1. Activated Sludge Method
Process and Principle:
The typical activated sludge method consists of an aeration tank, sedimentation tank, sludge recirculation system, and excess sludge discharge system.
Wastewater and recirculated activated sludge enter the aeration tank to form a mixed liquor. Compressed air, supplied by an air compressor station, is introduced into the wastewater in the form of fine bubbles through the air diffusion device placed at the bottom of the aeration tank. The purpose is to increase the dissolved oxygen content in the wastewater and to create turbulence in the mixed liquor, keeping it in suspension. The dissolved oxygen, activated sludge, and wastewater mix and come into sufficient contact, enabling the activated sludge to react properly.
In the first stage, organic pollutants in the wastewater are adsorbed onto the surface of microbial aggregates in the activated sludge due to their large surface area and the presence of polysaccharide viscous substances. Meanwhile, some large organic molecules are broken down into smaller organic molecules by the bacterial extracellular enzymes.
In the second stage, under sufficient oxygen conditions, microorganisms absorb these organic substances and oxidize them to form carbon dioxide and water. Some of the organic substances provide nutrients for the microorganisms to proliferate. As a result of the activated sludge reaction, organic pollutants in the wastewater are degraded and removed, the activated sludge itself proliferates, and the wastewater is purified. After the activated sludge purification, the mixed liquor enters the secondary sedimentation tank, where the suspended activated sludge and other solid materials settle, separating from the water. The clarified water is discharged as treated effluent. The concentrated sludge from the bottom of the sedimentation tank is discharged, with most of it being returned to the aeration tank as inoculated sludge to maintain the concentration of suspended solids and microorganisms in the aeration tank. The proliferated microorganisms are removed from the system as “excess sludge.” In fact, pollutants are largely transferred to these excess sludges from the wastewater.
Principle of the Activated Sludge Method (Simplified Explanation)
Microorganisms “consume” the organic substances in the wastewater, thereby converting the wastewater into clean water. It is essentially similar to the natural self-purification process of water bodies, but it is artificially enhanced, making the purification effect more efficient.
2. Biofilm Method
1. Types of Biofilm Method Processes
- Wetting Type: Biological filter beds, biofilter towers, biological rotating discs.
- Immersion Type: Contact oxidation, filter media immersed in the filter tank.
- Moving Bed Type: Bioactive carbon, sand particles suspended and flowing within the tank.
2. Principle
Since domestic wastewater contains a large amount of organic substances, the biofilm method relies on microbial films fixed on the surface of a carrier to degrade organic matter. Microbial cells can almost firmly attach, grow, and reproduce on any suitable surface in the aquatic environment. The extracellular polymers extending from the cells form fibrous tangled structures, so biofilms typically have a porous structure and strong adsorption properties.
The biofilm attaches to the surface of the carrier and is highly hydrophilic. Under conditions of continuous wastewater flow, a layer of attached water is always present on the outer surface of the biofilm.
Biofilms are highly concentrated with microorganisms. On the surface and within the biofilm, a large number of microorganisms and microfauna grow and reproduce, forming a food chain composed of organic pollutants → bacteria → protozoa (metazoans).
The biofilm consists of bacteria, fungi, algae, protozoa, metazoans, and other visible biological communities. As wastewater flows over the carrier surface, the organic pollutants in the wastewater are adsorbed by the microorganisms in the biofilm. Oxygen diffuses into the biofilm, where biological oxidation and other processes occur, leading to the degradation of organic matter.
In the biofilm, aerobic and facultative anaerobic microorganisms grow on the outer layer, while microorganisms in the inner layer are often in an anaerobic state. As the biofilm gradually thickens, if the thickness of the anaerobic layer exceeds that of the aerobic layer, it may cause the biofilm to shed. A new biofilm will then regenerate on the surface of the carrier. Through the periodic renewal of the biofilm, the biofilm reactor’s normal operation is maintained.
3. Renewal and Shedding of the Biofilm
An important aspect of maintaining the normal operation of a biofilm reactor is the renewal and shedding of the biofilm. The outer layer of the biofilm consists of aerobic and facultative anaerobic microorganisms, while the inner layer of the biofilm typically contains anaerobic microorganisms. As the biofilm gradually thickens, if the thickness of the anaerobic layer exceeds that of the aerobic layer, it will lead to the shedding of the biofilm, and a new biofilm will regenerate on the carrier surface.
The renewal and shedding process is as follows:
First, the process of anaerobic biofilm formation:
- Biofilm formation: The initial formation of a biofilm.
- Mature biofilm: As the biofilm matures, its thickness continues to increase. The interior, which is unable to receive oxygen, will shift to an anaerobic state. The biofilm consists of both anaerobic and aerobic layers.
- Aerobic biofilm: This is the primary site for organic matter degradation, with a typical thickness of about 2 mm.
Next, the thickening of the anaerobic biofilm:
- The accumulation of anaerobic metabolic products disrupts the balance between the anaerobic and aerobic layers.
- The continuous release of gaseous products weakens the biofilm’s attachment ability to the filler material.
- The biofilm becomes an aging biofilm with poor purification efficiency and is more prone to shedding.
Finally, the renewal of the biofilm:
- The aging biofilm sheds, and a new biofilm begins to grow.
- The newly-formed biofilm has stronger purification capabilities.