2/3/21 Ponti Road, Reservoir, Townsville – Trihalomethanes

On 2 March 2021 routine lab sampling detected 0.275mg/L THMs at Ponti Road Reservoir. This was a result of the increase in sodium hypochlorite dose required for prechlorination for manganese during the discoloration event (DWI-506-21-08862). Once pre-chlorination had
been established, sodium hypochlorite dose was reduced and THMs (and chlorates) have returned to normal levels in the network. Pre-chlorination of the filters has resulted in a net reduction of chlorine dose across the plant which has reduced DBP concentrations overall.

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”. Source: https://water.epa.gov/drink/contaminant