Biological Contact Oxidation: How to Size the Filler, Set the Air-Water Ratio and Keep the Bed From Clogging
Contact oxidation occupies the awkward middle ground between a biofilter and an activated sludge tank: a submerged bed of packing, aerated from below, holding a fixed film while roughly 2-5% suspended activated sludge also circulates in the water. That dual character is the point. It keeps the biomass concentration and shock tolerance of a biofilm system while running on the aeration hardware of an activated sludge plant, and it does both without any sludge return.
Where the Process Came From and Why It Took Off
Germany tried biological contact oxidation for wastewater treatment at the end of the 19th century, but the industrial base of the period had no suitable fillers and the idea stalled. What changed it was the synthetic plastics industry: lightweight honeycomb fillers appeared, and by the 1970s Japan, the United States and others were researching and applying the method in earnest. China began work on municipal sewage and industrial wastewater in the mid-1970s and moved into production use soon after. Modern installations frequently pair the process with JBM new-type combined biological filler, which accelerates the biological decomposition process while keeping operation and management simple, investment low, treatment efficiency high and the footprint minimal.
How It Differs From an Ordinary Biofilter
Three distinctions matter. The packing is entirely immersed in the wastewater, so the vessel is really a submerged filter rather than a trickling bed. Oxygen is supplied mechanically by blower aeration instead of relying on natural ventilation - conceptually it is an aeration tank that has been given microbial attachment media, which is why it also goes by aerated circulating filter or contact aeration tank. And a few percent of suspended activated sludge is present in the tank water and contributes to purification. The purification chemistry is the same as any biofilm process: the film adsorbs organics, and under aerobic conditions microorganisms oxidise and decompose them.
The biofilm here is built from zoogloea, filamentous bacteria, fungi, protozoa and metazoa. Filamentous bacteria, which in the activated sludge method are mostly a factor impairing normal biological purification, do useful work in a contact oxidation tank: they form a three-dimensional structure in the filler voids, greatly increasing the contact surface between biomass and wastewater, and because they oxidise most organics strongly and adapt well to load changes they are a powerful contributor to purification capacity.
Sizing the Filler
Filler choice is the single biggest design lever, because the contact surface between film and wastewater sets the volumetric load. Honeycomb fillers are the common choice, and pore size follows wastewater quality - BOD5, that is five-day biochemical oxygen demand, and suspended solids concentration - together with BOD load and oxygenation conditions. The usual guidance: at BOD5 of 100-300 mg/L, choose a 32 mm pore size; at 50-100 mg/L, use 15-20 mm; below 50 mg/L, 10-15 mm fillers are enough. Fillers should be light, strong, highly resistant to oxidation and corrosion, and must introduce no new toxicity. Glass-fibre and plastic honeycomb dominate, with rope, synthetic fibre, zeolite and coke also used, in honeycomb, mesh or inclined corrugated plate forms.
Honeycomb packing should be installed in layers, each about 1 m high, with pore size not less than 25 mm, and total filler-layer height around 3 m. The three-dimensional elastic filler now generally performs best: against rigid honeycomb it has greater pore variability and does not clog; against soft fillers its material lasts longer and does not adhere or agglomerate; against semi-soft fillers it offers a larger surface area, faster biofilm formation and lower cost. Typical figures are specific surface area 300 m2/m3, filler length 1-2.5 m and diameter 150 mm, with water depth in the tank able to reach 3-8 m.
Loading, Contact Time and Dissolved Oxygen
For aerobic contact oxidation, influent BOD should stay below 500 mg/L. On low-strength wastewater - BOD of 50-300 mg/L - a filler load of 2-5 kg BOD5 per cubic metre of filler per day is typical, with wastewater retention of 0.5-1.5 h and oxygen consumption in the oxidation tank of about 1-3 mg/L. Because the biomass is large and the treatment load high, dissolved oxygen can be held higher than in suspended growth; the usual requirement is residual dissolved oxygen of 2-3 mg/L in the effluent.
Formal design practice sets the effective contact time at 1.5-3.0 h, derives filler volume from the volumetric load and average daily sewage volume, and - where no test data exist - assumes a volumetric load of 1000 to 1500 g BOD5 per cubic metre per day for domestic sewage or municipal sewage dominated by domestic sewage. Influent BOD5 concentration is normally controlled within 150-300 mg/L. Dissolved oxygen in the contact oxidation tank is held at 2.5-3.5 mg/L with an air-water ratio of 15-20:1, and each compartment should not exceed 25 m2 in area so that water and air distribute evenly. At least two tanks or compartments should be provided, designed to run simultaneously.
The speed advantage is the headline number: contact oxidation needs only 0.5-1.0 h to achieve what 8 h of activated sludge treatment delivers. It gets there by relying on the biofilm, splitting the oxidation tank into two sections and adding a contact layer to the sedimentation tank - where the biofilm is removed by sedimentation while fine suspended solids are intercepted by the filter layer, using an upward flow velocity of 6.5-7.5 m/h and 15 minutes of retention in the clarification zone. Sludge separated from a contact oxidation tank carries many bubbles, so flotation suits it better than gravity settling.
Direct Versus Split-Flow Aeration
Two aeration arrangements exist, and the difference shows up in film behaviour. In the direct type, air is blown for aeration directly at the bottom of the filler. The upward flow disturbs the biofilm strongly, so it renews quickly and maintains high activity; with a stable influent load the film holds a workable thickness - generally about 1 mm is appropriate - and is not prone to clogging. In the split-flow type, the aeration device sits on one side of the tank and the filler on the other, and water circulates within the filler layer by pump or air lifting. Oxygen is supplied in compartments and is plentiful, but utilisation is lower and power consumption higher; because hydraulic scouring is weaker, aged biofilm does not slough off easily, the metabolic cycle lengthens, activity drops, and the difficulty of sloughing can also cause filler clogging.
The air-water ratio has a simple derivation worth knowing: 1 m3 of air contains 20% oxygen and air density is about 1 kg/m3, so each cubic metre of aeration carries roughly 0.1 kg of oxygen. Against a BOD of 150 g oxygen per m3 and 10% utilisation efficiency, a ratio of 1.5 kg oxygen to water meets the demand. Blast volume should not be excessive, or it will shock the biofilm forming on the fillers.
Why It Is Chosen, and What Still Needs Work
The advantages are well rehearsed but real: high purification efficiency, short treatment time, strong adaptability to variations in influent organic load, no sludge return, no sludge bulking, and easy operation and management. The persistent problem is that the biofilm between fillers in the tank sometimes clogs and needs improvement. Current work concentrates on controlling aeration intensity for different influent loads so clogging does not develop, and on reasonable oxidation tank types, shapes, dimensions and suitable filler materials. For three-dimensional elastic filler the design volumetric load can reach 2 kg/(m3.d) on general sewage with an air-water ratio generally of 15:1 and operating dissolved oxygen greater than 2 mg/L.