Containment is used when there is no need to remove the waste material and/or the cost of removal is prohibitive. The main purpose of containment is to prevent or control liquid or semi-liquid contaminated wastes from leaking or leaching into surrounding areas. Techniques include pumping, capping, draining, and the installation of slurry walls. The selection of various types of containment systems is based on the type of waste materials, and geo hydrological conditions of the waste site.
Solidification is used to transform the waste into a durable physical form that is more compatible for storage, landfill, or reuse. This can be achieved with or without chemical reaction. It creates barriers between waste components and the environment by either reducing permeability of the waste, reducing the effective surface area available for diffusion, or both.
Stabilization refers to a chemical technique used to render a waste less toxic or less harmful to the surrounding environment. It reduces the hazard potential of the waste itself. Examples of stabilization techniques include the ion exchange of heavy metals in the alumino-silicate matrix of a cementitious stabilization agent and the sorption of heavy metals on fly ash in aqueous systems.
Nonchemical solidification includes dewatering, mixing with adsorbents, and vegetative stabilization.
These processes represent alternatives to mine disposal, ocean disposal, or conventional landfilling. Most of these processes originated in the field of radioactive waste control and management.
There are many solidification and stabilization techniques used in the industry for different types of waste materials. Each of these techniques is usually recommended for use with specific waste constituents. Generally, stabilization/solidification techniques can be divided into the following six major categories:
Portland cement is obtained by heating limestone and clay or other silicate mixtures at high temperatures. The resulting clinker is ground to a highly uniform fine powder. The addition of water to portland cement initiates the cementation process.
Most hazardous wastes can be incorporated into a waste-cement system. The suspended pollutants would be incorporated into the final hardened concrete. During this solidification process, the concrete formation binds and strengthens the mass, coats and incorporates some contaminant molecules in the siliceous solids, and block pathways between pores. Due to the elevated pH of cement mixtures (in the range of 9–11), most multivalent cations are converted into insoluble hydroxides or carbonates. Thus, this process is highly effective for waste components with high levels of toxic metals.
These techniques involve mixing a siliceous (pozzolanic) material with other alkaline earth material such as lime or gypsum in the presence of water to produce a concrete-like mass. The most common pozzolanic materials used in the stabilization/solidification of wastes are fly ash, ground blast-furnace slag, and cement-kiln dust. Combined cement-pozzolanic processes can be used to give a better and more economic final waste product.
When using a thermoplastic process, the waste is dried, heated, and then dispersed through a heated plastic system. Bitumen is the most common matrix material. However, paraffin and polyethylene have also been employed.
Sorbent techniques involve the use of clay minerals or modified clay minerals with high specific surface areas to fix and immobilize hydrocarbon molecules. This immobilization usually takes place on the outer and inner surface of clay particles. Typical clay minerals used in this technique include sodium-montmorillonite and vermiculite.
Organic polymer techniques were introduced to solidify/stabilize radiological wastes to allow for transportation. The urea-formaldehyde (UF) system is the most widely employed organic polymer solidification technique. In this technique, the wet or dry waste is mixed with a prepolymer in a waste secondary containment system. After the two components have been thoroughly mixed, a catalyst is introduced. This process forms a mass that traps the waste. After curing, the polymerized mass becomes highly impermeable.
Encapsulation means coating the waste with inert materials. The coating materials are chemically stable, adhere to the waste, and resist biodegradation. High-density polyethylene (HDPE) and polybutadiene are used to perform encapsulation.
This is similar to encapsulation but the WHO calls it out as a separate process.
Inertization is often used for ash (from incineration of wastes) and pharmaceutical waste.
A manual published by the WHO gives this recipe:
65 percent pharmaceutical waste
5 percent water
15 percent lime
15 percent cement
One great thing about inertization is the lost cost of the equipment and relative ease of use. Workers should wear PPE, but other than that, not a lot is required. Concrete mixers are ubiquitous - finding a used electric one is pretty easy. A crushing machine may be employed to grind the raw waste to smaller particle sizes. Bins or feeders may be employed to hold the lime and cement.
The waste cement can go into almost any sturdy container. That’s one of the beauties of cement - it’s like liquid rock until it hardens. The disposal facility might have rules on what kind of containers you can use. But within reason (don’t make the containers too big), they will probably take anything.
55-gallon drums are often used. They can be metal or plastic (high-density polyethylene). They are cheap and easy to get and interchangeable. You don’t get locked into a drum provider. Drum rings and gaskets are interchangeable.
Containers are usually watertight, but not doubled-walled. The cement itself is sufficient to isolate the waste from the environment.
You do not want large chunks of waste going into the mixer. The more homogeneous the better. Small pieces of solid waste have a higher surface area to volume ratio than large pieces, and the inetization is going to be more effective on small pieces. Many process designers install size reduction machines upstream of the cement mixer for this purpose. A “crusher” (types include ball, cone, and gyratory) reduces particle size for pieces that are over an inch in dimension. Rod mills and ball mills can produce even smaller pieces, although at some point the process designer must decide when enough is enough.
Vitrification incorporates the hazardous contaminated waste into a chemical, durable glass-like product. The process of vitrification is similar to the natural occurring obsidian (silicate glass) originated from rapidly solidified magma. Because of the rapid cooling rate and high liquid viscosity of oxide and silicate, molecules cannot move sufficiently to form a crystalline structure. Hence a glass-like structure is formed. Waste glass is so stable that some vitrified waste, such as municipal incinerator ash, can be made into consumer products like ashtrays, building material, or insulation. Almost any types of waste can be vitrified into a durable glass.