May 24, 2006 | General

Turning Up The Heat On Sludge Recycling

BioCycle May 2006, Vol. 47, No. 5, p. 52
Wisconsin company generates glass feedstocks from paper mill sludge and municipal biosolids that have beneficial construction applications – reusing more than 350,000 tons since 1998.
David Blurton

DEFINED as a thermal process for converting minerals into glass, vitrification is an emerging technology for treating wastewater residuals. The process has a well established track record in the glass manufacturing industry and slagging furnaces used in coal-fired power generation.
Wastewater treatment residuals, such as paper mill sludge and municipal biosolids, possess characteristics common to both glass manufacturing and power generation, and therefore play two important roles in the vitrification process. First, the organic fraction provides the thermal energy required to accomplish vitrification. These organics are essentially a biomass fuel that is renewable through the cycle of water use and wastewater treatment. Secondly, the mineral fraction (ash, clays and mineral fillers) melts into a glass aggregate product with multiple beneficial construction and industrial applications.
Until recently, commercial vitrification of waste materials was limited to small-scale hazardous waste applications. In 1998, commercial-scale vitrification of high-volume industrial wastewater treatment residuals emerged with Minergy’s Fox Valley Glass Aggregate Plant (FVGAP) located in Neenah, Wisconsin.
FVGAP was developed partially in response to a request by Wisconsin’s Winnebago County Solid Waste Management Board, which was seeking to recycle sludge from area paper mills instead of consuming valuable landfill capacity. FVGAP receives and processes 350,000 tons of sludge per year from eight area paper mills. Energy from the process is recovered and converted into steam, which is sold to several adjacent paper mills. To date, FVGAP has processed more than 2.2 million tons of paper mill sludge.
The large-scale and configuration of the FVGAP is somewhat unique to meet the needs of the region, with its multiple paper mills and the close proximity to large steam users.
The first phase of the process includes receipt of the sludge from the various suppliers. Sludge is delivered to FVGAP in dewatered form, in the range of 25 to 45 percent solids (by weight). The sludge is composed of both organic and mineral (ash) material.
The second stage of the process includes partial drying of the sludge. Drying limits the amount of moisture included in the vitrification process, thereby reducing the physical volume of the system and maintaining high processing temperatures. FVGAP utilizes rotary steam tube dryers. Selection of equipment for other projects would be dependent on the characteristics of the sludge to be processed and the overall thermal cycle of the project.
In the next step of the process, the sludge passes from the drying system directly into a pair of cyclone boilers. In the cyclones, the organic component of the sludge is completely combusted, liberating a significant amount of heat energy. This heat, along with the heat from the cofired coal, results in temperatures of approximately 3000°F. At these high temperatures, the mineral component of the sludge melts and flows out of the processing system as molten glass. In addition, the high-temperature environment provides superior destruction efficiencies of any organic contaminants that may be contained in the sludge.
Finally, the molten 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 similarly to conventional quarried aggregates.
Minergy’s philosophy on the beneficial reuse of glass aggregate produced from vitrification of industrial wastes is to concentrate marketing efforts on high-volume applications. Experience has thus far shown that obtaining and maintaining a small market share of multiple high-volume markets is easier than dominating one or two high-value markets.
The process of water quenching the molten glass results in the formation of an environmentally inert aggregate. During the quenching process, heavy metals that may be present in the molten glass are physically sequestered into the glass matrix, resulting in very low leaching potential.
The Wisconsin Department of Natural Resources has granted Minergy three separate Conditional Grant of Solid Waste Exemptions for glass aggregate produced from the vitrification of paper mill sludge, municipal biosolids and contaminated sediments/soils. Minergy has also received Beneficial Use Determinations (BUD) from regulatory agencies in Illinois and Michigan for use of glass aggregate produced from vitrification of industrial sludges and municipal biosolids. Approved uses identified in the exemptions and BUDs include roadbed construction, blended cements/pozzolan substitute, construction backfill, blasting media, roofing shingles and asphalt pavement.
FVGAP has been successful at beneficially reusing more than 350,000 tons of glass aggregate produced since beginning commercial operations in 1998 by pursuing the local high-volume, low-value market. One market sector into which glass aggregate has recently been accepted is hot mix asphalt (HMA) paving. Ten percent FVGAP glass aggregate was incorporated into a mix design, which was verified by the Wisconsin Department of Transportation (WisDOT) and used on WisDOT and private projects. Because of the grading and angularity of the glass aggregate, it was primarily used as a substitute for the washed manufactured sand component of a typical mix design.
Test results indicate that substitution of the washed manufactured sand with Minergy’s glass aggregate generally resulted in an increase in the amount of recycled asphalt pavement (RAP) that could be included in the mixes and a decrease in the amount of natural sand needed. This is a result of the high angularity of the glass aggregate, which helps to build beneficial voids in mineral aggregate into the mix. Increased RAP contents and general coarsening of the sand fraction of the mixes resulted in added asphalt binder contents that were up to 0.5 percent lower than comparable designs using washed manufactured sand. In addition, the incorporation of the glass aggregate and increased RAP contents produced HMA mix designs that contained up to 37 percent recycled materials.
Significant interest in FVGAP led Minergy to develop a second generation vitrification technology that is applicable to individual on-site processing of high-volume waste streams to meet the economic and environmental needs of industry and municipalities. Called GlassPack®, the system is a combination of two innovative concepts. The first is a patented closed-loop oxygen enhanced combustion process that uses enriched oxygen to achieve temperatures that support vitrification, which eliminates the need for co-fire fuel, and provides complete destruction of organic compounds.
The second innovation is the GlassPack modular melter concept. The melter is a shop-fabricated unit that can be delivered to the construction site on a single truck shipment. The entire process is highly modularized to minimize footprint, field installation costs and construction schedule. The first commercial GlassPack application, designed to vitrify municipal biosolids into glass aggregate at the North Shore Sanitary District’s new Sludge Recycling Facility located in Zion, Illinois, is expected to be placed into service later this summer. This facility will be the first of its kind in the world and will convert 187 wet tons of municipal biosolids per day into glass aggregate.
Vitrification technologies can help municipalities and industry meet long-term sludge disposal and environmental goals. For example, by recycling the sludge into a useable product instead of placing it in a landfill, the Fox Valley Glass Aggregate Plant has saved more than 57 acres of county landfill space and eliminated more than 2.5 million local truck miles. Put into perspective, if one were to pile this amount of material on a football field, it would be a more than 1,000 feet tall.
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 landfill disposal, providing a cost-effective alternative for managing sludge, and offering a more comprehensive and integrated approach to solid waste management.
Dave Blurton is with Ideas That Deliver based in Neenah, Wisconsin.

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