General term for biological treatment methods in which microorganisms in wastewater grow along the surface of a solid (which can be called a carrier). Because the microbial population grows into a mucous membrane along the solid surface, hence the name. When wastewater is in contact with biofilms, contaminants are transferred from the water to the membranes, which are then treated.
The basic mechanism is the biological treatment of water.
The typical process of the biofilm method The biological device in the process (Figure 1) can be a biological filter, a biological turntable, a biological aerated filter or an anaerobic biological filter. The first three are used in aerobic biological treatment processes and the last one is used in anaerobic processes. The first biomembrane bioorganisms are the intermittent sand filter and the contact filter (puddle filled with fragments). Its operation is intermittent, filtering-leisure or water filling-contact-water release-leisure, they constitute a work cycle. They are the development of wastewater irrigation, based on the phenomenon of soil self-purification. Then came the continuously operating biological filter. After the arrival of new plastics, there have been new developments.
??The biofilter is the most widely used biological device in the biofilm method. The biological carrier used is a small piece of material (such as crushed stone, plastic fill) or plastic type block, which is stacked or stacked to form a filter bed, thus it is often called filter material. Unlike the general filter in water treatment, the filter bed of the biological filter is exposed to air, and the waste water spills onto the filter bed. There are many types of water dispensers, including stationary and mobile. The rotary water distributor is the most widely used. Two or more symmetrically arranged horizontal perforated tubes are needed as the main body, which can rotate around the center of the pool. The perforated tube is close to the surface of the filter bed and water comes out of the holes. The work of the water distributor is continuous, but the application of water to the local bed surface is intermittent, which inherits the concept of intermittent irrigation of sewage irrigation. Below the filter bed, there is a water catchment layer made of special ceramic bricks or blocks and concrete blocks. Below is the bottom of the pool. The catchment layer communicates with the outside of the pool, which is both for drainage and ventilation. When you are working, waste water flows from the top to the bottom of the filter bed along the carrier surface and comes into close contact with a large number of microorganisms and adhering water that grows on the carrier surface (Figure 2) exchange substances. Contaminants enter the biofilm and metabolites enter the water stream. The effluent contains flaky biofilm debris and must be separated using a settling tank. The dissolved oxygen required by the biofilm is obtained from the air directly or by means of a water jet. In ordinary biological filters, the biological mucosa is thicker, and the part close to the carrier is often in an anaerobic state. Contaminants enter the biofilm and metabolites enter the water stream. The effluent contains flaky biofilm debris and must be separated using a settling tank. The dissolved oxygen required by the biofilm is obtained from the air directly or by means of a water jet. In ordinary biological filters, the biological mucosa is thicker, and the part close to the carrier is often in an anaerobic state. Contaminants enter the biofilm and metabolites enter the water stream. The effluent contains flaky biofilm debris and must be separated using a settling tank. The dissolved oxygen required by the biofilm is obtained from the air directly or by means of a water jet. In ordinary biological filters, the biological mucosa is thicker, and the part close to the carrier is often in an anaerobic state.
??Filter bed depth is related to filtration rate and filter material. The depth of the crushed stone filtration bed is usually about 1.8 to 2 meters in a relatively long period of time. If the depth is increased, the surface layer of the filter bed is likely to block the water. The seepage rate is about 1 to 4 m 3 / (m 2 · day) If it increases, the water will accumulate on the surface of the bed. The first advance is the increase in the filtration rate. When the hydraulic head rate (ie seepage rate) increases to more than 8 ? 10 m 3 / (m 2 day), the scrubbing effect of water flow prevents biofilm from clogging the filter bed, and the loading rate of organic matter (measured by BOD 5 ) can be measured from 0, 2 kg / (m 3 · day) will increase to more than 1 kg / (m 3 · day). To meet hydraulic head rate requirements, incoming water is often diluted by backflow.
In order to stabilize the processing efficiency, a two-stage serial connection can be used. This kind of biological filter with process innovation, higher loading rate and unchanged structure is called high loading rate biological filter. Then it was found that when the depth of the filter bed was increased from about 2 meters to more than 8 meters, the ventilation improved, even if the hydraulic loading rate increased, the filter bed would no longer be blocked, the filter tank worked. ok, and the organic matter loading rate could also be increased to 1 kg/(M3 day) or so. Because the diameter of the plane of this type of filter is generally 1/6 to 1/8 of the height of the pool, and the shape is like a tower, it is called a tower filter. Since the advent of the plastic block,
“The biological turntable” appeared with the popularization of plastics. Tens of pieces, almost a hundred pieces of plastic or fiberglass-reinforced plastic discs are attached with a shaft and placed on the groove surface of a strip-shaped groove with a semicircular section. The diameter of the disk is usually no more than 4 meters, and the diameter of the groove is a few centimeters larger. There is a motor and a deceleration device to rotate the disk shaft at a speed of about 1.5 to 3 revolutions per minute, depending on the diameter of the disk, and the peripheral linear speed of the disk is about 15 meters per minute ( Figure 3). ).
Figure 3 Biological turntable
??The waste water flows from one end of the tank to the other. The disk axis is higher than the water surface, about 40% of the disk surface is submerged in water, and about 60% is exposed to air. When the disc shaft rotates, the disc surface is alternately in contact with the residual water and the air. The disc surface is covered by a membrane formed by the growth of microorganisms, and the biofilm is alternately in full contact with the sewage and air, continuously obtaining pollutants and oxygen, and purifying the sewage. The shear stress generated between the membrane and the disk surface due to rotation increases as the thickness of the membrane increases. To a certain extent,
??Compared with biological filter, the contact time of sewage and biofilm in biological spinning disc method is longer. And there is a certain degree of controllability. The sink is often divided into sections, and the turntable is often grouped together, which can avoid short flow, but also help to increase the loading rate and effluent quality, because the loading rate is gradually reduced. If the biological carousel produces a bad smell, it can be covered. Biological turntables are generally used when the amount of water is not large.
The biological aerated filter is equipped with a plastic block aeration tank. According to its process, it is also called biological contact oxidation method. Its work is similar to that of the aeration tank in the activated sludge method, but it does not return the sludge and the aeration method cannot be used. Typically, full tank bubble aeration is used. The biomass in the tank is much higher than the activated sludge method, so the aeration time can be shortened. The operation is relatively stable, and the problem of sludge expansion will not occur. There are also granular materials (such as sand, activated carbon). At this time, the water flows upwards, and the filter bed expands without clogging. Because the surface area is high, the biomass is large, the contact is sufficient,
The structure of the anaerobic biological filter is the same as that of the aerated biological filter, except that the aeration system is not required. Due to the high biomass, compared with the sludge digestion tank, the treatment time can be greatly shortened (the residence time of the sludge digestion tank is generally more than 10 days), and it can be used to treat low-concentration wastewater as urban sewage.