Points to note about ultrafiltration backwashing

Points to note about ultrafiltration backwashing

Ultrafiltration (UF) technology is an advanced water treatment technology, widely used in drinking water, industrial water, wastewater treatment and other fields. With the increase of the use of time, the ultrafiltration membrane will be contaminated, resulting in decreased flux, filtration efficiency. Backwashing is an important measure to restore the performance of ultrafiltration membrane.

01 Introduction to Ultrafiltration Technology
Ultrafiltration is a pressure-driven membrane separation technology that utilizes membranes with pore sizes between 0.01 and 0.1 microns to separate suspended particles, colloids and high molecular weight substances. Ultrafiltration membranes are usually made of polymer materials such as polyvinylidene fluoride (PVDF), polyethersulfone (PES), etc.

Ultrafiltration system components:
Water inlet system: including water inlet pump, piping and pretreatment equipment.
Ultrafiltration membrane module: the core part, including ultrafiltration membrane shell and membrane elements.
Produced water system: including produced water pipeline, water storage tank and distribution system.
Control system: including flow meters, pressure gauges, control valves and automation control equipment.
Backwash system: including backwash pump, backwash tank and chemical cleaning device.

02 Ultrafiltration membrane contamination and backwashing 
Ultrafiltration membrane contamination refers to the membrane surface and pores are covered with contaminants or blocked, resulting in a decline in membrane performance.
Common pollutants include:
Suspended matter and colloids: such as sediment, iron and manganese oxides.
Organic matter: such as dissolved organic carbon (DOC), oil and grease.
Biological contamination: e.g. bacteria, algae and biofilm.
Control systems: including flow meters, pressure gauges, control valves and automated control equipment.
Inorganic deposits: e.g. calcium, magnesium, silicate deposits, etc.

Backwashing Principle
Backwashing is a process that utilizes water flow in reverse through the ultrafiltration membrane to remove contaminants attached to the membrane surface, thereby restoring membrane flux. Backwashing includes physical backwashing and chemical backwashing.
Physical backwashing: the main use of reverse water flow to flush the membrane surface to remove loose pollutants.
Chemical backwashing: chemical agents, such as acids, alkalis, oxidizers, etc., are added to the backwash water to remove stubborn contaminants and biological contamination.
Manual operation and backwashing of ultrafiltration system

Filtration process
In the filtration state, when the water inlet to the membrane filtration tank reaches a certain water level, start the suction pump and enter the filtration process. Filtration includes vacuum priming, adjusting the amount of suction water to ensure constant flow filtration of the ultrafiltration device. In order to effectively flush the water production pipeline, water production tank and backwash tank, the process may take a certain period of time, in order to ensure the backwash water quality, please do not start the backwash pump. In order to avoid greater contamination of the membrane, the following two measures can be taken: the use of low-flow filtration, the stage of the membrane of the actual production of water for the design of the water 1/2; filtration period of regular aeration, aeration period of 0.5 ~ 1.0h, duration of 1min, intensity and backwash aeration intensity consistent.
Throughout the filtration process, we should regularly take water samples from the inlet water of the membrane pool, the production pipeline, the production pool and the backwash pool for water quality monitoring, and the test indexes include but are not limited to turbidity, particulate matter, CODMn, etc. The water samples should be taken from the production pipeline, the production pool and the backwash pool for water quality monitoring. 
There is no obvious difference between the water samples from the production pipeline, the production pool and the backwash pool, and the water from the backwash pool has no obvious impurities such as particles before backwash is carried out.
0.5mm filter is set in the backwash water pipeline.


The backwashing process is as follows
The backwash uses ultrafiltration water. Membrane system backwash cycle is 1-3h, backwash lasts 1-3min, water backwash at the same time bottom aeration.
Step 1: Close the water inlet valve, drop the liquid filtration, to reach the cleaning level when the suction pump is closed;
Step 2: under the condition of starting the scrubbing blower, open the air flushing valve of the membrane pool, air scrubbing on the outer surface of the membrane filaments, the air volume is 50m3/m2/h; under the condition of turning on the backwash pump, open the backwash valve, backwash the membrane filaments with water, backwash flux is 60-80L/m2/h. The air and water backwash time lasts for 1~3min;
Step 3: Close the air flushing valve of this membrane pool and close the backwash valve;
Step 4: Open the sewage valve, after the membrane pool drained off the design water volume, close the sewage valve;
Step 5: open the membrane pool water inlet valve, after reaching the filtration level, vacuum diversion, start the suction pump, re-enter the filtration process.

03 Precautions for ultrafiltration membrane backwashing    
Quality of backwash water: the backwash water should be free of pollution to avoid secondary pollution of ultrafiltration membrane. Usually use produced water or treated water.
Frequency of backwashing: Determine the frequency of backwashing according to the actual operation, generally 1-2 times a day, adjusted according to the degree of membrane contamination.
Backwash intensity and time: backwash water flow and backwash time should be appropriate to ensure effective removal of pollutants, while avoiding damage to the membrane. Usually, the physical backwashing time is 5-15 minutes, and the chemical backwashing time is 30-60 minutes.
Chemical selection and concentration: Select the appropriate chemical and concentration to ensure effective removal of pollutants, while not damaging the membrane material. The concentration of chemical backwashing agent is generally between 0.1-1%.
Temperature control: The temperature of the backwash water is moderate, generally at 20-40℃, too high or too low temperature may affect the backwash effect and membrane life.

04 Application examples of ultrafiltration membrane
Drinking water treatment
In drinking water treatment plants, ultrafiltration technology is widely used to remove suspended solids, colloids and microorganisms. Regular backwashing ensures that the membrane operates stably for a long period of time and guarantees the quality of the produced water. Physical backwashing combined with chemical backwashing effectively removes membrane contamination and extends membrane life.
Industrial water treatment
In industrial water treatment, such as in the electronics, pharmaceutical and food industries, ultrafiltration technology is used to ensure water quality by removing small particles and organic matter. Backwashing technology is particularly important in these applications to maintain the high throughput and efficient filtration capacity of the ultrafiltration membranes, reduce downtime and increase productivity.
Wastewater Treatment
In wastewater treatment, ultrafiltration technology is used to remove suspended solids and contaminants from wastewater. Backwashing technology effectively removes contaminants adhering to the ultrafiltration membrane, maintains membrane flux and treatment efficiency, and ensures that wastewater treatment meets discharge or reuse standards.

Ultrafiltration backwashing technology is the key measure to keep the ultrafiltration system running efficiently. Through the combination of physical backwashing and chemical backwashing, membrane contamination can be effectively removed, membrane flux can be restored, and membrane service life can be extended. In practical application, according to the water quality and the degree of membrane contamination, reasonably select the backwash water quality, backwash frequency, backwash strength and chemicals to ensure the long-term stable operation of the ultrafiltration system.
By continuously optimizing the backwashing technology and process parameters, the water treatment industry can further improve the performance and reliability of the ultrafiltration system, providing high-quality solutions for various water treatment applications.