Process Equipment for Treatment of Contaminated Soil and Sludge Waste
In the context of hazardous waste treatment under the POEO Act, it is imperative that the responsible person uses proper process plant and equipment competently to conduct the treatment. This approach would help assure that the favourable treatment results achieved at the laboratory or pilot scale trials can be repeated consistently at the full scale treatment and high treatment standards are maintained at all times.
Industry may practise either or both of the following methods in the immobilisation treatment of contaminated soil and sludge waste:
- Chemical fixation: Chemical reagents are used to convert the target contaminants contained in the waste to a chemically stable form(s) suitable for landfill disposal.
- Stabilisation/solidification: Cement and/or pozzolans reagents are used to transform the waste into a stable monolithic substance suitable for landfill disposal.
Unlike washed and clean aggregates used in cement concrete, contaminated soil and sludge waste including river sediments can be very heterogeneous with a mixture of materials of different particle size distributions, shapes, densities and surface properties (eg clayey and plastic). Such dissimilar characteristics and rheological properties can compromise the immobilisation treatment.
The responsible person must use a properly designed and engineered treatment plant that is adequately equipped with automatic or semi automatic control in respect of waste and chemical reagents handling. Avoid or prohibit any manual operation which is prone to human error and may be unreliable. A typical process flow diagram for the treatment of contaminated soil and sludge waste is in Annex A (PDF 14KB).
The mechanical mixer functions as the chemical reactor of the immobilisation treatment process. It should be equipped with a stationary mixing compartment and an agitator fitted with heavy duty mixing paddles/blades; and it can perform the following mixing duties:
- Provide positive agitation/stirring of the mix and achieve rigorous mixing e.g. turbulent flow within the mixing compartment.
- Adequately handle homogeneous and heterogeneous solids including soil, aggregates and sludges, and materials exhibiting plastic properties e.g. silt and clay.
- Capable of achieving a homogenous mix within minutes of mixing.
As a matter of OEH policy the tumbler type mixer e.g. small DIY rotating concrete mixer, rotating mixer mounted on a delivery truck, rotary hoe or bull dozer are not acceptable mixing devices for the immobilisation treatment of contaminated soil and sludge waste. Such machineries cannot discharge the above mixing duties for processing hazardous waste or sludge.
The following types of mixer (Perry, Chemical Engineers Handbook, McGraw Hill) could attain the above mixing duties and are considered suitable for the purposes of the immobilisation treatment of contaminated soil or sludge waste. They are commonly used in industry operations.
- Pug mill mixer.
- Paddle type mixer including Ribbon mixer and Turbine/Pan mixer.
However, the choice of mixer is a waste specific issue and the responsible person should conduct a test run before adopting the equipment for full scale treatment. OEH would consider and approve other types of mechanical mixing device on merits.