MBR membrane technology is gaining wider application, attracting attention for its stable and clear effluent. However, the maintenance workload can be a headache for many users. To minimize maintenance efforts during operation, several issues should be addressed during the design phase.
01. Is this wastewater suitable for the MBR membrane process?
MBR is not that; it is a microfiltration membrane, defined by the particle size it can allow to pass through. Therefore, for it, clogging issues are crucial. It is advisable not to use the MBR membrane method for wastewater with high levels of scaling, oil-containing substances, and sticky materials.
Wastewaters unsuitable for the MBR process include: emulsion/washing fluid/annealing fluid/coolant wastewater, surfactant wastewater, petroleum-based wastewater, and lipid wastewater (except for those with pretreatment measures).
02. The brand and corresponding quantities for the MBR have been determined.
Once the decision to use an MBR membrane is made, the next step is to choose a brand. Currently, there's a significant gap in quality between imported and domestically produced MBR membranes. While domestic brands can meet the needs by increasing the number of membrane fibers and reducing the design flux, corresponding issues arise one after another.
On the price front, imported membranes are about 3 to 5 times the cost of domestic ones. Provided the price is acceptable, it's best to opt for imported membranes; Mitsubishi and Seiko's films are quite good. Of course, domestic membranes can still be adequate if there's sufficient design margin. The key is for designers to have in-depth technical communication with suppliers. In terms of design flux, domestic membrane manufacturers typically already leave sufficient margin upon factory shipment. However, designers should still add more margin, with a personal belief that 50% or more is advisable. For instance, the author once designed a small wastewater treatment system with a domestic MBR brand, but the MBR design was 10 times the standard amount. To this day, after two years, it hasn't been cleaned and has been operating normally, with the effluent COD maintained below 30 mg/l.
The newly released flat membrane has superior anti-pollution performance compared to woven membranes. Flat membranes are categorized into PVDF and PTFE materials, with PTFE exhibiting strong anti-pollution properties, currently available only in imported products, with Japanese performance standards. The membrane design's flux reference is based on data provided by the supplier.
03. Design of MBR membrane components
1. Assemble the membrane sheets to form a membrane module, with special attention to the spacing between the membrane sheets, which should be sufficiently large, with an effective distance greater than 100mm (axial distance greater than 140mm). If the membrane itself has a high membrane fiber density, the effective spacing should be appropriately widened. The purpose of this is to ensure that the flushing airflow reaches the top membrane fibers smoothly, and it can also reduce the caking and retention between the membrane fibers, thereby reducing the cleaning frequency of the membrane module; for flat membrane sheets, a spacing of 60 to 80mm is sufficient; a larger spacing would result in excessive space occupation.
2. The membrane can be installed horizontally or vertically, depending on the available space. When installed horizontally, the membrane filaments should hang slightly, with a droop of 10mm. In other words, without applying tension, keep the filaments as straight as possible to prevent excessive debris between them. We recommend the vertical installation method.
3. Membrane components cannot be made too large, as larger components have a higher installation density. For the same volume of stirred air, they may appear insufficient. Additionally, when a significant amount of debris accumulates on the membrane sheets, it becomes necessary to rinse the membranes, using either a high-pressure water jet or tap water. Installing them too densely makes it difficult to reach the inner membrane layers. It is recommended that the individual membrane component's processing capacity not exceed 1.5 m³/hr.
Design Key Points of the MBR Aeration Equipment
1. Aeration devices can be mounted on the pool bottom (requiring a membrane component support bracket and a sliding guide rail for the membrane component) or integrated with the membrane component, each approach having its own advantages and disadvantages. The placement of the aeration tubes must be carefully considered. DN20 perforated pipes are used, with each membrane sheet gap corresponding to a single perforated pipe. The perforation size is φ2.0mm, and the spacing between perforations is 100mm. The perforation positions of adjacent pipes are staggered and interlaced. The holes are arranged in a single row, vertically upward. Many double-row and downward-sloping arrangements are considered inadvisable. Sedimentation sludge will not block the holes.
2. A rough estimate of the aeration volume can be made based on experience: a gas-to-water ratio of 24:1 is sufficient (with a standard pool depth of 3.5 meters). The fan's discharge head selection should be 0.01 MPa higher than the liquid level; a vent valve is installed at the fan outlet. The vent pipe diameter should be fully open to release 70% of the air volume. A silencer is added to the vent outlet. This device is used to control the dissolved oxygen (DO) level in the biochemical reactor and to protect the fan.
Each membrane module aeration is equipped with a separate regulating valve, and a separate control valve is also required for the aeration and oxygenation of the entire biochemical tank. A microporous aeration device is used to ensure the flexibility of adjusting the mixing air volume and the oxygenation air volume.
The mbr pool's DO control is set between 2.5-5, with a normal liquid level of approximately 3ppm. The DO level may vary with the liquid level, and it is not advisable to exceed 5.0ppm for extended periods.
Key Design Points of the 05.mbr Sump Pump
Under optimal conditions, it is preferred to match each membrane module with one pump for ease of observation and assessment of the module's status (pressure and flux). However, it is also acceptable to have multiple membrane modules sharing one pump, with a flowmeter installed on the suction piping of each module.
2. Install the suction pump as low as possible below the liquid level; the lower, the better. In the normal state of the membrane module, water can also be drawn through siphon. If the MBR tank is an underground tank, then construct an underground machine room to ensure that the suction pump has sufficient suction head. The main parameter below the MBR is the trans-membrane pressure difference, which varies slightly by brand, generally not exceeding 0.03 MPa. Here, the trans-membrane pressure difference may not be equal to the vacuum gauge reading and depends on the height difference between the pump's intake and the biochemical pool liquid level. If the vacuum gauge reads 0.03 Pa and the biochemical pool liquid level is 1 meter higher than the pump intake, the trans-membrane pressure difference is 0.04 MPa. If the pump installation height is 1 meter above the biochemical pool level, the trans-membrane pressure difference is only 0.02 MPa. The formula is: Trans-membrane Pressure Difference = Vacuum Gauge Reading (take positive) + (Biochemical Pool Surface Height - Suction Pump Intake Height).
3. Ensure that the suction pump outlet pipeline is equipped with a transparent flow meter and a sampling valve. The transparent flow meter allows for a direct observation of the water quality. A regulating valve should be added before or after each flow meter to adjust the water output of the membrane module.
4. Electrical Control: The MBR suction pump is typically set to run for 13 minutes and then stop for 2 minutes, effectively reducing the frequency of blockages. Specific start and stop times should be confirmed with the manufacturer. The pump can shut off and alarm when the pressure exceeds the limit on the electrical contact pressure gauge; the suction pump must be linked with the fan, and it should not operate when the fan is in the stopped state.
Chemical Immersion Cleaning
1. Under the right conditions, to reduce work intensity, the entire membrane module can be cleaned, which requires precise positioning of the module during loading and unloading. Water and air pipes should be equipped with detachable quick connections (air pipes not directly connected to the membrane module are not considered). These quick connections must be durable. I suggest using flange connections or branded double-acting ball valves for this purpose. The hoisting system for lifting and lowering the membrane module can significantly ease labor intensity; the hoist is branded for a 500kg capacity (but can actually handle up to 1 ton).
2. We will produce three chemical soaking tanks, which are spacious enough to accommodate the membrane components comfortably. The tanks should be 500mm taller than the membrane fibers, submerged, and each tank should be fitted with a perforated aeration pipeline and a protective platform.
The total depth of the immersion tank = the height of the pool bottom platform + the height from the bottom of the membrane module to the upper membrane fiber + 500mm extra height.
Three: Two storage tanks should be set up on the sides of the three immersion tanks, with capacities greater than the effective volume of the immersion tanks, for the purpose of recycling cleaning solutions.
Each soaking tank requires a plastic effluent pump for transferring the solution from the soaking tank to the storage tank or for discharge.
5. Consider the disposal of the waste liquid after washing. NaOH can be added as a reagent to the system, NaClO can be clarified and then directly discharged or stored for reuse, while citric acid can be slowly added to the biochemical treatment system.
Each immersion tank is designed with a high-velocity agitation air flow and equipped with a regulating valve.
7. The immersion tank should be equipped with a water supply pipe, which should be thick to avoid wasting time on filling with tap water. The filling time should ideally be around 10 minutes. Reference data indicates that at a tap water pressure of 2~3 kg, the flow rate of a DN50 tap water pipe is approximately 18~22 m3/hr.
8. Common Chemical Cleaning Agents and Concentrations:
NaOH (used for sterilization and removal of organic pollutants): concentration of 1% to 2%, soaking time of more than 2 hours.
Citric Acid (for removing inorganic scale, omit if not applicable): 2% concentration, soak time > 2 hours.
NACLO (10% liquid, for deep sterilization and restoring membrane fiber filtration function): concentration 5%, soak time >2 hours.
Soak single alcohol (95% industrial-grade alcohol) for 2 minutes to recover the film fibers after dehydration; omit if not dehydrated.
Step 1: Water Rinse → Water Soak → Alkali Soak → Citric Acid Soak → NACLO Soak → Water Rinse → Reset.
Note: Citric acid is an organic acid with unrestricted use; however, if it is not used within one month from the next use, it may mold and spoil during storage. It is recommended to use it in one go.
11. Note that after each cleaning, inspect the film filaments for breaks and tie the broken ones individually.
07. Online Backwashing
Firstly, do not over-rely on online backwashing. The backwashing of MBR is different from traditional backwashing. The majority of normal clogging in MBR is caused by the proliferation of microorganisms within the membrane fibers, while particle-induced hard clogging accounts for a small fraction.
2. However, online backwashing is still necessary to address abnormal blockages, such as the deterioration of sludge conditions, the misoperation of the MBR suction pump resulting in excessive flow, or the entry of fine particles causing a hard blockage. Backwashing remains highly effective.
The online backwashing of wastewater treatment equipment is automatically controlled by a PLC, performed once a day (provided the purchased membrane sheets support online backwashing; avoid using those that don't, as this can cause the membrane fiber pore size to expand). The backwashing water must be at least tap water quality, with the end equipped with a tap water filter, achieving a filtration precision of 50μm. The backwashing water volume should be approximately 3 to 5 times the normal filtration flow rate, and the pressure must not exceed 2.5 kg, otherwise it may damage the membrane fibers. It can be directly connected to the tap water pipeline without the need for a pressure pump, but it is essential to install a pressure gauge and flow meter.
