Chlorine and chloramines

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Chlorine compounds are use by water treatment plants to disinfect water. They must be removed from the water before using it to make beer (see Water for methods to remove these compounds).

Hypochlorous acid (HOCl) is formed by introducing chlorine gas into water. It decomposes into HCl and an oxygen radical that kills microbes by oxidation of the cell membranes.[1]

Chlorine is an effective sterilant by virtue of its strong oxidizing capability.48 It rapidly oxidizes protein constituents of bacteria (including spores), yeast, and viruses and probably acts by impairment of membrane function, preventing the uptake of nutrients, and by disruption of protein synthesis.[2] When liquid chlorine or hypochlorite are mixed with water, they are hydrolyzed to form hypochlorous acid (HOCl). 

Cl2 + H2O → HCl + HOCl
HOCl ↔ H+ + OCl−

Chloramines are significantly less volatile than free chlorine, although it does have a lower odor threshold (think swimming pool).[3]

Chlorine and chloramines are very effective disinfectants that act by oxidizing the cellular membranes of microorganisms and rupturing the cell.[3]

Oxidizing agents, such as chlorine in its various forms, including chlorine dioxide or ozone, should not be present in the brew water.[4]

Chlorination: Chlorine is either added as "free chlorine" or chloramines. Free chlorine produces hypochlorite ion (OCl-).[3] Cl2 + H2O <—> H+ + Cl- + HOCl (hypochlorous acid)

Residual halogen-based (e.g. chlorine or iodine) sanitizing agents that get into the beer can result in very nasty flavored phenolic compounds, which cause off-flavors at very low levels.[5]

Free chlorine also attacks reverse osmosis membranes and sterile filter membranes.[6]

Chlorine is most commonly available as chlorine gas and the hypochlorites of sodium and calcium. In water, they hydrolyze instantaneously to hypochlorous acid:[7]

  • Cl2 + H2O → HOCl + HCl
  • NaOCl + H2O → HOCl + NaOH
  • Ca(OCl)2 + 2 H2O → 2 HOCl + Ca(OH)2

Hypochlorous acid dissociates in water to hydrogen ions and hypochlorite ions: HOCl ↔ H+ + OCl– The sum of Cl2, NaOCl, Ca(OCl)2, HOCl, and OCl– is referred to as free available chlorine (FAC) or free residual chlorine (FRC), expressed as mg/L Cl2. As discussed later, chloramines are formed from the reaction of chlorine with ammonia compounds present in the water. These chlorine-ammonia compounds are referred to as combined available chlorine (CAC) or combined residual chlorine (CRC). The sum of free and combined available/residual chlorine is called the total residual chlorine (TRC).[7]

A part of the chlorine dosage reacts with ammonia nitrogen to combined available chlorine in a series of stepwise reactions:

  1. HOCl + NH3 ↔ NH2Cl (monochloramine) + H2O
  2. HOCl + NH2Cl ↔ NHCl2 (dichloramine) + H2O
  3. HOCl + NHCl2 ↔ NCl3

(trichloramine) + H2O These reactions are governed primarily by pH and chlorine-to-nitrogen weight ratio. Chloramine also has a germicidal effect, albeit lower than that of chlorine.[7]

Chloramines retain the oxidizing equivalents of the initial HOCl.[8]

Reaction with chlorine (in the form of hypochlorous acid):[9]

HSO3- + HOCl → SO42- + 2 H+ + Cl-
[sulfite and hypochlorous acid] [sulfate, hydrogen ion, and chloride]

Reaction with chloramine:[10][11]

SO2 + 2 H2O + NH2Cl → 2 H+ + SO42- + Cl- + NH4+
[sulfite, water, and chloramine] [hydrogen ion, sulfate, chloride, and ammonium]

Residual free chlorine can be reduced to harmless chlorides by activated carbon or chemical reducing agents. An activated carbon bed is very effective in the dechlorination of RO feedwater according to following reaction:[7]

  • C + 2Cl2 + 2H2O → 4HCl + CO2

Sodium metabisulfite (SMBS) is commonly used for removal of free chlorine and as a biostatic. Other chemical reducing agents exist (e.g., sulfur dioxide), but they are not as cost-effective as SMBS.[7] When dissolved in water, sodium bisulfite (SBS) is formed from SMBS:[7]

  • Na2S2O5 + H2O → 2 NaHSO3

SBS then reduces hypochlorous acid according to:[7]

  • 2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4

When chlorine is added to water, it destroys the membrane of many microorganisms and kills them.[12] Chlorine disinfects water but does not purify it: there are some contaminants it cannot remove.

Chlorine takes time to kill organisms. At temperatures of 18OC and above, the chlorine should be in contact with the water for at least 30 minutes. If the water is colder then the contact time must be increased. The turbidity and the acidity (pH) of the water have a significant effect on the efficiency of chlorine as a disinfectant. The turbidity should be < 5NTU and the pH level between 7.2 and 6.8.[12]

The optimum chlorine residual in a small, communal water supply is in the range of 0.2 to 0.5 mg/L.[12]

Dechlorination by sulfur dioxide (SO2) is the most common process to meet zero TRC effluent limits. Sodium bisulfite and sodium metabisulfite also have been used for chemical dechlorination. In the dechlorination process using SO2, sulfurous acid is formed first:[13]

  • SO2 + H2O → H2SO3

Sulfurous acid then reacts with the various chlorine residual species:[13]

  • H2SO3 + HOCl → HCl + H2SO4
  • H2SO3 + NH2Cl + H2O → NH4Cl + H2SO4
  • 2H2SO3 + NHCl2 + 2H2O → NH4Cl + HCl + 2H2SO4
  • 3H2SO3 + NCl3 + 3H2O → NH4Cl + 2HCl + 3H2SO4

It is common practice to overdose the sulfur dioxide to maintain a level up to 5 mg/L SO2 in the effluent. This ensures the reduction of all chlorine residual species.[13]

The taste and odour thresholds for chlorine in distilled water are 5 and 2 mg/L, respectively.[14]

THMs are not removed by agents used for dechlorination, only by carbon filtration.[15]

Trihalomethanes (THMs) are formed by chlorine reactions, and these are known to cause cancer and fertility problems.[16][17][18][3][19] THMs are only removed by carbon filtration.[20]

THMs (Tri Halo Methanes) are substances of concern in the brew water stream and should be removed to as great an extent as possible, preferably to below 10 ppb.[21]

It goes without saying that oxidizing agents like chlorine in its various forms or ozone should not be present in the brewing liquor.[21]

The preferred disinfection method is chlorine dioxide (ClO 2 ). The advantages of ClO 2 plants are that they are easy and safe to use and virtually no by-products (THMs) are formed. Disinfection with sodium hypochlorite or chlorine gas is far more hazardous and unwanted by-products are created.[21]

See also[edit]

References[edit]

  1. ↑ Kunze W. Hendel O, ed. Technology Brewing & Malting. 6th ed. VLB Berlin; 2019.
  2. ↑ Taylor DG. Water. In: Stewart GG, Russell I, Anstruther A, eds. Handbook of Brewing. 3rd ed. CRC Press; 2017.
  3. ↑ a b c d Palmer J, Kaminski C. Water: A Comprehensive Guide for Brewers. Brewers Publications; 2013.
  4. ↑ Eumann M, Schildbach S. 125th Anniversary review: Water sources and treatment in brewing. J Inst Brew. 2012;118:12–21.
  5. ↑ Fix G. Principles of Brewing Science. 2nd ed. Brewers Publications; 1999.
  6. ↑ Howe S. Raw materials. In: Smart C, ed. The Craft Brewing Handbook. Woodhead Publishing; 2019.
  7. ↑ a b c d e f g FilmTec™ reverse osmosis membranes technical manual. Dupont website. Updated April 2020. Accessed October 2020.
  8. ↑ Hawkins CL, Morgan PE, Davies MJ. Quantification of protein modification by oxidants. Free Radic Biol Med. 2009;46(8):965–988.
  9. ↑ Jolley, RL, and Carpenter, JH. "Aqueous Chemistry of Chlorine: Chemistry, Analysis, and Environmental Fate of Reactive Oxidant Species." 1982. doi:10.2172/5505533.
  10. ↑ Yiin, Boudin, et al. "Nonmetal redox kinetics: general-acid-assisted reactions of chloramine with sulfite and hydrogen sulfite." Inorg. Chem. 1987, 26, 21, 3435-3441.
  11. ↑ "Experiments in Removing Chlorine and Chloramine From Brewing Water" 1998
  12. ↑ a b c Reed B, Shaw R, Chatterton K. Chapter 11: Measuring chlorine levels in water supplies. In: Technical Notes on Drinking-Water, Sanitation and Hygiene in Emergencies. World Health Organization (WHO), Water, Engineering and Development Centre (WEDC); 2013. Accessed March 2024.
  13. ↑ a b c Harp DL. Current Technology of Chlorine Analysis for Water and Wastewater. Booklet No. 17. Hach; 2002. Accessed March 2024.
  14. ↑ https://cdn.who.int/media/docs/default-source/wash-documents/wash-chemicals/chlorine.pdf
  15. ↑ Moore N, Ebrahimi S, Zhu Y, Wang C, Hofmann R, Andrews S. A comparison of sodium sulfite, ammonium chloride, and ascorbic acid for quenching chlorine prior to disinfection byproduct analysis. Water Supply. 2021;21(5):2313–2323.
  16. ↑ Karim K, Guha S, Beni R. Comparative Analysis of chemical, physical and biological contaminants in drinking water in various developed countries around the world. J Water Resour Prot. 2020;12(8):714–728.
  17. ↑ Scherer T, Johnson R. Filtration: Sediment activated carbon and mixed media. North Dakota State University website. Revised Feb 2022. Accessed online Mar 2024.
  18. ↑ Briggs DE, Boulton CA, Brookes PA, Stevens R. Brewing Science and Practice. Woodhead Publishing Limited and CRC Press LLC; 2004.
  19. ↑ deLange AJ. Removing chloramines from water: Chloramines removal. MoreBeer website. 2013.
  20. ↑ Moore N, Ebrahimi S, Zhu Y, Wang C, Hofmann R, Andrews S. A comparison of sodium sulfite, ammonium chloride, and ascorbic acid for quenching chlorine prior to disinfection byproduct analysis. Water Supply. 2021;21(5):2313–2323.
  21. ↑ a b c Eumann M. Chapter 9: Water in brewing. In: Bamforth CW, ed. Brewing: New Technologies. Woodhead Publishing; 2006:183–207.
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