27/6/16 – Thangool (Queensland) – Chlorine
Incident Description: On the 27/6/2016 several customers from Thangool reported a strong chlorine taste and odour in the water supply to Banana Shire Council. When testing the mains in Thangool it was found that the residual chlorine was in exceedance of the ADWG guideline. Corrective and Preventative Actions: The mains supply to Thangool was tested and found to be within the guideline values. Water mains throughout Thangool were flushed to reduce the chlorine levels to within the ADWG levels and follow up monitoring and checking with affected customers was undertaken. Investigation after the incident found that the chlorine dosing equipment had a fault that allowed overdosing of chlorine during high flow periods. All routine maintenance and inspection had been performed according to manufacturer and legislative requirements. The dosing controller was replaced and commissioned to ensure that chlorine dosage was unaffected by mains flow.
Chlorine dissociates in water to form free chlorine, which consists of aqueous molecular chlorine, hypochlorous acid and hypochlorite ion. Chlorine and hypochlorites are toxic to microorganisms and are used extensively as disinfectants for drinking water supplies. Chlorine is also used to disinfect sewage and wastewater, swimming pool water, in-plant supplies, and industrial cooling water.
Chlorine has an odour threshold in drinking water of about 0.6 mg/L, but some people are particularly sensitive and can detect amounts as low as 0.2 mg/L. Water authorities may need to exceed the odour threshold value of 0.6 mg/L in order to maintain an effective disinfectant residual.
In the food industry, chlorine and hypochlorites are used for general sanitation and for odour control. Large amounts of chlorine are used in the production of industrial and domestic disinfectants and bleaches, and it is used in the synthesis of a large range of chemical compounds.
Free chlorine reacts with ammonia and certain nitrogen compounds to form combined chlorine. With ammonia, chlorine forms chloramines (monochloramine, dichloramine and nitrogen trichloride or trichloramine) (APHA 2012). Chloramines are used for disinfection but are weaker oxidising agents than free chlorine.
Free chlorine and combined chlorine may be present simultaneously (APHA 2012). The term totalchlorine refers to the sum of free chlorine and combined chlorine present in a sample.
Chlorine (Free) ADWG Guideline: 5mg/L (Chlorine in chloraminated supplies 4.1mg/L). Chlorine dissociates in water to form free chlorine, which consists of aqueous molecular chlorine, hypochlorous acid and hypochlorite ion.
Chlorine (Total) ADWG Guideline 5mg/L (chloraminated supplies 4.1mg/L): The term total chlorine refers to the sum of free chlorine and combined chlorine present in a sample