Principle of ultrafiltration membrane filtration

Hollow fiber ultrafiltration membrane is an important branch of membrane separation technology. It is characterized by separation at room temperature and low pressure. It has the advantages of low energy consumption, high filtration precision, large water yield and strong anti pollution ability. It can effectively filter out harmful substances such as bacteria, colloids, suspended solids, rust and large organic matter in water. The water purification principle of hollow fiber ultrafiltration membrane is shown in Figure 1 below:

Figure 1: filtration schematic

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Performance characteristics of ultrafiltration system

1. High filtration precision, ultrafiltration pore size in the range of 0.001-0.1m, effectively filtering out bacteria, colloids, suspended solids, rust, macromolecular organic matter and other harmful substances in water. Significantly reduce COD, SS, SDI and turbidity to ensure stable and up to standard effluent.

2. If ultrafiltration effluent is used as reverse osmosis influent, it can prolong the cleaning cycle and service life of reverse osmosis membrane and improve the water yield.

3. Full automatic control of system operation.

4. Compact structure and small floor area.

5. Low operating pressure and low operating cost.

6. The operation is simple, safe and easy to maintain.

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Standard process flow diagram

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Workflow:

Pneumatic butterfly valve opening status of each working procedure

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System operating instructions

1. Connect the ultrafiltration host and peripheral equipment (water pump, water tank, etc.) according to the process flow diagram. A security filter must be installed before the host water inlet.

2. After checking that the pipeline connection is correct, disconnect the unions connected to the ultrafiltration host, clean the pipeline connected to the host with water for 5-10 minutes until it is cleaned, pay attention to the direct discharge of the cleaning water, and do not enter the ultrafiltration to avoid large particle impurities damaging the ultrafiltration membrane, and check whether there is water leakage at each connection of the pipeline.

3. Turn down the water inlet valve of the ultrafiltration host, open all the water production valve, concentrated water return valve and backwash valve, and close the medicine washing water inlet valve and return valve, so as to avoid the impact on the ultrafiltration membrane caused by excessive water pressure during startup and damage the equipment. Then turn the "automatic / manual" knob to the manual gear on the panel of the electric cabinet, open the "ultrafiltration cross flow valve", then open the "ultrafiltration inlet valve", then slowly open the inlet valve, turn down the cross flow valve, adjust the inlet pressure P1 to be less than 0.2MPa, flush the ultrafiltration membrane for 5 ~ 10 minutes, flush the protective fluid in the ultrafiltration membrane, and check whether there is leakage in the interface and pipeline.

4. After the above cleaning, the ultrafiltration system can enter the formal commissioning operation, slightly open the inlet valve, open the outlet valve and cross flow valve, turn the "automatic / manual" knob to the automatic gear, and adjust the inlet valve, cross flow valve and water production valve so that the water production pressure is 0.02-0.05mpa and the water production flow is about the design flow, The concentrated water flow is about 10% - 50% of the influent flow according to the turbidity of raw water. At this time, the influent pressure is the initial influent pressure. When the water production flow is guaranteed, the lower the influent pressure, the better. The maximum influent pressure shall not exceed 0.3MPa and the backwashing pressure shall not exceed 0.20mpa.

5. Set the backwashing cycle according to the water quality of raw water. In the range of 60-90 minutes, the worse the water quality, the shorter the cycle. The forward washing is generally 15 seconds, the upper backwashing is 25 seconds, and the lower backwashing is 20 seconds.

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chemical cleaning

With the long-term use of ultrafiltration membrane components, when the water production of ultrafiltration membrane decreases by more than 20% or after 1-4 months of use, it is necessary to carry out chemical cleaning of ultrafiltration membrane, so as to remove pollutants on ultrafiltration membrane in time and prevent stubborn scaling of ultrafiltration membrane. Chemical cleaning is divided into:

a. Pickling: use citric acid to adjust the pH value of RO reverse osmosis water (or ultrafiltration water) in the solution tank to pH = 2

b. Alkali washing: use sodium hydroxide and sodium hypochlorite to adjust the RO reverse osmosis water (or ultrafiltration water) in the liquid medicine tank to pH = 12

Note: 1. After pickling, wash with alkali and clean with clean water until the outlet water pH = 7, otherwise the water flux will be affected.

Chemical cleaning operation method:

Put the agent into the chemical cleaning tank according to the amount, and prepare the cleaning solution.

Open the medicine washing water inlet valve and return valve, turn the "automatic / manual" knob to the manual position on the panel of the electric cabinet, start the medicine washing pump, adjust the water inlet valve to make the pressure displayed on the pressure gauge P = 0.20mpa, carry out positive flushing medicine washing for the ultrafiltration membrane, open the water production return valve 10 minutes after positive flushing medicine washing, and carry out water preparation and positive flushing medicine washing for 20 to 60 minutes.

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Precautions for operation

1. The designed water yield of ultrafiltration system is usually measured under the condition of 25 ℃ water temperature and standard pressure. If the water temperature rises or falls, the water yield also increases or decreases accordingly. For every 1 ℃ drop in water temperature, the water yield decreases by 2.13%, and vice versa.

2. Ultrafiltration membrane is best installed indoors to avoid sun and rain.

3. When the ultrafiltration system works normally, the water inlet pressure gauge value is 0.1-0.3mpa, and the pressure cannot be increased arbitrarily to increase the water volume, otherwise the membrane element will be damaged. The pressure value can be adjusted by ultrafiltration inlet butterfly valve.

4. Before startup, please confirm that all valves are in the correct position, and start the booster pump only after the inlet valve is turned down, otherwise it is easy to damage the pressure gauge, ultrafiltration membrane and other components. After starting the raw water pump, adjust the inlet valve to the inlet pressure of 0.1-0.3mpa.

5. When the ultrafiltration membrane is started for the first time, the air in the membrane module shall be driven away with low-pressure and low flow inlet water. The inlet water pressure shall be as low as possible, and water shall be fed from the bottom of the membrane. All produced water and concentrated water shall be discharged during flushing; Each start and stop in the future involves sudden change of system pressure and flow, impact on membrane modules, etc. Therefore, the number of start and stop of system equipment shall be reduced as much as possible, and the normal start and stop process shall be as stable as possible, especially the rise of inlet pressure and flow shall be slow;

6. If the ultrafiltration inlet pressure is higher than 0.3MPa, the inlet water quality fluctuates greatly or the ultrafiltration produced water quality does not meet the standard, stop the machine and check the causes.

7. When the transmembrane pressure difference of ultrafiltration membrane Δ When p ≤ 0.2MPa, the series operates normally and the transmembrane pressure difference Δ p> 0.2MPa indicates that the membrane is seriously blocked and needs to be shut down for treatment (chemical cleaning).

8. After the system is shut down, proceed as follows:

1) Prevent the membrane from drying and cracking after water loss, and the water yield will decrease irreversibly after water loss;

2) Take appropriate protective measures to prevent microbial breeding or wash regularly every 24 hours;

3) When it is not used for a long time (more than 5 days), please fill the ultrafiltration membrane with protective solution for storage to prevent the decline of water production caused by water loss of ultrafiltration membrane.

Formula in protective solution: (1% sodium pyrosulfite + 10% propylene glycol protective solution + 89% RO reverse osmosis water or ultrafiltration water).

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Maintenance

1. Clean or replace the filter bag or filter element of the security filter regularly. When the pressure drop of the security filter exceeds 50% of the initial pressure drop, replace the filter bag with a new one.

2. Chemical cleaning of ultrafiltration membrane shall be carried out regularly

1) Cleaning time

During the operation of the ultrafiltration system, colloidal particles, microorganisms and macromolecular organics in the influent are carried in the membrane pipe. These pollutants are deposited on the membrane surface, resulting in the decline of water production and water quality of the system. Chemical cleaning is required in case of the following conditions, so as to remove pollutants in time and restore membrane performance.

A. When the standardized water production decreases by more than 20%;

B. The standardized membrane pressure difference reaches 0.15Mpa;

(the above standard comparison conditions are taken from the operating performance of the system after the first 48 hours of operation.)

After the continuous operation of the system for 1-4 months, when the water yield of the system decreases by more than 20% of the initial water yield or the transmembrane differential pressure increases by more than 25% of the initial differential pressure, the ultrafiltration system shall be chemically cleaned.

2) Cleaning method: refer to VII. Chemical cleaning