BA AND CA SULPHATES REMOVAL BEFORE RO ??

[andredd]

hello everybody...!
i m asked by a client to recover with RO 80% of a water coming from DMF backwash + demi plant regeneration effluents (after neutralization). This water has a lot of sulphates (5000 ppm) and calcium (1250 ppm), some magnesium (550 ppm) and barium (0,5 ppm).

The RO design guy said that scaling is a big issue and that recovery can not be achieved even using the best antiscalant in the RO membranes. My question is: if the solubility constants Kps are so exceeded, so there is tendency for "self - precipitation" would it not be possible to precipitate (also adding chemicals etc) in same pre-treatment TSS clarifier the calcium sulphates, barium sulphates and then going to RO plant? any of you experienced this? or is it theory hard to put in practice? thanks a lot since now ! have a nice day !
Andrea

 

[bimr]

You should be able to precipitate calcium sulfate with lime. The calcium sulfate solubility is around 2700 mg/l. After that precipitation step, the water will still not be suitable for RO treatment. The calcium sulfate precipitation will use a lot of chemical. The treatment residuals will have to be dewatered and disposed of as well.

Your best process option if you want to recover the water is to use a brinc concentrator (evaporator).

http://www.gewater.com/products/equipment/thermal_evaporative/brine_concentration.jsp

Recycling demineralizer wastewater is not an RO application.

 

[andredd]

ok many thanks for your quick reply ! but apart from chemicals consumptions and disposal issues (also evaporator has costs, in terms of enery, and high also CAPEX cost)of sludge from clarifiers bottom, why do you say that recovering from regeneration effluents is not an application? still moneywise or what?

you mean, its better to run at , dunno, 50% recovery with simple RO system with good antiscalant rather than making it hard aiming to 80%?

pls advice thank you !!
Andrea

 

[bimr]

It is going to be a considerable expense to recycle the demineralizer waste. I am saying that the most practical approach considering cost and ease of operation/maintenance is the brine concentrator.

Assume you go with a RO system, what do you plan to do with the RO reject stream? I see little value in reducing your waste volume with a 50% recovery RO. You will just be concentrating the salts further, making a smaller volume of waste but something that is more difficult and expensive to dispose of.

Trying to precipitate solids prior to the RO system is generally not a good approach because suspended solids will foul the RO unit.

For the most part, the applications for RO are for water purification not waste treatment. There are only a few specialized RO waste treatment applications that are practical/economical.

One of the RO membranes that could be used here in this application is the tubular membrane.

http://www.kochmembrane.com/prod_tubular.html

You will most likely find that it is not cost effective.

You might consider revising your water treatment scheme.

If you go with a RO treatment followed by EDI, your waste residual volumes will typically be less than the use of a demineralizer.

If you use a brine concentrator, you can recycle almost 100% of the water back as DI water. The salts are concentrated into a dry powder and easily disposed of. Waste residual problems solved.

 

[andredd]

thank you . So at the end you are saying that the brine concentrator, working on 16 m3 / hr, in terms of CAPEX but ALSO on OPEX, would be less utility / power / overall cost consuming than RO ?
thank you

 

[bimr]

correct

 

[andredd]

ok many thanks

 

[DeltaCascade]

Alternately, waste the demin plant effluent as it may be about as, if not more, concentrated than RO reject waste water (there's nothing like a component mass balance to tell you that .. but it is work). You can recycle a portion of demin plant regen rinse waters which is of good quality (monitor conductivity), if worth the extra piping and perhaps, tank.

Lime softening will reduce calcium to about 35 ppm as CaCO3, a little higher if TDS is high, but soda ash (Na2CO3) feed may be needed to ensure alkalinity is in excess of hardness, or lime softening will not be that effective. That is another topic.

So, it is good to have the component balance in mind, and consider the waste stream qualities.

So the "RO guy" may be right, too.

If you need to recover that very high TDS ion-exchange demin plant wastewater, yes, evaporator may be best, as bimr said. But that must be some very expensive water (distillate) that you get harvest with the evaporator. And, evaporator will have a very concentrated waste stream to deal with ... and the evaporator will likely be subject to extensive fouling (that calcium and barium must go somewhere).

Good luck.
//

 

[andredd]

many thanx evbody for kind support ! i think i will go first to RO (45% max to have saturation 95% and be at limits of scaling) and then evaporator.

yes they must recover as

a) disposing outside not allowed
b) paying tankers not feasible also from logistic point fo view
c) evaporation pond already constructed and not possible to extend it

basically it has been a mistake of their EPC contractor which thought that 80% recovery was possible with RO just looking at the overall TDS salinity, but not considering that calcium and sulphates were accounting nearly 50% of overall salts......

Andrea

 

[andredd]

but, would it be better evaporators or crystallizers? which is the difference exactly? is it concerning the aimed target i.e. if the scope is recovering salts (not here) or water ?

my brine would enter the unit with some 62.000 TDS, mainly sulphates and calcium

thanks evbody since now !
andrea

 

[bimr]

Evaporation is a two step process. The first step is a brine concentrator. The second step is the crystallizer which will give you dewatered solids.

If you have a solar pond in a favorable climate, you can use only the brine concentrator step.

Not really sure what you stating. But on a power plant zero discharge application, I used RO/EDI on the make up water supply and evaporator on the wastewater

 

[andredd]

thanks for your reply very much. I do have a concern: dont the sulfates make big problems with scaling? as i know, they have inverse solubility relationship with temperature, dont they make CaSO4 / BaSO4 scaling quickly inside the evaporator? Did you not have problems such as frequent chemical cleanings etc?

 

[bimr]

The manufacturer of the evaporators know how to make a product that works. These systems are designed to evaporate the water. This process is well understood and widely used.

Contact this firm if you have any questions:

http://www.gewater.com/products/equipment/thermal_evaporative/brine_concentration.jsp

What type of project are you working on?

 

[andredd]

thanks a lot.
its a power plant aiming to recover water from ro brine, effluents regeneration e dmf backwash, but they want 80 % !!

 

[bimr]

DeltaCascade made a good recommendation as to recycle the demineralizer rinse water. That recommendation can be accomplished at small expense.

Beyond that, the cost to recycle the rest will be expensive.

What are you doing with the cooling tower blowdown? You should be thinking of a plant wide approach rather than looking at individual waste streams.

Use of RO on the waste streams is not practical. Assume you go with a RO system, what do you plan to do with the RO reject stream? I see little value in reducing your waste volume with a 50% recovery RO. You will just be concentrating the salts further, making a smaller volume of waste but something that is more difficult and expensive to dispose of.

If you are really serious about zero discharge. You will have to get rid of the demineralizer. Use RO on the raw water. The RO reject can then go to the cooling tower. Polish the RO effluent with an CDI. The RO/CDI to producing demineralizer water will give you the same high purity water quality with the least amount of wastes.

 

[andredd]

thanks for your advices. The point is that the plant is already existing , so wastes are "already ready" to be discharged to my "new" WWTP (its the new WWTP to ttreat the wastes coming from the existing plant). so i can not modify that much the WT approach in the power plant but only receiving those effluents and do as much as possibile....

 

[bimr]

Then you should contact one of the major RO system providers. They should be able to inform you that RO will not work. At that point, if your boss still wants to recyle, then investigate the evaporator.

GE makes all of those products. They should be able to help you.

http://www.gewater.com/products/equipment/ed_edr_edi/index.jsp

And just what are you doing with the cooling tower blowdown? That is the biggest waste stream in a power plant.

 

[andredd]

well , blowdown is one of the streams to this plant, but it accounts some 2 m3 / hr on a overall of 16. yeah i m contacting evaporators suppliers, its under process, lets see what they can do with caso4 and baso4 scaling.....

 

[bimr]

On the projects that I have been involved with, the cooling tower blowdown has always been the largest waste stream.

 

[andredd]

no, here main stream is RO reject (8 m3) then various streams 1,5 - 2 m3 each such ad DMF backwash, neutralized effluents, etc

did you have in mind different treatment for it?