I am not an Engineer so I decided to come here and hopefully one could help me.  We have a newly installed Reverse Osmosis Water System in our facility.  The piping is polypropylene and is 32mm in diameter.  If needed I can get the pump info but don't have it here now. 

We would like to know the minimum flow rate that is needed to generate a turbid flow.  Turbid flow is needed to prevent the buildup of biological material (biofilm i.e. bacteria, mold, etc.) from growing in the system. 

The answer or a place to go and find it for ourselves would be greatly appreciated. 


Do you need to know the pressure of the system, HP or RPMs of the motor (Pump), number of drops etc.?

I am no expert on pure water, but I can tell you that our system is designed for a velocity of 4.5 ft/sec (1.4 M/sec), and we have never experienced problems with biological growth.  It is very important to eliminate stagnant legs in the system by creating a continuously flowing distribution loop; at what point the velocity becomes unacceptably low, I could not say.

We also use UV sterilization and microfiltration to destroy and remove any biological matter that is present before the water enters the distribution system.

One recognized minimum standard flow rate in pure water systems is 3 feet per second.  You'll be good at that velocity.  A number of studies have indicated that lower flows are adequate and a number of major firms use the flow rate equivalent to a Reynolds number of 20,000.  Flow at this rate will retard biofilm growth, but not prevent it. As per other entries, having minimum dead legs, using UV lights in the loop, using diaphragm valves,etc are all important.  In any case, there will still be bacterial growth, its just a matter of how much how soon.  Depending on how much bacteria your process can tolerate, you will still need to sanitize periodically.  How often depends on your spec and the system.  If bacterial level is an issue, you need to sample regularly and track the bacterial levels to determine when to sanitize.  Since this is pp piping, you can not heat sanitize.  You will need to a chemical - peracetic acid is one.
This minimum flow rate would be what you would want in the return section of the loop when there is the maximum amount of drawoff from the drops.  You would make an estimate of the expected maximum flow, based on the total number of drops, the number of drops that might reasonable be expected to be open at one time, and the flow per drop. Your pump needs to provide that flow plus the flow in the return section of line equal to 3 fps.  That is the design case. The pump needs to provide the head to accomplish this. During periods with no draw off at the drops, or minimal draw, which is probably much of the time, the pump will run out on its curve and the "normal" flow rate through the system will be higher (probably 4 to 6 fps).

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