Sustainable energy management in the wastewater sector applies the concept of sustainable management to the energy involved in the treatment of wastewater. The energy used by the wastewater sector is usually the largest portion of energy consumed by the urban water and wastewater utilities. The rising costs of electricity, the contribution to greenhouse gas emissions of the energy sector and the growing need to mitigate global warming, are driving wastewater utilities to rethink their energy management, adopting more energy efficient technologies and processes and investing in on-site renewable energy generation.
Importance Among the water and wastewater services of a city, wastewater treatment is usually the most energy intense process. Wastewater treatment plants are designed with the purpose of treating the influent sewage to a set quality before discharging it back into a water body, without real concern for the energy consumption of the treating units of a plant. These facilities play the important role to protect not only the water systems but also the human health, preventing the discharge of pathogens normally present in the municipal sewage. Despite the key role of wastewater facilities, energy consumption can not be ignored anymore because of its contribution to greenhouse gas emissions and the need to reduce the emissions to mitigate global warming, as established by the Kyoto Protocol in 1997 and most recently, by the Paris Agreement. Moreover, there is uncertainty concerning energy costs. Because for a wastewater utility energy costs represent the second highest cost after labour, an increase in energy rate would further increase the operational budget of a municipality, and consequently, the service rates for consumers. Therefore, it is important for the wastewater sector to invest in strategies to limit the demand of energy from the grid in order to mitigate both costs and greenhouse gas emissions.
Global challenges Population growth, urbanisation, climate change and increasing demand for natural resources are among the future biggest challenges for the urban wastewater sector.
Population growth and urbanisation World population and population density in urban settlements are expected to grow by 32% and 66% by 2050, respectively. An increase in population is expected to increase the volume of wastewater and sewage sludge requiring treatment. To handle a higher volume of sewage and sewage sludge, it is necessary to upgrade the existing wastewater facilities and the collection networks to collect, store and treat the expected additional volume. Therefore, an increase in volume translates in high future investment costs for a sector that has a low cost recovery rate. Moreover, as shown by Mizuta, there is a direct correlation between the volume of sewage treated and the energy consumption of a wastewater treatment plant. Hence, a rise in energy demand is expected with the upgrading of the existing wastewater facilities and the construction of new ones to cope with future population increase.
Climate change Greenhouse gas emissions from human activities have almost doubled between 1970 and 2010. The increase in greenhouse gases is held responsible for the changes in climate that have caused impacts on natural systems across all continents and oceans. In particular, climate change has affected the water cycle, increasing the precipitation intensity and variability, increasing in this way the risks of floods and droughts in many areas. The changes that have affected the water cycle have had substantial impacts on the aquatic ecosystems, increasing and worsening the stressors already affecting these systems. Nutrient loading represents one of those stressors as, with the change in precipitation patterns, it is expected a substantial increase in nutrients entering aquatic ecosystems due to increased erosion events and frequent sewage systems overflows. Because wastewater treatment plants discharge the treated wastewater in aquatic ecosystems, the target nutrient load of these ecosystems affects the level of treatment that a wastewater treatment plant has to perform on the wastewater before being allowed to discharge it. Higher native levels of nutrients in the receiving water bodies force wastewater treatment plants to perform more stringent nutrients removal from the wastewater before dispose of it. In addition to stricter discharge limits, future standards for currently unregulated contaminants are most likely to be introduced. The combination of more stringent limits and new treatment requirements might further increase the already significant energy demand of these facilities. Population growth and climate change are increasing the energy needs of wastewater facilities. Because fossil fuels are still the most diffused source of energy, providing more than half of the global energy needs, the wastewater sector is still highly dependent on fossil fuel-based energy sources. Electricity, for example, is sourced from the electricity grid while gas and diesel are generally used on-site for heating and to run backup generators. High consumption of fossil-fuel energy makes the wastewater sector an indirect contributor to greenhouse gas emissions since fossil fuels are one of the biggest emitters of greenhouse gases, contributing for 65% of the CO2 emissions globally.
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