Urban water systems, especially wastewater treatment plants, are one of the largest energy consumers at the global municipal level.
It is estimated that these facilities alone consume an average of about 1% to 3% of the country’s total electricity output and account for a significant portion of the municipal energy bill.
The specific power consumption of modern systems should be 20-45 kWh per inhabitant equivalent supplied annually, even if the requirements of older systems are even higher.
This number does not include wastewater pumping and post-treatment of residues.
Wastewater and its by-products, on the other hand, contain various forms of chemical, thermal and potential energy.
Until recently, most plants’ only form of energy recovery was anaerobic post-digestion of process residues (sludge waste).
It uses chemical energy to produce biogas which is generally a sufficient amount of methane to meet about half the needs of the plant.
New strategic and technological approaches to urban water cycle management
After decades of unchanging practices, new strategic and technological approaches to urban water cycle management are being proposed.
Urban water systems and, in particular, wastewater treatment plants are one of the major energy consumers at the municipal level worldwide.
It has been estimated that on average these facilities alone may require about 1% to 3% of the total electrical energy output of a country and that over 20% of public utilities’ electrical energy consumption by municipalities is required for their operation.
Furthermore, energy use by utilities is expected to grow significantly in the next decade in some areas.
Source of greenhouse gas emissions
In Australia, for example, due to expected population growth of 25% by 2030, and the need to access new, more energy-intensive water sources mostly by reuse of treated wastewater, energy use by water utilities for enhanced effluent processing is estimated to grow between 130% and 200 ova existing levels to fulfill a steady precipitate consumption.
According to current technology levels, the specific power consumption of state-of-the-art Wastewater treatment plants should range between 20 and 45 kWh per PE (population equivalent) per year, even though some older plants may double these energy demands.
Other data show power consumption of 0.3-2.1 kWh / m3 treated wastewater in the EU and 0.41-0.87 kWh / m3 in the United States, depending on treatment type, plant size, terrain, etc.
Some assessments may include energy to pump wastewater into the facility, but they may not, so various values have been reported.
According to the Energy Handbook of the Ministry of the Environment of North Rhine-Westphalia (Germany), the optimum target for total electricity consumption of sewage treatment plants is set at 20 kWh / PE year, and the standard value is 26 kWh / PE year.
In general, the smaller the system, the higher the specific power consumption.
Power consumption depends not only on the size of the system but also on the design and technology.
Previous values refer to modern plants that include nutrient removal (N and P) and anaerobic sludge digestion.
Lack of nutrient removal process means lower energy requirements, but lack of anaerobic digestion increases consumption (e.g., for stabilization of aerobic sludge).
The values listed do not include upstream water supply cycles, sewage pumps (mainly pump energy), and post-treatment and disposal of residues (chemical and biological sludge).
As a result, these plants are not only very energy intensive and costly to operate but can also be seen as an important source of greenhouse gas emissions, the reductions of which have recently been made in the European Union and others.
Wastewater, on the other hand, contains energy in various forms, including chemical, thermal and kinetic.
The chemical energy contained in the wastewater is estimated to be about 10 to 14 kJ / g COD (1.67 to 2.33 kWh / m3 even assuming a diluted COD concentration of 600 mg / L) in various studies.
Thermal energy is about 21 MJ /.
When the wastewater temperature drops by 5 ° C, m3 (5.8 kWh / m3), and potential energy produces only 30 kJ / m3 (0.008 kWh / m3) for a drop of 10 m.
The first two numbers show the energy-saving potential of innovative waste, showing the fact that the energy stored in wastewater is about 6-9 times the electrical energy required for treatment.
Assuming that this energy is available, wastewater treatment plants can be converted into energy-neutral or even net energy producers.
The form of energy recovery from these plants, which until recently was almost unique, consisted of anaerobic post-fermentation of process residues (activated sludge, WAS) through which the chemical energy was biogas (mainly methane).
This remains one of the most important energy recovery options, generally sufficient to meet about half of the plant’s aggregate demand, but the efficiency of converting organically embedded chemical energy into an easy-to-use source is low. (30% to 60%).
However, the implementation of new process technologies today enables more efficient strategies for energy recovery from wastewater.
Relation of energy recovery with greenhouse gases
Significant additional energy recovery and improved greenhouse gas mitigation by leveraging the energy content of process residues to close the wastewater loop, in addition to using the chemical and thermal energy content to upgrade the wastewater.
Therefore, wastewater and its residual products can be considered a renewable energy source if addressed by appropriate technical solutions.
Energy required
The energy required to treat waste to the required standards (mainly electricity) accounts for a significant portion of the total cost of the city’s water cycle.
It is estimated that 30% to 35% of the total cost of US wastewater treatment facilities comes from electricity.
Energy reduction and recovery represent an important sustainability issue to maintain the required standards, as the introduction of additional process steps can significantly increase the energy demand of the processing plant.
So far, many efforts to adapt the mainstream process to accepting the mainstream’s degradation are mainly standardized energy monitoring procedures for detecting improvement capabilities and the lack of periodic energy audits.
More common reasons for high energy requirements in wastewater treatment plants are designed to treat drainage using an aerobic active sludge process (ASP), thereby a large compressed air flow in the biological tank A pump is required.
This component may be up to more than 50% and up to 70% of the total energy requirements of the conventional device. ASP is easy to handle, but the operation is more energy intensive.
Sludge drainage
In sludge drainage with a belt press, less than 4% of total plant energy can be required, but a small amount of final water can be reached.
The disinfection requirements are also quite different depending on the technology.
Simple chlorination does not require more than 0.3% of total plant energy, but UV disinfection can account for up to 25% of total electrical energy consumption in urban WWTP.
Sloped energy etching ignores external energy embedded in chemical substances that contribute to the process.
Chemicals consumed at the factory bring both power and energy consumption and field emissions.
The influence of their energy on the treatment facility was estimated at 7% of the total demand.
However, the cost of chemicals is generally evaluated as an economic time than energy costs (these depend on indirect factors such as transportation) and their facilities have pure energy costs or more.
Expansion ventilation is usually designed for small Wastewater treatment plants that are longer ventilated than traditional ASPs.
As a result, extended ventilation wastewater treatment plant uses more energy than 0.21 to 5.5 kWh / m3 average energy intensity than other common small plant shapes.
Axis Sludge Design is very similar to ASP. Bacterial cells are intrinsic phases.
Thus, cell tissue consumed is oxidized in carbon dioxide, water, and ammonia.
Aerobic digestion
Aerobic digestion is less capital than other post-process technologies and is more severe in operation.
However, the oxygen supply is more energy-consolidated, as the process must not be annoyed.
Aerobic digestion was evaluated as the largest energy consumer from a total semi-energy intensity sludge stabilization method of 0.52 kWh / m3 in small plants.
Aerobic stabilization was adopted in large plants while being usually adopted by normal wastewater treatment plants for its simplicity.
Energy requirements for post-processing may be adjacent to thermal technology that can be regarded as standardized baselines.
The minimum thermodynamic requirements for sludge drying can be calculated based on rational and latent heat necessary to evaporate water at about 0.63 kWh / kg.
In practice, such requirements (depending on the technology) range from 0.82 to 1.1 kWh per kg of evaporated water, and the total energy required for sludge drying averages about 2500 kWh / ton of dry sludge.
References
[1] Capodaglio, A. G., & Olsson, G. (2019). Energy issues in sustainable urban waste: Use, demand reduction and recovery in the urban water Sustainability, 12(1), 266.
[2] Capodaglio, A. G. (2021). Fit-for-purpose urban wastewater reuse: Analysis of issues and available technologies for sustainable multiple barrier approaches. Critical Reviews in Environmental Science and Technology, 51(15), 1619-1666.
[3] Wilberforce, T., Sayed, E. T., Abdelkareem, M. A., Elsaid, K., & Olabi, A. G. (2021). Value added products from wastewater using bioelectrochemical systems: Current trends and perspectives. Journal of Water Process Engineering, 39, 101737.
[4] Cecconet, D., Raček, J., Callegari, A., & Hlavinek, P. (2019). Energy recovery from wastewater: a study on heating and cooling of a multipurpose building with sewage-reclaimed heat energy. Sustainability, 12(1), 116.
[5] Hesham, A., Awad, Y., Jahin, H., El-Korashy, S., Maher, S., Kalil, H., & Khairy, G. (2021). Hydrochar for Industrial Wastewater Treatment: An Overview on its Advantages and Applications. J Environ Anal Toxicol, 11(3).
