Municipal wastewater treatment energy management

World population and population density in urban settlements are expected to grow by 32% and 66% by 2050, respectively.[6][7] 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.[8] Moreover, as shown by Mizuta,[9] 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.

Greenhouse gas emissions from human activities have almost doubled between 1970 and 2010.[10] 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.[10] 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.[11] 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.[12] 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.[13] The combination of more stringent limits and new treatment requirements might further increase the already significant energy demand of these facilities.[14]

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,[15] 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.[10]

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 biogas[16][17] and heat.[18][19] 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.[20] Composted sewage sludge finds applications in agricultural setting and urban gardens as soil amendment.[21] 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.[22] 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 facilities[23]

Since the recognition of anthropogenic causes of climate change in the late 1980s and the identification of the energy sector as one of the main contributors, there has been a global effort to investigate the energy consumption of human activities and their indirect contribution to greenhouse gas emissions. In Europe the energy analysis of the wastewater sector was conducted adopting mainly two strategies. Germany (MURL, 1999) and Switzerland (BUWAL, 1994), for example, developed energy management manuals for wastewater treatment plants and reduced their energy consumption by 38% and 50%, respectively.[24] These manuals provided wastewater utilities with energy targets to achieve. On the other hand, in 1999 Austria promoted benchmarking that allowed annual comparison of wastewater treatment plants energy performances. This comparison stimulated a competition among the wastewater treatment plants and the aspiration to improve their efficiency, which led Austria to be one of the first countries in the world to achieve energy neutrality in the wastewater sector. The energy benchmarking process has allowed wastewater treatment plants to identify their most energy-consuming assets and possible inefficiencies, and target them to reduce their energy demand. For example, the inefficiency of the aeration process identified by multiple studies[25][26][24] has allowed the development of more energy efficient oxidation units, with a possible energy saving of about 20% to 50% in some cases according to Frijns[27] and an EPA[28] study.

Increased energy efficiency has allowed wastewater treatment plants to comply with discharge limits reducing the energy demand even up to 50%[24] without affecting treatment performances. However, energy efficiency strategies by themselves are not sufficient to achieve independence from the electricity grid and fossil fuel-based energy sources. To achieve energy neutrality, multiple studies have looked at the feasibility of integrating a variety of renewable energy sources into wastewater treatment plants. The wastewater itself is a carrier of energy and a theoretical calculation, based on the characteristic of the sewage, shows that the composition of the embedded energy is 80% thermal energy and 20% chemical energy.[29] The thermal energy can be recovered as heat while the chemical energy is recovered as biogas.

Renewable energy generation on site, in addition to increased energy efficiency, has already allowed at least twelve plants worldwide not only to achieve energy neutrality but also to produce more energy than they need.[30] Energy efficiency programs and renewable energy generation have proven successful in diminishing the dependence of the wastewater sector from the energy grid, reducing treatment costs and the environmental impact associated with the grid connection.