POSTED

Posted by Deanna Cox on 5/17/2011 at 10:37:00 AM
Hello, I have a water chemistry of the following:

• TDS: 790
• Hardness: 500 ppm
• pH: 6.8
• Iron: 2.1
• Iron bacteria: Present
• Manganese: .051 ppm
• Chloride: 16.0 ppm
• Sodium: 91 ppm — let me know if there are other contaminants you wish to know the testing results on
• Silica: 17.9 ppm.

I am just wondering about the silica. What effect, if any, will this level of silica have to the other contaminants? Is this level of silica going to be an issue in any way? I would love to hear your thoughts.

RESPONSES

Marianne Metzger: 5/19/2011 12:40:51 PM
From what I've heard, when silica gets up over 20 mg/L it could cause some issues with other treatment processes that would include a media like ion exchange resin as it can build up on these media and reduce the useful life. I would be interested to hear what others have to say about this topic.

Jeremy Kern CWS I: 5/19/2011 4:27:50 PM
This level of silica is an issue if you are using reverse osmosis as far as the life of the membrane, but more importantly, you didn't indicate what treatment method, if any, you are currently utilizing. This is pertinent in that what you pretreat can often dictate what you can do about certain contaminants. Also, it would be helpful if you put up your whole water analysis because just what you've put up already will have a myriad of options for treatment. Also, is this just for domestic use or boiler makeup? What is end use? This is very important in figuring out what steps are necessary to produce what quality of water.

Deanna Cox: 5/20/2011 2:04:50 PM
Marianne, thank you for your comment. Those are things I need to figure out before I finalize equipment selection for this well.

Jeremy, I am still in the process of making the equipment selection — finding out what effect silica will have on certain media/treatment methods will hopefully help. It is for a residential home where these levels of iron, manganese and hardness will most definitely need to be taken care of all throughout the home. I will also probably be recommending a RO for the drinking water.

I have a 70 parameter analysis. What other analytes would be helpful to know?

My main questions are: Will this level of silica effect other methods of treatment (ie., resin media, carbon, RO membrane)? If so, what is an effective way to remove silica?

Stephen Carr: 5/20/2011 5:50:14 PM
The silica level reported is not very high. At the 7.8 pH mentioned, silica has a solubility in excess of 80 ppm even with very cold water. So even an RO with 67 percent recovery would not have the reject water exceed solubility. The high iron may have some effect on the silica stability, but I think the iron and manganese themselves are probably the biggest treatment challenges with this water.

Jeremy Kern CWS I: 5/23/2011 11:01:14 AM
Knowing a little more would help because it is hard to make an assessment without all of the information. To elaborate on this, typically you have two forms of silica, one ion exchange will not remove; both can be removed to a certain degree with reverse osmosis and ultrafiltration, but both life spans on the membranes and the ultrafiltration system in general will probably be short due to not only the silica, but the other contaminants if you don't pretreat.

Deanna Cox: 5/26/2011 12:09:37 PM
Thank you very much for the responses and thoughts on this issue. Jeremy, please let me know what other information I can give you. I will gladly release any information you might need should it help provide any answers to my question.

I collected the sample and am quite familiar with proper sampling requirements. I am a CWS-II, and have been in the water industry for six years. I work for a company who has tested and provides well water equipment for hundreds and hundreds of wells. I am not at all worried about proper equipment to take care of these levels of iron and manganese. This type of water is somewhat prevalent in certain parts of our service area and I am quite familiar with this.

However, I have not had much experience with this level of silica in the water and am not too familiar with this contaminant and how it responds. It may have been somewhat ignorant of me to think that just throwing out a general question about silica with only the basic chemistry outline would provide answers.

Please let me know what additional information you would like. Once again, if anyone has any experience with dealing with this level of silica and might have thoughts on any effect it might have, I would appreciate to hear.

Gary Schreiber: 5/26/2011 11:02:21 PM
Deanna, Good to see you are sill involved in the business.

I do not believe you will have any silica interference with removing the contaminants on the analysis by using ion exchange resin. The main silica effect on resin would be when anion exchange resin is used in the hydroxide form which is the usual resin of choice for silica removal.

Softening resin or anion resin used in their usual forms of sodium for softening and chloride form for dealkalization would not be affected by the silica. Feel free to contact me privately to discuss and we can bring the appropriate info as a result of that back to the board here.

Jeremy Kern CWS I: 5/31/2011 6:29:20 PM
The waters we have encountered in the SW of New Mexico and Texas that have 15-20 ppm of silica have often been troublesome and costly for the homeowner or business. Most often, without one or multiple backwashable UF filters, the silica has a tendency to precipitate and the life of undersink RO membranes are greatly reduced.

With this treatment method, the silica is generally reduced by 98 percent. I have also seen systems regionally that were used to disinfect wells injecting chlorine dioxide that reacted with the silica forming a white precipitant. We have had good luck with our systems using UF but that may not mean much for your area, we do not generally see levels of iron above 1 ppm and have never encountered iron bacteria.