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The third little pig’s house made of bricks saved him from the wolf. But it wasn’t the most eco-friendly. Manufacturing bricks is carbon-intensive and creates toxic air pollution. So engineers at RMIT University in Australia have proposed brick made partly with treated sewage waste in a new study published in the journal Buildings. The idea could be hard to digest for some, but would tremendously benefit the environment.
To make bricks, a mix of clay and concrete materials is heated at temperatures between 900 and 1200°C. This requires burning a lot of fuel. In South Asian countries, where brickmakers will burn coal, biomass and trash for this firing, brick kilns have a global warming impact equivalent to that of all passenger cars in the United States, and the pollutants they create kills tens of thousands of people each year. About 8 percent of global carbon emissions come from brick manufacturing, according to some estimates.
Bricks made from sewage would help to clean up the air. Once sewage is treated and dried at wastewater treatment plants, some of the solids go into making fertilizer. But a good 30 percent of it is stockpiled or sent to landfill. Incorporating the biosolids in bricks would put this waste to good use, and it reduces emissions from brick-making.
The RMIT University researchers collected three different biosolid waste samples from two treatment plants, and used them to make bricks containing 10, 15, 20, and 25 percent biowaste. They report that bricks containing 25 percent sewage solids required about half the energy to manufacture as regular bricks.
The biobricks would also be better for the environment in other ways. More than three billion cubic meters of clay soil are dug up around the world every year to produce around 1.5 trillion bricks. That is “equivalent to over 1000 soccer fields dug 440 m deep or to a depth greater than three times the height of the Sydney Harbour Bridge,” the researchers write. Biosolid bricks could reduce the need for such massive excavation.
Plus, 43 to 99 percent of heavy metals present in the biosolids remained trapped in the bricks, keeping them from leaching into the environment, the researchers found.
The bricks passed compressive strength tests. They were more porous than their conventional cousins, which made them more insulating. And as an added bonus they were cheaper to produce.
Further tests are needed before biobricks are produced on a larger scale because sewage waste in different parts of the world can have different compositions and chemical traits, the researchers say. But based on their study results, they propose that including a minimum of 15% biosolids content into 15% of brick production could “completely recycle all the approximately 5 million tonnes of annual leftover biosolids production in Australia, New Zealand, the EU, the USA and Canada. This is a practical and sustainable proposal for recycling all the leftover biosolids worldwide.”
Source: Abbas Mohajerani et al. Proposal for Recycling the World’s Unused Stockpiles of Treated Wastewater Sludge (Biosolids) in Fired-Clay Bricks. Buildings, 2019.
Photo: RMIT University