biological aerated filter (baf) is a new wastewater treatment technology developed in the late 1980s. in 1990, otv company of france built the world's first baf, which is called "submerged fixed biofilm aerated filter". because it overcomes the shortcomings of activated sludge process, such as large area, easy to emit odor and unstable operation, it has attracted much attention.
at present, hundreds of biological aerated filters have been built and operated all over the world. it belongs to the category of biofilm process and has some characteristics of activated sludge process. porous filters with a diameter of only a few millimeters are placed in the filter as the attachment propagation medium of the biological community, and air is supplied to the biological community through the air distribution system (also placed in the middle of the filter layer) under the filter layer (air source is blower). in addition to the biofilm on the filter material, a large number of suspended organisms similar to activated sludge are intercepted in the filter layer, and pollutants are also adsorbed and degraded. the upward flow pattern is often used in the direction of water flow, that is, the top of the bottom intake pool and the outlet of the pool, and some downward flow patterns are also used. the upward flow type uses perforated tube bottom to distribute water, while the reinforced concrete filter plate and filter head are installed at the top of the tank to prevent the loss of filter material and collect effluent. the downflow water distribution system with large resistance is adopted. light porous filter material has small particle size, large specific surface area, high volume load and greatly reduced filter area. because the direction of flow is consistent with the direction of filter material compaction, biological contact oxidation and solid-liquid separation can be accomplished simultaneously, and subsequent secondary sedimentation tanks can usually be omitted. with the process of filtration, the biofilm becomes thicker, aging and falling off, the suspended matter intercepted by the filter layer increases gradually, and the filtration resistance increases synchronously. it is necessary to wash back regularly to restore its purification ability.
the flushing mode is three-stage air-water backwashing, that is, first air washing, air-water combined flushing and then separate water washing. backwash air is provided by the blower through the air distribution system at the bottom of the pool. the direction of backwashing water flow is from top to bottom (up-flow filter) or from bottom to top (down-flow filter). the flushing water of the upflow filter is stored on the filter plate, and gravity flushing is carried out by using the effluent of the same group of filters without flushing pump.
in recent years, a number of upflow biological aerated filters (ubaf) have been constructed in china. their forms have changed, and their structures are similar to those of v-type filters for water supply. the water distribution and air distribution system is located at the bottom of the filter, and the reinforced concrete filter plate and the long handle filter head are used. in order to prevent the loss of filter material, a grid-shaped flow stabilizing plate is added before the outlet weir, and the top surface of the outlet weir is made into a 60 degree slope. the flushing mode is still three-stage air-water backflushing, requiring a backwashing pump. when there is a requirement for denitrification of effluent, two-stage biological aerated filter is usually used to control oxygen supply to create anoxic or aerobic environment in the filter layer, so that the dominant bacteria on the biofilm are aerobic heterotrophic bacteria, nitrifying bacteria and denitrifying bacteria, so as to achieve the purpose of carbon removal and nitrogen removal. chemical flocculation is the main method of phosphorus removal (adding iron or aluminium salts before filtration). phosphorus accumulating bacteria in the filter can remove phosphorus from wastewater by biological method under the condition of alternating anaerobic and aerobic conditions.
in addition, there are also examples of application of biological aerated filter in water pretreatment in china. it has successfully reduced cod, nh3-n, no2 and aoc of slightly polluted raw water, and improved the quality and biological stability of the effluent. biological aerated filter (baf) has many advantages, such as single process barrel, large hydraulic load and volume load, small land occupation, low investment, low operation cost and good effluent quality. it can be used not only in the treatment of domestic sewage and some industrial wastewater (such as beer, printing and dyeing, food processing wastewater), but also in the pretreatment of water supply. however, its pool structure is more complex, equipments are more, automation is higher, and higher construction quality and operation management level are required. based on engineering practice, this paper discusses some problems that should be paid attention to in the design and construction of upflow biological aerated filter (ubaf) for sewage treatment.
1. main parameters
influent water quality (mg/l): cod100-1000bod550-350ss50-350tkn15-60nh3-n10-40
water quality of effluent (mg/l):cod<40bod5<20ss<20nh3-n<10tkn<15
volume load ns: 2-6 kg bod5/(m3.d) 4-12 kg cod/(m3.d)
the value of ns is closely related to the quality of influent and effluent. the higher the volume load of organic matter, the higher the residual concentration of organic matter in effluent. for example, municipal sewage requires effluent bod510-20mg/l, ns is 3.5-5.0, and ns should be reduced to 2.5-3.2 when effluent bod55-10mg/l is required. when ns > 3, nh3-n removal was inhibited, and when ns > 4, nh3-n removal was inhibited significantly. when nitrification and denitrification are required, the nitrification load should also be considered, generally 0.3-0.8 kg nh3-n/(m3.d). therefore, the appropriate ns value should be selected according to the nature of raw water and treatment requirements.
hydraulic load: 3-6m3/m2.h
去除率∶cod>90%bod5>90%nh3-n>90%ss>90%
filter material: in addition to the requirements of particle size, density, porosity, mechanical strength, chemical stability, non-toxic, harmful substances and other aspects, the most important thing is the specific surface area. the larger the specific surface area, the more microbial biomass per unit filter material, the higher the biochemical treatment efficiency. the material can be made of lightweight ceramisite, anthracite, quartz sand, plastics, etc. the circular lightweight ceramisite filter material is better. the particle size is 3-6mm and the thickness of the filter is 2.5-4.5m.
flushing strength: water 4-10l/m2.s, gas 12-20l/m2.s, and filter expansion rate about 10%.
flushing mode: water and gas distribution with long handle filter head. first air wash for 3-5 minutes, then air-water combined wash for 3-5 minutes, and finally single water wash for 3-5 minutes. the sludge intercepted in the filter layer and the aged biofilm are discharged by flushing, but the flushing intensity should not be too high, so as to retain enough active biofilm for the next biochemical period.
aeration: it provides dissolved oxygen for microorganisms to grow and reproduce, and has the function of agitating the filter layer and promoting the falling off and renewal of aging membrane. oxygen demand is about 0.42-0.8 kg o2/kg bod5, which can be used for air distribution by perforated tube or air diffuser (aerator) mounted on the filter plate. in order to prevent water backflow, the bottom of backwash air main pipe and aeration main pipe should be raised to 500 mm above the highest water level of the filter.
work cycle: 24-48h
single cell filtration area: 50-100m2
2. pool type
in order to ensure the effect of backwashing, the area of a single tank should not be too large (<100m2). in the plane, rectangular and unilateral water distribution is usually adopted. the ratio of longitudinal length to transverse length is 1:1.2-1:1.5, and the length of longitudinal (short) side is less than 8m. a water distribution chamber should be set up at the front of the transverse (long) side to distribute air evenly. the inlet hole is located on the bottom plate of the filter. the top of the intake hole should be flat or slightly lower than the bottom of the filter plate, and the aperture (50-80 mm) should not be too large. there are no inlet holes more than one meter long at each end of a project design pool, and no balanced holes at the top of the filter beam. the inlet holes are located at 300 mm under the filter plate, and the diameter of the holes is d100 mm. as a result, serious jets occur in the middle of the backwash pool due to the barrier of the filter beams, and the air distribution is obviously not uniform.
3. filter plate and beam
the design and construction requirements of the filter plate and beam of the biological aerated filter are the same as those of the v-type filter for water supply. the design of the filter beam should not only ensure the longitudinal strength, but also have the necessary lateral stiffness to resist the horizontal force that may occur during the installation of the filter plate. the strength of the filter plate and the filter beam should be one level higher than that of the concrete of the tank body. the precision requirement of its manufacture and installation is very high. it is difficult for the general civil construction unit to achieve, so it should be produced by special steel mould for the professional manufacturer. horizontal die should be used for the filter plate, one piece per production, and the demoulding time is 24 hours (no less than 16 hours in summer, that is, two days and three demoulding times in high temperature). in order to shorten the production and installation cycle, some people have used abs plate to make the whole cast-in-situ filter plate of the bottom die, but because of the uneven bottom die, the distribution of gas and water is not uniform enough, and the cost is increased by 50-100% compared with the prefabrication. a unit uses a vertical die, which is said to produce five pieces at a time. however, it is difficult to remove the gas in the mould, and the produced filter plate has a lot of air bubbles and low strength; the steel bar position is difficult to fix, and there are many bars exposed; the edge of the filter plate is less compact, and there are many burrs, defects and angle drops, which lead to all rework. in a project, the filter beams were poured by the civil construction units of the filter. although the construction was painstaking, many pre-embedded bolts were inspected after demolition (the installation position of the filter plate was insufficient, one grid tank could not even fit 20 filter plates), the beam body was not straight (the filter rod in the prominent part was blocked, the supporting surface of the filter plate in the depression part was insufficient, and the stress was too concentrated), and the top surface of the beam was not straight. the maximum horizontal error is 19 mm, which directly affects the installation and service life of the filter plate.
4. filters and opening rate
biological aerated filter (baf) usually uses a small resistance water distribution system (long handle filter). although the influent of the filter has been pretreated, there are still many suspended substances in the filter, especially the viscous substances in domestic sewage. there are many plastic film fragments mixed in the water, which do great harm to the filter head. in order to avoid clogging, the filter slot should be wider than the feed water filter (2.0-2.5 mm), and the total area of each filter slot is about 250-350 mm2. the opening of the filter is larger than that of the feed water filter, about 0.011-0.015. the diameter of the air distribution hole is 2.0-2.5 mm, and the position should be under the screw of the filter rod or flat with the bottom of the filter plate. the distance between the air distribution hole and the water distribution strip hole at the bottom of the filter rod should be maintained above 150-200 mm. excessive opening ratio not only affects the uniformity of backwash, but also leads to the decrease of the stability of water distribution system (sensitive to minor changes in other factors in backwash system). in a project, the opening ratio is 0.027, and the position of the air distribution hole (d=4mm) of the filter head is low (238mm from the bottom of the filter plate and only 50mm from the water distribution strip hole). during commissioning, it is found that not only the washing is not uniform, but also a strong pulse occurs. when the air pressure changes, a large amount of air is ejected from the filter head (through the water distribution strip hole) in the area where the air cushion layer is lowered. that is the so-called "air cushion breakdown" phenomenon.
the filter head of v-type feed water filter has narrow gap (0.25-0.3 mm), small aperture ratio (about 0.008-0.01) and uniform water distribution. the filter material is generally homogeneous (0.9-1.2 mm) quartz sand. the gravel supporting layer can be simplified into one layer (diameter 2-4 mm, thickness 100-150 mm). the filter head of biological aerated filter has wide gap, large aperture ratio and strong flushing strength. the filter material is ceramsite filter material of 3-6 mm. the gravel supporting layer is suggested to be divided into three layers: 2-4 mm, 4-8 mm and 8-16 mm. each layer is 50-100 mm thick.
6. preventing clogging of filter head
upflow biological aerated filter (baf) feeds water from the bottom of the filter plate. there are many suspended substances (especially plastic film debris) in the water. clogging of the filter head during long-term operation is unavoidable, and it is difficult to clean once it occurs. a sewage treatment plant has caused such serious consequences as overturning the filter plate when backwashing. in addition to repairing manholes at the bottom height of the filter plate on the wall of the tank, the corresponding measures should be taken to flush the plug away from the filter head from top to bottom rapidly by using the water level of the filter when necessary.
7. compressed air system
biological aerated filter (baf) is usually controlled by pneumatic butterfly valve, just like v-type feed water filter. the latter is powered by compressed air. compressed air should be carefully filtered, dried and then piped to the solenoid valves of the pneumatic valves. the air tightness of pipelines in compressed air system should be paid attention to during construction because of many joints. otherwise, air leakage will lead to frequent start-up of air compressor, which not only wastes energy but also affects the life of air compressor. in order to reduce the starting frequency of air compressor, it is necessary to equip a larger storage tank in the system. pipeline material is best made of stainless steel pipe (or seamless steel pipe) and welded joints. the front end of the branch leading to each filter or equipment (backwash pump, blower) should have a control valve (flange connection) to facilitate sectional maintenance. the gas tightness of the joints of galvanized steel pipes (threaded joints) is poor; the inner wall of the joints is rusty in long-term operation, and once the rust spots fall off, the solenoid valves will be blocked. aluminum-plastic composite pipe is used in a project, although it can prevent rust, but because of its insufficient stiffness, it is easy to disengage at the joint under the action of air pressure during operation.
8. civil construction quality
the requirements for construction quality of biological aerated filter are the same as those of v-type filter for water supply. general civil construction units often do not understand or attach importance to this. design institutes and supervision companies must repeatedly emphasize that process monitoring should be strengthened and implemented step by step. especially in some key parts, such as the flatness of the wall, the verticality of adjacent wall, the central elevation, spacing and direction of the intake and intake holes, the level and elevation of the weir top of the intake and outflow overflow weir, the location and fixing degree of the reserved holes and embedded parts, the reliability of the formwork support, etc., should be checked and checked repeatedly. the flanges (supporting filter plate and stainless steel pressure plate) around the bottom of the inner wall of the tank should be poured at the same time. otherwise, the flanges should be separated from the concrete of the tank wall, and the joint surface is easy to leak when backwashing. installation and positioning of embedded steel plate bolts and waterproof sleeves is also a weak link. they must be fixed when the formwork is installed to prevent displacement in the process of pouring concrete. otherwise, it will bring great difficulties to subsequent installation of equipment and pipelines. a project found that there were more than ten embedded stainless steel bolts (fixed filter plate) missing after the wall of the watering pool, so the same caliber stainless steel expansion bolts were used to make up the deficiency. this is clearly wrong. because once the concrete in the bolt hole dissolves under the erosion of water, the friction force will be lost. in this case, chemical anchoring stainless steel bolt is recommended, but the cost is high. the air intake hole of a project adopts the form of pre-embedded d=100mm pvc short pipe, which is the same thickness as the wall of the pond (580mm). because the design documents do not require the installation accuracy of the hole, the construction units also neglect to install it improperly. as a result, the height of each hole is different (the maximum error of the actual measurement is 95 mm), and the direction is different (referring to the direction of short pipe, which is about half of the obvious deviation from the upper to the lower, left to right). it is uneven, which has a great impact on the uniformity of gas distribution.
9. air pipe blowing
there are compressed air pipe, backflushing pipe and aeration pipe in the air pipeline of biological aerated filter. after water flushing, pressure test and drainage of water, they all need to use air to sweep (aeration pipe sweeping should be done before aeration head installation). the operation is based on code for construction and acceptance of industrial metal pipeline engineering (gb50235-97). the blowing speed should not be less than 20m/s. the compressed air pipeline should be cleaned first, and then the backwash and aeration pipes should be cleaned. it is better to use installed air compressors, backwash blowers and aerators respectively. purge sequence should be in accordance with (from the gas source) first near and then far, step by step purge (the remaining section should close the control valve of the corresponding branch). the acceptance criteria are as follows: after the air outlet is visually inspected without water stain and dust, a wooden target plate coated with white paint is placed at this place, and no rust, dust, moisture and other impurities are found on the target plate within 5 minutes, which is qualified.
10. debugging
the following acceptance tests should be carried out in the process of water distribution and aeration system installation of biological aerated filter:
1) flatness test of filter plate installation
the appearance quality of the filter plate is one of the preconditions to ensure the installation quality. its purpose is to make the upper and lower surfaces of the filter plate as level and smooth as possible after installation. the air holes and water distribution strip holes on all filter rods are kept at the same height to improve the uniformity and stability of air-water backwashing. installation flatness test is carried out after the filter plate is installed. the upper surface of the filter plate is sampled by the level instrument (with the steel ruler of millimeter scale as the ruler). it is required that the flatness error of a single filter plate be less than 1 mm and that of a single filter cell be less than 5 mm.
2) air tightness test of filter plate seam
after the maintenance of the filter plate seam is completed, the inspection seam should be flat, without air holes and cracks; the installation of stainless steel pressure plate should be stable and firm. turn the plug on all the filter holders (except one of them is connected with the barometer), inject about 20 cm of deep water into the tank, start the back washer or air compressor to supply air, and adjust the pressure under the filter plate through the exhaust valve. the test pressure is 0.04-0.05 mpa and the constant pressure is 10 minutes. all the seams are qualified without bubbles. otherwise, the location of leakage point should be recorded in detail, and the drainage should be repaired and re-tested until qualified. in this test, the test pressure must be well controlled. excessive pressure will lead to overturning and breaking the filter beam.
3) testing of air distribution uniformity of filter head
remove the plug and screw on the filter. the installation of the filter head should be carried out by an experienced teacher using a torque sheet hand to prevent uneven exertion. check that the filter head is not damaged, the position is correct and the installation is firm. put water into the tank and the water level should be at the top of the submerged filter head (at this time enough water has been seeped through the filter head to the bottom of the filter plate). start the back washing fan to supply air to the pool. after the fan is running normally, observe and check whether the filter head is damaged, loosened (individual filter head has a particularly large outgassing), and whether the backwash air distribution is uniform.
4) aeration uniformity test of aeration head
after the installation of aeration head and its gas distribution pipeline is completed, check whether the installation position of bracket and aeration head is correct and whether it is loose or not. the water level is about 20 cm above the aeration head when water is injected into the pond. start the aerator, after its normal operation, observe and check whether the aerator head is damaged, loosened (individual aerator head is particularly large), and whether the air distribution is uniform.
epilogue
biological aerated filter (baf) is a new biological treatment process for water supply and sewage. it has the advantages of high treatment efficiency, good effluent quality and wide application range. it is worth further research and promotion. however, its structure is relatively complex and its construction quality is required to be high. it must be taken seriously in design and construction and properly solved.
source: polaris water treatment network
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