Abstract : This paper introduces the application of frequency converters in the oxidation ditch of the Hebi Qibin Wastewater Treatment Plant and points out several issues that should be noted in the application of frequency converters. Keywords : Frequency converter; Ober oxidation ditch; Programmable logic controller; Dissolved oxygen; Harmonic AC motor Variable frequency speed regulation is a new technology developed in the last 20 years. It has many advantages such as wide speed range, smooth speed regulation, good working characteristics, and low failure rate. This technology has developed rapidly and is widely used in China. This paper briefly introduces the application of frequency converters in the oxidation ditch of the Hebi Qibin Wastewater Treatment Plant. 1 Project Introduction The Hebi Qibin Wastewater Treatment Plant has a construction scale of 5×104 m3/d and adopts the Ober oxidation ditch treatment process. The process requires the removal of carbon source organic matter in the wastewater and has a certain phosphorus and nitrogen removal function to meet the effluent water quality requirements. This project has two sets of Ober oxidation ditches. The process flow diagram of the wastewater treatment plant is shown in Figure 1. 2. Process Description of Aeration in the Aubel Oxidation Ditch and the Significance of Using Variable Frequency Drives The Aubel oxidation ditch consists of three concentric elliptical channels. Wastewater enters through the outer channel, mixes with returned sludge, and then flows from the outer channel into the middle channel and then into the inner channel, circulating dozens to hundreds of times in each channel before finally flowing out through the adjustable weir gate in the central island to the secondary sedimentation tank. Rotary disc aerators are installed across each channel. When the aeration discs come into contact with the water, they break the wastewater into fine droplets, providing strong oxygenation and agitation. The aeration process can be broadly described as follows: During their lifespan, bacterial flocs in wastewater can adsorb and consume bacteria, decomposing organic matter. Aeration provides sufficient oxygen to these flocs to meet their needs for survival and reproduction. The aeration system must be controlled because: the required oxygen level for the flocs must be provided in a timely manner. The amount of oxygen available in biological treatment has a certain range; insufficient oxygen reduces the decomposition effect, prolongs treatment time, and may even lead to biological treatment failure; excessive oxygen causes bacterial peroxidation, reducing activity and hindering treatment. Therefore, using a frequency converter to control the rotation speed of the rotary disc aerator in the wastewater pump station, thereby controlling the oxygen supply, is highly significant. Furthermore, as the main oxygen supply equipment in the wastewater treatment plant, it operates continuously year-round, consuming a large proportion of electricity. Minimizing power consumption while meeting oxygenation requirements plays a crucial role in reducing operating costs. [align=center][b]For more details, please click: Application of Frequency Converters in the Oxidation Ditch of Hebi Qibin Wastewater Treatment Plant[/b][/align]