Treatment evaluation

Treatment of materials to make them meet environmental criteria for a given application is a common practice. The tools to be applied for verifying compliance with regulatory criteria has some pitfalls:

• If a given test, usually a single step procedure, indicates that a material does not meet the criteria for one or a few substances, the focus may be to cure the issue for the substances that proved to fail. However, a treatment may change the leaching conditions of a material due to alterations in the overall behaviour of the material, which implies that a broader range of substances needs to be verified to ensure compliance with the regulatory criteria. An example is the Pb leachability from a highly alkaline material (pH > 12). Bringing the pH down through carbonation of the material will bring the Pb leachability down dramatically, but at the same time may lead to an increase in oxyanion mobility (e.g. As, Sb, Se, Mo, SO4, V).
• A thermal after treatment of MSWI bottom ash to reduce the Cu leaching (Cu associated with dissolved organic carbon – DOC) will bring down the Cu leaching by at least an order of magnitude, but the oxidation occurring as a consequence of heating the material to some 500 °C will transform part of the Cr (III) present to more mobile chromate with the possibility of exceeding a release criterion. For more information see the section on “Role of dissolved organic carbon” under Material information – Geochemical modelling)
• Generally, the application of chemical complexants does not lead to long-term stable conditions and thus should be avoided.

Understanding the processes that control leaching is helpful in designing treatments that will have a long-lasting effect on the release behaviour of a material. The combination of pH dependence leaching (EN14429 or EPA 1313) in combination with geochemical modelling will provide means to identify chemically stable controls of leachability that work over a broader range of exposure conditions that may occur in the foreseen application. Aging of materials that can impact leaching as freshly treated materials may have very different leaching results than field aged materials. Important processes include:

(i) carbonation (for alkali materials),

(ii) redox changes (oxidation by exposure to air; reduction by microbial processes);

(iii) leaching/depletion of solubility controlling elements (e.g., Ca depletion impacts As leaching);

(iv) potential blending with other materials during use or disposal that can change the leaching chemistry.

A practical comparison is to run a pH dependence test before and after treatment to evaluate the consequences of the treatment on the overall release behaviour of the raw material vs the treated product. The same option applies to verifying environmentally acceptable release behaviour of additives to material mixes, that were not used before and now due to increased demand for recycling and beneficial use of materials are becoming more prominent. Ensuring safe use requires insight in long-term release behaviour of the use of alternative materials.

Many examples of such questions exist:

• Use of incinerator bottom ash as a admixture to cement and concrete
• Application of industrial slags as sand replacement in concrete
• Application of waste derived materials in fertilizers and growing media
• Application of industrial slags in mixtures applied in road works

More information on environmental assessment:

Regulatory approaches
Uncertainty
Treatment evaluation
Laboratory – Field comparison
Waste classification

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