Application of water treatment sludge as a low-cost and eco-friendly catalyst in the biodiesel production via fatty acids esterification: Process optimization

Abstract One of the ways to produce biodiesel is through fatty acid esterification via heterogeneous catalysis. The water treatment plants in Manaus - Amazonas/Brazil produce about 20 tons of sludge per day. In order to establish an application to this abundant waste as a new heterogeneous catalyst, acid and heat treatments were performed and characterized by X-ray diffraction, X-ray fluorescence, infrared spectroscopy, thermogravimetric analysis, surface area, and scanning electron microscopy. The sludge is mainly composed of Si, Al, and Fe. Only the sludge without heat treatment showed S levels after acid treatment, proving the sulfation in these materials. The obtained materials were applied as catalysts, where the activity was evaluated by fatty acid esterification with methanol. The process was optimized using the best catalyst (WTS3M) and oleic acid, where under the following reaction conditions: 15:1 methanol:acid molar ratio, 5 wt.% of catalyst, 100 °C and 3 h of reaction, resulted in conversions higher than 97%. The application of water treatment sludge as a catalyst with high catalytic activity for methyl oleate synthesis and the possibility of regeneration were demonstrated. This study will thus aid in the cost-effective reduction of water treatment sludge and increase the potential for additional biofuel production.