Vitrification—a mineral recovery process—has for centuries been almost exclusively associated with converting earthen minerals into common everyday glass products, such as light bulbs, windows, bottles and fiberglass insulation. That is, until now, as commercial-scale vitrification of high-volume municipal wastewater treatment residuals has been initiated at the North Shore Sanitary District (NSSD).
Located just north of Chicago, the NSSD owns and operates more than 100 miles of intercepting sewer lines and pumping stations that collect and convey wastewater to its three treatment plants in Gurnee, Waukegan and Highland Park, Ill. As of October 2006, all three plants now send their dewatered sludge to NSSD’s new sludge recycling facility, located in Zion, Ill.
Opting for a sustainable solution
In 2001, the NSSD was faced with a decision that every sanitary district confronts sooner or later: continue its current biosolids management practice or change direction and pursue a more sustainable alternative.
At the time, the NSSD landfilled its sludge and was facing the permitting process for a landfill expansion. After an extensive search and review of the best environmental practices used around the globe, the NSSD opted for a cutting-edge bioenergy technology, developed by Minergy Corp., to vitrify, or melt, its biosolids into an inert glass aggregate, a product that can be beneficially reused by the local construction industry.
At the heart of the NSSD’s sludge recycling facility is Minergy’s GlassPack vitrification process. According to Bob Paulson, business development manager for Minergy, the GlassPack technology utilizes a patented closed-loop oxygen-enhanced combustion process to convert the biosolids into a renewable fuel source and produce the marketable glass aggregate product. Oxygen-enhanced combustion provides significant process benefits, including higher radiant heat energy, higher thermal transfer efficiency, improved ignition characteristics and greater flame stability. These process efficiencies lead to greater heat transfer to the biosolids and produce the temperatures necessary to sustain the melting process.
“The district’s choice of this vitrification process over our past practice of landfilling dewatered biosolids was due to the environmental benefits of the technology, particularly the beneficial reuse of the glass aggregate produced,” said NSSD General Manager Brian Dorn. “By recycling the district’s biosolids into an inert glass product that can be used for construction purposes, the district will preserve open lands for other uses besides landfilling sludge.”
The closed-loop process uses more than 90% pure oxygen injection to achieve melter temperatures in excess of 2,300˚F. At these high temperatures, the biosolids’ mineral component melts and flows out of the processing system as molten glass. The lava-like material is collected and cooled quickly in a water- quench system to form the glass aggregate product. In this form, the glass aggregate stores and handles just like naturally quarried sand and gravel aggregate.
The vitrification process is configured so that thermal energy liberated from the organic fraction is recovered from the melter flue gas using a conventional thermal oil heat recovery system. The recovered heat is transferred directly to the sludge drying circuit where the district’s daily production of up to 187 tons of wet sludge (17 to 20% dry solids content) is converted into 35 tons of granulate (92% dry solids content) without the need for a supplemental fuel source. When processed by GlassPack, the 35 tons of granulate feedstock will produce about 7.5 tons of glass aggregate each day.
Under normal operating conditions, natural gas is used only during a brief startup period as the unit is brought up to operating temperature. Once at operating temperature, natural gas is progressively replaced as a fuel source by dry granulate to the point when the energy released from the dry granulate is adequate to sustain the process. The sustained autothermal operation of the process is the combined result of the NSSD’s sludge quality, sludge pre-drying and closed-loop oxygen-enhanced configuration.
Emissions under control
“The GlassPack system is environmentally friendly,” Paulson said. “One of the main benefits of the closed-loop oxygen-enhanced process is exhaust volume reduction. In this configuration, the volume of air emissions from the GlassPack system is less than 10% of sludge incineration systems currently operating in the U.S.”
To illustrate the extent to which the GlassPack process reduces emissions, the facility is located in Lake County, Ill., a U.S. EPA-designated Ozone Non-Attainment Area, and was successfully permitted as a minor emission source without add-on NOx or volatile organic compound control technology.
The minimal volume of exhaust gas had added benefits, as it allowed the district to address a local community concern regarding mercury emissions.
“With the small exhaust gas volumes generated by the GlassPack process, the district voluntarily included additional mercury control technology to the plant’s design to address concerns of the local environmental community, even though it was not required by state regulators,” Dorn said.
Beneficial reuse of glass aggregate
An ancillary benefit to the process’s high operating temperatures is that organic compounds present in the sludge are completely destroyed. On the other hand, heavy metals present in the sludge are physically sequestered in the glass matrix formed by quenching the molten glass in water. The result is an exceptionally hard, environmentally inert aggregate that has many beneficial uses. The Illinois EPA has determined that the glass aggregate produced from NSSD sludge is no longer a solid waste when used in utility trench fill, road bed construction or blended cement applications.
Minergy also has received approval from state environmental regulatory agencies in Wisconsin and Michigan for the use of glass aggregate produced from biosolids in other beneficial applications, including pozzolan substitute, construction backfill, blasting media, roofing shingles and asphalt pavement.
Minergy’s philosophy on the beneficial reuse of glass aggregate produced from vitrification of wastes is to concentrate efforts on high-volume applications. Experience has thus far shown that obtaining and maintaining a small market share of multiple high-volume markets is more practical than dominating a couple of low-volume markets. Although high-volume markets typically offer low return value, it is unlikely that entering into these markets will upset existing forces of supply and demand.
Environmental benefits
Preserving open land by reducing the need to landfill has been at the forefront of the NSSD’s strategic and environmental objectives. Dorn believes the NSSD plant will be a model for other municipalities around the country and globe seeking a sustainable solution to sludge disposal.
The glass aggregate process offers environmental and economic benefits to municipal wastewater treatment systems, sludge-producing industries and surrounding communities. These benefits include reducing long-term dependence on land spreading or landfill disposal, providing a cost-effective alternative for managing sludge, and offering a more comprehensive and sustainable approach to solid waste management.
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