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INTRODUCTION
Most of the water treatment plant (WTP) residuals generated in developing countries are: (i) discharged directly into water bodies, which is not reported due to the scarcity of water sources, siltation of water bodies, water quality deterioration and adverse impacts on aquatic biota (Zhao & Babatunde 2007); (ii) disposed in landfills, which is an expensive method that requires large areas and can contaminate the soil and groundwater if not well operated; or (iii) discharged into publicly owned treatment works (POTW), which can cause operational problems if the POTW was not designed for this purpose and also transfers the sludge disposal problem from the WTP to the wastewater treatment plant.
Today, the principle of industrial ecology (Korevaar 2004) has emerged; sludge should not be considered rejected material but raw material that can be used in productive processes. Furthermore, ever stringent environmental legislation and rising logistical costs have brought into focus the beneficial uses of WTP sludge (US Environmental Protection Agency 2011).
There are several benefits of using WTP sludge in the civil construction industry, such as: eliminating discharges to surface water or POTWs; reducing or eliminating disposal to landfill (American Water Works Association/American Society of Civil Engineers/US Environmental Protection Agency 2011; Lu et al. 2013); reducing sludge disposal costs; reducing environmental impact of clay mining because the sludge is a substitute for clay; and creating opportunities for sanitation companies and brick industries in the global market, which encourages sustainable growth across all industry sectors, including partnerships between businesses (green economy, industrial symbiosis, etc.).
A study by Cantó et al. (2002) was performed at the Saint Joan Despí WTP in Barcelona, Spain. To use WTP sludge to make ceramic bricks, an experimental plant was built, which used heat to dry the sludge and break it down into powder. The material consisted of silicon dioxide (SiO2, 50%), aluminum oxide (Al2O3, 13%), calcium oxide (CaO, 14%), potassium oxide (K2O, 3%) and organic matter (10%). By mixing 70% of the powder produced with 30% clay, the ceramic material obtained was porous and could be used for acoustic and thermal insulation.
Rouf & Hossain (2003) demonstrated the viability of using sludge rich in arsenic and iron as a substitute for clay to manufacture bricks. Different mixtures of clay-sludge were tested,...