Effect of low temperature on RO performance
1. Water flux Normally, for every 1 degree change in temperature, the water production volume changes by about 3%, this is mainly due to changes in the viscosity of water. (Higher water temperature) Low water viscosity is more conducive to water flow through the membrane, that is: water flux changes with temperature decreases. 2. Desalination rate The decrease in water temperature will increase the desalination rate of the system (different types of membrane elements have different degrees of increase), which means an improvement in the quality of the produced water. Note: The above discussion of the impact of temperature on the membrane system refers to the same operating conditions, that is, the same raw water salt content, pH, system operating pressure and recovery rate, etc. How to avoid the impact of low temperature on the Ro System Based on the above, the impact of low temperature on the RO system is mainly the lack of system output caused by reduced water flux. So how to avoid it? Generally speaking, there are the following ways: 1. Increase the temperature of raw water Since low water temperature causes a reduction in RO System water flux, the most direct way to solve this problem is to increase the raw water temperature. The most common approach is to heat raw water in pipelines (e.g. using plate heat exchangers, etc.) or in pretreatment water tanks (e.g. using steam coil heating, etc.). This method is more suitable for situations where there are heat sources in or near the Water Treatment System plant area. When heating raw water, attention should be paid to the selection of the location of the heating point. In real projects, it is often encountered that the temperature of the heated raw water drops again due to long pretreatment processes or the front water tank of the RO system being placed outdoors.
2. Increase operating pressure to make up for insufficient water production Since the water flux of the Ro Membrane system increases with the increase of operating pressure, when the water temperature is too low in winter, the system water production can be increased by increasing the operating pressure of the high-pressure pump. It is recommended to use a variable frequency pump to ensure the output balance of the RO system at different temperatures by adjusting the speed of the pump motor. Other advantages of using variable frequency pumps include: energy saving, increased system operation flexibility and system soft start. 3. Use low-pressure (large flux) RO Membrane elements Balancing the system water production by increasing the system operating pressure for a long time will inevitably increase the system energy consumption. At this time, the system operating pressure can be reduced by selecting low-pressure (large-flux) RO membrane elements, and at the same time, the RO system can be prevented from operating at ultra-high pressure or even extreme conditions for a long time. Operate under pressure. It is worth noting that the design of the RO system should comprehensively consider the temperature range of the system operation. While fully considering the low temperature factors, the changes in system performance at high temperatures should also be noted. For example: many systems will appear in summer due to the increase in water temperature. The membrane flux (system output) increases and the salt rejection rate decreases. At this time, the system should be adjusted according to the end user's requirements for produced water quantity and quality. Storage of RO membrane under low temperature conditions
The RO Membrane should be stored to avoid freezing. Once the Membrane Element freezes, it may cause irreversible damage to the performance of the membrane element. In addition, for unopened new membrane elements, if the ambient temperature is likely to drop below 0°C, please cover the packaging box with insulation material or increase the temperature of the storage room.
