Wastewater solids could make quality house bricks

Clay Technology magazine
21 Feb 2019

Wastewater solids could contribute to making quality housebuilding bricks. Ceri Jones looks at the pros and cons.

Vast stockpiles of biosolid waste could be safely incorporated into clay bricks, improving the eco credentials of brickmaking, says a team at RMIT University, Australia.

Biosolids are a useful byproduct of the sewage treatment process, when large bodies of wastewater sludge are subjected to thickening, anaerobic digestion, and dewatering to make it less, make it safe and reclaim the valuable water resources.

Resulting solid matter is often used in agricultural and forestry applications, but this still leaves around 30% of global stores going to waste in landfills or storehouses – representing more than 15 million tonnes a year in Australia, New Zealand, the EU, the USA and Canada.

According to the research team, led by Associate Professor and civil engineer Abbas Mohajerani, more than three billion square metres of clay soil is dug up each year for the global brickmaking industry, to produce about 1.5 trillion bricks. However, he says that a minimum of 15% biosolids content into 15% of brick production would ‘completely recycle all the approximately five million tonnes of annual leftover biosolids production in Australia, New Zealand, the EU, the USA and Canada’.

The combination of exploiting this stock, the subsequent reduction in the amount of virgin soil needed for brickmaking, and the resulting lower carbon emissions for firing, make a strong environmental case for using biosolid fillers.

The team believes that while it is viable to use up to 25% of biosolids in brickmaking, lowering the amount would yield positive results and have the desired environmental impact. ‘The bricks incorporating only 15% of biosolids used in this study are excellent quality bricks, suitable for use as standard high-quality bricks,’ the paper reads.

Shifting the foundations

The Australian team tested the integrity of fired clay bricks containing biosolid fillers at levels of 10, 15, 20 and 25%, using three types of waste obtained from two different treatment farms in Melbourne, which has a growing stockpile problem. X-ray fluorescence was used to check both the brick soil and biosolids, and a diffractometer to monitor their crystalline phases.

They were tested for compressive strength, recording a range of 35.5-12.04MPa and leachate analysis returned negligible results for heavy metal presence once fired, but more in-depth trials would be required to study leaching over time. They were also put through analysis of water absorption rates, density, liquid and plastic limits and linear shrinkage, with average test results compared against control bricks using only the soil.

‘By testing the Atterberg limits it was found that the liquid limit ranged from 53–70% in the biosolids, which is substantially greater than the brick soil at 32%. The plastic limit range of the biosolids samples was also
higher than the brick soil at 27–62% compared to 19%.

‘Scanning electron microscopy (SEM) images illustrate that biosolids-amended bricks have a higher porosity than the control bricks, which corresponds to the lower thermal conductivity values recorded for biosolids-amended bricks.’ This is as the firing burns away the organic matter, creating a larger pore volume. ‘The increase in pore volume ultimately resulted in lower compressive strength values, reduced density and increased shrinkage for the biosolids-amended bricks compared to the control bricks.’

Additionally, the energy required for firing is expected to decrease due to the higher organic content of the biosolids. One of the treatment plants produced biosolid matter which, when used at 25%, was estimated to save 48.6% of firing energy.

But due to the variation in wastewater treatments, biosolids present rather differing chemical properties, so while the team believes incorporating matter into bricks is possible, thorough testing must be undertaken first to establish the best blend and outcomes.

This project set out to determine an effective way to utilise biosolid waste, with funding from Melbourne Water. From here, the researchers are proposing that brickmakers make use of the data, which suggests that the switch can be easily achieved.

Specific methodologies and data for each type of biosolid and filler percentage can be found in the paper, A proposal for recycling the world’s unused stockpiles of treated wastewater sludge (biosolids) in fired-clay bricks, published in Buildings. Read it here: bit.ly/2WA7Y5A