During Water Week 2017, the Water Quality Assn. (WQA)...
Cost-effective and simple ammonia and chloramines removal
Rural and municipal water distribution systems have chlorinated drinking water supplies for more than 70 years. Chlorine residuals must be present at the end of a distribution system to maintain drinking water safety. It is known that chlorine dissipates with time, high temperature and the amount of organics present; therefore, greater amounts of chlorine are needed to maintain these residuals.
When chlorinating water that contains higher levels of organics, a complex family of chemical compounds is created. These compounds are known as disinfection byproducts (DBPs) and include bromodichloromethane, bromoform, chloroform, dichloroacetic acid and bromate, which also include trihalomethanes (THMs). The U.S. Environmental Protection Agency (EPA) has mandated that systems serving 10,000 or more customers must regulate the amount of THMs present. Consequently, the Safe Drinking Water Act Amendments of 1996 required the EPA to reduce the levels of DBPs, including THMs.
Many water distribution systems higher in organics have recently added ammonia compounds to comply with the new DBPs rule. Ammonia lowers chlorine’s reaction with organics, which in turn lowers the amount of DBPs and THMs created. Because these ammonia compounds can’t be removed by standard water treatment methods, municipalities and water districts must notify high-risk users such as kidney dialysis patients, aquarium owners, and pharmaceutical and biotech companies.
Chlorinating water that contains natural amounts of ammonia also produces chloramine compounds. Many regions throughout the world have high levels of naturally occurring ammonia. Water samples from Japan have contained ammonia levels as high as 14 to 18 parts per million (ppm). These forms of ammonia are produced naturally by the biological breakdown of nitrogenous matter that is present in highly organic soils and water.
The majority of chloramines, however, are generated by the addition of ammonia through water treatment facilities. These chloramines are often referred to as a monochloramine. When water is combined with chlorine and ammonia, three chloramine compounds can exist: monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine (NCl3). The predominant compound is affected by various reactions in water, such as pH, temperature, time, chlorine and ammonia ratios, and levels of organics. These newly created ammonia compounds attack rubber washers, toilet flappers and other household and commercial plumbing system components and are difficult to remove.
Conventional methods like reverse osmosis and carbon filtration don’t remove the ammonia from the chloramines, but synthetic zeolites have been found to do an excellent job of bonding with ammonia. Natural zeolites also remove ammonia, but can handle only one-fourth of the capacity of synthetic zeolites and have proven to be very inefficient to regenerate. Synthetic zeolites, such as Crystal-Right CR-100 and CR-200, remove ammonia efficiently and can regenerate the ammonia to waste using only salt brine and bleach during the regeneration process.
The application is so successful that pharmaceutical markets use Crystal-Right technology to remove ammonia and soften water prior to vapor compression stills. The final product is classified as a chemically and biologically pure distillate that usually meets or exceeds pharmacopoeia standards. If the ammonia is not removed after distillation occurs, it will “carry over” and the need for removal will be critical.
Believe it or not, chloramines testing is problematic and is not needed when determining success in ammonia removal. Available ammonia will be “tied up” as chloramines. Field testing for ammonia is not possible unless the chloramines bond is broken. Carbon prefiltration breaks the bond of ammonia and chlorine, allowing the ammonia reading to be present. The chlorine side is removed or adsorbed by the carbon, allowing the ammonia to be tested. Carbon prefiltration is not required on non-chlorinated waters containing ammonia. This process is only needed to remove chlorine from the chloramines bond. Some carbon manufacturers claim higher reduction rates of chloramines with shell-based and catalyst carbons, but our field testing and other observations have yielded only the removal of the chlorine bond of the chloramines without a significant decrease of ammonia.
Models below show chloramines removal with the use of Crystal-Right synthetic zeolite.
The amount of carbon and Crystal-Right media needed is determined by the Crystal-Right capacity. Crystal-Right has an estimated 28,000 ppm ammonia removal per cu ft. Regeneration of Crystal-Right is performed with salt (NaCl) and bleach (NaOCl ), while the carbon filter uses only periodic backwashing. The amount of bleach needed to assist in the “offing” of the ammonia is determined by the amount of ammonia in the incoming water and the information learned through pilot testing. A note of caution applies here. With the regeneration of Crystal-Right media, there is potential for large amounts of ammonia gases to vaporize during the regeneration cycle. Take precautions to keep waste vapors from confined spaces and public areas.
Ammonia removal appears to be a function of monovalent ion exchange sites associated with the Crystal-Right media. This would explain why these functionalities are independent of hardness ion exchange. Hardness and other multivalent ions do not affect the monovalent sites. Although Na+ is monovalent, due to mass action, the sodium ion displaces both monovalent and multivalent ions to regenerate the ion media.
Bleach is sodium hypochlorite (NaOCl) and hydrolyses according to the following equation:
NaOCl + H2O = HOCl- + Na+ + OH-
The purpose of bleach in the regeneration process is to supply a source of hydrogen ions. This is important in water softening because zeolites raise the pH of acidic waters. Also, because the media can be regenerated with Na+, the H+ can be displaced too. Thus, there are multiple exchange sites—some capable of reacting preferably with monovalent ions and some with di- or trivalent ions.
Why is H+ necessary for the regeneration of ammoniated zeolite when Na+ alone is effective in displacing H+ from zeolite treating acidic water? The ammonia specie (without water) is shown here as the equilibrium:
NH3 + H2O = NH4+ + OH-
NH3 + H+ = NH4+
Because of the equilibrium, NH3 may be residing in combination with the ionic form at the ion exchange site. This equilibrium could inhibit the regeneration of the monovalent site by Na+; however, if H+ is also present near the local site during regeneration then it can stabilize the NH4+ and allow it to be displaced by Na+. This does not preclude the possibility that H+ may compete for the monovalent site nor the formation of chloramines causing ammonia to form a stable molecule that enhances displacement.
Ammonia does not “bleed through” at the same time hardness does, likely due to the ammonium ion being as monovalent as the hydrogen ion. Therefore the ammonium ion does not necessarily compete for the same sites as calcium and magnesium. In the case of monovalent sites, Na+ can displace NH4+ and H+ from those ion specific sites due to the mass action effect.
Pilot plant operations are the best measurable way to determine what the final outcome will be for a given system. Conduct a running pilot study for an extended amount of time while keeping the operational flow down to 4 gal per minute (gpm)/sq ft. A 10-in. diameter tank yielding a 0.5 sq ft area will operate at 2 gpm, or 4 gpm/sq ft. A final test should be completed with a 10 x 54 vessel and 1.5 cu ft of HAC carbon followed by 1.5 cu ft of Crystal-Right zeolite.
Residential removal of chloramines and ammonia is achieved by the same method: carbon pre-filter followed by Crystal-Right water conditioner. Higher flow rates are possible, but each water system will need trial applications to determine the peak flow and bleach requirements. In many residential applications, additional bleach can be manufactured by a variety of products known to the industry as the Sanitizer Series of water treatment equipment. Water-Right uses patented technologies customized for the regeneration of Crystal-Right media by converting the salt into a salt and chlorine mixture by means of electrodialysis. This ability to manufacture “bleach” with only salt water eliminates the need for additional feed pumps, and simplifies the operational system.
Using the right process, ammonia and chloramines removal can be simple and cost-effective for the commercial or residential application. This technology has just begun to emerge and will continue to advance in the near future.