low pH problem in spray humidifier systems

[Lucaz]

Hi,

I have come up against a problem on a site that we manage which has 7 spray humdifiers.

The problem is as follows:

All humidifiers have low pH: the table below shows the analysis:


(pH) 4.5 4.5 4.5 3.9 5.3 5.0 6.0

(Cond)480 1194 566 948 210 466 48.4

(CL-) 160 200 160 180 80 160 20


NOTE: cond=conductivity, CL- = Chlorides


All the systems are fed by reverse osmosis water which has a pH of 7.3

All gas burners have been checked to see if they are exhaling too much Carbon Dioxide. (They are not)

(CO2 would cause lower pH)

I would expect the systems with the least bleed off (ie higher conductivity) would concentrate up and pH would increase. But as above, it does not appear in this way..

This is becoming a headache....I cannot figure out why the systems are suffering from the low pH problem. The systems are being dosed with Bromine but there is no pH builder (at the moment!) being dosed.

Sodium Metabisulphite is dosed into the RO system (I dont know why...the RO system is managed by another contractor)


nothing else (as far I know) is dosed into the humidifier systems (or the RO system)

Can anyone come up with a solution for this??

your help is much appreciated..the client wants answers yesterday (dont they all!!)

Many thanks


PS further analysis and information can be posted if needs be...


Lucaz

 

[deposition]

A comprehensive water analysis should be done, at least on one of the low pH (~4) samples to diagnose the problem. My guess is something to do with the pretreatment of the RO. If you can break down the cations and the anions in the water analysis, it will shed more light to the problem.

 

[deposition]

Another factor I forgot to bring up is bacteria. I don't know the operating temperature. If the temperature is below 120 F, you may have anaerobic bacteria problem. In other words, acid producing bacteria may be present in your system.

 

[bimr]

It is not clear what you are doing from your description. It would be extremely helpful to have a water analysis as suggested by deposition.

However, it is likely that the low pH is caused by CO2 which will not be removed by the RO unit. Alkalinity is rejected by the RO and CO2 passes through causing the product water pH to drop. It is common practice to strip out the CO2 after a RO unit and/or to increase the pH.

Sodium Metabisulphite is used to remove chlorine and should have no effect on the pH.

 

[Lucaz]

Hi all,

Thanks for your posts. In response to the questions raised:

The dipslides on the systems do not show any signs of a bacterial problem which would be big enough to cause pH lowering on this scale...(all are below 10^2 count) Bromine levels are almost always between 1-3ppm (although low pH will affect the amount of free Bromine available)

System water temperatures for the humidifiers are between 16-20 degrees celsius. We also carry out servicing of 10 other humidifiers on the site, all of which are supplied on softened water make-up and have similar water chemistry as a cooling tower (ie higher dissolved solids/conductivity = higher pH). None of them suffer this low pH problem.

Regarding the full water analysis suggested by Deposition, I have carried out further analysis on the systems. One system (listed as ABTB) did have further analysis done:


ABTB Humidifier:

RO Make up water TOTAL HARDNESS: <10 ppm

ABTB pH = 4.1

ABTB total hardness: 80 ppm

ABTB Conductivity: 765 microseimens

ABTB TDS: 535.5ppm

ABTB Chlorides: 110ppm

ABTB Bromine: 0.6ppm

ABTB M Alkalinity: 0ppm

ABTB soluble iron: 0.3ppm



Do I still require further water analysis (if so, what exactly needs to be tested for??) or is this info more helpful?

BTW, bimr, a method of stripping out the C02 on RO water...what would this entail? All I can think of is to dose the RO water with Caustic...am I on the right lines here?

Also,

(forgive my lack of knowledge about RO systems..Its not my strongpoint and to be fair I am the only one in the company who has come across spray humidifiers fed on RO!!)

how does alkalinity get "rejected" in the RO process?

I am beginning to think that Co2 is the problem here... Are there any papers/good tech advice pointers that are available to outline best practice when running systems on RO make up water? Other contractors who design and build RO systems have suggested that its due to C02 and advise me to dose caustic proportionally so the pH is built up in the humidifiers...however, the client is also wondering if there are any other methods available to combat the pH problem...

My next step I presume is to advise that we put a caustic dosing system on the RO outlet pipe which feeds the humidifiers......but before I go in that direction....are there any other ideas I should be aware of?

Many thanks to you all so far

 

[quark]

Water pH of 4.5 indicates dissolved CO[sub]2[/sub] to saturation. Didn't you check the pH of RO permeate water? Like bimr suggested, this tends to be acidic due to dissolved CO[sub]2[/sub] . Either you should have a degassifier or maintain the feed water pH nearer to 8.5 This higher pH favors formation of carbonate and bicarbonate ions which can be removed by RO membrane.

Take care that the higher pH in the feed water enhances scale formation on the membranes. You should also use a descalent in the feed water.

 

[bimr]

The selection of the method to remove the CO2 depends on the economics of your system. An air stripper (degassifier) is more economical on larger flow systems and on systems having signficant concentrations of CO2.

If you have a small flow capacity RO system, it is probably more economical to add an alkali. You should investigate the use of soda ash rather than caustic as it is less hazardous to handle. The RO supplier should be able to help you with this.

An RO unit has one influent stream and two effluent streams (product and reject). The salts are rejected by the RO memebrane and leave the RO unit in the reject stream. CO2 passes through the RO membrane and leaves the RO unit in the product stream.

The effect of alkalinity and CO2 on pH is simple to understand. Water pH is the ratio of the alkalinity divided by the free CO2. So if you remove the alkalinity and keep the CO2 the same, the pH is reduced.


pH = Alk/CO2

Raw water =

 

[Lucaz]

Hello again,

After reading the latest posts, Here is the analysis of the RO permeate:

ph = 7.1

TOTAL Hardness: 4ppm

Cond: 37 microseimens

TDS: 25.9ppm

Chlorides: 25ppm

M Alkalinity: 20ppm

Soluble Fe: <0.1ppm



As the pH is 7.1 and as there is dissolved Co2 in the permeate, would I then be right in thinking that in the case of the Humidifier listed above (known as ABTB) is concentrating up (ie higher TDS compared with permeate) then this would mean a concentrating effect on the Co2 in the system water (hence a lowering of the pH??)

The RO system on site is a large mulitple membrane system which feeds 7 large humidification units. So Degassing would probably be the best option....)

Any more ideas on this???

 

[bimr]

The use of a stripper would depend on the flow rate of your system. Also note that stripping is not without its own problems. Unless you use a vacuum stripper, the air stripper is a location where contaminants may enter the water stream.

What are you using these humidifiers for? Power plant perhaps? What is the water flow rate? What is the CO2 concentration?

Usually, when you have systems of this sort such as cooling tower, the alkalinity will concentrate as the concentration cycles increase causing the pH to increase. It is unusual for CO2 to increase as well because it will strip to the air. As the alkalinity increases, acid is then used to maintain a maximum alkalinity (and pH).