Mount Stromlo Water Treatment Plant

The original Mount Stromlo Water Treatment Plant (WTP) was commissioned in June 1967. It contained facilities for the chlorination, fluoridation and pH correction of all water drawn from the Cotter River system.

The 2003 bushfire damage to the Cotter River catchment meant that the existing water treatment plant was not adequate to treat the potentially turbid (cloudy) water drawn from the dams in the burnt-out catchment. A substantial upgrade of the water treatment plant was undertaken to ensure the quality of Canberra’s drinking water was protected during the Cotter River catchment’s lengthy recovery. The new, more sophisticated Mount Stromlo WTP was commissioned in November 2004.

Many of the old Mount Stromlo WTP components, such as the chlorination and fluoridation systems, were incorporated into the new water treatment plant as well as some additional treatment methods that enabled the plant to treat water from a wider variety of raw water sources, including direct filtration and dissolved air flotation and filtration.

In 2007, ultraviolet (UV) light disinfection was installed at the treatment plant, increasing the ability of the plant to treat water sourced from the Murrumbidgee River.

Mount Stromlo WTP also has a mini-hydro which generates electricity that is fed back in to the network. 

The water treatment plant can produce up to 250 megalitres (million litres) per day. The treatment process is shown in the shaded area below:

Stromlo process

Step 1 – Pre-treatment which involves adding alum and a polymer coagulant to the water.

Step 2 – Coagulation: the substances added in step 1 encourage solid particles in the water that could host viruses and bacteria to group together to make larger particles, this is called coagulation.

Step 3 – Flocculation: coagulation results in larger particle clumps called flocs. Flocculation is a process of separating the flocs out of the water. It involves mixing and circulating the water and causing the flocs to float to the top in a sludgy froth.

Step 4 (optional) – Dissolved air flotation: this optional step involves dissolving air in the water which causes suspended matter such as oil or solids to float to the surface where they can be more easily removed through the filtration process. This process is used when the raw water quality is relatively poor, although it is generally not used which makes operation of the plant more cost effective.

Step 5 – Filtration: this involves pumping the water through filters to remove any remaining particles. Water is filtered through anthracite coal and sand.

Step 6 – Fluoridation by sodium silico fluoride to protect teeth (under direction from ACT Health).

Step 7 – Ultraviolet disinfection: In 2007, this additional disinfection step was installed at the treatment plant. UV disinfection works by exposing water to UV light which kills or inactivates microorganisms. This additional step also increases the ability to use water from the Murrumbidgee River for town supply. Before the UV system was in place, water extracted from the Murrumbidgee River needed to be restricted to ensure the drinking water quality levels could be maintained. This reduced the time period and the amount of water that we could draw from this source.

Step 8 – Chlorine disinfection: Chemical dosing to add chlorine in to the water works to kill remaining microorganisims in the water. Chlorine continues to disinfect the water as it moves through the distribution network ensuring the water remains high quality.

Step 9 – Adjusting the water’s pH levels to balance it with lime and carbon dioxide.