An innovative trial in Queensland’s Pine River is revealing how shellfish reefs can improve water quality through natural filtration. The Unitywater shellfish reef trial, delivered in partnership with the University of the Sunshine Coast (UniSC), OzFish, and Healthy Land and Water, has demonstrated measurable nutrient reductions nearly two years after installation.
This first-of-its-kind Australian trial was designed to test whether shellfish could offset nutrients downstream from Unitywater’s Murrumba Downs Wastewater Treatment Plant, a step toward the utility’s goal of ensuring all nutrients from wastewater are diverted or offset from waterways by 2040.
Unitywater Executive Manager of Sustainable Infrastructure Solutions, Mike Basterfield, said the project is helping the organisation understand how nature-based systems can complement engineered treatment.
“The trial tests the value of shellfish reefs in abating nutrients downstream from our Murrumba Downs treatment plant,” he said. “These results could shape future investment in nature-based solutions.”
How does the shellfish reef filtration process work?
Each reef is constructed from durable oyster baskets developed by OzFish volunteers and filled with about 18 kilograms of recycled oyster shells sourced from restaurants and shucking operations. The shells are sun-cured for 12 months to remove impurities and prepare them for attachment of oyster spat before deployment.
Once installed, scientists measure performance by analysing nitrogen absorption in the shellfish tissue, deposition in surrounding sediment, and natural removal from the water column. These metrics adhere to well-established marine ecology methods for quantifying nutrient cycling and storage.
In the first six months of monitoring, the reefs achieved a total nitrogen reduction of 7.24 kilograms.
“Shellfish filter water by separating organic material for food and storing nutrients in their body tissue,” Mr Basterfield said. “That ensures the nutrients do not re-enter the waterways.”
According to modelling from Unitywater and UniSC, installing around 154 reefs, covering an area about the size of two rugby league fields, could offset nitrogen discharge from a treatment plant serving 10,000 people.
What are researchers discovering about reef performance?
UniSC marine ecologist Associate Professor Ben Gilby said the project is not only improving local water quality but also helping to restore a critical ecosystem.
“More than 95 per cent of shellfish reefs have disappeared along Australia’s coastline,” Dr Gilby said. “Historically, they played a large role in nutrient removal. This trial shows reefs are filtering water, improving quality, and storing nutrients within shellfish tissue and the reef structure itself.”
Gilby’s team used nitrogen tracing in both shellfish flesh and surrounding sediment to measure nutrient retention and transformation. The results confirm that these living reefs act as both filters and nutrient sinks, reducing concentrations that would otherwise contribute to algal blooms or oxygen depletion downstream.
What is next for the Unitywater shellfish reef program?
Following State Assessment Referral Agency (SARA) approval, Unitywater plans to expand the trial by installing two additional reefs in the North Pine River between Lawnton and Murrumba Downs. The new sites will test how depth, salinity and flow affect performance and provide data for scaling nature-based infrastructure.
“These new sites will help us test reef performance under different conditions and give us a clearer picture of how to scale this solution,” Mr Basterfield said. “Our commitment to net zero by 2040 includes offsetting all wastewater nutrients, and this is one of the innovative solutions that can help get us there.”
The results so far indicate that engineered and ecological systems can coexist, combining wastewater treatment precision with the resilience of natural ecosystems. For Unitywater, this is a model of nature-based infrastructure that delivers measurable outcomes in nutrient reduction and biodiversity restoration.