|
|
| Fig. 1: Energy demand ranges (upper and lower limits) of some steps in wastewater treatment process. [1] (Image source: J. Smith) |
Wastewater treatment is critical to public health as humans require clean water for consumption as well as cleaning, sanitation, and food preparation. [1,2] Wastewater treatment involves collecting contaminated water, transporting it to a treatment plant, cleaning the wastewater, and then distributing treated water to residential and industrial sites of use. [1,2] The wastewater treatment process must remove a variety of contaminants, such as physical objects, chemicals like pharmaceuticals, and biological pathogens such as bacteria and viruses. [2]
Wastewater treatment is a complex process often described in three stages: primary, secondary, and tertiary treatment. [1,2] Primary treatment has a lower energy demand and involves processes like sedimentation, in which gravity is used to separate large objects from wastewater. [1,2] A larger amount of energy is consumed in secondary treatment, during which wastewater is aerated. [1,2] Aeration involves exposing the water to oxygen to break down dissolved organic matter. [2] Aeration also assists with flocculation, a process in which small particles come together to form larger ones that are easier to remove. [2] Further energy is consumed in tertiary treatment, which involves processes like nitrification. [1,2] Energy is also consumed in the process of treating sludge before it is released into the environment. [1,2]
The quantitative metric used to evaluate energy consumption by wastewater treatment depends on the desired analysis. For instance, energy consumption by water treatment is frequently reported in kWh consumed per m3 of water treated. [1] This can be a useful unit for assessing the energy demand of a country's wastewater treatment with respect to its total energy use. However, this metric does not account for differences in the contamination level of the water purified at a given treatment plant. [1] Furthermore, different wastewater treatment methods consume different amounts of energy, as do the different stages in the multi-step wastewater treatment process (Fig. 1). [1] Therefore, energy consumption by wastewater treatment plants depends on the processes they use. [1,2] For comparing energy efficiency across treatment plants, regions, or countries, a more useful metric might be the amount energy consumed per mass of contaminant removed. [1] Examples of this metric are kWh per kg of chemical oxygen demand, biological oxygen demand, or nitrogen removed. [1]
Let's examine energy demand using each metric.
To assess the overall contribution of wastewater treatment to a country's energy use, kWh/m3 is a reasonable metric. Secondary wastewater treatment, an energy intensive stage, consumes on average 0.2 kWh/m3 of energy in the US. [2] According to the WHO, each human needs approximately 0.0025 m3 of water per day. [2] In 2021, roughly estimating the US population at 330 million people, we have for total energy use for secondary wastewater treatment
| 0.0025 m3 person-1 d-1 × 365 d y-1 × 0.2 kWh m-3 × 330 × 106 people | = | 6.02 × 107 kWh y-1 |
In 2021, the US consumed 92.97 exajoules of energy. [3] Converting this to kWh
| 92.97 × 1018 J y-1 3.6 × 106 J kWh-1 |
= | 2.58 × 1013 kWh y-1 |
we find that the energy demand of secondary treatment for supplying sufficient water to the 2021 US population would comprise (excluding complex social, geographic, political, and other factors affecting water access and distribution)
| 6.02 × 107 kWh y-1 2.58 × 10 13 kWh y-1 |
= | 0.00023% |
of the country's energy consumption.
To compare the energy demands of wastewater treatment between countries, kWh/kg of chemical oxygen demand (COD) removed from the water is a more effective metric. [1] Spain consumes on average 0.97 kWh/kg of COD removed from water. [1] Italy consumes on average 0.85 kWh/kg of COD removed from water. [1] These numbers suggest that wastewater treatment is, on average, less energy intensive in Italy than in Spain.
In order to assess the energy demand of wastewater treatment and to compare energy efficiency between different purification sites, it is important to use an appropriate quantitative metric. [1]
© Jordyn Smith. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Longo et al., "Monitoring and Diagnosis of Energy Consumption in Wastewater Treatment Plants. A State of the Art and Proposals For Improvement," Appl. Energy 179, 1251 (2016).
[2] A. K. Plappally and J. H. Lienhard V, "Energy Requirements For Water Production, Treatment, Ed Use, Reclamation, and Disposal," Renew. Sustain. Energy Rev. 16, 4818 (2012).
[3] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.