Generally speaking, water bodies are more likely exposed to natural pollutions and most of them are not suitable for drinking purposes. Even waters of springs and wells may be considered as pure and clean waters are at risk of getting polluted by natural pollutants such as Cyanide, Arsenic elements or human-made pollutants like Nitrate and microbial pollution, which are unable to be seen. Furthermore, water resources should be qualified after they are chosen and quantified.  They are then treated  in terms of the amount and type of pollution, if necessary.

Wastewater treatment plants  range from large-scale domestic and industrial  plants, which are used to treat surface water for removal of turbidity and microbial pollution to small package-size ones which are located under  sinks. Size, volume, type of process and equipment employed  in these plants completely depend on the volume and type of pollutants. Thus, consulting engineers determine the type of treatment process according to the quality and quantity of  wastewater, using exact experiments and technical, economical, and environmental analyses and then as-built drawing are provided in the primary and secondary studies. Finally, the next stages are supervised.