Once defined the next stage is to classify the waste. Waste might be classified in three ways:

• According to where it comes from - 'arisings'
• According to its pollution potential - 'environmental impact'
• According to its state - 'liquid or solid or gaseous'

Waste may be classified according to its source, e.g. commercial, industrial, municipal. The Department of the Environment estimates that dry waste arisings in the UK totals around 400 million tonnes per year.

• Inert: 39% (159m tonnes) of total waste arisings are inert, e.g. glass, plastic, metals, rubble
• Hazardous (special): 1% (2.5m tonnes) of total waste arisings are hazardous, e.g. laboratory waste, spent chemicals
• Putrescible: 60% (239m tonnes) of total waste arisings is putrescible, e.g. unconsumed food, agricultural waste, sewage

No one knows the total levels of liquid waste arisings in the UK - although some three and a half billion tonnes of wastewater are treated in sewage works each year. The bulk of industrial effluents is currently discharged to water courses, the sea, or even by some operators into landfill. Only the volume to landfill is known with confidence - three million tonnes. Increasing volumes are undergoing sophisticated treatments. Handling and the transportation of liquid waste presents a different sort of challenge to solid waste. The total volume of gaseous emissions in the UK is also difficult to quantify. However, total annual CO₂ emissions from industry are 160 million tonnes - or one and a half time the amount of waste sent each year to landfill.

Figure 3 is a schematic of the general government classifications, while Table 1 provides descriptions of other recognised waste classifications.