Electro-Composting: An Emerging Technology

Abstract

This study focuses on electrical stimulation for composting. Using the PSALSAR method, a comprehensive systematic review analysis identified 22 relevant articles. The examined studies fall into four main systems: electric field-assisted aerobic composting (EAAC), electrolytic oxygen aerobic composting (EOAC), microbial fuel cells (MFCs), and thermoelectric generators (TEGs). Apart from the main systems highlighted above, bioelectrochemically assisted anaerobic composting (AnC_{BE, III}) is discussed as an underexplored system with the potential to improve the efficiency of anaerobic degradation. Each system is described in terms of key materials, composter design, operating conditions, temperature evolution, compost maturity, microbial community, and environmental outcomes. EAAC and EOAC systems accelerate organic matter decomposition by improving oxygen distribution and microbial activity, whereas MFC and TEG systems have dual functioning due to the energy generated alongside waste degradation. These innovative systems not only significantly improve composting efficiency by speeding up organic matter breakdown and increasing oxygen supply but also support sustainable waste management by reducing greenhouse gas emissions and generating bioelectricity or heat. Together, these systems overcome the drawbacks of conventional composting systems and promote future environmental sustainability solutions.

1. Introduction

The growing global issue of organic waste has intensified the need for sustainable and effective waste management strategies. Traditional aerobic composting, while commonly used, faces significant challenges, including long processing times, inefficient oxygen distribution, and environmental concerns. These issues have facilitated the development of electro-composting technologies [1]. Electro-composting refers to the application of electrical stimulation via direct voltage, electrolysis, or thermoelectricity to accelerate and enhance composting processes through microbial and chemical mechanisms [2]. Such advanced systems in the sustainable management of organic waste aim to enhance the efficiency of composting, and simultaneously, they address environmental concerns such as greenhouse gas (GHG) emissions and nutrient recycling [3].

Among the most researched methods, electric field-assisted aerobic composting (EAAC) is a promising electro-composting system through which a direct current is used to elevate compost temperatures, improve microbial activity, and reduce the length of the composting process [1,3,4,5,6]. Studies have demonstrated that EAAC systems can increase compost temperatures by 5–10 °C in comparison with conventional methods, thereby improving both oxygen utilisation and compost maturity [2]. Additionally, similar to EAAC, electrolytic oxygen aerobic composting (EOAC) employs electrolysis to produce oxygen in situ. EOAC has a higher electricity consumption than EAAC, but it stands out due to the provision of oxygen inside the compost pile. Therefore, it is a process that has been demonstrated to enhance the degradation of organic material by reducing the formation of anaerobic zones and maintaining aerobic conditions within the compost pile [7].

Unlike EAAC and EOAC, microbial fuel cells (MFCs) produce electricity instead of consuming it. MFCs have been known for a long time already, but their integration with composting remains poorly investigated. The combination with composting enables a dual-purpose technology, which generates bioelectricity and simultaneously decomposes waste [8]. While MFCs frequently serve as a bioelectricity generation system, much evidence indicates their important role in compost stabilisation and organic matter decomposition [8]. The system provides a novel approach to waste management and energy recovery, in which extracellular electron transfer aligns with key principles of electro-composting [9]. Recent research has applied electro-composting in a diverse set of configurations using substrates such as rice hulls, oil cake (from mustard plants), leaf mould, grass trimmings, and chicken faeces [10].

Similar to MFCs, the implementation of thermoelectric generators also allows for electricity production. This enables the utilisation of heat produced during the composting process [11].

This review systematically evaluates the most prominent electro-composting systems (EAAC, EOAC, MFCs, and TEGs) in terms of their environmental impact, operating efficiencies, and technological advancements. Apart from these technologies, there is a rare case known as “Bioelectrochemically assisted anaerobic composting (AnC_{BE, III})”, which remains relatively unexplored. Despite their lack of attention, the aforementioned systems offer innovative insights and potential advancements in the field of electro-composting, particularly in the optimisation of waste treatment and energy recovery processes. Traditional composting methods face challenges such as extended processing time, poor aeration, methane release, and limited energy recovery. Electro-composting technologies directly address these issues by accelerating microbial degradation, enabling in situ oxygenation, and even recovering usable electricity or biogas. The authors of this study provide a comprehensive evaluation of the role of electro-composting in the promotion of sustainable waste management methods by contrasting existing systems and highlighting future innovations.

2. Materials and Methods

The PSALSAR technique was employed in a systematic literature study by the authors of [12] to investigate the integration of electrical technology in composting. Initially, a protocol was developed to determine the research topic, objectives, and scope, focusing on the impact of electrical stimulation technologies on composting processes. Boolean operators were employed to identify the primary search terms, including “compost* AND electri*” or “compost* AND electro*”, which were then applied to the Web of Science database. A vast number of studies (7791 items in total) were identified in the search results; however, many articles were unrelated to the topic. Therefore, a method for conducting a more specific search was sought. To refine the dataset, the results shown in the Web of Science database were skim read to determine frequent keywords. Since it was not feasible to read 7791 articles, the authors extracted possible search terms from the first 250 articles of high relevance. This assisted in defining a core vocabulary relevant to electro-composting, and based on this, 17 refined search terms were created, which are shown in

Table 1. Number of articles for each search term before applying exclusion and inclusion criteria.

Search Terms	Result
compost* AND electrode	399
compost* AND “electric field”	48
compost* AND electrolyte	221
compost* AND “fuel cell”	104
compost* AND “electrical current”	16
compost* AND “microbial stimulation”	9
compost* AND electrostatic	111
compost* AND MFC	64
compost* AND bioelectrochemical	26
compost* AND “electr* bacteria”	7
compost* AND “electro catalyst”	3
compost* AND electrically evoked	7
compost* AND electrification	5
compost* AND electromagnetic	142
compost* AND “Electro* Biofilm”	4
compost* AND electro-based	2
compost* AND electrotechnology	1

. Following this, the Web of Science database search was repeated with the extracted search terms. The flow of article selection is illustrated in Figure 1, which follows the PSALSAR structure with explicit inclusion/exclusion criteria. A preliminary list was created by compiling important terms from different parts of these articles, including titles, abstracts, keywords, and main sections. A total of 17 particular search terms, such as compost* AND electrode, compost* AND “electric field”, compost* AND MFC, and compost* AND “thermoelectric composting”, were selected following a comprehensive cross-comparison and repetition removal process. The number of articles was reduced to 984 after applying these 17 keywords. The outcomes for each search term are represented in Table 1. For the selection of relevant articles, specific inclusion and exclusion criteria were applied. Articles published between 1977 and 2023 were included to capture the historical and contemporary advancements in the field of electro-composting. Studies that examined the application of electrical stimulation in the composting process were included. The articles were sorted to retain only “original research” types of articles, and non-English publications were excluded as well. The remaining 403 articles were screened in more detail, which involved two stages: an initial title and abstract screening that omitted 312 irrelevant articles and a comprehensive examination of the remaining articles. In this regard, an extensive analysis was conducted to classify all articles into studies on composting processes (91 articles) and articles that employed electrical technologies in combination with composting (22 articles). As electro-composting is an emerging field, it has been addressed by relatively few researchers using diverse terminologies and reactor setups, which creates a challenge in collecting a broad dataset. The particularly low number of EOAC-, TEG-, and AnC_{BE, III}-related studies indicates a research gap. The final 22 articles were systematically analysed based on the main parameters, including key materials, composter design, operating conditions, temperature evolution, compost maturity, microbial community, and environmental outcomes. Table 1 illustrates the number of articles obtained per search term before applying the criterion, while Figure 1 illustrates the screening process using a PSALSAR-related flowchart. Figure 2A,B,D illustrate publication trends, and Figure 2C illustrates the abundant keywords. Additionally, Figure 2C shows a word cloud that graphically depicts the key terms used in the 22 selected articles. The abstract and keywords of these articles served as input data for the WordClouds.com software (https://www.wordclouds.com, accessed on 31 March 2024). The most common terms were “electric composting” and “aerobic composting,” which appeared more frequently. The size of each term in the word cloud indicates the number of times it is used in the text being analysed.

3. Results and Discussion

In the reviewed articles, four main electro-composting systems, including electric field-assisted aerobic composting (EAAC), electrolytic oxygen aerobic composting (EOAC), microbial fuel cells (MFCs), and thermoelectric generators (TEGs), were covered in more than one article. The remaining system concerns a rare case, for which only one article was found. This includes bioelectrochemically assisted anaerobic composting (AnCBE, III). Seven critical parameters, including important materials for the systems setup, design of the composters, operating conditions, temperature evolution, compost maturity, microbial communities, and key features for each system, were compared.

3.1. Electric Field-Assisted Aerobic Composting (EAAC)

EAAC is particularly suitable for composting nutrient-rich waste such as poultry manure, as it accelerates microbial activity and reduces greenhouse gas emissions, making it ideal for semi-urban composting facilities [2,3]. The authors noted that several abbreviations have been used in the literature. This includes EAAC for electric field-assisted aerobic, EAC for electric field-assisted composting, and AEFAC for alternating electric field-assisted aerobic composting. To make it less confusing, the authors decided to exclusively use the term EAAC. EAAC includes systems that apply controlled electrical stimulation to improve microbial metabolism, organic matter degradation, and overall composting efficiency. Usually, an external electric field is applied to induce microbial electrochemical activity. Figure 3 illustrates the concept of electric field-assisted aerobic composting, which incorporates electric fields to enhance composting efficiency and increase microbial activity. EAAC is a new technique that shows promise for boosting compost quality, decreasing emissions of greenhouse gases, and making composting more economically viable.

3.1.1. Materials for the EAAC Setup

Chicken manure and rice hulls were important in the studies because of their widespread availability and their specific contribution to the composting process, as chicken manure provides a high nitrogen content and rice hulls are rich in carbon. To maintain 65–70% moisture, chicken manure and rice hulls were mixed with dewatered sewage sludge and mature compost in a ratio of 5:2:2:1 (w/w) [2]. Additionally, the authors of [1,4] used sawdust together with poultry manure in a 3.5:1 ratio in a 50 L reactor. With this ratio, they achieved a C/N ratio of 20, which is similar to research in [2], and a moisture content of 65%. Another substrate that has been tested in EAAC is food waste. The authors of [13] used food waste mixed with chicken manure, mature compost, and rice husks in a 6:2:1.5:1 (v/v) ratio, resulting in a 60% moisture content. A study [14] also used food waste, which they mixed with rice straw and shredded to between 0.5 and 1 cm in a reactor using 20 L foam boxes. Altogether, the works mentioned above demonstrate the suitability of EAAC for a wide range of substrates. Apart from these conventional substrates, the findings from [15] showed the possibility of further improving EAAC by using supplements. In the study, Fu et al. [15] used biochar with chicken manure at a ratio of 9:1 (w/w). Moreover, the study in [16] utilised chicken and rice husk at a ratio of 7:2 (w/w) for electric field-assisted composting and conventional composting, and another reactor used chicken and rice husk, including biochar mixed at a ratio of 7:2:0.8 (w/w) for biochar-added electric field-assisted aerobic composting. The biochar was prepared from bamboo at 600 °C, crushed, and sieved to 0.5–2 cm.

3.1.2. Design of EAAC Composters

Although they vary in shape, size, and material, the design of electric field-assisted aerobic composting (EAAC) systems is similar in most cases. Most authors used a well-insulated reactor containing an anode and cathode for electrical stimulation. The research in [3] used 200 L cylindrical casks with a DC voltage for electric field-assisted aerobic composting (EAAC), including a stainless-steel sheet (150 cm × 50 cm × 0.3 cm) as the positive electrode and graphite (diameter 10 cm, height 60 cm) as the negative electrode. A similar study [2] used 4.5 cm thick foam rectangular reactors with stainless-steel plate electrodes. The authors of [1] used a 50 L bench-scale reactor with graphite as electrodes. The study in [13] used 150 L cube reactors. A power source is required to actively stimulate the underlying microbiome. For this, a conventional potentiostat can be used, as indicated by the authors of [5]. To allow for more diverse configurations, it is also possible to apply a multi-potentiostat, as reported by the authors of [16]. Although most studies have rather similar approaches, there are some cases where EAAC is improved due to the application of biochar. For example, the authors of [15] applied direct current. However, in contrast to this, the authors of [2] have also shown the application of alternating current. Apart from reactor size, shape, and input materials, systems deviate with respect to the electrode material and the voltage applied. In this regard, the study in [14] can be highlighted, which used 2 V DC EAAC reactors with carbon felt and Fe plates as electrodes. In another example, the authors of [13] used array electrodes for Pin-EAC and Flat-EAC configurations, where Pin-EAC referred to the reactors with array electrodes and Flat-EAC referred to the reactors with flat electrodes. Comparing these two types of electrodes, Mi et al. [13] found that using array electrodes greatly raised the composting temperature and enhanced the germination index, resulting in a more successful and faster composting process. With respect to the voltage, there are some variations. Some systems work with low values such as 1 V [3] or 2 V [1,5,14]. Other systems applied higher values of up to 5 V [2,3,7] or even 10 V, as reported in [13].

3.1.3. Operating Conditions During EAAC

Studies on optimising composting systems suggest several approaches to increase microbial activity and organic matter breakdown. The work in [3] obtained samples from three random places at a 20 cm depth to test the physicochemical parameters, including moisture content and volatile solids, every five days. They experimented with multiple DC voltages (1–5 V) while the maximum temperature was almost the same between 2 V and 5 V, and ultimately selected 2 V; moreover, the study in [3] compared non-aerated composting with alternating aeration. A small pump was employed to deliver oxygen. Days 1–13 employed an alternate aeration technique, with the air turned on and off intermittently every 24 h at a flow rate of 1.5 L/min. During days 14–30, there was no active aeration. On days 10 and 20, the substrates in each reactor were emptied, mixed uniformly, and promptly returned to the reactors. The authors of [15] mixed biochar with chicken manure and rice hulls to maintain high moisture levels. The authors of [1] tested 2 V and 5 V DC voltages, continuous aeration at 0.2 L/min/kg dry weight, and periodic mixing and sampling for 42 days. The authors of [13] suggested online temperature sensors to better keep track of reactor performance. An interesting approach is presented by the authors of [5]. To improve the operating conditions, they evaluated different electrolytes. At a voltage of 2 V, they mixed each of the electrolytes FeCl3 (5% w/w), KCl (6.9% w/w), and NaHCO₃ (0.8% w/w) with the composting substrate to mitigate greenhouse gas emissions. As a result, they found that FeCl3 was successful in decreasing emissions of ammonia; however, it increased the nitrous oxide emissions, illustrating the importance of carefully considering the trade-offs when choosing electrolytes for composting.

3.1.4. Temperature Evolution During EAAC

Several studies have found an impact of electric fields on temperature. The alternating electric field-assisted aerobic composting system exhibited more uniform water distribution, resulting in higher initial temperatures (90.85 ± 0.76 °C by day 3) compared with conventional aerobic composting (73.95 ± 1.15 °C by day 4) and sustained temperatures over 80 °C for four days [7]. The authors of [15] compared EAAC without biochar as well as biochar without electric stimulation. The combination of electric fields and biochar raised the temperature to 71.3°C on day 4, while the controls remained at 68.2 °C (EAAC) and 59.8 °C (biochar). The combination of EAAC and biochar maintained temperatures above 50 °C for 16 days. This enabled temperatures of up to 77.5 °C. Another study that also applied biochar in EAAC achieved 72.6 °C, and it remained above 55 °C for more than 7 days, which confirms the positive impact of biochar on EAAC [16]. Another study [7] has already been applied at full scale. The resulting high-temperature composting system reached 91 °C and remained above 80 °C for 3 to 5 days [7]. Although there are systems that reach high temperatures, there are counterexamples. Low temperatures were observed in the studies [3,17]. In the study in [3], an EAAC system reached just 65 °C, and it stayed above 55 °C for 15 days. However, this was again better than the control. The control showed its temperature peak at 58 °C and remained above 55 °C for just 10 days [3]. In the case of [17], EAAC piles reached 67.8 °C for 10 days, while the control piles just reached 58 °C for 5 days [17]. In the study in [6], the EAAC system temperature rose to 68.5 °C, while the conventional aerobic composting control peaked at a temperature of 63.1 °C, which was probably due to better organic carbon degradation in the EAAC system [6].

3.1.5. Compost Maturity in EAAC

Compost maturity is an important metric in all composting-related investigations. Compared with conventional composting, EAACs with alternating electric fields increase humic acid (HA) and fulvic acid (FA) molecules by 429.58 ± 10.95%. These are heterogeneous molecules with a high polymer composition that serve an important role in improving nutrient accumulation and humification in composting. The EAACs have 65% higher electron-accepting and electron-donating capabilities, which speed up lignin oxidation and increase quinones/aromatic compounds. In the same approach, compost maturity was reached in 20 days, with 28% higher humification index (HI) and 38% higher germination index (GI) contents [7]. In another study [3], EAAC, together with biochar, sped up compost maturation by 33%, producing more HA- and FA-like compounds and stabilising dissolved organic matter (DOM) faster than the respective control [3]. The authors of [13] used a so-called Pin-EAAC, which refers to reactors with array electrodes. They compared the pin electrodes with flat electrodes and found a 13% higher GI for pin electrodes than for flat electrodes. On the other hand, reactors with flat electrodes showed increased humification and a 33% quicker maturity period [13]. Due to the implementation of EAAC systems, advanced composting technologies can help to manage waste more sustainably by increasing humification, microbial activity, and composting time.

3.1.6. Microbial Communities in EAAC

Electric fields improve microbial activity by stimulating the growth of electroactive bacteria such as Geobacter and Shewanella by 3.4 times compared to the conventional aerobic composting. These organisms facilitate direct electron transfer to electrodes, which accelerates organic matter oxidation and compost maturation [3]. Researchers indicate that bacterial communities in EAAC are rich in Bacillus, Navibacillus, Ureibacillus, and Thermobifida. Ureibacillus and Navibacillus bacteria were the most prevalent in EAACs with alternating electrical fields (52.36% on day 3 and 46.54% on day 41) compared with conventional aerobic composting (2.37% and 12%). High temperatures supported thermophilic bacteria such as Pseudomonas, Ardenticatena, Firmicutes, and Bacillus [7]. In the study in [3], denitrifying bacteria rose from 3.8% to 8.1% in the control system, but in EAAC, they decreased to 2.6%. EAAC did not include the main denitrifying microorganisms identified in conventional composting, such as Nitrospira and Denitrobacter. Bacillus and Alcaligenes, genera that both contain electroactive species, were observed as the primary denitrifiers in the respective EAAC approach. Another study [17] described that the abundance of Pseudomonas and Bacillus increased to 2.66% and 15.6%, respectively, in EAAC compared with 1.88% and 4.36% in conventional aerobic composting, while Acinetobacter’s abundance gradually decreased after 18 days in both systems.

3.1.7. Major Impacts of EAAC

Hyperthermophilic composting (HTC) and EAAC experiments improve efficiency, microbial activity, and environmental impact. The authors of [7] state that compared with conventional HTC, EAAC saves money by ageing compost at 80 °C using thermophilic microbes, as the high temperature speeds up the composting process, shortens the maturation period, and increases microbial activity, which causes faster decomposition and cheaper overall expenses. The authors of [3] discovered that employing EAAC at 2 V direct-current voltage decreased maturation time by 33%, greenhouse gas emissions by 70%, and oxygen utilisation by 30 ± 9% and boosted electroactive bacteria abundance by 3.4 times. The authors of [15] found that biochar increases electrical conductivity, lowers methane and nitrous oxide emissions, and accelerates compost maturation by 25%. Moreover, another study reported that EAAC promotes moisture migration and microbial activity. It leads to faster compost maturation, particularly in the cathodic zone [2]. EAAC inhibits nitrification, increases N₂O-consuming genes, reduces heavy metal bioavailability by 83.7%, and promotes metal-resistant bacteria [17]. Acidic electrolytes reduce ammonia (NH₃) emissions by 72.1% while increasing N₂O emissions due to altered nitrifier activity [5]. Modern composting processes increase efficiency, microbial activity, and environmental impact; however, emission management requires careful consideration.

3.1.8. Comparative Evaluation of EAAC

The authors of [1] reported a reduction of up to 75.5% in N₂O emissions using bioelectrochemical nitrification inhibition. Compared with MFC-based composting, EAAC does not generate recoverable energy but provides better control over compost temperature and aerobic stability. While thermophilic conditions are often highlighted for EAAC, this effect has not been highlighted in the articles found on EOAC and, therefore, remains to be investigated.

3.2. Electrolytic Oxygen Aerobic Composting (EOAC)

EOAC systems mitigate oxygen deficiency in dense compost piles by generating oxygen via electrolysis, making them particularly useful in systems treating compacted or low-aeration substrates [18]. The electrolytic oxygen aerobic composting system shown in Figure 4 utilises electrolysis to generate oxygen on site, ensuring optimal aerobic conditions for composting. The authors of [18,19] conducted analyses of EOAC, which they compared with conventional composting. They advocate for the use of electrolytic oxygen to enhance the quality and efficacy of composting.

3.2.1. Materials for the Setup of EOAC

The study in [19] used plastic bucket reactors for EOAC. Electrolysis was performed with direct current. The authors of [18] employed a raw material ratio of 6:3:1 for chicken manure, mature compost, and rice husk. Electrodes made of a rectangular steel plate and a graphite rod were connected to apply a voltage. Electrodes were positioned at the bottom and the top of the EOAC reactor. The study in [18] focused on oxygen distribution and microbial activity at different depths, whereas the study in [19] focused on the chemical composition and transformation of DOM.

3.2.2. Design of EOAC Composters

Both of the studies mentioned above utilised composters with direct electric fields and electrodes to efficiently disperse oxygen. The authors of [18] used cylindrical plastic bucket reactors (50 cm in diameter, 80 cm tall) with cotton insulation to improve thermal insulation and maintain stable composting temperatures. Similarly, the authors of [19] employed rectangular reactors of 50 cm in length, 40 cm in width, and 90 cm in height. Three strategically placed sampling ports at 15 cm, 40 cm, and 65 cm from the bottom enabled accurate vertical profiling of composting material in the reactors [19]. This design helped in understanding the composting process at various depths and allowed for more accurate monitoring of temperature and other parameters. Although the design is similar to an EAAC, it is a different process. An EAAC aims at an electrical stimulation of microbes. In contrast, an EOAC aims at improved oxygenation. However, both concepts overlap in their functionality, as some of the EAAC systems use voltages that are set high enough to cause electrolysis and thereby oxygen formation. Therefore, the authors of this review suggest that a clearer distinction between EAAC and EOAC should be established in future research.

3.2.3. Operating Conditions During EOAC

The authors of [18] applied a voltage of 10 V (DC) to the EOAC reactor. In contrast, the control reactors received adequate aeration but remained without electrical stimulation. Amongst others, the study discovered that oxygen dispersion influences microbial activity and compost maturity at varying depths. The research in [19] collected compost solid samples from different fractions of the reactor at various depths at various time points for a total duration of 30 days. They investigated chemical processes and transformations and discovered molecular changes such as microbial activity, chemical transformations, oxygen dispersion, and moisture migration during EOAC, indicating that the bottom layer experienced a higher abundance of common thermophilic bacteria (such as Cerasibacillus, Lactobacillus, and Pseudogracilibacillus), which could encourage compost maturation. This comparison demonstrates the huge impact of operational settings on chemical process parameters and the underlying biology.

3.2.4. Temperature Evolution During EOAC

The authors of [18] found that the composting process was divided into two phases: (1) temperature-raising during the first 6 days; (2) thermophilic phase/humification for the following 24 days. They assessed molecular alterations in dry organic matter (DOM). They found that low O/C (oxygen-to-carbon) and high H/C (hydrogen-to-carbon) compounds were preferentially decomposed during EOAC. The decomposition in EOAC was higher than in conventional composting controls. During the EOAC process, temperatures rapidly increased to the thermophilic phase (>50 °C) at all three locations measured. The bottom temperature (84.1 ± 1.3 °C) was significantly higher than the middle and top [19].

3.2.5. Compost Maturity in EOAC

According to [18], higher maturity is linked to effective oxygen distribution and microbial activity, particularly near the pile’s bottom. The authors of [19] found that EOAC increases the breakdown of complex organic molecules and the creation of humus. EOAC promotes compost maturity by physically oxygenating the compost more evenly through the pile of compost and chemical reactions.

3.2.6. Microbial Community of EOAC

The study in [18] found that due to the positioning of the electrodes, thermophilic bacteria (Cerasibacillus, Lactobacillus, and Pseudogracilibacillus) were more prevalent at the bottom of the compost. However, this was likely affected by better insulation in that area. A second study was published on EOAC, which investigated how the microbial community interacts with DOM, and it was discovered that various microbial processes produce chemical changes [19]. These changes include converting dissolved organic matter into humic substances, decomposition of organic matter, and generating metabolites, including alcohols, gases, and acidic substances.

3.2.7. Major Impacts of EOAC

The authors of [18,19] agree that EOAC composts are faster, that they use higher amounts of oxygen, and that they achieve higher compost quality than conventional composting. According to the first article [18], optimising the oxygen distribution can improve compost maturity and composting efficiency. The second article [19] demonstrated how EOAC alters the chemical structure of DOM due to the decomposition of complex organic compounds, such as lignin and cellulose, into simpler molecules, such as sugars and organic acids. It is vital to highlight that, even when reduced, lignin improved the compost quality.

3.2.8. Comparative Evaluation of EOAC

EOAC provides direct in situ oxygen via water electrolysis, eliminating mechanical aeration. The authors of [18] observed improved microbial diversity, reduced anaerobic zones, and accelerated formation of humic substances in EOAC systems. Temperature profiles reached up to 84.1 °C at the bottom of the compost bed, surpassing traditional methods. Despite its simplicity, EOAC is under-represented in the literature and lacks optimisation for energy input and electrode placement. Unlike MFCs, TEGs, and AnC_BE, EOAC is not designed for energy recovery but rather for compost enhancement. To reach the necessary voltage for H₂O electrolysis, a comparatively high energy input is required. The resulting oxygenation might result in an improved performance compared with EAAC. However, a direct comparison has not yet been found. EOAC has been described as ideal for compact or urban composting setups where passive aeration is ineffective.

3.3. Microbial Fuel Cells (MFCs)

MFCs address the dual challenges of waste treatment and energy recovery in low-resource settings, particularly for remote or decentralised waste streams such as livestock manure [20]. Integrating microbial fuel cells (MFCs) with composting is unique as they simultaneously enhance substrate degradation while generating electricity. In contrast to conventional composting systems, which entirely rely on microbial respiration for organic degradation, MFCs use microbial electrochemical interactions to accelerate degradation [21,22]. This interaction takes place via electroactive bacteria such as Geobacter and Shewanella, which can transfer electrons from organic substrates to electrodes, thereby accelerating the biodegradation rate and generating electrical energy [8]. Furthermore, MFC-integrated composting systems can improve waste stabilisation by controlling moisture content, maximising C/N ratios, and maintaining aerobic and anaerobic balance [20]. This section reviews studies on MFC efficiency and the importance of optimising environmental factors and microbial interactions. These findings suggest that MFCs can alleviate environmental concerns, but it has also been highlighted that they improve sustainability and efficiency. Figure 5 schematically demonstrates the dual functionality of the microbial fuel cell system, which not only treats organic waste but also produces bioelectricity.

3.3.1. Materials for the Setup of Compost MFCs

Among the different setups published, there are similarities but also differences with respect to material and system characteristics. The authors of [8] used an acrylic chamber and carbon felt as electrodes. Similarly, the authors of [23] used carbon cloth as electrodes; however, unlike the study in [8], a 10 wt% platinum-coated carbon black was utilised as a micro-film electrode. The authors of [23] also reported on the implementation of a proton exchange membrane (PEM) directly in the soil as part of a microbial fuel cell setup. The work in [8] pretreated their electrode as well but with a different purpose. They soaked the carbon felt in 10% hydrogen peroxide for 3 h to stimulate microbial adherence. The authors of [21] utilised cow manure together with 37% of miscellaneous organic matter. Air-dried, pulverised, and blended manure was employed. Eight titanium electrodes with 189 cm² and a 1571 cm² main electrode were employed in the composting tub with a volume of 3.6 m³. The compost was prepared from cow excreta, which was mixed with a microbial additive containing Jiangella spp., Actinomadura spp., Glycomyces spp., Thermobifida fusca, Bacillus spp., Planifilum spp., and Mechercharimyces spp. in a ratio of 10:1 [22]. In a quite extraordinary MFC approach, researchers aimed to create a sustainable sanitation solution that could treat human waste and generate electricity [20]. They constructed the MFC by using locally available concrete blocks. This approach not only addressed sanitation challenges but also contributed to waste management and energy generation in the community. As electrodes, they used graphite granules, stressing their sustainability and cost-efficiency. The studies in [8,23] concentrated on electrochemical efficiency using advanced materials, whereas the authors of [20] investigated practical and scalable solutions for resource-constrained places.

3.3.2. Design of MFC Composters

Several designs were used to integrate MFCs with composting. The researcher in [23] utilised cylindrical single-chamber MFCs with 50 mL anode chambers and aluminium mesh as current collectors. They used hot-pressed membrane electrode assemblies at 120–140 °C and 6.89 MPa pressure to achieve good electrode contact with the proton exchange membrane (PEM) in a compact, efficient design. The electrode membrane assemblies were designed to provide proper contact between the electrodes and the proton exchange membrane (PEM), which is critical for optimal electron transfer and overall efficiency of the microbial fuel cell. According to the authors, such a configuration improves the MFC’s ability to produce electricity by facilitating the flow of protons and electrons throughout the system. To obtain further insight into the impact of MFCs on chemical parameters, the authors of [8] used solid MFC setups for C/N ratio and moisture content measurements. SMFCs generate electricity using solid organic waste as a substrate, such as agricultural waste, animal manure, and urban waste, instead of wastewater. They applied reactors with different sizes, which included smaller chambers with 200 cm³ volume and 18 cm² of electrode surface, as well as larger chambers with 2000 cm³ of volume and 65 cm² of electrode surface. They were especially interested in analysing pH. They discovered that the ideal pH range of solid MFCs was between 6 and 8, which greatly enhanced the generation of electricity. In the case of the latrine MFC by the authors of [20], composting and urine treatment were combined with an electrical treatment using anoxic anodes and cathodes. Additionally, they included aerobic nitrification chambers. Therefore, the system contained both aerobic and anaerobic chambers. The MFC latrine contained an anode in the anaerobic chamber, which oxidised organic matter and generated energy. The cathode was put in an aerobic chamber, where nitrification occurred, converting ammonium to nitrate for further processing. To create compost, solid human faeces were processed separately in the composting toilet and aerobically decomposed by thermophilic bacteria. Both liquid and solid waste were effectively treated by this integrated system, which also produced electricity and compost. The electrodes were composed of graphite granules. In their setup, the authors of [20] composted their solid waste under aerobic conditions, while the liquid waste was treated in the MFC. This integrated technology uses oxygen-exposed composting to treat liquid waste while also producing electricity. In the case of [21], which was already introduced further above, the authors constructed a polymethyl methacrylate single-chamber air cathode with a diameter of 120 mm and a height of 120 mm. A carbon mesh anode with a surface of 113 cm² was coated with glass fibres to prevent short circuits. A larger carbon mesh disk with a Pt catalyst cathode-generated power was used. The system was aerated from the top, and it maintained anaerobic conditions at the bottom.

3.3.3. Operating Conditions in Compost MFCs

MFCs in electro-composting work under various conditions to enhance microbial activity and electricity generation. The authors of [23] boosted microbial activity in vermicompost soil by moistening it with sugarcane bagasse and by adding leaves. After a day of acclimatisation, multimeters were used to measure the resulting voltages. Resistors were used to calculate the resulting power and to create polarisation curves. The anode chamber received a composting chamber effluent, which included anode-respiring bacteria that degraded organic matter and transferred electrons. The nitrification chamber transformed ammonium from urine into nitrate, which the cathode reduced to nitrogen gas [20]. The research in [21] investigated moisture, phosphate buffer solution, catalysts, and electrode areas. The researchers tested different PBS concentrations (50, 100, and 200 mM), adjusted the moisture content to 60%, 70%, and 80%, compared runs with and without a catalyst, and used 28 cm² and 113 cm² electrode surfaces. They found that the proper moisture, PBS, and catalyst concentration had a significant impact on MFC power output as well as on waste degradation, and the best performance was attained with a moisture content of over 80%, a concentration of 100 mM PBS, and the addition of 0.1 mg/Pt cm² catalyst.

3.3.4. Temperature Evolution in Compost MFCs

In their study, the authors of [20] implemented their system in a tropical climate, which sustained high temperatures of around 32 °C. The warm temperature improved microbial activity but also resulted in performance variations due to variations in the external variables, including variations in ambient temperature, levels of humidity, and the frequency of latrines utilised by the community. The compost temperature reached 60 °C after 13 days. Following this, the temperature dropped to 40 °C over the next 12 days. This experiment yielded a steady 0.5 V DC voltage. The changes in electrical current and compost temperature indicated that electron generation and electrical current were related to bacterial activity [22]. In the work of [8], they hot-pressed membrane electrode assemblies at 120 °C to 140 °C to ensure adhesion and component functionality. The authors performed the manufacturing with a slow cool-down phase. This cooling procedure guaranteed that the electrodes remained in good contact with the proton exchange membrane, which was critical for efficient electron transfer and overall performance of the MFC. After cooling, the assemblies were integrated into the MFC setup. These studies show that temperature control, whether in controlled laboratory settings, natural climates, or high-temperature fabrication processes, has the potential to improve the performance of electro-composting.

3.3.5. Maturity in Compost MFCs

Maturation experiments in compost MFCs were used to demonstrate how maturation processes and conditions influence the overall system performance. The studies in [8] found that solid microbial fuel cells performed best with the following conditions: a C/N ratio of 31.4:1; 60% of moisture content with a power density of 4.6 mW/m². They further described that excessive and low C/N ratios or moisture levels reduced microbial activity, stressing the importance of a well-balanced substrate. Wang et al. (2014) [21] discovered that MFCs operate more effectively when the carbon-to-nitrogen (C/N) ratio is lowered. Among multiple variables, the presence of a catalyst in the cathode showed the biggest effect on the breakdown of organic matter. This catalyst contributes to MFC efficiency by speeding up electron transport. Other parameters, such as moisture content, phosphate buffer solution concentration, and electrode size, had a lower impact. The authors of [20] discovered that the MFC latrine’s performance increased after a year, with power increasing from 0.18 μW to 6.75 μW and resistance lowering to 0.5 kΩ, indicating biofilm formation and microbial community. The findings from the authors of [22] found that mature compost had a constant temperature, pH of 8.0, 14.7% water content, 35.7% carbon, 1.9% nitrogen, and a C/N ratio of 19.1. Finally, the study in [23] used vermicompost soil matured over one month and enhanced the soil with nutrients and microbial flora in the anode chamber to boost the MFC performance, which provided a mature microbial community and high organic content.

3.3.6. Microbial Community of Compost MFCs

A study in [23] investigated a vermicompost MFC and highlighted the E. coli strain CCFM8333, the Bacillus cereus strain BUU2, and the Pseudomonas monteilii strain CIP104883. According to [23], these strains enhanced the MFC’s performance by oxidising organic molecules and transferring electrons. Unfortunately, the other studies on MFCs did not dwell on taxonomy. The authors of [8] discovered that optimal composting increased the C/N ratio and moisture of their solid microbial fuel cells’ microbial activity, but unfortunately, they did not describe related taxa. The investigations by the authors of [20] discovered that their MFC latrine harboured anaerobic bacteria in the anode chamber, which degraded organic matter, while nitrifying bacteria in the nitrification chamber transformed ammonium to nitrate. Denitrification at the cathode converted nitrate to nitrogen gas. The authors of [22] described that the anode received its electrons due to bacterial degradation processes, where changes in voltage, electrical current, and compost temperature were related to bacterial activity. The work of [21] investigated the anode microbial community using DGGE. In their case, increasing the anode area and decreasing the PBS concentration increased the microbial diversity.

3.3.7. Major Impacts of MFCs

The real-world influence of MFCs in waste management goes beyond laboratory-scale examinations. Field experiments, such as the MFC latrine system in Ghana, have demonstrated that MFCs may work as self-sustaining waste treatment systems, producing both compost and electricity [20]. MFCs optimise redox conditions to decrease greenhouse gas emissions, including methane (CH₄) and nitrous oxide (N₂O) [23]. Experimental settings have an impact on the ability of MFCs to create energy and remove waste. The authors of [23] used MFCs with vermicompost soil to eliminate 66% more COD with a maximum power density of 4 mW/m². As a result, the authors were able to power an LED light with their MFC latrine [20]. According to the authors of [8], optimal composting conditions at a C/N ratio of 31.4:1 and a moisture content of 60% resulted in a maximum power density of 4.6 mW/m². In their study, the authors of [21] enhanced the performances of microbial fuel cells by using a platinum (Pt) catalyst and produced an output voltage of 544 ± 26 mV and a power density of 349 ± 39 mW/m². The MFC with Pt catalyst values was higher compared with the non-catalyst. According to [22], MFCs have a power density of 0.06 mW/m² in composters and 0.07 mW/m² in agricultural fields. The process not only produces electrical energy but also improves soil fertility and lowers carbon emissions through the degradation of organic matter and bacterial activity. These investigations demonstrate that substrate composition, electrode materials, and operating conditions all have an impact on MFC electro-composting efficiency.

3.3.8. Comparative Evaluation of MFCs

MFCs require minimal to no external energy inputs and instead generate electricity during composting. For instance, the authors of [22] used composted cattle manure and titanium electrodes to produce a stable voltage of 0.5 V and a power density of 0.06 mW/m² in a composting tub, while a field setup reached 1.1 V and 0.07 mW/m². Similarly, the authors of [20] deployed an MFC latrine system using human waste and local materials in Ghana. Their system increased in performance from 0.18 μW to 6.75 μW over one year, ultimately powering an LED light. MFC-based composting systems enable energy harvesting combined with microbial waste degradation. The authors of [8] reported that optimising parameters such as moisture content (60%) and C/N ratio (31.4:1) led to improved electrogenic performance, with a power density of 4.6 mW/m², highlighting the system’s electrogenic focus over compost enhancement. Comparing all related articles, it becomes obvious that MFCs are less effective than EAAC and EOAC in maintaining thermophilic phases and reducing composting time. They are well suited for decentralised rural setups. MFCs could further be deployed in passive monitoring applications for remote or off-grid compost facilities.

3.4. Thermoelectric Generators (TEGs)

TEG systems leverage heat generated during composting to generate electricity, providing a solution for passive energy recovery in large-scale, heat-intensive composting operations [11]. The thermoelectric generator system illustrated in Figure 6 integrates waste treatment with energy recovery, utilising heat generated from composting operations to produce power. The authors of [11,24] explored thermoelectric generators, which is a novel method for improving waste management’s energy efficiency and sustainability.

3.4.1. Materials for the Setup of TEGs

Both the studies mentioned above utilised aluminium thermoelectric generators, which effectively converted temperature differences into electrical voltage. In a more complex system, the authors of [24] used multiple thermoelectric couplings (n- and p-type materials) encapsulated in ceramics to improve durability and performance. The study in [11] employed a simple design of side-by-side thermoelectric sheets (40 × 40 × 4 mm; TGM-199-1.4-1.5), heat pipes, and cooling fins to cool the TEG’s cold end.

3.4.2. Design of TEG Composters

The study by the authors of [24] employed an electrical insulator to connect multiple thermoelectric couplings in series and thermally in parallel. In comparison, the second article shows a stripe-shaped TEG with ten thermoelectric sheets connected in series. Metal sheets conduct heat to the hot end, and a water-cooled aluminium pipe keeps the cold end cool. Additionally, heat-conductive silicone gel allows for system-wide heat conduction [11].

3.4.3. Operating Conditions During TEGs

The authors of [24] employed composting temperature gradients in a cycle of 33 days. This gradient dictates the system’s voltage output, and temperature difference stability influences efficiency regarding electricity generation. The compost (hot end) and water-cooled pipe (cold end) were kept at different temperatures, and in turn, this temperature difference influenced the system’s voltage output, which ranged from 6.9 to 14.4 V. A regulator was used to maintain a stable voltage output [11]. The studies utilised aerobic composting to achieve high temperatures for thermoelectric conversion.

3.4.4. Temperature Evolution During TEGs

Although heat was converted into electricity, both studies on TEGs achieved compost temperatures higher than 55 °C, which is characteristic of active composting and excellent for TEG operation. The authors of [24] investigated and described in detail temperature profiles in three different stages to document the effects of external temperatures on compost temperature stability throughout time. In contrast, the study in [11] discovered a temperature difference of 20–45 °C between compost and ambient temperature. This temperature gradient increased the voltage output from 6.9 V to 14.4 V.

3.4.5. Compost Maturity in TEGs

According to the authors of [24], TEG implementation helps to stabilise the compost pile temperature, which promotes microbial activity. A second article used the GI to assess compost maturity. On day 12, the GI was 107%, which increased to 118% by day 15. In contrast to this, the control group had a GI of just 88% on the 15th day [11].

3.4.6. Microbial Community of TEGs

The regular temperature settings of the TEG setup foster a diverse and active microbial community, which is required for effective composting [24]. The authors of [11,24] did not perform any taxonomical investigation of the microbial community in their studies.

3.4.7. Major Impacts of TEGs

TEG composting results in effective thermal energy recovery and lower operational costs. The authors of [24] presented a cost-effective thermoelectric heat recovery device that utilises thermal energy generated during composting. Similar to this, the authors of [11] emphasise how the TEG system recovers heat from waste. The authors of [11] named this concept a self-powered EAAC (sp-EAAC). According to [11,24], the implementation of a TEG increases composting efficiency, reduces costs, and recovers heat from waste, making the process more competitive and sustainable.

3.4.8. Comparative Evaluation of TEGs

TEGs recover heat released during composting without interfering with microbial processes. The authors of [24] designed a 120 modular TEG panel that captured 11.3 V and 175 mW/m² from compost gradients up to 20 °C. Similarly, the authors of [11] reported even higher output from livestock manure composting, achieving over 7 W of power, stabilised at 6.0 V using a DC–DC converter. Due to the limited number of studies, a comparison to other energy/biogas-yielding systems such as MFCs and AnC_BE remains elusive. One might ask whether it is possible to integrate TEGs with MFCs since both systems rely on electrodes, which are incorporated into the compost. However, the combination has not yet been addressed. Similar to the MFC, AnC_BE does not enhance compost quality as well as EAAC or EOAC systems. Due to the similar setup to MFCs, the application scenarios are the same.

3.5. Three-Chamber Bioelectrochemically Assisted Anaerobic Composting (AnC_{BE, III})

The AnC_{BE, III} system provides anaerobic degradation solutions while enabling internal electron transfer, which is suitable for treating high-solid organic waste in anaerobic settings with minimal energy input [25]. In contrast to an MFC, it does not generate electricity but requires a small amount of energy. This amount of electrical power, though, is smaller than that required for an EOAC; therefore, it does not release additional oxygen. It is distinguishable from an EAAC due to its anaerobic conditions, which enables it to produce methane as an additional resource, similar to a biogas plant. This makes it technologically more sophisticated and more difficult to implement. AnC_{BE, III} is an unexplored system that combines anaerobic composting and electrochemical processes to improve organic waste treatment and bioenergy generation [25]. The three-chamber bioelectrochemically assisted anaerobic composting process is depicted in Figure 7, highlighting its chambered design for effective anaerobic decomposition and energy recovery. This innovative system enhances organic matter degradation, accelerates compost maturity, and improves energy recovery. Compared with its predecessor, AnC_{BE, II}, the three-chamber AnC_{BE, III} system has demonstrated superior performance in terms of total chemical oxygen demand (TCOD) removal, power density, and sludge stabilisation [25]. AnC_{BE, III} has a substantial advantage in that it enhances electron transfer efficiency and accelerates hydrolysis reactions. The system removes 42.3% of TCOD in 42 days, outperforming AnC_{BE, II} (30.3%) and traditional anaerobic composting (23.7%). Furthermore, bioelectrogenesis in AnC_{BE, III} enhances the solubility of dissolved organic matter (DOM), increasing the availability of biodegradable substrates for microbial metabolism. This action speeds up the decomposition of organic materials and helps to keep the treated sludge stable. AnC_{BE, III} also has the capacity to remove hazardous organic contaminants, specifically polycyclic aromatic hydrocarbons (PAHs). Studies show that the system degrades PAH chemicals significantly, with naphthalene degradation reaching 53.8%. This shows that AnC_{BE, III} could be used in sludge treatment operations where the removal of persistent organic contaminants is critical. In addition to organic matter degradation, the system produces a power density of 7.0–8.6 W/m³, surpassing AnC_{BE, II}’s 4.7–5.7 W/m³. This demonstrates the potential of AnC_{BE, III} for incorporation into waste-to-energy plans, particularly in municipal and industrial sludge treatment facilities.

Future studies should concentrate on optimising reactor designs, electrode materials, and microbial communities to improve AnC_{BE, III} efficiency. Furthermore, large-scale research is required to assess the system’s long-term operational reliability and economic viability.

3.5.1. Important Materials for the Setup of AnC_{BE, III}

Material improvements make the AnC_{BE, III} reactor more functional than its predecessor, AnC_{BE, II}. The setup used advanced electrodes, which significantly improved organic matter breakdown and bioelectrochemical activity. The cathode and anode electrodes in the anodic chamber are titanium wire-twisted graphite fibre brushes (STS4024 K, Toho Tenax, Tokyo, Japan). The anodic and cathodic chambers are separated by two proton exchange membranes (PEMs). The anodic chamber has a water bath and a port at the top for gas collection [25]. Future research could consider exploring alternative electrode materials such as carbon-based nanostructures and conductive biofilms to further enhance electron transfer and bioelectrogenesis.

3.5.2. Design of the AnC_{BE, III} Composter

The authors of [25] describe the AnC_{BE, III} composter as consisting of a cylindrical anodic chamber (80 mm diameter, 100 mm height, 380 mL volume) and two cubic cathodic chambers (60 mm × 70 mm × 100 mm, 320 mL volume). The cylindrical anodic chamber enhances the efficient decomposition of substrates, while the dual cathodic chambers improve proton exchange and energy recovery. Future design enhancements could focus on developing modular systems that allow for scalability in larger waste treatment applications. Additionally, integrating both aerobic and anaerobic processes in hybrid designs could further optimise waste decomposition.

3.5.3. Operating Conditions During AnC_{BE, III}

The system uses dewatered sludge, and anaerobic conditions were maintained by purging the reactors with nitrogen. To sustain a stable cathode potential, the catholyte contained potassium ferricyanide (32.9 g L⁻¹) and monopotassium phosphate (27.2 g L⁻¹) [25]. Further research could investigate alternative catholytes, such as microbial metabolic by-products or bio-based redox mediators, to reduce the environmental impact and improve sustainability.

3.5.4. Temperature Evolution During AnC_{BE, III}

Temperature regulation is crucial for maximising bioelectrogenesis in AnC_{BE, III}. During AnC_{BE, III}, a water bath provided thermal insulation for the anode chamber. This enhances organic matter decomposition and energy generation during composting due to stabilised microbial activity involving bioelectrochemical processes [25]. Future research could focus on integrating thermoelectric generators (TEGs) to recover heat from the composting process and maintain self-regulated temperature conditions.

3.5.5. Compost Maturity in AnC_{BE, III}

In their study, the authors of [25] found that the initial TOC degraded dramatically within the first 24 days of testing. The AnC_{BE, III} removed 41.2 ± 0.4% of TOC during the 42-day test, surpassing the AnC_{BE, II} (30.3 ± 0.5%) and AnC results. The enhanced bioelectrochemical activity facilitates the process of humification, thereby improving the stability of compost and its capacity for nutrient retention. Future research should consider examining optimised operational parameters, such as regulated aeration cycles or the inclusion of biochar amendments, to further advance compost maturity and enhance applications related to soil fertility.

3.5.6. Microbial Community of AnC_{BE, III}

The study in [25] disclosed that bioelectrogenesis improved electricity and organic matter degradation. Bioelectrogenic bacteria, such as Geobacter and Shewanella species, are predominant in the anodic biofilm, enabling effective electron transfer [25]. However, detailed taxonomic information about the microbial consortia is still lacking. Future research could utilise metagenomic and transcriptomic methods to better characterise the dynamics of these microbial communities and optimise their interactions to improve the systems’ performance.

3.5.7. Major Impacts of AnC_{BE, III}

A novel three-chamber bioelectrochemically assisted anaerobic composting system improves energy generation and organic matter breakdown. It outperforms the two-chambered AnC_{BE, II} in terms of power density (7.0–8.6 W/m³) and TCOD removal (42.3% vs. 32.1%). The AnC_{BE, III} technique improves organic solubilisation while stabilising and lowering treated sludge volume. Future applications of AnC_{BE, III} could include decentralised wastewater treatment, large-scale processing of organic waste, and the enhanced recovery of bio-based energy products. Furthermore, examining its potential in the degradation of emerging contaminants, such as pharmaceutical residues and microplastics, could significantly broaden its environmental impact.

3.5.8. Comparative Evaluation of AnC_{BE, III}

The AnC_{BE, III} system achieved a power density of 7.0–8.6 W/m³, which also enhanced the degradation of organic matter, as indicated by a higher TCOD removal rate of 42.3 ± 0.5% [25]. AnC_BE outperforms MFCs and TEGs in energy yield but requires multi-chamber reactors and strict environmental control. MFCs and TEGs are suitable for industrial or centralised bioenergy platforms and less appropriate for rural settings due to their complexity.

3.6. Comparative Performance Evaluation of Electro-Composting Systems

A comparative analysis was conducted across several key performance indicators, as listed in the

Table 2. Comparative performance of electro-composting systems (EAAC and EOAC), which require electricity to improve the composting performance.

Parameter	EAAC	EOAC
Composting Efficiency	High: 33% reduction in composting time and enhanced humification, 28% higher HI, and 38% higher GI [3,7]	High: Enhanced oxygenation and degradation rate [19]
Energy Use	Moderate: Requires external DC power (2–10 V) [3]	High: Requires continuous electrolysis for O2 production [19]
GHG Emissions (CO2, CH4, N2O)	Reduced CH4 and N2O by up to 75.5% [1]	Significantly lower CH4 and CO2 emissions due to optimal oxygenation [18]
Economic Costs	Moderate initial investment but low operating costs [3]	High capital costs, but moderate operating costs [18]
Operational Complexity	Medium, requires electrode maintenance and controlled aeration [4]	High, complex setup with electrolysis equipment [18]
Composting Temperature	65–91 °C	60–84 °C
System Type	Aerobic	Aerobic
Oxygen Control	Electrode-driven oxygen boost	In situ oxygen via electrolysis
Energy Recovery	None	None
Electrodes Used	Yes	Yes

and

Table 3. Comparative performance of electro-composting systems that allow for the production of electricity or biogas (MFC, TEG, and AnC_{BE, III}).

Parameter	MFC	TEG	AnCBE, III
Composting Efficiency	Moderate: COD removal up to 66%, C/N ratio = 31.4:1, moisture = 60% [8,23]	Moderate: GI improved from 107% to 118% in 15 days [11]	High: 42.3% TOC removal in 42 days, surpassing AnCBE, II [25]
Energy Use	Low: Generates energy rather than consuming it [23]	Low: Compost heat is converted to electricity [24,25]	High: Requires energy for microbial stimulation [25]
GHG Emissions (CO2, CH4, N2O)	CO2 reduction due to microbial metabolism, minor CH4, and N2O impact [20]	Neutral impact, does not alter microbial metabolism [11]	Potential to mitigate CH4 emissions due to microbial electron transfer [25]
Economic Costs	Moderate initial investment, low operational costs [20]	High cost due to thermoelectric material requirements [11]	High capital costs, and specialised materials increase expenses [25]
Operational Complexity	High, requires optimised moisture levels and has limited scalability [22]	Medium, scalable, but requires temperature control [24]	A high, complex system with multiple chambers and membranes [25]
Composting Temperature	30–60 °C	55–80 °C	35–55 °C
System Type	Hybrid anaerobic–aerobic	Aerobic	Anaerobic
Oxygen Control	None	Not applicable	Anaerobic; oxygen excluded
Energy Recovery	Electricity and biogas	Electrical via thermoelectricity	Biogas
Electrodes Used	Yes	No	Yes

below, to provide a comprehensive evaluation of various electro-composting systems. Among all systems, EAAC shows the most favourable cost–benefit profile for large-scale use. The study in [11] reported a setup cost of USD 36/t and energy savings of up to USD 135/t due to reduced aeration. MFCs use low-cost materials but produce limited power, making large-scale cost-effectiveness uncertain. TEGs recover passive heat energy but require substantial surface area of 5708 m² for 1 kWh, posing material and space constraints [24]. These results are summarised in the tables below, which provide a structured summary of each system’s advantages and drawbacks.

4. Conclusions

Electro-composting technologies offer substantial advantages over conventional composting by improving decomposition efficiency, reducing emissions, and enabling energy recovery. However, cost, complexity, and scalability challenges persist, particularly for energy-generating systems. Among the reviewed systems, EAAC and EOAC focus on biological and electrochemical stimulation, while MFCs, TEGs, and AnC_{BE, III} incorporate waste-to-energy applications. AnC_{BE, III}, although underexplored, shows promise for anaerobic bioelectrochemical composting. Comparatively, EAAC systems are most mature for scale-up, while EOAC and AnC_{BE, III} require further exploration of electrode materials and optimisation. MFCs hold potential in hybrid wastewater–energy setups, but cost and efficiency remain barriers. TEGs are promising for passive energy recovery but are still under-researched. Despite their potential, EOAC, TEGs, and AnC_{BE, III} lack large-scale trials, limiting our understanding of their operational stability and scalability. Energy-harvesting systems such as MFCs, TEGs, and AnC_{BE, III} suffer from economic and technical inefficiencies. Future research should focus on low-cost and durable electrode materials, integration with smart systems, and conducting LCA and economic assessments to evaluate the systems’ performances in urban and rural settings.