A technique that uses the residue left over from wastewater treatment in the production of concrete has been developed by the São Carlos School of Engineering at the University of São Paulo, Brazil. Ten percent of the mix used to make the concrete is composed of the sludge left over from sewage water treatment combined with polyaluminum chloride, a coagulant.

"The results have been satisfactory: the volume of concrete is expanded, without a reduction in resistance or an increase in cost. Moreover, using the waste residue this way keeps it from ending up in our waterways," research coordinator Valdir Schalch told Tierramérica.

The presence of heavy metals like lead and cadmium in the sludge makes it recommendable to use this concrete only for public roads and sidewalks, where "the metals remain encased." On the other hand, "we don’t recommend using it to manufacture concrete blocks for the construction of housing and other buildings," since there are no regulatory standards on metal content, added Schalch.