Biofilm Reactor Selection: Film Thickness, Carrier Hydraulics and the Sloughing Cycle That Governs Performance

A biofilm reactor asks a different question of the same bacteria. Instead of keeping microorganisms suspended and washing them out at a controlled rate, you give them a surface and let them build a structured community that stays in the vessel for months. What you gain is resilience and a much longer food chain. What you take on is a hydraulic problem, because the packing that holds the film also holds back the flow.

A Fixed Film Is an Ecosystem, Not a Filter

The film is a layered consortium of aerobic bacteria, anaerobic bacteria, facultative species, fungi, protozoa and algae growing on a solid that goes by filter media, carrier or packing depending on who is describing it. Working outward from the media there is an anaerobic layer, an aerobic layer, an attached water layer and the moving water layer. Organic matter is adsorbed from the attached water, broken down by the aerobic layer, and passed inward for anaerobic decomposition; the moving water then shears off aged film so a fresh one can grow.

That structure is why the process is described as an artificial intensification of soil self-purification. It sits alongside the activated sludge process as one of the two main aerobic biological treatment routes, and it targets dissolved and colloidal organic pollutants rather than settleable solids. The earliest reactors - intermittent sand filters and contact filters filled with broken stone - ran in fill-and-rest cycles inherited directly from sewage irrigation.

Layer by Layer: What Sits Between Carrier and Bulk Water

Three properties govern behaviour. The film is strongly hydrophilic and carries an attached water layer. It is densely populated, supporting a food chain that runs organic pollutants to bacteria to protozoa and metazoa. And it matures into two distinct zones: an aerobic film, generally 2 mm thick, where most organic degradation happens, over an anaerobic film near the carrier. As thickness grows, oxygen can no longer reach the depths and the inner zone turns anaerobic. Anaerobic metabolites then upset the balance between the two zones, gas escaping from the reaction weakens adhesion to the packing, and the film becomes aged, less effective and prone to sloughing.

Operating a biofilm plant well means acting on that cycle: slowing the ageing process, controlling the thickness of the anaerobic film, and above all preventing the film from detaching all at once. A reactor that sloughs uniformly stops performing for days.

Hydraulic Loading and the Clogging Threshold

Loading numbers explain most of the difference between reactor types. An ordinary biological filter runs at a filtration rate of around 1-4 m3/(m2.day). Push an ordinary bed much past that and it ponds and clogs. High-rate beds tolerate more: once the hydraulic loading rate rises above 8-10 m3/(m2.day), the scouring action of the water keeps the film thin enough that the bed stays open. That scouring is the mechanism, not a side effect, and it is why high-rate configurations depend on recirculation to function at all.

Four Reactor Families, Four Contact Regimes

Classification follows how the film meets the water. Packed types include biological filters - ordinary, high-rate and tower configurations - and rotating biological contactors. Immersed types include contact oxidation and the biological fluidised bed, alongside aerobic biological fluidised-bed variants.

The biological filter is the workhorse: crushed stone or plastic modules stacked into a bed that sits open to the air, with wastewater sprinkled over it, most often by a rotary distributor of perforated pipes sweeping close to the bed surface. Any given spot is wetted intermittently even though the distributor runs continuously - another inheritance from irrigation practice. Below the bed, a water-collection layer of brick or specially made ceramic or concrete blocks drains effluent and provides ventilation at the same time. Wastewater flows down the carrier surface exchanging material with the microorganisms and attached water; pollutants enter the film and metabolic products leave with the flow. Oxygen comes from air directly or via the water. Effluent always carries sloughed film, so a sedimentation tank sits downstream. In ordinary filters the mucous-membrane layer is thick and the zone near the carrier is frequently oxygen-free.

Rotating biological contactors appeared once plastics became cheap. Dozens of discs, generally not exceeding 4 m in diameter, are strung on a shaft lying just above a semicircular trough a few centimetres larger. A motor and reduction gear turn the shaft at roughly 1.5-3 rpm depending on disc diameter, with peripheral linear velocity around 15 m/min. About 40% of each disc is submerged and 60% exposed, so the film alternates between wastewater and air. Rotation generates shear between film and disc; past a certain thickness the film peels and leaves with the water. Compared with a filter, contact time is longer and more controllable, troughs and disc banks are staged to prevent short-circuiting and to let load fall stage by stage, and odour is manageable by covering the unit. They suit installations where the water volume is not large.

The biological aerated filter is an aeration tank fitted with plastic modules - the same thing as biological contact oxidation under a different name. It runs like an activated sludge aeration tank but needs no returned sludge, and the aeration method cannot simply be carried over: whole-tank bubble aeration is generally used instead. Biomass is far higher than in activated sludge, so aeration time shortens; operation is stable and sludge bulking does not occur. With granular media such as sand or activated carbon in upflow, the bed expands and resists clogging, and the combination of high surface area, abundant biomass and full contact improves efficiency further. Drop the aeration system and the same vessel becomes an anaerobic biological filter, where the high biomass cuts treatment time dramatically against the ten-day-plus residence of a conventional sludge digester. That variant makes sense on low-concentration streams such as municipal sewage.

Start-up, and What to Watch

Two prerequisites govern whether a film forms at all: a carrier substance to grow on, and nutrients - organic matter, nitrogen and phosphorus. Once organisms attach, adsorption creates the thin surface water layer whose organic concentration is far below the influent, so organics diffuse continually from the moving water into the film while oxygen travels the same path inward. Carbon dioxide and other inorganic products move the other way, out through the attached water layer into the bulk flow or the air.

For municipal sewage at 20 degrees C, roughly 30 days separate initial attachment from a mature film, and the ecosystem only reaches balanced, stable degradation capacity at the end of that period. Plants that commission in warm weather and expect immediate performance are usually disappointed for exactly this reason.

Where Biofilm Beats Suspended Growth

The advantages are consistent: strong tolerance of swings in water volume, composition and temperature; good treatment effect with useful nitrification; roughly three-quarters of the sludge volume of the activated sludge process, with easier solid-liquid separation; and lower power costs. Small, decentralised installations benefit most, because the microbial phase is more diverse, the food chain is longer, slow-growing organisms survive more easily, and each stage in staged operation develops its own dominant species.

Measured results back this up. At the Xiufeng Industrial City sewage treatment station, a biological aerated filter and a biological contact oxidation tank both removed BOD5, CODcr and suspended solids at rates generally above 80%, with the aerated filter slightly ahead. Biomass in the aerated filter was substantially higher than in the contact oxidation tank - sludge mass concentration about twice as high, and microbial density one to two orders of magnitude greater.