Around the world, treated sewage sludge is typically sent to landfill or stockpiled, impacting the environment and using up valuable land space.

Derived from the sewage treatment process, biosolids are mainly a mix of water and organic materials.

The treatment process stabilises these biosolids by reducing organic matter and micro-organisms to make them safe to use in a variety of applications including as fertiliser, in land rehabilitation or as a construction material such as bricks.

Fired-clay bricks made using biosolids offer a sustainable solution for both the wastewater treatment and brickmaking industries.

According to research published by a team from RMIT University in the ‘Green Building Materials Special Issue’ of Buildings, biosolids bricks required only around half the energy of conventional bricks during manufacturing, were cheaper to produce, and had lower thermal conductivity.

With countries around the world producing millions of tonnes of biosolids each year that go to landfills or stockpiles, there is significant opportunity to create a new beneficial reuse market with bricks for construction. Australia produces 327,000 tonnes of biosolids every year.

Excavation of soil for brickmaking was another issue addressed by the RMIT study.

According to lead investigator associate professor Abbas Mohajerani, more than 3 billion cubic metres of clay soil are dug up each year for the global brickmaking industry to produce about 1.5 trillion bricks. He believes using biosolids in bricks could be the solution to these big environmental challenges.

Mohajerani, who is a civil engineer in RMIT’s School of Engineering, adds that brickmaking using treated sludge offers a practical and sustainable proposal for recycling the biosolids currently stockpiled or going to landfill around the globe.

About 5 million tonnes of the biosolids produced in Australia, New Zealand, the EU, US and Canada currently go to landfill or stockpiles each year.

Even a minimum 15 percent biosolids content in 15 percent of bricks produced could use up this 5 million tonnes.

The RMIT research analysed the physical, chemical and mechanical properties of fired-clay bricks incorporating 10-25 percent biosolids content. These bricks passed compressive strength tests and analysis revealed that heavy metals were largely trapped within the brick.

Being more porous than standard bricks, the biosolids bricks have lower thermal conductivity.

In terms of energy use during brick firing, the energy demand was cut by up to 48.6 percent for bricks incorporating 25 percent biosolids, thanks to the organic content of the biosolids; this could considerably reduce the carbon footprint of brick manufacturing companies.

A comparative Life Cycle Assessment and an emissions study conducted as part of the research confirmed that biosolids bricks offered a sustainable alternative to the environmental impacts of biosolids management and brick manufacturing.