Many years of experience in the application of aeration facilities for the biological treatment of wastewater and accompanied by the accumulation of data necessary for the calculation and design of structures. All the major provisions of the method of calculation based on the results of numerous theoretical and experimental research and testing in the specific conditions of use. The purpose of the calculation is to determine the aeration facilities required to achieve the desired degree of purification, volume structures and the amount of air volume produced during cleaning of excess sludge to be constant excretion of biological treatment system. Factors contributing to the capacity of aeration tanks are flow entering the wastewater treatment and the duration of their stay (it is also called the period of aeration) in the aeration tanks. Since the duration of aeration is measured in hours (by current standards, it should not be less than 2 h), then for calculating the capacity of the aeration tanks are average hourly flow rate of sewage over a period duration equal to the period of aeration during which comes from the maximum daily inflow of water. The duration of the aeration will depend on the type and mode of operation of the aeration structures, nature of impurities and their original concentration, the desired degree of purification plants maintained in a dose yl, oxygen conditions, water temperature and others.
The currently applied method of calculation of aeration is based on the following assumptions.
1. Select the type of aeration plants (aeration tank propellant, aeration, mixing, aeration dispersed with water inlet), and its mode of operation (with the regeneration of the activated sludge or not) be based on the characteristics of the wastewater to be treated (the nature of contaminants, their concentration and mode Income and expenditure sinks) and the required depth of cleaning.
2. The duration of aeration is a function of the simultaneous impact of factors such as initial and final concentrations of contaminants in the waste water, the nature of contaminants dose sludge in the aeration tank, the rate of oxidation contaminants activated sludge maintained in the construction of the concentration of dissolved oxygen, the hydrodynamic conditions in the aeration tanks. Since the degree of influence of these factors on the nature of the biological processes will depend on the type of structure, and the method of calculating the duration of aeration will be different for different types of structures.
3. the estimated duration of aeration, ie duration of stay of wastewater in aeration facilities, determined design flow of wastewater.
4. The required aeration capacity without regenerators determined based on the calculated flow rate and the duration of aeration without recirculation flow activated sludge, as its mixing with the supplied water for wastewater leads to a proportional reduction in the initial concentration of the waste water due to its dilution, practically without changing the total load contaminants on activated sludge.
5. Capacity aeration in the presence of activated sludge regenerators, and the regenerator itself determined based on the activated sludge recirculation flow therein. The duration of regeneration is regarded as the difference between the duration required for the oxidation of contaminants, and the length of stay of wastewater in the aeration tank where contamination has were withdrawn from the water and a particular active digested sludge.
6. Selection of the aeration system shall be based on the capacity of sewage treatment plants, technical and economic efficiency of the aeration system, and the availability of opportunities for selected aeration equipment, its durability and reliability.
7. Calculation of the aeration system involves determining the necessary amount of air (oxygen), the design parameters of its submission to the aeration tanks (flow, pressure, flow intensivnst), the number of air diffusers – aerators for a given oxygen regime and hydrodynamic conditions for mixing the mixed liquor in the aeration tank. The method of calculating SNIP 2.04.03-85.
Sludge index characterizing the sedimentation capability of sludge depends on the load on the mud, and thus the load of the sludge will depend on the dose of the sludge in the aeration structure. Experience shows that as can be in the range of 3-5 g / l – in the extended aeration; 3-4 g / l – the low load; 2.5-3.5 g / l – in the middle, and 2-3 g / l – a high load.
The aeration must not be below a certain limit in order to ensure proper mixing of the contents of the aeration tank sludge regardless of the need for oxygen. The minimum intensity is in the range of 48 m3 / (m2’ch) with a depth of immersion aerator ha = 0.5 m and 2.5 m3 / (m2 * hr) at ha = 6 m. On the other hand, the aeration rate must not exceed certain values because of the high content of air in the liquid aeration actual performance drop compared with the calculated values it. The maximum values depend on the intensity ratio of the area of aerators to the area of the bottom of the aeration tank, ie by fcJfa ,, and change ot.5 m3 / (m2 * h) with /, D ‘= 0.05 to 100 m3 / (m2 * h) at fa / fal = 1. If the calculated value J “exceeds the maximum allowed for these conditions, it is necessary to increase the area of aerated zones, ie increase the number of aerators. Ja If less than the minimum of its value, it is necessary to increase the air flow.
The calculation is in the selection duct diameter pipelines and determination of head loss in them. In order to save metal should seek to reduce the diameter of the pipe, but at the same time, the loss of pressure in them should not be too large to avoid, unnecessary power consumption. The total value of local resistance and resistance to friction in the duct should not exceed 0.3-0.35 m. Calculation of mechanical aeration system is to select the type of aerator, aerators amount both in terms of providing the required oxidizing power, and from the point of view of the requirements ensure efficient mixing mixed liquor in the aeration zone.