1. Chapter 6.4 Stabilisation and solidification of hazardous wastes
TRP Chapter

2. Immobilisation
Immobilisation is a collective term for a range of treatment processes that primarily aim to make hazardous waste safe for disposal by reducing the potential for waste component leaching
Stabilisation: techniques by which hazardous wastes are converted into a more stable form
Solidification: techniques that create a solid mass of either the original waste, or waste that has been stabilised
S/S often used in combination
S/S feedstock is often the residue from other treatment processes
Slide 2 Immobilisation
The umbrella term ‘immobilisation’ includes physical and chemical methods both for solidifying and for stabilising wastes.
Stabilisation is a general term to describe techniques or methods whereby hazardous wastes are converted into a more stable form. Stabilisation changes the chemical states of waste constituents to forms with lower aqueous solubility. The process usually improves the handling and physical characteristics of wastes.
Solidification refers to techniques which immobilise wastes in a solid mass. This may be either the original waste or waste which has first been stabilised. Solidification changes the physical form of the waste and usually causes chemical binding of the waste constituents into the S/S matrix.
Just as immobilisation is used in combination with other techniques (eg physico-chemical treatment), stabilisation and solidification are often used in combination and often used to treat residues from other treatment processes eg sludges from heavy metal precipitation or dewatering or fly ash from incineration (including of municipal solid wastes).
TRP Chapter

3. Aims of S/S of hazardous waste
Reduce potential for hazardous waste leaching
Conversion of pollutants into less toxic form
Decrease in waste surface area
Reduction of pollutant mobility
Formation of solid mass with no free liquid
Improvement in handling and physical characteristics of waste
Should normally be considered as a pre-landfill treatment process
Slide 3 Aims of S/S of hazardous waste
As the slide shows, the major aim of S/S techniques is to reduce the potential for hazardous waste components to leach.
An effective immobilisation process is one where potential pollutants are effectively locked into the stabilised or solidified waste structure and isolated from the wider environment, making them unavailable for leaching. This is achieved by various means:
by converting the pollutants into a less toxic form
by decreasing the surface area of the wastes
by reducing the mobility of the pollutants in the waste
An additional benefit is the improvement in handling that may occur.
TRP Chapter

4. Waste pre-treatment Adjustment of physical characteristics
particle size, shape and distribution by size screening and/or reduction
moisture content
homogeneity
viscosity
Adjustment of chemical characteristics pH
toxicity
removal of toxic constituents
destruction of toxicity
reduction of toxicity
Slide 4 Waste pre-treatment
Various pre-treatment processes may be required to transform the waste into a form suitable for S/S. Some of the types of pre-treatment that may be needed are listed on the slide.
As was shown on slide 3, S/S processes are often used in combination with other methods of hazardous waste treatment including physical and chemical methods (see also Chapter 6.2 Physico-chemical processes); biological treatment (see also Chapter 6.3 Biological methods) and also thermal methods (see Chapter 6.5 Thermal treatment).
The pre-treatment needed to render the wastes suitable for S/S techniques may include one or more of these.
TRP Chapter

5. Stabilisation Chemical reaction Acid/alkali neutralisation Chelation
Complexation
Oxidation/reduction
Precipitation
Hydroxides
Silicates
Sulphides
Chemisorption
Ion exchange
Slide 5 Stabilisation
Stabilisation of waste reduces its hazard potential by converting the waste into a less soluble, less mobile or less toxic form. This may include using some of the chemical and physical methods shown on the slide.
Waste pre-treatment and waste stabilisation may include the same types of processes.
Stabilisation may be the final stage prior to landfilling or, commonly, stabilisation may be followed by solidification.
TRP Chapter

6. Solidification May be used to treat original or stabilised wastes
Types of binders used:
Cement-based
Portland cement, cement kiln dust
Lime/limestone/quicklime
Lime/fly ash, lime kiln dust
Lime/ other natural and artificial pozzolana based systems
Thermoplastic materials
Asphalt (Bitumen), Paraffin, polyethylene
Thermosetting polymers
Polybutadiene, (poly)urea-formaldehyde, polyvinylesterstyrene
Slide 6 Solidification
Solidification techniques immobilise either the original or the stabilised waste through the formation of a solid mass.
Solidification normally involves chemical bonding between the waste and a binder. Some of the types of binders used are shown on the slide. Organic systems based on thermoplastics and thermosetting polymers are expensive and are not widely used.
There are two broad solidification approaches. Solidification binders may be mixed with the waste in the landfill (or waste which has been pumped into a pit) where the material then solidifies in-situ. Recent techniques use a batch process where the material is first allowed to set, allowing its strength and leaching characteristics to be tested, before it is sent to landfill as a solid.
TRP Chapter

7. Additives Activated carbon Emulsifiers and surfactants
Lime, fly ash & kiln dust
Oxidants
Reducing agents
Selected clays
Soluble silicates
Slide 7 Additives
S/S processes may be used in conjunction with sorbents or other additives to improve the immobilisation of specific contaminants. Additives can be particularly useful for cement or pozzolan processes, to decrease the mobility of contaminants in their porous products.
Activated carbon is the sorbent usually chosen for organics, although S/S methods are not ideally suited to organic wastes treatment (see slide 12).
Emulsifiers and surfactants are applied to allow the incorporation of immiscible organic solutions.
Lime, fly ash and kiln dusts are added to maintain alkaline conditions. They may also act as solidification binders by undergoing cement-like reactions in the S/S waste.
Oxidants and reducing agents are employed to convert contaminants to less toxic chemical states.
Selected clays are used to sorb liquid and bind specific anions or cations.
TRP Chapter

8. Key factors Characteristics of waste chemical properties
composition and concentration
acidity/alkalinity
oxidation/reduction potential
solubility
Physical properties
state (liquid, sludge or solid)
particle size, shape & distribution
solid content
viscosity
Characteristics of binders
Mode of processing
Slide 8 Key factors
The slide shows some the factors which influence the effectiveness of the S/S process, and the number of treatment stages needed.
The chemical, biological and physical characteristics of the waste are key. Appropriate treatment methods must be matched to these. The choice of binders and additives also depend on the waste characteristics.
In S/S, hazardous waste constituents are dispersed throughout an inorganic or organic binder matrix and this physically isolates them from groundwater. The effectiveness of the isolation depends on the permeability of the matrix and this is determined by the interactions between the waste and the binders and the processing method used to form the S/S monolith. The degree of mixing of waste constituents throughout the matrix is an important factor.
The long-term durability of treated waste is an important factor in determining the effectiveness of the S/S treatment. The aim is that the S/S waste should remain as a solid monolith over extended time periods.
In terms of controlling the leaching of metals, the acid neutralisation capacity is a key factor. The S/S waste should produce a local pH within the pores in the material’s microstructure that produces low solubility, typically in the range pH8-10 for most metals.
Discussion: Trainers should discuss the relative influence of the key factors on the use of S/S processes
TRP Chapter

9. Waste assessment Waste sampling and characterisation to determine:
type of contaminants
levels of contamination
spatial distribution of contaminants
presence of possible interference effects
S/S is best suited to largely inorganic wastes
Slide 9 Waste assessment
As the previous slide showed, there are key factors which influence the treatment choices when conducting S/S methods and the effectiveness of the process.
Waste sampling and characterisation are conducted to determine the type, levels and spatial distribution of the contaminants, plus the presence of any constituents that might interfere with the process.
In general S/S technologies are very effective at treating wastes that are primarily inorganic. Highly organic wastes can occasionally be treated, providing it is undertaken with care, but other techniques are usually more appropriate. Organic molecules can interfere with the hydration reactions of cement and other solidification binder systems. They also tend not to bind into the inorganic materials formed by S/S.
Wastes being considered for S/S should first be tested and their treatability assessed.
TRP Chapter

10. Performance tests Physical tests Chemical tests
Moisture content
specific gravity
bulk density
permeability
porosity
strength
durability
Chemical tests pH
acid neutralisation capacity
oxidation/reduction potential
total organic carbon
oil & grease
volatile organic compounds
metal analysis
Slide 10 Performance tests
There are a number of physical tests that can be used to measure the performance of S/S technologies on the wastes they have treated. Some of these tests may be required by regulations while others may be used to provide additional assurance that a given S/S process is appropriate for its intended use.
In the USA, the EPA generally considers a S/S material as satisfactory if it has a compressive strength of at least 50 pounds per square inch. The minimum strength required, however, should be determined from the design loads to which the material may be subjected.
Durability testing evaluates the ability of a material to withstand environmental stresses such as freezing and thawing or wetting and drying. Weight loss or the number of such cycles that the material can withstand without failing can provide an indication of its physical stability.
Extraction tests refer to leaching tests that generally involve agitation of ground or pulverised waste forms in a leaching solution. The leaching solution may be acidic or neutral, and it may vary during the test. Extraction tests may involve one-time or multiple extractions. In either case, leaching is assumed to reach equilibrium by the end of one extraction period. Therefore extraction tests are generally used to determine the maximum, or saturated, leachate concentration under a given set of test conditions.
Leaching tests measure the potential of a stabilised and/or solidified waste to release contaminants to the environment. In all tests, the waste is exposed to a leaching solution and the amount of contaminant in the leachate is measured and compared to a previously established standard. This standard may be a regulatory standard or it may be baseline leaching data for the untreated waste.
Testing can be expensive, especially when applied to a large number of samples, and replicated to ensure statistical validity.
Leaching/extraction tests
TRP Chapter

11. Properties of S/S treated waste
1. FORMULATION:
Waste type, % and composition
Binder composition
Water content
2. PROCESSING OF MIX
Mixing method
Hydration conditions
Age
Disposal conditions
Slide 11 Properties of S/S treated waste
The properties of the stabilised wastes once they have been treated by these methods depend on the factors shown on the slide.
TRP Chapter

12. Re-use applications for S/S waste
Likely to be limited because of:
Unreliable long term durability
Poor mechanical properties
Perception of risk
May be possible to use as inert fill
Should be seen as landfill pre-treatment method
Disposal - should not be with mixed MSW
Compatibility with disposal environment should be tested
Slide 12 Re-use applications for S/S waste
As a general rule, S/S techniques should be seen as a landfill pre-treatment stage. (See also Chapter 6.6 Land disposal)
Although there may be posible ways to use S/S treated waste, there are a number of factors to consider. The lack of knowledge about long-term performance and environmental impact of the S/S treated wastes is the most important of those. In view of the wide range of possible scenarios in use situations, it is often impossible to predict the environmental consequences of using waste products based on limited testing data. The perception of risk associated with use of S/S waste will typically limit the desirability and the opportunities.
The leaching potential may make the S/S treated waste suitable for use only in specified controlled circumstances eg quarry rehabilitation, road base material.
Where the S/S treated waste is unsuitable for use, or where use is impractical or not permitted, it should be landfilled in a segregated site. Disposal in a clay-lined cell is the preferred option.
S/S waste should not be landfilled with MSW as the acid leachates produced by the mixed wastes will adversely affect the S/S waste and its ability to retain hazardous components.
Compatibility with the disposal environment should form part of the design and treatability-testing programme.
TRP Chapter

13. Wastes typically treated by S/S
Air pollution control residues
Metal sludge wastes
Dredging sludge
Filter press cake
Tannery wastes
Contaminated soils
Lagoon sludge
Plus other PREDOMINANTLY inorganic
wastes - all are likely to contain some organics
Slide 13 Wastes typically treated by S/S
The slide gives some examples of types of wastes that are commonly treated by S/S. The list is not exhaustive and many different waste types have been treated, although these probably represent the major types.
Waste incineration ash and air pollution control residues are commonly solidified in Europe while in the USA contaminated soil is extensively treated by S/S.
Note that all of these wastes are predominantly inorganic, although they are likely to contain some level of organic contamination. As has been stated, highly organic wastes are not generally suitable for treatment by S/S techniques.
TRP Chapter

14. Case study - UK TRP Chapter 6.4 14 Slide 14 Case study UK
The next four slides describe an S/S facility in the UK.
There are no specific requirements for the properties of S/S wastes in the UK, but the EC Landfill Directive (which has been adopted into UK law) requires broad testing of wastes to determine their classification and thus to which category of landfill (hazardous, non-hazardous or inert) they are sent for disposal. Given the higher costs and lower availability of hazardous waste landfill, where S/S processes can enable hazardous wastes to be downgraded to non-hazardous, the techniques may become more widely used in future.
TRP Chapter

15. Schematic of waste S/S plant
Part 1: Stabilisation
Waste effluent
5000 tonnes
Alkali waste
pH > 12
Inorganic
solids,
liquids
and
sludge
TOC < 1000 mg/L
Stock
storage
tank pH
20-30%
solids
Vacuum
filtration to
produce
filter
cake
9000 tonnes
acid waste
pH > 7-8
Slide 15 Schematic of waste S/S plant Part 1 - Stabilisation
This is a schematic diagram of a typical S/S waste plant that was operating until recently in the UK. This slide shows a diagram of the stabilisation stage.
Incoming wastes to the plant are primarily inorganic liquids, solids or sludge. Wastes with a TOC (total organic carbon) greater than 1000 mg/L were not accepted for treatment.
Wastes were separated into alkali (pH>12) and acid wastes (pH~7-8) and stored in large basins. These were then blended in a large tank known as the stock storage basin. The pH of this was maintained in the region between 8.5 and 9.5 by careful blending so that metals would precipitate out of solution.
In this way alkaline wastes were effectively used to treat acid wastes. The solids content of the stock storage basin was typically 20-30% and the whole mixture was kept agitated to stop settlement. A stream from the stock storage basin fed a vacuum filter press that produced the stabilised waste for solidification.
Incoming waste storage blending stabilised
waste waste
TRP Chapter

16. Schematic of waste S/S plant
Part 2: Solidification
stabilised
waste
filter
cake
Landfill
disposal
in clay
lined
cell
15% OPC + 15% PFA
+ 70% filter cake
(~ 50% solids)
Output ~ 6T
per hour
Slide 16 Schematic of waste S/S plant Part 2 - Solidification
The stabilised waste was produced at a rate of approximately 6 tonnes per hour from the filter press.
This was fed to a high-energy mixer where it was mixed with the solidification binders. Normally a mix of ordinary Portland cement (OPC) and pulverised fuel ash (PFA) was used although the exact mix varied. The cement represents a major cost to the process and therefore there was a driver to keep this to a minimum.
The solidification binder materials hydrate in the water contained within the filter cake. No additional water was added. The product formed initially had a clay-like consistency and was transported directly to landfill for disposal in a clay lined monofill cell.
Stabilised High energy Mono-disposal
waste mixer
TRP Chapter

17. Waste types treated by UK plant
Type of waste % of whole
Sulphuric acid
Hydrochloric acid
Chromic acid
Mixed/other acids
Al-chloride solutions
Fe-chloride solutions
Solid/liquid cyanides
Caustic solutions
Neutral sludges
Lime sludges
Other sludges
Filter cakes
Paint stripper washings 1.7
Ferrous sulphate
Others
Slide 17 Waste types treated by UK plant
The waste types treated by this plant are summarised in this slide.
As can be seen, a wide range of acid and alkaline wastes were treated.
TRP Chapter

18. Treated waste specification
28 day strength from any 1 day of production will
not be less than 700 kPa. No individual sample to
have strength less than 350kPa
Permeability at 28 days less than 1x10-7 m/s
No supernatant after S/S waste standing 24 hours
Leaching properties:
Depends on test but typically includes limits on
pH, TOC, Total cyanide, total phenol, ammonia
Heavy metals (Zn, Hg, Cr), total metal limit and total
organic or organo-metallic pesticides
Slide 18 Treated waste specification
The slide shows the performance specification agreed between the contractor and the local authority for the S/S treated materials produced by the UK plant.
The requirements covered strength, permeability and leaching of the treated waste. It was also required that no supernatant or free standing water would form on the surface of the treated waste.
TRP Chapter

19. Key considerations
Waste reduction and avoidance by generators should always be a priority
Role of on-site vs off-site technologies
Need to consider residues from treatment processes and their disposal
Transitional technologies may be used until final high-quality installations are available
Slide 19 Key considerations
There are some key factors to take into account before making any decision on treatment or disposal. Trainers should refer to slides 30, 31 and 32 of Chapter 6.1 when summarising each of the Chapters dealing with treatment options.
These are also a useful basis for classroom discussions and individual study projects.
In many cases there will be additional factors which are important, and these should be developed by individual trauners based on local knowledge and issues.
In each case, technology evolution will be important. A simple modular approach to technology can enable continuous adaptation as the overall waste management system becomes more comprehensive. Conversely, installation of advanced technology can itself help to determine the development of the strategy. In all cases the investment cost (purchase, installation, operation and monitoring) must be carefully determined. The return on investment will depend on the fee structure for wastes accepted, and this is often linked to government policy on industrial development.
TRP Chapter

20. Chapter 6.4 Summary Stabilisation and solidification techniques
Reduce potential for hazardous waste leaching
Improve handling and physical characteristics
May require pre-treatment of wastes eg to change particle size, pH
Stabilisation is usually followed by solidification
Should be considered as a pre-landfill treatment process
TRP Chapter