Wastewater Treatment Glossary: The Biofilm Process
Biofilm process (biomembrane process). The biofilm process is also known as the fixed-film process.
Structure of the biofilm on biological filter media (cross-sectional view)
* It is a category of aerobic biological wastewater treatment technology on a par with the activated sludge process; it is a fixed-film method and represents the artificial reproduction and intensification of the soil self-purification process.
* It mainly removes dissolved and colloidal organic pollutants from wastewater.
* Main categories: biological filters include ordinary biological filters, high-rate biological filters, tower biological filters, etc.
aerobic biological fluidized-bed process, etc.
The biofilm process is a method of treating organic sewage using microorganisms (i.e., the biofilm) that grow attached to the surfaces of certain solid materials. The biofilm is an ecosystem composed of highly dense aerobic bacteria, anaerobic bacteria, facultative bacteria, fungi, protozoa, algae, etc., and the solid medium to which it attaches is called filter media or carrier. From the media outward, the biofilm can be divided into an anaerobic layer, an aerobic layer, an attached water layer, and a moving water layer. The principle of the biofilm process is that the biofilm first adsorbs organic matter in the attached water layer, the aerobic bacteria in the aerobic layer decompose it, and it then enters the anaerobic layer for anaerobic decomposition; the moving water layer washes away the aged biofilm so that new biofilm can grow, and so on repeatedly to achieve the purpose of purifying the sewage.
A general term for biological treatment methods in which microorganisms in wastewater grow along the surface of a solid (which can be called a carrier). It is so named because the microbial community grows into a mucous-membrane-like layer along the solid surface. When wastewater contacts the biofilm, pollutants transfer from the water onto the film and are thereby treated. For its basic mechanism, see biological water treatment methods.
Typical process flow of the biofilm process The bioreactor in the flow (Figure 1) can be a biological filter, a rotating biological contactor, a biological aerated filter, or an anaerobic biological filter. The first three are used for aerobic biological treatment processes, and the last for anaerobic processes. The earliest biofilm bioreactors were the intermittent sand filter and the contact filter (a tank filled with broken pieces). They operated intermittently: filtration-rest or filling-contact-draining-rest, forming one working cycle. They were developments of sewage irrigation and were based on the soil self-purification phenomenon. Then continuously operating biological filters appeared. After the advent of new plastics, there were further developments.
The most commonly used bioreactor in the biofilm process. The biological carrier used is small pieces of material (such as crushed stone or plastic packing) or plastic modules, stacked or piled into a filter bed, hence often called filter media. Unlike ordinary filters in water treatment, the filter bed of a biological filter is exposed to the air, and wastewater is sprinkled onto it. Distributors come in many forms, some fixed and some movable, with the rotary distributor being the most widely used. It consists mainly of two or more symmetrically arranged horizontal perforated pipes that can rotate around the center of the tank; the perforated pipes are close to the surface of the filter bed, and water flows out from the holes. The distributor works continuously, but the application of water to any local part of the bed is intermittent, inheriting the concept of intermittent watering in sewage irrigation. Beneath the filter bed is a water-collection layer laid with bricks or specially made ceramic or concrete blocks, and below that is the tank bottom. The water-collection layer communicates with the outside of the tank, both draining water and providing ventilation. During operation, wastewater flows down along the carrier surface through the filter bed, closely contacting the large number of microorganisms and attached water on the carrier surface for material exchange; pollutants enter the biofilm, and metabolic products enter the water flow. The effluent carries sloughed-off biofilm debris and needs a sedimentation tank for separation. The dissolved oxygen required by the biofilm is obtained from the air directly or through the water flow. In ordinary biological filters, the biological mucous-membrane layer is relatively thick, and the part close to the carrier is often in an oxygen-free state. The depth of the filter bed is related to the filtration rate and the media. The depth of crushed-stone filter beds was mostly around 1.8-2 meters for a considerable time; if the depth is increased, the surface layer of the filter bed easily clogs and ponds water. The filtration rate is around 1-4 m3/(m2.day); if increased, the bed surface also easily ponds water. The first breakthrough was the increase of the filtration rate. When the hydraulic loading rate (i.e., filtration rate) is raised above 8-10 m3/(m2.day), the scouring action of the water flow keeps the biofilm from clogging the filter bed, and the organic loading rate (measured by BOD5) can be raised from around 0.2 kg/(m3.day) to over 1 kg/(m3.day). To meet the hydraulic loading requirement, the incoming water is often recirculated for dilution; to stabilize the treatment efficiency, two-stage series operation can be used. Such a biological filter -- with an innovative flow, an increased loading rate, and unchanged structure -- is called a high-rate biological filter. It was then found that when the filter-bed depth was increased from around 2 meters to over 8 meters, ventilation improved, and even if the hydraulic loading rate was increased, the filter bed no longer clogged, the filter worked well, and the organic loading rate could also be increased to around 1 kg/(m3.day). Because the plan diameter of such a filter is generally about 1/6-1/8 of the filter height and its shape resembles a tower, it is called a tower filter. After the advent of plastic modules, ventilation, clogging, and other issues were no longer problems, and the filter-bed depth and filtration rate could be designed as needed.
It appeared with the popularization of plastics. Dozens to nearly a hundred discs of plastic or fiberglass are strung on a shaft and laid flat on the surface of a strip-shaped trough with a semicircular cross-section. The disc diameter generally does not exceed 4 meters, and the trough diameter is a few centimeters larger. A motor and reduction gear rotate the disc shaft at a speed of about 1.5-3 rpm, depending on the disc diameter, with the peripheral linear velocity of the discs at about 15 m/min.
Wastewater flows from one end of the trough to the other. The disc shaft is above the water surface, with about 40% of the disc surface immersed in water and about 60% exposed to the air. As the disc shaft rotates, the disc surface alternately contacts wastewater and air. The disc surface is covered with a film-like substance formed by microbial growth; the biofilm alternately makes full contact with wastewater and air, continuously obtaining pollutants and oxygen and purifying the wastewater. Shear stress is generated between the film and the disc surface due to rotation, increasing as the film thickens; when it reaches a certain point, the film peels off the disc surface and is carried away with the water.
Compared with the biological filter, the rotating biological contactor process gives a relatively long contact time between the wastewater and the biofilm, with a certain degree of controllability. The water trough is often divided into sections and the discs into groups, which both prevents short-circuiting and helps improve the loading rate and effluent quality, since the loading rate decreases stage by stage. If the rotating biological contactor produces odor, it can be covered. Rotating biological contactors are generally used when the water volume is not large.
An aeration tank fitted with plastic modules. According to its process, it is also called the biological contact oxidation method. Its operation is similar to the aeration tank in the activated sludge process, but it does not require returned sludge, and the aeration method cannot be carried over; generally, whole-tank bubble aeration is used. The biomass in the tank is much higher than in the activated sludge process, so the aeration time can be shortened. Operation is relatively stable, and the problem of sludge bulking does not occur. Granular media (such as sand or activated carbon) are also used; in this case, the water flows upward, the filter bed expands, and it does not clog. Because of the high surface area, abundant biomass, and full contact, the aeration time can be shortened and the treatment efficiency improved; this is still in the research stage.
Its structure is identical to that of the biological aerated filter, except that no aeration system is required. Because of the high biomass, compared with the sludge digestion tank, the treatment time can be greatly shortened (the residence time of a sludge digestion tank is generally over 10 days). It may be adopted when treating low-concentration wastewater such as municipal sewage.
* Prerequisite: a supporting carrier substance -- packing, also called filter media.
* Nutrients -- organic matter, N, P, and others.
After microorganisms attach and gather on the packing surface to form a biofilm, due to the adsorption of the biofilm, a thin water layer exists on its surface. The organic matter in this water layer has already been oxidized and decomposed by the biofilm, so the organic-matter concentration in the water layer is much lower than that in the influent. When wastewater flows past the biofilm surface, organic matter transfers from the moving wastewater into the water layer attached to the biofilm surface and is further adsorbed by the biofilm; at the same time, oxygen in the air also passes through the wastewater into the biofilm water layer and transfers inward.
The ecosystem composed of bacteria and various other microorganisms on the biofilm, as well as the biofilm's function of degrading organic matter, both reach balance and stability.
For municipal sewage, at 20C, it generally takes about 30 days for the biofilm to develop from initial formation to maturity.
Property: highly hydrophilic, with an attached water layer present.
Highly dense microorganisms: various bacteria as well as microscopic animals. These microorganisms play the main role in removing organic pollutants from wastewater, forming a food chain of organic pollutants -- bacteria -- protozoa (metazoa).
1) As the biofilm thickness continuously increases, the deep interior that oxygen cannot penetrate will turn 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-matter degradation, generally 2 mm thick.
1) The increase of anaerobic metabolic products destroys the balance between the anaerobic film and the aerobic film.
2) The continuous escape of gaseous products weakens the adhesion of the biofilm to the packing.
3) It becomes an aged biofilm with poorer purification function and a tendency to slough off.
1) After the aged film sloughs off, a new biofilm grows again.
2) The new biofilm has a stronger purification function.
1) Slow down the aging process of the biofilm.
2) Control the thickness of the anaerobic film.
4) Try to prevent the biofilm from sloughing off all at once.
(1) Strong adaptability to variations in water volume, water quality, and water temperature; (2) good treatment effect with good nitrification function; (3) small sludge volume (about 3/4 that of the activated sludge process) and easy solid-liquid separation; (4) low power costs.
a. Using the biological aerated filter and biological contact oxidation tank of the biofilm process gives a good treatment effect on the sewage of the Xiufeng Industrial City sewage treatment station, with removal rates of BOD5, CODcr, and suspended solids generally above 80%. The treatment effect of the biological aerated filter is slightly better than that of the biological contact oxidation tank.
b. The amount of biofilm and biomass in the biological aerated filter is significantly higher than in the biological contact oxidation tank, with the sludge mass concentration about twice as high and the microbial density 1-2 orders of magnitude higher.
Carriers for microbial growth and aggregation (generally called packing) are set up inside the sewage treatment structure. Under aerated conditions, microorganisms attach and gather on the packing surface to form a biofilm. When aerated sewage (the aeration device consists of a water-treatment aeration blower and an aerator) flows past the packing at a certain velocity, the microorganisms in the biofilm absorb and decompose the organic matter in the water, purifying the sewage while the microorganisms also multiply and the biofilm thickens accordingly. When the biofilm grows to a certain thickness, the oxygen diffusing into its interior is limited; its surface remains aerobic, while the inner layer becomes anoxic or even anaerobic, eventually causing the biofilm to slough off. Subsequently, new biofilm continues to grow on the packing surface, and the cycle repeats, purifying the sewage.
* The formation process of the inoculated microbial biofilm: sewage containing nutrients and inoculated microorganisms flows over the packing surface; after a certain time, the microorganisms attach to the packing surface and multiply and grow, forming a thin biofilm.
The microorganisms on the biofilm decompose organic matter under conditions of dissolved oxygen and carry out their own metabolism, so the resulting inorganic substances such as carbon dioxide travel in the opposite direction, i.e., transferring from the biofilm through the attached water layer into the flowing wastewater or the air. In this way, the organic-matter content of the effluent decreases and the wastewater is purified.
In small-scale, decentralized sewage treatment, the biofilm sewage treatment process is widely used and has more advantages than the activated sludge process, specifically: 1) In terms of the microbial phase, the microorganisms participating in the purification reaction in various biofilm processes are diverse, the microbial food chain is longer, microorganisms with longer generation times survive more easily, and each stage in staged operation can form dominant species; 2) In terms of the treatment process, various biofilm processes have strong adaptability to changes in water quality and quantity, the sludge has good settling performance and is easy to separate from the liquid, low-concentration sewage can be treated, and maintenance is easy and energy-saving.
Classified by the way the biofilm contacts the wastewater:
Packed type includes biological filters and rotating biological contactors.
Immersed type includes the contact oxidation method and the biological fluidized bed.
