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. dioxin, PCB, organophosphates, halogenated hydrocarbons
 Excessive fertilizers, especially nitrogenous compounds, phosphates, sodium and potassium salts.
 Acids or acid-forming compounds (a pH less than 5,5 increases metallic pollution

Many of these factors interact. Acid rain dissolves out of rocks and soil poisonous forms of aluminium, mercury, lead, cadmium, and selenium, or other metals such as copper, nickel and lead, cadmium and selenium, or other metals such as copper, nickel, and lead from drinking tanks, tea urns, and hot water tanks. Organisms may convert inorganic mercury to organic forms, which are readily absorbed by the body

Water treatments commonly used

 Aeration (oxygenation) by wind, mechanical aeration, or by increasing turbulence in flow. Aeration is also achieved by trickle columns and vegetation, phytoplancton, or injected air.
 Settling: spreading flow in still water ponds or rush beds to allow particles to fall out, filter out, or flocculate
 Skimming and sieving to remove large organic particles.
 Filtration via sand beds or charcoal-fiber columns, soils, the roots of aquatic plants.
 Coagulation or flocculation by using chemical additives (lime, salt, ferric sulphates) or organic (bacterial) gels.
 Biological removal by bacteria, phytoplancton, and higher plants.
 pH adjustment by adding calcium (as lime) or sulphur compounds as needed.

Filtration

A classical and widely used filter is sand. Many cities used sand filters followed by chlorinating to clean settled and treated raw sewage water sedimentation. Filtration by slow drip through 1,2 m of sand (top half fine, bottom half-coarse) is used even in temporary rural camps for water filtration.
Activated charcoal, often from bones or plant such as willow or coconut husks, is also used as a fine filter in homes and where purity is of the essence. Fine dripstone (fine-pored stone) is used in water cleaners and coolers to supply cool water in homes.
Trickle filters through sand and gravel columns actually feed resident bacteria, which remove the surplus nutrient.