Excess of RO water production 4

[jisita]

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….

 

[semo]

Can you not use a VFD on the RO feed pump(s) to vary the flow through the unit as needed?

 

[quark]

What is the duration of your peak consumption? At the present rate of production and with two 60kL tanks, you can supply RO water for 17.6 hours at maximum production rate. You should take advantage of your huge storage capacity to reduce the flowrate of RO system.

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,

 

[VER]

quark is correct, you can always stop the R.O.No need to flush and/or sanitize if shutdown is only for short duration (e.g.< 1 day). You may consider automating your shutdown and start up of R.O. by installing level switches in your storage tank. R.O. to shutdown when high level switch is energized and start when the low level is energized. We do this all the time in our installations specially in Hotel and industrial users.

 

[cub3bead]

RO systems I design are set up to be cycled based on water demand. My preference is to automatically stop on high tank level and alrm on low tank level so the operator can give the system a once over before returning to service. This operating mode assumes your permeate storage tank has sufficient capacity to prevent the need to start/stop frequently.

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.

 

[jisita]

Thanks for your answers,

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.

 

[quark]

Jisita,

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,

 

[Digiorno]

Depending on your system, VER's response would work.

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.

 

[jmj5152]

Group: Would he not benefit by using the R/O water as cooling water as suggested? He would drastically reduce chemical addition/treatment costs, etc., compared to well water treatment? He seems to have a tough contractor who does not want his client to optimize.

 

[abcmex]

Jose,
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

 

[cub3bead]

The answer to jmj5152's question depends on where you draw mass balance boundary at and the number of cycles of concentration the cooling tower(s) operates at.

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.