Technical Discussion: Key Operating Points of the BAF Process

1. Structure and working principle of the BAF process The Biological Aerated Filter (BAF), also known as a submerged biological aerated filter, draws heavily on the basic design concepts of the biological contact oxidation process and rapid filtration systems used in water treatment. By utilizing a special filtering medium, it combines functions such as oxygen supply through aeration, rapid filtration, and periodic backwashing. And it removes organic matter, SS, as well as nitrogen and phosphorus through different operating modes. 1.1 Structure and composition of the BAF process ； 　　The structure of an aerated biological filter is basically the same as that of a conventional rapid filter, but it is equipped with an aeration system. The packing material used in it differs significantly from that used in ordinary biological filters; it is similar to the filtering media used in conventional rapid filters, but with larger particle sizes and a high porosity, which results in a higher specific surface area and a lower density. 　　An aerated biological filter consists of five components: a water distribution system, an aeration system, a support layer, a biological media layer, and a backwashing system (including backwashing air and backwashing water). 1.1.1 Water and air distribution systems in the BAF process ； 　　The water distribution system of an aerated biological filter includes inlet water distribution, backwash water distribution, and the collection of filtered effluent. Regarding water distribution to the inlet, depending on the direction of water flow during the operation of the aerated biological filter, its distribution system can be installed either at the upper or lower part. Since the BAF is always in an aeration state, the water distribution device located at the upper part and operating in a downflow manner has a significant impact on the uniformity of water distribution during its operation. Due to aeration, for the water distribution system located in the lower areas, the homogenizing effect of turbulence generated by continuous aeration contributes to improved uniformity in water distribution. 1.1.2 Operation mode of the BAF process 　　Depending on the direction of water flow, there are two possible operation modes: one is the upflow type (UBAF) ； Two are downflow type (SBAF) ； 　　When the BAF operates in upflow mode, the water flow moves from the bottom of the BAF upward through the filter media, coming into contact with the air that flows upward as well. BAFs operating in this manner have a higher hydraulic load due to their greater capacity for removing pollutants, which results in slightly poorer water quality; the SS content is higher compared to those operating in a downward flow mode. Therefore, when stricter treatment requirements are needed, additional sedimentation separation equipment should be installed. 　　When the BAF operates in a downflow mode, its water flow passes through the filter media from the top of the BAF, moving from top to bottom, and comes into counter-current contact with the air flowing from bottom to top. BAFs operating in this manner have a low hydraulic load, but they are effective at removing SS from the influent; as a result, the SS content in the effluent is low, and generally no subsequent sedimentation separation equipment is required. 2. Key points of BAF process design and operation control ； 　　The structural design of a BAF mainly includes its tank body, filter media, support layer, as well as water and air distribution devices. 2.1 Design of the filter tank structure ； 　　Like ordinary rapid filtration tanks, the basic function of a BAF tank is to hold the amount of water to be treated and to support the filter media bed inside the tank. The planar shape of BAFs is mostly square or rectangular (with circles being rare). The area of each cell is generally preferably no more than 50 m2, to facilitate uniform water distribution during filtration and backwashing, as well as to reduce the volume of the backwashing water storage tank. The number should generally be no less than two units (operating in parallel), and should not exceed 8 to 12 units to facilitate operation and management ； Total area of the filter bed: A = q/vT; the total height of a BAF is generally 5–7 m, of which the height of the filter media layer is 1.5–4 m, the height of the supporting layer is 0.3–0.4 m, the height of the water collection space at the top is 0.8–1.0 m, the height of the water distribution area at the bottom is 1.2–1.5 m, with an additional overheight of 0.3–0.5 m.

Reply to 1# Yu Ru Yu Cheng: 1. At the beginning of operation of the facility, when the BAF filter is in the stage of cultivating microbial sludge, the wastewater inflow rate should be controlled at around 30%. At the same time, phosphates (such as trisodium phosphate) must be added to the tank daily to keep the total phosphorus content in the wastewater at around 0.1–0.5 ppm. Sampling and analysis are carried out once a day, along with the measurement of the total N content. If this value is below 2.0 ppm, urea must be added to keep the total N content within the range of 2–5 ppm. Once the biofilm in the BAF filter is established, approximately after 15 days, the water flow rate can be gradually increased to the designed level, while less urea and phosphate can be used, or none at all. The BAF filter has been put into normal operation. 2. Adjust the air intake opening to control the air volume, so as to keep the dissolved oxygen level in the BAF filter at 2-5 ppm; samples should be taken for analysis once a day. 3. The BAF filter needs to be backwashed after operating for about 10 days.

On the 3rd floor, my BAF tank keeps the dissolved oxygen concentration above 8 mg/L, and the results are much better than those achieved when using mg/L as the measurement standard.

Are there any specific documents? Could you send me one?:P

You can take a look at this post. http://bbs.hcbbs.com/thread-1390108-1-1.html