Energy recovery from wastewater in Mexico: A systematic review

Abstract

The usage of fossil fuels to generate energy and the lack of wastewater treatment in Mexico are two issues that can be addressed at the same time while developing wastewater treatment technologies that incorporate energy recovery in their process train. We carried out a systematic review based on the PRISMA methodology to identify and review studies regarding energy recovery using wastewater as a substrate in Mexico. Peer-reviewed papers were identified through Scopus, Web of Knowledge, and Google Scholar, using a timeframe of 22 years that represented from 2000 to 2022. After applying the selection criteria, we identified 31 studies to be included in the final review, starting from 2007. The kind of energy product, type of technology used, substrate wastewater, amount of energy produced, and main parameters for the operation of the technology were extracted from the papers. The results show that methane is the most researched energy recovery product from wastewater, followed by hydrogen and electricity, and the technology used to archive it is an up-flow anaerobic sludge bed (UASB) reactor to produce methane and hydrogen. In addition, microbial fuel cells (MFCs) were preferred to produce electricity. According to our data, more energy per kgCOD removed could be obtained with methane-recovering technologies in the Mexican peer-reviewed studies compared with hydrogen recovery and electricity production.

1 Introduction

The environmental impacts of fossil fuels are particularly evident for developing countries (Solarin, 2020), especially those whose economies and energy production rely on them significantly. Mexico is very rich in petroleum, which affects how energy is produced. Fossil fuels represent around 85% of the energy generated to meet energetic demands (Masera and Sacramento, 2022). Mexican crude oil reserves are decreasing for a country that depends so heavily on fossil fuels (Martínez Hernández and Aguilar, 2021). Deforestation, landscape changes, hazardous atmospheric emissions, and effluents that pollute water and soil and destroy biodiversity are just a few of those already known environmental impacts of the mismanagement of the oil industry (Rico et al., 2007). The current governmental administration has not been clear about the future of renewable energies in the country, whereas the actions around the investment extraction and exploitation of oil and gas have been vigorous (Catalán, 2020). Since 2015, Mexico confirmed its willingness to meet the compromises of the 2030 Agenda from the United Nations (SRE, 2019). Thus, clean energy usage in Mexico should be encouraged to mitigate the greenhouse gas emissions used in energy production.

Water and energy demands in Mexico continue to grow despite the slowing of population growth. Nearly 50% of the wastewater produced in Mexico is discharged into the environment with no treatment whatsoever (CONAGUA, 2019); hence, there is a need to develop and install wastewater treatment technologies to achieve a cleaner future. In addition to having the capacity to clean wastewater from pollutants, wastewater treatment processes have the potential to be used to harvest energy and other value-added products, which bring with them environmental and economic advantages (Zarei, 2020). The potential of the country to recover energy from wastewater is astronomical and should be taken into account when installing new technology because it tackles one of the biggest general environmental problems in the country. For instance, several large-scale successful cases have been reported throughout Mexico generating renewable energy while treating wastewater (Ramírez-Higareda et al., 2019). Furthermore, Mexico is an upper-middle-class country (The World Bank, 2023), a member of the North American Free Trade Agreement (NAFTA), the Organization for Economic Cooperation and Development (OCDE), and the G20, where there is an opportunity to implement technologies to recover energy.

Using wastewater as an alternative energy source can mitigate certain environmental impacts and cover certain of the country’s energy demands, although it would represent a minimal part of them when compared to fossil fuel energy generation and other alternative energy sources currently operating. Furthermore, Mexico is a country full of resources that are suitable for the task of providing alternative energy sources like biomass, hydroelectric power, wave energy from the oceans, 11,122 km of shores (INEGI, 2020), and photovoltaic energy. In terms of solar irradiation, the country has an annual average of 5.3 kWh/m² per day (CONAGUA-JICA, 2012). Nonetheless, the development of these particular technologies also addresses, at the same time, in addition to the issue of energy production without the use of fossil fuels, the possibility of attacking the transcendental problem of discharging untreated wastewater into the environment.

The development of technology capable of recovering energy from wastewater is vital in a country like Mexico, which faces the sanitation and energy supply problems mentioned earlier. Thereby, to advance in the development of technology, there is a need to have a more general perspective of the state of the question regarding the relationship between wastewater treatment, energy recovery, and technology development.

This review focused on how much energy can be recovered from wastewater producing methane, hydrogen, and electricity, which are the most prominent methods used to recover energy from wastewater.

2 Methods

To achieve this study’s goals, the authors conducted a systematic review following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) (Page et al., 2021) guidelines. This methodology has been successfully right applied when dealing with subjects around and about wastewater and wastewater treatment (Lorick et al., 2020; Agyekum et al., 2022; Emenekwe et al., 2022; Muzioreva et al., 2022).

2.1 Eligibility criteria

For the studies to be included in this review they had to meet the eligibility and exclusion criteria considered in Table 1 and Table 2. We focused on studies that explore methods and technologies that recover energy from wastewater and wastewater byproducts in Mexico.

Only articles focused on energy recovery using wastewater were taken into account. Energy recovery using byproducts from wastewater treatment or co-digestion process were discarded. Co-digestion was not explored in this work in order to explore research specifically on wastewater treatment and energy generation with the aim of promoting technological development in this area in which there is a lack of work to be done. Only peer-reviewed scientific articles focused on wastewater and energy recovery were taken into consideration. Additionally, all grey literature, technical reports, conference papers, reviews, news, and thesis were rejected from this study. We took only into consideration the papers within the timespan of 2000–2022. This work included works written in English and Spanish, and studies in another language were rejected. Regarding document type, only peer-reviewed studies that assess research carried on in Mexico or with Mexican wastewater were included in this study. Publications regarding a bigger region, such as Latin America or Global South were excluded from this work. The term “wastewater” in this work includes industrial and domestic wastewater.

2.2 Data sources and search strategy

Two databases of journals and one search engine were used: Scopus, Web of Science, and Google Scholar, respectively. The latter was used to identify peer-reviewed studies in Spanish because the majority of the most important databases in Mexico and Latin America (e.g., Redalcyt and Dialnet) lack an advanced search component. The search of studies was defined by search strings shown in Table 3.

In the two databases of journals and one search engine, the search string was refined using only the “Article” document type. For Scopus and Web of Science, the timeframe was defined between 2000 and 2022, although there were any articles before 2000 using the search string in Scopus. Furthermore, “Mexico” was used to reduce the search result to a Country. All articles were in the final stage of publication. All data retrieved from the databases and the search engine was first exported to an excel document.

2.3 Study selection

The last date the database was scanned was November 15th of 2022. Figure 1 describes the selection process that was carried out for this work. A total of 313 articles were identified from Scopus, Web of Science, and Google Scholar to be screened and selected for inclusion.

FIGURE 1

Before the screening process, 61 works were removed: 29 representing duplicates and 32 representing other than peer-reviewed articles (thesis, news, congress articles). The title and abstract were read from the 313 articles to be screened and 281 works were excluded for not fulfilling the eligibility criteria. The remaining articles were assessed by full-text reading and 21 were excluded: 14 were out-of-scope articles, 6 were reviews, and 1 was not found. Additionally, 19 publications were included in bibliography searches. At the end of the selection process, just two articles in Spanish were included (Pérez-Grijalva et al., 2018) (Gómez-Paredes et al., 2020).

2.4 Data extraction and quality

The data of interest from the 31 studies selected for this review were extracted in a Microsoft Excel document to be further analyzed. Supplementary Table S1 exhibits the data collection. The latter included data from authors, publication year, magazine, keywords, name of the study, type of technology, type of energy recovery, type of wastewater, energy recovered, and specific data about the technology and specific case of the study carried around. If possible, missing parameters were calculated from data from the publication.

All strategies to gather data were discussed between the authors to ensure the validity of data and quality of itself. Furthermore, after the eligibility criteria and search strings were settled, the authors independently gathered the data and tougher resolved all the concerning disagreements that arouse. The authors had no competing interests.

3 Results

The gathered data from the 31 studies were used to assess the state of the question regarding the development of an energy recovery process using wastewater as a substrate. For every study, we added an index number to refer to it (N1 to N31). The oldest study assessed was from 2007, even though we limit the search to 2000. Table 4 summarizes the studies selected.

We realized a pattern of technologies being researched. There were three main categories in which the studies were divided based on the type of energy or energy product the researchers tried to develop: 1) methane (also known as biomethane); 2) hydrogen (also known as biohydrogen); and 3) electricity. It is worth remarking that study N24 was the only one that used bio-crude.

3.1 Publication trend

The results of our research of the 31 papers that we utilized in this review regarding the kind of product or direct energy recovered in those studies are presented in Figure 2. As the figure displays, there was a peak of research around the topic of this review in the year 2020, where biogas, including methane and hydrogen, was the most investigated subject. This (2020) was the year with the greatest number of studies produced. In the next 2 years (2021 and 2022, up to November), there was a decline of 50% in the first year and 90% in the second year. No studies were detected under our selection criteria from 2008–2011. The kind of energy recovery the most studied was electricity which ranged from 2012–2022, the year 2019 being the most active in the topic. Also, more than half of the studies were published in the past 4 years, since 2019.

FIGURE 2

Another thing to take into consideration is the COVID-19 pandemic: the worldwide non-COVID-19 research was reduced by 18% by 2021 (Raynaud et al., 2021). This could be due to editors dedicating less time to reviewing non-COVID-19 research or editors rejecting more papers than usual. Also, the impact of the restrictions during the pandemic needs to be taken into consideration. Most of the research obtained in this work was produced in closed environments such as laboratories.

The following are the publication trends concerning scientific journals and authors. The journals in which the selected papers were most published were Bioresource Technology, with three publications, and Revista Mexicana de Ingeniería Química (Mexican Journal of Chemical Engineering), with another three publications. They are followed by journals with two publications: Biomass and Bioenergy, International Journal of Hydrogen Energy, Water (Switzerland), and Water Science and Technology. The authors with more publications in the selected studies are Alzate-Gavira, Liliana, with seven publications, and Domínguez-Maldonado, Victor F., with four. Both of them are part of the Renewable Energy Unit of the Yucatán Center for Scientific Research A.C.

3.2 Technologies researched

Even though we identified four main energetic recovery products, the technology used to produce them and treat the wastewater at the same time was very diverse. They were 16 different types of technologies applied in the studies reviewed. Figure 3 exhibits the proportion of the applied technologies. Microbial fuel cells (MFCs) were the most researched technologies, appearing in 27% of the studies, closely followed by up-flow sludge blanket (UASB) reactors, being on 24% of the studies. Anaerobic packed bed reactors (APBRs) were the third most used technology in the reviewed studies and represented 9%. Photobioreactors (PBRs), constructed wetlands (CWs), anaerobic batch tests (ABTs), anaerobic batch test with granular activated carbon (ABTGAC), high-rate algal ponds (HRAPs), anaerobic stirred tank reactors (AnSTRs), and microbial electrolysis cells (MECs) each represented 4% of the type of technology used in our research. “Others” refer to technologies that showed only one time in our process. They were as follows: anaerobic sequencing batch reactor (AnSBR), Bioclave bioreactor, dark fermentation reactor, algae harvesting reactor, electrochemical cell, and septic tank.

FIGURE 3

3.2.1 Technology trends

There were 15 selected studies that were conducted between 2020 and November 2022. That means almost half of the selected studies were conducted in the recent 3 years. One of the evident trends is microbial electrolysis and fuel cells, which accounted for almost half of the technologies represented in the studies. The development of hybrid embedded technologies, such as constructed wetlands + MFC, dark fermentation microbial cells, and using an MFC to treat high organic strength industrial wastewater, represent highlights in the recent reseach (N12, N16, and N18). It is important to note that UASB reactors continue to be a trend. They represent almost a third of these 2020–2022 selected studies. Furthermore, the use of anaerobic technologies represents almost two-thirds of these studies. This agrees with the fact that for these recent studies, the use of high-strength industrial wastewater, those from common Mexican industries, such as maize processing (nejayote), chocolate processing, and tequila vinasses, is widely utilized as the raw material because of their massive organic loading.

3.3 Type of energy product

There were four principal types of energy products identified in the research papers we reviewed: methane, hydrogen, electricity, and bio-crude. From the 31 studies, 38 cases were extracted because almost half of the studies (N2, N5, N6, N10, N13, N14, N15, N18, N21, N23, N24, N26, N29, and N30) tested more than one technology or different variations of the same. By far, the most energy product produced through wastewater in our selected works was methane, with 17 cases. A total of 11 studies encompassed subjects of electricity recovery, eight were about hydrogen recovery, and one was about bio-crude. There were only five studies where different kinds of energy products were tested in unison: N5, both methane and hydrogen; N13, biohydrogen and electricity; N14, both methane and hydrogen; N15, biogas, methane, and hydrogen; and N16, in which biohydrogen and electricity were tested in the same reactor.

3.4 Type of wastewater used as a substrate

Three main types of wastewater were spotted in our review. As regarded in Figure 4, the great majority (17) of studies targeted industrial wastewater. The subcategories of industrial wastewater, based on the cases previously presented, are displayed in Supplementary Table S1: vinasses with seven cases: N3, N5, N8, N9, N14, N15, and N20; industrial reactor effluent (I.R. effluent) with seven cases: N5, N10, N14, two in N15, N26, and N29; maize processing with four cases: N10, N16, N28, and N29; chocolate processing with two cases: N1 and N11; pig farming with two cases: N4 and N12; a mix of industrial with one case: N17; and acid whey with one case: N26. We refer to “reactor effluent” as the cases in which wastewater exits a previous reactor in the treatment train used in the study. Eight studies fell into the category of domestic wastewater: N6, N7, N18, N21, N24, N25, and N30. The subcategories were two: domestic with seven cases: N7, two in N18, N21, N24, N25, and N30 and domestic reactor effluent (D.R. effluent) with five cases: two in N6, N21, N24, and N30. Finally, six studies were identified into the category of synthetic wastewater. The subcategories were synthetic with nine cases: two in N2, N13, N19, three in N23, N27, and N31 and synthetic reactor effluent with one: N13.

FIGURE 4

3.5 Energy recovery

To connect and compare energy and pollutant removal technologies, taking into account their diversity and different configuration, the authors decided to evaluate them separately by the type of energetic product they recovered in the subsequent sections. However, the use of a baseline measurement was necessary to connect the biological byproducts of wastewater treatment with the energetic products and their potential to generate energy. Therefore, kWh/kg CODremoved was used as a unit to measure the capacity of systems to recover energy at the same time as removing pollutants from water. Only 13 studies were included in this part of our study because the lack of data from the articles prevented our team from calculating this parameter. Figure 5 represents the calculated kWh/kgCODremoved in these 13 studies. The estimations that Suhartini et al. (2019) proposed to calculate the energy yields for methane were used, and the estimations from IDIALHY for hydrogen (Idealhy.eu, 2022) were used. Figure 5 represents the calculated energy recovered in the studies. Four studies used hydrogen as the source of energy, all shown in yellow: N2 (Alzate-Gaviria et al., 2007), N7 (Chacón-Carrera et al., 2019), N13 (Estrada-Arriaga et al., 2021), and N15 (García-Depraect et al., 2020b; México reafirma compromisos de la Agenda 2030, 2022), the values of which are below the average that was 2.48 kWh/kg COD_removed in this review. The best performance was attributed to N6 (Carrillo-Reyes et al., 2021), using an anaerobic stirred tank and effluent from a domestic reactor. According to our data, more energy per kgCOD removed could be obtained with methane-recovering technologies.

FIGURE 5

3.5.1 Methane production

Methane production was the most studied energy recovery process. Table 5 summarizes some of the technologies used in its production in addition to the type of wastewater and some important basic parameters. Because of the heterogeneity of the data and the lack of it in some cases, the results presented have their units as presented by the authors. The OLR varied from 0.99 to 29.2 kg COD/m³d and reached production rates of 0.437 m³ CH₄/kgVS for a batch test from a lactate fermentator (N8) or 0.316 L CH₄/g COD added for a UASB treating the effluent from a CSTR treating tequila vinasse (N14). The most frequently used technology to produce methane was UASB, with the studies from the section handling the subject. Most of the substrate to generate methane is highly strong wastewater coming from industry, except for one case (N6).

3.5.2 Hydrogen production

Hydrogen production, as represented in the studies reviewed, was carried out as a primary wastewater treatment process using high-strength industrial wastewater or synthetic wastewater resembling it. As presented in Table 6, the kind of technology used to recover hydrogen was wildly variable. Because of the heterogeneity of the data, or the lack of it in some cases, the results presented have their units as presented by the authors. OLR as high as 52.1 kg COD/m³ L and 107 gVS/kg d were presented in two works (N14 and N15) that both used a continuously stirred tank. It is worth mentioning that MFCs, combined with dark fermentation, and MECs have been present in hydrogen recovery from wastewater since 2014 in Mexico. As shown in the aforementioned case with methane, wastewater from industries with high organic loads is primarily used to produce hydrogen. Hydrogen production varies from 0.204 L H₂/L/d produced by a photobioreactor treating synthetic domestic water (N19) to 11.7 L H₂/L/d by a continuously stirred reactor treating effluents in a lactate fermentation tank (N15).

3.5.3 Electricity production

The studies within our review that focused on recovering electricity from wastewater mostly used microbial fuel cells in seven out of nine studies. As presented in Table 7, the most used ion separation membrane was Nafion 117 and, when compared with Ultrex CMI-7000 and Ultrex CMI-7001, produced a volumetric power density of 205.5 mW/m³, as presented in the N23 study. Double chamber types were the most numerous, followed by air cathodes, and the distance between electrodes was never more than 10.1 cm. The maximum voltage generated ranges from 83 to 820.35 mV. There was one hybrid pilot-scale study (N18) in which two vertically constructed wetlands and activated carbon sheets as anode and cathode were used. In addition to generating a power density of 6.4 and 9.7 mW/m² with two different species of plants (Canna hybrids and Z. aethiopica, respectively), it also had COD removal percentages of 98.9 and 98.1 in treating domestic wastewater.

3.6 Further research suggested in the studies

Most of the further research suggestions regarding energy recovery presented in the selected studies revolved around using new or modified variables over the same trials. In the case of methane production, Burboa and Alvarez suggested that future trials should include a large period in a continuous instead of a batch mode to “evaluate the if the capacity of carbon materials to act as electron conduits is maintained, because the adsorption of undesirable compounds may limit the mass transfer to and from the material” (Burboa-Charis and Alvarez, 2020). Moreover, Carrillo-Reyes et al. (2021) suggested that future trials in their thermophilic anaerobic digestion system must use protein removal from the microalga-bacteria aggregates biomass during anaerobic digestion with the intention to reduce the inhibition produced by free ammonia from the protein degradation. Furthermore, they suggest that future investigation should be based on a higher organic loading rate (OLR> 2 kgVS m⁻³ d⁻¹) because no inhibition by the accumulation of volatile fatty acids was observed in the system. For the case of hydrogen production, García-Depraect et al. (2020a) advised that further studies on lactate-type fermentation in acidogenesis must focus on both “the structural diversity and functionality of microbial communities involved in the different fermentation stages” to produce hydrogen and in the next step methane in a more efficient way. For the case of electricity production, Chacón-Carrera et al. (2019) suggested that future attempts to exploit bipolar membranes of microbial electrochemical systems must take notice of using, for instance, another type of cathode material with greater surface area (they used platinum electrode 1 cm²) using an acid buffer to benefit oxidation of acetate in the reactor and wastewater with higher COD levels and good conductivity

4 Discussion

One of the main findings in this review was that there are three principal ways in which Mexican research addresses the issues of both wastewater and energy recovery: methane, hydrogen, and bioelectricity. We found only one study that directly addressed bio-crude. This agrees with the study by Meneses-Jácome et al. (2016), who found that biohydrogen and MECs were the most researched in Mexico. Although before that year, less than a third of the studies gathered in this work had been published yet.

Possibly, Mexico has not bet on energy recovery wastewater treatment technologies because it has enormous potential through the alternative sources of energy mentioned before. Nevertheless, the fact that technology is being created in the country with the capacity to treat wastewater and at the same time recover energy from it is transcendental because it addresses two fundamental issues: the overabundant amount of untreated wastewater that is discharged into the environment and the consequences of the excessive use of oil to generate energy. In addition, research has shown successful exercises to power wastewater treatment plants using the energy recovered from wastewater (Gutierrez, 2018). These could encourage municipal water utilities to apply these technologies.

The decline of studies from last year (2021) in the publication of studies regarding the topic of our research could be caused by the number of studies focusing on co-digestion instead of using only wastewater as substrate. Co-digestion was the most discarded study in the screening process at the beginning of this work. Also, this could be attributed to the change of approach in the legislation and political action toward energy policy. As mentioned before, the Federal Administration has changed toward a heavily centralized fossil fuel energy production from which the CFE (Federal Electricity Commission) has greatly benefited. Also, it could be attributed to the COVID-19 worldwide pandemic, when non-COVID-19 research fell by around 18%.

One reason for this many studies using industrial wastewater could be that it represents the most organic concentrate wastewater, which can be used to produce more energy with less volume of the substrate. In the studies, domestic wastewater was used more commonly in studies where MECs and MFCs were researched. The kind of application was different from those producing methane or hydrogen. Energy recovery by producing electricity using MECs was specifically designed to be decentralized wastewater treatment plants, where the energy is produced during the wastewater treatment. It is worth noting that energy recovery in the form of electricity was depicted in most cases as a way to be neutral in terms of energy, not to be energy positive. Except for one study (N16), all of the studies producing electricity using MECs used domestic wastewater.

In Mexico, the research conceptually regarded as energy recovery from wastewater began in 2007, according to our gathered data. The first study identified as “energy recovery” from wastewater found in this work was a 2007 study (Alzate-Gaviria et al., 2007), and it addressed topics around hydrogen generation. Additionally, the first study identified in Mexico concerning MFC was published in 2014. Research on biogas production from wastewater has been carried out in Mexico since 1990 (Guyot et al., 1990). The turning point was identified when scientific research began to consider recovering energy as a concept, not only biogas production, as was the previously mentioned work by Alzate-Gaviria et al. (2007).

The microbial communities and the operational conditions are presented as research gaps most commonly. At each site where the same biological wastewater treatment and energy recovery technology is managed, the general conditions of the surrounding environment diverge from each other, which not only results in well-known changes in operational conditions but also results in a wide variety of local microorganisms and substrates. The composition and the relationship between its members is an issue that deserves further research: how the microbial community affects wastewater treatment and energy recovery technology performance? How the composition of the microbial community affects the technology yield? Which are the perfect operational conditions that allow for improvement in energy recovery yields for each heavily organic-loaded wastewater in the country?

As previously mentioned, recovering energy through methane generated from wastewater is a well-studied and implemented technology in Mexico (Ramírez-Higareda et al., 2019), whereby the majority of studies revolved around it. The fact that methane production technologies were mostly fed with wastewater coming from a high-pollutant industry is expected because it is known by the industry sector that it is not only a way to regenerate the quality of water but also to generate “free” renewable energy (Hamawand, 2015). Additionally, large-scale agribusinesses know that they can rely upon bio-methanation to recover a fair amount of energy through anaerobic wastewater treatment and co-digestion and initial construction investment. Juice producers, pig farms and slaughterhouses, chocolate processing plants, and tequila industries treat their wastewater with anaerobic processes to produce biogas. Also, Mexico already has a great human capital trained to operate anaerobic biogas-producing technologies.

5 Conclusion

Research on energy recovery technology using wastewater treatment processes in Mexico is still an ongoing field of research. As in all other research areas, the COVID-19 pandemic had a negative effect on the volume of research. Even so, the increase in research on this topic had a substantial growth since 2019, compared to all the years since 2007. The technologies’ operating conditions and the reactors’ microbial community composition are the most numerous gaps in Mexican research. Since anaerobic processes for wastewater treatment are the most used in Mexico for energy recovery in the industry, it is consistent that most of the articles included in this study revolved around it. We can also highlight the growing interest in the development of hybrid systems, using MFCs and MECs as their main treatment and energy recovery components.

Although Mexico has great potential for energy generation through renewable sources, leaving fossil fuels behind, energy recovery through wastewater treatment systems helps to mitigate in unison the major problem of untreated wastewater discharges to the environment and to leave aside fossil fuels with all the negative impacts they entail.

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