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Stabilisation & Solidification

Solifidication/Stabilisation (S/S) is typically a process that involves the mixing of a waste with a binder to reduce the contaminants leachability by both physical and chemcial means and convert the hazardous waste into an environmentally acceptable waste form for land disposal or reuse/recovery. S/S has been widely used to treat and dispose of hazardous and mixed waste streams, as well as remediation of conatminated sites like other immobilisation technologies. S/S does not destroy the waste and contaminant(s) it contains, but tends to prevent it’s release into the environment.

 

Definition of Stabilisation and Solidification

Solidification refers to techniques that encapsulate a waste, forming a solid material and does not necessarily involve chemical interaction between the contaminants and the solidifying additives. The waste is normally treated to entrap the waste materials in a solid and/or crystalline matrix. For example, solidification consists of encapsulating the insoluble resdiue from an air pollution control (APC) washing process with cement to form a monolithic material that reduces the porosity and hydraulic conductivity of the material and hence it’s leachability.

 

Stabilisation refers to techniques that chemcially reduce the hazard potential of a waste by converting the contaminant to less soluble, mobile or toxic forms. The physical nature and handling characteristics of the waste are not necessarily changed by stabilisation. The waste is treated so as to complex or bind the contaminants into a stable, insoluble form.

 

Inorganic binders such as cement are effective in immobilising heavy metals through chemical and physical containment mechanisms, but are not as effective in immobilising most organic contaminants. Many substances in the waste significantly affect the setting and hardening characteristics of binders, especially cement-based cementing systems.

 

The mixing of waste and binders can be carried out through either ex-situ or in-situ processes. A wide range of mixers are available for ex-situ mixing, including pugmills, mortar mixers or concrete mixers. In situ methods are wideley used for remediation of conaminated sites and can be classified into the following three categories:-

 

·          backhoe-based methods,

·          drilling/jetting/augering/trenching methods and

·          shallow area methods.

 

Selection of the mixing method is based on the depth of the contaminant and the characetristics of the contaminated media.

 

Scientific basis of S/S

Stabilization and Solidification (S/S) have different goals. Solidification aims to minimise the spread of pollution by converting the contaminated materials into solid impermeable mass with a low surface to volume ratio. This is often achieved by adding a binder such as cement and quicklime to the waste material. Stabilisation is a form of solifification where reagents are added which converts the contaminant to a less soluble form by chemical reaction or pH adjustment. The combined process of solidification and stabilisation are often termed “waste fixation or encapsulation”.

 

In hazardous waste disposal or contaminated land remediation, treated waste must meet certain standards for safe disposal or re-use by removing the hazardous characteristic of the waste. This usually involves passing concentration-based standards such as the landfill waste acceptance criteria or remediation standards as agreed by the regulator. A binder is often used to stabilize the contaminants in the waste. Portland cement is a commonly used binder because of it’s availability and low cost. Supplementary cementing materials such as coal fly ash and ground blast furnace slags are often used to partially replace portland cement to improve the performance of the treated waste and to reduce the cost of the binder.

 

Types of binders

 

Inorganic binders

The two principal types of inorganic binders are Cement binders and Pozzolanic binders (lime, kiln dust, fly ash, etc). A pozzolan is a siliceous or siliceous and aluminous material, i.e containing silica or silica and alumina which in itself possess little or no cementious value but which will in finely divided form and in the presence of water, reacts chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementious properties. The most common inorganic binders are:

 

·                      Portland cement

·                      Lime/ fly ash

·                      Kiln dust (lime and cement)

·                      Portland cement/ fly ash

·                      Portland cement/ lime

·                      Portland cement/ sodium silicate

 

 

Cement Processes

During the cement based S/S process, the reaction forms a granular or monolithic solid that incorporates the waste materials and immobilises the contaminants. The solid matrix forms because of hydration of silicates in the cement, yielding calcium-silicate hydrate (C-S-H). The major crystalline compounds present in portland cement are Tricaclcium Dicalcium Silicate (C3S), while Tricalcium Aluminate (C3A) and Calcium Aluminoferite (C4AF) are present in small quantities. The cementation process binds free water, increases the pH and alters other chemcial properties of the mixture, reduces surface area and increases strength. All these mechanisms contribute  to the overall performansce characteristics of the treated waste.

 

The hydration of portland cement is a series of simultaneous and consecutive reaction between water and solid cement constituents which occur in the setting and hardening process. Anhydrous portland cement consists of angular particles (usually 1 to 50 μm) with a chemical composition of the primary clincker materials (U.S. EPA, 1993) that correspond approximately to C3S, C2S, C3A and C4AF, where C=CAO, S=SIO2, A=AL2O3, F=Fe2O3, S=SO3 and H=H2O.

 

Once the cement powder comes into contact with water, tricalcium aluminate (C3A) immediately hydrates, causing the rapid setting which produces a rigid structure. In an idealised setting, the water hydrates the calcium silicate and aluminates in the cement to form calcium silicate hydrate (C-S-H). Densely packed fibrils of silicate grows out from the cement grains and interlace to harden the mixture entrapping inert material and unreacted grains of cement. Hydration of tricalcium and dicalcium silicates results in the formation of Tobermerite (C-S-H) and Crystalline Calcium Hydroxide (CH). These compounds account for the strength development after the initial settings. The setting rate is controlled by the amount of gypsum added to the cement. If sufficient gypsum is present, sulfates combine  with tricalcium aluminate to form calcium aluminate sulfate which coates the cement particles and retards hydration reactions.

 

 

Pozzolanic Processes

Pozzolanic reaction, while not identical, are similar to Portland cement reactions.

 

 

 

1.     CH + S –H2O-> CxSyHz

                                                   (C-S-H of varying stoichiometry)

 

2.     CH + A –H20-> CxAyHz

                                                  (hexagonal and cubic aluminate hydrates)

 

3.     CH + A + S –H2O-> CxAySzHw

                                                         (Hydrogernets)

 

4.     CH + S + A –H2O-> CxAy(SH)zHw

                                                                                       (ettringite and derivatives)

 

 

 

The above reaction (Hydrated Lime with Flysashes) yields products whose properties are similar to the reaction products of Portland cement. The difference is that Pozzolanic reactions consume lime rather than produce it, as with portland cement hydration.

 

 

Ettringite Formation Effects

Ettringite also know as Calcium Aluminate Sulfate Hydrate is a needle like crystal which appears within minutes of cement hydration (see equation)

 

 

 

C3A + 3CSH2 + 26H -> C3A.3CS.H32

                                                                (ettringite)

 

 

 

Its formation is typically required early in the curing process to control setting rate. However the ettringite then dissolves and precipitates as calcium sulfate. Due to the high content of water of hydration, ettringite increases the volume of solids where it forms. If formed while the S/S treated waste is still plastic, the material can accommodate the expansive salt. However, if the ettringite forms after the grout has become rigid, cracking can occur and will reduce the strength of the product. The formation of this salt, with its larger amount of water of crystallisation and consequently larger increase in volume can be destructive to the S/S treated product.

 

 

Organic binders

Organic binders are mainly used to solidify radioactive or hazardous orgnaic wastes that cannot be destroyed thermally. Organic binders used for S/S include the following:

 

·                      Asphalt (bitumen)

·                      Polyethylene

·                      Polyesters

·                      Polybutadiene

·                      Epoxide

·                      Urea Formaldehyde

·                      Acrylamide Gel

·                      Polyolefin Encapsulations

 

There are two basic types of organic S/S processes. These are:

·                      Thermoplastic, and

·                      Thermisetting

 

 

Thermosplastic

Thermoplastic processes involves blending with a polymer such as asphalt, polyethylene or other thermoplastic binders. Thermoplastic is a polymer that becomes pliable or moldable above a specific temperature and returns to solid state upon cooling. Liquid and volatile phases associated with waste are driven off and the waste is contained in a mass of cooled, hardened thermoplastic.

 

Thermosetting

Thermosetting polymers involves mixing waste with reactive monomers which join to form a solid incorporating the waste. Unlike thermoplastics, thermosetting polymers form irreversible chemical bonds during the curing process. Thermoset bonds break down upon melting and do not reform upon cooling, e.g., urea formaldehyde.

 

Studies have been carried out to investigate the potential of the thermosetting polyester polymer to solidify/stabilise phenol, a primary constituent in many organic hazardous wastes. Most of the polyester polymer solidified phenol specimens showed no measurable amount of phenol in the leachate after the extraction procedure test. Results also showed that compressive and tensile strengths of solidified waste reduces with increasing phenol content. Full details can be found in article by Vipulanandan, C., Krishnan, S., titled: Solidification/stabilization of phenolic waste with cementitious and polymeric materials.

 

Additives

Additives are added to improve the immobilisation of specific contaminants. They can also be incorpoaretd to mitgate the effects of certain inhbitors. It should be noted that many additives may work for one constituent but have the opposite effect for a different constituent.

 

Key Issues and Environmental Risk

 

Accident Risk

There is always the risk of accidents when dealing with hazardous waste. Wastes are heterogeneous in nature and are potentially aggressive to plant and equipment. Any failure in the management of the waste, from the process of characterisation and checking of wastes, to operational control for reactions and mixing of wastes, will significantly increase the risk from unwanted or runaway reactions. Combinations of inappropriate equipment and poor inspection and maintenance procedures also increase the accident risk through, for example, tank overfill situations where level indicators may not be working or have not been correctly calibrated.

 

Relationship to BAT

Article 13 of the Waste Directive (WFD) (2008/98/EC) requires Member States to take the necessary measures to ensure that waste management is carried out without endangering human health, without harming the environment and, in particular:

 

·          without risk to water, air, soil, plants or animals;

·          without causing a nuisance through noise or odours; and

·          without adversely affecting the countryside or places of special interest.

 

Article 18 of the same directive also prohibits the mixing of hazardous waste either with other categories of hazardous waste or with other waste, substances or materials. Where this is to be carried out, the mixing operation must conform to best available techniques. Therefore for any waste S/S activity, an assessment of the appropriate measures including BAT will be needed to support any application.

 

Waste hierarchy

Where waste is produced, the WFD requires that waste be recovered, re-used or used as a source of energy in preference to disposal.

 

With regard to the S/S treatment activities involving disposal, this raises the question of whether these

activities constitute the appropriate means of dealing with the waste. Clearly, where an opportunity to

recover a waste exists, then disposal or treatment may not be the appropriate measure.

 

Cross-Media Transfer Potential

 

·          Under dry and/or windy environmental conditions, both ex situ and in situ S/S processes are likely to generate fugitive dusts.

·          S/S processes can produce gases, including vapors that are potentially toxic, irritating, or noxious.

·          Leaching of contaminants or excess reagents to ground water from treated waste that is disposed on site.

·          Long-term degradation of the stabilised mass, creating the potential for solidified wastes, reagents, VOCs, and other contaminants to be released from the treated waste.

 

 

BAT for Avoiding Potential Cross-Media Transfers During Solidification/Stabilisation

Operational Controls or practices that can be implemented to reduce dangerous emissions include:

 

·          Plan site remediation for times of year with relatively cooler temperatures and lower wind speeds to minimize volatilisation and particulate matter emissions.

 

·          Fugitive dust emissions should be controlled during excavation by spraying water to keep the ground moist. During excavation, material handling and preporcessing activities, meteorological conditions should be strongly evaluated. Soil stockpiles should be covered with reinforced plastic sheets to prevent fugitive dust emission and rainwater infiltration.

 

·          Vapor treatment systems should be used to the extent possible to control the movement of these vapors. VOC emissions associated with these activities that exceed acceptable regulatory limits should be controlled by capturing these emissions and then treating the captured vapor/air to the extent practicable. Hand-held organic vapor analyzers provide quick readings on VOCs. Effective VOC, methane, and odor emissions should be controlled by using covers, foam suppressants, enclosures, vapor collection systems, gas flares, or other methods as appropriate.

 

·          Mixing, crushing, or conveying activities should generally be conducted under an environment where the off gases, volatiles, dusts, etc. are all captured inside a hood. The VOC emissions associated with these activities should be controlled by capturing and then treating the captured vapor/air. The vaporized organic contaminants can be captured by condensation of the off-gas, passing through a carbon absorption bed or other treatment system.

 

·          Maintain lower speeds with all vehicles on unpaved roads.

 

·          Materials that are removed during prescreening activities should be disposed of properly.

 

·          Control placement and shape of storage piles. Place piles in areas shielded from prevailing winds. Shape pile in a way that minimises surface area exposed to wind.

 

·          During excavation, use larger equipment to minimize surface area/volume ratio of material being excavated.

 

·          During dumping, minimise soil drop height onto pile, and load/unload material on leeward side of pile.

 

·          Cover all loads being moved by truck, open piping, or other conveyance with roofs or other structures that will eliminate or reduce the likelihood of particulate release into the atmosphere. Increase freeboard requirements and repair trucks exhibiting spillage due to leaks.

 

·          "Blanket" the emitting source with foam, thus forming a physical barrier to emissions. Also isolate emitting surface source from wind and sun, further reducing particulate and volatile emissions.

 

·          Air emission monitoring must be conducted to detect and quickly act on potential crossmedia transfer. Monitoring (visual or olfactory) upwind/downwind concentrations of ambient dust/particulates or odorous target compounds. Hand-held organic vapor analysers provide quick readings on presence of organic vapors.

 

·          Periodic visual inspection of pipes and joints for corrosion and leaks could provide early detection and prevent major leaks and spills. Reagent delivery piping should be regularly checked to ensure tight fittings. This will reduce the likelihood of releases of VOCs.

 

·          Wastes should be homogenised as much as practicable before processing. This can improve the efficiency of the stabilisation activities, and may help to reduce spillage and other problems related to encountering irregular masses during the mixing process.

 

 

 

Site restoration (prevention of emissions to land)

IPPC in common with Waste Management Licensing (now EPR) requires that, on completion of activities, there should be no pollution risk from the site. As required by the Environmental Permitting Regulations 2010, prevention of both short and long-term contamination of the site requires the provision and maintenance of surfacing of operational areas, measures to prevent or quickly clear away leaks and spillages, maintenance of drainage systems and other subsurface structures

 

Physical Tests

Visual observation: for surface spalling, grain exfoliation, crack development, colour, surface pore size and conduction, salt efflorescence.
 
 Permeability (Hydraulic Conducivity) Test: measure of flow of fluid through the pore structure of the S/S treated waste.
 
Strength Tests: An indciation of how well a material will hold up under mechanical stresses caused by over-burden or earth moving equipment. Better strength provides better physical barriers from the containment of contaminants but S/S treated waste strength should not be mistaking for the degree of containment stablisation.
 
Unconfined Comporessive Strength (UCS): This is a measure of the shear strength of an S/S treated material/waste without lateral confinement. The minimum required strength should be determined from the design loads to which the material may be subjected. This European Standard, a part of the BS EN 12390-4 series, specifies the requirements for the performance of compression testing machines for the measurement of the compressive strength of hardened concrete.
 
Immersion compressive Test: To test the strength of simulate  the performance of the S/S treated waste in a saturated disposal environment
 
Durability Test: This evaluates the ability of a material to withstand environmental stresses such as freezing and thawing or wetting and drying. The ability of the material to withstand such conditions or cycles is an indication of its physical stability. Other performance tests such as UCS and permeability can be conducted on the material after each cycle to determine the change in performance due to climate stresses.
 

Leaching/Extraction Tests: Leaching tests measure the potentials of a stabilised waste to release contaminants to the environment. The waste is exposed to a leachant and the amount of contaminant in the leachate (or extract) is measured and compared to a previously established standard, which may be regulatory standard of baseline leaching data. Treated waste may give reduced contaminant concentration in the leachate due to waste dilution with binders (independent of any immobilisation mechanism). This should be taken into consideration when using leaching tests.