Ion exchange for ammonia removal in drinking water

[benalizzy]

I run a small municipal water plant that has sand filters, ion exchange softeners, and chlorination.

The problem is we have too much ammonia, over 2 parts, which causes problems in disinfection, nitrites, and copper problems. The softeners do not currently remove ammonia. I would like to use ion exchange to remove ammonia rather than going to RO or nanofiltration because of cost. Any ideas on the cheapest, effective alternatives? We only pump at 200 gpm about 8 hours a day. The water after the softeners is also agressive and needs some treatment.

 

[ccor]

Consider using breakpoint chlorination by adding another chlorination point upstream of the filters (assuming they are not biological filters). Dose this point past the “breakpoint” where you just start to see a free-chlorine residual. This will not count towards your CT so the rest of the plant would operate as usual. This converts the ammonia to N2 gas and HCl (pH adjustment may be needed depending on your alkalinity). Cheap and easy to operate.

 

[bimr]

The softeners that should be removing the ammonia, since ammonia is removed preferentially to calcium. However, the calcium will then displace the ammonia as the softening run continues. A softener will remove ammonia during the initial portion of the service cycle but will then release the ammonia as the resin bed becomes exhausted with calcium.

If you want to remove the ammonia with ion exchange, you can achieve that by operating water softeners in series. Unfortunately, that is not a practical solution as water that has been completely softened will be very corrosive. Therefore, for your application, ammonia removal with ion exchange is probably not practical from a corrosion standpoint.

Rather than breakpoint chlorination, you should consider chloramination. Chloramination uses 3:1 to 4:1 chlorine to ammonia feed ratio by weight. Breakpoint chlorination uses 10:1 chlorine to ammonia feed ratio by weight.

The least expensive method to make the water less corrosive is to raise the water pH. Not sure why you are using water softeners in the first place, maybe radium removal? From what you have presented, water softeners do not seem to be practical.

 

[Karbone]

There is also another idea I can throw out there which is to raise the pH and aerate. At higher pH Ammonia is in the neutral form and will be air-stripped because it is voaltile. There is a balance to keep though because Chlorination is more effective at lower pH. Do some treatability testing.

Good luck,

karbone

 

[ccor]

I agree that conversion to chloramines could be cheaper from a chemical addition standpoint. However, CT is difficult to achieve with combined chlorine, typically requiring a 400 - 500% increase in clearwell size. I have only heard of one system that uses it for primary disinfection and its because they have miles of pipeline before the first tap. The operation both the plant and distribution system is much more intense with chloramines, usually only recommended for small systems when they have an interconnect with a system that uses chloramines.

Air stripping will also work but is usually only economic when other VOC's are present.

 

[benalizzy]

I don't know that breakpoint chlorination to relieve ammonia is the solution. The amount we would have to dose would be large and the effect on the plant and the system could be harsh. We have to soften our water because it is very hard, plus it removes arsenic. We use chloromines now for dissinfection. All of this leaves us with nitrite problems if chlorine levels are to low and copper problems when they are to high. I need to reduce ammonia so I can reduce chlorine and eliminate copper problems.

 

[bimr]

Again, the least expensive method to make the water less corrosive is to raise the water pH. I would suspect that your copper problem is more a result of corrosion from the lack of hardness than from ammonia. The ammonia should be tied up with chlorine.

Air stripping the ammonia is probably also not practical because of the cost of chemicals to raise and lower the pH.

Why don't you post the complete water analysis?

 

[sbush]

Your application seems perfect for a macroporous zeolite media filtration/adsorption process. Zeolites are aluminosilicate minerals having ion-exchange properties. Zeolites have a high affinity for the ammonium-ion and ammonium sorption is good at low temperatures. For your application, I would recommend a pressure vessel measuring 72" diameter containing a 42" depth of, ANSI/NSF-61 listed, zeolite media with an effective size close to that of filter sand. The zeolite unit is designed similarly to your existing softener. The zeolite unit processes raw water until ammonia leakage increases. Before ammonia leakage becomes unacceptable, the unit is regenerated with a brine solution similar to the existing softener process, in fact, the existing brine system could possibly be used for both systems. The cost for a zeolite unit would be comparable to the existing softener unit. If you are interested, I recommend pilot testing to determine performance and runtime for your specific application.

As to the aggressive nature of the softened water, have you considered blending soft water with hard water? Ten States standards recommends blending to around 85 mg/L hardness for drinking water.

S. Bush

http://www.water-eg.com

 

[bimr]

benalizzy,

Have you looked into the arsenic removal process where you co-precipitate the arsenic with iron and filter the resulting floc? You already have the filter, you would only need to install the chemical feed.

And as sbush suggests, you should not be softening the entire water flow. Municipal water is not usually softened to lower than 120 mg/l.

 

[cub3bead]

If you want to air strip the ammonia you will have to raise the pH to near 11.0. If you do it, please add the caustic and air strip after the softener unless you want to deal with a bunch of CaCO3 solids.