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Graphical abstract Highlights • Metal nanoparticles (NPs) were added to anaerobic digestion (AD) of poultry litter. • Adding Ni NPs (12 mg/L) increased CH 4 production by 38.4% (368 mL CH 4 /g VS). • Fe, Co and Fe 3 O 4 NPs added individually also increased CH 4 with no effect on H 2 S. • Higher concentrations of NP mixtures greatly reduced H 2 S with no effect on CH 4. • A method for extraction of NPs from digestate for SEM detection was developed. Abstract The effects of metal nanoparticle (NP) addition during anaerobic digestion (AD) of poultry litter was tested using two sequential experiments: Exp. A) four NPs (Fe, Ni, Co, and Fe 3 O 4) at three concentrations; and Exp. B) NP combinations (Fe, Ni, and Co) at four concentrations. Scanning electronic microscopy (SEM) and elemental analysis were used to confirm NP inclusion after dispersion (before AD) and track nanoparticles post-AD, and new technique for NP extraction post-AD was developed. Before AD, NPs ranged from 30.0 to 80.9 nm for Fe, Ni, and Co, and 94.3 to 400 nm for Fe 3 O 4. Methane production increased with NPs addition compared to poultry litter-only, with the highest increases observed with NPs concentrations (in mg/L) of 12 Ni (38.4% increase), 5.4 Co (29.7% increase), 100 Fe (29.1% increase), and 15 Fe 3 O 4 (27.5% increase). Nanoparticle mixtures greatly decreased H 2 S production. The SEM post-AD detected Fe, Ni, and Fe 3 O 4 at concentrations ≥100 mg/L. [ABSTRACT FROM AUTHOR]

This study determined the effect of incorporating a microbial electrolysis cell (MEC) with an anaerobic digester (AD) in a single chamber. The study evaluated three treatments: a combined AD-MEC operated for 23 days (AD-MEC-23); a combined AD-MEC operated with the MEC running for 5 days followed by no MEC for the subsequent 18 days (AD-MEC-5); and an AD operated for 23 days (AD-only). Food waste was the digestion substrate at an inoculum to substrate ratio of 1:1 (VS basis). Cumulative methane and hydrogen during the batch test in AD-MEC-23 (9.4 L CH4 , 3.39 L H2 , 2.8 L CO2 ) was higher than AD-MEC-5 (7.6 L CH4 , 2.2 L H2 , 4.6 L CO2 ), and AD-only (7.4 L CH4 , 0.2 L H2 , 5.8 L CO2 ). The results also showed that using the MEC continuously inside the digester (AD-MEC treatment) reduced CO 2 concentration to approximately 4% at the end of the experimental period, thereby, increasing the useful gases (CH 4 and H 2 ) concentrations to a maximum of 95.8%, with an average of 71.9% CH 4 , 17.4% H 2 and 10.7% CO 2 over the 23-day digestion period in the AD-MEC-23 reactor. Additionally, the COD removal in AD-MEC-23 was 12% higher than AD-only. The volumetric current was 108.7 A/m 3 based on MEC volume and 17.3 A/m 3 based on total AD-MEC volume, while the current density was 1.35 A/m 2 (cathode surface area). The total energy produced from the AD-MEC-23 (414 kJ) was higher than AD-MEC-5 (325 kJ) and AD-only (295 kJ), with an increase of energy production of over 400% output energy compared to the input energy to power the MEC. The results showed that the novel MEC design incorporated into an AD reactor increased the biogas quality, overall energy production, and waste treatment. [ABSTRACT FROM AUTHOR]

The current study examines the impact of using solar energy, specifically the use of a solar greenhouse to surround the digester and a second inner greenhouse to heat the digester inlet (System A), by simulation and validation for warming biogas digester. The simulation study compares the heat balances of System A and System B, which had no heating resource. Both systems were assumed to be located in Xianyang, Shaanxi province, northwest China. The simulation and modeling were undertaken using EPBSS, OpenStudio, and SketchUp. The simulation had the following results: in System A, the monthly average temperature of the process during the winter was maintained high enough (≥9.8 °C) to support biogas production for the whole year, in contrary to System B, which had a temperature of <8 °C for 5 months of the year. The results also show that System A could only be viable in regions where the average temperature during the winter is above −5 °C. The validation study was conducted on an actual digester with the same model and location as System A, and the results were within the range of results from simulation results (≤0.34 °C observed difference). Consequently, this model could be used for predicting the geographic locations where System A’s design could be used. Economic analysis of System A showed that the installation of the two greenhouses could increase revenue from biogas by US$1,290 per year, which could cover the costs of the greenhouses within 14 months of operation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Conductive NPs potential to improve AD performance was critically discussed. • Impact of different NPs and sizes on AD biogas quantity and quality are elucidated. • The interactions of AD microorganisms and DIET with the aid of NPs were reviewed. • The role of NPs in biochemical pathways direction and metabolites utilization are covered. Anaerobic digestion (AD) is a biochemical process that converts waste organic matter into energy-rich biogas with methane as the main component. Addition of electric electro-conductive, such as that nanoparticles (NP), has been shown to improve biogas generation. Interspecies electron transfer and direct interspecies electron transfer (DIET) using conductive materials is one of the mechanisms responsible for observed increases in CH 4. This article discusses the effect of the type and size of electro-conductive NPs on improving microbial degradation within AD systems, as well as the effect of electro-conductive NPs on microbial community shifts and syntrophic metabolism. Limitations and future perspectives of using NPs in an AD system is also discussed. [ABSTRACT FROM AUTHOR]

• Tracked nanoparticle (NP) use, degradation, and uptake in an AD-hydroponics system. • NP use increased CH 4 production by 23.7% during anaerobic digestion (AD). • NP addition temporarily ceased H 2 S production, with an average reduction of 56.3%. • The AD treated with NPs had 1,160–19,400% higher NP concentrations in the effluent. • Harvested lettuce had 21.0–1,920% more NP uptake, yet, well below toxic thresholds. Nanoparticle (NP) use can increase biological activity and adversely impact the environment. This study was the first to quantify biogas increases with NP mixtures during continuous anaerobic digestion (AD) of poultry litter and NP uptake in crops through tracking: 1) CH 4 and H 2 S production from a NP mixture (Fe, Ni, and Co) in 30 L continuous digester (AD1) for 278 days compared to a control digester (AD2) without NP addition, 2) NP degradation during digestion, 3) using AD effluent with and without NP addition as a fertilizer, and 4) plant uptake of NPs. With NP inclusion, CH 4 production increased by 23.7%, and H 2 S was reduced by 56.3%. The AD1 effluent had 1,160–19,400% higher NP concentrations and the lettuce biomass had 21.0–1,920% more NPs than lettuce fertilized with the AD2 effluent. This study showed that the effects of NPs remaining in the AD effluent must be considered. [ABSTRACT FROM AUTHOR]

Currently, there are challenges with proper disposal of cassava processing wastewater, and a need for sustainable energy in the cassava industry. This study investigated the impact of co-digestion of cassava wastewater (CW) with livestock manure (poultry litter (PL) and dairy manure (DM)), and porous adsorbents (biochar (B-Char) and zeolite (ZEO)) on energy production and treatment efficiency. Batch anaerobic digestion experiments were conducted, with 16 treatments of CW combined with manure and/or porous adsorbents using triplicate reactors for 48 days. The results showed that CW combined with ZEO (3 g/g total solids (TS)) produced the highest cumulative CH4 (653 mL CH4/g VS), while CW:PL (1:1) produced the most CH4 on a mass basis (17.9 mL CH4/g substrate). The largest reduction in lag phase was observed in the mixture containing CW (1:1), PL (1:1), and B-Char (3 g/g TS), yielding 400 mL CH4/g volatile solids (VS) after 15 days of digestion, which was 84.8% of the total cumulative CH4 from the 48-day trial. Co-digesting CW with ZEO, B-Char, or PL provided the necessary buffer needed for digestion of CW, which improved the process stability and resulted in a significant reduction in chemical oxygen demand (COD). Co-digestion could provide a sustainable strategy for treating and valorizing CW. Scale-up calculations showed that a CW input of 1000–2000 L/d co-digested with PL (1:1) could produce 9403 m3 CH4/yr using a 50 m3 digester, equivalent to 373,327 MJ/yr or 24.9 tons of firewood/year. This system would have a profit of $5642/yr and a $47,805 net present value. [ABSTRACT FROM AUTHOR]

[Display omitted] • FNA pretreatment can significantly improve the solubilization of organic waste. • pH change within the system can adversely impact the FNA pretreatment performance. • Effective FNA concentration may decrease due to NO 2 – consumption by denitrifiers. • FNA-pretreated organic waste does not always show a higher biogas production in AD. • FNA at 1.0–5.2 mg FNA-N/L effectively controlled H 2 S-odor in the organic wastes. This work investigated the effectiveness of free nitrous acid (FNA) in enhancing organic waste solubilization to improve biogas production in anaerobic digestion (AD). The results indicated that FNA pretreatment can enhance soluble organic content and control H 2 S odor in tested organic wastes, including food waste, sewage sludge, and their combination. However, a significant decrease (>50 %) in FNA concentration was found in the reactors, possibly due to denitrifier-driven NO 2 – consumption. Biochemical methane potential (BMP) tests showed a 25 ± 8 % enhancement in CH 4 production in the reactors fed with mixed substrate pretreated with 2.9 mg FNA-N/L. However, the presence of NO 2 – (325.6–2368.0 mg N/L) in some BMP reactors, due to carryover from FNA pretreatment, adversely affected CH 4 production (>55 %) and prolonged lag time (>4.2 times). These findings are valuable for researchers and practitioners in waste management, offering insights for implementing FNA pretreatment to enhance the biodegradability of organic wastes in AD. [ABSTRACT FROM AUTHOR]

Biological desulfurization of biogas from a field-scale anaerobic digester in Peru was tested using air injection (microaeration) in separate duplicate vessels and chemical desulfurization using duplicate iron filters to compare hydrogen sulfide (H2S) reduction, feasibility, and cost. Microaeration was tested after biogas retention times of 2 and 4 h after a single injection of ambient air at 2 L/min. The microaeration vessels contained digester sludge to seed sulfur-oxidizing bacteria and facilitate H2S removal. The average H2S removal efficiency using iron filters was 32.91%, with a maximum of 70.21%. The average H2S removal efficiency by iron filters was significantly lower than microaeration after 2 and 4 h retention times (91.5% and 99.8%, respectively). The longer retention time (4 h) resulted in a higher average removal efficiency (99.8%) compared to 2 h (91.5%). The sulfur concentration in the microaeration treatment vessel was 493% higher after 50 days of treatments, indicating that the bacterial community present in the liquid phase of the vessels effectively sequestered the sulfur compounds from the biogas. The H2S removal cost for microaeration (2 h: $29/m3 H2S removed; and 4 h: $27/m3 H2S removed) was an order of magnitude lower than for the iron filter ($382/m3 H2S removed). In the small-scale anaerobic digestion system in Peru, microaeration was more efficient and cost effective for desulfurizing the biogas than the use of iron filters. [ABSTRACT FROM AUTHOR]