Effectiveness of  RO (water treatment facility) backwashing

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Dear fellows,

I would like to ask the effectiveness of  RO (water treatment facility) backwashing.  

Is there a way to calculate or say how long the membrane replacement would be postponed with backwashing?  and is the backwashing is a necessity of RO system? 

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4 Answers

  1. Dear Nomundari,

    I am not sure if you are Nomundari I know from the univesity in Vienna ? The term backwashing can lead to misinterpreatations, becuase it would implement to wash against the forward direction ( which is feed to permeate). Never wash from the permeate side to the feed side. The possibility to wash from the outlet (concentrate side) to the inlet side but only on the feed-conncentrate side, is always possible and has the  advantage, that brine seals (V-Chevron seals) flap in and there will be some flow between membrane element and pressure housing, which is normal a very stagnant area (a 8" membrane shows a stagnant volume of around half a  liter)- if heavy biofilming- Billions of bacteria are in this area and even during disinfection-sanitisation, stay as they are. If you are the Noumndari I know, send me an e-mail: walter.lintner@usf-water.com. Kind regards Walter

  2. I will add to Raphael and Pablo's comments. Yes, it depends on several factors, such as, membrane types, Feed water quality, Pre-treatment measures to include micron size pre-filters employed, chemical pH adjustments, as well as, proprietary anti-scalants. Other factors include your recovery rates (ratio of Feed water/Permeate/Concentrate flows) and how often you perform cleanings, as the more you clean, the more the membranes degrade over time.

    From my experience, I find that a "tech hack" is to utilize your "Clean-in-Place (CIP) System" to collect Permeate water and heat it up with a built-in heating element in the CIP tank to about 100 F and circulate the flow of water through the membranes in reverse at a conservative low flow for back-washing.  Likewise, if you find that the face of the 1st pass elements to be fouled, you can also replace (switch) these with elements on the last stage, but flip them over so that the fouled ends face the discharge end before you conduct a normal flow rinse cycle with heated water. In addition, you should make it a standard operating procedure (SOP) to flush your membranes with Permeate water before shutting down the plant for an extended period of time for maintenance, etc. Elements should only soak in Permeate water. 

    Implementing all these procedures, can help extend the life of the membranes significantly. However, this also depends on how your R.O./ Desal system is designed to include the infrastructure for a CIP system with built-in heater element and valving to configure for regular cleanings and or flushing scenarios.  I have been able to extend membrane average service lives from 4-6 years to 9-11 years.  Your performance will be reflected in your "Normalized Data program software" which you can trend. Do not forget to conduct "Conductivity Profiles" for each element to ensure integrity for fouling or leaking "O" rings and connectors.  Hope this helps. Postulating is fine, but I find that experimenting is the best way to achieve results. Nothing ventured nothing gained. Should anyone wish for consult, you are more than welcome to contact me. Best regards.     

  3. The answer depends on the type of the membrane, the quality of the water, the way of operation, the type of fouling, Etc. It cannot replace good and suitable pretreatment. It is possible to use it when a  relatively small tank of the product is located on the roof of the RO room, so every time the plant shuts down, the backwash starts to operate automatically. It is suggested to design properly the thank and it is good for a system that may be shut down from time to time. It is important to note that the operation may be effective only on low fouling. An established thick layer of fouling will not be removed by the backwash and large fouling particles may damage the membranes.

    1 Comment

    1. Dear Raphael Semiat, thank you very much!

  4. Hello Nomundari,

    In reverse osmosis, the backwashing is said ``flushing´´ due to the fact that the water flows in the same direction as operation. It´s completely neccesary to make a flushing everytime the RO stops, or every 12 hours working.

    How long the membranes should be replaced depends on the inlet water quality and depends on whether you make the chemical cleaning when it´s needed. 

    I normally use the normalized permeate flow, which includes mathematical formulas about pressure, conductivity, temperature, etc. to know when the chemical cleaning is neccesary.

    https://puretecwater.com/reverse-osmosis/reverse-osmosis-normalization

    If you need some example, I can give you some sheet to use. It´s very easy. Taking care of this, the membranes life will increase.

    Best regards,

    Pablo

    1 Comment

    1. Dear Mr. Pablo Ruiz Molina, thank you very much! Much appreciated, I think i found what I was looking for.