Spanish researchers have developed a way to use human waste as an admixture in ceramics, a method that could have industrial and environmental benefits.

Researchers at the Department of Physics and Nuclear Engineering at the Polytechnic University of Catalonia, based in Barcelona, Spain, claim that structural ceramics can be made that implement both sewage sludge and forest waste in the manufacturing process. Led by Professor Martin Devant, the research looks to make use of the large quantities of sewage sludge being generated, approximately 0.4kg of sludge per person, per day, he claims.

The method involves incorporating forest waste, such as sawdust from shredded furniture, into a binary mixture of illitical clay, which enables sewage to be assimilated into an extrudable mix. The use of forest waste enhances the ceramic properties, says Devant. ‘The key is that with the addition of forest waste, you can add more sludge.’

The researchers found the ideal composition to be 10% sludge, 10% forest waste and 80% clay, which yielded a compression strength of 96kp/cm2. ‘This is the bulk compression to finally obtain a perforated brick with 50kp/cm2, is an acceptable minimum,’ says Devant.

The mixture would be suitable for producing ceramics with low thermal conductivity and high porosity, which would be the same as ‘a ceramic brick, because all the organic compounds are thermally destroyed, but with a high porosity,’ says Devant. He adds that the bricks would be perfectly suitable for applications such as thermal isolation walls.

Devant said that the process, which entails firing the brick at 980ºC, is only as energy intensive as a normal brick factory. ‘The method is cost effective, because the brick factory will be paid to recycle the sludge,’ he says. The researchers also tested the environmental aspects of production using leaching and outgassing tests.

The team has produced a tool to design tailor-made bricks with customised physical properties. ‘We observed a dependence of the compressive strength according to the clay fraction. We observed a dependence of the technological limit with the sludge fraction, and we think that this can be used for other sludge and clays.’ Though research in this area has been done before, Devant claims, ‘Nobody has used forest waste and sludge at the same time’.

Christopher Hall, Emeritus Professor of Materials at the University of Edinburgh, UK, says, ‘It is important to be cautious about the durability of the product. This is an example of a long-term property that is not easy to assess, and if changes are made to composition or firing temperature then there may be unanticipated consequences.’