Abstract of the construction and operation experience of BAF treating urban sewage: Multiple issues such as backwashing, aeration, and filter material selection that arise in the design and construction process of aerated biological filters are elaborated separately, combined with the author's Based on practical experience, the precautions in the design and construction of aerated biological filters are discussed for reference when designing this process. This is to enable the mature design of the aerated biological filter process to be promoted and applied as soon as possible to avoid blind application of design manuals or hidden process risks due to lack of practical experience. At the end of the article, the application of this technology in Wenzhou, Zhejiang is briefly introduced for reference by colleagues. The aerated biological filter is derived from the V-type filter of Deliman Company. Deliman Company applies the aerated biological filter to Dalian Malanhe Sewage Treatment Plant. Due to its technological advancement and technical uniqueness, it was soon widely used throughout the country and other places. According to preliminary statistics, there are more than 30 10,000-ton sewage plants across the country that use aerated biological filters as sewage (deep) treatment, with a total treatment capacity of over 100×104m3/d. According to the results of the investigation, the design and construction of the process are summarized, and the insights are as follows: 1. The impact of pretreatment on aerated biological filters. According to the process characteristics of aerated biological filters, the sewage entering the filter should be pretreated. Avoid clogging the filter head or filter layer. Dalian, Shenyang and other places use SEDIPAC?0?3 3D (referred to as S3D) for pretreatment, which includes three parts: aeration sand settling, air flotation oil removal and inclined tube sedimentation [1]; while Guangdong Xinhui and Jiangmen use hydrolysis and acidification. It is actually a primary sedimentation tank between hydrolysis acidification and high-efficiency sedimentation tanks. The bottom area is hydrolyzed and the upper part realizes mud and water separation; other pretreatment processes such as diatomaceous soil and wetland bed are also designed to remove floating and suspended solids. Theoretically, the sewage entering the filter should not contain floating objects such as plastic pieces to avoid entering the filter water distribution area and causing blockage of the filter head. For this reason, some sewage treatment plants set up fine grids on the water distribution channels entering the filter, with a grid gap of 2mm. (or smaller). The concentration of suspended solids (SS) in the pretreatment effluent determines the backwash cycle of the filter to a large extent. In operation practice, it is found that if the sewage entering the filter carries too much suspended solids, the backwash cycle of the filter will even be shortened to Within 12 hours, it will affect the next-level filter and even the effluent; while sewage with high concentrations of ammonia nitrogen and soluble pollutants but low suspended solids (SS) enters the filter, and the filter effluent quality is very good and the operation is stable. The flushing cycle is 7-10 days and there is no abnormality. 2 The impact of filter plate construction quality on the application effect of aerated biological filters. The core part of filter construction is the construction of the water distribution area, including the construction of water distribution gas chambers, support beams, filter plates, etc., among which the installation of filter plates Reaching the design level tolerance is the most difficult to achieve. Generally, the horizontality tolerance of each filter plate (about 1m2) is ±1mm, the horizontality tolerance of the filter plate of a single filter is ±5mm, and the horizontality tolerance of the filter plates of the same group of filters is ±10mm. The filter plate leveling requirements are very high, the construction is difficult, the workload is large and cumbersome, and it is very difficult for the construction unit to meet this requirement in actual construction. In view of this, relevant design and construction units should attach great importance to it and invest more energy in construction meticulousness, new construction methods, and the application of new materials. During the construction of cast-in-place or prefabricated filter plates, the positioning of the embedded parts should be well controlled and the levelness should be well controlled. The quality of the project (including construction accuracy) should not be affected in order to meet the construction deadline. Since the construction of the filter head (cap) and aeration system must wait until the filter plate is installed, the level of the filter plate directly affects the levelness of the filter head and aeration system. The level of the filter head and aeration system is closely related to the filter backwash (mainly air washing) and aeration. The air volume is large at the high part of the filter plate, and there is little or even no air at the low part of the filter plate. After the filter head is assembled, it must be The backwashing effect of the filter was tested. The backwashing effect did not meet the design requirements. When assembling the seam-filled filter plate, the joints of the filter cap and filter plate must be processed. The overall cast-in-place filter plate must adjust the level of the adjustable filter rod. . 3. The influence of aeration method and selection of aeration equipment on the operation effect of aerated biological filters. Factors such as filter plate levelness, aeration tube levelness, air volume (air-to-water ratio) and other factors often affect the aeration uniformity of the filter. . Uneven aeration will lead to a reduction in the actual utilization volume of the filter material and a high actual load on the filter material layer, which will affect various indicators of the filter water and also reduce the oxygen utilization rate.
Aeration equipment includes single-hole diaphragm diffusers, tube-type micropore diffusers, disc-type micropore diffusers, perforated pipes, filter head aeration, etc. In terms of aeration effect, single-hole membrane aeration is used. The aeration effect of the filter is better. As long as the design and construction are reasonable, it can even be achieved with an air supply volume of 1:1 to make the air distribution on the filter surface of the 200m2 (referring to the total area of ??the multi-cell filter) basically uniform. Filter head aeration is used in secondary filters in Sichuan, Liaoning and other places, and it is said to be relatively successful. Since I have no personal experience in using filter head aeration, it is difficult to make too many comments here, but one thing is certain. Filter head aeration requires very high levelness of the filter plate, otherwise it will be difficult to achieve uniform aeration. It is recommended that when assembling the supporting layer of the filter, the construction organization must be well organized to prevent pebbles from damaging the filter cap or aerator. After the supporting layer is assembled, the aeration uniformity and backwash uniformity must be tested to see if they meet the requirements. You must rework when required, and you can't take any chances. The aeration method can use one blower to supply air to one filter (i.e. one to one), or one fan (or multiple fans and one main pipe to supply air to the outside) to supply air to multiple filters. (That is, one drags many). Due to the different installation levels and different sewage interception capacities of different filters, there is a risk of uneven aeration in one-to-multiple aeration. One-to-one (or one-to-two) is a better choice. Each fan can be equipped with one frequency converter. The device adjusts the air volume. The aeration fan should be a Roots blower. The fan is cooled by water. Gear oil is added to both sides of the fuselage to run more smoothly. 4. The Importance of Backwash Optimization Design As the operation time increases, the amount of sewage intercepted by the filter increases greatly, and the effluent quality may deteriorate accordingly. At this time, the filter should be backwashed. Whether the backwash design and construction is reasonable has a crucial impact on the operating results of the aerated biological filter. The following factors should be considered during design: ① Select the appropriate filter plate opening ratio. If the opening ratio is designed to be too small, the resistance during backwashing will increase, and in severe cases, the filter plate may be damaged; if the opening ratio is too large, a stable and suitable air cushion layer cannot be formed, which will cause uneven backwashing; ② Select a suitable filter tank For the backwash air distribution and water distribution system, if the filter area is large, it is recommended to set up a backwash air distribution and water distribution chamber (there are corresponding design specifications, which will not be described here). However, the air distribution and water distribution chamber requires more structural processing, especially during construction. This is troublesome, so a considerable number of aerated biological filter projects have arranged perforated pipes in the lower part of the filter, or even directly connected the backwash air inlet pipe to the filter water distribution area (i.e., the space under the filter plate), and have achieved success, but should Be cautious (there are some unsuccessful project applications); ③ Choose the appropriate backwash intensity. According to the design parameter values ??of Dalian Malan River and the empirical data we have obtained in actual operation, the backwash and air wash intensity of the filter should be appropriate. Large, it is recommended to set the value within the intensity range of 60-90 m/h to achieve the effect of scrubbing the filter material and micro-expansion of the filter material. Moderate water washing intensity is sufficient. ④ Rapid dewatering before backwashing should be considered in the design. The pipeline system of the aerated biological filter is many and complex, but the setting of rapid dewatering pipelines is very important. At present, domestic projects using aerated biological filters, except for Jiuquan sewage treatment plant, are all upflow aerated biological filters. Most of the suspended solids intercepted during operation of the filter are in the water distribution area and the lower filter material layer space. Due to the obstruction of the upper filter material layer, these substances are not easily discharged from the backwash drainage. On the contrary, they are easier to be discharged from the vent pipe (i.e., rapid dewatering pipe). Therefore, it is recommended to use electric venting to drain about 1/5 of the water in the pool before backwashing, and then backwash. The backwashing effect is good. Of course, the impact of filter plate levelness on backwashing is crucial. This has been mentioned before and will not be repeated here. Generally speaking, in the early stage of operation of the filter (referring to the operation soon or in the debugging and film-hanging period), due to the small amount of biofilm, the aeration and backwash resistance is small, and the backwash intensity should be small. As the time the filter is put into use increases, the adhesion ability of microorganisms increases, the amount of microorganisms increases, and the difficulty of backwashing increases. At this time, the backwashing time should be extended or other measures should be taken to ensure the backwashing effect. The backwash duration, applied to the design parameters of water supply, is within 15 minutes, but the feedback from actual operations is not optimistic. The backwash duration in Dalian, Xinhui, and Cixi all exceeds 20 minutes, with the highest being 50 minutes. Therefore, the design consideration is between 15 and 40 minutes, and the load impact of the filter backwash on other filters and the loss of backwash water volume are also considered. 5. Selection of filter materials for aerated biological filters. As a carrier for microbial growth, filler has added a new meaning to the aerated biological filter: filter material. It not only has the function of providing a carrier for the survival of microorganisms, but also has the filtration function of the general filter process. Therefore, the selection of filter materials is crucial to the normal and stable operation of the aerated biological filter [2].
5.1 Selection of filter material particle size The particle size of filter material for aerated biological filters is generally 2 to 8 mm. In order to achieve the purpose of denitrification, some aerated biological filters divide their filter material layer into two areas, the lower anoxic area and the upper aerobic area; some even have the entire filter material layer operating in anoxic condition. The final effluent return flow is introduced into NO-3-N denitrification and denitrification. The primary denitrification filter DN pool of Fengle Wastewater Treatment Plant in Jiangmen, Guangdong is of this type. The particle size of the filter material selected for this type of filter should be large, generally between 5 and 8 mm. Selection of particle size range for multi-stage aerobic filters: considering the need for sufficient dirt-holding space for the first-level filter, the particle size is 4 to 6 mm, and the second-level filter is preferably 2 to 5 mm, as long as the backwash can meet the requirements and the backwash However, it is recommended that the particle size should be smaller to achieve better processing results. 5.2 Characteristics of filter materials. The hardness (strength) of a good aerated biological filter filter material must meet the requirements. Otherwise, if the filter material is worn and damaged in the filter, it will fill the gaps in the filter material, reduce the dirt-holding space and cause adverse reactions. Difficulty flushing. At present, ceramsite filter materials are widely used in China. Compared with other lightweight filter materials, this type of filter material has a larger accumulation volume and is less likely to expand and loosen during backwashing. On the one hand, it is difficult to clean the filter material to meet the requirements; on the other hand, it is difficult to clean the filter material. As a result, backwashing consumes more water, gas, and time. The surface of the filter material should be rough and microporous, without glaze and bright. When other indicators are guaranteed, the larger the specific surface area, the better, which is more conducive to film hanging and increasing biomass. The use of filter materials with low specific surface area will limit the increase in biomass in the filter, which will have a great impact on the quality of the treated water. It is recommended that the selection of filter material increase the control of the above aspects. As for the selection of other indicators, it can be determined according to actual needs and parameters recommended by the filter material manufacturer. 6. Biophase Characteristics of Aerated Biological Filters Activated Sludge Method When filamentous bacteria breed in large numbers, it is easy to have difficulty in solid-liquid separation, resulting in mud inclusion in the effluent, resulting in poor effluent quality. In fact, filamentous bacteria have a large specific surface area, good adsorption capacity, strong survivability, and strong ability to degrade pollutants. However, due to their light weight, it is difficult to achieve solid-liquid sedimentation separation. The aerated biological filter uses filter (fill) material as a carrier for microorganisms to survive, turning harm into benefit. Therefore, the microbial population in the filter is particularly large. The biofilm flocs are rich in filamentous bacteria, with long hyphae and high density. . The following introduces the microbial phase of the secondary aerobic biological filter we observed in the Xinhui Sewage Treatment Plant for reference: Primary filter: The bacterial jelly mass is bright yellow, rich in filamentous bacteria, bell worms, and branch worms , there are more polycystic worms, less rotifers and nematodes; the secondary filter: rich in filamentous bacteria, more nematodes, less rotifers, and occasionally erythrozoans (huge in size under the microscope) can be seen. In the secondary filter water, small red worms can be seen with the naked eye under sunlight, and the water is clear and transparent. 7 The operating effect of the aerated biological filter and the impact of backwashing on the filter operating effect The following takes the operation status of the sewage treatment plant in Binhai Park, Wenzhou Economic and Technological Development Zone as an example to analyze the operating effect of the aerated biological filter. The scale of this project is 5×104m3/d. The currently operating aerated biological filter is the first phase of the project, with a treatment capacity of 2×104m3/d. Industrial wastewater, mainly synthetic leather, accounts for a large proportion of the raw water. , the concentration of ammonia nitrogen in the incoming water is high. Considering that the aerated biological filter has good nitrification performance, the diatomaceous soil + aerated biological filter composite process is used in the process. The filter part is divided into two stages: CN tank (decarbonization, Partial nitrification) and N (nitrification), each filter stage is equipped with 12 filter cells, and was officially put into operation in December 2005. The treatment effect is shown in Table 1: Table 1 Inlet and outlet water quality index mg/L project CODCr BOD5 NH3-N PO43-P SS inlet water quality 350 160 60 5 200 Actual outlet water index <30 <5 <5 <0.5 <5 Due to pre-treatment The effluent after treatment is very clear, the suspended solids (SS) in the pretreated effluent is less than 40 mg/l, and the backwash cycle is currently controlled at 7-10 days. However, the quality of the primary filtered water after backwashing is poor, and it is not suitable to discharge the water immediately after backwashing. Currently, measures are taken to improve the quality of the primary filtered water after backwashing for 2 hours, otherwise it will have a greater impact on the quality of the effluent water.
8 Conclusions and Suggestions The effluent quality of the aerated biological filter process is relatively stable, the treatment effect is relatively good, and the treatment effect is better for low CODCr and high NH3-N wastewater; the biomass is large, and the residence time to complete the nitrification of NH3-N only needs 2~ 4h; it occupies a small area, and there is no odor in the area where the factory filter is located. As long as the noise reaches the standard, it can be built in a prosperous area of ??the city or in the suburbs of the city, saving pipe network investment. The effluent quality of aerated biological filters is good and the operation is stable. With the improvement of automation level, the application of aerated biological filters will become more and more widespread. From the perspective of the application of aerated biological filters, the best choice is to apply them to the in-depth treatment of micro-pollution source water and sewage. That is, the inlet water of the filter is secondary treatment water. The main reason is that it is difficult to obtain good pretreatment. guarantee. Considering the actual situation in China, when the front end is connected to the first-level enhanced treatment, the floating objects must be removed thoroughly, and the concentration of suspended matter entering the filter should be as low as possible. For this reason, the surface load value of the primary sedimentation tank (high-efficiency sedimentation tank) must be appropriate, and The impact of changes in water volume on the pre-system should be considered as much as possible, so that the process can maximize its strengths and avoid weaknesses (it has high removal efficiency for soluble pollutants, especially ammonia nitrogen, but is not resistant to suspended solids), and the treatment efficiency can be fully utilized, which is also more beneficial. Promotion and application of this technology.