MBR membrane bioreactor integrated equipment

Characteristics of MBR membrane bioreactors

1. High pollutant removal rate, strong resistance to sludge expansion, stable and reliable effluent water quality, and no suspended solids in the effluent;

2. The membrane bioreactor achieves separate control of the sludge retention time (SRT) and hydraulic retention time (HRT) of the reactor, thus simplifying its design and operation;

3. The mechanical interception of the membrane prevents the loss of microorganisms, allowing the bioreactor to maintain a high sludge concentration, thereby increasing the volumetric load, reducing the sludge load, and exhibiting strong shock resistance;

4. Due to the long SRT, the bioreactor also acts as a "sludge nitrification tank," significantly reducing sludge production, resulting in low excess sludge production and low sludge treatment costs;

5. Due to the interception of the membrane, the SRT is extended, creating an environment conducive to the proliferation of slowly growing microorganisms, such as nitrifying bacteria, which can improve the nitrification capacity of the system and improve the efficiency of treating refractory macromolecular organic matter and promote its complete decomposition;

6. The activated sludge in the MBR aeration tank will not be lost with the effluent. During operation, the activated sludge will change due to changes in the concentration of incoming organic matter and reach a dynamic equilibrium, making the effluent stable and resistant to shock loads;

7. The large hydraulic circulation leads to uniform mixing of the wastewater, resulting in good dispersion of the activated sludge and significantly increasing the specific surface area of the activated sludge. The high dispersion of activated sludge in the MBR system is another reason for improving the water treatment effect. This is difficult to compare with the formation of large bacterial flocs in ordinary biochemical water treatment technology;

8. Membrane bioreactors are easy to integrate, easy to automate, and convenient to operate and manage;

9. The MBR process omits the secondary sedimentation tank, reducing the footprint.

Application of membrane bioreactors in the treatment of nitrogen fertilizer wastewater

Membrane bioreactors are a new and efficient wastewater treatment technology that combines membrane separation and biological treatment. The denitrification mechanism of industrial nitrogen-containing wastewater includes two basic processes: nitrification and denitrification. Nitrification refers to the process of converting ammonia nitrogen into nitrate nitrogen, which is mainly accomplished by two types of aerobic autotrophic bacteria: nitrite bacteria and nitrate bacteria.

MBR membrane technology first uses activated sludge to remove biodegradable organic pollutants in water, and then uses membrane modules to forcibly retain activated sludge and most suspended solids in the bioreactor, achieving solid-liquid separation of purified water and activated sludge, thereby enhancing the biochemical reaction and improving the wastewater treatment effect and effluent water quality.

The MBR treatment process has a significant effect on COD treatment in nitrogen fertilizer wastewater. The cultivation and domestication stage of its biochemical strains is short, showing stable effects from the 3rd day; both the stable treatment stage and the normal operation stage can maintain a high COD removal rate, with the removal rate basically above 90%; the average outlet COD is controlled below 30 mg/L.

Advantages of membrane bioreactors

1. High treatment efficiency, effluent can be directly reused. Due to the high-efficiency separation effect of the hollow fiber membrane on the mixed liquid of the bioreactor, the sludge and effluent can be completely separated, so the SS and turbidity of the effluent can be close to zero. At the same time, since the loss of activated sludge is almost zero, the concentration of activated sludge in the bioreactor can be 2-6 times higher than that of traditional processes, greatly improving the nitrogen removal capacity.

2. Stable system operation, simple process, less equipment, small footprint. Due to the high concentration of activated sludge in MBR technology, the device has a large volumetric load; it has good shock resistance to influent fluctuations and stable operation. This process not only greatly reduces the volume of the bioreactor-aeration tank, miniaturizing the equipment and structures, but also eliminates the need for a primary sedimentation tank and a secondary sedimentation tank, reducing the footprint of the system.

3. Long sludge age, small amount of excess sludge. When the sludge concentration is high and the influent load is low, the nutrient-to-microorganism ratio (F/M) in the system is low, and the sludge age becomes longer. When F/M maintains a certain low value, the growth of activated sludge is close to zero, which reduces the cost of treating excess sludge.

4. Convenient operation and management, easy to achieve automatic control. Since membrane separation can completely retain activated sludge in the bioreactor, the hydraulic retention time (HRT) and sludge retention time (SRT) in the bioreactor are completely separated, so they can be flexibly and stably controlled; at the same time, it is very easy to achieve automatic control, improving the automation level of wastewater treatment.

Basic classification of MBR

Separate MBR refers to the separate setting of the membrane module and the bioreactor. In the separate MBR, the mixed liquid of the bioreactor is pressurized by a pump and then enters the membrane module. Under pressure, the membrane filtrate becomes the system's treated effluent, while the activated sludge and macromolecules are retained by the membrane and returned to the bioreactor. The membrane modules used in separate MBR are generally plate-type and tubular-type.

Separate MBR operates through liquid circulation cross-flow. Its characteristics are: stable and reliable operation, easy operation and management, and easy membrane cleaning, replacement, and addition. However, in order to reduce the deposition of pollutants on the membrane surface, the liquid flow rate provided by the circulation pump is very high, resulting in high power consumption.

Integrated MBR refers to the membrane module being placed inside the bioreactor. According to the requirements of the biological treatment process, it can be divided into two forms: the first has two bioreactors, one is a nitrification tank and the other is a denitrification tank. The membrane module is immersed in the nitrification reactor, and the mixed liquid to be filtered is updated between the two tanks by a pump.

Integrated MBR utilizes the shear force of upward gas-liquid flow during aeration to achieve cross-flow effect on the membrane surface. Some also use impeller stirring near the integrated membrane module and the rotation of the membrane module itself (such as rotary membrane modules) to achieve cross-flow effects on the membrane surface. Compared with separate systems, the biggest advantage of integrated systems is low energy consumption. Some scholars believe that integrated systems are inferior to separate systems in terms of operational stability, operation and management, and cleaning and replacement.

Membrane technology combined with biological reactors for wastewater treatment has developed into three types of membrane bioreactors: those used for solid separation and retention (membrane separation bioreactors), those used for bubble-free aeration in the reactor (membrane-aeration bioreactors), and those used to extract priority pollutants from industrial wastewater (extractive membrane bioreactors).

Integrated MBR refers to the membrane module being placed inside the bioreactor. According to the requirements of the biological treatment process, it can be divided into two forms: the first has two bioreactors, one is a nitrification tank and the other is a denitrification tank. The membrane module is immersed in the nitrification reactor, and the mixed liquid to be filtered is updated between the two tanks by a pump.

Integrated MBR utilizes the shear force of upward gas-liquid flow during aeration to achieve cross-flow effect on the membrane surface. Some also use impeller stirring near the integrated membrane module and the rotation of the membrane module itself (such as rotary membrane modules) to achieve cross-flow effects on the membrane surface. Compared with separate systems, the biggest advantage of integrated systems is low energy consumption. Some scholars believe that integrated systems are inferior to separate systems in terms of operational stability, operation and management, and cleaning and replacement.

Membrane aeration bioreactors (MABR), bubble-free MBRs, were first reported by Cote.P in 1988. They use gas-permeable dense membranes (such as silicone rubber membranes) or microporous membranes (such as hydrophobic polymer membranes), in plate or hollow fiber modules, to achieve bubble-free aeration to the bioreactor while maintaining gas partial pressure below the bubble point. Because the gas being transferred is contained within the membrane system, the contact time is increased, greatly improving oxygen transfer efficiency. Since the gas and liquid phases are separated by the membrane, this facilitates better control of the aeration process and effectively separates the aeration and mixing functions. Because the oxygen supply area is fixed, this process is not affected by factors such as bubble size and residence time in traditional aeration systems. Subsequently, Keith Brndle et al. in the UK conducted further research, such as using spiral silicone rubber tubes for bubble-free aeration in sequencing batch biofilm reactors, achieving high-efficiency aeration.

Extractive membrane bioreactors (EMBR) combine membrane extraction and biodegradation. They use membranes to extract toxic, poorly soluble priority pollutants from toxic industrial wastewater, and then use specific bacteria to biodegrade them separately. This protects the specific bacteria from the effects of ionic strength and pH in the wastewater, optimizing the function of the bioreactor. Currently, membrane aeration bioreactors and extractive membrane bioreactors are still in the laboratory stage and have not yet been applied in practical engineering.

Biomass separation membrane bioreactors (BSMBR, or MBR) use membrane modules as a substitute for the secondary sedimentation tank in traditional biological treatment systems. They use membrane modules for solid-liquid separation, with the intercepted sludge being returned to the bioreactor and the permeate being discharged. Currently, membrane separation bioreactors are widely used in practical engineering.