Over the last 8-10 years, the regulatory agencies have come to a realization that simple excavation and disposal within a lined landfill is not a viable option, with many sites within the US requiring remediation. Landfills are filling up to capacity, liners are leaking and the permitting and/or expansion is getting harder and harder due to regulatory requirements and public opposition, such as NIMBY (Not In My Back Yard).
Options such as thermal desorption/incineration and In Situ Stabilization (ISS) are becoming more and more viable and approved as methods for remediation by regulatory agencies. Thermal desorption/incineration typically generates secondary wastes such as scrubber water, ash and stack emissions that must be handled as regulated hazardous waste. The final product typically ends up in a municipal landfill as daily cover over incoming trash like wastes. Therefore, this option is not optimum in the treatment of heavily contaminated sites.
The Benefits of ISS Remediation
ISS presents a more viable option, as it results in little or no generation of additional wastes. In general, ISS includes the addition of cementitious compounds (Portland cement) to the contaminated material that results in a cement-like final product. Other additives such as quick lime, silica fume, bentonite and ground granulated blast furnace slag can also be introduced to the contaminated media as means of pre-treatment or as a more economical reagent in comparison to Portland cement. The addition of ISS reagents to the contaminated material results in the binding of contaminants, rendering the final product inert and unable to release the contaminants. ISS has been used extensively for the treatment of wastes, wastes containing coal tar, PCB’s, semi-volatile and volatile compounds, heavy metals and other organic and inorganic compounds. Typical performance criteria include meeting leachability criteria (TCLP, SPLP), permeability (typ. 1 x 10-6) and Unconfined Compressive Strength (UCS-typ. 50 psi). The criteria goals are in consideration of the binding up of contaminants (leachability), desire to render the material incapable of groundwater intrusion (permeability) and strength in support of potential future site development (UCS). The remedial targets and associated reagent addition take into account contaminant type, concentration, pre-treatment requirements and future site use.
To ensure that a treatment scheme and associated reagent recipe will successfully treat the targeted waste, a bench scale Treatability Study is performed. The lab study uses samples obtained from the site and several reagent recipes are formulated based upon site contaminants and past performance knowledge. Bench scale testing is performed and a correlation developed for the implementation of full scale ISS using a mix that successfully met the performance criteria.
Grout and Reagent: Know Your Options
The performance of ISS can be carried out using two forms of reagent mixes – liquid grout or dry reagent. Based on existing groundwater data and lab results dictating water addition to the contaminated material/reagent mix, a determination can be made as to how to deliver the reagent to the contaminated material. If the groundwater is relatively deep and not contributing to the moisture content of the final mix, the reagent must be delivered in a grout form. The grout is generated using highly specialized batch plants that precisely meter both water and reagents on a weight basis. The batch plant has mechanisms that can be adjusted to deliver the water and reagents in accordance with the successful mix design developed during the Treatability Study. The mixed water and reagents, or grout, are then stored into a batch tank before being pumped to the machine performing the mixing. The dry reagents are delivered to the site and pneumatically offloaded into temporary storage silos that are connected to the batch plant.
If groundwater is present in the contaminated material in fairly shallow depths, reagent can be added to the material in a dry form. Using this method, the pneumatic tankers offload the dry reagent into a specialized reagent dust box that is fitted with a filter system to avoid particulate loss during offloading. The amount of reagent is added on a per pound basis, taking into account the weight of contaminated material, weight of reagent and results of the Treatability Study.
How It Works
There are a variety of methods to fully mix the reagents with the contaminated material. The choice of machinery is based upon depth of material to be treated, type of material, industrial setting, overhead obstructions, underground obstructions and Engineer specifications. In general, when performing ISS to depths of 12-feet or less, the use of a standard excavator is preferred. At these depths, the excavator will provide the adequate amount of mixing energy required to result in a fully homogeneous mixture. In addition, the excavator is not a highly specialized piece of equipment and can easily be replaced should a breakdown occur, can remove obstructions if encountered and requires a standard level of operator skill. The excavator can be used for both dry and grout reagent additions. If more mixing energy is required, the excavator can be fitted with a specialized blender head that is driven by a dedicated diesel motor and hydraulic system. While these machines offer increased mixing energy, they are very prone to breakage should an unknown underground obstruction be encountered.
When mixing depths are greater than 12-feet, the machine of choice to perform the mixing is either an excavator fitted with a drilling attachment (Delmag, Bauer, SoilMech) or a crane mounted auger platform known as a Hain Platform. Both utilize the same method for reagent delivery where grout is fed to the machine and into a hollow bar attached to the auger containing injection nozzles. The grout is piped up to the top of the hollow bar (Kelly bar) which then travels down the bar under pressure to the auger fitted with nozzles. The reagent is injected into the material to be treated as the auger is advanced to the full depth of treatment. Once reaching the bottom, the auger is plunged up and down the full depth of the column a number of times specified by the project Specifications. This is typically done 3 to 4 times to ensure a fully homogenous mix in the column.
The excavator mounted drill rigs are limited on diameter auger being utilized as well as overall depth of mixing. There is no hard-and-fast rule as to depth, but this is highly dependent on auger diameter choice and hardness of material being stabilized. These machines are more versatile than the crane fitted with the Hain Platform and require far less site preparation and machine assembly on site. Limitations include auger diameter and depth which is primarily a result of the limited amount of torque available.
The crane mounted Hain Platform uses a fully dedicated diesel engine and transmission that affords far more torque than the excavator mounted drill rig. The available torque allows the machine to rotate up to a 12-foot diameter auger at depths in excess of 80-feet. This type of machine is used for very deep ISS projects or where mass production is desired at shallower depths. The operation of this machine requires a highly trained equipment operator that is proficient at both crane and Hain Platform operation. While this machine requires far more assembly and site preparation, it is the machine of choice when performing at great depths into hard material.
Why It Works
As stated earlier, the performance of ISS results in little or no extra waste when properly executed. While there is some material swell generated during the process, this treated material is typically utilized on site where fill material is required to raise grades. ISS obviates the requirement for material excavation, hauling and landfilling and is accepted nationwide as a viable means of site remediation. This method of remediation is particularly used on sites where the end use has been defined such as the future construction of large buildings, big box stores, solar fields, municipal parks and sports fields. While the treated material layer is considered inert material, it is typically capped with a clean layer of fill material to further reduce public exposure.
Great Lakes E&I possesses the technical knowledge, personnel and equipment to perform ISS using all equipment and methods defined above. Great Lakes E&I presently has ongoing ISS projects as well as backlogged ISS work to be performed later in 2017.