Sequestering Agent for Softening

[jartgo]

I'm familiar with ion exchange softening. How does the sequestering agent work? Apparently the Ca and Mg doesn't precipitate out? It just stays "sequestered" in the water, thereby not causing the normal problems associated with hardness? Is there a hardness limit when this is an option in lieu of ion exchange? Thanks in advance.

 

[GarySCWSVI]

Sequestering agents do not soften the water and are not a replacement for ion exchange softening.

Gary Schreiber, CWS VI
The Purolite Co.

 

[jartgo]

I guess I'm more interested in what it does do. What does the sequestering agent do?

 

[semo]

In a nutshell, sequestering agents bind with the mineral (Fe, Mn, Ca, Mg) to keep them in suspension. This prohibits (delays) them from falling out and causing buildup, stains, discoloration, etc.

There are limits to the effectiveness of sequestering agents based upon the water quality. At high mineral levels, the agents are not very effective. With long detention times in your storage, they are also not very effective. Testing should be performed to determine if they work.

 

[orenda1168]

As a point of interest, we carry a sequestrant that will handle up to 7,000 ppm calcium/magnesium hardness at a treatment level of about 1-quart per 10,000 gallons, and is temperature stable to about 140F.

If you are interested, please contact me through

http://www.orendatech.com.

Orenda

 

[jartgo]

So we have a resin representative stating the sequestering agents do nothing and a sequestrant representative stating that his product can "handle" hardness to 7000ppm.

Any other unbiased discussion/description? I'm mostly interested in how it works/how effective it is. I saw a small plant yesterday that was using a sequestering agent and it was referred to by the plant operator as "softener."

bimr, where you at?

 

[orenda1168]

Jthompson:

Gary Schreiber has answered your question regarding sequestering agents not being "water softeners".

Semo has answered your question about the functioning of sequestrants.

To restate my earlier comment that our seqestering agent can "handle" up to 7,000 ppm calcium/magnesium hardness, please substitute "sequester" for "handle".

Virtually all of these products, regardless of their specific chemistry, function at a less-than-stoichiometric (threshold) treatment level. In general, these materials are effective at inhibiting hardness deposition, providing consideration is given to the water chemistry and hardness composition.

If you are looking for commercial users of these products for good/bad recommendations, contact any manufacturer utilizing large volumes of water, particularly for cooling and/or process purposes. The odds are good that they will have the experience you seem to be looking for.

If you continue to have a problem with this subject, I suggest you pick up a Betz or Nalco water treatment handbook as either will provide you with authoritative answers to your questions.

Orenda

 

[bimr]

There is an extensive discussion of water treatment chemicals in Water Quality & Treatment published by the AWWA. A pilot test is normally recommended because the actions of the sequestering agents are unpredictable due to the variable water quality parameters such as TDS, pH, hardness, alkalinity, temperature, chlorine concentrations, and DO as well as the materials of construction of the piping system (steel, ductile, plastic, etc.) and other aspects such as the velocity of the fluid in the pipe.

Here are a few select passages from the book so you know what to expect.

“Much misinformation exists about the mechanisms of action of many of the inhibitor formulations. For example, the sequestration of ferrous iron to prevent red-water formation is often interpreted as “corrosion inhibition,” but in fact, it is only mitigating the symptom of the corrosion. Many polyphosphate chemicals will convert one form of corrosion to another, for example, alleviating tuberculation and replacing it with uniform corrosion. That is not corrosion inhibitions.”

“The way in which sodium silicate acts to form a protective film is still not completely understood, but there is evidence it sorbs on or reacts with existing oxide, hydroxide, or carbonate pipe deposits.” The effectiveness of sodium silicate as a corrosion inhibitor depends on water quality properties such as pH and bicarbonate concentrations. It is also more effective under higher-velocity flow conditions.”

“Silicate has been found to be effective when used in combination with chlorination to sequester iron in groundwater and allow control of copper and lead levels by pH adjustment.”

“Orthophosphate is known to have significant effect on the aggregation behavior of ferric hydroxide.”

“Orthophosphate usually forms insoluble passivating films on the pipe, reacting with the metal pipe itself (particularly with lead, iron and galvanized steel) in restricted pH and dosage ranges.”

 

[bimr]

Here is something about calcium and magnesium:

"Although not yet comprehensively investigated, polyphosphates when mixed with orthophosphate, may assist in the formation of orthophosphate films by complexing calcium and magnesium in hard waters that otherwise could cause unwanted orthophosphate complexation or precipitates."

The chemical treatment suppliers such as Betz typically operate cooling tower cooling systems at high temperatures and high calcium concentrations using chemical treatment programs that involve the addition phosphate chemicals to the cooling water. Note that these chemical treatment programs are always done in combination with pH and alkalinity control.

This chemical treatment scheme will not work in a municipal water distribution system because the cost of the treatment chemicals will be too expensive.

 

[semo]

Not trying to insult anyone; but, I've had several chemical reps tell me what their products can do and seen brochures with data on same products only to test them and get failure. I've also had reps tell me their products won't work after reviewing the water quality and system data.

No rep can say their product will work 100% of the time, especially without knowing about the water and system being treated. Also notice that Orenda said "inhibit" not "prohibit" hardness deposition. This goes back to Gary's comment that sequestering agents are not a sub for softening.

As I mentioned and Bimr reiterated, if you are really interested in chemicals do a pilot test. And, I'll also mention, jar testing does not suffice. The total water quality and system variables affect the performance.

 

[jartgo]

Thanks for the responses. Just to reiterate, I'm not currently designing a project, I was just at a plant yesterday (looking at some sedimentation basin issues) and the operator was just giving me the quick and dirty tour, pointed at the sequestering agent and referred to it as "the softener." Not being an expert and hesitant to question him, I left scratching my head wondering how you get softening out of a 55 gallon barrel. I have the AWWA book and will look through it as well.

 

[GarySCWSVI]

John,

If you will be at the WQA Convention in March in Las Vegas, stop by our booth.

Gary Schreiber, CWS VI
The Purolite Co.