3.2. Structure and properties of sewage

Water is turned into waste water by the addition of water-carried waste from private households, business, and industry. The mixture of water and waste is generally called sewage. The composition of sewage depends on it source. Domestic sewage consists of used water containing human excreta and all kinds of substances typical of domestic activities and hospitals. Industrial sewage is composed of used water and solid and liquid wastes resulting from the operation of slaughterhouses, dairies, tanneries, paper mills, food processors, and various other industrial enterprises. Many of these waste waters may be harmful to public health and cause epidemics of various kinds.

Sewage contains solid and liquid pollutants in suspension and solution. Untreated or improperly treated sewage that is discharged into rivers, lakes and the open sea, may cause the formation of sludge banks of obnoxious matter. Sludge banks are primarily found in so-called dead spaces of two- or three-dimensional flow fields, and this may also be the case in waste water treatment plants. From a chemical point of view, sewage contains matter of animal, vegetable and mineral origin. Organic matter contributes 40 to 70 % of the solids in urban sewage and is readily decomposed by biological or chemical methods. This decomposition process may take place under conditions that require oxygen, thereby endangering fish life in rivers, lakes and coastal waters. Other decomposition process does not require oxygen, but may result in objectionable odors and poisoning of the water, that will also endanger fish life.

Waste water is collected and transported to treatment plants by a system of pipes and conduits called sewers. Sewers are graded so that the sewage may flow downstream at a rate sufficient to prevent particulates from settling out.

Sewage of chemical manufactures is polluted with various organic and inorganic substances. For example, depending on kind of manufacture, characteristic pollutions can be:

Source of pollution	Structure of pollution
Manufactures of mineral and inorganic salts	Inorganic acids, alkalis, salts (fluorides, sulfates, phosphates, etc.)
Manufactures of the basic organic and petrochemical synthesis	Fat acids, aromatic compounds, spirits, aldehydes, ethers, etc.
Oil refining factories, the enterprises on thermal processing of firm fuels	Mineral oil, oils and pitches, phenols, surface active substances, etc.
Manufactures of synthetic pitches, polymers, synthetic fibres, etc.	High-molecular substances, monomers, particles of polymers, etc.

Except for the dissolved organic and inorganic substances sewage can contain colloidal impurities, and also suspended (fine dispersed and rough dispersed) substances, which density can be less or more the density of water. In some cases sewage contains the dissolved gases (hydrogen sulphide, etc.). More often sewage represents the complex systems containing mixes of various substances.

The degree of damage of sewage depends on toxicity of substances polluting it. Sewage of high toxicity contains salts of heavy metals, cyanides, hydrogen sulphide, cancerogenic substances, benzene and many other substances.

Sewage of chemical manufactures can contain inflammable and explosive substances, and also the compounds, polymerized in a water solution, that can lead to contamination of pipelines.

It is necessary to take into account aggression of sewage in relation to materials of pipelines, sewer collectors and devices of clearing constructions. Aggression of sewage depends from рН, presence of some salts and gaseous products. Sewage of manufactures of acids, phosphoric and other fertilizers, oil refining factories, manufacture of synthetic fat acids and etc. concern to number of very aggressive sewage.

Frequently sewage contains the substances possessing sharp, unpleasant smell (sulfides, disulfides, hydrogen sulphide, merkaptides, etc.). A lot of sewage is painted owing to pollution by dyes or the substances having painting.

Presence of surface active substances in sewage causes their foaming ability.

Hit of household waters in industrial sewage leads to biological pollution of them.

The temperature of sewage can change over wide range.

Methods of research of sewage. Complex structure, wide interval of impurities concentration, change of structure of sewage in time complicate the research of qualitative and quantitative structure of sewage. Research of sewage of unknown structure (for example, biologically cleared waters) is especially complex.

Frequently at the analysis of sewage there are the difficulties connected to presence of accompanying and preventing substances, not stipulated in standard techniques.

Special value has application of automatic devices, which:

- Allow to increase labour productivity of chemists-analysts;

- To lower cost of analyses;

- To carry out the continuous control over structure of sewage and work of clearing constructions;

- Immediately to fix any infringements.

The general parameters of pollution. To the general parameters of sewage pollution are concerned:

- The parameters, describing the general properties of water (organoleptic - color, taste, smell, transparency, turbidity; physical and chemical - density, optical density, chromaticity, рН, temperature, electrical conductivity, alkaliness, acidity viscosity, surface tension, etc.);

- Insoluble impurity (the content of the suspended substances);

- Dissolved substances (the general content of organic and inorganic impurities, "organic" carbon, definition of bichromate and permanganate oxidability, biochemical consumption of oxygen, etc.).