Hazardous wastes have become an important environmental and public health issue which concerns many countries in the world. In the modern framework of hazardous waste management, a four pronged strategy has been adopted:
Minimizing the quantity of waste
Recycling of industrial waste
Treatment of the waste
Collection, transport and disposal of waste in an environmentally sound manner.
All four of these approaches are important and are not exclusive of each other. When dealing with a given hazardous waste problem, often there is a need to utilise a combination of the four general approaches outlined above. We will discuss, in brief, each of these four approaches.
The first priority in hazardous waste management is to reduce the quantity of waste to minimum. Three major waste reduction schemes which are often used can be summarized as below:
i) Process Modification:
Often the industrial process can be altered in such a way that the use of raw materials is optimized and the amount of-hazardous waste is reduced to barest minimum. For example, in zinc electroplating, the sulphate salt is substituted by the chloride compound with slight modification of the process; this can eliminate the cyanide problem.
ii) Waste Concentration :
The waste can be concentrated using evaporation, precipitation or decantation techniques which means that the volume of waste can be considerably reduced using these methods. Incineration, viz., oxidation of inflammable-waste is often practiced in order to reduce the volume of waste to be handled. It is an excellent method of waste disposal, but the cost of operation usually exceeds the net gains.
iii) Waste Segregation : Segregating the hazardous waste streams from non-hazardous streams decreases the volume of hazardous wastes, thus, making it easier to treat.
Recycling Industrial Wastes
Many substances in refuse wastes have value. They include glass, wood fibre from paper products, and metal. Scientists have developed ways of recycling many wastes so they can be used again. Almost all materials are recyclable. However, in some more energy will be expended in recovery than the recovered value warrants.
The two broad ways of processing hazardous waste are waste reuse and waste recycling. We shall briefly deal with them.
i) Waste Reuse:
In some cases waste material can be used as a raw material with very little processing. Transfer of the waste “as is” without reprocessing, to another facility is known as waste reuse or waste exchange. Unwanted materials of commence such as outdated chemicals or untested materials not meeting the high quality control requirements of purchasing industry, can be reused without processing. Process wastes such as cardboard for making paper pulp, copper or other metal salt solutions for metal recovery, oils that can be used as fuels. This includes a variety of other materials that can be reused as industrial feed stocks.
ii) Waste Recycling :
Recycling differs from reuse in that the waste must first be treated before it can be used in a manufacturing process. When a transfer of waste “as is” is not possible, reprocessing the waste for material recovery is known as recycling. For example, baghouse dust from scrap steel processors, containing upto 25 per cent zinc oxide, can be combined with waste sulphuric acid to make galvaniser’s pickle acid. The spent pickle liquor containing 8-10 per cent zinc sulphate and some iron salts is then usable, as fertiliser in agricultural fields. Use of waste organic solvents is the best example of recycling waste.
Treatment of Hazardous Wastes
After material recovery, the waste water containing hazardous waste chemicals should be detoxified and neutralised through treatment. There are many technologies available for treating hazardous wastes before they are ultimately disposed of. Their aim is to modify the physical and/or chemical properties of the wastes so that they are rendered harmless. Selection of a treatment process depends on many factors such as the nature of the waste, the desired characteristics of the output stream, and economic and energy considerations. The treatment technologies can be divided into the following groups, namely:
Physical treatment :
Is conducted using various methods such as phase separation. Phase separation includes three steps, namely: lagooning, prolonged storage in tanks and sludge drying in beds. Lagooning and tank storage are collectively used to separate particulate impurities.
Chemical treatment :
Is used to facilitate complete breakdown of hazardous wastes and more usually to modify the chemical properties of the wastes, e.g., to reduce water solubility or to neutralise acidity or alkalinity. The techniques involve oxidation, chemical reduction, neutralisation, heavy metal precipitation, oil/water separation and solvents/fuels recovery.
Biological treatment :
The gross impurities obtained from treatment of sewage are
collectively known as sludge, which is given biological treatment, before disposal. This is known as sludge processing which has become important since improvements in industrial waste water treatment. The typical technologies for sludge processing include conditioning, digestion, composting, thickening or dewatering and solidification.
i) Conditioning : In this step the sludge is exposed to atmosphere for a stipulated period until a desired consistency is reached.
ii) Digestion : In this process the sludge is treated with bacteria which break down the long chain compounds into simpler ones
iii) Composting : In this step the organic matter in the waste sludge is converted into a usable stable material.
Pprocesses convert the liquid waste into insoluble, rock-hard material and are used as pre-treatment prior to landfill disposal. This is usually done by mixing the waste with various reactants to produce a solid mass. The basic aim of solidification process is to immobilise the hazardous constituents of the waste, so that these do not leach out at the landfill disposal site,
Collection, Transport and Disposal
Waste disposal is a multiphase activity, the different stages of which, i.e. collection, interim storage, transport, treatment and disposal are highly interdependent, both technically and organizationally. Safe collection and transport of hazardous waste form a critical link in the chain between its point of generation and its place of treatment and disposal. In many respects, the same precautions apply to hazardous waste in transit as apply to the carriage of dangerous goods; however, additional problems arise from the hazardous nature of certain wastes because:
i) waste in general has no perceptible economic value to the generator;
ii) the chemical and physical properties of a waste may not be precisely known because it is frequently a complex mixture from which all economically useful components have been extracted :
iii) mixing of non-compatible wastes for convenience in transit could create an acute hazard, either immediately or on treatment and disposal (for example, a mixture of ether waste containing a sodium residue with an aqueous ether waste will explode).
Therefore, for a safe and secure disposal of hazardous waste, there should be a proper collection, transport and storage system. The non-compatible wastes should be segregated and transported separately.
DISPOSAL OF HAZARDOUS WASTE
The final disposal of the hazardous wastes also needs to be carefully planned. There are four different ways in which hazardous wastes can be finally disposed.
Dumping at sea
We shall now discuss each of the above method of disposal of hazardous wastes.
The disposal of hazardous waste by landfilling is an important method of disposal in many countries. Landfilling means storing harmful substances under the ground. This involves hauling the refuse to an area allocated for this purpose. In India such areas range from unsanitary open dumps to properly operated sanitary landfills. Open dumps are a poor method of waste disposal because they cause environmental problems. For example, they can ruin the appearance of all area and provide a home for rats and other rodents who spread disease. If garbage is exposed, it rots and smells foul. Most dumps allow some burning, which fills the surroundings with smoke. In addition, rain water can drain through refuse and carry harmful substances to streams.
Properly operated sanitary landfills cause little damage to the environment. The area to be filled with waste must be lined with a nonporous substance such as clay, or high density polyethylene (HOPE)—plastic membrane to prevent the wastes from leaking to the surrounding areas. The wastes are packed and dumped at the site and covered with earth each day. They cover of earth prevents insects and rodents from getting into refuse. Operators of these sites forbid burning. In time, sanitary landfill sites become filled up, many communities then cover the site for a final time and use the area for recreational purpose.
A typical landfill site consists of an artificial double liner at the bottom and a cover at the top. The above design of landfill site does not have any provision for monitoring and repair of the site. In the recent past, a new concept has developed in which the landfill site is constructed on a structure consisting of concrete cells. The cell is a space for plant personnel to visit and observe any fault and repair the same.
Incineration burns waste products. This is another method many industries and large cities use if they do not have enough vacant areas for disposal sites nearby. Most hazardous wastes are detoxified in this process. This is also an excellent method of waste minimization, waste detoxification and disposal, but its cost of operation is very high, if the heat content of waste is not reutilized.
Incineration is a ‘process for the high-temperature oxidation of gaseous, liquid or solid wastes, convening them into gases and an incombustible residue. The flue gases are released to the atmosphere with or without recovery of heat and with or without cleaning; and any slag or ash produced is deposited in a landfill. In general, incineration may be considered as an alternative method of detoxifying some non-recoverable highly toxic wastes. It is an excellent method of reducing waste volume, and in addition offers the possibility for recovering the heat content of the waste. In some communities heat from municipal waste incineration is used to produce steam. This steam drives turbines that produce electric power. Recycling of heat thus reduces the cost of operation of incinerators.
Generally, the wastes having inflammable characteristics are incinerated. The following types of wastes are commonly treated in hazardous waste incinerators:
Solvent waste and sludges
Waste mineral oils
Varnish and paint wastes and sludges
Plastics, rubber and latex waste sludges and emulsions
Oils, emulsions and oil/water mixtures
Mineral oil sludges
Grease and wax wastes
Acid tar and spent clay
Organic wastes containing halogen, sulphur or phosphorus compounds.
Wastes having high chlorine, sulphur, nitrogen and phosphorus contents, polychlorinated biphenyls (PCB) and those containing heavy metals and carcinogenic substances need special incineration technologies and precautions. A large number of municipal incinerators lack adequate air pollution control devices. Burning in many of these devices may release gases and solid particles that may harm human health, damage property and kill plants. The flue gases from ordinary incinerators can be dangerous in the absence of pollution control devices. Furthermore, as you have read earlier, incineration sometimes becomes a costly affair.
Dumping at Sea
Another method of disposal of hazardous wastes involves dumping wastes at deep sea, designed to prevent contamination of groundwater.
Disposal at sea, of waste generated on land, is based on the misconceived notion that-the enoromous volume of water available for dilution, enables the seas to be used as a dump without permanent damage. However, this is an erroneous conviction. The decision to choose this method of disposal is generally based on financial considerations. The site of disposal is determined by the geographical location of the waste producer.
Disposal of waste at sea is controlled by international legislation and by the national legislation required for the ratification of the international legislation. To prevent pollution of the seas by the direct discharge of waste, the international legislation bans the dumping of extraordinarily hazardous wastes such as organic silicon compounds, halogenated organics, mercury and its compounds, cadmium, carcinogenic waste and plastics into the sea. The last of these can seriously disturb fishing and navigation.
It maybe excessively expensive to dispose off certain hazardous wastes, such as radioactive nuclear wastes, in an environmentally acceptable manner at landfill still sites or incinerate them at thermal treatment plants. These wastes are generated in all operations associated with the use of nuclear energy for national defence or peaceful purposes such as mining of radioactive ore, production of nuclear fuel, laboratory experiments and medical treatment. Underground disposal may provide an .environmentally and economically viable option in case of radioactive wastes. The underground disposal of hazardous waste is acceptable only in inactive or partially active mines that meet specific geological and technical criteria. Worldwide, only one deep-mine disposal facility is currently in operation: a worked-out halite/potash salt mine at Herfa Neurode in the Federal Republic of Germany (now united Germany).
Salt mines are often used for radioactive waste disposal because the excellent properties of salt deposits prevent the interaction of wastes with other geological formations. The very existence of a salt deposit is a proof that the underground site has been unaffected by water for .millions of years. Salt is impermeable to liquids and gases. Due to its hygroscopic nature, salt is capable of absorbing water entering ,the formation from outside and of repairing minor fractures by re-crystallization, thus maintaining the original impermeability. This feature is frequently supplemented by impermeable upper strata consisting of wastes, usually rock, from mines or other industries.
The atmosphere in salt mines is extremely dry, so metal equipment and containers do not rust. There is no risk of methane explosions as in coal mines. Bursting of carbon dioxide gas inclusions in the salt mines may be observed during excavation of rocks but this does not pose a risk, particularly after mining operations have ceased. Thermal conductivity of salt is good. Salt is strong, permitting the excavation of spacious, stable galleries. In addition, salt has a certain plasticity under pressure, allowing the dispersion of strain and increasing the overall stability.
Thus, in principle you have learnt that there are four methods of waste disposal.
Landfilling of solid wastes
Incineration of inflammable organic wastes
Dumping of wastes at sea, and
Underground disposal usually of radioactive wastes.