Recent advances in biological technologies for anoxic biogas desulfurization

Anoxic desulfurization of biogas rich in hydrogen sulfide through feedback control using biotrickling filters: Operational limits and multi-omics analysis

Biodesulfurization is crucial for sustainable biogas purification from hydrogen sulfide (H2S). This study investigates the operational limits of anoxic biotrickling filters (BTFs) for treating biogas with high H2S concentrations (up to 20,000 ppmv) using nitrite, along with simulated interruptions in H2S supply. The BTF achieved a maximum elimination capacity of 312 g S-H2S m-3 h-1 with an H2S removal efficiency of 98 % at an empty bed residence time of 284 s. A proportional-integral-derivative (PID) feedback control system was successfully employed to maintain an H2S outlet concentration close to the requisite setpoint (100 and 500 ppmv) by adjusting the nitrite flow rate, thereby minimizing its accumulation. Continuous nitrite feeding after interruptions in H2S supply was essential to avoid H2S release due to sulfate-reducing bacteria. Multi-omics analyses, combining metagenomics and proteomics, revealed Sulfurimonas as the dominant sulfur-oxidizing bacteria, which downregulates most enzyme genes involved in nitrogen and sulfur metabolism in response to substrate starvation. These findings underscore the resilience of BTFs under extreme conditions and the value of multi-omics approaches in understanding microbial population dynamics, positioning BTFs as a robust solution for large-scale biogas purification.

Assessment of aerobic-anoxic biotrickling filtration for the desulfurization of high-strength H2S streams from sugarcane vinasse fermentation

The increasing demand for renewable energy has heightened interest in biogas production from agro-industrial residues, such as sugarcane vinasse-a byproduct of ethanol production. During vinasse fermentation, sulfate reduction generates biogas with high hydrogen sulfide (H2S) concentrations, reaching up to 50,000 ppmv. This study assessed the performance of two bench-scale biotrickling filters (BTFs) treating synthetic sulfide-rich acidogenic off-gas (7000 ppmv) from mesophilic sugarcane vinasse fermentation. The systems were packed with materials of high (950 m2 m-3, BTFH) and low (460 m2 m-3, BTFL) specific surface areas and inoculated with sulfur-oxidizing bacteria (SOB). Operational conditions included decreasing empty bed residence times (EBRTs) of 9, 6, and 4 min and nitrate-to-sulfur ratios of 0.1, 0.3, and 0.5, respectively. Both BTFs achieved complete H2S removal at the shortest EBRT, with elimination capacities (ECs) exceeding 140 g S-H2S m-3 h-1. However, BTFH exhibited reduced EC at higher H2S loads due to elemental sulfur (S⁰) accumulation, resulting in clogging, pH instability, and diminished denitrification activity. Despite these challenges, the system demonstrated resilience by restoring nitrate reduction and H2S oxidation. This study underscores the efficacy of hybrid aerobic-anoxic BTFs for treating H2S-rich biogas and highlights the critical role of packing material selection and nitrogen-to-sulfur ratio control for long-term operational stability.

Bio-electrochemically assisted sulfide, phosphorus, and nitrogen remediation in continuous anaerobic digestion of dairy manure with improved biogas production

Anaerobic digestion (AD) is an industrial practice to properly manage and valorize dairy manure, whereas impurities in biogas and excessive nutrients in digestate always require post-treatment. In this study, integration of bio-electrochemical (BEC) treatment with AD of dairy manure was proposed to simultaneously improve biogas production, reduce hydrogen sulfide (H2S) release, and remediate nutrients in digestate. A continuous stirred tank reactor (CSTR) and a BEC unit using stainless steel mesh electrodes at applied voltages of 0.5-0.8 V were integrated for continuous AD treatment of liquid dairy manure. At a relatively short hydraulic retention time of 20 d and a high voltage of 0.8 V, the biogas production of CSTR-BEC significantly outperformed that of the control operated in an open circuit mode. The methane (CH4) content in the biogas from CSTR-BEC at 0.8 V reached 71.1%, leading to a specific CH4 yield of CSTR-BEC (238.6 mL/gVS) higher by 42.5% than that of the control. The higher applied voltage of 0.8 V in CSTR-BEC also secured significant aqueous sulfide and gaseous H2S removals of 58.6% and 89%, respectively. Meanwhile, stronger electrochemical reactions in CSTR-BEC resulted in efficient removals of soluble and total phosphorus from dairy manure at a range of 49.5-63.7%. The compositional analysis of cathode precipitates implies that the release of iron ions from the sacrificial anode for further precipitation and adsorption might be the main route for sulfide and phosphorus removal. The average power consumption of the BEC unit (1.024 kWh/m3/d) at 0.8 V was 7.9-fold that at 0.5 V, whereas the net energy gain of CSTR-BEC (7.42 MJ/m3/d) was still comparable to that of the control because of the improved CH4 production. This bio-electrochemically assisted AD system offers a promising perspective in cleaner bioenergy production with concurrent considerable contaminants remediation from dairy manure.

Pilot-scale biogas desulfurization through anoxic biofiltration

In this study, the performance of a pilot-scale biotrickling filter (BTF) for anoxic hydrogen sulfide (H2S) removal from real biogas was evaluated over 226 days. The BTF, inoculated with activated sludge from a nearby wastewater treatment plant, operated in an industrial environment with raw biogas from an anaerobic digester fed with municipal solid waste. The operating strategy was based on controlling nitrate consumption by sulfur-oxidizing nitrate-reducing (SO-NR) bacteria. H2S-inlet loads (H2S-IL) ranged from 0.43 to 61.73 g S-H2S m³ h⁻¹ , with biogas flow rates between 0.25 and 3.5 m³ h⁻¹ and H2S concentration peaks of 11,000 ppmv. The pH was maintained at 7.0 using a 25 %v·v⁻¹ NaOH solution. The H2S removal efficiency (H2S-RE) exceeded 93.9 % for most of the period, with a maximum elimination capacity of 35.17 g S-H2S m⁻³ h⁻¹ at an IL of 37.20 g S-H2S m⁻³ h⁻¹ (H2S-RE > 95 %). Sulfate selectivity was over 85 % during normal operation. High H2S-IL led to excessive elemental sulfur accumulation without nitrate, which was later oxidized to sulfate during low H2S-IL periods, preventing maintenance shutdowns. Overall, the results demonstrate the efficacy of anoxic BTFs in removing H2S under variable biogas conditions at a pilot scale.

Harnessing the potential of the microbial sulfur cycle for environmental biotechnology

The sulfur cycle is a complex biogeochemical cycle characterized by the high variability in the oxidation states of sulfur. While sulfur is essential for life processes, certain sulfur compounds, such as hydrogen sulfide, are toxic to all life forms. Micro-organisms facilitate the sulfur cycle, playing a prominent role even in extreme environments, such as soda lakes, acid mine drainage sites, hot springs, and other harsh habitats. The activity of these micro-organisms presents unique opportunities for mitigating sulfur-based pollution and enhancing the recovery of sulfur and metals. This review highlights the application of sulfur-oxidizing and -reducing micro-organisms in environmental biotechnology through three illustrative examples. Additionally, it discusses the challenges, recent trends, and prospects associated with these applications.

Elemental sulfur biorecovery from phosphogypsum using oxygen-membrane biofilm reactor: Bioreactor parameters optimization, metagenomic analysis and metabolic prediction of the biofilm activity

This work investigated elemental sulfur (S0) biorecovery from Phosphogypsum (PG) using sulfur-oxidizing bacteria in an O2-based membrane biofilm reactor (MBfR). The system was first optimized using synthetic sulfide medium (SSM) as influent, then switched to biogenic sulfide medium (BSM) generated by biological reduction of PG alkaline leachate. The results using SSM had high sulfide-oxidation efficiency (98 %), sulfide to S0 conversion (∼90 %), and S0 production rate up to 2.7 g S0/(m2.d), when the O2/S ratio was ∼0.5 g O2/g S. With the BSM influent, the system maintained high sulfide-to-S0 conversion rate (97 %), and S0-production rate of 1.6 g S0/(m2.d). Metagenomic analysis revealed that Thauera was the dominant genus in SSM and BSM biofilms. Furthermore, influent composition affected the bacterial community structure and abundances of functional microbial sulfur genes, modifying the sulfur-transformation pathways in the biofilms. Overall, this work shows promise for O2-MBfR usage in S0 biorecovery from PG-leachate and other sulfidogenic effluents.

Biodesulfurization of landfill biogas by a pilot-scale bioscrubber: Operational limits and microbial analysis

Biogas serves as a crucial renewable energy vector to ensure a more sustainable energy future. However, the presence of hydrogen sulfide (H2S) limits its application in various sectors, emphasizing the importance of effective H2S removal techniques for maximizing its potential. In the present study, the limits of a pilot-scale bioscrubber for biogas desulfurization was study in a real scenario. An increase in the superficial liquid velocity resulted in significant improvements in the H2S removal efficiency, increasing from 76 ± 8% (elimination capacity of 6.2 ± 0.5 gS-H2S m-3 h-1) to 97.7 ± 0.5% (elimination capacity of 8 ± 1 gS-H2S m-3 h-1) as the superficial liquid velocity increased from 50 ± 3 m h-1 to 200 ± 8 m h-1. A USL of 161.4 ± 0.5 m h-1 was able to achieve outlet H2S concentrations as low as 3 ± 1 ppmv (H2S removal efficiency of 97 ± 1%) for 7 days. High superficial liquid velocity favoured the aerobic H2S oxidation reducing the nitrate demand. The maximum EC reached throughout the operation was 50.8 ± 0.6 gS-H2S m-3 h-1 (H2S removal efficiency of 96 ± 1%) and a sulfur production of 60%. Studies in batch flocculation experiments showed sulfur removal rates up to 97.6 ± 0.9% with a cationic flocculant dose of 75 mg L-1. Microbial analysis revealed that the predominant genus with sulfo-oxidant capacity during periods of low H2S inlet load was Thioalkalispira-sulfurivermis (61-69%), while in periods of higher H2S inlet load, family Arcobacteraceae was the most prevalent (11%).

Sustainable Recovery of Platinum Group Metals from Spent Automotive Three-Way Catalysts through a Biogenic Thiosulfate-Copper-Ammonia System

This study explores an eco-friendly method for recovering platinum group metals from a synthetic automotive three-way catalyst (TWC). Bioleaching of palladium (Pd) using the thiosulfate-copper-ammonia leaching processes, with biogenic thiosulfate sourced from a bioreactor used for biogas biodesulfurization, is proposed as a sustainable alternative to conventional methods. Biogenic thiosulfate production was optimized in a gas-lift bioreactor by studying the pH (8-10) and operation modes (batch and continuous) under anoxic and microaerobic conditions for 35 d. The maximum concentration of 4.9 g S2O32- L-1 of biogenic thiosulfate was reached under optimal conditions (batch mode, pH = 10, and airflow rate 0.033 vvm). To optimize Pd bioleaching from a ground TWC, screening through a Plackett-Burman design determined that oxygen and temperature significantly affected the leaching yield negatively and positively, respectively. Based on these results, an optimization through an experimental design was performed, indicating the optimal conditions to be Na2S2O3 1.2 M, CuSO4 0.03 M, (NH4)2SO4 1.5 M, Na2SO3 0.2 M, pH 8, and 60 °C. A remarkable 96.2 and 93.2% of the total Pd was successfully extracted from the solid at 5% pulp density using both commercially available and biogenic thiosulfate, highlighting the method's versatility for Pd bioleaching from both thiosulfate sources.