ABSTRACT
Binary zinc oxide (ZnO) nanocomposites with different noble metals, silver (Ag) and ruthenium (Ru), were prepared from an aqueous leaf extract of Callistemon viminalis. The biosynthesized photocatalysts were characterized and examined for their photocatalytic disinfection against Escherichia coli isolated from hospital wastewater. The influence of the different noble metals showed a difference in physicochemical characteristics and photocatalytic efficiency between Ag-ZnO and Ru-ZnO. The photocatalytic degradation of methylene blue and photocatalytic disinfection were found to be in the order Ag-ZnO > Ru-ZnO > ZnO. The photocatalytic disinfection of Ag-ZnO reached a 75% reduction in 60 min, compared to 34 and 9% reductions of Ru-ZnO and ZnO, respectively. The kinetic reaction rate for the photocatalytic disinfection of Ag-ZnO was found to be 2.8 times higher than that of Ru-ZnO. The outstanding photocatalytic activity of Ag-ZnO over Ru-ZnO was attributed to higher crystallinity, greater UVA adsorption capacity, smaller particle size, and the additional antimicrobial effect of Ag itself. The C. viminalis-mediated Ag-ZnO nanocomposites can be a potential candidate for photocatalytic disinfection of drug-resistant E. coli in hospital wastewater.
ABSTRACT
The recovery of energy from municipal solid waste (MSW) has gained popularity in many industrialized countries, but its adoption in economically developing countries, especially in Africa, has been slow. While capital investments and technical requirements for waste-to-energy (WtE) systems are among the most important causes for this slow adoption, the unavailability of data on the thermochemical quality of MSW as a potential feedstock for energy recovery is also a limiting factor. In this paper, Harare, a typical African city, was selected as a case study. The evaluation was based on the analysis of the MSW's composition, moisture as-discarded, thermochemical properties and energy content. The results show that the quality of the MSW is comparable to that in regions outside Africa where WtE has been a success. The combustible fraction exceeded 75 wt% making it ideal for thermal treatment without requiring supplementary fuel. With an MSW throughput of 421,757 tonnes year-1 (11.1% of which is recycled), and a lower heating value of 10.1 MJ kg-1, the energetic potential was estimated at 3.8 × 106 GJ. MSW thermal treatment via conventional technologies can reduce the waste throughput to landfills by up to 40%, provide up to 112 GWh year-1 of electrical energy, and increase the annual share of electrical energy produced from bio-fuels and wastes from 1.3% to at least 2.2%. These benefits make thermal MSW treatment a suitable option for waste disposal in African cities.
