Stabilisation involves the addition of reagents to a contaminated material (e.g. soil or sludge) to produce more chemically stable constituents; and
Solidification involves the addition of reagents to a contaminated material to impart physical/dimensional stability to contain contaminants in a solid product and reduce access by external agents (e.g. air, rainfall).
Available Binders and Products A range of binder systems are available that include cement, lime and a variety of mineral additives. Binders may both solidify and stabilise, depending upon their selection. Cement or lime-based binders are popular and utilise hydraulic reactions to immobilise contaminants. These binders typically result in a treated material that has a pH in the range 10-12 units, causing many metallic pollutants to precipitate as insoluble salts. Cement- solidified waste forms can be engineered to have low permeability (typically 10-6 m/s to 10-9 m/s) and the strength of weak rock (7).
The main hydration product from the use of cement is calcium silicate hydrate gel (known as C-S-H). This phase binds a waste form into a solidified monolithic mass or a granular material with a soil-like consistency, depending on design objectives. The C-S-H gel has a significant sorptivity for many inorganic and organic pollutants and can ‘lock’ contaminants into its chemical structure. It is also primarily responsible for modifying the s/s products’ pore structure, resulting in reduced porosity and permeability, and the immobilisation of contaminants, breaking the source-pathway-receptor linkage.
Determining the viability of s/s Although contaminants are not removed or destroyed by s/s, the technology can have a number of advantages over alternative risk management options, including: • S/s can be used to meet the requirements of the Landfill Directive
• Treatment can be completed in a relatively short time period
• Diverse and recalcitrant contaminants, such as heavy metals and dioxins can be treated
• S/s can be performed in-situ or ex-situ and occupy a relatively small footprint on-site
• Different mix designs can be used across a site to target material or contaminant type(s) and concentration(s)
• The geotechnical properties of a soil may be significantly enhanced enabling re-use as an engineering material
The eventual cost of remediation will be strongly influenced by the choice of binder materials, the throughput of treated materials, the nature and concentration of contaminants and the physical properties of the material to be treated. Other costs that may apply include the maintenance of measures to protect the treated material, such as capping and (any)
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long-term monitoring requirements.
Following the effective design and implementation of s/s, the pollutant linkage is broken. However, the release of contaminants will eventually occur over extended timescales and the durability of a risk management strategy that includes s/s will need to be effectively demonstrated. Key issues that may need to be addressed include the structural integrity of the treated material, the buffering capacity of the system against ingress of acidic groundwater, and the rate and time scale of contaminant release.
The efficacy of s/s The effectiveness of s/s to manage risk is critical and will be dependent upon a good knowledge base. EA Guidance recognises this and requires that expert knowledge is used and that the materials to be treated are fully characterised. Nevertheless, key factors for consideration include: • Selection of the most appropriate binder formulation. • Effective contact between the contaminants and binder reagents (i.e. thorough mixing, using plant that is fit for purpose).
• A high degree of chemical and physical consistency of the feedstock.
• Control over external factors such as temperature, humidity, which may affect setting, strength development and the durability of the product.
• The absence (or control) of substances that inhibit s/s processes and can affect the product properties.
• Good engineering design of the site as a whole, including the boundaries of the treated material.
A key inclusion in the EA Guidance is the use of treatability trials in the field to demonstrate the effectiveness of the remedial design chosen.
As previously mentioned, the use of s/s has been far greater in the USA, where the technology is considered an effective remedial treatment for contaminated land and wastes and is recommended by the US Environmental Protection Agency (USEPA) as a best demonstrated available technology (BDAT). A large amount of work on the application of s/s to contaminated soil has been undertaken by the USEPA and binders used in Superfund projects include cement, phosphate, lime, proprietary additives, polymers, iron salts, silicates and clays (8).
A number of large sites in Europe have employed s/s as a remedial option, including treatment of oil sludge pits in Brest, France, and a number of tar pits in Belgium (9).
Treatment of Waste There are two routes for the treatment of waste streams by s/s, either as part of the waste-generating process, dealing with a specific waste stream, such as ash generated by incineration plants; or through a centralised facility, such as the former processing plant at Thurrock, Essex (10). At Villeparisis, near Paris, approximately 1M
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