I. Overview

Anaerobic bioreactors are one of the primary equipment used for treating organic pollutants in wastewater through anaerobic microorganisms. They are characterized by low treatment costs (no need for aeration), ability to handle high-concentration organic pollutants wastewater, recoverable biogas, and small land area required (high volumetric load, tall equipment). As research deepens, the special effects of anaerobic bioreactors in treating challenging organic wastewater have also garnered significant attention.

The most widely used anaerobic bioreactor in the world is the UASB anaerobic bioreactor, which is also 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, featuring a relatively stable anaerobic biofilm, shows a disadvantage in 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, and their market share has been increasing year by year.

CASB (Patent No. ZL 200720037207.1) is also a new, high-efficiency anaerobic bioreactor developed on the basis of UASB, 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 for the same volume. However, EGSB typically has a massive "brain case" designed for gas, solid, and liquid phase separation. If this "brain case" is not large enough, the phase separation is ineffective, which adds significant construction challenges. EGSB also features an external recycle system, which fluidizes the anaerobic organisms inside but increases energy consumption. IC does not require a large "brain case" or an external recycle system, but it needs a taller design. This additional height is primarily for phase separation and for enabling internal recycle using biogas generated within the reactor, but it doesn't participate in the fluidization of anaerobic organisms, thus consuming some of the reactor's effective volume. CASB utilizes a special internal structure that eliminates the need for a large "brain case," external recycle system, or additional height, yet achieves superior fluidization and has a broader application range.

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

II. CASB Working Principle

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

As shown in Figure 1, Zone A in the CASB anaerobic bioreactor is the main reaction area. Here, influent water and anaerobic bacteria within the reactor are thoroughly mixed and react, making it the primary biogas-producing zone. In Zone A, the mixture of anaerobic bacteria and influent water moves upwards with the biogas, gradually purifying the water quality. By the time it reaches Zone B, most organic matter in the influent has been decomposed, resulting in a significant decrease in biogas production. In Zone B, the biogas produced in Zone A is separated and expelled through the biogas pipe, while the anaerobic bacteria and water flow, carrying a small amount of biogas, enter Zone C. Zone C is the secondary reaction area. Here, 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 Zone A, while the lighter ones, attached to a small amount of biogas, reach the three-phase separator with the effluent. During this process, the biogas is separated and expelled through the biogas pipe, the heavier anaerobic bacteria return to Zone C, and the lighter ones are carried out of the reactor with the effluent. In Zone 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 effluent.

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 mixed liquid density in the channel between Area A and B is substantially 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 are drawn back 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 rate 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. Consequently, the treatment efficiency can be more than double that of existing anaerobic biological reactors, and the investment can be reduced by over 50%.