Solidification Of Waste Management

Solidification continues to be sold around the world as an inexpensive alternative treatment technique to other methods of waste management, particularly in countries with relatively lax environmental regulations. This treatment technique is not recommended unless a thorough investigation of the process to be used is conducted.

Solidification (also referred to as encapsulation, fixation, and stabilization)
is a technique in which material is added to reduce free water and possibly reduce or slow potential contaminant release. This is considered a treatment technique because something must still be done with the solidified material. In the past, this technique was used prior to burial or road spreading (as surface material) of the solidified material. It has also been used to make reusable material, such as bricks or blocks. Probably the most promising application has been to make roadbed material (the dirt used under the surface of a road).

Typically, fly ash or cement has been used to solidify cuttings. However, many products have been marketed claiming successful results. The main problem with solidification has been that there have been no well-defined processes or quality control techniques used. One could argue that adding dirt to cuttings is a solidification technique, but obviously this form of dilution would not be of much advantage except to reduce free water.

In any solidification process, the solidified material must first pass some sort of leachate test. As a process control, the amount of solidification material added to the cuttings should always be equivalent to the amount required to pass the leachate test. Because there is such variability in the consistency of the feed material, this has rarely been achieved. The second result that must be obtained is that the solidified material must resemble whatever material is being made. If the product desired is a brick, then a good brick should be made. If the product desired is road-grade material, then the material should pass the tests for good road-grade material.

Solidification leachate standards are given in Louisiana’s regulations (29b). If no regulation exists in the area where this technique is to be applied, then La. 29b is a good standard to follow. It assumes that the material is to be solidified and buried. The standard calls for a minimum compressive strength of 20 psi to be developed in a test block. The test block is then crushed, and sized particles of the block are immersed in water to test the leaching of oil and salt. The leachate limits for oil and chloride are less than 10 mg/L and 500 mg/L, respectively. There are other tests as well. It should be noted that almost no solidification process can be economically applied if this standard is met.

Assuming that the leachate standards can be met, then the second criterion is to meet standards set for the particular product. Again, no standard has ever been used in previous work, so there is no history to recall. But there is a road-grade material standard published by the American Association of State Highway and Transportation Officials. In the standard, there are specifications for materials used for embankments and subgrades (M 57-80) and for aggregate and soil-aggregate subbase, base, and surface courses (M 147-65), as well as a classification of soils and soil-aggregate mixtures for highway construction purposes (M 145-91). These specifications discuss desirable particle or aggregateb size, liquid or moisture content, and plasticity (relating to clay content and wet/dry cycling and expansion/contraction). A quick review of the standards would lead most readers to conclude that cuttings would probably not make good roadbed material.

But this technology continues to be sold, especially in developing countries and remote areas. The attractiveness seems to lie in the view that roads can be made from cuttings and that waste can be converted to a usable product. The promise has not come to fruition in the more developed countries.
Another seemingly promising technology has been conversion of the waste material into bricks or other construction slabs or blocks. Again, the technology has not proven reliable or economical if leachate standards are met. Salt readily leaches out of bricks and concrete, as can be seen in any tropical climate (salt is used to speed up setting time in building construction). In Venezuela, a wall was built of concrete blocks as part of an encapsulation/solidification process. Years later oil could be seen and smelled at considerable distance from the wall. Unfortunately, this result is the rule rather than the exception.

While the advantage seems obvious (if elusive), there are some disadvantages:

  • Poor or uncontrolled application of the technology
  • Leaching of contaminants
  • Increases in volume of waste (if it cannot be used as a product)
  • Labor intensiveness (may not be a problem if labor is cheap and unemployment is high)
  • Long-term liability with the product (would you want to live in a house made from solidified cuttings bricks?)
  • And the main disadvantage: the unfulfilled promise that this technology really works.

This treatment technique is not recommended unless a thorough investigation of the process to be used is conducted.

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