2019/20: Woodhill Reservoir – Chlorine
2/2/20: Woodhill water dosing facility dosed above 5mg/L for a period of 6 minutes (6:42am – 6:48am). Root-cause analysis showed that this was due to a downstream reservoir ceasing calling for water and the chlorine dosing station continued to dose for 6 minutes into the main – resulting in a very short-term chlorine exceedance. When the high levels were detected by the
analysers the dosing unit disabled. This triggered a page to the on-call officer.
No customer complaints were experienced, and it is likely that no customers were affected due to dilution, flows and the time of day.
SCADA reprogramming and testing of the new code and operation proved successful in addressing the underlying issue and there have been no more similar events since.
This issue occurred in October 2019, however a fault in the programming had meant that the issue was experienced again. Note that this has not occurred again, and all testing and monitoring demonstrates the system now works as designed.
18/10/19: Woodhill water dosing facility dosed above 5mg/L for a period of 15 minutes (3:30am – 3:45am). Root-cause analysis showed that this was due to a downstream reservoir ceasing calling for water and the chlorine dosing station continued to dose for 6 minutes into the main. The system worked as designed, and when the high levels were detected by the analysers, the dosing unit disabled, and this triggered a page to the on-call officer. No customer complaints were experienced, and it is likely that no customers were affected due to dilution, flows
and the time of day. SCADA reprogramming was performed to try and prevent a recurrence.
https://www.logan.qld.gov.au/downloads/file/2924/drinking-water-quality-management-plan-annual-report-2019-2020
GENERAL DESCRIPTION
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