How to figure out contaminations of RO Membrane Elements?
Eventually, during normal operation, the reverse osmosis membrane components get contaminated by suspended or low-solubility compounds found in feed water. Calcium carbonate, calcium sulphate, metal oxides, silica, organic or biological deposits are the most prevalent materials deposited on the surface of membrane components.
The content of the feed water influences the character and pace of the salt scale on the membrane element surface. Salt scale is a progressive component that, if not controlled early, can have a detrimental influence on the performance characteristics of reverse osmosis membrane elements in a very short period of time.Monitoring overall system performance on a regular basis is critical for detecting membrane element fouling. The impact of fouling on membrane flow is a slow process that is dependent on the type of the pollutant.
Heavy pollution of reverse osmosis membrane elements: Causes and Features
Although the reverse osmosis system will be designed with a certain degree of surplus to ensure that in an emergency, the water production or desalination rate of the reverse osmosis system will not be reduced, and the pressure difference of the reverse osmosis system will increase, resulting in insufficient water supply and safety in production.But it is exactly because of the existence of these surpluses that sometimes-latent flaws cannot be recognized in time, which can finally lead to substantial contamination of reverse osmosis membrane materials.
Contaminant elimination in RO systems
To eliminate pollutants, cleaning, washing, or modifications to system operational parameters are necessary. Contaminant removal should typically take place under the following conditions:
1-When compared to the estimated flow under normal pressure, the normalized filtrate flow should decrease by 15%.
2-The electrical conductivity of the permeate should be enhanced by 15%, and the salt passage by 15%.
3-The pressure drop in the RO pressure vessel should be reduced while the product water constant flow and recovery should increase by 15%.
Heavy pollutant removal in RO systems
Below is a list of common pollutants and ways for removing them.
1: Scale of calcium carbonate: Calcium carbonate deposits primarily forms from any sort of feed water when there is a failure in the anti-scalant addition system or the acid injection pH control system, resulting in a high pH level in the feed water. Early diagnosis of calcium carbonate scaling is critical for preventing damage caused by crystals on the active membrane layers.
Calcium carbonate scale discovered early in the process can be eliminated by reducing the pH of the feed water to 3.0 - 5.0 for one or two hours.Longer term calcium carbonate scale accumulations can be eliminated by recycling 2 percent citric acid and a pH of at least 4.0 via the RO membrane components.
2: Scale of calcium sulphate: The best method for removing calcium sulphate scale from the reverse osmosis membrane element is to use sodium tripolyphosphate (Tetrasodium salt of ethylenediaminetetraacetic acid).
3: Metal Oxides as Pollutants: The calcium carbonate scale removal technique is commonly used to remove precipitated hydroxides (e.g., ferric hydroxide).
Cleaning and flushing of RO membrane elements
Cleaning solution recycled from the feed side flushes reverse osmosis membrane components in pressure vessels at low pressure and relatively high flow. This necessitates the use of a chemical cleaning method for the membrane components.
Cleaning general techniques for reverse osmosis membrane elements:
1. Flush the pressure vessel for a few minutes with clean free chlorine generated water from the cleaning tank (or an equivalent source).
2. Using generated water, combine a new volume of the specified cleaning solution in the cleaning tank. The volume of the cleaning solution is regulated by the number and size of membrane components.
3. It is advised that the temperature be controlled and the pH level of the cleaning solution be kept within limits.
4. After flushing, empty and wash the cleaning tank and refill it with the created clean water for cleaning.
5. Flush the membrane elements for a few minutes with clean free chlorine generated water from the cleaning tank (or an equivalent source).
6. After cleansing the reverse osmosis system, start the system with the filtrate and concentrate valves until clear water devoid of any foam or residuals from the cleaning agents passes through the system (usually within 15-30 min).
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