Process comparison between activated sludge method and biofilm method

At present, the representative processes in the application of biological treatment of sewage mainly include activated sludge method and biofilm method. Below, based on years of production practice and theoretical study, the two process methods are analyzed and compared. 

1. Activated sludge method 

Process and principle 

The typical activated sludge method is composed of aeration tank, sedimentation tank, sludge return system and excess sludge removal system.

The sewage and the returned activated sludge enter the aeration tank together to form a mixed liquid. The compressed air sent from the air compressor station enters the sewage in the form of fine bubbles through the air diffusion device laid at the bottom of the aeration tank. The purpose is to increase the dissolved oxygen content in the sewage and also make the mixed liquid in a vigorous agitation. State, in a suspended state. Dissolved oxygen, activated sludge and sewage are mixed and fully contacted, so that the activated sludge reaction can proceed normally. 

In the first stage, the organic pollutants in the sewage are adsorbed by the activated sludge particles on the surface of the micelles, due to its huge surface area and the action of polysaccharide viscous substances. At the same time, some macromolecular organic matter is decomposed into small molecular organic matter under the action of bacterial extracellular enzymes. 

In the second stage, microorganisms absorb these organic substances under sufficient oxygen conditions, and oxidize and decompose them to form carbon dioxide and water, and part of them are provided for their own proliferation and reproduction. As a result of the activated sludge reaction, the organic pollutants in the sewage are degraded and removed, the activated sludge itself can multiply and grow, and the sewage can be purified and treated. The mixed liquid after the purification of activated sludge enters the secondary sedimentation tank, where the activated sludge and other solid substances suspended in the mixed liquid are precipitated and separated from the water, and the clarified sewage is discharged out of the system as treated water. The sludge that has undergone sedimentation and concentration is discharged from the bottom of the sedimentation tank, and most of it is returned to the aeration tank as inoculated sludge to ensure the concentration of suspended solids and microorganisms in the aeration tank; the proliferated microorganisms are discharged from the system, which is called "surplus" sludge". In fact, pollutants are largely transferred from sewage to these remaining sludge. 

The principle of the activated sludge method is a vivid statement: microorganisms "eat" the organic matter in the sewage, so that the sewage becomes clean water. It is essentially similar to the self-purification process of natural water bodies, but after artificial enhancement, the effect of sewage purification is better.

2. Biofilm method 

1. Type of biofilm process. 

Wetting type: biological filter, biological filter tower, biological turntable.

Immersion type: contact oxidation, the filter material is immersed in the filter tank.

Fluidized bed type: biological activated carbon, sand medium suspends and flows in the pool. 

2. Principle:

Since domestic sewage contains a large amount of organic components, the biofilm method relies on the microbial film fixed on the surface of the carrier to degrade organic matter. Because the microbial cells can adhere, grow and reproduce firmly on almost any suitable carrier surface in the water environment , The extracellular polymers extending from the inside of the cell make the microbial cells form a fibrous tangled structure, so the biofilm usually has a porous structure and has a strong adsorption performance. 

The biofilm is a highly hydrophilic substance attached to the surface of the carrier. Under the condition of continuous flow of sewage, there is always an attached water layer on the outside. 

Biofilm is a highly dense substance of microorganisms. A large number of microorganisms and micro-animals grow and multiply on the surface and inside of the film, forming a food chain consisting of organic pollutants → bacteria → protozoa (metazoa). 

Biofilm is composed of bacteria, fungi, algae, protozoa, metazoa, and other visible biological communities. When the sewage flows through the surface of the carrier, the organic pollutants in the sewage are adsorbed by the microorganisms in the biofilm, and diffuse into the biofilm through oxygen, and biological oxidation occurs in the film, thereby completing the degradation of organic matter. 

The surface layer of the biofilm grows with aerobic and facultative microorganisms, while the microorganisms in the inner layer of the biofilm are often in an anaerobic state. When the biofilm gradually thickens and the thickness of the anaerobic layer exceeds the aerobic layer, it will cause the biofilm The new biofilm will be regenerated on the surface of the carrier, and the biofilm will be renewed periodically to maintain the normal operation of the biofilm reactor. 

3. Renewal and shedding of biofilm.

An important part of maintaining the normal operation of the biofilm reactor is the renewal and shedding of the biofilm. Aerobic and facultative microorganisms grow on the surface of the biofilm, while the microorganisms in the inner layer of the biofilm are often in an anaerobic state. Thickening, when the thickness of the anaerobic layer exceeds the aerobic layer, it will cause the biofilm to fall off, and new biofilm will regenerate on the surface of the carrier. 

The update and fall-off process is as follows: 

First, the emergence of anaerobic membranes:

One is biofilm;

The second is that the thickness of mature biofilms generally increases, and the inner depths where oxygen cannot penetrate will transform into an anaerobic state; both are composed of anaerobic and aerobic membranes;

The third is that the aerobic membrane is the main site for the degradation of organic matter, and its thickness is generally 2 mm. 

Secondly, the thickening process of anaerobic membrane:

One is the increase in anaerobic metabolites, which leads to the breakdown of the balance between anaerobic and aerobic membranes;

The second is the continuous escape of gaseous products, which weakens the ability of the biofilm to adhere to the filler;

The third is to become an aging biofilm, its purification function is poor, and easy to fall off. 

Again, the update of biofilm:

First, the aging film falls off, and the new biofilm will grow again;

The second is the strong purification function of the new biofilm. 

3. Comparison of activated sludge method and biofilm method 

1)     Advantages and disadvantages of activated sludge method. 

For a long time, the secondary biological treatment of urban domestic sewage has mostly adopted the activated sludge method, which is currently the most widely used secondary biological treatment process in various countries in the world. It has the following characteristics: 

First, the traditional activated sludge method is often used, which often requires high capital construction costs, high operating costs, high energy consumption, complicated management, and sludge expansion. The process equipment cannot meet the requirements of high efficiency and low consumption. 

Second, as the sewage discharge standards continue to be strict, the requirements for the discharge of nutrients such as nitrogen and phosphorus in the sewage are relatively high. The traditional sewage treatment processes with the function of nitrogen and phosphorus removal are mostly based on the activated sludge method. Multiple anaerobic and aerobic reaction tanks are connected in series to form a multi-stage reaction tank. The purpose of nitrogen and phosphorus removal is achieved by increasing the internal circulation. This is bound to increase the cost of capital investment and energy consumption, and make operation and management more complicated. 

Third, the activated sludge method produces a large amount of excess sludge, which requires harmless treatment of the sludge, which increases investment. 

2)     The advantages and disadvantages of the biofilm method. 

The biofilm method is also a common method for the secondary biological treatment of municipal wastewater. Compared with the activated sludge method, it has the following characteristics: 

First, the biofilm has strong adaptability to changes in sewage water quality and water volume, and is easy to manage, and sludge expansion will not occur. 

The second is that the microorganisms are fixed on the surface of the carrier, and the microorganisms with a long generation time can also multiply, and the organisms are relatively richer and stable, and the remaining sludge is less. 

The third is the ability to treat low-concentration sewage. In addition, the disadvantage of the biofilm method is that the biofilm carrier increases the system investment; the specific surface area of the carrier material is small, the volume of the reaction device is limited, the space efficiency is low, and the treatment efficiency is lower than that of the activated sludge method when treating urban sewage; The amount of microorganisms on the solid surface is difficult to control, and the operation flexibility is poor; relying on natural ventilation to provide oxygen is not as good as activated sludge to provide sufficient oxygen, which is easy to produce anaerobic.