Hazardous Waste Management & Treatment
Hazardous waste means a solid waste or combination of solid wastes which because of its quantity, concentration or physical, chemical of infectious characteristics may:
- Cause or significantly contributes to an increase in mortality or an increase in serious irreversible or in capacitating illness or
- Pose a substantial present or potential hazard to human health or the environment when improperly treated, stored, transported or disposed of or otherwise managed (HWM, 1994)
Hazardous wastes refer to wastes that may, or tend to, cause adverse health effects on the ecosystem and human beings. These wastes pose present or potential risks to human health or living organisms, due to the fact that they:
- are non-degradable or persistent in nature;
- can be biologically magnified;
- are highly toxic and even lethal at very low concentrations.
The criteria used to determine the nature of hazard include toxicity, phytotoxicity, genetic activity and bio-concentration
Criteria for Identification of Hazardous Waste
By using either or both of the following criteria, we can identify as to whether or not a waste is hazardous:
i. The list provided by Government agencies declaring that substance as hazardous
ii. Characteristics such as ignitibility, corrosivity, reactivity and toxicity of the substance
Characteristics & Classification of Hazardous Waste
Characteristics of Hazardous Waste
United States Environment Protection Agency (USEPA) has classified waste into the category of hazardous waste if it poses one of the four characteristics:
Ignitability: (EPA code D001)
- It is a liquid having flash point less than 60 degree Celcius
- It is not a liquid and is capable of catching fire under standard temperature and pressure through friction, absorption of moisture, spontaneous chemical changes etc.
- It is inflammable
- It is a strong oxidizer
Example - Naphtha, thinner, epoxy resins, adhesives and oil based paints
Corrosivity: (EPA code D002)
- Having pH less than 2 or greater or equal to 12.5
- It is a liquid that corrodes steel at a rate greater than 6.35 mm per year at a temperature of 55 degree celcius
Reactivity: (EPA code D003)
- If normally unstable
- React violently with water
- When mixed with water generates toxic gases, vapours, fumes that pose danger to human health and environment
- Cyanide or sulfur bearing wastes
- Explosive at standard temperature and pressure
Example - gunpowder, sodium metal or wastes containing cyanides or sulphides
Toxicity: (EPA code D004) -Wastes that is capable of releasing toxic gases, fumes or any other end product (conducted in a certified laboratory)
Classification of Hazardous Waste
Substances that emit ionising radiation are radioactive. Such substances are hazardous because prolonged exposure to radiation often results in damage to living organisms
Most hazardous chemical wastes can be classified into four groups: synthetic organics, inorganic metals, salts, acids and bases, and flammables and explosives
The principal sources of hazardous biological wastes are hospitals and biological research facilities. The ability to infect other living organisms and the ability to produce toxins are the most significant characteristics of hazardous biological wastes
These wastes may be liquid, gaseous or solid, but most often they are liquids. Typical examples include organic solvents, oils, plasticisers and organic sludges
Explosive hazardous wastes are mainly ordnance (artillery) materials, i.e., the wastes resulting from ordnance manufacturing and some industrial gases
Household hazardous wastes
Generic household hazardous material include non chlorinated organic, chlorinated organic, pesticides, latex paint, oil based paints, waste oil, automobile battery and household battery.
Hazardous Waste Management Process
Similar to Solid waste management; hazardous waste management also follows the same flow from generation to disposal. But the process used are specific for hazardous wste and thus techniques are specially designed to handle them with least risk to human and environment.
Major source – industries
Hazardous waste generation outside the industry is irregular and very less in amount, rendering the waste generation parameter meaningless. The only practical means to overcome these limitations is to conduct a detailed inventory and measurement studies at each potential source in a community
Spillage of containerised hazardous waste must also be considered an important source
2. STORAGE & COLLECTION
Onsite storage practices are a function of the types and amounts of hazardous wastes generated and the period over which generation occurs.
Usually, when large quantities are generated, special facilities are used that have sufficient capacity to hold wastes accumulated over a period of several days. When only a small amount is generated, the waste can be containerised, and limited quantity may be stored.
Corrosive acids or caustic solutions are stored in fibreglass or glass-lined containers to prevent deterioration of metals in the container.
(i) Wastes stored in large-capacity tanks are either drained or pumped into collection vehicles; (least human exposure)
(ii) Wastes stored in sealed drums or sealed containers are loaded by hand or by mechanical equipment onto flatbed trucks
For short-haul distances, drum storage and collection with a flatbed truck is often used. As hauling distances increase, the larger tank trucks, trailers and railroad tank cars are used.
3. TRANSFER & TRANSPORT
Typically, hazardous wastes are not compacted (i.e., mechanical volume reduction) or delivered by numerous community residents. Instead, liquid hazardous wastes are generally pumped from collection vehicles and sludge or solids are reloaded without removal from the collection containers for transport to processing and disposal facilities
For example, neutralisation of corrosive wastes might result in the use of a lower-cost holding tank on transport vehicles.
4. HAZARDOUS WASTE PROCESSING
Processing of hazardous waste is done for purposes of recovering useful materials and preparing the wastes for disposal
Processing can be accomplished on-site or off-site. The variables affecting the selection of processing site include the
- characteristics of wastes,
- the quantity of wastes,
the technical, economical and environmental aspects of available on-site treatment processes and the availability of the nearest off-site treatment facility (e.g., haul distance, fees, and exclusions). The treatment of hazardous waste can be accomplished by physical, chemical, thermal or biological means
Physical, chemical and thermal treatment operations are the most commonly used. (Biological treatment processes are used less often because of their sensitivity.)
5. HAZARDOUS WASTE DISPOSAL
Regardless of their form (i.e., solid, liquid, or gas), most hazardous waste is disposed off either near the surface or by deep burial (Geological repositories)
Controlled landfill methods have been proved adequate for disposing of municipal solid waste and limited amounts of hazardous waste, they are not suitable enough for the disposal of a large quantity of hazardous waste, due to the following reasons:
- possible percolation of toxic liquid waste to the ground water;
- dissolution of solids followed by leaching and percolation to the ground water;
- dissolution of solid hazardous wastes by acid leachate from solid waste, followed by leaching and percolation to the ground water;
- potential for undesirable reactions in the landfill that may lead to the development of explosive or toxic gases
- volatilisation of hazardous waste leading to the release of toxic or explosive vapours to the atmosphere;
- corrosion of containers with hazardous wastes
Hazardous Waste Treatment Hierarchy
A three stage hierarchy of options for handling hazardous wastes are:
1) Process manipulation, recycle and reuse options that reduce the production of hazardous waste (most desirable options)
2) Processes that convert hazardous waste to less hazardous or non hazardous substances that include
- a) Incineration
- b) Land treatment
- c) Ocean and atmospheric assimilation
- d) Chemical, physical and biological treatments
- e) Thermal treatments
3) Landfill, underground injection, arid region unsaturated zone, surface impoundments, salt formations etc
Physical and chemical treatment
Filtration and separation - Filtration is a method for separating solid particles from a liquid using a porous medium. The driving force in filtration is a pressure gradient, caused by gravity, centrifugal force, vacuum, or pressure greater than atmospheric pressure.
Suspended solid particles less than 100 ppm (parts per million) concentration are removed from an aqueous stream . Primary aim is to produce a clear aqueous effluent
Aim is to concentrate the solids into a phase or solid form for disposal or further treatment
Process by which the soluble substance is converted to an insoluble form either by a chemical reaction or by change in the composition of the solvent to diminish the solubility of the substance in it. Settling and/or filtration can then remove the precipitated solids
the process has a wide applicability in the removal of toxic metal from aqueous wastes by converting them to an insoluble form. Example - arsenic, barium, cadmium, chromium, copper
Chemical oxidation and reduction
When electrons are removed from an ion, atom, or molecule, the substance is oxidised and when electrons are added to a substance, it is reduced. Such reactions are used in treatment of metal-bearing wastes, sulphides, cyanides and chromium
Oxidising agents are sodium hypochlorite, hydrogen peroxide, calcium hypochlorite, potassium permanganate and ozone.
Reducing agents are used to treat wastes containing hexavalent chromium, mercury, organometallic compounds
This oxidation and reduction treatment tends to be more suitable for low concentration (i.e., less than 1%) in wastes
Solidification and stabilisation
AIM OF SOLIDIFICATION AND STABLIZATION
- To reduce the mobility of pollutants, thereby making the waste acceptable under current land disposal requirements
- To decrease surface area across which pollutants can transfer or leach,
- To limit the solubility or detoxify the hazardous constituent
Solidification : materials are added to the waste to produce a solid. It may or may not involve a chemical bonding between the toxic contaminant and the additive.
Stabilisation : waste is converted to a more chemically stable form. stabilisation represents the use of a chemical reaction to transform the toxic component to a new, non-toxic compound or substance
Chemical fixation - transformation of toxic contaminants to a new non-toxic compound. It , do not involve chemical bonding of the contaminant to the binder
Encapsulation: This is a process involving the complete coating or enclosure of a toxic particle or waste agglomerate with a new substance (e.g., S/S additive or binder). The encapsulation of the individual particles is known as micro-encapsulation, while that of an agglomeration of waste particles or micro-encapsulated materials is known as macro-encapsulation.
Evaporation is used in the treatment of hazardous waste and the process equipment is quite flexible and can handle waste in various forms – aqueous, slurries, sludges and tars. Evaporation is commonly used as a pre-treatment method to decrease quantities of material for final treatment. It is also used in cases where no other treatment method was found to be practical, such as in the concentration of trinitrotoluene (TNT) for subsequent incineration.
Ozone and UV radiations have been used to detoxify industrial organic wastes, containing aromatic and aliphatic polychlorinated compounds, ketones and alcohols
Incineration can be regarded as either a pre-treatment of hazardous waste, prior to final disposal or as a means of valorising waste by recovering energy. It includes both the burning of mixed solid waste or burning of selected parts of the waste stream as a fuel. The concept of treating hazardous waste is similar to that of municipal solid waste
This is defined as the chemical decomposition or change brought about by heating in the absence of oxygen. This is a thermal process for transformation of solid and liquid carbonaceous materials into gaseous components and the solid residue containing fixed carbon and ash. The application of pyrolysis to hazardous waste treatment leads to a two-step process for disposal.
- In the first step, wastes are heated separating the volatile contents (e.g., combustible gases, water vapour, etc.) from non-volatile char and ash.
- In the second step volatile components are burned under proper conditions to assure incineration of all hazardous components
Land treatment: This is a waste treatment and disposal process, where a waste is mixed with or incorporated into the surface soil and is degraded, transformed or immobilised through proper management. The other terminologies used commonly include land cultivation, land farming, land application and sludge spreading.
Waste characteristics: Biodegradable wastes are suitable for land treatment. Radioactive wastes, highly volatile, reactive, flammable liquids and inorganic wastes such as heavy metals, acids and bases, cyanides and ammonia are not considered for land treatment.
Ease of biodegradation depends on its
- Chemical structure,
- molecular weight,
- water solubility and
- vapour pressure
pH, salinity, aeration, moisture holding capacity, soil temperature,
bacteria, actinomycetes, fungi, algae and protozoa
The factors affecting waste degradation are soil pH (near 7), soil moisture content (usually between 30 to 90 %), soil temperature and nutrients
As discussed earlier; biological processes are not much preferred for Hazardous waste treatment.
Enzymatic Systems for Hazardous Waste Treatment
Enzymes capable of transforming hazardous waste chemicals to non-toxic products can be harvested from microorganisms grown in mass culture. Such crude enzyme extracts derived from microorganisms have been shown to convert pesticides into less toxic and persistent products.
The factors of moisture, temperature, aeration, soil structure, organic matter content, seasonal variation and the availability of soil nutrients influence the presence and abundance of enzymes
The microbiology of hazardous wastes differs from that of composting in the use of inoculums. The reaction is that certain types of hazardous waste molecules can be degraded by only one or a very few microbial species, which may not be widely distributed or abundant in nature
Aerobic and anaerobic treatment
Hazardous materials are present in low to high concentration in wastewaters, leachate and soil. These wastes are characterised by high organic content (e.g., up to 40,000 mg/l total organic carbon), low and high pH (2 to 12), elevated salt levels (sometimes, over 5%), and presence of heavy metals and hazardous organics. Hazardous wastes can be treated using either aerobic or anaerobic treatment methods.
In aerobic treatment, under proper conditions, microorganisms grow. They need a carbon and energy source, which many hazardous wastes satisfy, nutrients such as nitrogen, phosphorus and trace metals and a source of oxygen.
The process is mediated through enzyme catalysis and depends on maintaining a balance of population within a specific set of environmental conditions.