Any of the widely accepted disposal alternatives has more potential environmental issues than vitrification. The facilities have highly effective air emissions control devices, resulting in maximum ambient concentrations that are below ambient air quality standards. For mercury emissions, NSSD has voluntarily agreed to keep levels below the detectable threshold. Based upon this parameter, NSSD would discharge less than 1.77 pounds of mercury per year.

The vitrification process offers many environmental and economic benefits to municipal wastewater treatment plant owners. These benefits include eliminating long-term dependence on landfill disposal, providing agencies with a cost-effective alternative for managing biosolids, and providing public agencies with a more comprehensive and integrated approach to solid waste management.
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Document Type: Research Article

Publication date: 2007-01-01

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