Specifying a Biological Aerated Filter: Media, Backwash and the SS Ceiling

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. The trade it makes is simple and worth stating up front: you give up the secondary clarifier, and in exchange you accept a hard ceiling on what you are allowed to feed it.

The process removes more than carbon

The process can remove SS, COD and BOD and perform nitrification, denitrification, phosphorus removal and the removal of AOX (harmful substances). That list is why BAF shows up in upgrade projects: one vessel does what would otherwise need a bioreactor plus a clarifier plus a tertiary polish. 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.

Where the configurations came from

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. 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.

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. Among them, BIOSTYR is one type of biological aerated filter process, and the earliest wastewater treatment plant using Biostyr biological filter technology was built in France in 1990. Biofor (biological filtration and oxidation reactor) is another type of biological aerated filter process, and it is the third-generation biofilm reactor designed by Degremont specifically for wastewater treatment plants, following the trickling filter and the Biodrof dry filtration system. Its filter floor is fitted with 25UB33e nozzles, whose anti-clogging design optimises filtration performance through uniform water distribution.

Chinese practice added its own variant. 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. It uses lightweight granular media with a specific gravity generally around 0.1 and a particle size of about 4-5 mm.

Numbers that decide whether the filter survives

The influent suspended solids requirement is the one that bites. Influent suspended solids should be 50-60 mg/L, ideally combined with enhanced primary treatment such as a hydrolysis-acidification tank. Generally SS should be no more than 100 mg/L, preferably no more than 60 mg/L, so the influent needs pretreatment. Skip that and large amounts of impurities and SS enter the aerated filter and clog the aeration and water distribution systems.

Media and hydraulics follow. Media are mostly shale ceramsite with a diameter of 5 mm and a layer height of 1.5-2 m, with filter layer heights of 2.0 m or 1.8-3.0 m also suggested. 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. Oxygen utilisation efficiency can reach 25% and effluent SS is generally less than 10 mg/L. The air-to-water ratio is usually controlled at (1-3):1 and the backwash cycle is about 24-48 hours, mostly using combined air-water backwashing.

Against conventional activated sludge, the footprint is one third, investment saving is 30%, there is no sludge bulking, and oxygen transfer efficiency is high. One-off investment is one quarter lower than with conventional methods.

Where it has actually been built

On the Chinese mainland, biological aerated filters moved slowly at first. The Malan River Wastewater Treatment Plant in Dalian was China's first municipal wastewater treatment plant to adopt the biological aerated filter process, with average effluent COD and NH3-N of 33 mg/L and 2 mg/L respectively. The Eastern Suburb Wastewater Treatment Plant in Xinhui, Guangdong adopted a hydrolysis plus biological aerated filter treatment process. In upgrading, 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. In the 40,000 cubic metres per day plant in Xinhui, Guangdong, a BOT concession project, the process was applied successfully for the first time in a domestic sewage treatment project in China, running hydrolysis plus a two-stage biological aerated filter with a CN tank and an N tank forming the two stages. For low-temperature sewage, the Xining No. 2 Wastewater Treatment Plant has a minimum winter water temperature of about 6 degrees Celsius, and a biological aerated filter plus A2/O treatment process was recommended in the feasibility study report. In reclaimed water reuse, the Dalian project adopts the French Degremont A3D plus BIOFOR process technology, treating 120,000 tonnes of sewage per day with 40,000 tonnes of effluent reused for urban greening, construction and industry.

Phosphorus is where biology runs out

Relying entirely on biological phosphorus removal makes it difficult to meet discharge standards, because phosphorus removal and nitrogen removal are inherently contradictory. 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. So chemicals such as ferric chloride are usually dosed. The filter has strong tolerance to hydraulic shock loads, allowing recirculation and chemical dosing during operation, which relatively reduces the chemical dosage. The same logic applies to combined schemes: catalytic ozonation plus biological aerated filter is used for advanced treatment of petrochemical, coal chemical and dyeing and finishing industry wastewater, and ozonation plus BAF is common in the advanced treatment stage of large and medium-sized industrial wastewater projects.