Precautions during reverse osmosis chemical cleaning process

In the actual cleaning process of the reverse osmosis system, attention should be paid to controlling the actual value of flow rate (flow rate) to not exceed the specified value of a single membrane element. For online cleaning, a non segmented cleaning method is usually adopted. During the non segmented cleaning process, the final end is in a high-speed cleaning state, while the front end is in a low-speed state. If the flow rate in the front section is blindly increased, it will cause the rear end to be in an excessively high speed state, which is extremely easy to cause damage to the membrane components. The main reason is that the concentrated water at the end is directly circulated to the cleaning tank through the concentrated water pipe, which is combined with the atmosphere, causing a sharp change in the pressure difference of the rear membrane element and a sharp increase in flow rate.
For segmented cleaning, the flow rate of different sections is obviously different, especially in the 3:2:1 arrangement device, where the flow rate values differ by three times. Therefore, it is necessary to adjust the outlet flow rate of the circulating cleaning pump in a timely and correct manner during the cleaning process.
When selecting cleaning agents, attention should be paid to their actual effectiveness. An effective method is to take some dirt and put it into the cleaning agent, gently shake the liquid intermittently for 30-60 minutes. If the dirt is completely dissolved, it indicates that the cleaning agent is very effective. It is advisable to use a non segmented cleaning method, which has the same effect as segmented cleaning; If the dirt is partially dissolved and the residue is loose and suspended, it indicates that the cleaning agent is still effective. It can be cleaned in sections or not; If it partially dissolves but the residue is relatively hard, then these dirt should be from different parts of the RO shell pressure vessel. The dirt inside the security filter can serve as a reference. In short, the results of this work are straightforward, highly persuasive, simple, convenient, and easily repeatable.
The cleaning agent should first rule out its inability to have oxidizing properties, and pay attention to the effect of pH value on the decomposition of RO membrane during the cleaning process. In practical operation, pH values are often neglected in strict testing and control, leading to a decrease in membrane performance after cleaning, especially in desalination rate, which can doubt the performance of cleaning agents. Chemical cleaning drugs that generally do not present pure solvents or reagents, except for oxidizing properties and pH values, usually do not cause damage to the performance of the membrane. Therefore, the selection range of drug types should be large, not limited to the recommendations of membrane manufacturers or products formulated by professional cleaning companies, and there is no need to increase cleaning costs.
The important performance of cleaning agents lies in their strong solubility in dirt. Emphasizing high-speed cleaning is actually emphasizing its high-speed flushing function, but it should be understood that the inner membrane components of pressure vessels are in series, with a diameter of about 200mm in the space between the two membranes, and the flow rate cannot be compared to the cleaning flow rate on the membrane surface. If the cleaning agent does not have strong solubility in dirt, the dirt stripped by high-speed hydraulic flushing will accumulate in the space between the two membranes. After normal startup, these dirt will rise and enter the downstream membrane elements, causing an increase in pressure difference of a certain membrane element. The result is that the pressure difference is far from the average value, and the performance loss of the membrane element is extremely fast. Therefore, it can be said that blindly pursuing and emphasizing high-speed cleaning does not fully evaluate the effectiveness of cleaning agents in terms of scale solubility. Of course, high-speed offline cleaning for single membrane components is another matter.
During the cleaning process, it is recommended to be particularly cautious when closing the production water valve to ensure the cleaning effect and ensure the cross-sectional flow rate on the membrane surface. Taking a two-stage RO device as an example to illustrate the inadequacy of this approach. Figure 8-47 shows the trend of changes in water pressure and head loss during the cleaning process with the production water valve closed. During the cleaning process, increase the cross-sectional flow rate on the membrane and fully open the concentrated water valve to ensure that the concentrated water flows into the cleaning water tank as soon as possible. Therefore, the pressure of the concentrated water at the outlet of the second section will be released to atmospheric pressure.
The connecting pipes for the water production in the first and second stages of the RO device are interconnected water pipes that serve as connecting containers to ensure that the water production pressure in any part of the first and second stages is equal. The actual pressure for normal cleaning is 300kPa. The osmotic pressure of the cleaning solution is generally lower than 100kPa, so water production is inevitable. Closing the water production valve causes the water production pressure to rise to over 100kPa, resulting in the back pressure of the two-stage membrane element exceeding the limit, which can easily damage the membrane element.
Some cleaning procedures and requirements cannot be simplified or omitted, such as dismantling the front and rear end plates of pressure vessels or selecting a container to cycle and push out a membrane element to scrape off pollutants, which is extremely meaningful in determining the type and amount of pollutants. When there are many pollutants, the membrane components are first sequentially removed and the RO produced water is used to focus on external flushing of the pollutants at both ends of the membrane, while simultaneously flushing the pressure vessel and end plate, and then reset for normal online cleaning procedures.
Observing the pH value and color changes of the cleaning solution during the cleaning process is crucial for the cleaning effect and membrane performance. It is important to adjust the pH to the target value in a timely manner using acid or alkali. Rapid detection of the substance composition in the cleaning solution at the cleaning site is generally not achievable in a timely manner, but the color of the cleaning solution will change to a certain extent with the circulation process. This provides a certain reference for determining the cleaning effect and the amount of substance composition in it. Due to the function of cleaning the microporous filter, when the color of the cleaning solution changes to a constant and transparent state, it indicates that the cleaning is approaching the end point. Otherwise, the preparation should be repeated.