External-pressure ultrafiltration membranes have a filtration direction from the outside to the inside. This structure allows suspended solids, colloids, and other impurities in the feed liquid to be retained on the outer side of the membrane, while the permeate flows out from the inner side. The larger external surface area of the membrane provides more filtration channels, thereby improving filtration efficiency. In practical applications, the recovery rate of external-pressure ultrafiltration membranes can typically reach over 95%, which is significantly higher than that of internal-pressure ultrafiltration membranes. For example, when treating surface water with a high concentration of suspended solids, the turbidity of the permeate from external-pressure ultrafiltration membranes can be stably maintained below 0.1 NTU, that while from internal-pressure ultrafiltration membranes may be around 0.2 NTU.

The design of external-pressure ultrafiltration membrane modules allows them to be used in reverse, i.e., filtering from the inside out. This flexibility enables external-pressure ultrafiltration membranes to be more adaptable to different water qualities and operating conditions. For example, when treating feed water with high concentrations of suspended solids, reverse flushing can effectively remove pollutants from the outer side of the membrane, extending the membrane's service life. In addition, external-pressure ultrafiltration membranes can maintain good performance even under high-pressure differential conditions, with higher mechanical strength to resist damage to the membrane structure from external pressure.

Due to the filtration direction from the outside to the inside, suspended solids in the feed water are retained on the outer side of the membrane and do not enter the internal channels of the membrane. This structure makes external-pressure ultrafiltration membranes particularly effective in treating water with high suspended solids content. For example, when treating municipal sewage or industrial wastewater, external-pressure ultrafiltration membranes provide stable filtration performance, effectively removing suspended solids, colloids, and microorganisms from the water, ensuring stable and reliable effluent quality. In contrast, internal-pressure ultrafiltration membranes are more prone to internal channel blockage when treating water with high suspended solids content, leading to reduced filtration efficiency.

The filtration direction of external-pressure ultrafiltration membranes, from the outside to the inside, means that pollutants in the feed water directly contact the outer surface of the membrane filaments. This structure requires the membrane filaments to have high strength to resist the adhesion and erosion of pollutants. For example, when treating feed water containing hard particles, these particles can wear the membrane filaments, reducing their strength and potentially causing damage. Moreover, the outer surface of the membrane filaments is easily covered by pollutants, forming a pollution layer that reduces the membrane's filtration performance. Studies have shown that when treating surface water with a suspended solids concentration of 100 mg/L, the flux decline rate of external-pressure ultrafiltration membranes is about 30% faster than that of internal-pressure ultrafiltration membranes, indicating that external-pressure ultrafiltration membranes are more susceptible to pollution.

To maintain the filtration performance of external-pressure ultrafiltration membranes, regular backwashing is necessary. Backwashing involves reversing the flow of water to flush from the inside of the membrane to the outside, removing pollutants from the outer side of the membrane. However, backwashing operations are relatively complex and require precise control of frequency, duration, and pressure. If backwashing is not performed correctly, it can damage the membrane filaments and further degrade membrane performance. For example, in practical applications, the backwashing pressure is typically controlled at around 0.2 MPa, with a backwashing duration of 30 seconds to 1 minute. Excessive backwashing pressure or duration can deform or damage the membrane filaments, while insufficient backwashing frequency will fail to effectively remove pollutants from the outer side of the membrane, affecting filtration efficiency.

Although external-pressure ultrafiltration membranes are adaptable to water with high suspended solids content, they still have certain requirements for feed water quality. Since the outer surface of the membrane filaments directly contacts the feed water poor, feed water quality, containing large amounts of organic matter, microorganisms, or minerals with high hardness, can exacerbate membrane surface pollution and even clog membrane pores. Therefore, pre-treatment of the feed water, such as coagulation, sedimentation, or sand filtration, is usually required before using external-pressure ultrafiltration membranes to remove most suspended solids and impurities, reducing the risk of membrane pollution. For example, when treating municipal sewage, coagulation and sedimentation should be performed first to reduce the suspended solids concentration in the feed water to below 50 mg/L, which can effectively extend the service life of external-pressure ultrafiltration membranes.

Internal-pressure ultrafiltration membranes use patented membrane-forming technology, resulting in membrane filaments with high strength and toughness that can effectively resist wear and impact from hard particles in the feed water. For example, when treating groundwater with high hardness minerals, the membrane filament breakage rate of internal-pressure ultrafiltration membranes is much lower than that of external-pressure ultrafiltration membranes. In addition, internal-pressure ultrafiltration membranes have strong pollution resistance. The smaller internal pore channels of the membrane filaments prevent pollutants from entering the interior, reducing the risk of membrane pollution When. treating surface water with a suspended solids concentration of 20 mg/L, the flux decline rate of internal-pressure ultrafiltration membranes is about 20% slower than that of external-pressure ultrafiltration membranes, indicating a significant advantage in anti-pollution performance.

The operation and maintenance of internal-pressure ultrafiltration membranes are relatively simple, without the need for complex backwashing equipment and procedures. In practical applications, internal-pressure ultrafiltration membranes require less frequent backwashing, typically once every 24 hours, compared to external-pressure ultrafiltration membranes, which may need more frequent backwashing. Moreover, internal-pressure ultrafiltration membranes provide stable product water quality, effectively removing turbidity, suspended solids, colloids, and microorganisms from the water. For example, when treating municipal, sewage the permeate turbidity of internal-pressure ultrafiltration membranes can be stably maintained below 0.2 NTU, meeting national standards.

Internal-pressure ultrafiltration membranes have strong adaptability to feed water quality changes and can effectively handle water sources with significant water quality fluctuations. For example, when treating slightly polluted surface water, even if the suspended solids concentration and water quality composition of the feed water change, internal-pressure ultrafiltration membranes can still maintain stable filtration performance and product water quality. This is because the filtration direction of internal-pressure ultrafiltration membranes is from the inside out, with pollutants retained on the outer side of the membrane, preventing internal clogging and ensuring stable filtration.

The filtration direction of internal-pressure ultrafiltration membranes, from the inside out, makes it easier for suspended solids and colloids in the feed water to form a pollution layer on the inner side of the membrane, hindering water flow and resulting in lower filtration efficiency. In practical applications, the recovery rate of internal-pressure ultrafiltration membranes is usually around 90%. In contrast, external-pressure ultrafiltration membranes can achieve a recovery rate of over 95%. For example, when treating surface water with high suspended solids content, the permeate turbidity of internal-pressure ultrafiltration membranes may be around 0.2 NTU, while that of external-pressure ultrafiltration membranes can be stably maintained below 0.1 NTU.

Due to their structural characteristics, internal-pressure ultrafiltration membranes cannot perform air-washing operations. Backwashing involves reversing the flow of water to flush from the outside of the membrane to the inside, removing pollutants from the inner side. However, because the inner wall of internal-pressure ultrafiltration membranes is relatively smooth, pollutants can easily adhere to the inner surface, making it difficult to completely remove them with water backwashing alone. Studies have shown that when treating surface water with a suspended solids concentration of 20 mg/L, internal-pressure ultrafiltration membranes typically require backwashing once every 24 hours, but even so, the flux decline rate remains relatively high. In contrast, external-pressure ultrafiltration membranes can combine air-washing and water-washing to more effectively remove pollutants from the outer side of the membrane, extending membrane service life.

Despite the relatively smooth inner wall of internal-pressure ultrafiltration membranes, pollutants in the feed water can still form complex retention layers on the inner side during actual operation. These pollutants not only reduce the membrane's filtration performance but can also lead to membrane pore blockage, further affecting filtration efficiency. For example, when treating wastewater containing large amounts of organic matter and microorganisms, the inner wall of internal-pressure ultrafiltration membranes is prone to biofilm formation, accelerating the flux decline rate. Moreover, since pollutants on the inner wall of internal-pressure ultrafiltration membranes are difficult to completely remove, even with backwashing, it is challenging to restore the initial filtration performance.

External-pressure and internal-pressure ultrafiltration membranes each have their own advantages and disadvantages and are suitable for different application scenarios.

External-pressure ultrafiltration membranes have the advantages of high filtration efficiency with a recovery rate of over 95%, suitability for treating water with high suspended solids content, and reversible membrane modules with strong adaptability. However, they also have some disadvantages, such as high membrane filament strength requirements, susceptibility to pollution, the need for regular backwashing with complex operations, and requirements for feed water quality with high pre-treatment needs.

Internal-pressure ultrafiltration membranes have the advantages of high membrane filament strength, strong pollution resistance, simple operation and maintenance, stable product water quality, and suitability for treating water sources with large water quality variations. However, their filtration recovery rate is relatively low, generally around 90%, they cannot perform air-washing, and their backwashing effectiveness is limited. Although the inner wall is smooth, pollutants can still form complex retention layers during actual operation.

In practical applications, the choice between external-pressure and internal-pressure ultrafiltration membranes should be based on a comprehensive consideration of specific water quality conditions, treatment requirements, and economic budgets.