The Reverse Osmosis Membrane Element is a deep filtration method, and the surface of the reverse osmosis membrane element will inevitably have the precipitation of colloids, microorganisms, impurity particles and insoluble salts on its surface. Therefore, once the reverse osmosis devices used in various fields are put into use, they will eventually need to be cleaned, Just the length of the cleaning cycle is different.
Generally speaking, membrane system failures caused by organic matter account for 60% to 80% of all system failures. Organic matter in influent water is adsorbed on the surface of membrane elements, which will cause flux loss. In many cases, the adsorption layer formed on the membrane surface acts as another separation barrier to the dissolved salts in the water, blocking the membrane channel and causing the membrane system to be polluted by organics.
The following cleaning procedures are specifically for the cleaning of fouled membrane systems in the presence of organic substances such as humic and fulvic acids, antiscalants or oils. It is generally recommended to take the following six steps to clean the membrane elements with organic contamination, and these six cleaning steps need to be repeated, one for high pH cleaning solution and the other for low pH cleaning solution.
1. Prepare the cleaning solution as following table:
|No.||Cleaning fluid||Addition volume to prepare 379 liters of solution||PH regulation|
|1||Citric acid||7.7kg||Adjust pH to 3.0 with ammonia|
|2||STPP||7.7kg||Adjust pH to 10.0 with vitriol and ETDA salt 3.18kg|
|3||Sodium tripolyphosphate||7.7kg||Adjust pH to 10.0 with ammonia|
|4||SDBS||0.97kg||Reverse osmosis product water(ECF) 379L|
2. Low flow input cleaning fluid
When the cleaning pump mixes the cleaning liquid, preheats the cleaning liquid and replaces the raw water in the element with the cleaning liquid, it is carried out under the operating conditions of low flow and low pressure, and the required pressure only needs to be sufficient to make up the pressure loss of the feed to concentrate.
When the raw water is replaced, the cleaning fluid should appear in the concentrated water pipeline, and the cleaning liquid can be circulated back to the cleaning liquid tank for 30 minutes. If the color of the cleaning solution still changes, drain the cleaning solution and prepare a new cleaning solution according to step 1.
Stop the operation of the cleaning pump and allow the RO Membrane Elements to be completely immersed in the cleaning solution. The soaking time is about 1~15 hours (soaking overnight will be better). In order to maintain the temperature of the soaking process, a lower circulating flow rate can be used. The soaking time depends on the severity of the pollution. For mild pollution, soaking for 1~2 hours is sufficient.
5. High flow pump circulation
High flow cycle for 30 minutes. High flow rate washes away contaminants removed by cleaning fluid. Under high flow conditions, there will be problems with excessive pressure drop, the maximum allowable pressure drop for a single element is 1 bar (15psi), and the maximum allowable pressure drop for a multi-element pressure vessels is 3.5bar (50psi).
Qualified pretreated effluent can be used to flush the cleaning fluid unless there is a corrosion problem (eg. Still seawater will corrode stainless steel pipes). To prevent precipitation, the minimum flushing temperature is 20℃, and the system flushing time is about 1 hour.
7. Repeat step 2 to step 6 with pH 2 HCI solution
8. Reboot the system
It is necessary to wait for the components and the system to stabilize, and then record the operating parameters after the system is restarted. The time for system performance to return to stable after cleaning depends on the level of previous contamination.
(1) In order to maximize the cleaning efficiency, the temperature of the cleaning solution must be higher than 25℃. The elevated temperature helps the cleaning solution to remove organic contaminants from the membrane surface.
(2) Some organics such as oils are very difficult to clean, in order to remove them, experiment with various cleaning soak times for optimum efficiency. In addition, the most effective cleaning solutions usually contain surfactants such as Na-SDS or some commercial cleaners contain surfactants and detergents that help remove oil contamination. Please consult the chemical supplier.
(3) If the organic pollution is caused by the addition of excessive flocculant in the pretreatment part, it is more effective to use an acidic cleaning solution first and then an alkaline cleaning solution. In order to determine the proper sequence of cleaning solutions, try to obtain samples of system contaminants, and qualitatively confirm which sequence is better for the sample. If the effect is comparable, it is usually more appropriate to first wash the organics with a high pH cleaning solution.