1. Pre-treatment

Proper control and regulation of disposal of toxic solid wastes would encourage the industry to become aware of the expenditure involved and methods would become more effective. Three types of processes can be used to render a hazardous waste less hazardous or non-hazardous:

• i) Physical: such as carbon adsorption, distillation, ultrafiltration, etc.
• ii) Chemical: such as neutralisation, fixation into solids that can be easily disposed of, etc.
• iii) Biological: such as activated sludge process, trickling filters, land farming, etc.

The segregation will enable adoption of suitable methods for different types of wastes. For example, waste oils are not reclaimed due to the high cost of collection and treatment and the relatively low cost of fuel oil. But if such waste oils can be used in boilers, besides reducing some cost of boiler operation, oil could be saved along with avoidance of nuisance on landfill sites.

2. Detoxification

Toxic substances are seldom chemically inert and hence can be converted to a non-toxic or less toxic compound by chemical treatment. However, solids have often to be brought into solution before subjecting it to chemical treatment followed by effective sludge disposal. Some of the toxic metals have a resale value, and hence effort should be made to recover and reuse them.

3. Prevention of Water Pollution from Landfills Receiving Toxic Waste

As wastes are commonly disposed on land, it should be ensured that water pollution (surface water by runoff & groundwater by leachates) does not occur from such sites. It could be ensured by:

• i) proper consolidation of the waste to reduce spaces and permeability
• ii) disposing the waste at a sufficient depth to prevent water pollution
• iii) depositing waste in the impervious zone at a certain depth so that leachate will not gain access to aquifer; geologic investigations would be useful in such cases
• iv) providing a layer of impervious soil on top and sides to prevent percolation of water
• v) mixing with municipal solid waste for composting if the waste is amenable for biological decomposition

4. Oil and Tarry Wastes

They are highly viscous making it difficult to permeate through the pore space in the soil. Less viscous oils may travel fast posing pollution problems. In the case of viscous wastes, it is noticed that they turn into an emulsion (if filled with water) or foam (if filled with air). In either case, viscosity would further be increased and the rate of percolation decreased, thereby reducing leaching effect.

5. Incineration

It is an effective way of disposal of combustible organic and chemical wastes. The residue left is quite small which can be disposed of. Incineration has the advantage that when the toxic constituents are burnt or otherwise decomposed they cease to be toxic.

During incineration, the organic chemicals are converted to gas, moisture and simple compounds of $N_2$, S, P, $CI_2$. At the incineration temperature, toxic materials may alter physically so that the toxic constituents are not so readily available, e.g., a powder converted to slag, when the toxic constituents would not be easily soluble in water. Though incineration is likely to cause air pollution, technology is available to keep it below permissible limits.

6. Disposal in Sea

This method may not always be possible and at times, ocean currents may wash it back to the shore. Sea provides a large dilution and if disposal is carried out in deep regions the waste would not easily reach the surface layers. Every care should be taken to ensure that the disposal is done by taking all the necessary precautions. Disposal by incineration at sea on specially designed ships could be carried out in special cases such as polychlorinated byphenyls.

7. Authorisation for Disposal

In most of the developing countries, legislation to control and regulate the disposal of solid toxic wastes does not exist. The solid toxic waste, when converted to liquid (i.e. dissolved in water), can partly be regulated by the use of the Water Pollution Control Act.

Independent authorities with sufficient regulatory powers and infrastructure would be needed to ensure safe disposal of toxic solid wastes. Before any toxic solid waste is disposed of, approval for the method and sites proposed to be used must be taken from a regulatory agency which can have a list of common types of wastes for landfilling or incineration. Complete information about the sources of the waste should be provided.

The disposal facility may be provided by the concerned industry or it may be provided by the authority or it can be privately operated which can levy fees for disposal. Even if the site is owned by the industry, approval for its location and use will be essential. If the site is operated by the authority or private party, unauthorised disposal of other toxic wastes may take place. In such cases, the carriers of waste should be licensed by the authority. A standing committee of advisers will be useful in dealing with unusual wastes, especially if they are in need of immediate disposal.