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Although there are a variety of commercially available biological and chemical treatments for removal of hydrogen sulfide (H2S) from biogas, managing biogas H2S remains a significant challenge for agricultural digesters where labor and operational funds are very limited compared to municipal and industrial digesters. The objectives of this study were to evaluate headspace aeration for reducing H2S levels in low cost plug flow digesters and to characterize the relationship between the liquid surface area and H2S oxidation rates. Experiments with replicate field scale plug flow digesters showed that H2S levels decreased from 3500 ppmv to <100 ppmv when headspace oxygen levels were 0.5 to 1%. Methane production was not affected by aeration rates that resulted in headspace oxygen levels of up to 1%. Pilot scale experiments using 65 to 104 L desulfurization units showed that H2S oxidation rates increased with increases in liquid surface area. These results support the hypothesis that H2S oxidation rates are limited, in part, by the surface area available for oxygen transfer, and can be increased by growth of biofilms containing H2S oxidizing bacteria. Maximum removal rates corresponded to 40 to 100 g S m-2 d-1 of liquid surface area at biogas retention times of 30 to 40 min.

BACKGROUND Poultry litter (PL) has a high solid and organic nitrogen content that is converted into NH3 during anaerobic digestion (AD), requiring the dilution of PL influent prior to AD. With nutrients extracted from the AD effluent, the water could be reused for influent dilution or for other purposes, while creating a designer nutrient product from the PL matrix. The study goal was to determine the effectiveness of an ammonia (NH3)‐stripping and scrubbing system to remove and capture NH3 from anaerobically digested PL. After NH3 stripping, a novel, patent‐pending dual‐vessel NH3‐scrubber system was designed to simultaneously capture the stripped NH3 while producing higher quality biogas through partial removal of non‐combustible carbon dioxide (CO2) and corrosive hydrogen sulfide (H2S). RESULTS: The stripping reactor reduced total ammonia nitrogen (TAN) by 84.6–90.4% when loaded with an ammonium chloride solution or screened PL digestate after 4 h, while the scrubber captured 73.3–76.2% of the stripped NH3 when proxy biogas streams (59–60% CH4, 40% CO2) with two H2S concentrations (0.1 or 1% H2S) were used. The scrubber reduced CO2 and H2S concentrations in the biogas by 3.9% and 23–29%, respectively. CONCLUSION: The study demonstrated a novel NH3 stripping‐and‐scrubbing system that successfully reduced TAN in anaerobic digestate, captured the stripped NH3, and reduced CO2 and H2S concentrations in biogas. The novel design can be used to increase the adoption of AD for PL by providing a useful NH3 fertilizer, wastewater with less NH3, and a biogas stream with less H2S. © 2020 Society of Chemical Industry (SCI) [ABSTRACT FROM AUTHOR]

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]

Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm 3 ) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH 4 and H 2 S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH 4 potential than dairy manure on a volatile solids basis. While no synergistic effect on CH 4 production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH 4 production. Initial H 2 S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H 2 S concentrations (0.10–0.14%) and higher CH 4 content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH 4 yield (372 ± 12 L kg −1 VS). The co-digestion mixture containing 40% radish had a lower specific CH 4 yield (345 ± 2 L kg −1 VS) but also showed significantly less H 2 S production at start-up and high quality biogas (58% CH 4 ). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH 4 production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH 4 production of manure digesters in the fall/winter. [ABSTRACT FROM AUTHOR]

Pilot-scale digesters (850 L) were used to quantify CH 4 and H 2 S production when using forage radish cover crops as a co-digestion feedstock in dairy manure-based digesters. During two trials, triplicate mixed digesters were operated in batch mode with manure-only or radish + manure (27% and 13% radish by wet weight in Trial 1 and 2, respectively). Co-digestion increased CH 4 production by 11% and 39% in Trial 1 and 2, respectively. As H 2 S production rapidly declined in the radish + manure digesters, CH 4 production increased reaching high levels of CH 4 (⩾67%) in the biogas. Over time, radish co-digestion lowered the H 2 S concentration in the biogas (0.20%) beyond that of manure-only digestion (0.34–0.40%), although cumulative H 2 S production in the radish + manure digesters was higher than manure-only. Extrapolated to a farm-scale (200 cows) continuous mixed digester, co-digesting with radish could generate 3150 m 3 CH 4 /month, providing a farmer additional revenue up to $3125/month in electricity sales. [ABSTRACT FROM AUTHOR]

Hydrogen sulfide (H 2 S) in biogas is produced during anaerobic digestion of organic wastes and can lead to corrosion of generators used for energy production. Recently, there has been interest in utilizing biochar as a substitute for activated carbon (AC) to adsorb and remove H 2 S from biogas. This study was the first to test iron (Fe)-impregnation of biochar to increase H 2 S removal from biogas using a biogas scrubbing column. Two biochar substrates, corn stover biochar (CSB) and maple wood biochar (MB), with and without Fe-impregnation were evaluated for H 2 S adsorption from biogas. The H 2 S adsorption capacity was quantified using dynamic breakthrough experiments. Iron-impregnated maple biochar (MB-Fe) had the highest H 2 S saturation adsorption capacity (23.9 mg H 2 S g−1 biochar), which was 3.9 times higher than the unmodified MB (6.1 mg H 2 S g−1 biochar). The Fe-impregnated corn stover biochar (CSB-Fe) saturation H 2 S sorption capacity (8.2 mg H 2 S g−1 biochar) was 2.5 times higher than the unmodified CSB (3.3 mg H 2 S g−1 biochar). The presence of iron oxide (Fe 3 O 4) was observed through X-ray diffraction and scanning electron microscopy in the Fe-impregnated biochar substrates, with ferrous sulfate (FeSO 4) as one of the end products in the saturated biochar surface and elemental sulfur in the pores. The study showed how Fe-impregnation can result in a multifold increase in H 2 S adsorption from biogas compared to unmodified biochar. ga1 • Iron-impregnation resulted in up to 3.9 times increase in H 2 S adsorption capacity. • Adsorption capacity of unmodified MB was 2.5 times higher than CSB. • H 2 S adsorption capacity of unmodified biochar was comparable to activated carbon. • Iron impregnation increased the Fe content of biochar from 1100 ppm to 29,700 ppm. • Fe 3 O 4 in the impregnated biochar was converted to FeSO 4 after reaction with H 2 S. [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]

• Unmodified biochar added to a digester resulted in 90.5% H 2 S removal efficiency. • The biochar particle size did not significantly affect the H 2 S removal efficiency. • Iron-impregnated biochar resulted in complete removal of H 2 S from the biogas. • Activated carbon was not effective when used as an in-situ desulfurization agent. • Biochar addition did not significantly affect the methane yield during digestion. Corn stover biochar (CSB) and maple biochar (MB) were added into anaerobic digesters and evaluated for hydrogen sulfide (H 2 S) reductions. This was the first study to show Fe-impregnated biochar can eliminate H 2 S production. The novel study evaluated biochar addition on H 2 S reduction and nutrient concentrations using three experiments to test the effect of: 1) biochar concentration, 2) biochar particle size, and 3) Fe-impregnated biochar using triplicate lab-scale reactors. At the highest biochar dose (1.82 g biochar/g manure TS), H 2 S production was 90.5% less than the control treatment (351 mL H 2 S/kg VS). Biochar particle size did not significantly affect H 2 S concentration. The Fe-impregnated biochar (0.5 g biochar/g manure TS) reactors had no H 2 S detected in the CSB-Fe system. Methane (CH 4) in the biochar and control treatments were not significantly different in all three experiments. The results show that biochar added to digesters can significantly reduce H 2 S production without affecting CH 4 production. [ABSTRACT FROM AUTHOR]

Hydrogen sulfide (H2S) is a corrosive trace gas present in biogas produced from anaerobic digestion systems that should be removed to reduce engine-generator set maintenance costs. This study was conducted to provide a more complete understanding of two H2S scrubbers in terms of efficiency, operational and maintenance parameters, capital and operational costs, and the effect of scrubber management on sustained H2S reduction potential. For this work, biogas H2S, CO2, O2, and CH4 concentrations were quantified for two existing H2S scrubbing systems (iron-oxide scrubber, and biological oxidation using air injection) located on two rural dairy farms. In the micro-aerated digester, the variability in biogas H2S concentration (average: 1938 ± 65 ppm) correlated with the O2 concentration (average: 0.030 ± 0.004%). For the iron-oxide scrubber, there was no significant difference in the H2S concentrations in the pre-scrubbed (450 ± 42 ppm) and post-scrubbed (430 ± 41 ppm) biogas due to the use of scrap iron and steel wool instead of proprietary iron oxide-based adsorbents often used for biogas desulfurization. Even though the capital and operating costs for the two scrubbing systems were low (<$1500/year), the lack of dedicated operators led to inefficient performance for the two scrubbing systems. [ABSTRACT FROM AUTHOR]

Co-digestion of dairy manure with waste organic substrates has been shown to increase the methane (CH4) yield of farm-scale anaerobic digestion (AD). A gummy vitamin waste (GVW) product was evaluated as an AD co-digestion substrate using batch AD testing. The GVW product was added at four inclusion levels (0%, 5%, 9%, and 23% on a wet mass basis) to a co-digestion substrate mixture of dairy manure (DM), food-waste (FW), and grease-waste (GW) and compared to mono-digestion of the GVW, DM, FW, and GW substrates. All GVW co-digestion treatments significantly increased CH4 yield by 126–151% (336–374 mL CH4/g volatile solids (VS)) compared to DM-only treatment (149 mL CH4/g VS). The GVW co-digestion treatments also significantly decreased the hydrogen sulfide (H2S) content in the biogas by 66–83% (35.1–71.9 mL H2S/kg VS) compared to DM-only (212 mL H2S/kg VS) due to the low sulfur (S) content in GVW waste. The study showed that GVW is a potentially valuable co-digestion substrate for dairy manure. The high density of VS and low moisture and S content of GVW resulted in higher CH4 yields and lower H2S concentrations, which could be economically beneficial for dairy farmers. [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]