The Biofilm Process: How a Fixed Film on a Carrier Replaces Suspended Sludge
A fixed film rather than a suspension
The biofilm process, also called the biomembrane process or the fixed-film process, is an aerobic biological wastewater treatment technology that sits alongside the activated sludge process. It is an artificial and intensified version of the soil self-purification process, and it mainly removes dissolved and colloidal organic pollutants. Instead of keeping microorganisms suspended, it lets them attach and grow on the surface of solids. That community, highly concentrated aerobic bacteria, anaerobic bacteria, facultative bacteria, fungi, protozoa and algae, is the biofilm, and the solid it clings to is the filter media or carrier.
Structurally, working from the media outward, the film is an anaerobic layer, an aerobic layer, an attached water layer and a moving water layer. The film first adsorbs organics out of the attached water layer; aerobic bacteria in the aerobic layer decompose them, and the products pass into the anaerobic layer for anaerobic decomposition. The moving water layer washes away aged biofilm so a new one grows, and the cycle purifies the wastewater. The earliest biofilm reactors were intermittent sand filters and contact filters, tanks filled with broken material, run on a filter-rest or fill-contact-drain-rest cycle. They evolved from sewage irrigation and were based on soil self-purification. Continuously operated trickling filters followed, and after new plastics emerged the range widened again.
How the film forms, ages and renews
Two prerequisites have to be met: a supporting carrier, meaning packing or filter media, and nutrients, meaning organics, N, P and others. Inoculation is simple in principle. Sewage containing nutrients and inoculated microorganisms flows over the packing surface; after a certain time the microorganisms attach, multiply and grow into a thin biofilm. Maturity is the point at which both the ecosystem on the film and its degradation function for organics reach balance and stability. For municipal sewage at 20 degrees C, the film generally takes about 30 days from initial formation to maturity.
A mature film is highly hydrophilic and carries an attached water layer, and the microorganisms in it are highly concentrated, forming a food chain of organic pollutants, bacteria and protozoa or metazoa. As thickness keeps increasing, the deep interior where oxygen cannot penetrate turns anaerobic, so a mature biofilm generally consists of an anaerobic film and an aerobic film, and the aerobic film, generally about 2 mm thick, is the main site of organic degradation.
Aging follows a predictable path. More anaerobic metabolites upset the balance between the anaerobic and aerobic films; the continuous escape of gaseous products weakens the film's attachment to the media; and it becomes an aged biofilm with poorer purification function that is prone to sloughing. Renewal then happens naturally: the aged film sloughs, a new biofilm grows, and the new film has stronger purification function. Operational art lies in slowing the aging process, controlling the thickness of the anaerobic film, and trying to keep the film from sloughing all at once.
Reactor shapes
The trickling filter is the most commonly used bioreactor. Its carrier is small pieces, gravel or plastic packing, or plastic blocks, piled or stacked into a filter bed, hence the name filter media. Unlike ordinary filters, the bed is exposed to air and wastewater is sprinkled onto it. The rotary distributor is the most widely used: two or more symmetrically arranged horizontal perforated pipes rotate around the tank centre, discharging close to the bed surface. The distributor works continuously, but watering of any local bed surface is intermittent, inheriting the intermittent irrigation concept. Below the bed sits a water-collecting layer of brick, special ceramic blocks or concrete blocks, then the tank bottom; the collecting layer connects outside for both drainage and ventilation. Effluent carries sloughed biofilm debris and needs a settling tank. In a conventional trickling filter the biofilm layer is thick and the part close to the carrier is often anaerobic. Gravel beds were mostly 1.8-2 m deep for a long time, and increasing depth or filtration rate ponds the bed surface. The filtration rate is about 1-4 m3/(m2day); the first breakthrough was raising it, and when the hydraulic loading rate rises above 8-10 m3/(m2day) the scouring action of the water prevents the biofilm from clogging the bed. High-rate and tower biological filters are variants.
The rotating biological contactor appeared with the spread of plastics. Dozens to nearly a hundred plastic or FRP disks are strung on a shaft and laid flat in a strip tank with a semicircular cross-section. Disk diameter is generally no more than 4 m and the tank diameter is a few centimetres larger. A motor and reduction gear turn the shaft at about 1.5-3 rpm depending on disk diameter, with a peripheral linear speed of about 15 m/min. Wastewater flows from one end of the tank to the other, the shaft sits above the water surface, about 40% of the disk is submerged and about 60% is exposed to air, and as the shaft rotates the film alternately contacts wastewater and air, picking up pollutants and oxygen in turn. Shear stress between the film and the disk surface grows with film thickness until the film detaches and flows away. Compared with the trickling filter, contact time is longer and somewhat controllable; the tank is often divided into sections and the disks into groups, which prevents short-circuiting and raises loading and effluent quality because the loading rate drops stage by stage. A cover handles odour, and the unit is generally used for small flows.
The contact oxidation tank is an aeration tank fitted with plastic blocks, also called the biological contact oxidation method. It works like an activated sludge aeration tank but needs no sludge return, and aeration cannot be the same: whole-tank bubble aeration is generally used. Biomass in the tank is far higher than in the activated sludge process, so aeration time can be shortened. Operation is stable and there is no sludge bulking problem. Granular media such as sand or activated carbon are also used; then water flows upward, the bed expands and there is no clogging, and because of the high surface area, large biomass and full contact, aeration time is short and efficiency is high, though this remains at the research stage. The anaerobic biological filter is structurally similar to the biological aerated filter but has no aeration system; thanks to high biomass, treatment time drops sharply compared with a sludge digestion tank, whose retention time generally exceeds 10 days, and it may serve lower-concentration wastewater such as municipal sewage. By contact mode, the filled type includes the trickling filter and the rotating biological contactor, and the immersed type includes the contact oxidation method and the biological fluidized bed.
What happens at the film surface
Once the film exists, its adsorption leaves a thin attached water layer on the surface. Organics in that layer have already been oxidized and decomposed by the film, so its organic concentration is much lower than the influent. As wastewater flows past, organics transfer from the moving wastewater into the attached water layer and are further adsorbed by the film, while oxygen from the air also passes through the wastewater into the biofilm water layer and moves inward. The microorganisms decompose organics and metabolise under dissolved oxygen, and inorganic products such as carbon dioxide move in the opposite direction, out through the attached water layer into the flowing wastewater or air. Effluent organic content falls and the water is purified.
Why small plants prefer fixed film
Performance data back the preference. At the Xiufeng Industrial City wastewater station, the biological aerated filter and the biological contact oxidation tank both gave removal rates for BOD5, CODcr and suspended solids generally above 80%, with the biological aerated filter performing slightly better. In the biological aerated filter the amount of biofilm and microorganisms is clearly higher than in the contact oxidation tank: sludge mass concentration is about twice as high and microbial density is 1-2 orders of magnitude higher.
The advantages stack up for decentralised installations. The process adapts strongly to changes in water volume, quality and temperature; it nitrifies well; sludge production is small, about three quarters of the activated sludge process, and solid-liquid separation is easy; and power cost is low. Ecologically, biofilm processes host a more diverse microbial community with a longer food chain, so microorganisms with longer generation times survive and each stage of a staged system can develop its own dominant strains. For small-scale decentralized sewage treatment the biofilm process is used far more often than the activated sludge process, and it is easier to maintain and more energy-saving while handling low-concentration sewage.