The main principles of the choice of clearing circuits. Kinds of clearing constructions.

Set of factors influence on the choice of clearing circuit:

- Quantity of various kinds of sewage;

- An opportunity and economic feasibility of extraction of impurity;

- Requirements to quality of cleared water with the purpose of its using in systems of repeated and turnaround water supply;

- Quality of fresh water;

- Capacity of a reservoir, etc.

The circuit of clearing should provide the minimal flowing of sewage in reservoir, maximal use of cleared sewage in the technological processes, fuller extraction of valuable impurities.

To clearing apply three basic types of clearing constructions: 1) local (shop); 2) the general (factory); 3) regional, city. First two kinds are used more often.

Local (shop) clearing constructions are intended for neutralization of sewage directly after technological installations and shops. Installations of local clearing are continuation of technological process.

In these installations are cleared waters which without clearing cannot be directed to systems of repeated and turnaround water supply or on the general factory or regional clearing constructions. On these installations, as a rule, valuable impurities are recovered therefore usually the regenerative methods of clearing are applied: settling, flotation, extraction, rectification, adsorption, ion exchanging, reverse osmosis, etc. In some cases as local installations the installations of thermal neutralization of sewage by "fire" method are used.

Application of local installations in many cases allows repeatedly use cleared water in the same technological process.

The general (factory) clearing constructions exist at the large chemical and petrochemical enterprises. They include constructions of primary (mechanical), secondary (biological) and tertiary (additional cleaning) of sewage clearing. At many enterprises there are constructions of primary and secondary clearing. Installations of additional clearing are used for obtaining of water which can be repeatedly used in technological processes or in systems of turnaround water supply.

To constructionsof mechanical clearingare concerned lattices, sand traps, sediment bowls, flotation and filtration installations, etc. On these constructions basically rough dispersed impurities are removed, in some cases coagulation of colloid impurities is made.

To the constructionsof biological clearing, intended for biological destruction of organic substances, are concerned aerotanks, biological filters, biological ponds, etc.

Toadditional clearingof sewage are applied the reagent methods (for example, coagulation, floculation, co-settling of impurities), filtering, flotation, adsorption, ion exchanging, reverse osmosis, etc.

At the petrochemical and oil refining enterprises the general clearing constructions are constructions of petro-catching.

Many enterprises use the general clearing constructions for clearing of not only industrial, but also of household and atmospheric waters.

In connection with necessity of creation of non-discharge manufactures value of constructions for additional cleaning of sewage sharply grows.

Depending on degree of pollution sewage can be directed directly on constructions of primary, secondary or tertiary clearing. For example, the sewage which are not containing suspended particles, at once are given on biological clearing.

Regional or city clearing constructions are intended, basically, for mechanical and biological clearing of household sewage. The industrial sewage subjecting to joint clearing with household sewage of settlement, should not break work of constructions, i.e. should not contain:

1) More than 500 mg / l of suspended and flow emerging substances;

2) The substances, capable to block pipes of canalization or settle on the walls of pipes;

3) The substances, destroying the material of pipes;

4) Combustible impurities and dissolved gaseous substances, capable to form explosive mixes in sewers and constructions;

5) Harmful substances in the concentrations interfering biological clearing of sewage or their flowing in reservoir.

The temperature of these waters should not exceed 40⁰C. The industrial sewage, which can contain radioactive, toxic and bacterial pollutions, before flowing in reservoir should be disinfected and neutralized.

It is obvious, that sewage clearing on large regional clearing constructions is economically more expedient, than at local less large stations.