Aerobic wastewater treatment

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The most common method of this aerobic treatment is activated sludge process, which has different types. This process is one of the main methods of treating human sanitary wastewater. You may be interested to know that this process was discovered by two British engineers in 1913.
Despite the different types of methods of this treatment process, but the basis of the treatment method is all the same and that is the loss of 50 to 70% of all suspended solids after entering the treatment plant. Non-use of chemicals in this method makes it a permanent advantage over other methods of wastewater treatment. There are various parameters in the design of activated sludge processes, including hydraulic retention time (HRT), cell retention time (SRT), and feed to microorganism ratio (F / M).
The main components of the activated sludge process are aeration pool and sedimentation pool. In the aeration pool, the incoming wastewater is thoroughly mixed with the microbial mass and is called mixed liquor. In this part, the desired oxygen is supplied by the ventilation compressor. This liquid must remain in the pool for 5 to 7 hours to become floating sludge and its suspended particles are returned to the aeration pool and the rest is sent to the sludge treatment unit. There are different types of activated sludge process, some of which we will mention.
Conventional active process
Among all types of activated sludge processes, conventional activated sludge process with mold flow is the most used in the treatment of various types of wastewater, especially municipal wastewater. In this method, an initial sedimentation unit is used before the aeration tank and also walls are installed in the aeration tank, with the help of which the aeration tank becomes a channel with a small width and a long length. During this process, sewage enters from the beginning of this canal and after passing through the entire length of the canal, it enters the secondary sedimentation tank. In fact, the flow in a conventional activated sludge process is a combination of a complete and mold mixing process.
Extended Aeration
This process is used to treat industrial wastewater, which often contains complex soluble organic matter. The design of ventilation equipment is based on the amount of mixing required. The advantages of this process include a long residence time in the ventilation pool, and uniformity, which protects the system from organic load shocks. Due to the digestion of a number of bacteria, less sludge is produced, which is not used in the initial settling tank.
Gradual aeration process
The gradual ventilation system is very similar to wastewater treatment plants. In this method, the amount of air injection at the beginning of the pool is more than the end, the possibility of anaerobic conditions at the beginning of the pool will be less. In this method, the intensity of aeration to the sewage is reduced along the pool because the sewage initially needs more oxygen and with the reduction of pollution, the need for oxygen in it is also reduced. This method is more suitable for communities up to 100,000 people and provides more optimal conditions. The residence time in the aeration tank is usually 4 to 8 hours and aeration using diffusion.
Complete Mixed Activated Sludge
In this method, the incoming wastewater is immediately mixed with oxygen and bacteria in the aeration pond and the organic load, MLSS concentration and oxygen are uniform throughout the reactor. In this system, the wastewater is deposited and the activated sludge returns to the aeration pond from several points. The operation of this process is relatively simple and the concentration of organic matter in it is low. The flow in this method is continuous and mixed with liquid stirring and the aeration time is 3 to 5 hours.
Contact Stabilization Process
In this method, two separate aeration tanks are used for wastewater treatment. After stabilizing the solute and particulate matter, part of the activated sludge is mixed with the incoming wastewater and aerated inside the contact pool. The residence time in the contact pool is relatively short and the microorganisms in this pool quickly remove the solution and the particulate and colloidal organic matter remain in the activated sludge flasks and are oxidized. After settling in the secondary pond, the biological clots leave the system and return to the other part of the stabilization pond.
Cross process
The cross process is a modified form of the aeration process for nitrogen-deficient wastewater. In this method, which uses anaerobic digester sludge and supernatant to improve the settling quality of biological flasks and supply the required nitrogen, part of the secondary sludge return sludge is mixed in a nitrification reactor with anaerobic digester supernatant. This returned sludge returns to the main piston flow aeration reactor.
Pure oxygen process
The use of a 3 or 4 stage closed reactor in this process is the most obvious feature of this method. Inlet wastewater, return activated sludge and pure oxygen are added to the first stage. The gas flows on the surface of the liquid and the mixed liquid from one stage to another in one direction. The partial pressure of oxygen in the space above the liquid surface in the first stage is about 40 to 60% and in the last stage is about 20%.
The ability to perform nitrification in the pure oxygen process is limited. This decrease occurs due to the accumulation of carbon dioxide in the air above the liquid, which leads to a decrease in the pH of the liquid. This system has a shorter hydraulic residence time and a higher volumetric load, and the amount of oxygen supply is 2 to 3 times higher than conventional aeration systems. Important advantages of this method are the reduction of space and the amount of exhaust gases, if necessary, odor control and VOC.