Sizing a Biological Contact Oxidation Tank: Filler Choice, BOD Load and Retention Time

A biofilm process that borrows from activated sludge

Biological contact oxidation purifies organic wastewater using the biofilm attached to a carrier, the filler. It sits between the activated sludge process and the biofilter and takes something from each. Fillers are submerged in the wastewater, mechanical equipment supplies oxygen instead of natural ventilation, and roughly 2-5% suspended activated sludge remains in the tank and also purifies the wastewater. The result behaves like a biofilm process with activated sludge characteristics, which is why it holds biomass far better than a trickling filter and tolerates load swings better than a conventional aeration tank.

The history explains why it took so long to spread. Germany tried it at the end of the nineteenth century, but with no suitable filler available it went nowhere. By the 1970s the synthetic plastics industry had produced lightweight honeycomb fillers, and Japan and the United States began applying the process in earnest; China started researching it for municipal sewage and industrial wastewater in the mid-1970s and moved into production use from there. The technology waited on a material, not on an idea.

How the tank works

Filler fills the tank and bottom aeration oxygenates the sewage and keeps it moving, so wastewater and media stay in contact and the uneven-contact problem of earlier designs disappears. Oxygen comes from blower aeration. Once the biofilm reaches a certain thickness the organisms nearest the filler wall run out of oxygen and switch to anaerobic metabolism; the gas they produce, plus the scouring from aeration, strips the film off and a new one grows. The sloughed film leaves with the effluent.

Purification follows the usual biofilm route: the film adsorbs organic matter and, under aerobic conditions, microorganisms oxidise and decompose it. The film itself is built from zoogloea, filamentous bacteria, fungi, protozoa and metazoa. Filamentous bacteria deserve a note here. In activated sludge they are usually the organism that ruins settling, but in a contact oxidation tank they build a three-dimensional structure through the filler voids, expanding the contact surface between biophase and wastewater, tolerating swings in water-quality load and oxidising most organic matter aggressively. In this process they are an asset, not a nuisance.

Two arrangements exist, and the choice shows up in the power bill. In the split-flow type the aerator sits on one side of the tank and the filler on the other, with water circulating through the media by pump or air lift. Oxygen supply is generous and favours film growth, but oxygen utilisation is lower, power consumption higher, and hydraulic scouring is weak, so aged film does not shed easily, the metabolism cycle stretches out, activity falls and the media can block. The direct type blows air at the bottom of the filler. Rising air disturbs the film directly and strongly, renewal is faster, activity stays higher, and with a stable inlet load the film holds a workable thickness and resists clogging, generally controlled at about 1 mm.

Choosing the filler

Filler selection is the main lever on purification capacity, and pore size is chosen from the wastewater character, principally BOD5, the five-day biochemical oxygen demand, and suspended solids concentration, together with BOD load and oxygenation conditions. A practical mapping looks like this:

• BOD5 of 100-300 mg/L: pore size 32 mm.
• BOD5 of 50-100 mg/L: pore size 15-20 mm.
• BOD5 below 50 mg/L: pore size 10-15 mm.

Where honeycomb filler is used the pore size should not be less than 25 mm. The material itself must be light, strong, strongly resistant to oxidation and corrosion, and must bring no new toxicity. Glass cloth and plastic honeycomb are the usual choices; rope, synthetic fibre, zeolite and coke also appear. Forms include honeycomb, mesh and inclined corrugated plate.

At present the three-dimensional elastic filler suits this process best. Against rigid honeycomb it offers large variable porosity and does not clog; against soft fillers it lasts longer and neither sticks nor agglomerates; against semi-soft fillers it gives a larger surface area, rapid biofilm attachment and lower cost. Typical data: specific surface area 300 m2/m3, filler length 1-2.5 m, diameter 150 mm. Used together with the newer JBM combined biological filler, the process can accelerate biological decomposition, with the advantages of simple operation and management, low investment, high treatment effect and a minimised footprint.

Load, retention and oxygen

BOD load follows substrate concentration. For low-BOD wastewater of 50-300 mg/L, a rate of 2-5 kg (BOD5) per cubic metre of filler per day applies, wastewater retention time runs 0.5-1.5 h, and oxygen consumption in the oxidation tank is about 1-3 mg/L. Because biomass is large and the treatment load high, dissolved oxygen can be held higher, and residual dissolved oxygen in the oxidation tank effluent is generally required at 2-3 mg/L. When aerobic contact oxidation is used at all, inlet BOD should be less than 500 mg/L.

Design practice adds several more conventions worth keeping on the checklist. The number of biological contact oxidation tanks or compartments should be no less than 2, designed to operate simultaneously. Effective contact time of sewage in the oxidation tank is generally 1.5-3.0 h. Total filler layer height is generally 3 m, and when honeycomb filler is used it should be installed in layers of 1 m each. Inlet BOD5 concentration should be controlled within 150-300 mg/L. Dissolved oxygen in the contact oxidation tank is generally maintained between 2.5-3.5 mg/L with a gas-water ratio of 15-20:1. To keep water and air distribution uniform, the area of each oxidation tank cell should generally not exceed 25 m2. Water depth can be 3-8 m, the design volumetric load of three-dimensional elastic filler can reach 2 kg/(m3.d) for general sewage, the gas-water ratio is generally taken as 15:1, and operating dissolved oxygen is greater than 2 mg/L. Where no test data exist, volumetric load for domestic sewage or municipal sewage mainly consisting of domestic sewage is generally taken as 1000~1500g BODs/(m³·d).

The staging logic is worth keeping in view because it explains the footprint. Splitting the biochemical process in two, fast adsorption and biosynthesis first, slower oxidation second, raises purification capacity. Contact oxidation needs only 0.5-1.0 h to achieve what the activated sludge process achieves in 8 h, and it does that by relying on the biofilm, dividing the oxidation tank into two stages and adding a contact layer to the sedimentation tank. That contact layer removes biofilm by sedimentation while fine suspended solids are intercepted by the filter layer, with the sedimentation tank taking an upward flow velocity of 6.5-7.5 m/h and a 15 min retention time in the clarification zone. The sludge separated from the contact oxidation tank contains many bubbles, so flotation separates it better than settling does.

What still goes wrong

The outstanding problem is clogging between the fillers in the tank, and it has not been fully solved. The research directions follow directly from that: control aeration intensity against different inlet loads to eliminate clogging, and study reasonable oxidation tank shapes, sizes and suitable filler materials. Beyond that the process keeps its advantages: high purification efficiency, short treatment time, strong adaptability to variations in inlet organic load, no need for sludge return, no sludge bulking problem, and convenient operation and management. It also remains the natural biological step after physicochemical treatment, the point at which ammonia nitrogen, nitrous acid, nitrate and hydrogen sulfide are removed before the water moves on to whatever follows.