Shutdown Considerations for RO Systems (manual)

May 29, 2021

                                   


1.    When shutting down a RO system, the system should be thoroughly flushed at low pressure with sufficient quality flushing water to displace all the brine from the pressure vessels .
Acceptable water for flushing is pre-treated feed water (refer to Table 1) or RO product water. 
Water used for flushing must not contain any oxidants. Maintain the flush water solution pH between 3.0 to 8.5 at all times.

Table 1: Suggested flushing water for various RO feed water treatment systems
RO feed water type Flushing water
Sea water Pre-treated feed water
Brackish water Pre-treated feed water
Wastewater RO product water
High pH feed water (such as 2nd pass high pH feed water) Pre-treated feed water without NaOH 1st pass product water without NaOH

2.    Ensure membrane elements are kept wet, properly sanitized, and protected from freezing during the shutdown period.
3.    Ensure guidelines for temperature and pH of the preservative solution are observed during the shutdown period.
Take care that product back pressure never exceeds 0.03 MPa at any time. Assess product back pressure on an individual stage basis. Product back pressure is defined as product pressure minus feed resp. brine pressure.
CAUTION. When multiple RO trains are running in parallel, and one train requires a shutdown, assure that the train to be shutdown is properly isolated from the common header piping using check valves or isolation valves. It is critical that pressure relief valves be present and installed on each train permeate line.
4.    Under any circumstances, do not expose any membrane elements to chlorine or other chemical oxidants. Any such exposure may damage the membrane, possibly resulting in an irreversible increase in salt passage.
5.    Take extra care to avoid chlorine exposure:,
    •    When disinfecting piping or pre-treatment equipment upstream of the membrane
    •    When preparing cleaning or storage solutions
    •    Take extra caution to ensure that no trace of chlorine is present in the feed water to the RO membrane elements.
    •    If residual chlorine is known to be present in the RO feed, it must be removed with sodium bisulfite (SBS) solution in stoichiometric excess, allowing sufficient contact time to accomplish complete dechlorination.



Short-term Shutdown


Definition
Short-term shut-down is for periods where an RO plant must remain out of operation for more than one day, but fewer than four days, with the RO elements remaining loaded in the vessels.

Prepare each RO train as follows:
1.    Flush the RO section with flushing water, while simultaneously venting any air from the system feed piping.
2.    When the pressure vessels are filled with flushing water, isolate the train by closing all isolation valves.
Repeat steps 1 and 2 above every 24 hours. 
3.    For detailed instructions of flushing procedures, see “TMM-250: Flushing Procedure” stated in “Toray RO Handling Manual”. 

 

Long-term Shutdown


Definition
Long-term shut-down is for periods where an RO plant must remain out of operation for more than four days with the RO elements remaining in the pressure vessels.

Prepare each RO train as follows:
Case. A) Sufficient flushing water is available.
1.    Flush the RO system with flushing water for 0.5-1.0 hour, while simultaneously venting any air from the system.
2.    When the pressure vessels are filled with flushing water, isolate the train by closing all isolation valves.
3.    Repeat steps 1 and 2 at least every two days. 
*For detailed instructions of flushing procedures, see “TMM-250: Flushing Procedure” stated in “Toray RO Handling Manual”. 

Case. B) Flushing water is not available.
1.    Circulate the permeate through the system. While circulating permeate through the system, inject the RO system, flush line with a 500 to 1000 mg/L (maximum) SBS solution. This solution will serve to inhibit biological growth during the shut down period. Circulate for 30–60 minutes.
2.    Make sure the RO system is filled with the SBS solution. To prevent the solution from draining from the system, close all system isolation valves.
3.    The pH of the preservative solution should never be allowed to drop below 3.0. Regularly check the pH. If the pH drops below 3.2, drain and replace the preservative solution as soon as possible. 
4.    If measuring the preservation solution's pH is not possible, repeat steps 1 and 2 with fresh solution. 
    •    Every 30 days if the temperature is less than 27°C (80°F)
    •    Every 15 days if the temperature is equal to or greater than 27°C (80°F)  

Note: Any contact of the SBS solution with air (atmospheric oxygen) will oxidize SBS to sulfate, and the preservative solution pH will begin to drop. Take special care to keep the SBS preservative solution isolated from atmospheric oxygen. If the SBS is allowed to revert to sulfate, the potential for a biological activity will increase.



[Notice]
The data and information contained in this document are based upon rigorous technical testing by Toray, and is to the best of our knowledge reliable. Toray cannot control design and operating conditions, and consequently Toray does not assume any liability for results obtained or damage incurred through the application of the information provided herein. No liability, warranty or guarantee of final product performance by Toray is implied by the information provided in this document.
This manual supersedes all previous versions. Technical modification of products or production technology may necessitate changes to information in this manual without prior notice. Please verify that your version of the manual is the latest version available by either contacting Toray, or checking online at www.water.toray.

[Remarks]
This content is extracted from Toray’s “Toray RO Handling Manual (Version. Apr. 2021)” (the Manual). Please refer to other contents stated in the Manual. When using this content, please read the Terms and Conditions of Use stated in the manual and agree to them before using this content.
(http://s3-ap-northeast-1.amazonaws.com/water.toray/knowledge/manual/pdf/RO_HandlingManual.pdf)