Introduction to the Biological Aerated Filter (BAF) Process
The Biological Aerated Filter (BAF) is an aerobic wastewater treatment process created by introducing the concept of filtration used in drinking water treatment into the biological contact oxidation process. It appeared in Europe in the late 1970s and early 1980s [2].
The process can remove SS, COD and BOD and perform nitrification, denitrification, phosphorus removal and the removal of AOX (harmful substances). The biological aerated filter is a new process that integrates biological oxidation with the interception of suspended solids. Granular media are packed as biofilm carriers and combined with aeration, so that microorganisms degrade pollutants while solid-liquid separation is achieved, eliminating the need for a secondary clarifier [4-5].
The Biological Aerated Filter (BAF) originated in Europe and America in the late 1970s and was developed by OTV, a subsidiary of France's CGE. The world's first biological aerated filter was commissioned in France in 1981 [6] [15]. Engineering applications began in Europe in the late 1970s and early 1980s, underwent considerable development in the mid to late 1980s and were basically mature by the early 1990s, reaching scales of several hundred thousand tonnes per day and developing the capability for nitrogen and phosphorus removal [6-8]. Depending on the tank structure and the biofilm carrier used, there are currently several types of biological aerated filter, including Biocarbone, Bioput, Biofor, Biostyr, Biobead and Colox [15]. Among them, BIOSTYR is one type of biological aerated filter process [6], and the earliest wastewater treatment plant using Biostyr biological filter technology was built in France in 1990 [15]. Biofor (biological filtration and oxidation reactor) is another type of biological aerated filter process [6].
The world's first biological aerated filter was commissioned in France in 1981 and was subsequently widely applied in European countries. American countries such as the United States and Canada introduced this process in the late 1980s, and Japan, South Korea and Taiwan, China also introduced the technology one after another. A number of international environmental protection companies promoted it. On the Chinese mainland, biological aerated filters are still in the promotion stage. The Malan River Wastewater Treatment Plant in Dalian was China's first municipal wastewater treatment plant to adopt the biological aerated filter process (designed by the Northeast Municipal Engineering Design and Research Institute), and the Eastern Suburb Wastewater Treatment Plant in Xinhui, Guangdong adopted a hydrolysis plus biological aerated filter treatment process (designed by the MCC Maanshan Institute). The technology has also been used for treating some industrial wastewater in China, and many domestic research and design institutes have carried out experimental studies on biological aerated filters.
BIOSTYR is a registered water treatment technology of France's OTV, named after the new lightweight floating medium it uses, BIOSTYRENE (mainly composed of polystyrene, with a specific gravity of less than 1 g/cm3). The following uses a reactor that removes BOD and SS and provides nitrification and denitrification as an example to explain its process structure and basic principles.
The BIOSTYR process is an upflow biological filter and a new-generation innovative wastewater treatment process that is reliable in operation, highly automated, produces good effluent quality, has strong shock resistance and saves energy; the process is mature and efficient.
As wastewater passes through the media layer, the pollutants it contains are intercepted by the media layer and degraded and transformed by the biofilm attached to the media. At the same time, dissolved organic matter and specific substances are also removed. The sludge produced is retained within the filter layer, allowing only purified water to pass through, so that complete biological treatment can be achieved in a single closed reactor without the need for a downstream secondary clarifier for sludge settling.
In the field of advanced industrial wastewater treatment, combined processes are widely used. For example, the "catalytic ozonation plus biological aerated filter" combined process is suitable for the advanced treatment of petrochemical, coal chemical and dyeing and finishing industry wastewater, and the upgrading project of the wastewater treatment facility in the western industrial park of the Pingxiang Economic and Technological Development Zone adopted this technology [9]. In addition, the ozonation plus BAF process is also often used in the advanced treatment stage of large and medium-sized industrial wastewater projects [11].
The air supply system of the filter has two sets of pipework. The process air pipe placed inside the media layer is used for process aeration (oxygenation aeration being mainly supplied by aeration blowers) and divides the media layer into two zones: an aerobic zone in the upper part and an anoxic zone in the lower part. Depending on the raw water quality, the treatment objectives and the requirements, the height of the media layer differs and the proportions occupied by the aerobic and anaerobic zones change accordingly. The air pipe at the bottom of the filter is the backwash air pipe.
The process has the following features:
In addition, this technology also has broad applications and practices in related fields.
Biofor (biological filtration and oxidation reactor) is the third-generation biofilm reactor designed by Degremont specifically for wastewater treatment plants, following the trickling filter and the Biodrof dry filtration system.
The influent flows from the bottom of the filter towards the top. Upflow filtration continuously provides positive pressure conditions over the entire height of the filter, which offers many advantages compared with downflow filtration.
It is worth noting that the technologies and standards in this field are also continuously developing and improving.
Air and water flow in the same direction. The filter floor of the Biofor biological filter is fitted with 25UB33e nozzles, whose anti-clogging design optimises filtration performance through uniform water distribution.
Zou Weiguo and colleagues at the Shanghai Municipal Engineering Design Institute developed a biological aerated filter called BIOSMEDI, which uses pulsed backwashing with air and water flowing in the same direction and can be used for the pretreatment of slightly polluted source water or for the advanced treatment of wastewater.
The BIOSMEDI biological filter is a new type of biological filter developed by the Shanghai Municipal Engineering Design and Research Institute for slightly polluted raw water. It uses lightweight granular media as the filtration medium; the specific gravity of the media is relatively low, generally around 0.1, and the particle size is about 4-5 mm, with both specific gravity and particle size selectable according to actual needs. This medium offers a series of advantages, including wide availability, a large specific surface area, a surface suitable for microbial growth, a low price (300-500 yuan per cubic metre) and good chemical stability.
On this basis, industry experts have also carried out a great deal of research and improvement.
Raw water enters the air chamber through the inlet valve and passes through hollow tubes into the filter layer, where the resistance of the media makes the influent distribute evenly across the filter. The air distribution pipe is installed at the bottom of the filter layer and distributes air through perforated pipes. After organic matter and ammonia nitrogen in the water are removed in the filter layer, the effluent passes through inverted nozzles into the clear water zone at the top and is discharged.
Backwashing of the filter uses a pulse method. First the inlet valve and the aeration pipe are closed and the backwash air pipe at the bottom of the filter is opened, forming an air cushion beneath the filter layer. When the air cushion reaches a certain height, the air in it is rapidly discharged through a valve or by siphoning, so that the downward flushing water flow through the filter layer suddenly increases and the media layer abruptly expands downward. After several pulses, the suspended matter attached to the media can be detached; the sludge discharge valve is then opened and the filter effluent is used for water rinsing, which effectively cleans the media.
(2) Low-cost, high-performance media: the media are widely available, have a large specific surface area and a surface suitable for microbial growth, are inexpensive (generally less than 500 yuan per cubic metre) and are chemically stable; their large specific surface area facilitates oxygen mass transfer and greatly improves oxygenation efficiency, so that perforated pipe air distribution is sufficient, saving on project investment.
As an efficient biofilm process integrating biological oxidation with solid-liquid separation, the biological aerated filter (BAF) has a series of notable features [6].
(1) One-off investment is one quarter lower than with conventional methods; (2) the footprint is one tenth to one fifth that of conventional processes and operating costs are one fifth lower; (3) influent suspended solids should be 50-60 mg/L, ideally combined with enhanced primary treatment such as a hydrolysis-acidification tank; (4) the media are mostly shale ceramsite with a diameter of 5 mm and a layer height of 1.5-2 m (filter layer heights of 2.0 m or 1.8-3.0 m have also been suggested) [6] [8]; (5) the counter-current arrangement with water flowing downward and air upward means no secondary clarifier is required.
Compared with the conventional activated sludge process, the biological aerated filter offers a high organic loading rate, a small footprint (one third that of the conventional activated sludge process), lower investment (a saving of 30%), no sludge bulking, high oxygen transfer efficiency and good effluent quality [6-7]. Its microbial concentration is high, reaching 10-15 g/L, and the bacterial community shows clear spatial gradient characteristics, achieving carbon removal and nitrification in sequence. The granular media of the biological aerated filter continuously cut air bubbles, improving oxygen mass transfer efficiency; oxygen utilisation efficiency can reach 25% and effluent SS is generally less than 10 mg/L. Biological aerated filters mostly have closed or semi-closed structures, so biochemical reactions are little affected by ambient temperature, making them suitable for cold regions [12]. However, the requirements on influent SS are fairly strict (generally SS no more than 100 mg/L, preferably no more than 60 mg/L), so the influent needs pretreatment. At the same time, the backwash water volume and head loss are relatively large, and problems such as media clogging and relatively high aeration energy consumption exist during operation [4-7].
As a process integrating biological oxidation with the interception of suspended solids, the biological aerated filter dispenses with a downstream settling tank (secondary clarifier) and features high volumetric and hydraulic loading, short hydraulic retention time, low capital investment, good effluent quality, low operating energy consumption and low operating costs [6-7]. The air-to-water ratio of a biological aerated filter is usually controlled at (1-3):1, the backwash cycle is about 24-48 hours, and combined air-water backwashing is mostly used [12].
As an efficient biofilm wastewater treatment technology, the biological aerated filter (BAF) has been widely applied in the upgrading and advanced treatment of municipal wastewater treatment plants, domestic sewage treatment, industrial wastewater treatment (including refractory organics), reclaimed water reuse, village and town sewage treatment and the pretreatment of slightly polluted source water [3] [12-13]. It offers a small footprint, high treatment efficiency and good effluent quality [12].
Biological aerated filters have a fairly wide range of applications and can play a role in advanced water treatment, the treatment of slightly polluted source water, the treatment of refractory organics, the nitrification of low-temperature sewage and the treatment of low-temperature slightly polluted water.
In the case of low-temperature sewage, the Xining No. 2 Wastewater Treatment Plant has a minimum winter water temperature of about 6 degrees Celsius; in order to solve the nitrification problem, a biological aerated filter plus A2/O treatment process was recommended in the feasibility study report.
In terms of the upgrading of municipal wastewater treatment, the Tangjiaqiao Wastewater Treatment Plant in Chongqing adopted a pre-denitrification two-stage biological aerated filter process, raising daily treatment capacity to 60,000 cubic metres with effluent quality stably meeting Grade 1A standards [10]. In addition, the Malan River Wastewater Treatment Plant in Dalian uses the Biofor process, with average effluent COD and NH3-N of 33 mg/L and 2 mg/L respectively [12].
In the 40,000 cubic metres per day wastewater treatment plant project in Xinhui, Guangdong (a BOT concession project), the process was applied successfully for the first time in a domestic sewage treatment project in China. Its process is hydrolysis plus a two-stage biological aerated filter (with a CN tank and an N tank forming the two stages), and the project has already been put into operation.
In the treatment of refractory organics, the wastewater treatment project of Tsingtao Brewery (Xuzhou Jinbo) Co., Ltd. used a hydrolysis-acidification plus biological aerated filter treatment process; from an operational point of view, the selected process meets the requirements.
In the field of reclaimed water reuse, the Malan River Wastewater Treatment Plant project in Dalian adopts the French Degremont A3D plus BIOFOR process technology. The effluent quality meets Grade 3 standards, with 120,000 tonnes of sewage treated per day, of which 40,000 tonnes of effluent can be reused for urban greening, construction, industry and other purposes.
In the reclaimed water reuse project in Linfen, Shanxi, the effluent from secondary treatment is used as the water source; in order to address the ammonia nitrogen indicator, the project uses a biological aerated filter as the pretreatment unit.
At the same time, the supporting processes and equipment involved are also being continuously optimised and upgraded.
In addition, in the field of black water treatment for scattered households, the patent for "a device for the treatment and resource utilisation of black water from scattered households", granted to Anhui Tongyuan Environment Energy Saving Co., Ltd. in February 2026, also makes the biological aerated filter a key link in its combined process (anaerobic fermentation tank, aerobic nitrification tank, denitrification tank, biological aerated filter, cascade constructed wetland and water storage tank), where it is used to further degrade pollutants, improve effluent quality and achieve resource recycling. [1]
The oxygen utilisation efficiency of BAF can reach 25%, with a low aeration volume and relatively low operating costs; at the same time, it mostly has a closed or semi-closed structure, so biochemical reactions are little affected by ambient temperature, making it suitable for cold regions. There is also no sludge bulking problem and operation and management are convenient [12].
In wastewater upgrading and advanced treatment, BAF can serve as a terminal water quality safeguard unit, effectively removing pollutants such as COD and ammonia nitrogen. The interception effect of its media generally keeps effluent suspended solids (SS) below 10 mg/L, and a secondary clarifier is usually unnecessary [12].
BAF can be used not only on its own but also effectively combined with other processes (such as catalytic ozonation) to form combined processes such as "O3 plus BAF" for treating refractory industrial wastewater. By improving the biodegradability of the wastewater, it further enhances pollutant removal and achieves advanced treatment and compliant discharge [9] [12].
Since its appearance in Europe in the late 1970s and early 1980s, BAF technology has continued to develop and mature, and its scope of application has expanded from the upgrading of municipal wastewater treatment plants and domestic sewage treatment to industrial circulating water treatment, rural sewage treatment and resource utilisation. It has received widespread attention for its high treatment loading and relatively low investment and operating costs [2-3] [7].
The Biostyr biological aerated filter is an upflow submerged biological filter known as the second-generation BAF process. It offers reliable operation, good effluent quality and strong resistance to shock loads, but has the shortcomings of high pretreatment costs and a relatively large sludge volume [15]. The optimisation and distribution of microbial populations in biological aerated filters is one research topic [16]. Studies comparing the effects of different media such as ceramsite, zeolite and gravel on BAF treatment performance show that ceramsite media perform better in removing COD, ammonia nitrogen and antibiotics [17]. Research also focuses on issues such as rapid biofilm start-up, backwashing patterns and the domestic production of filter media in BAF, so as to further broaden its range of application [6] [15].
In order to give the biological aerated filter a longer operating cycle and to reduce the number of backwashes and energy consumption, any process using BAF must pretreat the influent; otherwise large amounts of impurities and SS in the raw water will enter the aerated filter and clog the aeration and water distribution systems, causing serious consequences for system operation. This is especially true when the filter is used for secondary treatment, where chemicals often have to be dosed in order to meet this requirement. The use of chemicals not only increases operating costs, but some chemicals will also reduce alkalinity and thereby affect nitrification, which is an issue that must be considered when applying the BAF process.
In biological phosphorus removal technology, a system that combines nitrogen removal with phosphorus removal is unfavourable for phosphorus removal, because phosphorus removal and nitrogen removal are inherently contradictory. For example, if dissolved oxygen is too low, the phosphorus removal rate falls, nitrification is limited and sludge settleability is poor; if dissolved oxygen is too high, the increased dissolved oxygen returned to the anaerobic zone limits denitrification, while an excessively high nitrate nitrogen concentration affects the release of phosphorus in the anaerobic zone. This is because the release of phosphorus must take place in an anaerobic environment, and the presence of nitrate nitrogen indicates that the environment is anoxic rather than anaerobic.
Judging from the operating process of the biological aerated filter, relying entirely on biological phosphorus removal makes it difficult to meet discharge standards. If biological phosphorus removal alone were adopted, the high-loading advantage of the biological filter would be lost and investment would become excessive. Therefore, chemicals such as ferric chloride are usually dosed in this process to assist phosphorus removal. The biological filter has a strong tolerance to hydraulic shock loads, allowing the treated water to be recirculated in excess and chemicals to be dosed during operation, so that chemical treatment and biological treatment are applied simultaneously in the system to achieve phosphorus and nitrogen removal, which relatively reduces the chemical dosage and lowers operating costs.
