24 Stabilization, Solidification, chemical fixation, encapsulation

AG Murugesan

 

epgp books

Objectives:

 

The objectives of the study are

  • Understand the concept of Waste Management Techniques through Stabilisation or solidification, Fixation and encapsulation
  • Management of environmental hazards of improper hazardous waste management pollution due to its improper handing & disposal.
  • Adoption of scientific and effective waste management techniques and further improvement and improvising of the technology

 

1.0 Introduction to waste treatment

 

The processes such as Stabilisation/ solidification, Fixation and encapsulation are some of the remediation techniques used for the management and disposal of wide range of hazardous wastes. In these processes, the wastes are immobilised using different chemical or additives. The contaminants are also encapsulated in a medium to stabilize it. According to Environmental Protection Agency (EPA, 1989), the process of solidification and stabilization makes the handling of waste friendly/ easy and improves the quality of wastes. The process helps in reducing the surface area and mass of the wastes and also restricts the solubility of hazardous substances in the waste. Stabilization is general process through which the wastes are changed to less toxic and less mobile form. The process of solidification on the other hand treats the waste materials while increases the solidity and structural integrity of waste. The process of solidification does not render degradation of the hazardous wastes but removes the transport of waste by restricting its mobility.

 

The transforming of the hazardous waste operation is carried such that the treated hazardous waste will have less effect to contaminate the environment by reducing the leaching potential. The leaching potential can be defined as Natural process by which water soluble substances is washed out from soil or wastes. The process of leaching makes the waste less toxic.

 

The mechanisms involved in the treatment of hazardous waste are:

  1. Solidification/Stabilisation – Solidifying waste using solidifying, adsorption agents and other additives.
  2. Immobilisation or chemical fixation – Binding the contaminants chemically within the matrix to restrict the mobility of waste or its leachability.
  3. Encapsulation – Capturing the waste and reducing its reactive potential and overall volume.

 

These processes are executed in situ as well as ex situ. In in situ process, the stabilizing process is directly injected into the natural soil. In the ex situ process, the soil is excavated and subjected to the treatment process and finally after the remediation it is backfilled. The process of stabilization or solidification generally appropriate for that kind of soil which is polluted with metals, other inorganic compounds, radionuclides etc. volatile organic compounds (VOCs) are usually not preferred for this process because the VOCs have tendency to evaporate into the atmosphere during the process of mixing. The process of stabilization or solidification is further categorised into aqueous, polymer and vitrification process. USEPA has recognised the process vetrification as Best Demonstrated Available Technology for 57 different types of hazardous wastes listed in The Resource Conservation and Recovery Act (RCRA). According to USEPA, 2001, around 25% of the superfund remediation locations have been treated using vitrification techniques.

 

2.0 Application of stabilization or solidification process

 

The process of Stabilization or Solidification is utilized in broad range of medium such as, in the sediments, soil and sludge. The effectiveness of the Stabilization or Solidification process is according to Superfund policy is, determined by carrying a treatability study on similar kind of waste in order to guarantee the effectiveness of the process by 90-99% (EPA, 1993). The effectiveness of the process is selective for certain concentrations. For example, at lower to moderate concentration, the solidification of metals such as zinc, lead and copper with the help of cementitious substance is effectual. At the higher concentration of these metals, there is hindrance in the process of the immobilization; hence it proves to be unsuccessful (EPA, 1993). Moreover, certain combinations and allocation of the pollutants through the soil also prove to influence the success of the stabilization or solidification process.

 

3.0 Description

 

Stabilisation or the solidification treatment processes are used to alter chemical properties of wastes in order to facilitate or enable further treatment; to convert the waste to nontoxic/non-hazardous for disposal; or to solidify or stabilize the wastes for ease of handling or reduced leachability or to render them non-degradable.

 

The some of the common methods of chemical treatment include

 

1.      solidification/stabilization,

 

2.      fixation

 

3.      Encapsulation

 

These and other treatment methods are continuously evolving are devised by theory, stoichiometry, experimentation, accident, or combinations and identifying new waste-specific chemical treatments is a lucrative and rapidly growing global enterprise.

 

4.0 Solidification/Stabilization

 

Effectiveness: Effectiveness of the stabilisation method is tested by its effectiveness to prevent the contamination to the environment. Some of the parameters to verify the effectiveness are performed by examining the physical and chemical factors.

 

Physical tests are performed for the characterization of the waste materials before and after the stabilisation or solidification treatment process.

 

The chemical tests are carried out chiefly for the evaluation of the performance of particular treatment techniques.

 

Table 1 Physical Test:

 

 

Table 2 Natural Sorbents and Their Capacity for Removal of Specific Contaminants from different phases

 

Table 4: Compatibility of Some Waste Categories with Different Stabilization or Solidification Techniques

 

Chemical tests

 

Under chemical tests generally leaching tests is performed as an endorsed Toxicity characteristic leaching procedure (TCLP) for the identification of chemical constituents in the stabilised waste.

 

The waste stabilised should meet the Best Demonstrated Available Technology (BDAT) standards of before their disposal to the secure landfill.

 

4.1 Addition of Reagents and Additives:

 

Process can be grouped into the following:

 

a)      cementitious reagent processes, and

b)      surface adsorption reagent processes (organophilic clay and thermoplastics or other synthetic polymers).

 

Cementitious reagents are the most common commercially employed S/S process options due, in part, to low cost and availability.

Figure 1 Conceptual in situ S/S model

 

4.2 Technology Advantages and Limitations

 

Stabilisation/ Solidification methods have resulted in development of reliable information regarding effectiveness and other factors that are typically considered in the process to evaluate and select site remedial actions.

 

Must: In considering use of technology, a sound understanding of site conditions is important, as well as an understanding of the practical outcomes and limitations of the technology.

 

The treatment technology is applicable for a relatively broad range of contaminants and may be feasible when limitations to other technologies are imposed by site or contaminated material conditions.

 

General non-site-specific advantages and challenges of S/S technology are listed below.

 

Advantages

 

• effective in treating many inorganic contaminated materials

 

•  effective in treating some materials contaminated with organic chemicals

 

• option for treating mixed contaminants

 

•  often reaches fixed treatment end point in a relatively short period of time

 

• can improve structural property of soil, waste, and sludge (e.g., strength) to facilitate consideration of land beneficial reuse

 

• applicable for in situ or ex situ treatment

 

• has been applied in dry or wet conditions, reducing dewatering and waste management issues

 

• generally, uses simple, readily available equipment and materials

 

• It is compatible with different types of soils

 

• on-site management of contaminated materials conserves space and does not require transportation off site

 

• may be more cost-effective than excavation and off-site disposal

 

Challenges

 

• contaminants are not destroyed or removed; long-term monitoring may be required

 

• effectiveness for certain contaminants may require more testing, research and developing a viable design

 

• The soil treated undergo increase in the volume remarkably

 

• potential changes in physical setting (e.g., groundwater flow, mounding)

 

• when mixed contaminants are present long term prediction and behaviour is to be studied

 

• Uniform mixing of the waste with the stabilizing agents are again  difficult process

 

• Later modifications after applying at the site are sometimes difficult to assess and treat.

 

• Landscape management to refill the unearthed material could pose a challenge

 

• The stabilization or solidification techniques cannot be redeveloped in In situ.

 

• Long-term efficiency cannot be ensured through this process

 

5.0 Encapsulation

 

Encapsulation is a process by which the hazardous waste is immobilized in a solid block and sealed inside a plastic or steel drum. The containers should be cleaned previously. Containers that had been used previously to contain or transport hazardous waste must be avoided. The waste that needs to be encapsulated must be filled to 75% of container’s capacity. The remaining space is filled with a mixture of cement or cement/lime mixture, plastic foam or bituminous sand. The proportion of lime, cement and water mixture is around 15:15:5 (by weight). Sufficient water is required to maintain the consistency of the mixture. Before the filling process, the drum lids must be cut open and bent back to ease the filling process. Post filling the lids are brought to original position and sealed by seam or spot welding. The sealed drums should be placed at the base of a landfill and covered with fresh municipal solid waste. For ease of movement, the drums may be placed on pallets which can then be put on a pallet transporter. Encapsulation is popularly used for pharmaceutical wastes.

 

6.0 Inertization

 

Inertization is a type of encapsulation and is again used for pharmaceutical wastes. However, inertization process require preprocessing such as removal of packaging materials, paper, cardboard and plastic, from the pharmaceutical waste. Even pills should be removed from blister packs before the inertization process. The pharmaceutical waste compounds are grounded and mixed with lime, cement and water to form a homogenous paste. The paste is then transported to landfills in liquid form in containers. From the concrete mixer trucks, the waste is decanted into the landfills containing municipal solid waste. The paste then sets out into a solid mass. Required precautions such as protective clothing, masks, gloves must be used by the workers performing inertization. The process is cheap since it does not involve any sophisticated equipment. The main requirements are a grinder or road roller to crush the pharmaceuticals, a concrete mixer, and supplies of cement, lime and water. The approximate ratios by weight of materials used in inertization is as follows:

 

•        pharmaceutical waste: 65%

 

•        lime: 15%

 

•        cement: 15%

 

  • water: 5% or more to form a proper liquid consistency.

 

7.0 Summary

 

At the end of this module, we have studied about

 

  • stabilization and solidification process used for solid and hazardous waste disposal additives used for stabilizing the waste
  • technological advances in stabilization and solidification Challenges involved in stabilization and solidification
  • Concept of encapsulation and inertization

 

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References:

 

    1. ITRC (Interstate Technology & Regulatory Council). 2011. Development of Performance Specifications for Solidification/Stabilization. S/S-1. Washington, D.C.: Interstate Technology & Regulatory Council, Solidification/Stabilization Team. www.itrcweb.org.
    2. Heiser, J., and Dwyer, B.P., "Summary Report on Close-Coupled Subsurface Barrier Technology Initial Field Trials to Full-Scale Demonstration", Brookhaven National Laboratory, Upton, New York, September 1997, BNL-52531.
    3. CPCB (Central Pollution Control Board). 2001. Criteria for Hazardous Waste Landfills, Ministry of Environment & Forest, 2001.
    4. IL&FS (IL&FS Ecosmart Limited). 2010. Technical EIA Guidance Manual for Common Hazardous Waste Treatment Storage and Disposal Facilities. Ministry of Environmental and Forest.
    5. http://apps.who.int/medicinedocs/en/d/Jwhozip51e/3.3.html