I. Overview

Anaerobic bioreactors are one of the primary equipment used for treating organic pollutants in wastewater through anaerobic microorganisms. They are characterized by their low treatment costs (no need for aeration), ability to handle high concentrations of organic pollutants, recover biogas, and occupy minimal space (high volume load, tall equipment). As research advances, the special effectiveness of anaerobic bioreactors in treating difficult organic wastewater has garnered significant attention.

The most widely used anaerobic bioreactor in the world is the UASB (Upflow Anaerobic Sludge Blanket) bioreactor. Known as the second-generation anaerobic bioreactor, it is characterized by mature technology and simple manufacturing. However, with the application of fluidized bed reaction theory, the UASB bioreactor, which features a relatively stable anaerobic biomass bed, shows a disadvantage of low reaction efficiency. In contrast, mainstream third-generation bioreactors like EGSB and IC, which also utilize the fluidized bed reaction theory, have significantly advanced the application scope and reaction efficiency of anaerobic bioreactors, with their market share increasing year by year.

CASB (Patent No. ZL 200720037207.1) is also a novel, high-efficiency anaerobic bioreactor developed on the basis of UASB technology, and it represents an improvement over the third-generation anaerobic bioreactors like EGSB and IC. Visually, CASB, EGSB, and IC are all taller than UASB, so they occupy less floor space in the same volume. However, EGSB typically features a massive "braincase" which is essential for the separation of gas, solid, and liquid phases. If the "braincase" is not sufficiently large, the separation effectiveness is compromised, posing significant construction challenges. EGSB also has an external reflux system that fluidizes the anaerobic biomass within the reactor but increases energy consumption. IC does not require a large "braincase" or an external reflux system but needs a taller structure to provide phase separation and facilitate internal reflux using biogas generated by the reactor itself. This taller section, however, does not participate in the biomass fluidization process, thus consuming some of the reactor's effective volume. CASB utilizes a unique internal structure that eliminates the need for a large "braincase," external reflux system, or additional height, yet it achieves superior fluidization and has a broader application scope.

Therefore, our company plans to use a CASB reactor to treat this wastewater.

Section 2: CASB Working Principle

As shown in Figure 1, the A zone in the CASB anaerobic bioreactor is the main reaction area. The incoming water is thoroughly mixed and reacts with the anaerobic bacteria within this zone, making it the primary biogas-producing area. In the A zone, the mixture of anaerobic bacteria and incoming water moves upwards with the biogas, gradually purifying the water quality. By the time it reaches the B zone, most of the organic matter in the incoming water has been decomposed, resulting in a significant decrease in biogas production. In the B zone, the biogas produced in the A zone is separated and expelled through the biogas pipe, while the anaerobic bacteria and water flow, carrying a small amount of biogas, enter the C zone. The C zone is the secondary reaction area, where the organic matter in the water is further decomposed by the anaerobic bacteria, with a small amount of biogas produced. The heavier anaerobic bacteria fall directly into the A zone, while the lighter ones, attached to a small amount of biogas, reach the three-phase separator with the effluent. Upon passing through the separator, the biogas is separated and expelled through the biogas pipe, the heavier anaerobic bacteria return to the C zone, and the lighter ones are carried out of the reactor with the effluent. In the D zone, the heavier anaerobic bacteria form an unstable anaerobic bed to continue decomposing organic matter, while the lighter ones are discharged from the reactor with the effluent.

As shown in Figure 1, Area A in the CASB anaerobic bioreactor is the main reaction zone, where incoming water and anaerobic bacteria in the reactor are thoroughly mixed and react, making it the primary biogas-producing area. In Area A, the mixture of anaerobic bacteria and incoming water moves upwards with the biogas, gradually purifying the water quality. By the time it reaches Area B, most of the organic matter in the incoming water has been decomposed, and the biogas production significantly decreases. In Area B, the biogas produced in Area A is separated and released through the biogas pipe, while the anaerobic bacteria and water flow, carrying a small amount of biogas, enter Area C. Area C is the secondary reaction zone, where the organic matter in the water is further decomposed by the anaerobic bacteria, with a small amount of biogas produced. The heavier anaerobic bacteria directly fall back into Area A, while the lighter ones, attached to a small amount of biogas, reach the three-phase separator with the outflow. As it passes through the separator, the biogas is separated and expelled through the biogas pipe, the heavier anaerobic bacteria return to Area C, and the lighter ones are carried out with the outflow to Area D. In Area D, the heavier anaerobic bacteria form an unstable anaerobic bed to continue decomposing organic matter, while the lighter ones are discharged from the reactor with the outflow.

As shown in Figure 1, there are vertical channels between Area A and B, Area B and C, and Area C and A, respectively. Among the three channels, the methane content in the channel between Area A and B is significantly higher than the other two channels. Consequently, the mixture density in the channel between Area A and B is much lower than the other two channels. This creates a pressure difference between Area A and C, with the pressure in Area C being greater than that in Area A. As a result, anaerobic bacteria and water from Area C return to Area A, thereby forming an internal circulation from Area A to B, B to C, and C back to A.

The internal circulation increases the upward flow velocity in Zone A, thereby expanding the contact area between organic matter and anaerobic bacteria, accelerating the rate of water purification. It dilutes the incoming water concentration, reducing the concentration gradient of organic acids within the reactor, and improves the survival environment for anaerobic bacteria, enhancing their degradation rate. Due to the clever design of the CASB, its internal circulation volume can be several times greater than that of existing anaerobic reactors. Additionally, the effluent water quality is high and stable, allowing for a single-stage achievement of the designed anaerobic effluent requirements without the need for multi-level settings. As a result, the treatment efficiency can be more than double that of existing anaerobic biological reactors, with investment reduced by over 50%.