2016/18: “Trihalomethane investigation at Rosslynne WFP
Rosslynne Reservoir refilled quickly during June to September 2016 which resulted in the water quality in the reservoir changing – particularly in organic type. The type of organics in the reservoir were difficult to remove through the existing treatment process. Because of chlorination used for disinfection at the plant, there were elevated levels of disinfection by-products namely Trihalomethanes. Extensive investigation of the organics and quantities of Trihalomethane produced at the plant determined the issue is likely to persist for many years and the removal of organic type is a costly activity. The investigation included review of other water agencies in America and Europe to identify alternative technology. The technology most suitable for the treatment of Trihalomethanes is a PAXTM system, using tank mixing and aeration to release the volatile compounds from the treated water to tank air space, and venting the air to external atmosphere. The project has undergone assessments of financial and environmental impacts, prior to design and installation during 2018.” Western Water
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
“Trihalomethane detection in the Riddells Creek network, 1 February 2017 (Riddells Creek)
Issue – Refer to prior incident content for background discussion regarding Trihalomethane detection at McDonalds Tank.
The routine sample for the Riddells Creek network on 1 February 2017 detected Total Trihalomethanes of 0.28mg/L, above the ADWG limit (0.25mg/L). DHHS was notified on the day of the reported detection in accordance with the requirements of Section 18.
Actions – To prevent formation of Trihalomethanes, the treatment process at Rosslynne Water Filtration Plant was modified to reduce organics, and both the disinfection system at the plant and the Rosslynne network’s booster chlorination stations were also modified.
Outcome – External consultants conducted a review of the catchment, reservoir, plant and distribution network to assess the source of the organics and options to prevent future issues. Treatment options to prevent Trihalomethane formation during future refilling events included other types of chemical dosing and improvements to the disinfection system and storage tank management.” Western Water