Pollutants can be divided into either solid, liquid and gaseous (based on the phase) or physical, chemical and biological. They can also be divided into biodegradable and non-biodegradable.
The actual classification that engineers and scientists use depends on the medium (water, air, soil, etc.) for which we are studying pollution.
Here are some examples of pollutants of different types:
1. Solid: (for water and soil/land pollution): metals, plastics, wood, leaves, human waste, animal waste, etc.
2. Liquid (for water and land pollution): Oil and other petroleum fragments (gasoline, diesel, kerosene, etc.), human/animal waste, chemicals (Acids, bases, etc.), etc.
3. Gaseous (for air pollution and also water pollution): hydrogen sulphide, carbon dioxide, carbon monoxide, Nitrogenous gases (NOx), sulfurous gases (SOx), etc.
4. Biological (for water, air and land pollution): pollen, bacteria, fungi, worms, virus etc.