Fig. 2.34. Aero tank with pneumatic aeration:

1-air post; 2-filter-carrying channel; 3-filter-carrying plate.

Process of clearing of sewer water in aero tank can be divided into three stages. At the first stage, the initial one, at once after the mixing of fresh sewer water with active mule the adsorption of contamination of sewer water by mule and oxidation of simply oxidized substances takes place. As the result the sharp decreasing of sewer water БПК is seen (on 40-80%) and total consuming of diluted oxygen on processes of oxidation and, in such way, its deficit approaching to be one. The first stage usually lasts 0.5-2 hours.

At the second stage of process the oxidation of slowly oxidized substances and regeneration of active mule take place, that is the renewal of its properties, which considerable weakened at the end of the first stage. Speed of oxygen consumption at the second stage is considerably lower than at the first.

At the third stage of process nitrification of ammonium salts takes place and speed of oxygen consumption grows again.

Aero tanks can be by hydraulic mode of their work (aero tanks of ideal ousting, aero tanks of ideal mixing, aero tanks of intermediate type) or by the size of loading on active mule (highly loaded, ordinal, and low loaded). As a rule, highly loaded ones provide not full clearing, and low loaded at total clearing provide auto oxidation of surplus active mule.

The mixture of water and active mule from the aero tanks is directed to the secondary settling tanks for settling (separation) of active mule. In the process of sewer water clearing the volume of active mule increases due to growing of its biomass. Part of active mule continuously pumped in aero tanks for supporting the

necessary concentration (circulating active mule), and the rest is directed to the mule compactors (surplus active mule).

Fig.2.35. Schemes of aero tanks:

a-of ideal ousting; б-of ideal mixing.

Fig.2.36. Schemes of aero tanks work: a-of corridor ideal ousting; б-of of mixer with spraying supply of sewer water and active mule: 1-supply of sewer water; 2-cleared water; 3-circulating mule; 4-mule mixture; 5-air; 6-aerotank; 7-secondary settling tank; 8-surplus mule.

Very often circulating active mule contains unoxidized contamination, and that’s why this mule is directed into the regenerators where the contamination is oxidized and the adsorptive capacity of the active mule is renewed. As a rule, the part of aero tank is taken for the regenerators. Separate structures in which the mineralization of active mule takes place are named aerobic mineralizators.

If mixture of sewer water with active mule is aerated longer than as usually, the volume of active mule firstly grows to the maximum value and than decreases and reaches the initial value. Such case appears because the part of active mule during the long lasting aeration has time to mineralize (auto oxidize). This process is called total oxidization or continued aeration. Aero tanks allow achieving high degree of sewer water clearing with leading of organic substances content in cleared sewer water to 15mg/l by БПК_повне, Volume of aero tank is determined by average hour sewer water rising for the period of aeration in hours of maximal rising. Existing methods of ear tanks calculation take into account the process of biological clearing in them till the beginning of nitrification stage. Duration of aeration of city sewer water during this accounts nearly 6-8 hours. Expends of circulating active mule at the calculation of secondary settling tanks and aero tanks without regenerator’s calculation are not taken into account.