Calculation of pneumatic aeration systems in aeration tanks

Summary

The pneumatic aeration system of the aeration tank at the wastewater treatment facilities is considered. Prime costs of electricity at the treatment facilities go towards ensuring the operation of the aeration system, for which reason the problem of the optimal air distribution over all the operational zones is raised. At the same time, either the lack of air, even at some treatment stages, or its excess leads to a decrease in the treatment efficiency. Providing for the controlled air supply only with the use of measuring instruments is quite expensive; therefore, it is proposed to use a mathematical model for aerodynamic calculation of an aeration system with non-fixed air supply through aerators. Based on the results of the aerodynamic calculation, the adaptive control of air distribution in the aeration system with a change in the flow and quality of incoming wastewater, pressure-flow characteristics of aerators, pressure, temperature and humidity of the outside air can be provided. An example of the aerodynamic calculation of a pneumatic aeration system is given.

Key words

air blower unit , pneumatic aeration system , flow distribution task , wastewater treatment facilities , fine-bubble aeration , aerodynamic calculation

Control valves in controlled pneumatic aeration processes

Summary

Through the 2013–2019 program of the Ministry of Energy and Public Utilities of the Samara Area «Samarskie Kommunal’nye Sistemy» Company has developed and has been successfully implementing the project of upgrading the municipal wastewater treatment facilities. In order to improve the energy efficiency retrofitting the systems of aeration and air distribution, equipping with compressors with controlled air supply to the aeration tanks within 45–100% range is envisaged. The general concept of control has been substantiated that includes complex level of process control and a two-loop automation scheme «aeration tank gate valve control – air blower group». The method of selecting control valves for the air supply control system for pneumatic wastewater aeration has been developed and substantiated. The method is based on the dependence of compressed media flow through the control valve stated in IEC 60534-2-1 International Standard. Under full-scale conditions with the use of precision equipment the flow characteristics of IRIS (DN300) control valve manufactured by Egger were studied. The average percentage error of the method was 6.79%. The method error does not matter much for the automated control since PID-control provides for supplying the required air amount according to the condition of compliance to О₂, NH₄ set values and current measurements of the supplied air amount. The classification of control valves is presented based on the type of the dependence of K_v/K_vs valve capacity coefficients from the rate of valve opening. Technological modes were identified: comparison of the theoretical and experimental flow characteristics of the control valve, pressure difference at the control valve, pressure difference in the «control valve – aerators» system, aerial environment temperature before the valve. The suggested method of the control valve calculation is a part of the «Energy efficiency of the controlled air supply to the aeration tanks» comprehensive mathematical model.

Key words

energy efficiency , automated control system , membrane aerator , wastewater treatment facilities , pneumatic aeration system , control valves , flow coefficient , diaphragm gate valve

Process concentrate treatment by flotation at CHP plants

Summary

The problem of processing oily wastewater generated at power plants is discussed. Much attention is paid to the purification of process concentrate that is characterized by higher temperature (55–70 °С). A pneumatic flotation unit used for the process concentrate treatment at the South CHP plant operated by Lenenergo OJSC is considered. The major drawback of the unit is pointed out: the changes in the rubber properties result in the changes in air bubble dimensions, and the efficiency decreases. A possible solution to the problem can be the use of an ejection aeration system with a disperser. The experiments in a laboratory unit on determining the size distribution of the bubbles generated by the ejection aeration system at 55°С were conducted. In addition, model oily wastewater was subject to purification in a unit with the pneumatic and ejection aeration systems. The average size of the finely dispersed bubbles generated by the ejection aeration system with a disperser was 60 μm, and that of the coarse bubbles was 105 μm. It is shown that the use of the ejection aeration system with a disperser provides for reducing the residual concentration of oil products from 4 to 1 mg/l or reducing the purification time from 25 to 15 minutes.

Key words

flotation , ejector , pneumatic aeration system , air bubble size , dispersant , process condensate

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