Soils washing is the process of separating and recovering usable materials from earth or ground material that has been excavated during construction and demolition activities, greenfield stripping, brownfield sites or collected from road sweepings or hydrovac waste streams. “Soils” can vary widely in composition depending on the site from which the material was excavated and can include a nonhomogeneous mixture of rubble, rock, sand, topsoil, grass, clay, silt, etc.
With soils washing, the aggregate (sand, gravel and rock) is separated from the rest of the material and cleaned of any contaminants for reuse in new construction projects. This helps to promote sustainable resource management and to minimize environmental impact.
Why soils washing: the importance of soils washing in the construction industry
As increased focus is placed on sustainability in construction practices, the importance of C&D recycling and soils washing is also expected to increase. This is due to several key factors:
Demand, circular economy and public perception
Supporting a growing population requires modernized infrastructure. Federal money, such as that from the Infrastructure Investment and Jobs Act (IIJA) is being allocated toward these infrastructure projects, particularly those that are better for the environment.
The IIJA includes $2.1 billion to address legacy pollution, including cleanup of contaminated brownfield sites. More communities and agencies will turn to demolition and remediation experts to help return polluted properties to good use.
The concept of a circular economy, where resources are reused and recycled to create a closed-loop system, is also gaining traction. Soils washing plays a vital role in promoting a circular economy by reintroducing materials into the production cycle.
Finally, consumer demand is also placing emphasis on the importance of C&D recycling and soils washing projects. Consumers want to demonstrate reductions in their total supply-chain emissions. Likewise, investors want evidence of progress toward achieving NetZero goals. Soils washing as part of C&D recycling helps to work toward these initiatives.
Adopting soils washing practices can enhance the public image of construction companies and developers, differentiating themselves in the market. Demonstrating a commitment to sustainability and environmental responsibility, as well as actively promoting a circular economy, can lead to increased brand loyalty and customer trust.
Environmental regulations, cost savings and resource conservation
Environmental regulations, particularly in relation to landfilling waste, are another driving factor toward sustainable construction practices. Governments worldwide are imposing stricter environmental regulations on waste disposal and encouraging recycling and reuse to reduce landfill waste.
Landfilling is a leading source of uncontrolled methane emissions, which is 84 times more potent than carbon dioxide. Landfills that are not designed to code can leach toxic chemicals and other organic pollutants that compromise soil and groundwater. Non-biodegradable materials, such as plastics and hard foams, migrate, damaging streams, lakes and oceans. Waste infiltrates areas where wild animals would otherwise thrive, causing premature death and lowering birth rates.
Landfilling takes up and devalues land that otherwise would be used for more productive purposes, including housing, parks or other greenspaces and businesses. Quality of life is diminished for people whose homes and businesses are located near landfills due to the associated odors and unsightliness.
Depending on the location, landfilling or hauling waste to the landfill can be costly. Soils washing can be a cost-effective alternative to traditional disposal methods, especially if the recycling operation is local to centers of construction activity.
Soils washing also helps limit the strain on natural resources by preserving high-quality virgin reserves for higher-spec aggregate applications.
Innovative technologies
Ongoing advancements in soil washing and recycling technologies are making these processes more efficient and effective. New technologies are being developed to handle a wider range of contaminants and materials, further increasing their importance.
How to wash soils: the challenges with soils washing
Washing soils, especially when contaminated, can present several challenges that need to be addressed to ensure effective and environmentally sound remediation, including:
Type and concentration of contaminants
The type and concentration of contaminants in the soil can vary widely, requiring different treatment methods and technologies. Some contaminants, such as heavy metals or persistent organic pollutants, may be more challenging to remove or neutralize effectively.
Soil characteristics
The composition and characteristics of the soil, such as texture, organic content and pH, can affect the efficiency of washing processes. Certain soils may be more prone to sticking and may require specific equipment layout approaches that minimize change in direction.
Energy consumption
Some soil washing techniques are extensive and large scale and require significant energy inputs, which can increase the overall cost and environmental impact of the process.
Residual contamination
It can be challenging to completely remove all contaminants from the soil, leading to concerns about residual contamination and the potential for long-term environmental impact.
Regulatory compliance
Meeting regulatory requirements for soil washing, including disposal of contaminated water and soil, can be challenging and will require permits or approvals from regulatory authorities. Furthermore, the standards for acceptable levels of contamination in washed soil for reuse in construction projects is currently very restrictive. As a result of the strict standards, and sometimes a lack of discretion on the part of qualified persons, suitable soils are being landfilled instead of being reused for construction projects.
Cost
Soil washing can be a costly process, particularly for large-capacity systems. It requires specialized material processing equipment and water treatment systems if dealing with contaminated water. Extensive quality control measures are required daily to test for incoming feed material, outgoing recycled material and waste water. In general terms, it costs twice as much to process clay-bound contaminated soils as it does uncontaminated soils.
All these factors can affect the type of equipment and systems needed for washing excavated soils from construction or greenfield sites, road sweepings or hydrovac waste.
Soils washing techniques
Because the composition of soils can vary vastly depending on collection source, location and material makeup, processing soils for reclamation can include any or all of the following:
- Sorting and Separation
- Material Size Reduction
- Washing and Classifying
- Sampling for Quality
- Tailings and Water Management
Sorting and Separation
The sorting and separation process isolates the material that can be recycled and removes it from the rest of the debris. This is commonly achieved with a scalper or Vibrating Grizzly Screen, which removes oversize material that can block or damage downstream processing equipment. Overband magnets are also used at various stages of the process to remove ferrous metals that may be present in the material stream ahead of downstream processing equipment or stockpiling. Eddy current separators can be added to divert valuable, non-ferrous metals such as copper from the washing process.
Material Size Reduction
Material size reduction is the process of reducing large lumps of material into smaller sizes for ease of handling downstream. Jaw Crushers and Impact Crushers are typically used for material size reduction.
Washing & Classifying
Washing removes silt fines and clays from fine aggregate, while classifying removes lightweight organics, plastics, paper and wood using density separation.
Equipment for these processes include Hydrocyclones, Separators™, Fine Material Screw Washers, Dewatering Screens, Hydrosizers™ and Flat Bottom Classifiers.
Coarse aggregate, particularly clay-bound aggregate, will be subjected to an aggressive scrubbing action to liberate sound aggregate from deleterious materials.
Fine aggregate can also be scrubbed to remove surface contaminants, increase durability and sand equivalency, making the material more suitable for construction projects.
Equipment for these processes include Attrition Cells, Log Washers, Blade Mills and Coarse Material Screw Washers.
Sampling for Quality
Sampling is important for verifying a finished product meets required specifications. Sample collection can be made simpler with the use of mechanical Sampling Systems. These machines automatically collect representative samples from a moving material stream without having to stop the belt. Sampling Systems improve accuracy, repeatability, safety and efficiency. Extensive sampling is required when handling contaminated soils, including feed sampling, product sampling and water sampling.
Tailings and Water Management
The wet processing of aggregate derived from C&D soils will inevitably generate a waste stream consisting of the process water, silt fines and clays that were removed.
Tailings and water management equipment, such as Thickeners, Centrifuges and Filter Presses provide a safe and efficient means to handle and store this waste material by separating the solids from liquid. This produces a clear liquid stream and drip-free solid cake. The liquid can be reused as process water in the washing stage, reducing freshwater requirements, while the solids can potentially be sold as backfill or used for site reclamation.
For contaminated soils, additional process steps, including water treatment, material sampling and daily quality control checks are required to identify and remove suspended and soluble contaminants from the process stream. It also helps to provide safe aggregate for reuse, safe working conditions for wash plant operators and safe disposal of contaminants to protect our society and the environment.
Best practices for soils washing
Washing soils, particularly contaminated soils, is a complex process that requires careful planning, execution and monitoring to ensure effective remediation while minimizing environmental impact. Here are some best practices for washing contaminated soils:
Secure source of feed material in quantity and quality
Quality and consistency in the output comes from quality and consistency in the input.
Assess incoming feed material
Conduct a thorough assessment to identify the soil characteristics, type and extent of any contamination. Provide adequate space for storage of different incoming materials. Typically, more space is required for incoming feed materials rather than washed stockpiles.
Prepare the soil
Pre-treat soils to optimize the effectiveness of washing processes, such as removing large debris or segregating soils based on contamination levels. Process materials in campaigns so the system can run predictably and consistently.
Wash the soil
Use mechanical methods such as screening, washing, scrubbing and density classification to remove contaminants from sound aggregates. Have a clear definition of “clean” sand when designing a system to produce marketable products. Use water recycling and tailings management systems to maximize water recovery and dewater tailings into a manageable by-product.
Monitor and control
Monitor aggregate and water quality throughout the washing process to ensure that contaminants are being effectively removed. Implement control measures to minimize the spread of contaminants and mitigate potential environmental impacts.
Manage the waste
Properly manage and dispose of dewatered tailings and wastewater according to regulatory requirements.
Prioritize safety
Implement safety measures to protect workers and the surrounding environment from exposure to any contaminants. Provide appropriate training and personal protective equipment for workers. Design a processing system with maintenance access in mind.
Document and Report
Keep detailed records including incoming feed material data sheets, sampling records, analysis data and disposal records. Report to regulatory authorities as required and seek approval for disposal or reuse of treated soils.
Strive for continuous improvement
Evaluate the effectiveness of the washing process and identify opportunities for further improvement. Stay informed about new technologies for ongoing optimization and become active in local or national trade associations.
Soils washing is poised to play a crucial role in the future of the construction materials industry. Embracing these practices can not only help meet regulatory requirements and reduce environmental impact, but also provide economic benefits to stakeholders and improve the company’s reputation in the construction sector.
