Biological Aerated Filter (BAF): Principles and Applications
Biological Aerated Filter
This process removes SS, COD, BOD, achieves nitrification, denitrification, phosphorus removal, and the removal of AOX (harmful substances). The biological aerated filter (BAF) is a new process that integrates biological oxidation with the retention of suspended solids.
1) One-time investment is 1/4 lower than traditional methods; 2) the footprint is 1/10 to 1/5 of conventional processes and operating cost is 1/5 lower; 3) the influent requires suspended solids of 50-60 mg/L and is best combined with enhanced primary treatment such as a hydrolysis acidification tank; 4) the media is mostly shale ceramsite, 5 mm in diameter, with a layer height of 1.5-2 m; 5) the counter-current flow (water down, air up) eliminates the need for a secondary sedimentation tank.
Compared with the conventional activated sludge process, the biological aerated filter offers advantages such as high organic loading, small footprint (1/3 of conventional activated sludge), low investment (saving 30%), no sludge bulking, high oxygen transfer efficiency, and good effluent quality. However, it has strict requirements for influent SS (generally SS <= 100 mg/L, preferably SS <= 60 mg/L), so the influent must be pretreated. At the same time, its backwash water volume and head loss are both relatively large.
As a process that integrates biological oxidation with the retention of suspended solids, the biological aerated filter saves a downstream settling tank (secondary clarifier) and features high volumetric and hydraulic loading, short hydraulic retention time, low capital investment, good effluent quality, low energy consumption, and low operating cost.
BIOSTYR is a registered water-treatment process technology of the French company OTV, named after its novel light-weight suspended media-BIOSTYRENE (mainly polystyrene with a specific gravity below 1 g/cm3). The process structure and basic principle are illustrated below using a reactor that removes BOD and SS and provides nitrification and denitrification.
The BIOSTYR process is an upflow biological filter, a new-generation innovative wastewater treatment process that is reliable in operation, highly automated, produces good effluent quality, has strong shock-load resistance, and saves energy. The process is mature and highly efficient.
Wastewater passes through the media layer; pollutants in the water are intercepted by the media and biodegraded and transformed by the biomass attached to the media. At the same time, dissolved organics and specific substances are also removed. The sludge produced is retained in the filter layer while only the purified water passes through, so complete biological treatment is achieved in a closed reactor without a downstream secondary clarifier for sludge settling.
The bottom of the filter is fitted with influent and sludge discharge pipes; the middle and upper section is the media layer, generally 2.5-3.5 m thick. To prevent media loss, a concrete baffle with filter nozzles is installed above the filter bed; the nozzles can be removed from the plate surface without draining the bed, facilitating maintenance. The space above the baffle serves as a storage zone for backwash water, with a height determined by the backwash head.
A recirculation pump in this zone pumps the filter effluent to the distribution gallery and then back to the filter bottom to achieve denitrification; processes that do not require denitrification have no such recirculation system. The space between the bottom of the media layer and the filter bottom is reserved for media expansion during backwash regeneration.
The aeration system of the filter has two pipelines. The process air pipe placed inside the media layer provides process aeration (mainly oxygen-enriched aeration supplied by the blower) and divides the media layer into two zones: an upper aerobic zone and a lower anoxic zone. According to the raw water quality, treatment purpose, and requirements, the media layer height differs and the proportions of aerobic and anoxic zones change accordingly. The air pipe at the filter bottom is the backwash air pipe.
The process has the following characteristics:
Upflow filter with bottom-channel influent distribution and top effluent;
Perforated-pipe aeration, saving equipment investment and maintenance costs;
Filter nozzles at the top of the filter, in contact with treated water, easy to maintain;
Gravity backwash, no backwash pump required;
Process air and backwash air share the same blower;
Aeration pipes can be arranged in the middle or at the bottom of the media layer, enabling nitrification and denitrification functions to be completed in the same tank;
Biofor (biological filtration oxidation reactor) is the third-generation biofilm reactor designed by Degremont for wastewater treatment plants, following the trickling filter and the Biodrof dry filtration system.
Compared with other types of biological filtration processes, Biofor mainly has the following characteristics:
Influent flows from the bottom to the top of the filter; upflow filtration continuously provides positive-pressure conditions over the full height of the filter, offering many advantages over downflow filtration.
2) Uses a special filtration and biofilm support medium: Biolite biological filter media
It ensures a high biofilm concentration and large retention capacity and lengthens the operating cycle.
Biofor uses a special aeration head: it not only supplies oxygen efficiently but also saves energy and is safe, easy to operate, and easy to maintain.
4) Fully uniform fluid distribution
Air and water flow in the same direction. The Biofor biological filter plate is equipped with 25UB33e filter nozzles whose anti-clogging design optimizes filtration through uniform water distribution.
BIOSMEDI process
Zou Weiguo and others at the Shanghai Municipal Engineering Design Institute developed a biological aerated filter called BIOSMEDI, which uses pulse backwashing and co-current air-water flow and can be used for pretreatment of slightly polluted raw water or advanced wastewater treatment.
1) Smaller filter bed resistance; co-current air-water flow avoids the relative cancellation of water and air velocities in counter-current flow that wastes energy. In addition, the more uniform media particle size greatly increases the porosity of the bed and reduces the head loss during filter operation.
Principle of the BIOSMEDI biological filter:
The upper part of the filter uses a reinforced concrete slab (with inverted nozzles for effluent air and water on the slab) to resist the buoyancy of the media and the operating resistance. In the lower part of the media layer, a concrete or steel plate separates a space below the bed to form an air bag that becomes an air chamber during backwashing.
Raw water enters the air chamber through the inlet valve and passes through a hollow tube into the media layer; the media resistance makes the filter influent uniform. The air distribution pipe is installed below the media layer and air is distributed through perforated pipes; after the media removes organics and ammonia nitrogen from the water, the effluent passes through the inverted nozzles into the upper clear-water zone and is discharged.
Filter backwashing uses a pulse flushing method. First close the inlet valve and aeration pipe and open the backwash air pipe at the filter bottom to form an air cushion layer below the media. Once the air cushion reaches a certain height, the air is instantly evacuated through a valve or siphon, causing the flushing water flow from top to bottom in the bed to suddenly increase, so the media layer suddenly expands downward. After several pulses, the suspended matter attached to the media falls off; then the sludge discharge valve is opened and the biological filter effluent is used for water rinsing, effectively cleaning the media.
The BIOSMEDI biological filter is a new biological filter developed by the Shanghai Municipal Engineering Design Institute for slightly polluted raw water. It uses light granular media as the filter medium; the media specific gravity is small, generally around 0.1, and the particle size is about 4-5 mm. Both specific gravity and particle size can be selected as needed. This media has a series of advantages: wide availability, large specific surface area, a surface suitable for microbial growth, low price (300-500 yuan/m3), and good chemical stability.
2) Low-price, high-performance media; the media is widely available, has a large specific surface area, a surface suitable for microbial growth, is cheap (generally below 500 yuan/m3), and has good chemical stability. The large specific surface area favors oxygen mass transfer and greatly improves oxygenation efficiency; aeration can be done with simple perforated pipes, saving engineering investment.
3) A unique pulse backwash form; conventional water backwash and air-water backwash are both hard to make effective. This filter uses a unique pulse backwash method that needs no dedicated backwash pump or blower, making it a high-efficiency, low-energy backwash form.
The biological aerated filter has a wide range of applications and performs well-even irreplaceably-in advanced water treatment, slightly polluted source water treatment, refractory organic treatment, nitrification of low-temperature sewage, and low-temperature slightly polluted water treatment.
For low-temperature sewage, the Xining No. 2 Wastewater Treatment Plant, where the minimum winter water temperature is about 6 degrees C, recommended the biological aerated filter + A2/O process in its feasibility study to solve the nitrification problem.
In the 40,000 m3/d wastewater treatment plant in Xinhui, Guangdong (a BOT concession project), the process was first applied successfully in a domestic sewage treatment project in China. Its process is hydrolysis + two-stage biological aerated filter (with CN and N tanks at two stages), and the project is in operation.
For refractory organic treatment, the wastewater treatment project of Tsingtao Beer (Xuzhou Jinbo) Co., Ltd. used the hydrolysis acidification + biological aerated filter process, and in operation the chosen process met the requirements.
For reclaimed water reuse, the Dalian Malan River Wastewater Treatment Plant project used the French Degremont A3D + BIOFOR process technology; effluent met the tertiary standard, treating 120,000 tons/day, of which 40,000 tons/day of effluent can be reused for urban greening, construction, and industry.
In the Linfen, Shanxi reclaimed water reuse project, the secondary treatment effluent served as the source water; to address its ammonia nitrogen indicator, the project used the biological aerated filter as a pretreatment unit.
In China it has been applied in pig farm manure wastewater treatment, printing and dyeing wastewater treatment, casings processing wastewater treatment, starch wastewater treatment, and other projects.
The world's first biological aerated filter was put into operation in France in 1981 and was then widely used across Europe. The United States, Canada, and other American countries introduced the process in the late 1980s, and Japan, South Korea, and Taiwan, China also introduced it successively. Major environmental companies such as Degremont of France, Philipp Muller of Germany, and Veolia of France all promoted it worldwide as a flagship product. In mainland China the biological aerated filter is in the promotion stage. The Dalian Malan River Wastewater Treatment Plant was China's first municipal plant to adopt the BAF process (designed by the Northeast Municipal Engineering Design Institute), and the Xinhui East Suburb Plant in Guangdong adopted the hydrolysis-biological aerated filter process (designed by the MCC Ma'anshan Institute). Some of China's industrial wastewater treatment also uses this technology. Many domestic research and design institutes have carried out experimental studies on BAF. As BAF spreads worldwide, many scholars have conducted detailed research on its structure, function, start-up, and media, achieving many results.
As a brand-new water treatment process, the biological aerated filter is being promoted. Based on research and application, many issues still await study:
The characteristics of the biofilm and ways to start it up quickly; the relationship between biological oxidation and filtration functions; the law of biofilm detachment during backwashing; further broadening the application scope of BAF and studying how it can be combined with other processes in advanced water treatment, slightly polluted source water treatment, refractory organic treatment, nitrification of low-temperature sewage, and low-temperature slightly polluted water treatment.
Research on the core medium-the filter media-in BAF will also promote its application scope in China. The BIOSTYR and Biofor processes are fairly powerful, but problems remain for large-scale application in China, such as patent issues, and their relatively high investment also hinders their wide application in China.
Therefore, localization of research and production of special filter media will be the key to the wide application of BAF in China.
To give the biological aerated filter a long operating cycle and reduce the number of backwashes and energy use, processes using BAF must pretreat the influent; otherwise large amounts of impurities and SS in the raw water will enter the filter and clog the aeration and water distribution systems, with serious consequences for operation. Especially when the filter is used for secondary treatment, chemicals often must be added to meet this requirement; chemical use not only increases operating costs but some chemicals also reduce alkalinity, affecting denitrification-an issue to consider when applying BAF.
In biological phosphorus removal technology, a system combining denitrification and phosphorus removal is unfavorable for P removal, because P removal and N removal are an irreconcilable contradiction: if DO is too low, the P removal rate drops and nitrification is limited with poor sludge settleability; if DO is too high, the increased DO in the returned anaerobic zone limits denitrification, and high NO3-N concentration can affect P release in the anaerobic zone. P release is best under anaerobic conditions; the presence of NO3-N means only a facultative environment.
From the BAF operating process, achieving P removal purely biologically is hard to meet discharge standards. Using biological P removal loses the high-load feature of the biofilter and causes excessive investment; it is therefore best to remove P by adding FeCl3. Because the biofilter withstands hydraulic shock loading, the treated water can be returned in excess and chemicals added during operation, applying chemical and biological treatment together in the system to achieve P removal and N removal and relatively reduce chemical dosage, lowering operating costs.
