
498 CHEMICALS AND THE ENVIRONMENT: SOURCES AND IMPACT
Conventionally, wastes are considered as being predominantly either solid, liquid or gaseous
but as illustrated in Table 16.3, they may be multi-phase. Solid waste comprises liquid slurries,
sludges, thixotropic solids and solids of varying particle sizes; it may be heterogeneous. Typical
examples are given in Table 16.4.
Table 16.3 Some physical forms of waste
Phase Examples
Gas
Gas–vapour – SO
2
; NO
x
; HCl; CO; hydrocarbons
Gas–liquid Mist Acid mist carryover; chromic acid; oil mists; tar fog
Gas–solid Fume Metal oxides, cement dust
Gas–liquid–solid – Paint spray
Liquid
Liquid Solution Metal plating effluent; ‘spent’ acids; wash-waters
Liquid–gas Foam Detergent foam
Liquid–liquid Emulsions Oil-in-water (e.g. suds); water-in-oil
Liquid–solid Slurry Aqueous effluent from fume scrubbing
Suspension
Solid
Solid – Asbestos insulation; heat treatment salts,
pulverized fuel ash; refuse
Solid–liquid Sludge, Filter cake
wet solid Sewage sludge
Waste treatment prior to disposal may introduce phase changes which result in quite different
pollution control considerations. For example, the gases generated by incineration of a solid
waste can be scrubbed with liquid in order to meet an acceptable discharge criterion; hence, in
addition to ash for disposal, a liquid effluent stream is produced and requires treatment. Other
waste treatment processes may result in the liberation of flammable or toxic gaseous emissions
as exemplified in Table 16.5.
Pollutants may enter the environment via air, water or land and prove:
• Damaging to the environment or public health and well-being.
• Damaging to buildings and materials of construction (Table 16.6).
• Wasteful of valuable resources.
• Illegal.
• Technically difficult and expensive to deal with (e.g. to clean up spillages and rectify damage).
• Harmful to company reputation.
The range of measures applicable to control are summarized in Chapter 17. Recycling and
recovery are potentially attractive measures; hence solvents, mineral oils, metals, e.g. lead, copper,
nickel, mercury, and glass are commonly recycled. However, recycling options generally depend
upon favourable economics, particularly low collection costs.
The two different, but related, considerations in waste disposal are hazard control and loss
prevention in the treatment and disposal operations, and the control of environmental hazards.
With gas and liquid streams the control of on-site hazards arising from the chemical properties
and processing operations generally follows the principles summarized in earlier chapters. The
measures necessary with ‘solid’ wastes may, however, differ, particularly if they are heterogeneous
in nature and disposed of on land.