Hazardous waste is waste that is dangerous and a potential threat to humans and the environment. To tackle this problem, certain modern technologies are utilized commercially that have been mentioned in detail with their pros and cons. These technologies mainly comprise physical/chemical processes, biological treatment, soil remediation, treatment of volatile compounds, and mixed waste treatment procedures. The physical/chemical processes have been further divided into 4 core processes that are the use of solid catalysts, adsorption & flocculation, supercritical water oxidation (SCWO), and Advanced oxidation processes (AEPs). The soil remediation process involves ex-situ thermal treatments, ex-situ chemical treatments & bioremediation, and in situ methods. To treat volatile compounds, we may use air venting, oxidation, photo-oxidation, use of microwave discharge plasma reactor, or use of fixed bed bioreactors.
All these processes are core to maintaining an environment that is both healthy and safe and waste should be disposed off properly by employing these techniques to avoid any potential hazards.
Hazardous waste (HW) is the type of waste that has the potential to create harmful effects on humans and the environment. HW is also generated both from natural processes and anthropogenic activities such as manufacturing, chemical production, and other industrial activities. With the continuous increase in production and consumption of industrial products, the quantity and toxicity of hazardous waste is increasing, posing major challenges for its management. Hence there is an ever-increasing demand for cost-effective and environmentally protective solutions. The approaches to managing hazardous waste attempt to reduce the volume of hazardous substances, reduce it to fewer toxic forms and use engineering controls to eliminate or reduce its exposure. US-Environmental Protection Agency has developed a framework for cradle to grave management of hazardous waste, which means because of its harmful nature, it should be safely managed at each step from production, transport, treatment, storage, and disposal. This article further describes different methods for the treatment of different types of hazardous wastes (wastewater, soil, organic compounds).
Physical and Chemical Methods for Hazardous Wastewater Treatment
To handle the hazardous wastewater, there are some physical and chemical treatments available which include:
- Use of solid catalysts: Solid catalysts boost the rate at which hazardous substances degrade. Basically, three things are involved: 1) Solid particle is surrounded by a layer through which diffusion takes place, 2) occurrence of diffusion through pores takes place at the catalytic site, and 3) the reactants adsorb.
- Adsorption and Flocculation: Adsorption can be a physical or chemical process where molecules of impurity stick to solid surfaces such as activated carbon. Flocculation uses fine particles that bind with impurities to form agglomerates that sediment. The chemical treatments including flocculation and adsorption can be used for treating wastewater, but how efficient they are in treating hazardous wastewater has still not been fully explained.
- Supercritical Water Oxidation (SCWO): SCWO is of great interest as it is capable of efficiently oxidizing many harmful substances such as polychlorinated hydrocarbons, without producing nitrogen oxides.
- Advanced Oxidation Processes (AOPs): AOPs are based on hydroxyl radical. Catalytic and Photolytic systems generate this radical which helps in the degradation of the main toxic substances and pollutants.
Biological Treatment of Hazardous waste:
There is a huge scope of biological treatment methods of hazardous waste because they are economically favourable and environmentally friendly. Microorganisms play a vital role in purifying the water by two techniques: 1) immobilization on beds and 2) over-use of their natural bio-accumulative properties. Microorganisms also have the quality of oxidation and reduction of some species which we can’t ignore because these qualities can be used in precipitating or liberating metals. Moreover, they help in the control of air pollution and treatment of soil.
Soil Remediation and Treatment
Soil remediation and treatment techniques include in-situ or ex-situ techniques. In-situ means that soil is treated in its original place, while ex-situ means removing the soil from the subsurface to treat the soil. The following types of in-situ and ex-situ methods are used to treat hazardous waste in the soil.
- Ex-situ thermal treatment: effective, but costly and harmful for the soil.
- Ex-situ chemical treatment and Bioremediation: Both are good at remediation of pollutants but can produce toxic by-products.
- In situ methods: where the soil is treated on-site, it does not cause much harm to the soil, but is slow.
Treatment of Volatile Compounds
Volatile Compounds can be treated by the following methods:
- Air venting: This is done with activated carbon adsorption. This is a common method used for the safety of workers who are exposed to volatile emissions. The disadvantage of this method is that regeneration or replacement of the adsorption beds is required.
- Oxidation: This is used for the detoxification of airborne volatiles.
- Photo Oxidization: Ultraviolet light is used in this method. The radiation source used can be a Xenon lamp. There is a directly proportional relationship between absorption and destruction rate.
- Use of microwave discharge plasma reactor at ATM: This is done for the destruction of trichloroethylene and 1,1,1-trichloroethane which are the most widespread volatile compounds (VOCs).
- Use of fixed bed bioreactors: The gaseous hazardous substances could be cleaned via gas-phase bioreactors. There is a competition between gas-phase bioreactors and traditional aqueous phase bioreactors.
Selected Mixed Waste Treatment Applications
When hazardous waste is mixed with different kinds of waste, special treatment methods are used for their treatment. Mainly, mixed waste constitutes radiological and chemical hazardous substances. Electrochemical Processes are used for the treatment of mixed waste.
- As compared to refractory metal oxides, carbonaceous species, both solids, and liquids, get oxidized more quickly.
- The refractory metal oxides, such as PuO2, can get ionized and then dissolved.
- Extra reducing agent could be added to diminish the corrosive traits of species with higher valency.
Written by: Sibgha Batool
Sibgha Batool is currently doing her MPhil in Environmental Sciences from Forman Christian College (A Chartered University).
Edited by: Anam Riaz and Mansoor Ahmad Majeed .