After the reverse osmosis membrane system operates for a period of time, sometimes the treatment capacity decreases significantly and cannot 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).

Performance of membrane products:

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 air washing, 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 factors:

one

work 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 viscosity and water permeability of the membrane will also be lower than the expected value. To solve these problems, it is necessary to consider the fluctuation range of the lowest water temperature and water quality in the whole year at the beginning of the design of the membrane system, and leave a certain safety margin when selecting the membrane area.

two

lectotype design

Due to insufficient design experience and excessive consideration of cost, the safety margin of the selected membrane area is insufficient, and the estimation of the water production flux of the membrane is too high, which will make the membrane unable to reach the design capacity. Whether MBR membrane, UF membrane or R0 membrane, sufficient membrane area must be ensured.

three

Installation and 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. Especially welding slag, sharp or filamentous objects, which 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.

four

Operation management

Correctly adding chemicals, especially scale inhibitors, is very important for RO system. In addition, it is also necessary to wash or take medicine washing in time according to the working conditions to restore the performance of the membrane. 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 short service life of membrane or frequent cleaning is reflected in two aspects:

First, the membrane itself is permanently damaged and cannot produce enough and 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 will 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 diaphragm from cracking under the impact of water hammer, the details of reducing water hammer shall be designed.

2. Membrane products

If the uf/mbr membrane adopts the conventional homogeneous membrane wire, the wire is easy to break, the anti pollution and anti-aging properties are poor, and the backwashing and tensile strength are low. It is recommended to select the lining reinforced PVDF film with long service life and high tensile strength. RO membrane has small circulation 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 membrane. The solution is as follows: for MBR membrane with lining, the impurities in the tank shall be cleaned before entering the water; For UF and RO systems, the front pipeline system must be flushed, and then the prefilter must be operated before water is fed.

4. Operation management

Incorrect or timely dosing, especially untimely dosing of scale inhibitor in RO system, will lead to serious blockage of membrane. 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 pressure drop of membrane group, and carry out restorative cleaning in time according to the working conditions to 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 when 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 the large RO system, it can basically offset more than 70% of the annual depreciation cost of the 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 slow down 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 more than 15% of energy at least 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. Appropriately increase the number of membranes and reduce the operating pressure. By appropriately increasing the membrane area, the operating pressure of membranes and power consumption can be reduced to a certain extent.

5. When MBR or UF produced water is mixed with RO produced water, the quality of RO produced water is better. Generally, it can be mixed with a certain proportion of produced water with lined 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.