In Fiscal Year (FY) 2015, the SWANA Applied Research Foundation’s (ARF) Waste-to-Energy (WTE) Group identified the topic of researching innovations in WTE ash management. In particular, two innovations were of interest to the group:
- New technologies that are being used to enhance the recovery of ferrous and non-ferrous metals—as well as enable the recovery of precious metals and rare earth metals—from WTE ash.
- A new method of WTE ash management that is being tried in the US—namely, the controlled mixing of WTE bottom ash with fly ash to render the mixture non-hazardous and the processing of the remaining bottom ash for metals and minerals recovery.
The findings of the ARF research regarding these two innovations are summarized in this article.
New Technologies to Enhance Metals Recovery From WTE Ash
The 77 WTE facilities in the US process almost 30 million tons of municipal solid waste (MSW) each year and generate more than 14.3 million MWh of electricity for sale (Michaels and Shiang 2016). The roughly 7 million tons of ash generated as a result of the combustion process, along with the non-combustible materials in the waste and the solid materials collected through the air pollution control (APC) systems, are collectively referred to as WTE ash.
WTE ash is comprised of the following two residue streams (Millrath et al. 2012):
- Bottom Ash: Bottom ash is the material that either falls through the furnace grate (grate siftings) or remains on the grate (grate ash) after the waste is combusted. Grate ash is discharged into a water quenching tank. Grate siftings consist of fine particles that fall through the furnace grate during the combustion process. Bottom ash also includes heat recovery ash that is collected in the heat recovery system of the facility, which includes the boiler, economizer, and superheater and is discharged into the bottom ash system. Bottom ash is generated at a rate of 0.20 to 0.25 tons per ton of waste processed through a WTE facility (Crillesen and Skaarup 2006).
- Fly Ash: Fly ash refers to ash that becomes entrained in the flue gas. The residues from the APC system include pollutants, reaction byproducts, condensates, and unreacted chemicals as well as the fly ash. (Lime, activated carbon, and other chemicals are used to remove pollutants from the combustion flue gas before it is emitted to the atmosphere.)
Bottom ash is, by far, the larger of the two ash streams—typically representing 80 to 90% by weight of the ash produced by WTE facilities (Bourtsalas 2015). Bottom ash is similar in appearance to a porous, grayish, silty sand/gravel mix and is essentially a mixture of minerals, metals, and small amounts of unburned organic materials.
A fact that is often overlooked regarding WTE facilities is their ability to recover ferrous and non-ferrous metals from products and packaging discards that are not collected in source-separation recycling programs.
In this regard, WTE facilities can play a significant role in the recovery and recycling of metals from MSW. (WTE facilities should more properly be called “Resource Recovery Facilities” as they recover both materials and energy from the waste stream.) For example, it is estimated that, despite a strong and established commitment to curbside recycling programs, approximately two thirds of all metals discarded by households in Switzerland end up in the municipal waste processed through WTE facilities (Bunge 2015).
This figure is corroborated by US data. As Figure 1 illustrates, about two thirds of the ferrous metals and almost two thirds of the non-ferrous metals generated in US communities served by WTE facilities remain in the waste after recycling and are found in the WTE ash.
One reason for this is that more than half of the metals in consumer products are small in size and are usually found in composite products along with other materials (such as plastics, ceramics, or textiles). Only metal items (such as beverage containers and silverware) that are relatively large and free of composite material are successfully recovered through source-separation curbside collection programs. Alternatively smaller metallic items, such as batteries, bottle tops, and can lids are disposed of in the household waste along with multi-materials packaging such as flexible beverage and food containers.
In the US, WTE facilities recover more than 730,000 tons of ferrous metal each year for recycling (Michaels 2014). All of these metals are recovered from the bottom ash produced at these facilities.
The following conclusions were reached regarding the conventional recovery of metals from WTE bottom ash based on this research.
- The recovery of metals from WTE bottom ash can play a significant role in a community’s recycling program. About two thirds of metals generated by residential households end up in the mixed waste mainly because they are not targeted for recovery in source-separation recycling programs.
- When metals are processed through a WTE facility, about a third are oxidized and can no longer be recovered. Only metals in their pure or native form can be economically recycled.
- Even after accounting for metals oxidation, the recycling of metals from WTE bottom ash can account for more recycling tonnages than are typically diverted through source separation recycling programs.
- Over 25% of WTE ash can consist of metals in both oxidized and pure forms. Recoverable metals greater than 2 millimeters in size typically represent about 11% of the WTE ash by weight or 2.5% of the MSW combusted.
- Conventional WTE ash processing systems typically target the recovery of native metals greater than 12 millimeters (0.47 inches) and recover about 50 pounds of metals per ton of waste combusted.
A primary purpose of the ARF research was to provide useful and timely performance and cost information regarding new technologies that are being used to enhance the recovery of ferrous and non-ferrous metals—as well as enable the recovery of precious metals and rare earth metals—from WTE bottom ash.
In this regard, it is noteworthy that metals smaller than 7 millimeters could not be separated from WTE bottom ash until a few years ago. Conventional WTE bottom ash metals recovery systems typically target ash particles greater than 12 millimeters (0.47 inches) in size. By treating the fine bottom ash as well, around 90% of the pure or native metals are now recoverable (Popara and Kyriakidou 2013). These recovery rates are significantly higher than the 80% and 20% rates that are typically achieved for ferrous and non-ferrous metals recovery respectively using conventional metal recovery systems.
There are two approaches that are being used on a commercial scale to achieve these high recovery rates.
- Wet Ash Dry Processing Systems: The vast majority of WTE facilities throughout the world quench their bottom ash following the combustion process. For this reason, there is significant interest in advanced metals recovery systems that are designed to process wet ash. Dry processing systems that rely on the physical properties of the wet ash particles—such as particle size and density—are used to process this wet bottom ash.
- Dry Ash Dry Processing Systems: A handful of WTE facilities have converted their ash processing systems from wet systems to dry systems that use air to cool the ash. Currently, there are several facilities in Japan, and three plants in Switzerland that utilize the dry discharge method.
When WTE bottom ash is quenched in a wet ash processing system, the metal oxides formed through oxidation react with water, which results in the metals being coated with ash. This inhibits their recovery by magnets and reduces the value of the recovered metals (Szczepkowski 2013).
This problem can be avoided if the WTE bottom ash is cooled with air rather than water and is kept dry. Since dry bottom ash is a free flowing material, it provides for the optimal separation and recovery of metals and minerals from the ash mixture. As a result, it enables higher capture of metals—especially non-ferrous metals—and the potential for recovering precious metals such as gold and critical metals such as germanium and tellurium (Ammann 2011). Also, since no water is added to the ash, it weighs roughly 20% less than wet ash and, as a result, incurs lower hauling and disposal costs.
For these reasons there is a growing interest in the use of dry processing systems in some European countries. To date, however, only a small number of WTE facilities in Europe use the dry processing approach (Bunge 2015).
The five advanced metals recovery systems reviewed in the ARF research project are summarized in Table 2. One system—the Inashco system—is described below.
Inashco Wet Ash Metals Recovery System
Inashco is a Dutch company located in Rotterdam, the Netherlands, which was founded in 2008 to offer a patented technology developed by the Technical University of Delft for recovering and upgrading non-ferrous metals from the fines fraction of WTE bottom ash. Inashco currently has over 250 employees and operates over twenty-five ash treatment facilities that collectively treat over 5 million tons of WTE bottom ash per year.
The majority shareholder of Inashco is Waterland Private Equity Investments, while the Fondel Group, the Delft University of Technology as well as the senior management are still involved as minority shareholders. Fondel, a global supplier of raw materials to the stainless steel, aluminum, and base metal refining industries with over 50 years of experience in handling both primary and secondary metals is the exclusive trader of Inashco’s metal products while the Delft University of Technologies serves as Inashco’s R&D partner.
The Inashco WTE ash recycling system is an advanced metals and aggregates recovery system that targets non-ferrous metals found in the fines in WTE bottom ash. As shown in Figure 2, the system consists of two components: (1) an Advanced Dry Recovery (ADR) system located at or near the source of the WTE bottom ash, and (2) a Centralized Upgrading Facility (CUF) located in the Netherlands for separating the non-ferrous concentrate from the first component.
The “Advanced Dry Recovery” (ADR) technology was developed by the Delft University of Technology to separate the sticky, wet fine mineral fraction—which encapsulates valuable fine metal particles—from the coarser fraction of the bottom ash. The coarser fraction is then processed by conventional eddy current separators to recover non-ferrous metals from the mineral aggregates. Together, with conventional eddy current separators, the ADR produces a fine non-ferrous concentrate and a clean marketable mineral aggregate.
The ADR is designed to process the ash fraction smaller than 12 millimeters (0.5 inches) in size. As discussed above, this size fraction has been found to contain over 50% of most metals contained in the ash and is typically not targeted for metals recovery in conventional ash metals recovery systems. The ADR system can treat ash with very high moisture content and is able to recover metal particles down to 0.5 millimeter in size.
Prior to being processed through the ADR, the wet bottom or combined ash is screened to recover particles typically larger than 12 mm in size. Ferrous and non-ferrous metals are then recovered from this fraction using conventional equipment such as ferrous magnets and eddy current separators to produce one sub stream: +12 mm non-ferrous metal scrap.
The remaining fraction—less than 12 millimeters (0.5 inches) in size—is then processed through the ADR system, which relies on physical processes to produce four material sub streams:
- Ferrous metal concentrate;
- -12 mm Non-ferrous metal concentrate;
- -2 mm mineral products; and
- -12 mm mineral products.
Further product upgrading of the non-ferrous metal concentrate is performed in the Netherlands by Inashco’s proprietary Central Upgrading Facilities to create metal blends that maximize the value of recovered heavy and light non-ferrous metals.
A major advantage of the ADR technology is that it can be implemented as an extension to existing wet ash processing systems that rely on conventional ash processing equipment (screens, magnets, and eddy current separators) and can, as a result, improve the non- ferrous metal recovery from the ash by 50 to 100%.
The ADR technology also provides WTE facility owners with the capability of processing bottom ash which in most cases has not been aged. While aging reduces the moisture content, it also reduces the amount of aluminum in the ash due to the oxidation of aluminum into aluminum hydroxide. The ADR technology can handle high moisture content ash directly from the combustion ash quench tank, thereby reducing the need for intermediate storage of the ash to allow drying and enabling the recovery of more aluminum.
Aggregates recovered from the fine fraction of the WTE bottom ash by the ADR technology can be substituted for natural aggregate in concrete production, thereby reducing concrete costs. In this regard, an independent committee in the Netherlands has investigated the use of aggregate recovered from bottom ash in concrete since 2008. Based on the results of tests performed under the authority of the committee, up to 50% of aggregates in concrete applications without reinforcement may be substituted with treated bottom ash aggregates while up to 20% of the aggregates may be substituted in concrete utilized with reinforcement.
Case Study: LCSWMA—Inashco
In May, 2016, the Lancaster County Solid Waste Management Authority (LCSWMA) entered into a long-term contract with Inashco North America Inc. to construct a $14 million facility next to the Frey Farm Landfill that will recover a variety of metals from waste-to-energy ash.
LCSWMA owns two waste-to-energy facilities (located in Bainbridge and Harrisburg, PA) that burn MSW—producing enough renewable energy to power the equivalent of 45,000 area homes and businesses. After combustion, the remaining ash from both power plants is transported to LCSWMA’s Frey Farm Landfill in Conestoga, PA where it is used as an alternative daily cover for the landfilled waste.
The two WTE facilities owned by LCSWMA currently use in-line metal recovery systems that target larger metals pieces (greater than 12 millimeters) for recovery. As described above, the ADR system that will be constructed at the landfill recovery system will be designed to recover pebble-sized metals from the ash, down to 0.5 millimeters.
The Inashco ADR system will be housed in a facility roughly 100,000 square feet in size and will be designed to process around 178,000 tons of ash generated annually from LCSWMA’s two WTE facilities. It is anticipated that about 8,000 tons of metals—equal to 4.8% of the weight of the ash processed—will be recovered by the ADR-system each year. Construction of the facility is projected to start in 2017 with full operations beginning about a year later.
As indicated below, Inashco will pay LCSWMA a service fee per ton of ash processed. LCSWMA will also receive a percentage of the net revenue earned by Inashco from the sale of the recovered metals.
The LCSWMA executed three contractual agreements with Inashco to enable the implementation of the Inashco ash processing facility at the Frey Farm Landfill. Key terms in these agreements include the following:
- The term of the agreement is ten years with two available five-year renewal periods.
- LCSWMA must deliver a minimum of 165,000 tons per year of Acceptable Ash to the Inashco facility.
- To qualify as Acceptable Ash, ash must meet the following requirements:
(1) Particle size is 3 inches or less
(2) Unburned materials are less than 6% of ash by weight, and (3) Moisture content may not exceed 30% by weight.
- Inashco must process a minimum of 125,000 tons per year and use commercially reasonable efforts to maximize the amount of metals recovered through processing.
- Inashco will pay LCSWMA a Service Fee per ton for all Acceptable Ash delivered to the Facility. In addition to the Service Fee, LCSWMA will receive a percentage of the net revenues from the project.
- LCSWMA must complete site grading and other work for the facility and obtain a permit to expand the Frey Farm Landfill.
- Inashco will be responsible for securing all necessary permits to construct and operate the Facility. LCSWMA will provide support on the permitting efforts.
- Inashco and LCSWMA will use commercially reasonable efforts to achieve the Commercial Operation Date by the 425th day after Commencement
The Inashco system is designed to process 178,000 tons of ash per year from both WTE facilities owned by LCSWMA and recover an additional 8,000 tons of metal, or 4.8% of the ash processed. The Inashco facility will enable the ash metals recovery rate to be increased by 46%—for a total metals recovery rate of 15.3% of the ash processed.
The following conclusions were reached regarding the enhanced recovery of metals and minerals from WTE bottom ash:
- Advanced metal recovery systems can improve the metal recovery rates from WTE bottom ash by targeting metals that are less than 12 millimeters (0.47 inches) in size and utilizing new technologies that have been recently developed. These systems can increase the metals recovery rate from 11% to over 15% of the bottom ash.
- There are two distinct approaches that are used by advanced system—namely, (1) the processing of wet bottom ash, or (2) the conversion to a dry ash discharge system and the subsequent processing of dry bottom ash.
- Regarding the wet ash processing approach, a contract has just been signed between the LCSWMA and Inashco for the implementation of an ash processing facility at the LCSWMA’s Frey Farm Landfill. This facility will increase the ash metals recovery rate from 10.5 to 15.3% and will provide additional revenues to the LCSWMA over the 10-year contract period.
- The dry ash processing approach has been implemented in a number of WTE facilities in Japan and three facilities in Switzerland. In this regard, there appears to be a growing interest in this approach in Europe where the high costs of landfill disposal of ash can be offset by the 20% weight reduction and cost savings that can be realized by keeping the ash dry. On the other hand, there is no evidence to date of similar interest of US WTE systems in converting to dry ash discharge.
Separate Management of Bottom and Fly Ash
Most of the WTE facilities in the US have been designed so that fly ash and bottom ash are mixed and managed as a combined ash. The reason for this is that the combined ash tests non-hazardous and can be disposed as a non-hazardous waste while fly ash—if not mixed with bottom ash—may test hazardous and has to be managed as a hazardous waste.
In May 1994, the US Supreme Court ruled that WTE ash was not exempt from testing to determine whether or not it is a hazardous waste. The test required under the federal “Resource Conservation and Recovery Act” (RCRA) regulations to determine the “toxicity characteristic” of a waste is called the “Toxic Characteristic Leaching Procedure,” or TCLP. Years of testing combined ash from US WTE facilities have confirmed that the combined ash is a non-hazardous waste that can be managed and disposed of accordingly.
While bottom ash and fly ash are mixed together and managed as combined ash in the US, these streams are managed separately in Europe. Fly ash is listed as a hazardous waste in the European Union’s European Waste Catalogue. As a result, fly ash is managed as a hazardous waste and is treated to minimize release of contaminants before being landfilled or used as fill in coal or salt mines. In contrast to US practices, bottom ash from many European WTE facilities is beneficially reused.
As described below, a WTE facility in Pasco County (Florida) has implemented a new approach to WTE ash management that enables the recovery of most of the bottom ash for beneficial reuse while avoiding the need to manage fly ash as a hazardous waste. This approach involves mixing a portion of the bottom ash with fly ash to render the mixture non-hazardous and then processing the remaining bottom ash for the recovery of metals and aggregates.
Following the conduct of a successful pilot test program, on December 5, 2014, Pasco County, FL, received the first permit ever issued by a state government authorizing the use of WTE bottom ash as an aggregate in road construction.
Concurrent with this development, the County implemented a new method of ash management—namely, the screening of its WTE bottom ash into two fractions (less than 3/8 inch and greater than 3/8 inch) and only mixing a portion of the less than 3/8-inch bottom ash with the fly ash from the WTE facility to render the mixture non-hazardous. This new ash management approach essentially frees up the major portion of WTE bottom ash produced at the Pasco WTE facility for reuse as construction aggregates.
This new approach to WTE ash management in Pasco County eliminates the need to manage fly ash as a hazardous waste as is done in Europe and simultaneously allows the County to pursue many of the WTE bottom ash reuse strategies that have been used in Europe for many years.
The new Pasco County WTE bottom ash reuse program is described below.
The Pasco County Solid Waste Resource Recovery Facility began commercial operation in May 1991. The facility processes up to 1,050 tons per day (TPD) of MSW and generates 31.2 megawatts of renewable energy that is sold to Duke Energy.
On August 4, 2014, the County submitted an updated version of its Ash Management Plan to the Florida Department of Environmental Protection. According to the updated plan, the facility generates approximately 250 TPD of ash (approximately 25% of the processed waste), which is disposed in an adjacent ash monofill (Figure 3).
In early 2014, the Pasco County Solid Waste Department began a research project to investigate the potential for beneficially using ash from the Pasco County Resource Recovery Facility in road construction applications.
With the issuance of a “Research, Demonstration and Development” (RD&D) permit from the Florida Department of Environmental Protection, the Pasco County Road and Bridge Department constructed a 1,000-foot series of roadway test sections using WTE ash on the site of the West Pasco Landfill. Three roadway sections—each 200 feet long by 25 feet wide—were constructed, two with WTE bottom ash used as a partial aggregate replacement in asphalt and concrete pavements and a third where the ash was used as the base course layer for the road. Fifteen groundwater monitoring wells were installed on the Pasco County Solid Waste property and two months of data were collected to determine if there was any impact on local groundwater metals concentrations due to the roadway.
Based on extensive testing already performed by the University of Florida, no environmental impacts were anticipated or found as a result of this project.
Following this test program, Pasco County received a “Department Order” (Order) issued by the state of Florida’s Department of Environmental Protection (Department) on December 5, 2014, which approved the use of WTE bottom ash from the Pasco County WTE facility in County road construction projects under the conditions specified in the Order.
Approval of the beneficial use of WTE bottom ash for road construction in Pasco County represents the first approval of its kind in the state.
Following the receipt of the permit from the State to reuse bottom ash, Pasco County modified its WTE ash management approach to one where only enough bottom ash with fly ash to render the mixture non-hazardous. According to the WTE plant manager, the ash is mixed using a ratio of 25% bottom ash to 75% fly ash on a weight basis.
As indicated in Table 3, the Pasco County WTE facility processes 1,050 tons of MSW per day or 335,000 tons per year, generating 23.5% of the input waste ash—or 78,725 tons per year—as WTE ash. Of this amount, 20%—or 15,745 tons—is comprised of fly ash.
Based on the 25% bottom ash/75% fly ash ratio described above, a total of 5,428 tons of bottom ash are needed to render the fly ash/bottom ash mixture non-hazardous. After diverting 10% of the remaining bottom ash through metals recycling, this leaves a total of almost 52,000 tons per year—or 83% of the bottom ash generated—available for reuse.
Incorporating ash reuse into county road construction will result in costs savings due to the diversion of WTE ash from disposal as well as savings in the purchase of mine aggregates (in this case, lime rock) for road construction. The County estimates that the potential cost savings should be in the neighborhood of $50,000 to $100,000 per mile of two-lane road constructed.
As a result of the County’s research program, it is estimated that the construction of five miles of road base would require 200 days of usable bottom ash from the facility. This estimate is confirmed by the calculations presented in Table 4.
Based on this estimate, it is possible that all of the available WTE bottom ash could be used by the County Road and Bridge Department to meet the annual road construction demands in the County.
In this regard, County Administrator Michele Baker has stated that she looks forward to seeing the WTE bottom ash reuse process applied in future projects. “By being able to reuse this ash product as part of road paving that allows us to go full circle on the recycling stream,” Baker says, “It really just completes the whole cycle of that reuse of the waste.”
It has been recently reported that Pasco County is currently beneficially using ash in their capital improvements projects and is moving forward with efforts to mine WTE ash from their existing landfill with the objective of beneficially reusing the mined material and recovering an additional fraction of the remaining metals.
In this regard, it appears that the County is finalizing plans for the construction of a road for commercial use with WTE bottom ash. To date, however, no additional road construction has been completed other than the test road at the West Pasco Landfill as there appears that there is some reticence on the part of local road builders to incorporate WTE bottom ash in road construction.
One of the possible reasons for this is that the logistical and operational costs associated with the aging of bottom ash could be significant when conducting a full-scale construction project. As indicated above, the State of Florida requires that the ash be aged for three months before it is used in road construction. Another State requirement is that it can only be stored on site at batch plants for a period of no more than 120 hours.
Perhaps the main reason for the lack of progress in the commercial application of WTE bottom ash in road construction is the widespread availability of low cost primary aggregate which has more predictable and familiar structural and handling characteristics.
Despite the slow commercial application of the use of WTE bottom ash in Pasco County, other WTE communities in Florida—including Miami-Dade County, Hillsborough County, and Palm Beach County—have expressed interest in the reuse of their bottom ash in road construction.
The following conclusions were reached with respect to the separate management of fly ash and bottom ash in the US and Canada.
- The issuance of a WTE bottom ash reuse permit by the State of Florida is a watershed moment regarding WTE ash management in North America. By only mixing enough bottom ash with fly ash to render the mixture non-hazardous, the majority of WTE bottom ash that is currently disposed in landfills in North America can be diverted and reused as has been done in Europe for many years.
- The WTE bottom ash reuse project in Pasco County has the potential to divert an additional 52,000 tons of waste from landfill disposal each year. If this ash is used in road construction, it would mean that almost 94% of the waste processed through the County’s WTE facility would be diverted from landfill disposal and recovered in the form of energy or reusable materials and metals.
- The reuse of WTE bottom ash for road construction has the potential of providing cost savings—in the form of reduced construction aggregate costs and avoided tipping fees—as well as the environmental benefits of avoiding the mining and use of primary construction aggregates.
- The road departments of the county governments that have implemented WTE facilities have the potential to reuse all of the WTE bottom ash aggregate that can be produced by these facilities in local road construction projects.
- There may be some reticence on the part of local road departments to use WTE bottom ash as a construction due to the specific requirements associated with this activity including the need to stockpile the ash for three months before reuse. WTE facility owners will have to work to overcome this inertia as well at to get their states to follow Florida’s lead in approving the use of WTE bottom ash in road construction.
This article describes the successful implementation of two new innovations in the North American WTE marketplace—namely, the recovery of metals from the small size fraction of WTE bottom ash and the permitted reuse of bottom ash in road construction projects implemented in communities that host WTE facilities.
These innovations further demonstrate the ability of WTE facilities to recover both energy as well as materials—in the form of metals and minerals—from the waste stream and the important role that WTE can and should play in diverting waste from landfill disposal in many communities.
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