The reasons for this phenomenon are mainly related to the working environment of the membrane system, selection design, installation and construction, operation management and membrane product performance. In addition, it has a certain relationship with the membrane system (MBR, UF or RO) itself.
Major factor:
Membrane product performance.
For the UF system, the anti-fouling and hydrophilicity of the membrane itself have a greater impact on the flux. After the flux value is low for a period of time, the processing capacity of MBR and UF will decrease. For this, a reinforced PVDF membrane with strong anti-pollution and good cleaning and recovery can be selected, combined with air washing and continuous filament, the cleaning effect is very good. As for the MBR system, it is necessary to use a permanently hydrophilic, continuous-filament lining MBR membrane to ensure the water production and water quality.
Other important factors:
1. Working environment
The inlet water temperature is lower than the design temperature (usually due to seasonal changes), which will cause the membrane outlet performance to decline; changes in the types of pollutants, concentration and water viscosity in the water quality will also cause the membrane's water permeability to be lower than expected. To solve these problems, it is necessary to consider the fluctuation range of the lowest water temperature and water quality throughout the year at the beginning of the membrane system design, and leave a certain safety margin when selecting the membrane area.
2. Selection design
Due to insufficient design experience, excessive consideration of cost, insufficient safety margin of the selected membrane area, and overestimation of the membrane production water flux, etc., will cause the membrane to fail to reach the design capacity. Whether it is MBR membrane, UF membrane or R0 membrane, sufficient membrane area must be ensured. Insufficient pressure and uneven fluid distribution can lead to deviations of water or air flow, which will also affect the overall membrane performance, especially for UF and Ro systems. For RO systems, if the membrane surface flow rate is too low, it will cause contaminants to deposit and cause blockages. The solution is to improve the design of pumps, water circuits, and air circuits so that multiple sets of membranes can share the processing load and avoid some membranes that are overloaded for water production and some membranes are not functioning. If the design of the cleaning and maintenance function is insufficient, especially the self-rinsing function without product water, it can be solved by improving the design of air washing, water washing and chemical washing, and setting appropriate parameters such as washing frequency and water volume. There is no pre-filter protection device before entering the membrane, which will cause the membrane system to block. The solution is: for the MBR system, add a 13 mm ultra-fine grid; for the UF system, add a 100um level coarse filter; for the R0 system, add a 5um level precision filter. In UF and RO systems, microbial slime will also be blocked, which will affect the normal water production of the membrane. The solution is: add an ultraviolet sterilizer or add a fungicide; for the RO membrane system, use a phosphorus-free scale inhibitor to slow down the growth of microorganisms; regularly clean and maintain the system. The pre-treatment capacity is too low or the effect becomes poor, which leads to deterioration of the membrane water quality, which has a great impact on the membrane system, especially the RO system. The solution is to improve the pre-treatment to ensure the quality of the influent water of the membrane system; for the UF system, improve the effect of the pre-filter; for the RO system, ensure that the influent pollution index (SDI) is qualified.
3. Installation and construction
In the process of membrane system manufacturing and installation, failure to clean up the debris in time will result in excessive residual impurities, which will affect the water production performance of the membrane. Especially welding slag, sharp or filaments, etc., can cause damage to the membrane filaments and membranes in severe cases. For MBR membrane, it is necessary to clean the impurities in the pool before entering the water; for UF and RO systems, first flush the front piping system, run the pre-filter, and then enter the water.
4. Operation management
The correct addition of chemicals, especially scale inhibitors, is very important to the RO system. In addition, according to the working conditions, it is necessary to rinse in time or take medicine washing to restore the membrane performance. Insufficient design experience leads to a high recovery rate, or human operations exceed the design recovery rate, which can also lead to a decline in membrane processing capacity. For the UF system, cross-flow filtration can be used instead of dead-end filtration; for RO membranes, 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.
