Overview | Waste Water Composition | Waste Water Treatment
Biological Treatment of Waste Water | A Waste Water Treatment Plant

What is in Waste Water?

What is Water?

Chemistry and Physics

Water - from a chemical point of view - is a molecule made of the elements hydrogen (two atoms of H) and oxygen (one atom of O) giving H₂O. It has a molecular weight of about 18 g/mol¹.
The water molecule has a planar structure forming a 'V' with the two hydrogen atoms at the edges and the oxygen at the angle. Oxygen has a higher electro-negativity, dragging the single electrons of the H atoms closer to it. This leads to a concentration of electric loads and lets the water molecule be an electrical dipole. A positive load is located near the hydrogen atoms, while a negative load is concentrated near the oxygen atom.
The fact that water is a dipole means that it is not mixable with fluids that are not dipoles. Oils, fats, hydrocarbons and other molecules belong to this group.

Water can dissolve salts, which split into their ions upon contact with it. A very small amount of water splits into ions of its own, a positive H⁺ ion and a negative OH^- ion. The H⁺ ion migrates to the negative loaded part of a water dipole forming a more complex H₃O⁺ ion.
By nature, water has a concentration of 10^-7mol/l H⁺ ions (and OH^- ions). The concentration of H⁺ ions is represented by the pH value². The natural pH value is 7 (neutral), corresponding to the negative exponent of the basis 10 in the natural concentration of H⁺ ions.
Dissolving acids in water increases the concentration of H⁺ ions. Hydrochloric acid, for example, dissociates completely into H⁺ and Cl^-. The concentration of H⁺ ions may be 0.1 mol/l (=10^-1mol/l) giving a pH of 1.
Dissolving bases like potassium hydroxide, for example, lets the concentration of H⁺ ions decrease and that of OH^- ions increase. The pH value may then be about 14, corresponding to an H⁺ ion concentration of 10^-14mol/l.
Adding bases to a solution with low pH values or, the other way round, acids to a high pH solution neutralises the pH value. If the amounts of H⁺ and OH^- ions are equal, the neutralisation reaction gives pH 7 by forming water and salt.

Natural Water

In natural water there are minerals (salts), micro-organisms and hydrocarbons, all of which have a certain natural concentration in fresh and in waste water. Fresh water, supplied by water works usually is enriched with chlorine, to keep micro-organisms (especially harmful ones) from infecting people.
Natural water composition depends strongly on the local environment. If springs are in soil or stone rich in iron, then, of, course, the iron concentration is relatively high. Water in swamps contains a relatively high concentration of hydrocarbons and of micro- organisms.
A very important property of water is its hardness. Everybody knows that diving into water in the wrong angle can hurt seriously. However, hardness has a different meaning to that: it is a measure for the amounts of (earth-) alkalis (Na⁺, K⁺, Ca²⁺, Mg²⁺) and their carbonates (CO₃^2-, HCO₃^-). These ions are responsible for those white sediments that block pipes, vessels and heating devices in washing machines. The latter are very often found to be corroded, by the way (for the Germans: Der Lochfraß-Man Peter Bürgi läßt grüßen!).

Natural water forms little creeks, rivers, lakes and the sea. Seawater has a higher concentration of salts than ground water. Besides micro-organisms, water is populated by macro-organisms, beginning with insects, fish, animals, birds, and any kind of plants. Both fauna and flora are well adapted to their individual local environment. Seawater fish is usually not able to survive in a lake up in the mountain.

What can be found in Waste Water

Apart from natural components, waste water is contaminated with different things, such as dirt, dust and pieces of rubbish. Oils, fat and grease cover surfaces or are emulsified by surfactants. Acids or bases, eventually forming and reforming salts, are soluted. Faecal matter, containing earthlike dirt and micro-organisms, is flushed into it.

Many elements found in waste water are washed in from the streets by rain, as a 'component' of waste water can be especially substantial after long periods of dry weather - everything from newspapers, cigarette filters and fast food packaging to car exhausts and rubber particles worn from tyres.