Wulguru Reservoir (Queensland) – Trihalomethanes
Incident Description: Detection of THMs (265μg/L) in a treated water sample at Wulguru Reservoir from a sample taken on 23/9/2016.
Corrective and Preventative Actions: Due to water restrictions it has been hard to maintain chlorine residuals at optimal levels to the outlying parts of the network. Due to an E.coli
detection (DWI-7-506-00041) chlorine levels were dosed at a higher level in Yongala reservoir which had a knock on effect at Wulguru reservoir. The increased chlorine with higher
temperatures generated excessive THMs. Chlorine residuals were dropped slightly (so as not to compromise disinfection), reservoir levels were dropped slightly to increase turnover and a greater monitoring of THMs occurred.
February – May 2018DWI-7-506-00055. Townsville Drinking Water Scheme Wulguru. THMs (271 μg/L and 289 μg/L) at Wulguru and Dahl reservoir. THMs were then detected across much of the network in February. THMs were detected across much of the network in February due to high rainfall which resulted in low usage. Chlorine set-point was high to ensure adequate disinfection to the outlying areas. TCC reduced set-points at Douglas WTP from 5mg/L in February due to the rain and low usage to 3.2mg/L in May due to increased usage across the network. TCC were also able to reduce chlorine set-points at re-chlorination points. This coupled with the lowered temperatures and increased demand across the network lowered THMs across the region. (Report also stated THM’s in Transmission Reservoirs 310ug/L (max), 198ug/L (av.) & Reticulation 308ug/L (max), 182.5ug/L (av.)
Trihalomethanes Australian Guideline Level 250μg/L (0.25mg/L)
Why and how are THMs formed?
“When chlorine is added to water with organic material, such as algae, river weeds, and decaying leaves, THMs are formed. Residual chlorine molecules react with this harmless organic material to form a group of chlorinated chemical compounds, THMs. They are tasteless and odourless, but harmful and potentially toxic. The quantity of by-products formed is determined by several factors, such as the amount and type of organic material present in water, temperature, pH, chlorine dosage, contact time available for chlorine, and bromide concentration in the water. The organic matter in water mainly consists of a) humic substance, which is the organic portion of soil that remains after prolonged microbial decomposition formed by the decay of leaves, wood, and other vegetable matter; and b) fulvic acid, which is a water soluble substance of low molecular weight that is derived from humus”. US EPA