Biological Aerated Filter (BAF): An Introduction to a Core Wastewater-Treatment Process
Biological Aerated Filter
This process removes SS, COD, BOD, achieves nitrification, nitrogen removal, phosphorus removal and AOX (harmful substances) removal. The biological aerated filter is a new process integrating biological oxidation and suspended-solid interception.
1) One-time investment is 1/4 lower than traditional methods; 2) Footprint is 1/10-1/5 of conventional processes, and operating cost is 1/5 lower; 3) Influent suspended solids should be 50-60 mg/L, preferably combined with enhanced primary treatment such as a hydrolysis acidification tank; 4) Media are mostly shale ceramsite, 5 mm in diameter, with a layer height of 1.5-2 m; 5) Down-flow water and up-flow air in counter-current need no 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 (30% savings), no sludge bulking, high oxygen transfer efficiency and good effluent quality, but it requires strict influent SS (generally SS <=100 mg/L, preferably SS <=60 mg/L), so the influent needs pretreatment. At the same time, its backwash water volume and head loss are both relatively large.
As a process integrating biological oxidation and suspended-solid interception, the biological aerated filter saves the downstream sedimentation tank (secondary sedimentation tank), and features high volumetric and hydraulic loading, short hydraulic retention time, low capital investment, good effluent quality, low operating energy consumption and low operating cost.
BIOSTYR is a registered water-treatment process technology of the French company OTV, named after its new lightweight suspended media - BIOSTYRENE (mainly polystyrene, with a specific gravity less than 1 g/cm3). Below, a reactor that removes BOD and SS and provides nitrification-denitrification is used as an example to illustrate its process structure and basic principle.
The BIOSTYR process is an upflow biological filter, a new-generation innovative wastewater-treatment process that is reliable in operation, highly automated, good in effluent quality, strong in shock-load resistance and energy-saving; the process is mature and efficient.
Wastewater passes through the filter media layer; pollutants in the water are intercepted by the media layer and biodegraded and transformed by the organisms attached to the media, while dissolved organics and specific substances are also removed; the sludge produced is retained in the filter layer and only the purified water passes through, so complete biological treatment can be achieved in a closed reactor without a downstream secondary sedimentation tank for sludge settling.
The bottom of the filter has inlet and sludge-discharge pipes; the middle-upper part is the media layer, generally 2.5-3.5 m thick. To prevent media loss, a concrete baffle fitted 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 backwash water storage, its height determined by the backwash head.
A return pump in this zone pumps the filter effluent to the distribution gallery and then back to the filter bottom to achieve denitrification; processes not requiring denitrification have no such return system. The space between the bottom of the media layer and the filter bottom is reserved for media expansion during backwash regeneration.
The filter aeration system has two sets of piping: the process air pipe inside the media layer is used for process aeration (mainly oxygen-enriched aeration supplied by the blower) and divides the media layer into upper and lower zones - the upper is an aerobic zone and the lower an anoxic zone. Depending on the raw water quality, treatment purpose and requirements, the media layer height differs and the proportions of aerobic and anaerobic zones change accordingly; the air piping at the filter bottom is the backwash air pipe.
This process has the following characteristics:
Upflow filter, bottom channel influent distribution, top effluent;
Perforated-pipe aeration, saving equipment investment and maintenance cost;
Filter nozzles at the top of the filter, in contact with treated water, easy to maintain;
Gravity backwash, no backwash pump needed;
Process air and backwash air share the blower;
Aeration pipes can be placed in the middle or bottom of the media layer, allowing nitrification and denitrification 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 filter bottom to the top; upflow filtration provides positive-pressure conditions throughout the filter height, offering many advantages over downflow filtration.
2) Uses a specially made filtration and biofilm support media: Biolite biological filter media
ensuring high biofilm concentration and large interception capacity, and extending the operating cycle.
Biofor uses a specially designed diffuser: it not only supplies oxygen efficiently but also saves energy, is safe to use and easy to operate and maintain.
4) Fully uniform fluid distribution
Air and water flow in the same direction. The Biofor biological filter plate is fitted with 25UB33e nozzles whose anti-clogging design optimizes filtration through uniform distribution.
BIOSMEDI Process
Zou Weiguo et al. of the Shanghai Municipal Engineering Design Institute developed a biological aerated filter called BIOSMEDI, which uses pulse backwash and co-current air-water flow and can be used for pretreatment of micro-polluted raw water or advanced wastewater treatment.
The BIOSMEDI biological filter is a new type of biological filter developed by the Shanghai Municipal Engineering Design Institute for micro-polluted raw water. It uses lightweight granular media as the filter medium, with a relatively small specific gravity of about 0.1 and a particle size of about 4-5 mm; the specific gravity and particle size can be selected as needed. This media offers a series of advantages: wide source, large specific surface area, a surface suitable for microbial growth, low price (300-500 yuan/m3) and good chemical stability.
BIOSMEDI biological filter principle:
The upper part of the filter uses a reinforced concrete slab (with inverted nozzles on the slab for air and water outlet) to resist the buoyancy of the media and operating resistance. At the lower part of the media layer, a concrete or steel plate separates an air pocket below the media layer, forming an air chamber at the bottom during backwash.
Therefore, the localization of research and production of special filter media will be the key to the widespread application of biological aerated filters in China.
Filter backwash uses a pulse-flushing method: first close the inlet valve and aeration pipe, 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, instantly empty the air in the cushion rapidly through a valve or siphon, so the flushing water flow from top to bottom in the media suddenly increases, causing the media layer to expand downward abruptly. After several pulses, the suspended matter attached to the media detaches; then open the sludge-discharge valve and use the biological filter effluent for water rinsing, effectively cleaning the media.
1) Smaller media-layer resistance; co-current air-water flow avoids the mutual cancellation of water and air velocities in counter-current flow that wastes energy, and the relatively uniform media particle size greatly increases the porosity of the media layer and reduces head loss during operation.
2) Low-price, high-performance media; the media has wide sources, large specific surface area, a surface suitable for microbial growth, low price (generally below 500 yuan/m3) and good chemical stability; the large specific surface area favors oxygen mass transfer and greatly improves oxygenation efficiency, and air distribution can use simple perforated-pipe distribution, saving engineering investment.
3) Unique pulse backwash form; traditional water backwash and air-water backwash are hard to be effective, while this filter uses a unique pulse backwash that needs no dedicated backwash pump or blower - a high-efficiency, low-energy backwash form.
The biological aerated filter has a wide application range and plays a good, even irreplaceable, role in advanced water treatment, micro-polluted raw water treatment, refractory organic treatment, nitrification of low-temperature wastewater and low-temperature micro-polluted water treatment.
In low-temperature wastewater, the Xining No. 2 wastewater treatment plant, with a minimum winter water temperature of about 6 C, recommended the biological aerated filter + A2/O process in its feasibility study report to solve the nitrification problem.
In the Guangdong Xinhui 40,000 m3/d wastewater treatment plant (a BOT concession project), the process was first applied in a domestic wastewater treatment project in China and succeeded; its process is hydrolysis + two-stage biological aerated filter (with CN and N tanks), and the project has been put into operation.
In refractory organic treatment, the wastewater treatment project of Tsingtao Brewery (Xuzhou Jinbo) Co., Ltd. reused the hydrolysis acidification + biological aerated filter process; in operation, the selected process met the requirements.
In reclaimed-water reuse, the Dalian Malanhe wastewater treatment plant project used the French Degremont A3D + BIOFOR process; effluent quality reached Class III standards, treating 120,000 tons/day, of which 40,000 tons of effluent can be reused for urban greening, construction and industry.
In the Shanxi Linfen reclaimed-water reuse project, the secondary-treatment effluent served as the source; to address its ammonia nitrogen indicator, the project used a biological aerated filter as the pretreatment unit.
Domestically, it has been applied in piggery 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 commissioned in France in 1981 and then widely used across Europe. The Americas, including the United States and Canada, introduced the process in the late 1980s, and Japan, South Korea and China's Taiwan also introduced the technology successively. Major global environmental companies such as France's Degremont, Germany's Philipp Muller and France's VEOLIA all promoted it worldwide as a flagship product. In mainland China, the biological aerated filter is in the promotion stage. The Dalian Malanhe wastewater treatment plant is China's first municipal plant using the process (designed by the Northeast Municipal Engineering Design Institute), and the Guangdong Xinhui East Suburb plant used the hydrolysis - biological aerated filter process (designed by the MCC Ma Institute). Part of China's industrial wastewater treatment also uses this technology. Many domestic research and design institutes have conducted experimental studies on the biological aerated filter. As it spreads worldwide, many scholars have conducted specific research on its structure, function, start-up and media, achieving many results.
As a novel water-treatment process, the biological aerated filter is being promoted. Based on research and application, many issues still need study:
Characteristics of the biofilm and ways to start it quickly; the relationship between biological oxidation and filtration functions; the law of biofilm detachment during backwash; further broadening the application range of the biological aerated filter and studying how it combines with other processes in advanced water treatment, micro-polluted raw water treatment, refractory organic treatment and nitrification of low-temperature and low-temperature micro-polluted wastewater.
Research on the core medium - the filter media - of the biological aerated filter will also promote its application scope in China. The BIOSTYR and Biofor processes are functionally strong, but their large-scale application in China still faces problems such as patents and high investment, which hinder their widespread use in China.
Raw water enters the air chamber from the inlet valve, passes through a hollow tube into the media layer, and the media resistance makes the filter influent uniform; the air distribution pipe is installed below the media layer, air is distributed through perforated pipes, and after the media removes organics and ammonia nitrogen from the water, the effluent passes through inverted nozzles into the upper clear-water zone and is discharged.
To give the biological aerated filter a longer operating cycle, reduce backwash frequency and lower energy consumption, processes 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 distribution systems, causing 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 cost but some chemicals also lower alkalinity, affecting denitrification - an issue to consider when applying the BAF process.
In biological phosphorus removal technology, a system combining nitrogen and phosphorus removal is unfavorable for phosphorus removal, because phosphorus and nitrogen removal are an irreconcilable contradiction: if DO is too low, phosphorus removal drops, nitrification is limited and sludge settleability is poor; if DO is too high, the increased DO in the returned anaerobic zone limits denitrification, and high NO3-N concentration can affect phosphorus release in the anaerobic zone. Phosphorus release prefers an anaerobic environment; the presence of NO3-N means only a facultative environment.
From the BAF operating process, purely biological phosphorus removal can hardly meet discharge standards. Using biological phosphorus removal loses the high-load feature of the biofilter and causes excessive investment, so it is best to remove phosphorus by adding FeCl3; because the biofilter withstands hydraulic shock loads, the treated water can be over-returned and chemicals added during operation, applying both chemical and biological treatment in the system to achieve phosphorus removal and nitrogen removal, relatively reducing chemical dosage and thus operating cost.
