What Is the Biofilm Process?
The biofilm process (biomembrane process) is also called the fixed-film process.
Structure of the biofilm on biological filter media (cross-section)
Is a type of aerobic biological wastewater treatment technology alongside the activated sludge process; it is a fixed-film method and an artificial and intensified version of the soil self-purification process.
Mainly removes dissolved and colloidal organic pollutants from wastewater.
Main types: biological filters include the conventional trickling filter, the high-rate trickling filter, and the tower biological filter, etc.
Aerobic biological fluidized bed method, etc.
The biofilm process is a method of organic wastewater treatment using microorganisms (i.e., the biofilm) attached and growing on the surface of certain solids. The biofilm is an ecosystem composed of highly concentrated aerobic bacteria, anaerobic bacteria, facultative bacteria, fungi, protozoa, and algae; the solid medium it attaches to is called the filter media or carrier. From the media outward, the biofilm is divided into an anaerobic layer, an aerobic layer, an attached water layer, and a moving water layer. The principle is that the biofilm first adsorbs the organics in the attached water layer, which are decomposed by the aerobic bacteria in the aerobic layer and then enter the anaerobic layer for anaerobic decomposition; the moving water layer washes away the aged biofilm so a new one grows, and this cycle purifies the wastewater.
A general term for biological treatment methods in which microorganisms in wastewater grow along the surface of a solid (called a carrier). It is so named because the microbial community grows into a mucous membrane along the solid surface. When wastewater contacts the biofilm, pollutants transfer from the water to the membrane and are thus treated. Its basic mechanism is described under biological treatment of water.
Typical flow of the biofilm process. The bioreactor in the flow (Fig. 1) can be a trickling filter, a rotating biological contactor, a biological aerated filter, or an anaerobic biological filter. The first three are for aerobic biological treatment and the last for anaerobic. The earliest biofilm bioreactors were intermittent sand filters and contact filters (tanks filled with broken material). Their operation was intermittent: filter-rest or fill-contact-drain-rest formed a work cycle. They evolved from sewage irrigation and were based on the soil self-purification phenomenon. Then continuously operated trickling filters appeared. After new plastics emerged, further developments followed.
The most commonly used bioreactor in the biofilm process. The biological carrier is small pieces (such as gravel or plastic packing) or plastic blocks, piled or stacked into a filter bed, hence often called filter media. Unlike ordinary filters in water treatment, the trickling filter bed is exposed to air and wastewater is sprinkled onto it. There are many forms of distributors, fixed and moving. The rotary distributor is the most widely used. Its main body is two or more symmetrically arranged horizontal perforated pipes that can rotate around the tank center. The perforated pipes are close to the bed surface and water flows from the holes. The distributor works continuously, but watering of a local bed surface is intermittent, inheriting the concept of intermittent irrigation in sewage irrigation. Below the bed is a water-collecting layer made of brick, special ceramic blocks, or concrete blocks. Below that is the tank bottom. The collecting layer connects with the outside for both drainage and ventilation. In operation, wastewater flows down the carrier surface through the bed from top to bottom and exchanges substances through close contact with the large microbial population and attached water on the carrier surface. Pollutants enter the biofilm and metabolites enter the water flow. The effluent carries sloughed biofilm debris and needs a settling tank for separation. The dissolved oxygen the biofilm needs is obtained directly or through the water flow from the air. In a conventional trickling filter the biofilm layer is thick and the part close to the carrier is often anaerobic. The bed depth relates to the filtration rate and media. Gravel beds were mostly 1.8-2 m deep for a long time. If the depth is increased, the bed surface easily clogs and ponds. The filtration rate is about 1-4 m3/(m2day); if increased, the bed surface also ponds easily. The first breakthrough was raising the filtration rate. When the hydraulic loading rate (i.e., filtration rate) rises above 8-10 m3/(m2day), the scouring action of the water prevents the biofilm from clogging the bed, and the organic (measured by BOD5) loading rate can rise from about 0.2 kg/(m3day) to above 1 kg/(m3day). To meet the hydraulic loading rate, the incoming water is often diluted by recirculation. For stable efficiency, two-stage series can be used. This innovated flow with higher loading rate but unchanged structure is the high-rate trickling filter. It was then found that when the bed depth rises from about 2 m to above 8 m, ventilation improves and even with a higher hydraulic loading rate the bed no longer clogs and the filter works well, while the organic loading rate can also rise to about 1 kg/(m3day). Because the plane diameter is generally about 1/6 to 1/8 of the bed height, it looks like a tower and is called the tower biological filter. Since plastic blocks appeared, ventilation and clogging are no longer problems and the bed depth and filtration rate can be designed as needed.
It appeared with the spread of plastics. Dozens to nearly a hundred plastic or FRP disks are strung on a shaft and laid flat on a strip tank with a semicircular cross-section. The disk diameter is generally no more than 4 m and the tank diameter is a few centimeters larger. A motor and reduction gear rotate the disk shaft at about 1.5-3 rpm, depending on the disk diameter, with a peripheral linear speed of about 15 m/min.
Wastewater flows from one end of the tank to the other. The disk shaft is above the water surface; about 40% of the disk is submerged and about 60% exposed to air. As the shaft rotates, the disk surface alternately contacts wastewater and air. The disk surface is covered by a membrane formed by microbial growth; the biofilm alternately contacts wastewater and air fully, continuously obtaining pollutants and oxygen and purifying the wastewater. Shear stress arises between the membrane and the disk surface due to rotation, increasing with membrane thickness; at a certain point the membrane detaches from the disk surface and flows away with the water.
Compared with the trickling filter, the rotating biological contactor gives longer contact time between wastewater and biofilm, and is somewhat controllable. The tank is often divided into sections and the disks into groups, preventing short-circuiting and helping raise the loading rate and effluent quality since the loading rate drops stage by stage. If the contactor produces odor, a cover can be added. It is generally used for small flows.
An aeration tank fitted with plastic blocks. By its process it is also called the biological contact oxidation method. It works like the aeration tank in the activated sludge process but needs no sludge return, and the aeration method cannot be the same; generally whole-tank bubble aeration is used, and the biomass in the tank is far higher than in the activated sludge process, so the 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. Because of the high surface area, large biomass, and full contact, the aeration time is short, efficiency is high, and it is still at the research stage.
Its structure is similar to the biological aerated filter but without the aeration system. Because of the high biomass, the treatment time can be greatly shortened compared with a sludge digestion tank (retention time generally over 10 days), and it may be used for lower-concentration wastewater such as municipal sewage.
Prerequisites: the supporting carrier-packing or filter media
Nutrients-organics, N, P, and others
Formation of the inoculated microbial biofilm: sewage containing nutrients and inoculated microorganisms flows over the surface of the packing; after a certain time the microorganisms attach to the packing surface, multiply, and grow into a thin biofilm.
On the biofilm, the ecosystem composed of bacteria and other microorganisms and the biofilm's degradation function for organics both reach balance and stability.
For municipal sewage, at 20 degrees C, the biofilm generally takes about 30 days from initial formation to maturity.
Property: highly hydrophilic, with an attached water layer
Microorganisms are highly concentrated: various bacteria and microfauna that play the main role in removing organic pollutants from wastewater, forming a food chain of organic pollutants-bacteria-protozoa (metazoa)
1) The biofilm thickness keeps increasing; the deep interior where oxygen cannot penetrate turns anaerobic
2) A mature biofilm generally consists of an anaerobic film and an aerobic film
3) The aerobic film is the main site of organic degradation, generally about 2 mm thick.
1) More anaerobic metabolites upset the balance between the anaerobic and aerobic films
2) Continuous escape of gaseous products weakens the biofilm's attachment to the media
3) It becomes an aged biofilm with poorer purification function and is prone to sloughing.
1) The aged film sloughs and a new biofilm grows
2) The new biofilm has stronger purification function.
1) Slow the biofilm aging process
2) Control the anaerobic film thickness
a. The biological aerated filter and biological contact oxidation tank in the biofilm process gave good treatment of sewage at the Xiufeng Industrial City wastewater station; BOD5, CODcr, and suspended solids removal rates were generally above 80%. The biological aerated filter performed slightly better than the biological contact oxidation tank.
1) Strong adaptability to changes in water volume, quality, and temperature; 2) good treatment effect with good nitrification function; 3) small sludge amount (about 3/4 of the activated sludge process) and easy solid-liquid separation; 4) low power cost.
4) Try to control the biofilm so it does not slough all at once.
b. In the biological aerated filter the amount of biofilm and microorganisms is clearly higher than in the biological contact oxidation tank-the sludge mass concentration is about twice as high and the microbial density is 1-2 orders of magnitude higher.
In a sewage treatment structure a carrier for microbial growth and aggregation (generally called packing) is set; under oxygenation, microorganisms attach to the packing surface and form a biofilm. When oxygenated sewage (the oxygenation device consists of a water-treatment blower and aerators) flows over the packing at a certain velocity, the microorganisms in the biofilm absorb and decompose the organics in the water, purifying the sewage, while the microorganisms also multiply and the biofilm thickens. When the biofilm grows to a certain thickness, oxygen diffusion into its interior is limited; the surface remains aerobic while the inner layer becomes anoxic or even anaerobic, eventually leading to sloughing. Then a new biofilm continues to grow on the packing surface, and the cycle purifies the sewage.
After microorganisms attach to the packing surface and form a biofilm, because of the biofilm's adsorption a thin water layer exists on its surface; the organics in this layer have already been oxidized and decomposed by the biofilm, so its organic concentration is much lower than the influent. When wastewater flows over the biofilm surface, organics transfer from the moving wastewater to the water layer attached to the biofilm surface and are further adsorbed by the biofilm, while oxygen from the air also passes through the wastewater into the biofilm water layer and moves inward.
The microorganisms on the biofilm decompose organics and carry out their own metabolism under dissolved oxygen, so the inorganic products such as carbon dioxide move in the opposite direction, from the biofilm through the attached water layer into the flowing wastewater or air. Thus the effluent organic content decreases and the wastewater is purified.
For small-scale decentralized sewage treatment, the biofilm process is used far more than the activated sludge process and has advantages: 1) In the microbial community, the various biofilm processes have diverse microorganisms participating in purification, a longer microbial food chain, and microorganisms with longer generation times easily survive; in staged operation each stage can form dominant strains. 2) In the treatment process, various biofilm processes have strong adaptability to water quality and quantity changes, good sludge settleability and easy solid-liquid separation, can treat low-concentration sewage, and are easy to maintain and energy-saving.
By the contact mode of biofilm and wastewater, it is divided into:
The filled type includes the trickling filter and the rotating biological contactor
The immersed type includes the contact oxidation method and the biological fluidized bed
