Excess of RO water production
Excess of RO water production
(OP)
Hello everybody,
In our plant we have a 80-85 gpm Reverse Osmosis Unit (Supplied by a contractor that charges us for that consumption, even if we didn’t use it), but in average we are just consuming 55 gpm. We have two 60 m3 tanks for heating and storing the water. Since the RO can’t be stopped because of microbiology policies, when our tanks get full we send the excess permeated water back to the break tank of the RO.
Because sometimes we have peaks in permeated water demand, the production department didn’t allow us to reduce the capacity of the RO unit. So what can be done? Could it be possible to adecuate the capacity of the RO unit according to demand?
If this is not possible, do you think it would be convenient to send the excess permeated water (25 gpm) to our cooling towers (Mixing it with our deep well water?)
Thanks….
In our plant we have a 80-85 gpm Reverse Osmosis Unit (Supplied by a contractor that charges us for that consumption, even if we didn’t use it), but in average we are just consuming 55 gpm. We have two 60 m3 tanks for heating and storing the water. Since the RO can’t be stopped because of microbiology policies, when our tanks get full we send the excess permeated water back to the break tank of the RO.
Because sometimes we have peaks in permeated water demand, the production department didn’t allow us to reduce the capacity of the RO unit. So what can be done? Could it be possible to adecuate the capacity of the RO unit according to demand?
If this is not possible, do you think it would be convenient to send the excess permeated water (25 gpm) to our cooling towers (Mixing it with our deep well water?)
Thanks….





RE: Excess of RO water production
RE: Excess of RO water production
Suppose if you have a peak consumption of 80 gpm for 6 hours then the total extra requirement will be (80-55)*6*60 = 9000 gallons (approximately 1/4th of your storage capacity). If you have to produce this extra quantity in 24 hours (i.e 9000/(24*60) = 6.25gpm) then your plant capacity should be 55+6.25 = 61.25 gpm.
It is not a bad idea to use this water for cooling towers but why to use when not necessary?
Why is it that you can't stop an RO unit? We have been using this for purified water application and we do stop the system but the system should be flushed once a day atleast.
Regards,
RE: Excess of RO water production
RE: Excess of RO water production
I also always include a permeate flush of the membranes on shutdown to clear potential bad actors from the system. In the long run it protects the membranes from salt deposition.
Be careful turning the system down using VFDs to modulate flow. If you need to reduce capacity you likely will have to remove pressure vessels and membranes from service in order to keep the units balanced hydraulically. If VFDs are used they are normally for following feed water temperature variations so production can be maintained.
RE: Excess of RO water production
I talked with my RO contractor, and he didn't want to cycle the system based on water demand. Because he says that his membranes will get damaged, and that he will have to sanitize the equipment every time it gets stopped.
I would like to run the RO system (It consists in a softener unit, the ro and a ionic exchange) 6 hours, then stop it 2 hours, then run it 6 hours, then stop it 2 hours...
Do you think this is posible? The idea is to stop the RO unit when my tanks (120 m3) are 95% full, and restart it when they reach 65%.
Do you agree with the concerns of my contractor? We use the permeated water from the RO for making shampoo.
Thanks.
RE: Excess of RO water production
I don't think there will be a problem if you use tank level to control RO Plant operation. Also, there shouldn't be any problem with 2 hr duration. We have been doing it for our purified water system which is used for WFI generation.
If you are doing chlorination of feed water, then only source of bioload addition is your dechlorinating chemical(SMBS?), generally. It is better if you periodically test your dechlorination chemical for bioload. Further, if you have a UV system in the RO outlet then you need not worry.
Regards,
RE: Excess of RO water production
As for chlorine, your contractor may be using low pressure polyamide membranes which cannot see chlorine. Therefore, it is generally best practice to have the RO unit continually running to limit biological growth in addition to limiting scaling and variation in feed/waste flowetc.
Does he charge you for total production or just permeate production? If its only permeate, ask him if you can lower the product recovery ratio by lowering the feed pressure. This will reduce the permeate volume produced and lower operating costs.
For expert assistance on RO contact crane environmental or membrane systems corporation for more help.I've used both in my experience with RO and they can help you out.
RE: Excess of RO water production
RE: Excess of RO water production
The RO water could be mixed with the make water and you would save (if your makeup water is hard or has a lot of salts) chemicals, wastewater fees, you may even change the chemical program.
Kind regards
RE: Excess of RO water production
For instance if you only consider the cooling system as the system boundary then the lower salinity of the makeup would result in what you describe.
However, if you look at the plant water balance and have a RO at operating at 75% recovery (equals a cooling tower operating at four cycles)the tower will now operate at less than four cycles when drift loss and blowdown are accounted for. Had the tower been operating at 5 or more cycles then you have increased the total plant wastewater load when the RO concentrate and tower blowdown are combined.
The other problem is that the RO preferentially rejects the divalent cations and yields a soft, corrosive permeate. You may end up a chemical treatment scheme that actually costs more.
If you are currently operating the tower at less than 4 cycles and your RO at 75% recovery then you have either the opportunity to increase cycles or you may be getting set up to have RO problems in the future. This statement presumes that you are not operating the tower to protect a small load heat exchanger with either a high heat flux or high skin temperature.