Not all wastewater is suitable for MBR membrane treatment!

In traditional wastewater biological treatment technologies, solid-liquid separation is achieved in secondary sedimentation tanks through gravity. The separation efficiency depends on the settling performance of activated sludge; better settling performance leads to higher solid-liquid separation efficiency. The settling performance of sludge depends on the operation of the aeration tank. Improving sludge settling performance requires strict control of the operating conditions of the aeration tank, which limits the applicability of this method. Due to the solid-liquid separation requirements of the secondary sedimentation tank, the sludge in the aeration tank cannot maintain a high concentration, generally around 1.5~3.5g/L, thus limiting the biochemical reaction rate. Hydraulic retention time (HRT) and sludge retention time (SRT) are interdependent, and increasing the volumetric load and reducing the sludge load often conflict. The system also produces a large amount of excess sludge during operation, and its disposal cost accounts for 25%~40% of the operating cost of the wastewater treatment plant. Traditional activated sludge treatment systems are also prone to sludge bulking, resulting in suspended solids in the effluent and deterioration of effluent water quality.
　　　
　　In response to the above problems, MBR combines membrane separation technology in separation engineering with traditional wastewater biological treatment technology, greatly improving solid-liquid separation efficiency. The increased concentration of activated sludge in the aeration tank and the presence of specific bacteria (especially dominant bacterial groups) increase the biochemical reaction rate. Simultaneously, reducing the F/M ratio reduces the amount of excess sludge produced (even to zero), thus fundamentally solving many prominent problems existing in the traditional activated sludge method.

　　However, MBR is not a panacea. It belongs to microfiltration membrane and is defined by the particle size that can pass through; therefore, blockage is a key issue. For wastewater with high scaling tendency, oily substances, or viscous substances, it is recommended not to use the MBR membrane method. Wastewater types unsuitable for MBR include: emulsion/grinding/quenching/coolant wastewater, surfactant wastewater, petroleum wastewater, and lipid wastewater (except with pretreatment measures).

　　MBR membrane technology is becoming increasingly widely used, attracting attention for its stable and clear effluent, but its huge maintenance workload also causes headaches for many users. To minimize maintenance workload during use, the following points should be considered during the design phase:

Design Points for MBR Aeration Devices
　　1. The aeration device can be fixed to the bottom of the tank (requiring a membrane module support frame and membrane module sliding guide rails), 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 corresponding to 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 designs use double rows or downward angles, which I believe are not advisable, as settled sludge will not block the holes.

2. The aeration volume is roughly estimated. According to experience, a gas-water ratio of 24:1 can be used (conventional tank depth 3.5m). The wind pressure head of the blower is selected to be 0.01Mpa higher than the high liquid level. A vent valve is installed at the blower outlet. The vent pipe can fully open to discharge 70% of the air volume. A silencer is installed on the vent outlet. 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 is separately controlled by a separate control valve, using a microporous oxygenation aeration device to ensure flexible adjustment of the stirring air volume and oxygenation air volume.

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

Chemical Immersion Cleaning
　　1. If conditions permit, to reduce workload, the entire membrane module can be cleaned. This requires good positioning of the membrane module for entering and exiting the tank. Water and air pipes should use convenient detachable quick connectors (air pipes do not need to be considered if not integrated with the membrane module). These quick connectors should be durable. Flange connections or branded double union ball valve connections are recommended. A hoisting mechanism for the membrane module can effectively reduce labor intensity. The hoisting capacity should be 500kg (it can actually handle 1t).

2. Three chemical immersion tanks should be provided, large enough to easily accommodate the membrane module. The height should be 500mm higher than the submerged membrane fibers. Each immersion tank should have perforated aeration pipes and a protective platform. Total immersion tank depth = bottom platform height + height from the bottom of the membrane module to the upper membrane fibers + 500mm extra height.
　　3. Two storage tanks should be placed next to the three immersion tanks. Their capacity should be larger than the effective volume of the immersion tanks, for reuse of the cleaning solution.
　　4. Each immersion tank should be equipped with a plastic drainage pump to transfer the solution from the immersion tank to the storage tank or for discharge.
　　5. Consider the treatment method for the wastewater after washing. 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 slowly added to the biological treatment system.
　　6. The stirring air volume of each immersion tank should be designed for vigorous stirring, and a regulating valve should be installed.
　　7. The immersion tank should have a tap water inlet pipe. The pipe should be large to avoid wasting time on tap water injection. The filling time should be around 10 minutes. As a reference, with a tap water pressure of 2-3 kg, the flow rate of a DN50 tap water pipe is approximately 18-22 m3/hr.
　　8. Commonly used chemical cleaning agents and concentrations:
　　NaOH (used to sterilize and clean organic pollutants): concentration 1%~2%, immersion time >2h.
　　Citric acid (used to remove inorganic scaling, omit if not present): concentration 2%, immersion time >2h.
　　NaClO (10% liquid, used for deep sterilization, restoring membrane fiber filtration function): concentration 5%, immersion time >2h.
　　Alcohol (95% industrial alcohol) Single-piece immersion for 2 minutes, used to restore dehydrated membrane fibers, omit if not dehydrated.

9. Cleaning steps: Water rinse → Water soak → Alkali solution soak → Citric acid soak → NaClO soak → Water rinse → Reset;

10. Note: Citric acid is an organic acid and its use is unrestricted. However, if more than one month passes before the next use, it may become moldy and deteriorate during storage. It is recommended to use it once only;

11. Note: After each cleaning, check for broken filaments. For single broken filaments, tie a knot to repair;