Design Considerations for MBR in Wastewater Treatment

MBR membrane technology is increasingly widely used, and its stable and clear effluent has attracted much attention. However, the huge maintenance costs also worry many users; therefore, to minimize maintenance workload during use, the following issues should be considered during the design phase.

I. Is this wastewater suitable for use? MBR membrane technology

MBR is not a panacea. It belongs to microfiltration membrane and is defined by the particle size that can pass through. Therefore, its clogging problem is the key. For wastewater with easy scaling, oily substances, and high viscosity, it is recommended not to use MBR membrane technology.

Unsuitable wastewater types for MBR technology include: emulsion/grinding liquid/quenching liquid/coolant wastewater, surfactant wastewater, petroleum wastewater, and lipid wastewater (except with pretreatment measures).

II. Design of MBR membrane modules

1. Assemble the membrane sheets together to form a membrane module.

The distance between the membrane sheets should be large enough, with an effective distance greater than 100mm (axial distance greater than 140mm). If the membrane sheet itself has a high membrane fiber density, the effective distance should be appropriately increased. This is to ensure that the flushing airflow smoothly reaches the top membrane fibers, reduce the bonding and retention of substances between the membrane fibers, and reduce the cleaning frequency of the membrane module. The spacing of the flat membrane only needs 60-80mm. Too large a spacing will lead to excessive space occupation;

2. The membrane sheet can be installed horizontally or vertically, depending on the installation space.

During horizontal installation, the membrane fibers are slightly drooping, with a droop of 10mm. While ensuring that the membrane fibers are not under tension, they should be as straight as possible to prevent too much debris from accumulating between the fibers. Vertical installation is recommended.

3. The membrane module volume should not be too large.

Because a large membrane module will result in high installation density, the same amount of stirred air will appear insufficient. When a lot of wrapping materials accumulate on the membrane sheet, the membrane sheet needs to be sprayed and cleaned with a high-pressure water gun or tap water. If the installation is too dense, it is difficult to flush the inner membrane sheet. It is recommended that the treatment capacity of a single membrane module should not exceed 1.5m³/hr.

III. Design points for MBR aeration devices

1. The aeration device can be fixed to the bottom of the tank (requires a membrane module support frame and membrane module sliding guide rail), or it can be integrated with the membrane module. Each has its advantages and disadvantages. The position of the aeration pipe should be carefully considered. DN20 perforated pipes are used, with one perforated pipe for each membrane sheet gap. The hole size is Φ2.0mm, and the hole spacing is 100mm. The perforations of adjacent pipes are staggered, and the holes are arranged in a single row vertically upwards. Many double-row and downward-sloping designs are not advisable, as the settling sludge will not block the holes.

2. Rough estimation of aeration volume. According to experience, the air-water ratio is 24:1 (conventional tank depth 3.5m). The wind pressure head of the blower is selected to be 0.01Mpa higher than the highest liquid level; a vent valve is set at the blower outlet, and the vent pipe diameter can fully discharge 70% of the air volume. A silencer is installed on the vent, and this device is used to control the DO value in the bioreactor and protect the blower.

3. Each membrane module aeration is equipped with a separate regulating valve, and the oxygenation aeration of the entire bioreactor should be equipped with a separate control valve, using a microporous oxygenation aeration device to ensure flexible adjustment of the stirred air volume and oxygenated air volume.

4. The optimal DO control of the MBR tank is between 2.5 and 5. The normal liquid level is about 3ppm. When the liquid level is different, the DO will also change, and it should not exceed 5.0ppm for a long time.

IV. Design points for MBR suction pumps

1. If possible, each membrane module should be equipped with one pump to facilitate observation and judgment of the status (pressure and flux) of each membrane module. Multiple membrane modules can share one pump, but a flow meter should be installed on the water intake pipeline of each membrane module.

2. The suction pump should be installed as low as possible below the liquid level. Under normal conditions of the membrane module, water can be discharged by siphon. If the MBR tank is an underground tank, an underground equipment room can be built to ensure that the suction pump has sufficient suction head.

The main parameter of MBR is the transmembrane pressure difference, which varies slightly among different brands. It should generally not exceed 0.03Mpa. Here, the transmembrane pressure difference is not necessarily equal to the vacuum gauge reading. It also depends on the height difference between the pump inlet and the bioreactor liquid level. If the vacuum gauge reading is 0.03pa, and the bioreactor liquid level is 1m higher than the pump inlet, then the transmembrane pressure difference is 0.04Mpa;

If the pump installation position is 1m higher than the bioreactor liquid level, then the transmembrane pressure difference is only 0.02Mpa. Formula: Transmembrane pressure difference = vacuum gauge reading (positive) + (bioreactor liquid level height - suction pump inlet height).

3. A transparent flow meter and sampling valve must be installed on the outlet pipeline of the suction pump. The transparent flow meter can directly observe the water quality status. A regulating valve is added before or after each flow meter to adjust the effluent of the membrane module.

4. Electrical control: Generally, the MBR suction pump is set to run for 13 minutes and stop for 2 minutes, which can effectively reduce the frequency of clogging. When the pressure of the electrical contact pressure gauge exceeds the limit, the pump can stop and alarm; the suction pump should be able to interlock with the blower, and the suction pump will not work when the blower is stopped.

V. Chemical soaking cleaning

To reduce workload and enable cleaning of the entire membrane module, precise positioning for module insertion and removal is crucial. Water and air pipes should use convenient, durable quick-connect fittings, such as high-quality flange connections or well-known brand double union ball valves. A hoisting mechanism for the membrane module will effectively reduce labor intensity; a 500kg hoist (capable of lifting 1t) is recommended.

Three chemical immersion tanks are required, each large enough to comfortably accommodate a membrane module. The height should allow for complete submersion of the membrane fibers with an additional 500mm clearance. Each tank should have perforated aeration piping and a protective platform.

Total immersion tank depth = Bottom platform height + Height from membrane module bottom to topmost fiber + 500mm clearance.

Two storage tanks should be located next to the three immersion tanks. Their capacity should exceed the effective volume of the immersion tanks to allow for reuse of cleaning solutions.

Each immersion tank should be equipped with one plastic wastewater pump to transfer the solution from the tank to the storage tank or for discharge.

Consider the disposal method for wastewater. NaOH can be added to the system as a reagent. NaClO can be discharged directly after clarification or stored for reuse. Citric acid can be gradually added to the biological treatment system.

The air agitation volume for each immersion tank should be designed for vigorous mixing and equipped with a regulating valve.

Each immersion tank should have a tap water inlet pipe with a large diameter to avoid wasting time during water filling. The filling time should be around 10 minutes. With a water pressure of 2-3 kg, a DN50 tap water pipe has a flow rate of approximately 18-22 m³/hr.

Common chemical cleaning agents and concentrations:

NaOH (for sterilization and cleaning organic pollutants): Concentration 1%~2%, immersion time >2h;

Citric acid (for removing inorganic scaling, omit if not needed): Concentration 2%, immersion time >2h;

NaClO (10% liquid, for deep sterilization and restoring membrane fiber filtration function): Concentration 5%, immersion time >2h;

Alcohol ( 95% industrial alcohol) Single-piece immersion for 2 minutes, to restore dehydrated membrane fibers; omit if not dehydrated;

Cleaning steps: Water rinse → Water immersion → Alkali immersion → Citric acid immersion → NaClO immersion → Water rinse → Reset.

Citric acid, being an organic acid, has unrestricted use. However, if storage exceeds one month, it may mold and deteriorate; single-use is recommended.

After each cleaning, inspect the membrane fibers for breakage. Repair broken fibers individually by tying knots.

VI. Online Backwashing

MBR backwashing differs from traditional backwashing. Normal MBR clogging is mainly caused by microbial growth within the membrane fibers, while hard clogging from particulate matter accounts for a small percentage.

Online backwashing addresses abnormal clogging. For example, if sludge conditions deteriorate, the MBR suction pump flow rate is mistakenly set too high, or small particles enter, causing hard clogging, backwashing is still effective.

Online backwashing is automatically controlled by a PLC, once daily (provided the purchased membrane supports online backwashing). Backwashing water should be at least tap water, filtered through a 50μm filter at the end. The backwashing water volume is approximately 3-5 times the normal filtration flow rate. Pressure should not exceed 2.5 kg to avoid membrane fiber damage. It can be directly connected to the tap water line without a booster pump, but a pressure gauge and flow meter are essential.