Analysis of the principles advantages and disadvantages of MBR process

Principles Advantages and Disadvantages of the MBR Process

1. Introduction to the MBR Process

The MBR process embodies the water-saving concept of "treatment and reuse". The MBR (Membrane Bioreactor) process is developed by combining the traditional biological treatment process with membrane separation technology. It consists of two parts: biological treatment and membrane treatment.

The biological treatment part includes anoxic tanks and aerobic tanks; the membrane treatment part includes membrane tanks. The MBR membrane separation technology adopts ultrafiltration to replace the sedimentation tank in traditional biological treatment. It has a good solid-liquid separation effect and provides a reliable guarantee for solving the problem of reclaimed water quality.

The working principle of the MBR process is as follows: first, activated sludge is used to remove biodegradable organic pollutants in water, and then membranes are used for solid-liquid separation between the purified water and the activated sludge.

Hollow fiber membrane filaments are tubular with micro-pores on the tube wall, which can retain activated sludge and most suspended solids, resulting in clear and transparent effluent. To ensure the long-term continuous and stable operation of the membrane, a certain amount of aeration is required below the membrane. This not only meets the biological oxygen demand but also keeps the membrane filaments vibrating continuously, preventing activated sludge from adhering to the membrane surface and causing pollution.

2. Classification of MBR Processes

A membrane bioreactor is mainly composed of two parts: a membrane module and a bioreactor. Based on the combination mode of the membrane module and the bioreactor, membrane bioreactors can be divided into three types: separate membrane bioreactors, integrated membrane bioreactors, and hybrid membrane bioreactors.

2.1 Separate Membrane Bioreactor

In a separate membrane bioreactor, the membrane module and the bioreactor are set separately and operate relatively independently. They are connected by pumps and pipelines. 

In this process, the membrane module and the bioreactor are separated and operate independently, so mutual interference is minimal, and regulation and control are easy. Moreover, since the membrane module is placed outside the bioreactor, cleaning and replacement are more convenient. However, it has high power consumption. A booster pump is required to provide high pressure to create high-speed cross-flow on the membrane surface, which delays membrane fouling—this is the reason for its high power cost. The energy consumption per ton of effluent is 2–10 kWh, which is about 10–20 times that of the traditional activated sludge process. Therefore, research on integrated membrane bioreactors with lower energy consumption has gradually attracted people's attention.

2.2 Integrated Membrane Bioreactor

The integrated membrane bioreactor originated in Japan and is mainly used for domestic sewage treatment. In recent years, some European countries have also shown great interest in its research and application. In an integrated membrane bioreactor, the membrane module is directly placed in the bioreactor, and it is sometimes called a submerged membrane bioreactor (SMBR). The effluent is driven by negative pressure generated by gravity or pump suction, or by a vacuum pump. 

In this process, the membrane module is placed inside the bioreactor, which reduces the floor area of the treatment system. Additionally, the process uses a suction pump or vacuum pump for effluent extraction, so the power consumption cost is much lower than that of the separate membrane bioreactor— the power consumption per ton of effluent is about 1/10 of that of the separate type. If gravity-driven effluent is adopted, this part of the cost can be completely saved. However, since the membrane module is immersed in the mixed liquid of the bioreactor, fouling occurs relatively quickly, and cleaning is more troublesome, as the membrane module needs to be taken out of the reactor.

2.3 Hybrid Membrane Bioreactor

In a hybrid membrane bioreactor, the membrane module is also placed in the bioreactor, and effluent is discharged by gravity or negative pressure, but the type of bioreactor is different. A hybrid MBR is a hybrid treatment system where fillers are installed in the bioreactor. 

The installation of fillers in the hybrid membrane bioreactor has two purposes: first, to improve the impact load resistance of the treatment system and ensure the treatment effect of the system; second, to reduce the concentration of suspended activated sludge in the reactor, reduce the degree of membrane fouling, and ensure a high membrane flux.

In the hybrid membrane bioreactor, a large number of microorganisms attach and grow on the fillers, which ensures that the system has a high treatment effect and the ability to resist impact loads. At the same time, it prevents the concentration of suspended sludge in the reactor from being too high, which would affect the membrane flux.

3. Characteristics of the MBR Process

3.1 Small Floor Area, Space-saving

When treating high-concentration wastewater through biological treatment, the higher the treatment concentration, the larger the size of the treatment tank required. With the MBR process, due to the high sludge concentration, the system can operate under high load, thus significantly saving the floor area.

3.2 Stable Effluent Quality and High Transparency

Hollow fiber membranes can retain almost all microorganisms, especially those that are difficult to settle and have a slow reproduction rate. Therefore, the biological community in the system is highly diverse, the domestication and proliferation process of activated sludge is greatly shortened, the treatment depth and the system's impact resistance are enhanced, and the effluent quality is very stable.

3.3 Convenient Operation and Management, Simple Maintenance

In the traditional aerobic activated sludge treatment process, sludge bulking may occur when operating under high sludge load, leading to abnormal system operation and substandard effluent. In the MBR process, solid-liquid separation is achieved through membrane suction. Therefore, the impact of sludge bulking on the MBR effluent is much smaller than that on the traditional process, making operation and management very convenient.

It has a high degree of automation and simple maintenance.

3.4 Long Sludge Retention Time (SRT)

Membrane separation ensures that macromolecular refractory components in sewage have sufficient residence time in the bioreactor with a limited volume, which greatly improves the degradation efficiency of refractory organic matter. The reactor operates under high volume load, low sludge load, and long SRT, enabling almost no excess sludge discharge. Due to the long SRT, it is more suitable for the growth of microorganisms with a long generation time, which is beneficial for removing refractory organic matter in sewage.

3.5 Low Power Consumption

The suction pressure required for hollow fiber membranes is only about -0.1 to -0.4 kg/cm², resulting in low power consumption. Generally, no sludge reflux is needed.

3.6 Strong Impact Resistance

When the influent volume changes significantly in a short period, the membrane throughput can be increased for a short time to alleviate the impact. When the influent quality changes, the high sludge concentration can also help alleviate the impact within a certain range.

4. Analysis of Advantages and Disadvantages of the MBR Process

4.1 Advantage: No Sludge Bulking

In the MBR process, the BOD sludge load is low, and the sludge is in a high endogenous respiration phase. After endogenous metabolism of bacteria, only inert residues remain, resulting in a small amount of sludge production. The sludge yield of the MBR reactor is lower than that of the traditional activated sludge process. The sludge yield of the traditional activated sludge process is 0.5–1.0 KgMLSS/KgBOD, while that of the MBR process is only 0.1–0.3 KgMLSS/KgBOD. The low BOD sludge load and long SRT inhibit the proliferation of filamentous bacteria, solving the sludge bulking problem of the traditional activated sludge process (Adham).

4.2 Applicable Scenarios

(1) Projects with strict requirements on effluent quality;  

(2) Companies and projects that have requirements for greening and aesthetics;  

(3) Reconstruction projects with limited floor area;  

(4) Areas with strict requirements on effluent quality.