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MBR, also known as Membrane Bioreactor, is a new type of water treatment technology that combines activated sludge processes with membrane separation technology. Currently, MBR membranes are mainly divided into four types: hollow fiber membranes, flat sheet membranes, ceramic membranes, and tubular membranes.
One. Introduction
Based on materials, they can be classified into organic membranes and inorganic membranes, with distinct differences in suitable scenarios, advantages, and disadvantages.
1. Hollow Fiber Membranes (also called sheet membranes) are organic membranes, commonly made from PVDF (polyvinylidene fluoride). Recently, new PTFE (polytetrafluoroethylene) membranes have also become more common. PTFE, known as the "king of plastics," has more uniform pore distribution and better resistance to fouling and aging than traditional PVDF membranes, making its overall performance stronger.
2. Flat Membranes
Also using PVDF as the core material, the overall structure consists of an ABS substrate with flow channels, double-sided composite fabric, and a membrane sheet, all seamlessly welded together. They can be further divided into soft sheet membranes and flexible membranes. Flat membrane modules are mostly assembled with 304 stainless steel frames and can be configured with 10–250 membrane sheets as needed. The biggest advantage: if a single membrane sheet is damaged or water production drops, you can troubleshoot or replace just that one sheet, greatly reducing replacement costs later. This is a common choice for small to medium-sized wastewater projects.
3. Ceramic Membranes
These are inorganic flat membranes, mainly made from alumina, titania, or silica. They have extremely strong chemical stability and can resist strong acids, strong bases, and organic solvents. They also withstand high temperatures, have narrow pore size distribution, and high separation efficiency, making them suitable for treating highly corrosive and difficult industrial wastewater. Their lifespan is much longer than conventional organic membranes.
4. Organic Tubular Membrane
It's suitable for processes like ultrafiltration, microfiltration, and nanofiltration. The main highlight is its wide channels, allowing the feed liquid to flow turbulently inside the tubes, which means it doesn't need strict pre-treatment of the water. It can be cleaned chemically or even physically scrubbed, with low pressure loss and high filtration efficiency, making it suitable for high-concentration, high-impurity wastewater conditions.
Two. MBR Membrane Module Selection
Membrane flux is the primary factor when choosing a membrane. Unit: L/m²·h. The common industry range is 10~26 L/m²·h, but different water qualities must be distinguished carefully—don’t just use a one-size-fits-all value.
1. Domestic sewage: the water is relatively clean, so a membrane flux of 18~22 L/m²·h is recommended.
2. High-concentration industrial wastewater like electroplating or pharmaceuticals: pollutants are complex and can easily clog the membrane, so it’s recommended to lower the membrane flux to 12.5~15 L/m²·h.
Note: Make sure to test the raw water composition before installation. If the water contains a lot of grease, calcium and magnesium ions, or organic solvents, it can accelerate membrane clogging and corrosive damage. In that case, prioritize membranes made of fouling-resistant materials and enhance the pretreatment process upfront.
Three. Membrane Tank & Membrane Count Calculation
Number of membrane sheets n = Q ÷ N ÷ A x 1000
Q: daily water treatment capacity in t/d
N: flux of the membrane sheet in L/m²·d (as mentioned above, just convert to daily flux, and try to take the lowest value)
A: effective area of each membrane sheet (common ones are 1.5 or 0.8 m², but there are also 2, 1.6, 1.0, 0.9 m²)
Since the choice of membrane area is related to the construction of the membrane tank, you need to determine the tank depth first. For an effective water depth of 3-3.5 meters, you can choose 1.5 m² sheets; for 2.2-2.5 meters, go with 0.8-1.0 m² sheets. Generally, for concrete tanks consider 1.5, and for steel box tanks consider 0.8.
When choosing a membrane tank, the effective water depth determines the size of the membrane sheets, the tank length decides how many units can be placed, and the tank width determines how many membranes can be placed in a single unit.
For example, for flat membranes of around 1.5 square meters, the length is roughly between 1600-1800mm, the width between 470-550mm, and the thickness between 5-7.5mm. They’re pretty similar overall.
Taking an example of 100 tons/day of domestic sewage to calculate the number of membranes:
According to the formula: n = 100 ÷ 400 ÷ 0.8 × 1000 = 312.5 (pieces), which is not an integer, so you need to round it. Since 400 L/m²·d is already the minimum value, you can directly take 300 pieces here. If there are 100 pieces per module, you need 3 modules; if there are 150 pieces per module, you need 2 modules. Choose based on the size of the membrane tank.
The required membrane tank sizes for these two options are:
- 3 modules: 4 × 2.5 × 2.8 m or 3.5 × 2.5 × 2.8 m
- 2 modules: 3 × 3.2 × 2.8 m
The length of the membrane tank can be 1 meter per module, which ensures even placement of the modules. If there are more modules, you can compress them; if fewer, you can’t. Installation also needs to be considered.
The width of the membrane tank should consider the placement of the modules, the piping for the aeration and water collection systems, and whether it will be convenient for workers for installation and later maintenance, so you should allow for a relatively generous space. The common module width is calculated as 11-14 mm per membrane, with 200 mm reserved for the racks at each end, and 350 mm reserved for installation of the collection and aeration systems. The width needed for 100 membranes: 100 x 14 + 400 + 700 = 2500 mm.
Four. How the MBR Membrane Works
The regular operating pressure of the flat-sheet membrane is 0~35 KPa, and the system uses an intermittent self-priming pump mode: a 10-minute cycle with 8 minutes of suction and 2 minutes of downtime.
Under negative pressure, the mixed sewage passes through the membrane pores, where suspended solids, microorganisms, and bacteria are retained by the membrane, achieving sludge-water separation. This process brings two major advantages:
- The concentration of activated sludge in the tank can be maintained at 6000~12000 mg/L, significantly improving biochemical degradation efficiency;
- The filtration precision of the membrane is high, producing clear effluent with stable, compliant water quality.
However, membranes aren't万能. Pollutants smaller than the membrane pores can't be trapped and will gradually adhere to the inner walls of the pores, causing membrane fouling. This directly shows up as a drop in water production and an increase in operating negative pressure, which is the main reason regular cleaning is necessary.
Five. Complete MBR Membrane Cleaning Plan: Online Cleaning and Offline Cleaning Practical Guidelines
MBR cleaning is divided into online chemical cleaning (routine maintenance) and offline cleaning (deep repair), with the cycle and chemicals determined based on water quality and pollution level.
1. Cleaning cycle reference:
Domestic sewage (better water quality): Clean once every 3–4 months
Landfill leachate, high-concentration industrial wastewater: Clean once every 15–20 days
Basis for judgment: If the water production significantly drops or the operating negative pressure keeps rising, cleaning should be arranged immediately.
2. Online Chemical Cleaning (Mainstream Daily Cleaning Method)
Regular process: Alkali wash first → Acid wash later; for special conditions, it can be adjusted to acid wash first and then alkali wash. Dosing methods include gravity flow or dosing pump addition, with a uniform usage of 3~5L of chemicals per membrane.
(1) Alkali Wash (removes organic matter and microbial slime)
Chemical solution ratio (calculated based on 100% pure chemicals): Sodium hypochlorite 2000~5000 mg/L, Sodium hydroxide 1000 mg/L. Operation: Pour the solution into the suction line through the cleaning tank under static pressure and soak for more than 5 hours.
(2) Acid Wash (removes inorganic scale and calcium/magnesium ion deposits)
Chemical solution ratio (calculated based on 100% pure chemicals): Oxalic acid / Citric acid 1000 mg/L solution
⚠️ Key Reminder: When the raw water has a high content of calcium and magnesium ions, do not use oxalic acid; prioritize citric acid to avoid forming insoluble precipitates that could worsen clogging.
Operation: Use the same amount of alkali as in the washing, soak for 3–5 hours.
Note: If using non-pure chemicals, the dosage should be adjusted according to the actual purity of the chemical.
3. Offline Cleaning (Deep Maintenance)
Also called shutdown maintenance cleaning, it is a method for deep repair;
When to use: After online cleaning, if membrane flux still cannot be restored or the membrane is severely fouled; normally performed once a year.
