Reverse osmosis for cooling water blowdown 3

[cryotechnic]

I'm working on a review of one of our cooling systems.

One of the things is saving on supply water. I’ve been reading about reusing the blow-down water of the cooling tower as supply water.
Our system has a blow-down of app. 28 m3/h.
In a reusing system, the water is fed through a Reverse Osmosis system.

I have no experience with RO.
Before I ask suppliers to come over, I do my own study about this first of course.

I'm looking for a RO system of about 30 m3/h. What is app. the operational cost for a system like that?
What difficulties during operation do I have to expect? Is there a possibility to mess up the membranes?
What is the maximum of TDS allowed in the feed water?

How is the unit controlled? Is the control based on the water quality? Is the pump frequency controlled or is it switched on/off with pressure switches.

What is done with the concentrate? Can we dump the concentrate in the sewer without any problems?

Does anyone have more info about RO systems? Does anyone have experience with a system like this?

Thank you in advance.
Cryo

"Math is the ruler of your potential succes...."

 

[quark]

There are no limitations as to the inlet quality of water, provided you plan for a proper pretreatment scheme. 30m³/hr is somewhat huge requirement and costs associated are significant(both capital and revenue). The running cost depends upon your location.

Considering makeup water at 500ppm and a COC of 5, you have about 2500ppm TDS in the input water. If the system is designed for 50:50 then your reject stream will have 4500 ppm TDS and I doubt this is a safe discharge limit.

Magnesium silicate can be a problem as this is cooling tower water and the scale is very hard to remove.

You can measure final conductivity and pH for permeate control, either dump it or recirculate back if not within the limits.

I didn't see any variable pumping systems so far and they will not work, most likely. It is an on/off system.

Few references

http://www.membranes.com/

http://www.osmonics.com/library/library.htm

 

[cryotechnic]

Thank you for your help, I will have a look on these sites.

Cryo

"Math is the ruler of your potential succes...."

 

[EdStainless]

If you keep looking you will also find systems that use multiple stages, filtration, RO, evaporators and so on. I have seen systems that only reject solids, and yes it is usually hazardous waste because of the metals content.

You really need to look at your entire plant and various waste water streams. You don't want to start with your worst water and try to turn it into the best.

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[bimr]

The application that you are considering is not really a good application for RO. Successfull RO applications generally involve water treatment, not wastewater treatment.

1. One of the difficult problems with RO is removing the influent suspended matter that will foul the RO membranes. You will need extensive pretreatment to remove the suspended matter (down to micron size particles) prior to the RO equipment.

2. Cooling towers are usually operated to the limits of a water's scalling potential. That is the same thing that you have to be worried about when operating the RO equipment. After the water salts are concentrated within the RO membrane, you have to be concerned that the water will scale inside the membrane.

3. The RO membrane typically operates at 75% recovery. That means that 25% of the influent flow will end up as wastewater with 4X's the salt content of the raw water.

4. The use of RO technology has been problematic when used on cooling towers in the past.

5. You would be better off using a cooling tower sidestream treament approach such as sidestream filters, or sidestream lime softening system. You are probably going to have to use these processes in conjunction with an RO system anyway.

 

[MedicineEng]

You have to consider the following:
Unless your water provider charges you an absurd cost for the water, a system like this will never be economical. Also you should think that you are recovering partially the water by concentrating all the pollutants in the rejected stream and this raises issues of wastewater discharge quality.
I don't have experience with blowdown recovery water but I agree with the other posters that before you go to RO, you must have a huge pre-treatment before, otherwise your RO membranes will be blocked in a very short time.
I am just guessing, butwould it be an option to add some chemical to precipitate part of the solids and only then pass trough filters? But then you would have the solids to dispose...
I think that recovering blowdown water is one of those cases where "the cure is worse than the disease"...

 

[dogtop]

cryotechnic,

Take a look at Chapter 38 in this book:

http://www.amazon.com/Nalco-Handboo...9048944?ie=UTF8&s=books&qid=1181223652&sr=1-1

HTH

dogtop

 

[EdStainless]

Yes, it is involved and expensive, but I have visited sites (both power gen and chem process) that are zero liquid discharge.
When (not if) you are prohibited from using any new fresh water then recycling your existing water becomes your only source.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[6748]

The pretreatment requirement can be summed up to as below.

01. A sand or multimedia filter : This is to remove the suspended solids and also to reduce the turbidity. This will call for a raw water pump.
02. Antiscalant dosing system. To feed/dose scale prevention chemicals to ensure that the membranes will not get scaled or fouled.
03. A micron filter system : Ususally 5 micron sized catridge filters are employed.
04. Chlorination system : this is based on the water quality at the inlet and also the water quality level required at the outlet. For a cooling tower makeup use or re-use application Chlorination may not be required.
05. Memebrane feed water storage tank of suitable capacity.
06. A high pressure water feeding pump. For the cooling tower water quality, a pressure of 150 mts of water column may be sufficient. ie 15 Kg/cm2 g pressure.
07. Then a series / parallel layout of membrane and their housing. This design is based on the lmh specs of the membrane employed. lmh = litres of water per square meter per hour.
08. Separate tank for the product water ie permeate. This piping should not have any back pressure.
09. Separate tank or pipeline routed to the disposal point for the waste water ie concentrate.
10. If required, a pH correction system - by dosing a pH correcting chemical.
11. Interconnecting pipes / isolation valves / pressure switches for pump safety/ relief valves / online pH meters / alarms / pressure gauges / temperature gauges and tripping circuits etc.,are the basic requirement --- Instrumentation panel etc., to match the instrumentation standards of the site. From fully automatic to fully manual.

6748

 

[cryotechnic]

Thanks to all,
in our system we have a sand filter, a partial stream of the cooling water is filtered here.
Als we have chemical treatment, anti scaling and chlorination for bio control.
From what I've understood from your answers: It's a complex and expensive thing to use the osmosis.
Thanks.

Cryo

"Math is the ruler of your potential succes...."

 

[dfmorvan]

Hi Cryotechnic

Using RO to treat CT blowdown is feasible but will not be easy to maintain. As the other contributors hinted, there will be problems with heavy pretreatment and scaling, or more likely with fouling of the membranes. Also consider that you usually have some bugs in the blowdown water and that could cause biofouling of the membrane.
If your goal is to save water, I would suggest to use RO to produce the make up water for the CT.
That way you should be able to use a higher concentration ratio on the CT (depending on your original water chemistry and treatment), ie instead of concentrating say twice you could go up to 5 or 6. With demin water, you will then be limited not by scaling, but by quick bacteriological development and you will have to change your chemicals program to avoid corrosion.

 

[BenThayer]

as others have stated, take a look at your overall site water make-up.

dfmorvan has a good point with regards to improving the water quality of the make-up water in that it reduces your chemical usage.

we changed from well water to raw/untreated "city" water from their reservoir and went from about 3.5 cycles to 6.

and some cooling towers and/or sites that have zero blowdown might also have lousy demisters. or small unmonitored once through users. all that behaves like blowdown is not necessarily at the blowdown meterstation of the tower.

 

[DocMax]

Hi Cryotechnic
In my job experience in the water treatment plants design I made this kind of plants.
This is a good way for saving water, but it's expensive.
The process is:
1- Pretreatment: The best is a clariflocculation to remove the colloids.
2- Filtration by sand-anthracite filters to remove the suspended solids. If the water is rich in iron you should use pirolusite filters.
3- RO, antiscale and bisulfite dosing stations and cartridge filters (usually 5 microns).
obiously the recovery of RO depends of water quality, usually for brakish water 75% recovery is fine.
4- pH corretion by sodium hydroxide dosing unit. Permeate water from RO plant is very agressive for carbon steel piping.
5 - The booster pumps for RO feeding can be controlled by frequency devices (inverters).
If you would like to have a zero discharge plant you could add a second RO unit to treat the reject water from first RO unit. This second RO unit works as a loop, you concentrate the reject from first RO and the recovery will be 40-50%. The reject from the second RO unit will be mixed with the reject of first RO unit; but in this way the membranes for the second RO unit has to be for seawater uses.
In this way you can discharge only a little part of reject.

Regards
DocMax

 

[sbush]

As Bimr stated, this is not a good application for reverse osmosis due to high total dissolved solids and scale-forming compounds. There is, however, a membrane-based system that is suitable. You should be looking into electro-dialysis reversal (EDR). EDR is the "junk yard dog" of membrane-based separation systems. EDR will perform very well on applications where reverse osmosis can't handle the high TDS, high silica, and hardness issues. It's worth a look. For starters, one leader in EDR systems is ionics.com.

S. Bush

http://www.water-eg.com