Microbiological contamination is the main concern of any municipal water supplier. These contaminants can be bacteria or other organisms such as cryptosporidium and giardia intestinalis. Residual disinfection is needed to profide constant protection after the water leaves the treatment plant. A good disinfectant is a strong and lingering disinfectant – one that doesn’t lose effectiveness over time as the water sits in a tank or pipe.
Low-flow regions of piping or “dead legs” can be a particular problem in breweries because the chlorine/chloramine residual disinfectants have typically been removed to prevent off-flavors in beer, such as chlorophenols. Bacteria can form deposits or biofilms in low-flow regions that are subsequently hard to disinfect because the thickness of the deposit can prevent cleaners and sanitizers from reaching the entire colony.
However, with good sanitation practices in the brewery, getting rid of the chlorine and chloramine in the first place is the challenge. Chlorine disinfectant is either added as “free chlorine” or chloramine. Water may be chlorinated at several places in the initial treatment process and the chlorination level may be adjusted throughout the year. Free chlorine is the older method of chlorination that produces hypochlorite ion OCl– in the water to oxidize and kill organisms.
When chlorine is dissolved in water the following reaction takes place:
CL2 +H20 <-> H+1 + CL -1 + HOCl (hypochlorous acid)
Water reports often list chlorine as, “free chlorine” or “residual chlorine.” The definitions are as follows:
Free chlorine = 2 [CL2] + [HOCl] + [OCl-1]
Combined Chlorine = [NH2Cl] + 2 [NHCl2] + 3[NCl3]
Residual Chlorine = free + combined chlorine
At pH below 7.6, HOCl is prevalent over OCl-1. HOCl is a better oxidizer and is better able to penetrate the cell membrane of microbes because of its neutral charge. It is, therefore, a better germicide. Hypchlorite is added to water by adding sodium hypchlorite, calcium hypochlorite, or by bubbling chlorine gas through the water. The hypochlorite ion (free chlorine) is highly volatile and can be removed from water by heating it or simply allowing it to sit at room temperature in an open container for a long period of time. Fortunately for brewers, simply heating the water to strike temperature in an open kettle will drive off most of the free chlorine. However, it only takes very small amounts of free chlorine in brewing water to produce discernible chlorophenols in beer.
Unfortunately, hypochlorite can also react with (oxidize) organic compounds from decayed vegetation to form potentially carcinogenic compounds-known as disinfection by-products (DBPs). Many of those organic compounds are naturally occurring and prevalent in surface water sources like lakes and streams. These DBPS are undesireable in potable water supplies and are controlled by environmental regulation and the Clean Water Act in the US. Chloramines are much less likely to form DBPS, so water companies frequently use chloramine in place of chlorine. Unfortunately, chloramine has a lower odor threshold (3-5 ppm) than chlorine (5-20 ppm), and is mostly responsible for that swimming pool odor. But, some of these DBPs, like THM and HAA5, have odor flavor thresholds in parts per billion in beer, typically fish or pound-like. Chloramines are created by combing chlorine and ammonia in water. Chloramines exist in mono-, di-, and tri-chlormine forms, but the predominant for is monochloramine. It has been used for potable water disinfection since the early 1900s when it was found to provide a much more stable disinfectant in water distribution systems. Since the toxic effects of chlorine DBPs were discovered in the 1970s, chloramines have become a new standard for disinfection of water supplies containing significant organic content. It stays in solution longer, regardless of organic load and therefore works better as a residual disinfectant.
Chloramine is now used in the majority of large water treatment plants; though there is concern that chloramine still causes higher-than-desirable levels of DBPs. Thus other disinfection procedures such as ozonation and treatment with ultraviolet light are being used in some plants. Since boiling treatment requires fuel and time, the most cost-effective chlorine and chloramine removal options are UV degradation, activated carbon filtration (GAC), or metabisulfite treatment.
Chlorine/Chloramine Removal by Metabisulfite
Vintners have long used sodium metabisulfite and potassium metabisulfite (also known as Campden tablets) to suppress wild yeast in wine must. It is also useful as an anti-oxidant in beer. However, it is most useful for breaking down chlorine and chloramine in water. Metabisulfite forms sulfur dioxide when dissolved in water according to the equation:
K2S2O5 + H20 -> 2k+1 + 2SO2 + 2OH-1
(potassium form shown in this example)
It is the sulfur dioxide that reduces the chlorine to chloride, and in return is oxidized to sulfate.
The equation for chlorine breakdown by either sodium or potassium metabisulfite is:
Na2S2O5 + 2Cl2 + 3H2O -> 2Na+1 + 2SO4-2+ 6H+1+ 4Cl-1
K2S2O5 + 2Cl2 + 3H2O -> 2K+1 + 2SO4-2 + 6H+1 + 4Cl-1
Assuming 3 ppm of residual chlorine is present in the water, this reaction would use 4.7 ppm of K2S2O5 and result in 3 ppm of chloride, about 4 ppm of sulfate, and about 6 ppm as CaCO3 of alkalinity neutralized by the hydrogen ions.
The reaction of chloramine and metabisulfite is similar:
Na2S2O5 + 2H2NCl + 3H2O -> 2Na+1 + 2SO4-2 + 2H+1 + 2Cl-1 + 2NH4+
Again, assuming 3 ppm of residual chlorine, the reaction would require 9.4 ppm of K2S2O5 and create 3 ppm of chloride, 8 ppm of sulfate, 1.5 ppm of ammonium, and about 4.2 ppm of alkalinity as CaCO3 is reduced. The ammonium ion is a yeast nutrient. Any residual metabisulfite/sulfur dioxide will not harm the beer, but act as an antioxidant. Treatment numbers are given below:
Dosing Requirements for Metabisulfite Treatment
The unites are volumeless, although if the free chlorine concentration is 3 mg/L (ppm) then the corresponding potassium metabisulfite requirement would be 3/1.564 x total liters to be treated. A fudge factor of 20=30% more may be used to assure completion.
Mg of potassium metabisulfite
- Per mg free chlorine: 1.564
- Per mg Monochloramine: 3.127
Mg of sodium metabisulfite required
- Per mg free chlorine: 1.337
- Per mg Monochloramine: 3.127
Mg of sodium added*
- Per mg free chlorine: 0.323
- Per mg Monochloramine: 0.646
Mg chlorid added
- Per mg free chlorine: 1.0
- Per mg Monochloramine: 1.0
Mg sulfate added
- Per mg free chlorine: 1.35
- Per mg Monochloramine: 2.70
Mg ammonia added
- Per mg free chlorine: 0
- Per mg Monochloramine: 0.51
Alkalinity neutralized (mg as CaCO3, not ppm)
- Per mg free chlorine: 2.11
- Per mg Monochloramine: 1.43
Water systems that use chloramines may sometimes revert to chlorine disinfection during periods when their water supply has low organic content (typically spring or winter). Chlorine is more effective in killing microorganisms and is less costly than chloramine. The occasional disinfectant change helps maintain sanitary conditions in the distribution system. Water users may notice more chlorine aroma from the water when this change is performed.
Chlorine removal – UV Degradation
A relatively new technology for dechlorniation is ultraviolet light photolysis, in which high-energy photons break molecular bonds. UV light breaks down the chlorine and chloramine molcules into component ions, yielding chloride, ammonia, and water. Chlorine degradation is optimized at 180-200 nanometers (nm) and the chloramines are optimized across 245-365 nm wavelength. The typical dose recommended in the literature is about 20X the disinfection dose, i.e., about 600 millijoule (mJ)/cm2, with the spectrum centered at 245 nm wavelnegith for combined breakdown.
UV also has the added benefit of killing 99.99% of bacteria and viruses at this level, and the complete breakdown of total organic carbon (TOC), typically non-polar molecules, into polar or charged species that are more susceptible to ion exchange removal. In other words, a UV dechloration treatment will also help prevent fouling of downstream processes, such as ion exchange and reverse osmosis treatments.
The energy cost may be high, but there are corresponding benefits in lower maintenance and replacement cost for ion exchange and membrane technology media.
More from Water:
Water: A Comprehensive Guide for Brewers by John Palmer and Colin Kaminski. Copyright 2013 Brewers Association. Reprinted with permission from the publisher; all rights reserved by the original copyright holder.