Membrane flux
The flux continues to decline
Under normal circumstances, the ultrafiltration membrane has a relatively stable flux at the beginning of operation. As the use time increases, the membrane flux will gradually decrease, which is caused by factors such as membrane pollution. If after effective cleaning (such as physical cleaning and chemical cleaning), the membrane flux still cannot be restored to an acceptable level, and the flux continues to be lower than 30% – 50% of the initial flux, this may indicate that the performance of the membrane component has seriously declined and needs to be considered for replacement.
For example, the initial flux of a certain ultrafiltration membrane component is 100L/(m²・h). After running for a period of time and after multiple cleanings, the flux stabilizes below 30 – 50L/(m²・h). Even if the operating pressure is increased, it is difficult to increase the flux. At this time, the membrane component may be close to the end of its service life.
Unstable flux
When the membrane component has unstable flux, that is, the flux fluctuates greatly, it may be that the membrane surface is unevenly contaminated or the membrane structure itself is partially damaged. If this situation persists and cannot be solved by conventional cleaning and maintenance methods, it may also mean that the membrane component needs to be replaced.
Water quality
The quality of produced water does not meet the standard
The main purpose of ultrafiltration equipment is to ensure that the quality of produced water meets specific requirements. If the turbidity, suspended solids content, microbial indicators, etc. of the produced water do not meet the corresponding standards or process requirements, after excluding equipment operation errors (such as abnormal operating pressure, incomplete cleaning, etc.) and other external factors (such as sudden deterioration of raw water quality and ineffective pretreatment), it is likely that the retention performance of the membrane component has decreased.
For example, in drinking water treatment, the turbidity of produced water should be lower than 0.1NTU. If the turbidity of the produced water of the ultrafiltration equipment continues to be higher than this standard and the membrane component has been used for a long time, it is necessary to consider whether the membrane component needs to be replaced.
Abnormal permeate composition
For some special ultrafiltration applications, such as the retention of specific macromolecules, if the concentration of macromolecules that should have been retained in the permeate is found to be significantly increased, this indicates that the selective separation ability of the membrane component has decreased, which may be caused by the enlargement of the membrane pores or the damage of the membrane structure. In this case, the membrane component may need to be replaced.
Membrane integrity
Pressure decay test results
Pressure decay test is a common method for detecting membrane integrity. Fill the membrane assembly with water and pressurize it to a certain pressure (such as 0.3MPa), then close the water inlet and outlet valves and observe the pressure decay within a certain period of time (such as 10-15 minutes). If the pressure decays too quickly and exceeds the specified range (for example, the specified pressure decay rate is no more than 0.01MPa per minute, but the actual test decays by more than 0.02MPa per minute), it may indicate that the membrane is damaged or poorly sealed, and the membrane assembly may need to be replaced.
Bubble observation method results
For hollow fiber membrane assemblies, the bubble observation method can be used. Pass gas into one side of the membrane and observe whether there are continuous bubbles on the other side. If there are, it means that the membrane has a damaged point. If the membrane assembly is found to have a large number of damaged points, it is difficult to restore its normal filtration performance even if it is repaired, and it is necessary to consider replacing the membrane assembly.
Operation cost
Cleaning frequency and cost are too high
If the membrane assembly needs to be cleaned frequently (for example, it was originally cleaned once a month, but now it needs to be cleaned every week), and the cost of cleaning agents, energy (such as the electricity required for backwashing) consumed for each cleaning continues to increase, which greatly increases the operation cost. At the same time, the effect after cleaning is getting worse and worse (such as the flux recovery is not obvious, the water quality of the produced water is still not up to standard, etc.), from an economic point of view, it may be more appropriate to replace the membrane assembly.
Increased energy consumption
With the aging and contamination of the membrane assembly, in order to maintain a certain flux and water quality, it may be necessary to continuously increase the operating pressure. When the operating pressure is increased by more than 30% – 50% compared with the initial design pressure, and there is no sign of improvement, this not only means an increase in energy consumption, but also indicates that the performance of the membrane assembly has seriously declined and may need to be replaced.
Service life
Reaching or approaching the design service life
Ultrafiltration membrane assemblies of different types and materials have their design service life. For example, the design service life of polysulfone membrane assemblies is generally 3-5 years, that of polyethersulfone membrane assemblies is 5-7 years, and that of ceramic membrane assemblies can reach 8-10 years. If the membrane module has reached or is close to its designed service life, even if it can still basically meet the operating requirements, its performance changes should be closely monitored, and planning to replace the membrane module can be considered in advance.
