Packing, Air and Biofilm: Getting Biological Contact Oxidation Right on Industrial Waste

Between the activated sludge process and the trickling filter sits a hybrid that borrows the best of both: a tank filled with submerged packing, aerated from the floor, carrying a fixed biofilm while a small amount of suspended biomass circulates with the water. That is biological contact oxidation, and for plants fighting footprint limits, variable loads and a two-person operating crew it is often the most forgiving option on the table.

Where the idea came from

Germany tried the concept at the end of the 19th century and dropped it, because the packing available at the time was heavy, expensive and clogged. What changed the picture was the arrival of lightweight honeycomb plastic packing, and by the 1970s Japan and the United States were researching and applying the process in earnest. Chinese work on municipal and industrial applications began in the mid-1970s and moved into production quickly. Pairing the tank with newer combined media, the JBM combined biological packing among them, speeds up biological decomposition further while keeping investment, land take and operating skill requirements low.

Why the submerged filter behaves differently

Three features separate this process from a conventional biofilter. The packing is fully immersed, which is why the tank is also called a submerged filter. Oxygen comes from mechanical equipment rather than natural draught, so the tank is effectively an aeration tank that has been given surface for biomass to grow on, hence the alternative names aeration-circulation filter and contact aeration tank. And roughly 2-5% of suspended activated sludge remains in the tank and contributes to purification, which is why the method is described as a biofilm process with activated sludge characteristics.

Because the packing offers a large specific surface area and the tank is well oxygenated, biomass per unit volume is high and the volumetric load follows. Because that biomass is fixed and the flow is completely mixed, the tank shrugs off sudden swings in flow and strength. And because most of the biomass never leaves, surplus sludge is small and sludge bulking simply does not arise.

What grows on the packing

The film is a consortium: zoogloea, filamentous bacteria, fungi, protozoa and metazoa. Filamentous bacteria are a nuisance in activated sludge, but here they are an asset. They span the packing voids as a three-dimensional structure, multiplying the contact area between the biological phase and the wastewater, and they oxidise a broad range of organics while tolerating load swings better than most of the community.

Film thickness has to be managed rather than maximised. Once the layer thickens past the point oxygen can penetrate, the inner microbes switch to anaerobic metabolism; the gas they generate plus the scour from aeration strips the film off and a new one starts. The sloughed material leaves with the effluent, which is why solid separation downstream is never optional. In practice a thickness near 1 mm is the target.

Two aeration layouts and the trade-off between them

Split-flow aeration puts the aeration element on one side of the tank and the packing on the other, and relies on a pump or air lift to circulate water through the packed zone. Oxygen supply per compartment is generous and the film grows well, but oxygen utilisation is poor and power consumption high. Worse, the hydraulic scour is weak, so aged film does not release, the metabolic cycle stretches out and activity falls, and the packing eventually blocks.

Direct aeration blows air straight into the bottom of the packing. Rising air disturbs the film hard, renewal is fast and activity stays high; with a stable influent load the film holds a workable thickness and does not blind. For most industrial duties this is the layout to pick.

Design numbers that keep the tank out of trouble

Loading depends on substrate strength. On low-strength waste in the 50-300 mg/L BOD range, packing is loaded at 2-5 kg BOD per cubic metre per day and oxygen consumption in the tank runs about 1-3 mg/L. Influent BOD5 is best held between 150-300 mg/L, and aerobic contact oxidation only makes sense while influent BOD stays under 500 mg/L.

Dissolved oxygen is the other lever. With so much biomass present the tank tolerates a higher DO than activated sludge, and the usual requirement is 2-3 mg/L in the oxidation tank effluent; design guidance puts the contact oxidation tank at 2.5-3.5 mg/L with an air-water ratio of 15-20:1, and dissolved oxygen above 2 mg/L during operation. On timing, the tank needs only 0.5-1.0 h to match what 8 h in an activated sludge tank achieves, and with a two-section tank plus a contact layer the wastewater retention is designed at 0.5-1.5 h. Effective contact time of 1.5-3.0 h is typical for the full installation.

Geometry matters as much as loading. Use at least two tanks or compartments, running in parallel. Total packing height is generally 3 m, installed in 1 m layers when honeycomb is used, with a honeycomb pore size of not less than 25 mm. Keep each compartment under 25 m2 so water and air distribute evenly. Packing should be light, strong, resistant to oxidative corrosion and chemically inert; glass cloth and plastics dominate, with rope, synthetic fibre, zeolite and coke also in use, in honeycomb, mesh or inclined corrugated plate form. Three-dimensional elastic packing is currently the best match, with larger variable porosity than rigid honeycomb so it does not block, and a longer service life than soft packing without clumping.

A useful reference point: three-dimensional elastic packing is quoted at 300 m2/m3 specific surface area, 1-2.5 m packing length and 150 mm diameter, with water depth up to 3-8 m and a design volumetric load of 2 kg/(m3.d) on general sewage at a 15:1 air-water ratio. Staging the biochemistry also helps, first a fast adsorption-synthesis step and then a slower oxidation step, and adding a contact layer to the clarifier pushes up-flow velocity to 6.5-7.5 m/h with 15 min retention in the clarification zone. Residual problems remain: biofilm can still blind the packing voids, and the open questions are aeration intensity control matched to load, and tank shapes and packing materials better suited to the duty.