Test entry by subed TC for Holle Polle to read. 7 Sept 2001

In order to be able to treat water, we need to know what waste water is composed of, and what the difference is between waste water and the drinking water that we expect to come out of our taps

What is in Waste Water?

The Chemistry and Physics of Water

From the chemical point of view, water is a molecule made of two atoms of Hydrogen (H) and one atom of Oxygen (O) giving H₂O. It has a molecular weight of about 18 g/mol¹.

The water molecule has a two-dimensional structure forming a "V" with the two hydrogen atoms at the sides and the oxygen atom at the point. Oxygen has the higher negative electric charge, attracting the single electrons of the H atoms closer to it. This leads to a concentration of electric charges making the water molecule into an electrical dipole. A positive charge is located near the hydrogen atoms, while a negative charge is concentrated near the oxygen atom.

This dipolar characteristic means that water will not mix with fluids that are not dipoles. Oils, fats, hydrocarbons and other molecules belong to this group.

However, water is able to dissolve salts, which then split into their ions. A very small proportion of the water splits into ions of its own: a positive H⁺ ion and a negative OH^- ion. The H⁺ ion migrates to the negatively charged 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 neutral pH value is 7, corresponding to the negative exponent of the base 10 in the natural concentration of H⁺ ions.

Dissolving acids in water increases the concentration of H⁺ ions. Hydrochloric acid, for example, breaks down completely
into H⁺ and Cl^-. The resulting concentration of H⁺ ions may be as low as 0.1 mol/l (=10^-1mol/l) giving a pH of 1.

Dissolving alkaline substances, such as potassium hydroxide, in water, decreases the concentration of H⁺ ions and increases that of OH^- ions. The pH value may then be as high as 14, corresponding to an H⁺ ion concentration of 10^-14mol/l.

Adding alkalines to a solution with low pH values or, the other way round, acids to a high pH solution neutralises the pH value. When the amounts of H⁺ and OH^- ions are equal, the result is a neutral (pH 7) mixture of 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, as supplied by the water works, is usually enriched with chlorine, to keep micro-organisms (especially harmful ones) from infecting people.

The composition of natural water depends mainly on the local environment. If the springs are in soil or rock rich in iron, then, of, course, the
iron content is relatively high. Water in swamp areas contains a relatively high concentration of hydrocarbons and of micro-organisms.

A very important factor with water is its degree of hardness. Everybody knows how much a belly flop can hurt, but this is a different kind of 'hardness': it is a measure for the amounts of (earth-) alkalies (Na⁺, K⁺,
Ca²⁺, Mg²⁺) and their carbonates (CO₃^2-, HCO₃^-). These ions are responsible for the white scale that blocks pipes, lines kettles and boilers, and sticks to the heating elements in washing machines. The latter are very often corroded by this covering, by the way.

Natural water forms creeks, rivers, lakes and the sea. Seawater has a higher concentration of salts than ground water. Water is populated not only by micro-organisms, but also by macro-organisms - insects, fish, animals, birds, and all kind of plants. Both fauna and flora are well-adapted to their individual local environment. Saltwater fish would not usually be able to survive in a mountain lake.

What can be found in Waste Water?

Apart from natural components, waste water can be contaminated with various other substances, such as dirt, dust and pieces of garbage. Oils, fat and grease cover surfaces or are emulsified by surfactants ³ Chemicals of varying acidity and alkalinity, eventually forming and reforming salts, are dissolved in the water. Faeces are flushed away, containing soil and micro-organisms.

Many things in waste water are in there because they are washed from the streets by the rain. Especially after long periods of dry weather, there is a rapid increase in the proportion of these last components; not only about rubbish such as newspapers, cigarette filters and McDonald's packages, but also soot from chimneys, car exhaust and rubber particles worn from tyres, to name but a few more examples.

Links to further entries within this project

This entry is part of the
'Biological Waste Water Treatment' Project.

• An Overview of Waste Water

• Waste Water Treatment

• Biological Background for Waste Water
  Treatment

• A Waste Water Treatment Plant