Biofilm Reactors: Why Attached Growth Still Beats Suspended Growth for Small Plants

The biofilm process, also called the biomembrane or attached-film process, sits alongside activated sludge as one of the two main aerobic biological treatment routes. It is an artificial intensification of what soil does naturally: microorganisms colonise a solid surface, and wastewater passing over that surface gives up its dissolved and colloidal organic load.

How a biofilm is built and how it works

A biofilm is a dense ecosystem — aerobic bacteria, anaerobic bacteria, facultative bacteria, fungi, protozoa and algae packed into a mucous layer on a carrier that may be gravel, plastic packing or a purpose-made block. From the carrier outward the film divides into an anaerobic layer, an aerobic layer, an attached-water layer and a flowing-water layer.

The sequence repeats continuously. The film adsorbs organic matter from the attached-water layer; aerobic bacteria in the aerobic layer break it down; residues pass into the anaerobic layer for anaerobic decomposition; and the flowing-water layer strips away aged film so fresh film can grow. Microbial products such as CO₂ travel in the opposite direction, out through the attached-water layer into the flowing water or the air. Because adsorption keeps the attached water layer low in organics, there is always a concentration gradient pulling more out of the bulk flow.

Getting a film established

Three things have to be present: a carrier to grow on, nutrients including organic matter, nitrogen and phosphorus, and inoculation. Sewage carrying both nutrients and seed microorganisms is passed over the packing; after a period the organisms attach, proliferate and form a thin film. For municipal sewage at 20°C, maturation from formation takes roughly 30 days, at which point the ecosystem and its degradation function reach a working balance.

Thickness is the variable to manage

As the film thickens, oxygen can no longer penetrate to the depths near the carrier and that inner zone turns anaerobic. A mature film therefore carries both an anaerobic and an aerobic layer, and the aerobic layer — generally about 2 mm thick — does the bulk of the organic degradation.

Trouble starts when the anaerobic side overbalances. Accumulating anaerobic metabolites disturb the balance between the two layers, gaseous products escaping continuously weaken the film's grip on the medium, and the film ages, loses purification capacity and tends to slough. The operational art is to slow ageing, keep anaerobic thickness in check, and avoid a concentrated sloughing event that dumps solids into the effluent.

Reactor formats

Biofilm reactors are classified by how water meets the film. Packed types include biofilters and rotating biological contactors; immersed types include contact oxidation and the biological fluidized bed.

The biofilter is the most widely used. Its bed of small media, gravel or plastic, is open to the air and wastewater is sprayed over it, usually by a rotary distributor of two or more symmetrically arranged perforated pipes sweeping close to the bed surface. Discharge is continuous but any given patch of bed is watered intermittently, which carries forward the intermittent-irrigation logic of sewage irrigation. Air moves through the collection layer beneath, which also drains. Effluent carries sloughed film debris and needs a settling tank. Conventional beds run thick slime layers, often anaerobic near the carrier; gravel beds settled at about 1.8–2 m depth because deeper beds clog and pond at the surface.

Pushing hydraulic load past 8–10 m³/(m²·day) was the breakthrough: flow scouring prevents clogging and lets the BOD₅ load rise from about 0.2 to above 1 kg/(m³·day), usually with reflux dilution to meet the hydraulic target and two stages in series for stability.

The rotating biological contactor, RBC, appeared once plastics became common. Dozens of plastic or FRP discs, usually under 4 m in diameter, are strung on a horizontal shaft in a trough of semicircular cross-section, with roughly 40% of each disc submerged and 60% exposed to air. A motor and reducer turn the shaft at 1.5–3 rpm depending on disc diameter, giving a peripheral speed of about 15 m/min. As the discs rotate, the film alternately meets wastewater and air, taking up pollutants and oxygen in turn; shear grows with film thickness until the film sloughs and leaves with the flow. Compared with biofilters, RBC gives longer contact time and more controllability, and dividing the trough into sections and discs into groups prevents short-circuiting while raising load and effluent quality. Odour can be contained with a cover. RBC is generally a small-flow solution.

Contact oxidation is essentially an aeration tank filled with plastic blocks. It behaves like an activated-sludge aeration tank but needs no sludge return, and because biomass is far higher the aeration time can be shortened; whole-tank bubble aeration is typical. Operation is stable with no sludge-bulking risk. Granular media such as sand or activated carbon can also be used with upward flow, the bed expanding so it does not clog.

Where biofilm wins

For small-scale decentralised treatment the argument is strong on two fronts. Biologically, biofilm processes carry a more diverse microbial population with longer food chains, so long-generation-time organisms survive and dominant strains establish themselves at each stage of staged operation. In process terms they absorb swings in quality and flow, settle well, separate easily, handle low-concentration wastewater, and are easy to maintain and cheap on power.

The general advantages are consistent with that: strong adaptability to changes in flow, quality and temperature; good treatment with solid nitrification; low sludge production at about three-quarters of activated sludge, with easy solid–liquid separation; and low power cost.

Field results back it up. At the Xiufeng Industrial City wastewater station, a biological aerated filter and a biological contact oxidation tank both removed BOD₅, CODcr and suspended solids at above 80%, with the aerated filter slightly ahead. Biomass in the aerated filter was markedly higher — sludge mass concentration about double, and microbial density one to two orders of magnitude greater — which is the mechanism behind the performance gap rather than a footnote to it.