Rainwater carries dust particles from the atmosphere to the ground, it dissolves also a little oxygen, nitrogen, and carbon dioxide as it falls through the atmosphere. During electrical storms, lighting causes nitrogen, oxygen, and water vapor to form nitric acid. Traces of this are also found in rainwater. Rainwater also dissolves and washes various chemical compounds from roads, streets and fertilizers from fields.

There are 2 types of wastewater treatment:
1. Pre-treatment – treatment of wastewater before its discharging onto the sewer (this is used for strongly polluted industrial wastewater)
2. Full wastewater treatment – treatment of wastewater before its discharging into the recipient (flow, lake, sea)

Biological wastewater treatment
 Final aim is to remove organic matter from wastewater
 Main components or actors in biological WWT are substrate – an organic matter; and a biomass – microorganisms.
 Substrate - organic material contained in domestic WW, in agro-industrial WW, in food industry WW (all these WW are compatible with biological WWT) and can be decomposed by microorganisms. Substrate contains 3 main element that are crucial for bacteria life: N- nitrogen, P- phosphorus, and K-potassium.  The organic matter biodegradation can be either aerobic or anaerobic. Aerobic oxidation occurs in the presence of dissolved oxygen. A measure of the amount of oxygen needed for this degradation is the biochemical oxygen demand (BOD). The higher the quantity of degradable organic wastes the higher the BOD.
 Flowing streams (thus wall aerated) are able to regenerate themselves. In rapids, the moving water dissolves oxygen. Lakes with little or no flow can remain dead for decades.
 Eutrophisation – a problem that can arise in still waters, which is fed by nutrients (nitrates and phosphates). This may serve as nutrient for the growth of algae. These algal blooms are stimulated by the runoff of agricultural fertilizers and by phosphates in detergents. The result of the eutrophisation is that water is overloaded with algae, their relics after their die-off – this causes the bad odor and turbidity of the water.


The problem contaminants most likely to affect water quality are:
 Turbidity: silt and fine particles suspended in the water
 Bacterial or organic pollution from sewage and as decay products, e.g. E. coli, disease organisms and viral or protozoan pathogens, parasitic worm eggs and so on.
 Metallic pollutants such as chromium, cadmium, lead, mercury
 Biocides, e.g.