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.Cook S., Hall M., Gregory A. (2012). Energy use in the provision and consumption of urban water in Australia: an update. CSIRO Water for a Healthy Country Flagship, Australia. Prepared for the Water Services Association of Australia. A. P. f. t. W. S. A. o. A. CSIRO Water for a Healthy Country Flagship, CSIRO.
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.UNFCCC (2016). Paris Agreement. United Nations. Moreover, there is uncertainty concerning energy costs. Because for a wastewater utility energy costs represent the second highest cost after labour,Copeland, Claudia. (2017). Energy-Water Nexus: The Water Sector's Energy Use, Congressional Research Service. 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.UN (2015). World Urbanization Prospects: The 2014 Revision, (ST/ESA/SER.A/366), United Nations.UN (2015). World Population Prospects: The 2015 Revision, Key Findings and Advance Tables. Working Paper No. ESA/P/WP.241., United Nations. 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.World Bank. (2015). East Asia and Pacific Wastewater to Energy Processes: a Technical Note for Utility Managers in EAP countries, World bank. Moreover, as shown by Mizuta,Mizuta K., Shimada M. (2010). "Benchmarking energy consumption in municipal wastewater treatment plants in Japan." Water Science and Technology 62(10): 2256-2262. 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.IPCC (2014). Climate Change 2014 Synthesis Report Summary for Policymakers. IPCC Assessment Report. 5. 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.Bates, B. C., Z. W. Kundzewicz, S. Wu and J. P. Palutikof, Eds., 2008: Climate Change and Water. Technical Paper of the Intergovernmental Panel on Climate Change, IPCC Secretariat, Geneva, 210 pp. 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.Van DijK, A. (2015). "Water resources, climate change and energy". Climate, Energy and Water. C. U. Press: 6-27. 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.Stillwell A. S., Hoppock D. C., Webber M. E. (2010). "Energy recovery from wastewater treatment plants in the United States: A case study of the energy-water nexus." Sustainability 2(4): 945-962. The combination of more stringent limits and new treatment requirements might further increase the already significant energy demand of these facilities.Venkatesh G., Brattebø H. (2011). "Analysis of chemicals and energy consumption in water and wastewater treatment, as cost components: Case study of Oslo, Norway." Urban Water Journal 8(3): 189-202.
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.
Resource recovery The pressure to reduce greenhouse gas emissions to mitigate climate change and the increasing demand for natural resources is compelling the wastewater sector to develop innovative and more efficient practices to operate. Wastewater contains energy, nutrients and other organic and inorganic resources that can be successfully recovered and used in a broad range of applications. Energy can be recovered as biogasAskari, Mohammad Bagher, Fatemeh, Golestanian, Saman, Motraz, Motamedi Mirhosseiny, Leili (2015). "Advantages and Disadvantages of Biogas Energy." Bulletin of Advanced Scientific Research (5): 132-135%V 131.Kwaśny, J., Balcerzak, W.(2017). "Production logistics and participation of biogas in obtaining primary energy in Poland." Energy and Environment 28(4): 425-436 and heat.Funamizu N., Iida M., Sakakura Y., Takakuwa T.(2001). "Reuse of heat energy in wastewater: implementation examples in Japan." Water Science and Technology IWA Publishing 43: 277-285.Kollmann, René, Neugebauer, Georg, Kretschmer, Florian, Truger, Barbara, Kindermann, Helene, Stoeglehner, Gernot, Ertl, Thomas, Narodoslawsky, Michael (2016). "Renewable energy from wastewater – Practical aspects of integrating a wastewater treatment plant into local energy supply concepts." Journal of Cleaner Production Biogas is an energy source with a wide range of uses, whereas heat has proven valuable for heating and cooling applications of buildings. Among the nutrients, phosphate can be recovered as struvite for fertilisers, a very important application since phosphate resources are limited and depletable. Composted sewage sludge finds applications in agricultural setting and urban gardens as soil amendment.Cheng, Hefa, Xu, Weipu, Liu, Junliang, Zhao, Qingjian, He, Yanqing, Chen, Gang (2007). "Application of composted sewage sludge (CSS) as a soil amendment for turfgrass growth." Ecological Engineering 29(1): 96-104. Besides energy and nutrients, the effluent from the wastewater treatment plants can be reused as quality irrigation water both in agricultural and landscape applications. The reuse of treated water is especially valuable in countries with limited rainfall events or long periods of drought.Arlosoroff, Saul. (2007). "Wastewater Management, Treatment, and Reuse in Israel". Wastewater Reuse–Risk Assessment, Decision-Making and Environmental Security. M. K. Zaidi. Dordrecht, Springer Netherlands: 55-64. All these possibilities are changing the vision of wastewater management and the role of wastewater treatment plants. Wastewater treatment plants have the potential not only to treat the wastewater but also to become resource recovery facilitiesRömgens Ben, Kruizinga Eelco (2013). Wastewater management roadmap towards 2030: A sustainable approach to the collection and treatment of wastewater in the Netherlands. A. o. N. M. Association of Regional Water Authorities, Agentschap NL and DNV.
Energy Effective strategies adopted by wastewater utilities to reduce energy consumption and dependence from fossil fuel-based energy sources include increasing energy efficiency and generating renewable energy on site.
Energy efficiency