No Secondary Clarifier, No Sludge Bulking: What a Biological Aerated Filter Changes

A biological aerated filter does two jobs in one shell: biological oxidation and retention of suspended solids. Remove the downstream clarifier and a lot of the plant's civil cost goes with it. That is the pitch, and mostly it holds up - but the process carries a hard inlet condition that catches people out.

What You Get, and What It Costs You

BAF removes SS, COD and BOD, and delivers nitrification, denitrification, phosphorus removal and the removal of AOX (harmful substances). Against the conventional activated sludge process it offers high organic loading, a footprint of about 1/3, roughly 30% lower investment, no sludge bulking, high oxygen transfer efficiency and good effluent quality. The trade-off is inlet suspended solids: generally SS <= 100 mg/L, preferably SS <= 60 mg/L, so the influent must be pretreated. Backwash water volume and head loss are both relatively large. On the figures usually quoted, one-time investment is 1/4 lower than traditional methods, footprint is 1/10 to 1/5 of conventional processes, and operating cost is 1/5 lower.

Media is mostly shale ceramsite, 5 mm in diameter, in a layer 1.5-2 m high. Counter-current flow - water down, air up - removes the need for a secondary sedimentation tank. High volumetric and hydraulic loading, short hydraulic retention time, low capital investment, good effluent quality, low energy consumption and low operating cost come with the package.

BIOSTYR: Upflow on Light Media

BIOSTYR is a registered water-treatment process technology of the French company OTV, named after its novel lightweight suspended media BIOSTYRENE, mainly polystyrene with a specific gravity below 1 g/cm3. It is an upflow biological filter: reliable in operation, highly automated, good effluent, strong against shock loads, and energy-lean. Wastewater passes through the media layer, pollutants are intercepted by the media and biodegraded by the attached biomass, dissolved organics and specific substances are removed, and the sludge stays in the filter while only purified water leaves - so complete biological treatment happens in a closed reactor with no secondary clarifier.

Construction details matter here. Influent and sludge discharge pipes sit at the bottom of the filter; the media layer in the middle and upper section is generally 2.5-3.5 m thick. A concrete baffle fitted with filter nozzles caps the bed to prevent media loss, and the nozzles can be removed from the plate surface without draining the bed, which keeps maintenance simple. The space above the baffle stores backwash water at a height set by the backwash head. A recirculation pump in that zone pumps filter effluent to the distribution gallery and back to the filter bottom to achieve denitrification; plants that do not need denitrification have no such recirculation system. Space between the bottom of the media layer and the filter floor is reserved for media expansion during backwash.

Two air lines serve the filter. The process air pipe inside the media layer provides process aeration and splits the bed into an upper aerobic zone and a lower anoxic zone, with the proportions changing according to raw water quality and treatment purpose. The air pipe at the floor is the backwash line, and process air and backwash air share the same blower. Perforated-pipe aeration saves equipment investment and maintenance; filter nozzles at the top sit in treated water and are easy to service; backwash runs by gravity with no backwash pump.

BIOFOR and the Third Generation

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. Influent flows from the bottom to the top of the filter, and upflow filtration keeps positive-pressure conditions over the full height of the filter, which beats downflow on several counts. It uses a special filtration and biofilm support medium, Biolite biological filter media, which holds a high biofilm concentration and large retention capacity and lengthens the operating cycle. A special aeration head supplies oxygen efficiently and safely, and the filter plate carries 25UB33e nozzles whose anti-clogging design evens out distribution.

BIOSMEDI: Pulse Backwash

Zou Weiguo and others at the Shanghai Municipal Engineering Design Institute developed a biological aerated filter called BIOSMEDI, using pulse backwashing and co-current air-water flow, for pretreatment of slightly polluted raw water or advanced wastewater treatment. Co-current flow avoids the relative cancellation of water and air velocities that wastes energy in counter-current arrangements, and more uniform media particle size raises bed porosity and cuts head loss. The upper part of the filter uses a reinforced concrete slab with inverted nozzles to resist media buoyancy and operating resistance; below the media, a concrete or steel plate separates a space that becomes the air chamber during backwash. Raw water enters that chamber through the inlet valve and passes through a hollow tube into the media, whose resistance evens out the feed. Air is distributed through perforated pipes below the bed, and once the media has removed organics and ammonia nitrogen the effluent passes through the inverted nozzles into the upper clear-water zone.

Backwashing is the clever part. Close the inlet valve and aeration pipe, open the backwash air pipe at the floor, and build an air cushion below the media. Once it reaches height, dump the air instantly through a valve or siphon: downward flushing flow spikes, the bed expands suddenly, and after several pulses the attached solids fall off. Then the sludge discharge valve opens and filter effluent rinses the bed. No dedicated backwash pump or blower is needed. Media is light granular material with specific gravity generally around 0.1 and particle size about 4-5 mm, both selectable - widely available, chemically stable, and cheap at 300-500 yuan/m3.

Where BAF Has Actually Run

The application range is wide, and in some niches irreplaceable: advanced water treatment, slightly polluted source water, refractory organic treatment, nitrification of low-temperature sewage, and low-temperature slightly polluted water. For low-temperature sewage, the Xining No. 2 Wastewater Treatment Plant, where the minimum winter water temperature is about 6 degrees C, recommended a biological aerated filter plus A2/O in its feasibility study to solve nitrification. The 40,000 m3/d wastewater treatment plant in Xinhui, Guangdong - a BOT concession project - was the first successful application of the process to a domestic sewage project in China, running hydrolysis plus two-stage biological aerated filter with CN and N tanks at the two stages. Tsingtao Beer (Xuzhou Jinbo) Co., Ltd. used hydrolysis acidification plus biological aerated filter on refractory organics and met requirements in operation. The Dalian Malan River Wastewater Treatment Plant took the French Degremont A3D + BIOFOR route for reclaimed water reuse: effluent met the tertiary standard at 120,000 tons/day, of which 40,000 tons/day can be reused for urban greening, construction and industry. In the Linfen, Shanxi reuse project, secondary treatment effluent was the source water and the filter served as a pretreatment unit against ammonia nitrogen.

The world's first biological aerated filter went into operation in France in 1981 and spread across Europe. The United States, Canada and other American countries introduced the process in the late 1980s, and Japan, South Korea and Taiwan, China followed. Degremont of France, Philipp Muller of Germany and Veolia of France promoted it worldwide as a flagship product. In mainland China it is still at the promotion stage: Dalian Malan River was the first municipal plant to adopt the BAF process, designed by the Northeast Municipal Engineering Design Institute, and the Xinhui East Suburb Plant in Guangdong used hydrolysis plus biological aerated filter, designed by the MCC Ma'anshan Institute.

Phosphorus Is Where Biology Runs Out

Biological phosphorus removal inside a filter is awkward, because P removal and N removal are an irreconcilable contradiction. If DO is too low, the P removal rate drops, nitrification is limited and sludge settleability suffers. If DO is too high, the extra DO carried into the returned anaerobic zone limits denitrification, and high NO3-N concentration affects P release in the anaerobic zone - P release needs true anaerobic conditions, and NO3-N only gives a facultative environment. In practice, meeting the permit on biology alone is hard, and pushing biological P removal also throws away the high-load feature of the biofilter and inflates investment. Dosing FeCl3 is usually the better answer. Because the filter tolerates hydraulic shock loading, treated water can be returned in excess and chemicals dosed during operation, so chemical and biological treatment run together on P and N and the chemical dose comes down.

Open Questions

Research on the core medium - the filter media - will decide how far BAF spreads in China. The BIOSTYR and BIOFOR processes are capable, but patent issues and relatively high investment hold them back at scale, so localising research and production of special filter media is the practical key. One operating rule is non-negotiable: pretreat the influent. Skip it and impurities and SS in the raw water enter the filter and clog the aeration and water distribution systems, with serious consequences. Where the filter does secondary treatment, chemicals are often needed to hold the inlet condition - which raises cost, and some chemicals also cut alkalinity and hurt denitrification.