Wastewaters are numerous in kind and defy easy definition. It is difficult to evolve a universal classification for ‘waste’. However, roughly there are three basic classification methods, which are combined to form a waste classification system (Hagerty., 1973). They are physical classification, chemical classification and thermal classification. The last one is directly related to thermal processing requirements and focuses on the information needed to select, design, and operate a thermal processing system.
5.1 Physical Classification
This classification identifies the physical state of waste. It is needed for the selection and design of waste handling systems and the choice of processing equipment. Based on physical characteristics, typical waste materials are classified as gaseous, liquid, slurry, sludge, solid and containerized waste. In the present context, solid and liquid wastes will be discussed in this section.
Solid waste is wastes those which arise from human and animal activities that are normally solid and are usually discarded as useless or unwanted. The term as used in this context is all inclusive, and it encompasses the heterogeneous mass of throw aways from the urban community as well as the more homogeneous accumulations of agricultural, industrial and mineral wastes (Tchobanoglous, 1977). In discussing solid waste, generally and traditionally certain categories of wastes are well recognized as they are very common. For example, solid wastes include domestic, commercial, industrial, (due to construction and demolition) agricultural, institutional and miscellaneous. Many times domestic and commercial wastes cannot be differentiated and are considered together as urban wastes. Included in this category are the garbage materials which result from food preparation both in the homes and restaurants, and also the rubbish which is produced in residences and commercial establishments. Generally, the garbage consists of rapidly decomposable materials while the rubbish is either slowly decomposable or non-degradable. A second major category of waste is industrial, the refuse produced by industrial processes. Generally the character of the refuse produced in any manufacturing or processing operation will depend very much on the type, amount, or the character of wastes produced by the industry. Obviously, the wastes produced by steel manufacturers will differ considerably from those produced in the chemical industry. Usually the wastes produced by any food processing operation, the paper and plastics industries are almost similar to the paper and plastic packaging materials found in domestic rubbish. On the other hand, the metal processing industry will obviously generate metallic wastes, but in addition will also produce large quantities of slags, processing chemicals, and other residues, many of which are used in air pollution control and water pollution control activities. The wastes produced by chemical industries or other more specialized industries will in general depend upon the specific nature of the end product of the manufacturing process (Waste Classification Guidelines Part 1: Classifying Waste , 2009).
5.2 Chemical Classification
It is the key to selection of proper process design parameters, selection of materials of construction, choice of concentration equipment, design of incinerator hardware, the possibility of bye-product recovery, and the requirement of fuel gas and discharge water treatment systems. Based on this classification, the waste materials are broadly classified as clean waste and hazardous waste.
5.2.1 Clean Waste
This group covers hydrocarbons which contain only carbon, hydrogen and oxygen. The oxidation products are considered clean and can be discharged into the environment.
5.2.2 Hazardous Waste
The problem with hazardous wastes is that there is still no internationally accepted definition of a hazardous waste. However, technical criteria such as toxicity, flammability, corrosivity, ignitability and reactivity have been proposed and used to some extent to identify and designate the waste as hazardous. Though the hazardous wastes account for only a small proportion of all wastes generated by the industries, their impact can be disastrous as they not only affect the physical environment but also the biotic components. The United States alone generates about 60 million tons of hazardous waste every year, and the European Economic Community about half of that quantity. Roughly 10 to 20 per cent of the total waste produced can be considered as potentially hazardous. In general, there are industries like metallurgical, iron and steel, fertilizers, thermal power production, etc. generating large quantities of solid and liquid wastes containing low concentration that is at the trace level of hazardous constituents. On the other hand, there is another group of industries such as chlor alkali, pesticides, dyestuffs, electroplating, photographic chemicals, fine chemicals, etc. which generate comparatively small quantity of wastes, but they are highly toxic and hazardous. Approximately one million tons of solid waste is generated annually by the 131 industrial units surveyed. Nearly 22% of wastes from these industries are hazardous. It has been estimated that about 94% of these wastes are generated by process oriented (generating during the processing of raw materials to get the finished products) industries and 6% of it is pollution control oriented (generated from the treatment of gaseous and liquid effluents) industries. Maximum amount of hazardous waste is generates by chlor-alkali, dyes and pigment, organic chemicals, pesticide and plating industrial sectors. The inorganic chemical sector on the other hand, generated low volume-high toxic wastes. The synthetic drug manufacturing units in drugs and pharma sector generate maximum quantity of hazardous wastes. Thus, hazardous waste may be further classified into waste generating gaseous contaminants, waste containing alkali and alkaline earth metals and waste containing heavy metals.
5.3 Thermal Classification
This is used in the determination of energy requirements for processing and in the selection of system configuration. Based on the thermal properties, typical waste materials are classified as follows, where combustible wastes are a group that includes wastes which will sustain oxidation reaction without additional energy requirements, noncombustible wastes are groups that represent low-Btu value wastes, which will not sustain oxidation reaction without additional energy supply. These wastes may be further classified into following two categories which are the noncombustible wastes with no low the volatiles (the organics in the wastes contain no low volatiles which, when heated, will evaporate) and the noncombustible waste with low volatiles (the organics in the waste are heat sensitive and readily vaporized when heated). These two categories provide important information for the designing of evaporation and steam stripping systems. They are also important factors in the selection of pre-concentration equipment of wastes. For the successful application of thermal processing systems accurate and reliable information about the composition and characteristics of any waste to be processed is essential. Such data are necessary to understand the process and for the selection of process equipment.

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Classification of a wastewater by toxicity or hazard is important for safety and also to comply with legislation (Mumford, 1973). In practice, it is better to use the characterization ‘hazardous’ and ‘non-hazardous’ having regard to both toxicity and the environmental implications of the waste on disposal. Therefore the Control of Pollution Act distinguishes waste as poisonous, noxious or polluting and whose presence on land is likely to give rise to an environmental hazard. It also encompasses highly flammable or explosive materials which may have little toxicological significance (Agency, n.d.).
Classification may be further expanded taking into consideration individual chemicals. Chemical analysis will usually give the proportion of each chemical present, except that of organics which may be characterized by one or more physical properties as listed in Table 2.

Table 2: Physical properties of liquid wastes.
Alternatively the description may be limited to synthetic organic materials, natural organic materials, and inorganic materials and this characteristic corresponds to classification by origin. Finally, wastewater is usually wastes because it consists of unusable and often unpleasant mixture of chemicals.
Bibliography
Agency, T.E.P. About Hazardous & Non-Hazardous Waste by The Environmental Protection Agency, [Online], Available: http://www.lonestar.edu/16715.htm [19 February 2014].
Hagerty., D.J.P.J.L.a.H.J.E. (1973) Solid Waste Management, New York: Van Nostrand Reinhold Co.
Mumford, C.J. (1973) Industrial Pollution Control, Bussiness Books.
Tchobanoglous, G.T.H.a.E.R. (1977) Solid Wastes: Engineering Principles and Management issues, New York: McGraw Hill.