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    Topic review

    High-Solid Anaerobic Digestion

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    Submitted by: Xiomar Gómez

    Definition

    High-solid anaerobic digestion (HS-AD) and solid-state anaerobic digestion (SS-AD) are technologies presenting an outstanding capacity for treating organic wastes and requiring lower digester volumes. 

    1. Introduction

    The valorization of biogas for the production of energy or to upgrade this gas to achieve a quality similar to that of natural gas is another integral component of digestion technology. Biogas conversion or upgrading also has a clear impact on the capital investment and operating costs of this technology. Energy production from biogas is usually performed by combined heat and power units (CHP), allowing the efficient use of on-site biogas [1]. The applications of fuel cells and micro-turbines are increasing, but the costs associated with these later technologies are still too high.

    A large amount of water in wet digestion systems translates into lower methane productivity [2] since biogas yields are directly associated with the dry matter content of the feeding. Thus an increase in reactor productivity usually translates into increasing the solid content of the input material. Increasing the solid content in the digestion system exerts different effects on microbial performance; thus, the term “high-solid” is used to refer to an anaerobic process that are still considered wet digestion systems but which work with solid content values close to the higher limit of this solid range, and experience diffusion limitations [3][4]. High-solid anaerobic digestion seems to be a logical option for enhancing digestion performance, given that biogas production is directly associated with the mass of volatile solids fed into the digester. However, the strategy of working with a higher solid content implies a great variety of modifications in plant operation and the equipment needed, and the higher organic matter content significantly affects the performance of the anaerobic microflora.

    The addition of water is necessary to set a specific solid content for the feeding, and it is usually recirculated to avoid excessive consumption of this resource. The liquid digestate may be treated for the recovery of nutrients through struvite precipitation [5][6] and solids can find applications as organic amendments in croplands. However, the amount of solid digestate produced is still high, and it may prove difficult to ensure a proper final disposal option all year round. Thus, alternatives are needed for increasing the conversion of the degradation process and the final valorization of the digestate to avoid generating an additional problem for farmers with no possibility of finding a solution for the final disposal of the digestate. Anaerobic reactors capable of treating highly concentrated substrates without experiencing significant inhibitory problems would aid in increasing plant feasibility, facilitating digestate handling operations, and final disposal.

    2. Solid-State Anaerobic Digestion (SS-AD)

    Solid-state fermentation is another way of operating digestion technology. TS values are higher than those used in HS-AD and seem more suitable for treating agricultural residues and food wastes, due to the lower water demand. Agricultural wastes have an intrinsic capacity to act as a structuring agent during fermentation because of their high content of lignocellulosic material. Many agricultural residues are untreated or underutilized, creating climate change problems associated with the emission of greenhouse gases (GHG) during the uncontrolled degradation of this type of waste [7]. Solid-state fermentation has been successfully studied for producing enzymes, biosurfactants, proteins, and biofuels [8][9] and a great variety of valuable products [10][11][12][13]. Anaerobic digestion has also been evaluated under this configuration and the co-digestion of different residues, such as manure, food wastes, and agricultural wastes, has also been studied under SS-AD conditions [14][15][16].

    The separation of the digestion system into two stages, the first dedicated to the hydrolysis and acidogenesis of the feeding material and the second to the conversion into methane of the acidified liquor, is a way to overcome acidification problems and buffering accidental overloading. When applied to solid-phase systems, the first phase acts as a leachate bed reactor and the second one as a traditional CSTR or as an up-flow anaerobic sludge bed (UASB) system [17][18][19]. Liu and Liao [20] studied a two-stage process, with the first stage operating as a leachate bed reactor (LBR). These authors attained the conversion of the substrate in less than 6 days, with a 70.9% removal of VS from the leachate reactor. However, if the total mass of substrate loaded (10 kg) and the volume of the reactors are considered (70 L for the LBR and 35 L for the second methanogenic phase), the OLR applied would be equivalent to 1.5 g VS/Lr d—expressed in terms of the volume of the reactor—when the loading estimation is performed for a continuous system. Biogas production in the LBR displayed an evolution characterized by a peaking behavior immediately after the addition of the methanogenic leachate and a rapid decrease due to the excessive accumulation of VFAs. These authors also reported compaction to be a problem and hydrogen gas evolution was described during the initial recirculation stages, which indicates process imbalances.

    The biogas yields obtained under solid-state conditions are lower than those from wet digestion systems. The increase in solid content causes a decrease in the biogas yield [21]. This fact was demonstrated by Li et al. [22] when evaluating the co-digestion of corn stover and chicken manure under different configurations, that is, wet, high-solid, and solid-state digestion. These authors tested mixtures of substrates, but in general, the wet digestion system (at 5.1–5.6%TS) achieved higher methane yields than any of the other experimental set-ups working at higher solid contents. The methane yield was reported to be 0.219 L/g VS added for the wet system, whereas this value decreased to 0.208 L CH 4/g VS in the system with a high-solid content (10.1–11.2%TS) and further decreased to 0.148 L CH 4/g VS when evaluating solid-state digestion (20.1–22.4%TS). In addition, the optimum mixture composition for obtaining the highest methane yield was different for solid-state digestion, with a proportion of 1:1 (VS basis, corn stover/chicken manure), whereas for the other two digestion systems, this proportion was found to be 3:1.

    Kim et al. [23] studied the effect of moisture content in SS-AD using a bedding material composed of sawdust collected after 2–3 months of being used as cattle bedding. These authors evaluated this material as a substrate, which had a solid content between 17% and 30%. Although the values of methane yield reported were low for all cases tested, the system with a higher solid content presented a methane yield that was 29% lower than that at a TS content of 17%, thus corroborating the adverse effect associated with an extremely low water content. In solid-state fermentation, water activity (aw) has a determinant influence on microbial activity, having a fundamental role in the mass transfer of water and solutes across microbial cells [24]. Therefore, there is clear evidence on the limits imposed regarding the levels of inhibitory components and the water content of the system, and their removal from the liquid phase is a necessity during SS-AD to avoid excessive toxic effects. The strategy proposed by Takashima and Yaguchi [25] of introducing an ammonia-stripping stage in HS-AD systems treating sewage sludge seems reasonable and leads to an expectation of success in digestion systems operating at even higher solid contents. Indeed, this is what Farrow et al. [26] intended when digesting poultry manure under a solid-phase configuration using struvite precipitation with pH controlled at around 7.0 during the ammonia removal stage to avoid adverse effects on the microbial biomass. This strategy allowed biogas to increase by about 30% under batch conditions and by nearly 235% when operating under semi-continuous conditions, reporting biogas yields of 0.420 ± 0.050 L/g VS added. However, the OLR was extremely low for an SS-AD system (OLR of 1.5 g VS/Lr d) and it should be added that they also experienced a decrease in the biogas yield with the increase in OLR.

    3. The Effect of Adsorbents and Materials in Accelerating Anaerobic Degradation

    The addition of adsorbents and carbon conductive materials to anaerobic reactors has been evaluated with success to decrease the impact of inhibitory compounds [27][28]. Adding this type of supplement to digestion allows for the enhancement of biogas productivity without greatly affecting the energy demands of the process [29]. The use of biochar derived from the thermal processing of lignocellulosic biomass in digestion systems has awakened interest among the scientific community, given its proven benefits regarding the mitigation of the negative effects of VFA and ammonia [30][31]. Other materials, such as zeolites, activated carbon and various adsorbents (kaolin, silica gel, polyvinyl alcohol, among others) have also provided benefits in biogas production [32][33][34][35] but the costs associated with these initiatives need to be carefully evaluated.

    The mechanism of direct interspecies electron transfer (DIET) has been frequently proposed to explain the better performance of anaerobic digestion when carbon conductive materials are supplemented [36][37]. The enhancement is explained by the availability of a faster degradation route for the conversion of VFA [38][39][40], which is possible due to the prevalence of microbial species that become dominant due to the presence of materials that favor electron transport. Guo et al. [41] demonstrated the efficacy of adding GAC or magnetite on propionate degradation. These compounds favor the dominance of a syntrophic consortium by creating a DIET environment.

    The addition of nanoparticles to digestion systems has recently demonstrated benefits in biogas production and the reduction of conversion times. The mechanism and effects of nanoparticles in anaerobic digestion have been reviewed by Abdelsalam et al. [42] and Faisal et al. [43]. Nanoparticles cause microbial activity stimulation based on the higher bio-availability of metal components essential for enzymatic reactions, thus enhancing cellular growth. Nanoparticles of iron oxide and zero-valent iron enhance interspecies hydrogen transfer and direct interspecies electron transfer, explaining the excellent results obtained when they are supplemented into digestion systems [44]. Other metals (Cu, Co, Ag, Ni) and metal oxides have also been studied as supplements in anaerobic digestion in the form of nanoparticles [45][46][47][48]. Nanomaterials, in general, may become a useful ally in promoting substrate degradation due to their unique characteristics such as their high surface area, high reactivity, and specificity, and their increased number of active sites [49]. There is a wide variety of reports available in the literature on the benefits associated with the addition of conductive materials and adsorbents.

    Casals et al. [50] reported a threefold increase in methane production when supplementing iron nanoparticles (NPs). Abdelwahab et al. [51] studied the digestion of cattle manure and obtained a biogas yield of 0.953 L/g VS when evaluating a concentration of 15 mg/L of (Fe) NPs against a value of 0.589 L/g VS obtained from the control experiments. Not only was the biogas yield enhanced, but the presence of these particles also favored a lower production of H 2S, which is of great relevance regarding subsequent biogas up-grading operations. Similarly, Farghali et al. [52] studied the addition of iron oxide (Fe 2O 3) and titanium dioxide (TiO 2) nanoparticles, reporting a twofold increase in biogas yields and a decrease in H 2S production. The addition of magnetite NPs was studied by Ali et al. [53] and Zhong et al. [54], with the latter indicating that the presence of these particles was probably responsible for accelerating the transfer of electrons from acid oxidizers to syntrophic methanogenesis, stimulating acid oxidizers to degrade acetate into H 2/CO 2, and finally to facilitate methane production. These reports open a new line of research completely disrupting the current efficiency of digestion plants, improving performance, and offering a completely radical change in the valorization of biogas. However, other factors—more than just economic criteria and bioenergy production—must also be evaluated when considering organic waste treatment. Sociocultural ideas, environmental impacts associated with this technology, and local knowledge may appear as important constraints [55], necessitating careful assessment to avoid causing a negative perception in local communities.

    4. Temperature and Digestion Performance

    Temperature is a crucial parameter for increasing the degradation rate. Psychrophilic conditions refer to systems working at temperatures lower than 20 °C, mesophilic conditions range between 20 °C and 45 °C, and thermophilic conditions have temperatures higher than 45 °C [56]. Any increase in temperature will translate into a greater biogas production rate, and therefore it is reasonable to assume that ideal operation should be based on optimum temperature conditions. However, this is not always possible since capital investment and operating costs are also parameters that greatly influence plant profitability. Thus, operation at low temperatures has been studied to determine the decrease produced in process performance and evaluate ranges of feasible operation [57]. The absence of a heating system to reduce operating costs also leads to variable performance due to daily temperature variations, which may cause process instabilities [58], and extremely low activities in the winter season.

    SS-AD has been tested at temperatures below 34 °C. Since the main advantage of this technology is its simplicity, the installation of a heating system would add unnecessary operating costs. Avoiding these additional costs is vital if this technology is extensively applied in developing countries and/or tropical countries where excessive low ambient temperatures are not experienced. Ghosh [59] evaluated the fermentation of solid wastes around 25 °C, obtaining a yield of 0.26 L CH 4/g VS added, thus proving the suitability of this process even at this temperature. Operating at lower temperatures to establish optimum conditions for low-cost digestion systems is needed.

    Psychrophilic digestion has been studied by different authors, reporting lower biogas yields [60][61] and solid accumulation [62], but successful experiences have also been described, with gas yields similar to those obtained at higher temperatures, indicating that the process was not significantly affected by the increase in the OLR, as would be expected [63][64]. These reports are important as many small-scale digesters operate under this regimen. When the performance of these systems is analyzed, better yields are obtained than those expected from control laboratory conditions. This is probably explained by the well-established consortium attained after an extended operation time in industrial operating reactors [65]. Zhao et al. [66] studied digestion performance at 4 °C, indicating that the maximum treatment capacity was set at 4.33 g VS/Lr d of OLR. Therefore, low-temperature operating digestion systems may become a low-cost solution for the operation of decentralized reactors with a treatment capacity equivalent to that of more complex mesophilic and thermophilic reactors.

    However, it is undeniable that increasing the temperature of the process affects reaction rates; therefore, to speed up biological degradation, the temperature should be increased. Moving from a mesophilic to a thermophilic regimen has been implemented to improve the treatment capacity of the reactor and thus productivity. Thermophilic conditions allow higher degradation rates, thus achieving a greater capacity for treating organics and attaining higher pathogen destruction [67]. A temperature rise from mesophilic to thermophilic conditions reduces the required volume of the digester and significantly decreases capital investment costs [68]. This feature translates into a significant increase in the treatment capacity of the plant for reactors that are already operating at lower temperatures but also result in a higher energy demand. The feed needs to be heated up to the desired thermophilic conditions, requiring a greater amount of energy, and this demand is accentuated in the winter season. Thermal losses are also higher due to the greater temperature gradient associated with the process and the ambient temperature, making insulation crucial to avoid excessive energy losses.

    The entry is from 10.3390/environments8080080

    References

    1. Riley, D.M.; Tian, J.; Güngör-Demirci, G.; Phelan, P.; Villalobos, J.R.; Milcarek, R.J. Techno-Economic Assessment of CHP Systems in Wastewater Treatment Plants. Environments 2020, 7, 74.
    2. Ge, X.; Xu, F.; Li, Y. Solid-state anaerobic digestion of lignocellulosic biomass: Recent progress and perspectives. Bioresour. Technol. 2016, 205, 239–249.
    3. Zhang, Y.; Li, H.; Liu, C.; Cheng, Y. Influencing mechanism of high solids concentration on anaerobic mono-digestion of sewage sludge without agitation. Front. Environ. Sci. Eng. 2015, 9, 1108–1116.
    4. Zhang, Y.Y.; Li, H.; Cheng, Y.C.; Liu, C. Influence of solids concentration on diffusion behavior in sewage sludge and its digestate. Chem. Eng. Sci. 2016, 152, 674–677.
    5. Cao, L.; Wang, J.; Xiang, S.; Huang, Z.; Ruan, R.; Liu, Y. Nutrient removal from digested swine wastewater by combining ammonia stripping with struvite precipitation. Environ. Sci. Pollut. Res. 2019, 26, 6725–6734.
    6. Ryu, H.D.; Lim, D.Y.; Kim, S.J.; Baek, U.I.; Chung, E.G.; Kim, K.; Lee, J.K. Struvite Precipitation for Sustainable Recovery of Nitrogen and Phosphorus from Anaerobic Digestion Effluents of Swine Manure. Sustainability 2020, 12, 8574.
    7. Sadh, P.K.; Duhan, S.; Duhan, J.S. Agro-industrial wastes and their utilization using solid state fermentation: A review. Bioresour. Bioprocess. 2018, 5, 1–15.
    8. Melnichuk, N.; Braia, M.J.; Anselmi, P.A.; Meini, M.R.; Romanini, D. Valorization of two agroindustrial wastes to produce alpha-amylase enzyme from Aspergillus oryzae by solid-state fermentation. Waste Manag. 2020, 106, 155–161.
    9. Rodríguez, A.; Gea, T.; Sánchez, A.; Font, X. Agro-wastes and Inert Materials as Supports for the Production of Biosurfactants by Solid-state Fermentation. Waste Biomass Valoris. 2021, 12, 1963–1976.
    10. Yazid, N.A.; Barrena, R.; Komilis, D.; Sánchez, A. Solid-state fermentation as a novel paradigm for organic waste valorization: A review. Sustainability 2017, 9, 224.
    11. Pourkhanali, K.; Khayati, G.; Mizani, F.; Raouf, F. Isolation, identification and optimization of enhanced production of laccase from Galactomyces geotrichum under solid-state fermentation. Prep. Biochem. Biotechnol. 2020, 51, 659–668.
    12. Slaný, O.; Klempová, T.; Shapaval, V.; Zimmermann, B.; Kohler, A.; Čertík, M. Biotransformation of animal Fat-by products into ARA-enriched fermented bioproducts by solid-state fermentation of mortierella alpina. J. Fungi 2020, 6, 236.
    13. Xing, Q.; Dekker, S.; Kyriakopoulou, K.; Boom, R.M.; Smid, E.J.; Schutyser, M.A. Enhanced nutritional value of chickpea protein concentrate by dry separation and solid state fermentation. Innov. Food Sci. Emerg. Technol. 2021, 59, 102269.
    14. Ajayi-Banji, A.A.; Rahman, S.; Sunoj, S.; Igathinathane, C. Impact of corn stover particle size and C/N ratio on reactor performance in solid-state anaerobic co-digestion with dairy manure. J. Air Waste Manag. Assoc. 2020, 70, 436–454.
    15. Guilford, N.G.; Lee, H.P.; Kanger, K.; Meyer, T.; Edwards, E.A. Solid-state anaerobic digestion of mixed organic waste: The synergistic effect of food waste addition on the destruction of paper and cardboard. Environ. Sci. Technol. 2019, 53, 12677–12687.
    16. Pezzolla, D.; Di Maria, F.; Zadra, C.; Massaccesi, L.; Sordi, A.; Gigliotti, G. Optimization of solid-state anaerobic digestion through the percolate recirculation. Biomass Bioenergy 2017, 96, 112–118.
    17. Nizami, A.S.; Thamsiriroj, T.; Singh, A.; Murphy, J.D. Role of leaching and hydrolysis in a two-phase grass digestion system. Energy Fuels 2010, 24, 4549–4559.
    18. Xu, S.Y.; Lam, H.P.; Karthikeyan, O.P.; Wong, J.W.C. Optimization of food waste hydrolysis in leach bed coupled with methanogenic reactor: Effect of pH and bulking agent. Bioresour. Technol. 2011, 102, 3702–3708.
    19. Siciliano, A.; Limonti, C.; Curcio, G.M.; Calabrò, V. Biogas generation through anaerobic digestion of compost leachate in semi-continuous completely stirred tank reactors. Processes 2019, 7, 635.
    20. Liu, W.Y.; Liao, B. Anaerobic co-digestion of vegetable and fruit market waste in LBR+ CSTR two-stage process for waste reduction and biogas production. Appl. Biochem. Biotechnol. 2019, 188, 185–193.
    21. Ziaee, F.; Mokhtarani, N.; Niavol, K.P. Solid-state anaerobic co-digestion of organic fraction of municipal waste and sawdust: Impact of co-digestion ratio, inoculum-to-substrate ratio, and total solids. Biodegradation 2021, 32, 299–312.
    22. Li, Y.; Zhang, R.; Chen, C.; Liu, G.; He, Y.; Liu, X. Biogas production from co-digestion of corn stover and chicken manure under anaerobic wet, hemi-solid, and solid state conditions. Bioresour. Technol. 2013, 149, 406–412.
    23. Kim, E.; Lee, S.; Jo, H.; Jeong, J.; Mulbry, W.; Rhaman, S.; Ahn, H. Solid-state anaerobic digestion of dairy manure from a sawdust-bedded pack barn: Moisture responses. Energies 2018, 11, 484.
    24. Pandey, A. Solid-state fermentation. Biochem. Eng. J. 2003, 13, 81–84.
    25. Takashima, M.; Yaguchi, J. High-solids thermophilic anaerobic digestion of sewage sludge: Effect of ammonia concentration. J. Mater. Cycles Waste Manag. 2021, 23, 205–213.
    26. Farrow, C.; Crolla, A.; Kinsley, C.; McBean, E. Anaerobic digestion of poultry manure: Process optimization employing struvite precipitation and novel digestion technologies. Environ. Prog. Sustain. Energy 2017, 36, 73–82.
    27. Bardi, M.J.; Rad, H.A. Simultaneous synergistic effects of addition of agro-based adsorbent on anaerobic co-digestion of food waste and sewage sludge. J. Mater. Cycles Waste Manag. 2020, 22, 65–79.
    28. Achi, C.G.; Hassanein, A.; Lansing, S. Enhanced biogas production of cassava wastewater using zeolite and biochar additives and manure co-digestion. Energies 2020, 13, 491.
    29. Arias, J.G.; Sánchez, M.E.; Gómez, X. Enhancing anaerobic digestion: The effect of carbon conductive materials. C J. Carbon Res. 2018, 4, 59.
    30. Wang, G.; Li, Q.; Gao, X.; Wang, X.C. Sawdust-derived biochar much mitigates VFAs accumulation and improves microbial activities to enhance methane production in thermophilic anaerobic digestion. ACS Sustain. Chem. Eng. 2018, 7, 2141–2150.
    31. Sánchez, E.; Herrmann, C.; Maja, W.; Borja, R. Effect of organic loading rate on the anaerobic digestion of swine waste with biochar addition. Environ. Sci. Pollut. Res. 2021, 28, 1–11.
    32. Patel, V.; Patel, A.; Datta, M. Effects of adsorbents on anaerobic digestion of water hyacinth-cattle dung. Bioresour. Technol. 1992, 40, 179–181.
    33. Milán, Z.; Sánchez, E.; Weiland, P.; Borja, R.; Martın, A.; Ilangovan, K. Influence of different natural zeolite concentrations on the anaerobic digestion of piggery waste. Bioresour. Technol. 2001, 80, 37–43.
    34. Salam, B.; Biswas, S.; Rabbi, M.S. Biogas from mesophilic anaerobic digestion of cow dung using silica gel as catalyst. Procedia Eng. 2015, 105, 652–657.
    35. Fatima, B.; Liaquat, R.; Farooq, U.; Jamal, A.; Ali, M.I.; Liu, F.J.; He, H.; Guo, H.; Urynowicz, M.; Huang, Z. Enhanced biogas production at mesophilic and thermophilic temperatures from a slaughterhouse waste with zeolite as ammonia adsorbent. Int. J. Environ. Sci. Technol. 2021, 18, 265–274.
    36. Baek, G.; Kim, J.; Kim, J.; Lee, C. Role and potential of direct interspecies electron transfer in anaerobic digestion. Energies 2018, 11, 107.
    37. Wang, Z.; Wang, T.; Si, B.; Watson, J.; Zhang, Y. Accelerating anaerobic digestion for methane production: Potential role of direct interspecies electron transfer. Renew. Sustain. Energy Rev. 2021, 145, 111069.
    38. Cerrillo, M.; Viñas, M.; Bonmatí, A. Anaerobic digestion and electromethanogenic microbial electrolysis cell integrated system: Increased stability and recovery of ammonia and methane. Renew. Energy 2018, 120, 178–189.
    39. Arenas, C.B.; Meredith, W.; Snape, C.E.; Gómez, X.; González, J.F.; Martinez, E.J. Effect of char addition on anaerobic digestion of animal by-products: Evaluating biogas production and process performance. Environ. Sci. Pollut. Res. 2020, 27, 24387–24399.
    40. Cui, Y.; Mao, F.; Zhang, J.; He, Y.; Tong, Y.W.; Peng, Y. Biochar enhanced high-solid mesophilic anaerobic digestion of food waste: Cell viability and methanogenic pathways. Chemosphere 2021, 272, 129863.
    41. Guo, B.; Zhang, Y.; Yu, N.; Liu, Y. Impacts of conductive materials on microbial community during syntrophic propionate oxidization for biomethane recovery. Water Environ. Res. 2021, 93, 84–93.
    42. Abdelsalam, E.M.; Samer, M. Biostimulation of anaerobic digestion using nanomaterials for increasing biogas production. Rev. Environ. Sci. Biotechnol. 2019, 18, 525–541.
    43. Faisal, S.; Hafeez, F.Y.; Zafar, Y.; Majeed, S.; Leng, X.; Zhao, S.; Saif, I.; Malik, K.; Li, X. A review on nanoparticles as boon for biogas producers—Nano fuels and biosensing monitoring. Appl. Sci. 2019, 9, 59.
    44. Li, S.; Cao, Y.; Zhao, Z.; Zhang, Y. Regulating secretion of extracellular polymeric substances through dosing magnetite and zerovalent iron nanoparticles to affect anaerobic digestion mode. ACS Sustain. Chem. Eng. 2019, 7, 9655–9662.
    45. Zaidi, A.A.; RuiZhe, F.; Shi, Y.; Khan, S.Z.; Mushtaq, K. Nanoparticles augmentation on biogas yield from microalgal biomass anaerobic digestion. Int. J. Hydrog. Energy 2018, 43, 14202–14213.
    46. Abdallah, M.S.; Hassaneen, F.Y.; Faisal, Y.; Mansour, M.S.; Ibrahim, A.M.; Abo-Elfadl, S.; Salem, H.G.; Allam, N.K. Effect of Ni-Ferrite and Ni-Co-Ferrite nanostructures on biogas production from anaerobic digestion. Fuel 2019, 254, 115673.
    47. Hassaan, M.A.; Pantaleo, A.; Tedone, L.; Elkatory, M.R.; Ali, R.M.; Nemr, A.E.; Mastro, G.D. Enhancement of biogas production via green ZnO nanoparticles: Experimental results of selected herbaceous crops. Chem. Eng. Commun. 2021, 208, 242–255.
    48. Grosser, A.; Grobelak, A.; Rorat, A.; Courtois, P.; Vandenbulcke, F.; Lemière, S.; Guyoneaud, R.; Attard, E.; Celary, P. Effects of silver nanoparticles on performance of anaerobic digestion of sewage sludge and associated microbial communities. Renew. Energy 2021, 171, 1014–1025.
    49. Baniamerian, H.; Isfahani, P.G.; Tsapekos, P.; Alvarado-Morales, M.; Shahrokhi, M.; Vossoughi, M.; Angelidaki, I. Application of nano-structured materials in anaerobic digestion: Current status and perspectives. Chemosphere 2019, 229, 188–199.
    50. Casals, E.; Barrena, R.; García, A.; González, E.; Delgado, L.; Busquets-Fité, M.; Font, X.; Arbiol, J.; Glatzel, P.; Kvashnina, K.; et al. Programmed iron oxide nanoparticles disintegration in anaerobic digesters boosts biogas production. Small 2014, 10, 2801–2808.
    51. Abdelwahab, T.A.M.; Mohanty, M.K.; Sahoo, P.K.; Behera, D. Impact of iron nanoparticles on biogas production and effluent chemical composition from anaerobic digestion of cattle manure. Biomass Convers. Bior. 2020, 1–13.
    52. Farghali, M.; Andriamanohiarisoamanana, F.J.; Ahmed, M.M.; Kotb, S.; Yamashiro, T.; Iwasaki, M.; Umetsu, K. Impacts of iron oxide and titanium dioxide nanoparticles on biogas production: Hydrogen sulfide mitigation, process stability, and prospective challenges. J. Environ. Manag. 2019, 240, 160–167.
    53. Ali, A.; Mahar, R.B.; Soomro, R.A.; Sherazi, S.T.H. Fe3O4 nanoparticles facilitated anaerobic digestion of organic fraction of municipal solid waste for enhancement of methane production. Energy Source Part A 2017, 39, 1815–1822.
    54. Zhong, D.; Li, J.; Ma, W.; Qian, F. Clarifying the synergetic effect of magnetite nanoparticles in the methane production process. Environ. Sci. Pollut. Res. 2020, 1–9.
    55. Babalola, M.A. Application of GIS-Based Multi-Criteria Decision technique in exploration of suitable site options for anaerobic digestion of food and biodegradable waste in Oita City, Japan. Environments 2018, 5, 77.
    56. Borja, R.; González, E.; Raposo, F.; Millán, F.; Martín, A. Kinetic analysis of the psychrophilic anaerobic digestion of wastewater derived from the production of proteins from extracted sunflower flour. J. Agric. Food Chem. 2002, 50, 4628–4633.
    57. Wang, S.; Ma, F.; Ma, W.; Wang, P.; Zhao, G.; Lu, X. Influence of temperature on biogas production efficiency and microbial community in a two-phase anaerobic digestion system. Water 2019, 11, 133.
    58. Cheng, Q.; Huang, W.; Jiang, M.; Xu, C.; Fan, G.; Yan, J.; Chai, B.; Zhang, Y.; Zhang, Y.; Zhang, S.; et al. Challenges of anaerobic digestion in China. Int. J. Environ. Sci. Technol. 2021, 1–12.
    59. Ghosh, S. Solid-phase methane fermentation of solid wastes. J. Energy Resour. Technol. 1985, 107, 402–405.
    60. Muñoz, P. Assessment of Batch and Semi-continuous Anaerobic Digestion of Food Waste at Psychrophilic Range at Different Food Waste to Inoculum Ratios and Organic Loading Rates. Waste Biomass Valoris. 2019, 10, 2119–2128.
    61. Muñoz, P.; Cordero, C.; Tapia, X.; Muñoz, L.; Candia, O. Assessment of anaerobic digestion of food waste at psychrophilic conditions and effluent post-treatment by microalgae cultivation. Clean Technol. Environ. Policy 2020, 22, 725–733.
    62. Massé, D.I.; Gilbert, Y.; Saady, N.M.C.; Liu, C. Low-temperature anaerobic digestion of swine manure in a plug-flow reactor. Environ. Technol. 2013, 34, 2617–2624.
    63. Rajagopal, R.; Bellavance, D.; Rahaman, M.S. Psychrophilic anaerobic digestion of semi-dry mixed municipal food waste: For North American context. Process. Saf. Environ. 2017, 105, 101–108.
    64. Massé, D.I.; Saady, N.M.C. Dry anaerobic digestion of high solids content dairy manure at high organic loading rates in psychrophilic sequence batch reactor. Appl. Microbiol. Biotechnol. 2015, 99, 4521–4529.
    65. Jaimes-Estévez, J.; Zafra, G.; Martí-Herrero, J.; Pelaz, G.; Morán, A.; Puentes, A.; Gómez, C.; Castro, L.; Escalante Hernández, H. Psychrophilic Full Scale Tubular Digester Operating over Eight Years: Complete Performance Evaluation and Microbiological Population. Energies 2021, 14, 151.
    66. Zhao, H.; Yan, F.; Li, X.; Piao, R.; Wang, W.; Cui, Z. Impact of Organic Loading Rate on Performance and Methanogenic Microbial Communities of a Fixed-Bed Anaerobic Reactor at 4 °C. Water 2020, 12, 2586.
    67. Meegoda, J.N.; Li, B.; Patel, K.; Wang, L.B. A review of the processes, parameters, and optimization of anaerobic digestion. Int. J. Environ. Res. Public Health 2018, 15, 2224.
    68. Mirmasoumi, S.; Ebrahimi, S.; Saray, R.K. Enhancement of biogas production from sewage sludge in a wastewater treatment plant: Evaluation of pretreatment techniques and co-digestion under mesophilic and thermophilic conditions. Energy 2018, 157, 707–717.
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