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Circular economies should be the core backbone of the infrastructure for water, energy, and sanitation treatment.
FREMONT, CA: A wastewater treatment plant that consumes a great deal of energy and is carbon-intensive could be a significant energy source and a source of recycled organic and metallic materials, contributing to broad sustainable development goals, the circular economy, and water-energy-sanitation-food-carbon issues. Climate change will exacerbate water scarcity by 2030 for 40 percent of the world's population. The water sector consumes a lot of fossil fuels, causing it to emit more carbon dioxide (CO2). Water consumption accounts for 4 percent of total energy consumption, and wastewater treatment plants (WWTPs) account for 25 percent.
Screening, grit removal, primary settling tanks, aeration (or activated sludge), secondary settling tanks, filtration, disinfection, and sludge treatment are typical wastewater treatment processes. These units are typically energy-dependent and carbon-intensive. To achieve net-zero carbon, existing WWTPs can utilize energy-extracting technologies, such as anaerobic digesters and membrane reactors, salinity gradient or osmotic energy recovery processes, and fuel cells. The available technologies for extracting energy are reviewed and discussed in this section.
Anaerobic bioreactors: The AD process has been used in WWTPs practically as an afterthought for decades to stabilize sludge and produce biogas. The net positive energy is usually generated when sludge is co-digested with organic substrates, including food waste, fats, oils, and grease. The AD process can also be enhanced with hybrid or combined systems, such as integrating bipolar membrane electrodialysis to recover ions with solid oxide fuel cells to generate electricity, which could result in a net energy efficiency of 55 percent. Furthermore, CHP systems with AD allow for the onsite conversion of biogas into power, and thermal energy can be used to power the facility or the grid.
Fuel cells: There has been a growing interest in hydrogen fuel cells for clean trucks. Fuel cells are functionally the reverse of electrolysis cells. They are devices that convert chemical energy into electrical energy. They typically consist of an anode and a cathode connected by an external circuit and a chemical fuel. Hydrogen fuel cells are being considered for clean trucks. Wastewater is an acceptable feedstock for fuel cells. A mere 1 percent of solar energy can be converted into microbial systems such as fuel cells, desalination cells, and electrolysis cells. Based on an estimate, ten times the current energy production can be generated using microbial systems. In solar energy-based photosynthetic MFCs, certain specialized light-harvesting complexes function as photosynthetic units, whereas photosystems I and II act as the photosystems.
