Since toxic waste can be so harmful, great care must be taken while collecting, transporting, and storing hazardous materials. The vast majority of hazardous materials are transported by highway, rather than by train, ship, or plane. This helps ensure a comparatively easier clean-up effort in the event of a spill. Additionally, most treatment, storage, or disposal facilities (TSDF), are directly accessed by public highways.
The United States, as well as some other countries, utilize a ‘manifest system,’ in order to track hazardous waste from its birth to eventual disposal. This is sometimes referred to as a ‘cradle-to-grave’ manifest system. Serving as a kind of waste serial code, these manifests help lessen the opportunity for illegal dumping.
Unlike persistent toxins, some hazardous waste can be treated by chemical, thermal, biological, or other physical methods of purification. Chemical treatments can involve ion exchange, in which negative or positively charged ions are removed and traded within the molecular make-up of the waste. Precipitation, and chemical neutralization are also forms of treatment via chemical means. Thermal methods can involve incineration, which is dually productive in detoxifying organic wastes while also essentially eliminating them. Thermal equipment is highly specialized, and varies between solid, liquid, and sludge incineration. This can involve a fluidized-bed incinerator, multiple-hearth furnace, rotary kiln, or liquid-injection incinerator. Thermal hazardous waste incineration must consider the possibility of air pollution while burning these materials.
Some hazardous wastes are treated through biological means. An example of this would be ‘landfarming,’ in which waste is combined with surface soil and covered with a mixture of microbes and nutrients. Sometimes, specifically engineered bacteria is injected into the site to maximize decomposition. In a process called bioremediation, microbes are used to stabilize hazardous wastes and reduce opportunities for contamination. The land used for waste decomposition or remediation will not be used in order to grow crops or other materials meant for human-use. Additionally, considerations must be made for opportunities for pollution, such as how precipitation and the resulting runoff will be handled.
While the above methods all involve changing the molecular form of the waste material, physical treatment is intended to concentrate, solidify, or reduce the volume of the hazardous waste. This can include evaporation, flotation, filtration, sedimentation, or solidification. Solidification involves entrapping hazardous waste within an impermeable material that prevents leaching into the surrounding soil. Physical treatment within evaporation removes the water within the waste, leaving behind a highly concentrated residual that takes up less space. Sedimentation, flotation, and filtration are all methods that separate toxic particles from liquid, leaving behind a reduced toxic load for disposal.
Hazardous liquid waste is often held within an open lagoon or pit. These waste lagoons are lined with impervious clay soils and durable membrane liners to protect the surrounding soil and groundwater from contamination. Leachate collection systems are paramount within a waste lagoon, continuously removing collecting fluid from between the liners in the landfill and the lagoon itself. Additionally, monitoring wells are often installed around the lagoon to ensure the health of the surrounding groundwater. Waste lagoons can minimally treat hazardous waste through sedimentation, evaporation of volatile organic material, and surface aeration. Accumulated sludge or scum will be dredged or skimmed from the lagoon, and then transported to another form of treatment, such as thermal, physical, or chemical means. Waste lagoons are often subject to floods and overflow during severe weather events. When this happens, the toxins trapped within the waste can travel long distances, affecting not only crops and livestock but human populations located downstream. Open lagoons present serious risks even for distant ecosystems, but closely managed, covered lagoons are rarely mandated and often considered too expensive.
Many older, unlined waste piles and lagoons are located above aquifers used for public water supply, thus posing significant risks to public health and environmental quality. Many these old sites have been identified and scheduled for cleanup, or remediation, around the world.
Hazardous solid wastes are deposited in ‘secure’ landfills, similar to the sanitary landfills of normal waste. These secure landfills provide special considerations for the especially toxic nature of the materials found within. Secure landfills provide at least 3 meters, or 10 feet of separation between the bottom of the landfill and the underlying bedrock or water table. These landfills must have at least two impermeable liner layers separated by a leachate collection system. The double leachate collection system often consists of a network of pipes placed above each liner. The higher system is intended as primary prevention against accumulation of leachate, while the lower systems serve as backups. Collected leachate may be reused, like in the case of a bioreactor landfill, or is then pumped to a treatment plant. In order to limit the amount of rainfall infiltrating a closed landfill and overwhelming the leachate collection system, a cap or impermeable cover is placed over a finished landfill. Stormwater management systems are also used to collect and deflect rainfall that runs off the capped surface.
