After the membrane system operates for a period of time, sometimes the processing capacity decreases significantly and can not reach the design capacity. The reasons for this phenomenon are mainly related to the working environment of membrane system, type selection and design, installation and construction, operation management and membrane product performance. In addition, it is also related to the membrane system (MBR, UF or R0) itself.

A very important point: membrane product performance

        For UF system, the anti fouling and hydrophilicity of the membrane itself have a great impact on the flux. The flux value is low. After running for a period of time, the processing capacity of MBR and UF decreases. For this, the inner lining reinforced PVDF membrane with strong pollution resistance and good cleaning recovery can be selected, combined with continuous air washing, and the cleaning effect is very good. For the MBR system, the MBR membrane with permanent hydrophilicity and continuous silk lining should be selected to ensure the water yield and quality.

Other important influencing factors:

1. Working environment

If the inlet water temperature is lower than the design temperature (common in seasonal changes), it will lead to the decline of membrane effluent performance; The changes of pollutant types, concentrations and water viscosity in water quality will also lead to the lower than expected permeability of the membrane. To solve these problems, it is necessary to consider the fluctuation range of annual minimum water temperature and water quality at the beginning of membrane system design, and leave a certain safety margin when selecting membrane area.

2. Type selection design

Due to insufficient design experience, excessive consideration of cost, insufficient safety margin of selected membrane area and high estimation of water production flux of membrane, the membrane will not reach the design capacity. Whether MBR membrane, UF membrane or R0 membrane, sufficient membrane area must be ensured.

Insufficient pressure and uneven fluid distribution lead to water or gas flow bias, which will also affect the overall membrane performance, especially for UF system and RO system. For RO system, too low membrane flow rate will lead to pollutant deposition and blockage. The solution is to improve the design of pump, water circuit and gas circuit, so that multiple groups of membranes can bear the treatment load, so as to avoid overload water production of some membranes, while some membranes do not play a role. If the design of cleaning and maintenance function is insufficient, especially there is no water producing self flushing function, it can be solved by improving the design of gas washing, water washing and drug washing, and setting appropriate flushing frequency, water volume and other parameters. There is no pre filtration protection facilities before entering the membrane, which will lead to the blockage of the membrane system. The solution is: for MBR system, 13 mm ultra-fine grid is added; For UF system, 100um coarse filter is added; For R0 system, add 5um precision filter.

In UF and RO systems, microbial slime blockage will also occur, affecting the normal water production of the membrane. The solution is: add ultraviolet sterilizer or add bactericide; For RO membrane system, phosphorus free scale inhibitor is selected to slow down microbial breeding; Clean and maintain the system regularly.

The design of pretreatment capacity is too low or the effect becomes worse, resulting in the deterioration of water quality into the membrane, which has a great impact on the membrane system, especially the RO system. The solution is to improve the pretreatment to ensure the inlet water quality of the membrane system; For UF system, improve the effect of prefilter; For RO system, ensure that the inlet water pollution index (SDI) is qualified.

3. Installation construction

During the manufacturing and installation of membrane system, if the sundries are not cleaned in time, too many impurities will remain, which will affect the water production performance of membrane. In particular, welding slag, sharp or filaments, etc. will cause damage to membrane wires and membranes in serious cases. For MBR membrane, the impurities in the tank shall be cleaned before water inflow; For UF and RO systems, first flush the front pipeline system, run the pre filter, and then feed water.

4. Operation management

It is very important for RO system to correctly add chemicals, especially scale inhibitors. In addition, it is also necessary to wash or take medicine washing in time according to the working conditions to restore the membrane performance.

Insufficient design experience leads to high recovery rate, or manual operation exceeds the design recovery rate, which will also lead to the decline of membrane treatment capacity. Cross flow filtration instead of dead end filtration can be adopted for UF system; For RO membrane, it is necessary to increase the discharge of concentrated water, reduce the recovery rate, reduce the number of membranes in series or increase the return flow.

The service life of the membrane is shortened or frequent cleaning is required

The short service life cycle of membrane is reflected in two aspects:

First, the membrane itself is permanently damaged and can no longer produce sufficient water, which needs to be solved;

Second, the membrane is temporarily damaged after being seriously polluted. It needs frequent cleaning to restore the water production capacity. The actual use time and cleaning cycle are short, so it is difficult to meet the production requirements. This problem can be solved from the process design, such as improving the quality of the membrane itself, and strengthening the environment, manufacturing, construction and operation management.

1. Design

Inadequate protective design can lead to irreparable damage to the membrane. The solutions vary according to the types of damage. For example, in order to avoid scratches on membrane wires and membranes, lining enhanced PVDF membrane can be selected and prefilter can be installed; In order to prevent RO membrane from being oxidized by oxidizing substances, ORP instrument and automatic reductant dosing device are installed; In order to prevent overpressure physical damage, pressure protection facilities can be installed; In order to prevent the membrane wire and membrane from breaking under the impact of water hammer, the details of reducing water hammer shall be designed.

Lack of design experience or excessive saving of material cost will result in serious shortage of actual membrane area; Overload operation per unit area, serious pollution and frequent cleaning are also required. Therefore, the membrane area shall be selected and designed according to the stable membrane flux. For inexperienced water quality, the design data shall be verified through pilot test o

For R0 system, too many membranes in series will also cause serious membrane pollution and difficult to clean and recover. The solution is to reasonably design the number of RO membranes in series. When two-stage ro design is adopted, sectional cleaning is designed.

In addition, the pretreatment capacity is too small, resulting in excessive influent quality of the membrane and forced operation, which will also shorten the service life of the membrane. Therefore, it is necessary to improve the pretreatment process to ensure that the inlet water quality of the membrane system meets the requirements.

2. Membrane products

If the UF/MBR film is made of conventional homogeneous filaments, it is easy to break the wire, has poor resistance to contamination and aging, and has low backwash and low tensile strength. Long life and high tensile strength lined reinforced PVDF membrane is recommended.

RO membrane has small flow channel and poor anti pollution performance. Wide channel RO membrane shall be selected as far as possible, and anti pollution membrane shall be selected according to water quality.

3. Manufacturing and construction

During manufacturing and installation, the membrane system will be mixed with sundries, especially sharp foreign matters, which will scratch the membrane wire and diaphragm. The solution is: for MBR membrane with lining, the impurities in the tank shall be cleaned before water inflow; For UF and RO systems, the front pipeline system must be flushed, the prefilter must be operated, and then water can be fed.

4. Operation management

Incorrect or timely dosing, especially untimely dosing of scale inhibitor in RO system, will lead to serious membrane blockage. On the premise of correct design, operate and maintain correctly according to the operating procedures.

When the membrane system operates under high pressure difference, the microfiltration membrane wire will be broken and the RO membrane will be damaged. Therefore, it is necessary to inspect the transmembrane differential pressure and membrane group pressure drop, and timely carry out restorative cleaning according to the working conditions to strictly prevent overpressure differential operation.

During chemical cleaning, strong acid, strong alkali and strong oxidizing agents will cause irreparable chemical damage to the membrane. Therefore, the operator needs to master the chemical cleaning process to avoid strong acid and alkali environment; RO membrane shall not be cleaned with strong oxidizing agent.

5. Environment

If the water temperature is higher than 45 ℃, the service life of the membrane will be shortened. Generally, the water temperature shall be controlled below 40 ℃, and cooling measures shall be taken if necessary.

For RO system, the wastewater contains strong oxidizing substances or easily precipitated substances, which will shorten the service life of the membrane. When designing the system, from the perspective of its safety, increasing the safety margin of pretreatment and installing protective shutdown design can avoid such problems.

The operation cost of membrane system is high

1. Select ultra-low pressure reverse osmosis membrane

The operating pressure of conventional reverse osmosis membrane is 1.3 ~ 1.5 MPa, and that of ultra-low pressure reverse osmosis membrane is about 0.8MPa or even lower (closely related to water temperature), which can save more than 30% of power consumption. For large RO system, it can basically offset more than 70% of the annual depreciation cost of membrane, and the savings are more significant.

2. High pressure pump with frequency converter

The high-pressure pump equipped with frequency converter can not only reduce the impact of water hammer when the water pump is started, but also reduce the throttle energy consumption of the valve by setting a reasonable operating pressure, which can save energy by at least 15% quarterly throughout the year.

3. Reasonably calculate the dosage of scale inhibitor. By analyzing the water quality data and optimizing the dosage, the reagent cost can be saved by 20% or even higher.

4. Properly increase the number of membranes and reduce the operating pressure

By properly increasing the membrane area, the operating pressure of the membrane and power consumption can be reduced to a certain extent.

5. MBR or UF produced water is mixed with RO produced water

Ro produced water has better quality and can usually be mixed with a certain proportion of produced water lined with MBR membrane or UF membrane, so as to improve the system recovery rate and reduce the operation scale of RO system, so as to save the operation cost.