Clean Earth provides various treatment options for non-hazardous and hazardous soil contamination. The combination of our various technologies, locations, and experienced team will result in a soil treatment and recycling or disposal plan that works specifically for your project site.
Thermal desorption is an environmental remediation technology that is used to treat material with a broad range of hydrocarbon contamination. This process involves heating soil in a rotating dryer to remove or separate the contaminants from the soil. This process is often referred to as “low temp” thermal desorption to differentiate it from high temperature incineration. This method is primarily used to treat soil with high levels of contaminants, such as MGP soil, by heating it in a rotating dryer to destroy contaminants.
Low Temperature Thermal Desorption (LTTD) technology utilizes heat to physically separate contaminants from soil and media. LTTD units are designed to heat contaminated soils in the rotary kiln (primary treatment unit) to temperatures sufficient to volatilize the contaminants and desorb (physically separate) from the soil or media. The vaporized hydrocarbons are treated in a secondary treatment unit – afterburner/thermal oxidizer – where the hydrocarbons are destroyed prior to discharge to the atmosphere. Pre-processing of soil is often necessary when using thermal desorption and can include screening and blending. The oversized materials may be sized and introduced back into the feed material or decontaminated for beneficial use.
More About the Thermal Desorption Process
The direct-fired thermal desorption process begins with waste characterization via laboratory analytical as well as site history documentation. The profile and proper analytical must be provided to, and approved by, the thermal company’s compliance team in order for shipment to the treatment facility to begin.
Facilities often require incoming materials meet a specific set of criteria including moisture content, debris and sizing of any oversize brick or concrete that may be commingled with the material destined for thermal treatment.
Sometimes wood and hay are added in the field as a moisture control measure. This is not a good idea because thermal treatment is different from combustion - it doesn’t involve incineration of the soil itself. The soil is indirectly heated. This only creates a biochar in the soil media that can cause issues with contaminate release from the soil during the treatment process.
Thermal Desorption Units can accommodate a limited level of thermal energy release from the soil and material. Additional wood, hay, mulch, and other combustible organic matter factor into the overall fuel/contaminant loading limit of 2-3% that is typical of direct-fired thermal treatment units. Higher levels are possible but come at the cost of lower processing rates and efficiencies. Furthermore, hay or plastic sheeting can become airborne without combusting within the primary treatment unit. This may cause cyclones within the dust separation system and it reduces necessary airflow throughout.
Once received at the facility, the soil media is preprocessed if it is too wet to be treated or contains off-spec materials such as concrete or brick beyond the permissible size. The addition of lime kiln dust (LKD) to combat high moisture content is common as the added volume of water vapor in the process off-gas can result in lower waste throughput, because the water vapor must be handled by downstream treatment equipment along with off-gas and desorbed contaminants. The lower processing throughput is attributable to (1) higher gas flows, resulting in greater pressure drops through the thermal desorption system; and (2) thermal input limitations, because some of the heating input is used to vaporize the water in the waste feed, and the feed rate may need to be reduced to adequately heat the waste feed to achieve satisfactory desorption.
The LKD serves several purposes. First, the available Calcium Oxide within the LKD reacts with the moisture in the soil to create a mineral Calcium Hydroxide with a far lower specific heat than the water. Though the energy savings isn’t likely to completely offset the cost of the LKD the improvement in material handling does. The LKD addition improves the friability of clay soils and greatly lessens issues with sticking and bridging of soils within feed hoppers and chutes.
In addition, all material is sent through a screen and jaw crusher to make sure no material larger than two inches enters the thermal processing plant. During the pre-process crushing, the operator removes any plastic, trash or other debris commingled with the material. A large magnet is also present to remove metal debris from the material prior to entering the thermal process.
Once the material is fed into the primary treatment unit (PTU) (typically a rotary dryer) it immediately begins to be cleaned. Dryer temperatures are set based on the specific characteristics of the current waste stream and can range from 4000F (gasoline, TCE/PCE) to upwards of 1,0000F (coal tar, PCB’s). The water and contaminants are volatilized into the airstream which is directed through a mechanism that separates larger fines from the airstream to be returned to the drum. The airstream then is processed in a thermal oxidizer (Secondary Treatment Unit) which combusts and destroys the individual contaminants in the airstream. Depending upon the contaminant, the operating temperature of the oxidizer ranges between 1,5000F to 1,8000F. The burners for both the PTU and STU can be fired by natural gas, fuel oil, waste oil, or propane. The electrical requirement is typically 480 volt, 3 phase, varying in amperage depending on the size of the thermal desorption unit (TDU).
Continuous Emission Monitoring Systems (CEMS) are often used to ensure efficient treatment and regulatory compliance. Triboelectric dust/particulate sensors are often used to monitor baghouse filtration bag integrity. Carbon monoxide (CO) and oxygen levels are also monitored as an indicator of the oxidizer destruction removal efficiency with which the plant burners are tuned.
Thermal desorption plants are outfitted with thermocouples, motion detectors, and magnahelic pressure sensors throughout. The PoP stack testing of a TDU establishes treatment parameters (operating envelope) during the early stages of any project. Those established Oxidizer temperature minimums are maintained by the Programmable Logic Controls (PLC) to ensure adequate destruction of the volatilized compounds within the airstream. Furthermore, TDU components have multiple safety and compliance interlock mechanisms that ensure that soil feed cannot occur until oxidizer temperatures are at required minimums. Pressure sensors within the PTU, multicone cyclone dust separator, STU, baghouse, and pugmill also ensure negative pressure is maintained on the plant and thus controlling fugitive emissions.
As for the soil media in the treatment process, it resides in the PTU for 12-15 minutes, to ensure the material will be rendered clean for beneficial reuse. The final stage of air treatment entails fine particulate removal in the baghouse.
Upon discharge from the unit, material is rehydrated prior to being stockpiled for confirmatory sampling and analysis. Once confirmatory analysis reveals that the cleanup criteria have been met, the material is ready for beneficial reuse. The beneficial reuse of material varies based on the characteristics of the material and the material’s intended use (general vs structural fill) but is often an economical alternative to “virgin” backfill.
Costs and Benefits
Thermal treatment can be conducted at fixed facilities or at a Remediation Site with a portable unit. For large quantities, typically over 30,000 tons, onsite thermal treatment may be the most cost-effective solution.
Several factors need to be evaluated to determine what type and size thermal treatment unit is ideal for each project. Those factors include the property layout and location, available utilities, quantity of material to be treated, material characteristics and timeframe for treatment. Pricing is an important consideration in the decision-making process for an onsite thermal unit when determining if onsite thermal treatment is a viable option.
Bioremediation is a treatment process that provides a cost effective and environmentally conscious recycling solution for non-hazardous contaminated soils. Through a process called bioaugmentation, microorganisms are introduced into the contaminated environment as a granular product, with a proper nutrient mix to stimulate and foster their growth while breaking down the contaminants in the soil.
The process degrades organic contaminants, such as petroleum hydrocarbons, in the soil by the application of cultured microorganisms selected for their ability to metabolize the petroleum products.
Soil that is lightly contaminated with petroleum hydrocarbons is inoculated with engineered bacteria and nutrients that combine to completely break down soil contaminants in 7 to 14 days. Bioremediation is typically used on soil with lower levels of contaminants. The resulting treated soil is recycled as fill material or landfill capping or cover.
Chemical fixation is a soil remediation process used to treat hydrocarbon contaminated soil and/or metal contaminated soil. This process involves stabilizing contaminated material by mixing it with other earthen material, and then treating the blended material with certain chemical additives to formulate a particular material. This treatment binds the petroleum contaminants and prevents leaching. Chemcial fixation is best used for light to medium hydrocarbon contamination and/or contaminated soil impacted by metals.
Physical treatment of soil includes special sizing and segregation processes to remove unsuitable materials from incoming soils. The resulting soils generate an engineered material suitable for beneficial reuse as fill material or landfill capping or cover.
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