Biodiesel production from edible oil wastewater sludge with bioethanol using heterogeneous nano-magnetic catalysis

Abstract

Currently, most sludge from the wastewater treatment plants of edible oil factories is disposed to landfills, but landfill sites are finite and potential sources of environmental pollution. Production of biodiesel from wastewater sludge can contribute to energy production and waste minimization. However, conventional biodiesel production is energy- and waste-intensive. Generally, biodiesel is produced from the transesterification reaction of oils with alcohol (i.e. methanol, ethanol) in the presence of a catalyst. Homogeneously catalysed transesterification is the conventional approach for large scale production of biodiesel as reaction times are relatively short. Nevertheless, homogenous catalysis presents several challenges such as high probability of soap formation in the presence of water and free fatty acids, and difficulty of separation and reusability. The current study aimed to reuse wastewater sludge from the edible oil industry as a novel feedstock for both monounsaturated fats and bioethanol to produce biodiesel. Preliminary results have shown that the fatty acid profile of the oilseed wastewater sludge is favourable for biodiesel production with 48% (w/w) monounsaturated fats; the residue left after the extraction of fats from the sludge contains sufficient fermentable sugars, after steam explosion followed by an enzymatic hydrolysis, for the successful production of bioethanol [29% (w/w)] using a commercial strain of Saccharomyces cerevisiae. A novel nano-magnetic catalyst was synthesised from mineral processing alkaline tailings, mainly containing dolomite originating from cupriferous ores using a modified sol-gel technique. The biodiesel produced from the wastewater sludge performed well. The thermal efficiency was 30.75% (compared to 28.4% and 26.95 for conventional biodiesel and petroleum-based commercial diesel, respectively). The fuel consumption was higher than commercial diesel at a maximum brake power of 12.8 kW (0.15 and 0.31 kg/kW.h for commercial diesel and wastewater sludge biodiesel respectively) but it was more environmentally friendly in terms of gaseous emissions - both fuels showed a linear decrease in emissions proportionate to the increase in engine speed, reaching averages of 51.5 ppm/g/h and 89.9 ppm/g/h nitrous oxides, respectively at 2000 rpm, and 249.3 ppm/g/h and 310.4 ppm/g/h carbon monoxide, respectively at 800 rpm. Both the catalytic properties and reusability of the catalyst were investigated. A maximum biodiesel yield of 88% was obtained, which dropped to 61% after the fourth transesterification reaction cycle. The proposed approach has the potential to reduce material costs, energy consumption and water usage associated with conventional biodiesel production technologies. It may also mitigate the impact of conventional biodiesel production on food and land security, while simultaneously reducing waste.