by Ken Williamson, Alberta Agriculture, Red Deer, Alberta
Trihalomethanes (THMS) are disinfection by products that are formed when chlorine reacts with naturally-occurring organic substances in water. The major THM that is formed is the potential carcinogen, chloroform. The amount of THMs formed depends on the chlorine level, amount of Total Organic Carbon in the water, contact time, temperature and pH.
Chlorine in the form of liquid sodium hypochlorite (laundry bleach) or calcium hypochlorite (chlorine pellets), is a very common chemical in domestic water treatment. It is used to disinfect, remove iron, control iron bacteria, and oxidize hydrogen sulfide. When chlorination is used to disinfect dugout water there is likely the highest potential for the formation of THMS. Dugout water usually contains quite a bit of organic material and relatively high levels of chlorine are used to provide disinfection.
Under the current Guidelines for Canadian Drinking Water Quality the maximum acceptable concentration of THMs is 0.350 milligrams per litre. This level is currently under review and will likely be reduced in Alberta to 0.100 milligrams per litre.
Just how do typical chlorine-treated dugout waters stand up to these standards? From our limited testing, dugout water was able to meet the 0.350 mg/L limit, but not always the 0.100 mg/L standard. THMs were removed when the treatment system utilized relatively new activated carbon, in a 1 to 2 cubic foot filter. Chlorinated dugout water typically contained 0.100 to 0.300 mg/L THMS. If the activated carbon in the system was relatively new the THM level was reduced to almost nothing. When the carbon was old there was basically very little, or no, reduction in THMS.
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