Clear Water Lagoon

An alternative to mechanical/chemical filtration of water.

Mount Isa Water Board’s Clear Water Lagoon is the only documented natural filtration facility as part of the treatment process of potable water in Australia.

Figure 1 – Clear Water Lagoon: The primary components of the reed bed system.

The lagoon was partitioned off from Lake Moondarra in 1968, approximately 10 years after the Leichardt River was dammed downstream of Mount Isa City. It was built as a protected reservoir to overcome high turbidity inflows entering Lake Moondarra during the wet season.

The physical and chemical quality of the water was improved so significantly that in 1982 the decision was made to divert water through Clear Water Lagoon year round. Since water quality monitoring began, outflow has consistently complied with the Australian Drinking Water Guidelines.

Today, Clear Water Lagoon is considered an integral part of water treatment and filters on average 2,000 ML/month of water, supplying Mount Isa’s population (~22,000), mining and industry.

The system operates within a 100 mm tolerance to ensure the ecological functioning of the system is not disrupted. The main lagoon is on average 8 m deep, storing approximately 30 days of water. Col Popple pump station has the capacity to pump down 1.8 m which equates to 5 days supply.

Water from Lake Moondarra is pumped into the Clear Water Lagoon via an open earth channel which directs flows through a settling pond. The cascading nature of the channel allows the water to be partially aerated. A second channel directs settled water into the main lagoon. Aquatic vegetation and regulated flows are utilised in a similar way to constructed wetlands.

The dominant macrophytes in the lower flume, settling pond and main lagoon include Hydrilla vetricillata, Potamogeton tricarnatus, Typha orientalis and Vallisneria spiralis. These macrophytes are instrumental to the water clarification process as they act to trap suspended matter, altering micro-scale flow velocity patterns around the foliage and prompting settlement on the bed of the lagoon.

Water quality parameters (e.g. faecal coliforms, suspended sediment and soluble heavy metals: See figure 2) improve markedly as water travels through the system.

Despite the difficulties of maintaining and supplying potable water in this climatically variable region, Clear Water Lagoon consistently provides quality drinking water for Mount Isa City.

The relative low cost, simplicity and limited maintenance of this technology presents an opportunity for it to be replicated in small remote communities where other treatment options are not economically feasible, or where access to trained operators is limited.