Sustainable wastewater treatment solutions for water-smart circular economy

Abstract

The Protection of aquatic water bodies and human health is a paramount objective accomplished by wastewater treatment systems. Traditionally, pollutants are managed and removed in wastewater treatment plants (WWTPs), following a paradigm in which wastewater is considered a waste. Wastewater treatment requires significant amounts of resources, such as energy and chemicals, while sludge is produced, requiring further treatment. A decade ago, a new paradigm emerged, suggesting that municipal wastewater is a source of resources, particularly reclaimed water, materials (e.g., nutrients) and energy. Many processes applicable for this new paradigm already existed, and others have been further developed (struvite-crystallization, membrane contactors, air-stripping, ionic exchange, electrodialysis, direct osmosis, etc.). Recently, resource recovery processes have been extensively developed and investigated to optimize their operation. Reclaimed water can be used for recharging aquifers, irrigation in agriculture and cooling applications. Potential risks posed by the use of reclaimed water – and of other recovered wastewater resources – must be assessed and managed during the lifecycle of the application. For example, membrane separation processes are recognised as suitable for this application to remove pathogens and particles to ensure water quality. Traditional WWTP design is based on effluent quality requirements and investment costs, with energy efficiency being only rarely considered. Larger facilities exhibit lower normalized electric consumption than smaller WWTPs, and older ones normally consume more than modern facilities (although is process dependent). For instance, in Spain it is possible to find facilities with specific electric consumptions 5-10 times higher than in modern and optimized facilities. This clearly reflects the great margin for potential energy savings. Electricity consumption at WWTPs can be reduced by improving the processes and their operation, as well as through mechanical equipment improvement. The aeration of the biological process is the major electricity consumer; thus, control strategies have been deployed to its optimization. Also, less oxygen-demanding process alternatives have been explored, like the simultaneous nitrification-denitrification operated at very low dissolved oxygen concentration. Partial nitritation and deammonification processes with low oxygen consumption per nitrogen load removed, are especially suited for treating supernatant from sludge dewatering units. However, these low energy solutions might have a downside with direct greenhouse gas GHG emissions, especially N2O. Anaerobic digestion of sludge, usually applied in large WWTPs, produces biogas that can generate both electricity and heat for local use or external use, through combined heat and power production, or liquefied biogas for external use. It is also possible to increase biogas production through co-digestion of external substrates, advanced control or sludge pre-treatment. Thermolysis processes, piloted for sewage sludge treatment, enable also waste-to-chemicals applications. There are also other possibilities for energy recovery at WWTPs, such as thermal energy via heat exchangers and heat pumps, hydropower generation using turbines, and heat from sludge incineration. Energy can be also recovered by anaerobic digestion of microalgae grown in nutrient-rich wastewater. In this paper, the transition towards sustainability and water-smart circular economy is illustrated showing how current WWTPs can be turned into Water Resource Recovery Facilities (WWRFs). The incorporation of sustainable pathways and technologies, make energy-positive facilities achievable, thus, reducing their climate impact.