Waste is any substance or object which its owner discards or intends or is obliged to discard. There are three categories of waste, which depending on their physical state are divided into solid, liquid and gas.

The waste gases (or aerosols) they mainly include urban aerosols and gaseous waste from processing, they are usually solids of very small particle size and low weight, which can be transported through the air, but also liquid sprays (organic solvents, acids and other substances) which show a high evaporation tendency . The main components of gaseous waste are carbon monoxide (CO), carbon dioxide (CO2), Sulfur oxides (SOx) with the most important representative being sulfur dioxide (SO2), Nitrogen oxides (NOx) most commonly nitrogen monoxide (NO) and nitrogen dioxide (NO2), Hydrocarbons (HCs), Particulate Matter (PM10 i.e. less than 10 µm in diameter) and Ozone (O3). While the main sources of aerosols are power plants, domestic heating, vehicle traffic, refineries, paper mills, dyehouses, glass factories, foundries, hot mineral processing units (hydrogen – alumina), agricultural dryers products etc.

The waste that is in a solid state is the  solid wastes. The category of solid waste includes municipal waste, industrial waste (e.g. empty packaging, empty barrels, cartons, plastic wraps), building demolition waste, petroleum, livestock and agricultural waste, waste from mines and mines, excavation waste (from land and sea), sludge from urban sewage treatment and industry, hospital waste, tires and scrap etc.

The liquid waste they include the solid residues which are dissolved in a liquid medium (water or some organic solvent) and are today one of the main sources of environmental pollution. Main sources of liquid waste are domestic, municipal and industrial waste. In general, the main pollutants of liquid waste are organic biodegradable materials, organic non-biodegradable materials, nutrients, toxic substances (e.g. sulphides, chromates, arsenic salts, cyanides, phenols and their derivatives, organospheric , organic sulphur, halogens), heavy metals, other inorganic materials (e.g. sodium chloride), pathogenic microorganisms (e.g. Escherichiacoli etc. Specifically, the most important components of municipal and industrial liquid waste are suspended solids , the Biochemically Required Oxygen (BOD5), the Chemically Required Oxygen COD and nitrogen.

Indicative list of liquid waste analyses: