Towards energy self-sufficiency: biogas production in wastewater treatment plants

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ater is a vital resource in our society, not only for human consumption but for any kind of activity (food production, industrial processes…). During human activity, we contaminate water streams, so, before they can be returned to water bodies, such as lakes or rivers, they must be decontaminated. There is where wastewater treatment plants (WWTP) come into play. WWTP are the facilities where all the wastewater that we produce is treated to remove organic matter, solids and nutrients before it is returned to the environment [1]. During their operation , WWTP become important energy consumers. In the U.S., wastewater treatment accounts for 3 – 4% of national electrical demand, being the electric power consumption the highest operation cost in these plants (more than 30%) [2].

During their operation, WWTP generate a big volume of sewage sludge, around 250 grams of dry solids per m³ of treated wastewater. Sewage sludge is produced during the primary and secondary settling (Figure 1) and it is mainly composed of the organic matter and solids removed from wastewater, being usually rich in nutrients [3]. Furthermore, sewage sludge cannot be directly disposed, but it requires prior treatment and its management accounts for 30% of WWTP operating costs [2]. Therefore, it is crucial to apply strategies that enables a sustainable management of the sludge, allowing to get the most of it.

Anaerobic digestion is a widespread technology that allows generation of renewable energy in the form of biogas during sewage sludge treatment. During this process, microorganisms break down the organic matter present in the sludge into smaller molecules, producing biogas. Biogas is formed by a mixture of mainly methane and carbon dioxide and it can be used as a fuel for heat and electricity generation [3].

However, sewage sludge cannot be directly used from primary and secondary settling for anaerobic digestion. The generated sludge goes through a series of process in the WWTP (Figure 2) [3]. First, the sludge is sieved and thickened to reduce the content of water and hence the energy consumed during its digestion. The thickened sludge is then pumped into the digesters and continuously stirred in anaerobic conditions at mesophilic temperatures (35 – 42 °C) during a retention time of around 20 days. About a third part of the solid matter from the sludge is transformed into biogas during digestion, so the digested sludge becomes again very liquid and needs to be dewatered before its final disposal. Dewatering of the sludge is normally achieved by mechanical pressure or centrifugation, although sometimes it can be heat dried to remove even more water. The dewatered sludge can then be used for agriculture, disposed in a landfill or sent to an incineration plant [3]. Finally, as a result of the dewatering process, a liquid stream with a high concentration of contaminants is produced and recirculated to the entrance of the WWTP [4].

During the anaerobic digestion, biogas is produced by biological breakdown of the biomass. Typical composition of biogas from sewage sludge consists of 60 – 67% methane, 33 – 40% carbon dioxide and traces of other compounds such as hydrogen, nitrogen, siloxanes and hydrogen sulphide [5]. Due to its high methane content, this biogas is a source of energy that is commonly used in WWTP to cover some of their energy demand. Biogas can be used to produce heat directly in a boiler, just as we all do at home. However, the most common application for the biogas produced in wastewater treatment plants is its use in Combined Heat and Power (CHP) units (systems that produce both, electricity and heat, from a single fuel source) [6]. Combustion engines and micro turbines are the most widely used CHP technologies, although some alternatives such as fuel cells are attracting attention due to their higher electrical efficiency [7], [8]. The heat and electricity produced through these CHP units is used in the WWTP to reduce the energy demand. Heat autonomy is generally achieved with the biogas, and the electrical consumption can be reduced from 30 to 70%, depending on the WWTP size [3]. Furthermore, some technologies aim to go even further to maximize the biogas production by co-digestion of other organic wastes from the WWTP itself (such as the grease removed in the pre-treatment) [9] or from other sources (such as food wastes or municipal solid wastes) [10]. Lastly, the biogas produced can be upgraded (i.e. increasing its methane content) so it can be used as a vehicle fuel or even injected to the natural gas grid [2].

Summarising, water and energy are vital and inseparably connected resources. Wastewater treatment processes consume a large amount of energy, which is translated into environmental, social and economic impacts. Anaerobic digestion at WWTP is a technology that produce biogas, a green energy source that can help reducing the footprint of the water cycle and supposes a step-forward towards a more sustainable society.

References

[1]         J. Palatsi, F. Ripoll, A. Benzal, M. Pijuan, and M. S. Romero-güiza, “Enhancement of biological nutrient removal process with advanced process control tools in full-scale wastewater treatment plant,” Water Res., vol. 200, p. 117212, 2021, doi: 10.1016/j.watres.2021.117212.

[2]         Y. Shen, J. L. Linville, M. Urgun-Demirtas, M. M. Mintz, and S. W. Snyder, “An overview of biogas production and utilization at full-scale wastewater treatment plants (WWTPs) in the United States: Challenges and opportunities towards energy-neutral WWTPs,” Renew. Sustain. Energy Rev., vol. 50, pp. 346–362, 2015, doi: 10.1016/j.rser.2015.04.129.

[3]         N. Bachmann, J. la C. Jansen, D. Baxter, G. Bochmann, and N. Montpart, “Sustainable biogas production in municipal wastewater treatment plants,” IEA Bioenergy, 2015.

[4]         A. Thornton, P. Pearce, and S. A. Parsons, “Ammonium removal from digested sludge liquors using ion exchange,” Water Res., vol. 41, no. 2, pp. 433–439, 2007, doi: 10.1016/j.watres.2006.10.021.

[5]         A. Kiselev, E. Magaril, R. Magaril, D. Panepinto, M. Ravina, and M. C. Zanetti, “Towards Circular Economy: Evaluation of Sewage Sludge Biogas Solutions,” Resources, vol. 9, no. 91, pp. 1–19, 2019.

[6]         M. MosayebNezhad, A. S. Mehr, M. Gandiglio, A. Lanzini, and M. Santarelli, “Techno-economic assessment of biogas-fed CHP hybrid systems in a real wastewater treatment plant,” Appl. Therm. Eng., vol. 129, pp. 1263–1280, 2018, doi: 10.1016/j.applthermaleng.2017.10.115.

[7]         D. M. Riley, J. Tian, G. Güngör-Demirci, P. Phelan, J. Rene Villalobos, and R. J. Milcarek, “Techno-economic assessment of CHP systems in wastewater treatment plants,” Environ. – MDPI, vol. 7, no. 10, pp. 1–32, 2020, doi: 10.3390/environments7100074.

[8]         M. Gandiglio, F. De Sario, A. Lanzini, S. Bobba, M. Santarelli, and G. A. Blengini, “Life cycle assessment of a biogas-fed solid oxide fuel cell (SOFC) integrated in awastewater treatment plant,” Energies, vol. 12, no. 9, 2019, doi: 10.3390/en12091611.

[9]         M. S. Romero-Güiza, J. Palatsi, X. Tomas, P. Icaran, F. Rogalla, and V. M. Monsalvo, “Anaerobic co-digestion of alkaline pre-treated grease trap waste: Laboratory-scale research to full-scale implementation,” Process Saf. Environ. Prot., vol. 149, pp. 958–966, 2021, doi: 10.1016/j.psep.2021.03.043.

[10]      S. Vinardell, S. Astals, K. Koch, J. Mata-Alvarez, and J. Dosta, “Co-digestion of sewage sludge and food waste in a wastewater treatment plant based on mainstream anaerobic membrane bioreactor technology: A techno-economic evaluation,” Bioresour. Technol., vol. 330, no. January, p. 124978, 2021, doi: 10.1016/j.biortech.2021.124978.

Advanced oxidation processes for the removal of contaminants of…

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ater quality can be affected by contamination due to microorganisms, trace metals, toxic chemical compounds, changes in salinity, temperature or acidity. All this contamination is removed in water treatment plants, ensuring the required quality of the water for each purpose following the correspondent legislation. However, in recent years, there has been an increase in global concern about water pollution and especially about the rise of contaminants of emerging concern (CECs) [1]. Contaminants of emerging concern can be defined as organic micropollutants, natural or synthetic, that present potential risks to health and the environment and are not, or are just beginning to be, regulated by current legislation [2]. Within this group of contaminants, we can find everyday products such as pharmaceuticals and personal care products, surfactants, plasticizers or industrial additives. These pollutants can be found in urban water and effluents from wastewater treatment plants mainly originated in industries, hospitals, pharmaceutical factories and processes related to livestock and agriculture [3].

The main problem associated with emerging pollutants is that they are refractory compounds whose removal is not completely guaranteed with biological treatments in wastewater treatment plants. Once these pollutants and their metabolites reach aquatic ecosystems, they present toxic effects and bioaccumulation. Furthermore, the effect of some of these contaminants on human health is not fully understood, presenting a risk if they reach the human body [4]. As a consequence of their refractory behaviour, it is necessary to use alternative methods, capable of producing changes in their structure and degrade them, such as Advanced Oxidation Processes (AOPs) [1].

The concept of advanced oxidation processes was first defined by Glaze et al. in 1987 as processes involving the production and utilization of highly reactive transient species, especially the hydroxyl radical [6]. This species presents a great oxidizing power as a consequence of its high redox potential (2.80 V). The hydroxyl radicals generated by the process are responsible for oxidizing the contaminants of emerging concern present in the water by different pathways, in many cases achieving total mineralization (i.e., full degradation into carbon dioxide and water). There are many different advanced oxidation processes that can generate hydroxyl radicals by the action of light and/or chemical agents [7].

Recently, heterogeneous photocatalysis has become one of the most studied AOPs. This process is based on the use of a solid semiconductor that is activated by the action of light. When the catalyst is irradiated it absorbs light so electrons “jump” from the valence band to the conduction band, creating an electron-hole pair (e^–/h⁺). Of these two formed species, the hole can react with water, oxidising it to produce the desired hydroxyl radicals. One key factor of photocatalysis is the wavelength of the light used. The energy of the light has to be enough to overcome the energy bandgap of the photocatalyst. Although several semiconductors can be used as photocatalysts, titanium dioxide is the most widely employed [7]. However, with a bandgap of 3.2 eV, it requires ultraviolet light for its activation, which requires a lot of energy. Hence there is a lot of research working for being able to carry out photocatalysis with solar radiation, aiming for a more sustainable process [8].

AOPs are usually limited by the rate of radical production and not so much by the degradation reaction of contaminants. An important role in the rate is played by the presence of radical scavengers, mainly dissolved organic matter, that can have a much higher concentration than CECs, blocking their degradation. Therefore, this process must be placed at the end of the water treatment, when other contaminants that can inhibit the degradation have already been removed.

Summing up, CECs present a risk to the environment and human health so they are starting to be regulated and need to be removed in WWTPs. Advanced oxidation processes can be the solution to this problem, in which photocatalysis can play an important role, especially if it can be achieved using solar light. This process could be installed in the tertiary treatment of wastewater treatment to ensure the removal of these pollutants.

References

[1] M. Salimi et al., “Contaminants of emerging concern: a review of new approach in AOP technologies,” Environ. Monit. Assess., vol. 189, no. 8, 2017.
[2] T. Deblonde, C. Cossu-Leguille, and P. Hartemann, “Emerging pollutants in wastewater: A review of the literature,” Int. J. Hyg. Environ. Health, vol. 214, no. 6, pp. 442–448, 2011.
[3] A. Pal, K. Y. H. Gin, A. Y. C. Lin, and M. Reinhard, “Impacts of emerging organic contaminants on freshwater resources: Review of recent occurrences, sources, fate and effects,” Sci. Total Environ., vol. 408, no. 24, pp. 6062–6069, 2010.
[4] S. A. Fast, V. G. Gude, D. D. Truax, J. Martin, and B. S. Magbanua, “A Critical Evaluation of Advanced Oxidation Processes for Emerging Contaminants Removal,” Environ. Process., vol. 4, no. 1, pp. 283–302, 2017.
[5] https://www.slu.se/en/ew-news/2019/6/cec-policy-brief-english/
[6] Quiroz, M. A.; Bandala, E. R.; Martínez-Huitle, C.A. Advanced Oxidation Processes (AOPs) for removal of pesticides from aqueous media. In: Pesticides- Formulations, Effects, Fate. Editor: Margarita Stoytcheva. InTech, 2011.
[7] G. Divyapriya, I. M. Nambi, and J. Senthilnathan, “Nanocatalysts in fenton based advanced oxidation process for water and wastewater treatment,” J. Bionanoscience, vol. 10, no. 5, pp. 356–368, 2016.
[8] C. Sordo, R. Van Grieken, J. Marugán, and P. Fernández-Ibáñez, “Solar photocatalytic disinfection with immobilised TiO2 at pilot-plant scale,” Water Sci. Technol., vol. 61, no. 2, pp. 507–512, 2010.