What is a landfill?
Prior to the implementation of the Resource Conservation and Recovery Act (RCRA) in the 1970’s, most wastes from municipal and industrial sources were dumped in unlined pits with little environmental oversight. RCRA ushered in the era of the modern sanitary landfill, a repository for “solid waste” materials that allows them to be contained in a controlled environment, with particular attention paid to the potential for groundwater contamination.
In the United States, a typical landfill is constructed in cells, with each cell consisting of a compacted clay liner built over the native soil base at least three feet above the water table. In modern landfills, a leachate collection system is installed on top of the clay liner, which is then covered by a geosynthetic liner to help stabilize the wastes. Next, a graded permeable sand or gravel drainage layer is added to help direct liquids into the leachate collection system.
During its daily operation, refuse collection trucks deposit waste materials onto the working face of the landfill, where the waste is spread and compacted to reduce the volume. At the end of each workday, the open working face is covered with a daily cover material in order to minimize wind, rain and pest impacts.
After a landfill cell is filled to capacity, the compacted solid waste is covered with a final soil layer and then capped with a clay cap to prevent water infiltration. Many landfills now install methane collection systems during their construction and they also plant grasses and other plant materials to stabilize the slopes and prevent soil erosion.
How is foundry sand used in landfills?
Foundry sand is used in a variety of ways as a construction material in landfills. Clay-bonded foundry sands (typically called “green sands”) contain bentonite clays, and as a result, they have many physical characteristics that resemble natural soils. Clay bonded foundry sands have good hydraulic properties and can be easier to handle than some natural clays by themselves. As a result, some foundry “green sands” can be excellent substitutes for the clay lining material utilized in landfills.
Foundry sands containing synthetic binder systems (“resin bonded sands’) rather than clay binder systems tend to be more permeable than the “green sands” due to their lack of bentonite clay particles and because they are typically processed differently within the foundry. Foundry sands with higher permeability may be successfully utilized in leachate collection applications (aka: drainage layers) within the landfill cells themselves.
Many foundry sands (both green sands and resin bonded sands) have particle sizes and physical characteristics that would make them good materials for intermediate covers, which are placed over a cell prior to final closure. Foundry sands have also been used for final caps and closures.
In most states, foundry sand can be approved as Alternative Daily Cover (ADC) material in municipal solid waste landfills. Foundry sand is an easily spreadable inert material with no danger of flammability or organic waste content. As a result, it works well in this application and creates a formidable barrier between household wastes that may attract rodents, birds or other pests. Depending on the cost and availability of other cover materials, using foundry sands for ADC may be beneficial for both the foundries and the landfill. Many states and landfills recognize the use of foundry sand as a product when used in this application and therefore tipping and disposal fees are usually reduced or eliminated when foundry sand is used in this application.
How do landfills perform using foundry sand?
Solid waste landfills constructed with foundry sand perform no differently than landfills constructed with native soils. As with other geotechnical applications, the consistency of foundry sands can provide an advantage over soils for cell construction. Some landfill operators proactively source foundry sands for new cell construction, leachate collection and drainage systems, intermediate cover, or caps and closures, and may offer lower tipping fees accordingly.
Foundry sand has frequently been used as Alternate Daily Cover where it is readily available. Landfill operators tend to like it because it is typically a very clean and uniform material. It is important to note that foundry sands utilized for leachate collection, ADC, and other landfill construction needs are not simply being disposed of, despite the fact that their permanent home will be as part of a landfill. Foundry sands utilized in these applications are usually displacing virgin materials that were otherwise disposed of without consideration of their lifecycle.
Metalcasting companies, especially those aiming for ISO 14000 certification, are moving to embrace “zero waste to the landfill” sustainability targets. The same performance advantages that make many foundry sands good construction materials inside landfills are being documented in specifications and guidelines for other construction applications. As these trends converge, it is anticipated that the amount of foundry sand used inside the landfill footprint will diminish over time as more foundries have additional beneficial use options for their sands.
What are the technical issues associated with foundry sand in landfills?
Landfill capacity is determined by volume, not by weight, so denser materials are an advantage in all phases of landfill construction. By some estimates, soil and soil-like materials consume 15-30 percent of all landfill capacity, which provides a number of opportunities for foundry sands that are uncontaminated by other foundry waste streams.
On the USDA soil triangle, foundry sand is usually classified as a “silty loam” meaning that the particle size distribution is finer than most natural or manufactured sands. Resin coated sands may be coarser than green sands and thus have higher permeability factors. Gradation and permeability are the most critical testing elements for foundry sand usage as leachate collection media.
Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?
As with other materials going into solid waste landfills, foundry sands need to be tested to be sure they are non-hazardous, although few properly segregated foundry sands would be. Foundry sand to be used in landfill construction applications needs to have comparable physical characteristics as the soils, sands or other materials that it would replace.
ADC materials should be free from fine particles that create dust or problems relating to dust. Most landfills will require foundries to segregate suitable ADC sands from other foundry waste streams such as baghouse dusts.
There are many foundry sand monofills located at foundries in North America, some of which serve as storage cells for future beneficial use projects. In those industrial monofills, the usable sand and slag is typically segregated from other byproduct streams and especially from mixed foundry waste streams. Sites with commingled material are less likely to be appropriate for most beneficial use projects. In some cases, mixed sand streams may be candidates for Alternate Daily Cover, so long as they meet the appropriate testing requirements.
Are there any specific environmental issues associated with the use of foundry sand in landfills?
Landfills today are highly engineered structures that are dependent on aggregates and other materials that meet specific physical and chemical requirements. It is critical that foundry sands utilized in the construction of landfills be managed as a valuable aggregate rather than a waste material. Comparing commingled foundry waste to the engineered foundry aggregates utilized in landfill construction today is like comparing apples to oranges. Sands containing sludges or other commingled materials are not suited for most landfill applications, with the exception of some ADC applications. Only non-hazardous sands should be utilized in landfill applications and this may preclude some sands from bronze and brass foundries containing leaded alloys.
Recent research has convinced US EPA and many state regulators that properly segregated iron, steel and aluminum sands are safe for construction and manufactured soils applications outside of the controlled landfill environment. For more information on that research, please see the ENVIRONMENTAL PROFILE section of this website.
AFS-FIRST, US EPA and other partners are working to create more market opportunities for foundry sands outside of landfills. Many companies are working toward “zero waste” goals – and that typically means zero waste to the landfill. There is more attention being paid to the life cycle of materials in commerce, where arguably it makes little sense to indiscriminately bury a highly engineered material such as foundry sand in a landfill and lose its value to the economy in subsequent reuse.