Ultrafiltration Membrane Process in Water Treatment Applications – Operating Parameters

Properly understanding and implementing operating parameters are crucial for the long-term and stable operation of ultrafiltration systems. The main operating parameters typically include: flow rate, pressure, pressure drop, concentrate discharge, recovery ratio, and temperature.

1. Flow Rate

Flow rate refers to the linear velocity of the feed liquid (raw water) flowing across the membrane surface and is a critical operating parameter in ultrafiltration systems. A high flow rate not only wastes energy and generates excessive pressure drop but also accelerates the deterioration of ultrafiltration membrane performance. Conversely, a low flow rate increases the thickness of the concentration boundary layer on the membrane surface, leading to concentration polarization, which affects both water permeation rate and quality. The optimal flow rate is determined through experimentation. For hollow fiber ultrafiltration membranes, when the inlet pressure is maintained below 0.2 MPa, the internal pressure membrane’s flow rate is only 0.1 m/s, indicating laminar flow. External pressure membranes can achieve higher flow rates. In capillary-type ultrafiltration membranes, increasing the capillary diameter to around 3 mm can help reduce the concentration boundary layer, thus benefiting flow rate enhancement. However, it’s important to note that flow rates cannot be arbitrarily set and are related to the inlet pressure and raw water flow rate. Additionally, for both hollow fiber and capillary membranes, flow rates are not consistent at the inlet, and the flow rate at the outlet is approximately 10% of that at the inlet when the concentrate flow rate is 10% of the raw water flow rate. Increasing the capillary diameter and appropriately raising the concentrate discharge (recycle flow) can enhance flow rates, especially in concentration processes, such as the recovery of electrophoretic paint, where ultrafiltration rates can be effectively increased.

In the allowable pressure range, increasing the feed water amount and selecting the maximum flow rate are beneficial for ensuring the performance of hollow fiber ultrafiltration membranes.

2. Pressure and Pressure Drop

The operating pressure range for hollow fiber ultrafiltration membranes is 0.1 to 0.6 MPa. This refers to the working pressure commonly used for treating solutions within the domain of ultrafiltration. Different molecular weight substances require different ultrafiltration membranes and corresponding operating pressures. Generally, the internal pressure membrane of the hollow fiber is limited by a pressure resistance strength of less than 0.3 MPa, and the pressure resistance strength of the hollow fiber itself is also generally less than 0.3 MPa. Therefore, the operating pressure should be below 0.2 MPa, and the pressure difference between the two sides of the membrane should not exceed 0.1 MPa. The pressure resistance strength of the external pressure hollow fiber ultrafiltration membrane can reach 0.6 MPa, but for the external pressure membrane components with plastic shells, the operating pressure is also limited to 0.2 MPa. It should be noted that due to the larger diameter of the internal pressure membrane, it is prone to flattening and cutting at the bonding site when used as an external pressure membrane, which can lead to damage. Thus, internal and external pressure membranes cannot be used interchangeably.

When a certain pressure is required for the ultrafiltration permeate for use in the next process, stainless steel shell ultrafiltration membrane components should be used. These hollow fiber ultrafiltration membrane components can withstand pressures of up to 0.6 MPa, while the pressure of the ultrafiltration permeate can reach 30 meters of water column, or 0.3 MPa, but the pressure difference between the two sides of the hollow fiber ultrafiltration membrane must not exceed 0.3 MPa.

When selecting the operating pressure, apart from considering the pressure resistance strength of the membrane and shell, the compactness of the membrane and its resistance to fouling must also be taken into account. Higher pressure results in greater water permeation but also causes more accumulated substances to gather on the membrane surface, increasing resistance and leading to a decrease in water permeation rate. Additionally, particles entering the membrane pores can easily block the channels. In general, selecting a lower working pressure when possible is advantageous for fully utilizing membrane performance.

Pressure drop of hollow fiber ultrafiltration membrane components refers to the difference between the pressure at the feed inlet and the pressure at the concentrate outlet. Pressure drop is closely related to the feed water amount, flow rate, and concentrate discharge. Particularly for internal pressure-type hollow fiber or capillary-type ultrafiltration membranes, the flow rate and pressure on the membrane surface gradually change along the water flow direction. The larger the feed water amount, flow rate, and concentrate discharge, the greater the pressure drop. As a result, the pressure on the downstream membrane surface may not reach the required operating pressure. The overall water production of membrane components will be influenced. In practical applications, it is important to control the pressure drop value to prevent it from becoming too large. As operating time extends, resistance due to accumulated fouling increases, causing a higher pressure drop. When the pressure drop exceeds the initial value of 0.05 MPa, cleaning and flushing of the water pathways should be performed.

3. Recovery Ratio and Concentrate Discharge

In ultrafiltration systems, the recovery ratio and concentrate discharge are interrelated factors. The recovery ratio refers to the ratio of the permeate flow to the feed flow, while the concentrate discharge refers to the amount of water discharged that did not pass through the membrane. Since the feed water amount is equal to the sum of the concentrate and permeate flow, a larger concentrate discharge leads to a smaller recovery ratio. To ensure the normal operation of the ultrafiltration system, a minimum concentrate discharge and a maximum recovery ratio for membrane components should be specified. In general water treatment projects, the recovery ratio of hollow fiber ultrafiltration membrane components is around 50% to 90%. The selection of the recovery ratio depends on factors such as the composition and state of the feed liquid, the amount of substances that can be retained, the thickness of the fouling layer on the membrane surface, and the influence on permeate flow. In many cases, smaller recovery ratios can be used, and the concentrate can be recycled back into the raw water system by increasing the circulation volume to reduce the thickness of the fouling layer, thereby improving water permeation rate. This approach can sometimes lead to lower energy consumption per unit of water produced.

4. Operating Temperature

The water permeation capacity of ultrafiltration membranes increases with higher temperatures. Generally, the viscosity of aqueous solutions decreases with temperature, reducing flow resistance and correspondingly increasing water permeation rate. The actual temperature of the feed liquid at the operational site should be considered in engineering design. Seasonal variations, especially in colder temperatures, require temperature adjustments to avoid potential changes in water permeation rate of around 50%. Excessively high temperatures can also affect membrane performance. Typically, the operating temperature for hollow fiber ultrafiltration membrane components should be around 25±5°C. If operation at higher temperatures is necessary, heat-resistant membrane materials and shell materials can be used.

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading

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