Posted on September 08, 2022
The disposal of municipal solid waste (MSW) is a nationwide challenge. According to the EPA, total MSW in 2018 amounted to 292.4 million tons. That’s almost five pounds per person per day!
Less than a third of this waste gets recycled or composted. Fully half goes to landfills. A key component of MSW is the ash left over from incineration. It is commonly referred to as incinerator bottom ash (IBA).
Ultimately, the use of IBW in concrete diverts the material from landfills. When properly processed, the ash can actually enhance concrete’s desirable qualities. In one study, researchers found that 5% IBA improves strength while reducing permeability.
SCMs & Emissions: An Overview
The production of Portland cement is a carbon-intensive process. Supplementary cementitious materials (SCMs) reduce the percentage of Portland cement in concrete. In turn, this reduces the industry’s carbon emissions.
Coal fly ash has been a popular SCM for some time now. Ash from municipal incineration is another potential SCM. However, contaminants pose a problem. Incinerator ash may contain heavy metals, chlorides, sulfates and organic compounds. In concrete, these impurities can inhibit curing, reduce compressive strength and cause volume expansion.
SCMs have been prominent in high-profile projects for some time now. Consider the concrete used at One World Trade Center and its companion towers. Various admixtures included fly ash, slag cement, and silica fume. The massive Tappan Zee Bridge is another example. Its two cable-stayed decks are 3.1 miles long, and its eight towers are 419-ft high. SCMs replaced 55-65% of the Portland cement (OPC) in the concrete used. For some concrete projects, SCMs now replace as much as 70-80% of the OPC.
Incinerator Ash: Pros
The key benefit of processing incinerator bottom ash so it is a safe SCM is clear. It reduces the use of Portland cement which lowers the industry's carbon emissions.
Diverting incinerator ash to concrete plants also keeps it out of the nation’s landfills. The Environmental Protection Agency (EPA) maintains a database of these sites. There are more than 2,600 landfills receiving municipal solid waste (MSW). As of March 2022, Pennsylvania has 38 operational facilities.
Incinerator Ash: Cons
The possible presence of contaminants is IBA’s biggest drawback. Common heavy metals found in IBA include gold, silver, platinum, copper, stainless steel, zinc and lead. Proper removal safeguards human health and the environment.
Various methods address the problem of contaminants. One study suggests that biotreatment reduces the concentration of copper and cadmium by 75% and 69%, respectively. However, biotreatment is not effective in immobilizing zinc. Microbiologically induced calcium carbonate precipitation (MICP) is another option. It induces calcium carbonate precipitation to reduce the impact of heavy metals.
In general, SCMs are not always available in the quantities required in a given locale. Long-distance transport is not always a viable option. Transporting IBA comes at a price, both economically and environmentally. Consider the experience of the precast concrete sector, and its embrace of fly ash from coal-fired power plants. As plants close, supplies dissipate. This map shows the uneven distribution of the remaining coal-fired power plants.
Reducing IBA contaminants
Fortunately, incineration does not alter the properties of metals commonly found in IBA. Consequently, recoverable metals can replace virgin metals, preserving natural resources and reducing emissions. For example, by 2016, European metal recycling from bottom ash was reducing GHG emissions by an estimated 3.2 million tons per year.
Recycling rates in modern IBA processing facilities can exceed 80%. Methods like the use of eddie current (EC) separators recover metals which are then sold to refiners. There are both wet and dry IBA processing technologies. Crushing ash residues may further enhance metal recovery. Dry processing requires 4-6 weeks of aging to reduce moisture content below 10%. Some fine metals get lost in a dry recovery process.
Research also demonstrates the promise of electrodialytic remediation (EDR). It uses modest electric currents to separate suspended particulates into different electrolyte solutions. Acidification releases ionized metals that migrate in the electric field. EDR recovers valuable metals while also removing soluble salts. When combined with other methods, EDR makes incinerator ash a more viable SCM.
Case Studies
One company already specializes in processing IBW across three continents. Meanwhile, a South Korean company plans a pilot project to demonstrate the viability of IBA as an SCM.
Blue Phoenix Group
Pasadena-based Blue Phoenix Group is the world’s largest ash processor. It specializes in processing IBA from the U.S. to Australia, and from the U.K. to Europe. In the U.S., it is dedicated to unlocking the potential of IBW as a recycled aggregate. In the process, the company recovers valuable raw materials that keep them out of landfills.
Ash emanating from U.S. energy-from-waste plants is often a mix of bottom ash, fly ash, and boiler ash. Processing this combined ash presents special challenges. For example, the fly ash makes the combined ash fine and compact. This inhibits the efficacy of standard processing equipment. In response, Blue Phoenix has developed solutions customized for this combined ash. For example, advanced ballistic processing ensures “recovery of a majority of all ferrous and non-ferrous metals.”
Pilot Project by SK Ecoplant
In July 2022, SK Ecoplant announced plans to launch an IBA pilot project. The goal is to use up to 60% IBA to replace Portland cement. The plan is to use IBA to induce an exothermic reaction in the presence of expansion-reducing reactants and non-sintered inorganic binders. This will increase compressive strength. Ecoplant believes its product will be price competitive.
About PACA
The Pennsylvania Aggregates and Concrete Association (PACA) monitors industry innovation worldwide. PACA commits to promoting technologies that move the industry toward carbon neutrality.
For answers to your questions about your upcoming concrete project, please contact us.
