Biological Aerated Filters: What the Media, the Air and the Backwash Each Decide

What the process is asked to remove

The biological aerated filter removes SS, COD and BOD, and achieves nitrification, nitrogen removal, phosphorus removal and AOX, meaning harmful substances. It is a new process that integrates biological oxidation with suspended-solid interception, and that integration is its whole identity. Because solids are captured in the bed, the downstream secondary sedimentation tank disappears. Volumetric and hydraulic loading are high, hydraulic retention time is short, capital investment is low, effluent quality is good, and operating energy consumption and cost are both low.

Against the conventional activated sludge process the trade is explicit: organic loading is higher, footprint is a third of activated sludge, investment saves 30%, there is no sludge bulking, oxygen transfer efficiency is high and effluent is good. The price is a strict influent requirement. Influent suspended solids should be 50-60 mg/L, generally no more than 100 mg/L and preferably no more than 60 mg/L, so pretreatment is mandatory, often enhanced primary treatment such as a hydrolysis acidification tank. Backwash water volume and head loss are both relatively large. Media are mostly shale ceramsite, 5 mm in diameter, with a layer height of 1.5-2 m, and down-flow water with up-flow air in counter-current needs no secondary sedimentation tank.

BIOSTYR: upflow with lightweight media

BIOSTYR is a registered water-treatment process technology of the French company OTV, named after its lightweight suspended media BIOSTYRENE, mainly polystyrene with a specific gravity less than 1 g/cm3. It is an upflow biological filter, reliable in operation, highly automated, good in effluent quality, strong in shock-load resistance and energy-saving.

Wastewater passes through the filter media layer, where pollutants are intercepted and then biodegraded and transformed by the organisms attached to the media. Dissolved organics and specific substances are removed. The sludge produced stays in the filter layer and only purified water passes through, so complete biological treatment happens in a closed reactor with no secondary sedimentation tank. The bottom of the filter carries 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 sits above the bed, and the nozzles can be removed from the plate surface without draining the bed. The space above the baffle stores backwash water at a height set by the backwash head. A return pump in this zone sends filter effluent to a distribution gallery and back to the filter bottom for denitrification; processes that do not need denitrification omit it. Space between the bottom of the media layer and the filter floor is reserved for media expansion during backwash.

Two sets of aeration piping run the filter. The process air pipe inside the media layer supplies oxygen-enriched aeration from the blower and divides the layer into an upper aerobic zone and a lower anoxic zone; the proportions shift with raw water quality and treatment purpose. The piping at the filter bottom is the backwash air pipe. The result is a short feature list: upflow filtration with bottom channel distribution and top effluent; perforated-pipe aeration that saves equipment and maintenance cost; filter nozzles at the top in contact with treated water and easy to maintain; gravity backwash needing no backwash pump; process air and backwash air sharing one blower; and aeration pipes that can sit mid-layer or at the bottom so nitrification and denitrification happen in the same tank.

BIOFOR and BIOSMEDI: two other answers

Biofor, the 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. Influent flows from the bottom to the top, and upflow filtration keeps positive-pressure conditions throughout the filter height, which offers many advantages over downflow. It uses a specially made filtration and biofilm support media, Biolite biological filter media, giving high biofilm concentration and large interception capacity and extending the operating cycle. A specially designed diffuser supplies oxygen efficiently while saving energy, and the biological filter plate is fitted with 25UB33e nozzles whose anti-clogging design optimizes filtration through uniform distribution.

BIOSMEDI was developed by Zou Weiguo and colleagues at the Shanghai Municipal Engineering Design Institute. It uses pulse backwash and co-current air-water flow and can pretreat micro-polluted raw water or serve as advanced wastewater treatment. The media are lightweight granular material with a specific gravity of about 0.1 and a particle size of about 4-5 mm, both selectable. The media are cheap and practical: wide source, large specific surface area, a surface suited to microbial growth, a price of 300-500 yuan/m3 and good chemical stability. The upper part of the filter uses a reinforced concrete slab with inverted nozzles for air and water outlet to resist media buoyancy and operating resistance; the lower part separates an air pocket below the media layer, forming an air chamber at the bottom during backwash.

The pulse backwash is the distinctive part. The inlet valve and aeration pipe close, the backwash air pipe at the bottom opens and builds an air cushion under the media. Once the cushion reaches height, the air is dumped instantly through a valve or siphon, the flushing water flow through the media suddenly increases and the bed expands downward abruptly. After several pulses the attached solids detach, the sludge-discharge valve opens and filter effluent rinses the media clean. No dedicated backwash pump or blower is needed. Co-current air and water flow also avoids the velocity cancellation that wastes energy in counter-current operation, and the uniform particle size raises porosity and cuts head loss.

Where it has actually been built

Low-temperature nitrification is a classic BAF application. At Xining No. 2 wastewater treatment plant, where the minimum winter water temperature is about 6 degrees C, the feasibility study recommended biological aerated filter plus A2/O to solve nitrification. In the Guangdong Xinhui 40,000 m3/d plant, a BOT concession project, the process, hydrolysis plus two-stage biological aerated filter with CN and N tanks, was the first successful domestic application in a municipal wastewater project. Tsingtao Brewery (Xuzhou Jinbo) Co., Ltd. reused hydrolysis acidification plus biological aerated filter for refractory organics. For reclaimed-water reuse, the Dalian Malanhe plant used the French Degremont A3D plus BIOFOR process, reaching Class III effluent standards at 120,000 tons/day, of which 40,000 tons is reused for urban greening, construction and industry. The Shanxi Linfen reuse project used a biological aerated filter as pretreatment to address ammonia nitrogen. Piggery manure, printing and dyeing, casings processing and starch wastewater have all been treated this way domestically.

Historically, the world's first biological aerated filter was commissioned in France in 1981 and then spread across Europe. The Americas, including the United States and Canada, introduced it in the late 1980s, and Japan, South Korea and China's Taiwan followed. Degremont, Germany's Philipp Muller and France's VEOLIA promoted it worldwide as a flagship product. In mainland China it is still in the promotion stage: Dalian Malanhe is the first municipal plant using the process, designed by the Northeast Municipal Engineering Design Institute, and the Guangdong Xinhui East Suburb plant used hydrolysis plus biological aerated filter, designed by the MCC Ma Institute.

The problems that have not gone away

Several questions remain open: the characteristics of the biofilm and how to start it quickly; the relationship between biological oxidation and filtration; the law of biofilm detachment during backwash; and how BAF combines with other processes in advanced treatment, micro-polluted raw water, refractory organics and low-temperature nitrification. Research on the core medium, the filter media, will decide how far it spreads in China, since BIOSTYR and Biofor are functionally strong but face patent and high-investment obstacles.

Pretreatment is not optional. Without it, large amounts of impurities and SS enter the filter and clog the aeration and distribution systems, with serious operating consequences. When the filter is used for secondary treatment, chemicals often have to be dosed to meet the influent requirement, which raises operating cost, and some chemicals lower alkalinity and hurt denitrification.

Phosphorus removal is the awkward part. A combined nitrogen and phosphorus removal system is unfavourable for phosphorus removal because the two 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 affects phosphorus release in the anaerobic zone. Phosphorus release prefers an anaerobic environment, and the presence of NO3-N means only a facultative one. From the operating record, purely biological phosphorus removal can hardly meet discharge standards, and using it forfeits the high-load advantage of the biofilter, so investment balloons. The better route is chemical dosing with FeCl3: because the biofilter withstands hydraulic shock loads, treated water can be over-returned and chemicals added during operation, combining chemical and biological treatment to achieve both phosphorus and nitrogen removal while relatively reducing chemical dosage and operating cost.