Hazardous wastes; definition, classification and management techniques

Hazardous Waste

Hazardous waste is the waste which is potentially dangerous or harmful to human health or the environment. The universe has hazardous wastes is very large and diverse. Hazardous wastes can be liquids, solids or contained with gases. They can be by-products of manufacturing processes, discarded used materials or discarded unused products such as cleaning fluids or pesticides.

Classification

1) Listed wastes

A hazardous waste is a waste that is one of the four RCRA1 hazardous wastes lists. They are

• The F-list (non-specific source wastes)

This list identifies the wastes from many common manufacturing and industrial processes such as solvents that have been used for cleaning or degreasing. The F-listed wastes are known as wastes from non-specific sources.

• The K-list (source-specific wastes)

This list includes the certain wastes from specific industries such as petroleum refining and pesticide manufacturing. Certain sludge and wastewaters from treatment and production processes of specific industries are examples of source-specific wastes.

• The P-list and the U-list (discarded commercial chemical)

These lists which include certain commercial chemical products that have not been used but that will be discarded. Industrial chemicals and pesticides are the examples of commercial chemical products that appear on these lists which become hazardous waste when it discarded.

• M-list Wastes (discarded mercury-containing products)

This list includes certain wastes contain mercury such as fluorescent lamps, mercury switches and the products that house these switches and mercury-containing novelties.

2) Characteristic Hazardous Wastes

Wastes may be hazardous wastes if they exhibit four characteristics of a hazardous waste i.e. ignitability, corrosively, reactivity and toxicity.

Introduction

Waste can be mainly divided into two types according to their effect o human health

• Non-hazardous waste
• Hazardous waste

Non-hazardous waste does not have the serious effect on human health and environment so they can be managed by simple methods such as composting, burning, recycling, using as fertilizer etc. Hazardous waste, however, causes severe damage to human health as well as they pollute the environment. Also, the byproducts of their treatment are hazardous. So they should be managed in the proper way.

Waste Management Approaches

Hazardous waste can be managed with four methods

• Physical approach
• Chemical approach
• Biological approach
• Thermal approach

Physical methods

Physical methods involve changing the physical appearance of the waste material so that they can be changed to more suitable to dumping according to the dumping facilities. Mainly used physical approaches for waste management are:

• Drying
• Screening
• Grinding
• Evaporation
• Sedimentation
• Filtration
• Fixation
• Separation
• Centrifuging

Chemical methods

The chemical method involves the conversion of the waste containing harmful chemicals into the neutral substance or less harmful form. The method used depends on the nature of waste and condition of surrounding such as temperature, humidity etc. Some of the commonly used chemical treatment methods are as follows.

1. Neutralization

Waste acid treated with an alkali to form neutral salt e.g. sulfuric acid with sodium carbonate:

H₂SO₄+ (CO₃)^2- → (SO4)^2- + CO₂ + H₂O

• Oxidation

Using as common oxidizing agents such as hydrogen peroxide or calcium hypochlorite e.g.

Cyanide waste with calcium hypo chloride

• Reduction

It is used to convert the inorganic substances to a less mobile and toxic form e.g. reducing Cr (VI) to Cr (III) by the use of ferrous sulphate

• Hydrolysis

The decomposition of given hazardous organic substances e.g. decomposing certain organophosphorus pesticides with the sodium hydroxide.

• Precipitation

Particularly useful for converting hazardous heavy metals to less mobile and insoluble form of prior to disposal in a landfill e.g. precipitation of cadmium as its hydroxide by the use of sodium hydroxide:

Biological methods

Biochemical methods involve the use of microorganisms under optimized conditions to mineralize hazardous organic substances. Mainly two methods are used to decompose waste using biological methods

1. Aerobic method
2. Anaerobic method

Aerobic method

Aerobic digestion is mostly used for the treatment of wastewater. It is operated in high oxygen presence (0.5ppm dissolved oxygen).by this process, Sulphur, nitrogen, and phosphorous compounds are converted into low-energy end products such as sulphates, nitrates, and phosphates. The main steps involved in aerobic treatment are described below

Pre-treatment stage: large solid substances are removed in this stage. Grit chamber is used for this process.

Aeration stage: Aerobic bacteria digest the biological wastes in a presence of air.

Settling stage: Any undigested solids are allowed to settle. This forms a sludge which must be periodically removed from the system.

Disinfecting stage: Chlorine or similar disinfectant is mixed with the waste to produce an antiseptic output which is dumped in the specific site.

Anaerobic method

It is a collection of processes by which the microorganisms breaks down the biodegradable materialism the absence of oxygen. This process is mainly used for industrial or domestic purposes to manage waste. The byproduct can use as fuel. This process includes fermentation used in industrial to produce food and drink products as well as home fermentation etc. Bacteria used in this process are mainly acid-forming bacteria (acetogenins) and methane forming bacteria (methanogens). End products of this process are mainly decomposed liquor which requires further treatment before discharge and a gas comprising mainly methane and carbon dioxide.

Anaerobic digestion relies on the optimum balance between non-methanogenic and methane producing organisms in order to ensure that the energy in waste is not retained in the liquor in the form of partially decomposed organics.

Thermal methods

The thermal method involves the conversion of hazardous waste into less hazardous/non-hazardous ash by application of heat. Different thermal methods used are:

• Incineration
• Autoclaving
• Gasification
• Pyrolysis
• Boiling
• UV treatment
• Microwave use

Incineration and autoclave are mostly used in context of Nepal

Incineration

Incineration is a thermo-chemical reaction in which carbon, hydrogen and other elements in the waste mix with oxygen in the combustion zone and generates heat. A typical incineration plant uses primary and secondary chamber. The temperature of the chamber rises more than 1000^o. This process has secondary advantages which are

• It reduces volume of waste by 90% and weight by 75%(Municipal Solid Waste Incineration, 1999)
• The generated volume can be used for another purpose such as generating steam which can use for produce electricity or directly heating purpose

Auxiliary devices can be added to the plant to make it more efficient and effective. Auxiliary parts such as emission control unit, ash disposal unit, heat recovery system etc.

Autoclave

Autoclave process uses heat, steam and pressure of an industrial autoclave in the processing of waste. This process is versatile as it process processes waste either in batch or in the continuous process. It is also preferred because autoclaves process gives a very high pathogen and virus kill rate.

References:
1. Mackenzie L. Davis & David A. Cornwell, “Introduction to Environmental Engineering”, McGraw Hill.
2. Gilbert M. Masters, Standford University, “Introduction to Environmental Engineering and Science”, Printice Hall.
3. Stephan Konz, Kansas State University, “Work design”, Grid Publishing Inc., Colombus, Ohio
4. C. S. Rao, “Environmental Pollution Control Engineering”, New age International (P) Limited, Publishers, India.