Anoxic biogas biodesulfurization promoting elemental sulfur production in a Continuous Stirred Tank Bioreactor

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.

Recent advances in biological technologies for anoxic biogas desulfurization

Recovery of the energy contained in biogas will be essential in coming years to reduce greenhouse gas emissions and our current dependence on fossil fuels. The elimination of H₂S is a priority to avoid equipment corrosion, poisoning of catalytic systems and SO₂ emissions in combustion engines. This review describes the advances made in this technology using fixed biomass bioreactors (FBB) and suspended growth bioreactors (SGB) since the first studies in this field in 2008. Anoxic desulfurization has been studied mainly in biotrickling filters (BTF). Elimination capacities (EC) up to 287 gS m^-3 h^-1 have been achieved, with a removal efficiency (RE) of 99%. Both nitrate and nitrite have been successfully used as electron acceptor. SGBs can solve some operational problems present in FBBs, such as clogging or nutrient distribution issues. However, they present greater difficulties in gas-liquid mass transfer, although ECs of up to 194 gS m^-3 h^-1 have been reported in both gas-lift and stirred tank reactors. One of the major disadvantages of using anoxic biodesulfurization compared to aerobic biodesulfurization is the need to provide reagents (nitrates and/or nitrites), with the consequent increase in operating costs. A solution proposed in this respect is the use of nitrified effluents, some ammonium-rich effluents nitrified include landfill leachate and digested effluent from the anaerobic digester have been tested successfully. Among the microbial diversity found in the bioreactors, the genera Thiobacillus, Sulfurimonas and Sedimenticola play a key role in anoxic removal of H₂S. Finally, a summary of future trends in technology is provided.

Experimental and Theoretical Density Functional Theory Approaches for Desulfurization of Dibenzothiophene from Diesel Fuel with Imidazole-Based Heteropolyacid Catalysts

Oxidative desulfurization (ODS) has been proved to be an efficient strategy for the removal of aromatic sulfur compounds from diesel oils, which are one of the main sources of air pollution. Heteropolyacid catalysts are highly active species for ODS, but the promotion of their catalytic activity and clarification of their catalytic mechanism remain an important issue. Herein, a series of novel imidazole-based heteropolyacid catalysts are prepared by a one-pot method for multiphase deep ODS of fuel with hydrogen peroxide as an oxidant. The experimental results show that the desulfurization performance of the prepared imidazole-based heteropolyacid catalysts is high up to 99.9% under mild conditions. The catalyst also possesses excellent recovery performance, and the desulfurization activity remains at 97.7% after being recycled seven times. Furthermore, density functional theory calculation is first employed to clarify the origin of the high desulfurization activity, and the results show that with the imidazole-based heteropolyacid (HPW-VIM) as the catalyst, the energy barrier is much lower than that with phosphotungstic acid (HPW) as the catalyst.

Resilience of hollow fibre membrane bioreactors for treating H2S under steady state and transient conditions

H₂S removal performance by hollow fibre membrane bioreactors (HFMBs) was investigated for 271 days at ambient (20 ± 2 °C) temperature employing an inlet H₂S concentrations up to 3600 ppm_v and empty bed residence time (EBRT) of 187, 92 and 62 s. Different operating conditions including pH control (with or without), famine period, shock loads (4-72 h) and different biomass types (presence or absence of suspended biomass) were investigated. The H₂S flux and mass-transfer coefficient were significantly higher for the biotic HFMBs (R₁ and R₂) compared to the abiotic control (R₃) at all employed EBRTs. Significant differences in H₂S removal efficiency (RE) and elimination capacity (EC) were noted for different inlet H₂S concentrations, EBRTs, pH and biomass type. The HFMB achieved >99% RE at steady-state for biotic operation with an EC of 33.8, 30.0 and 30.9 g m^-3 h^-1 at an EBRT of 187, 92 and 62 s, respectively. Sulfate (92-93%) was the main sulfur species in the H₂S bioconversion process. The HFMB showed a good resilience to shock loads and showed quick recovery (<24 h) after withdrawal of the shock loads. The HFMB had a critical loading rate of H₂S about 135 g m^-3 h^-1 under transient-state.

Anoxic desulphurisation of biogas from full-scale anaerobic digesters in suspended biomass bioreactors valorising previously nitrified digestate centrate

Nitrification of centrate from anaerobic sewage sludge digestion presents a major opportunity as an electron acceptor in anoxic biogas biodesulphurisation. Nitritation and nitrification inhibition by free ammonia was detected at laboratory scale, but was avoided during the scale-up operation in a 4 m³ reactor treating ammonium loads up to 19 gN m^-3 h^-1. This nitrate-rich stream was fed to two pilot-scale suspended biomass bioreactors (SBBs) treating real biogas for 220 days. After an adaptation period of 21 days, nitrate and alkalinity concentrations in the liquid medium below 10 mgN L^-1 and 100 mgCaCO₃ L^-1 were found to limit hydrogen sulphide (H₂S) oxidation. Once controlled, 95% of the H₂S was removed in SBB1 and 90% in SBB2, at a gas residence time (GRT) of 5.9 min, treating average values of 321 ± 205 ppmv and 457 ± 205 ppmv, respectively. Outlet H₂S concentrations of 16 ± 24 ppmv in SBB1 and 46 ± 39 ppmv in SBB2 were obtained, which are below the requirements of biogas combustion heat and power engines. Unlike H₂S, siloxanes were not removed with these GRTs. The results demonstrate the feasibility of the combined process for H₂S treatment, potential valorisation of precipitated elemental sulphur and a reduction in the reagents currently used to control H₂S.

Biodesulfurization of thiosulfate by a Pseudomonas strain PSP1 and the investigation of underlying metabolic mechanisms

The increasing public expectations for good air quality have necessitated the development of biodeodorization technology. Among different malodorous pollutants, H₂S has attracted extensive attention and diverse biodesulfurization technology has been developed for efficient H₂S removal. In this study, a novel heterotrophic Pseudomonas strain, PSP1, was isolated from biogas slurry and its biodesulfurization ability was investigated. Culture conditions of 30 °C and 200 rpm were determined as the optimal for both cell growth and thiosulfate conversion. Under such conditions, the highest OD₆₀₀ value was observed as 6.74 and 50 mM thiosulfate within 10 h. PSP1 was found to convert thiosulfate to sulfane which could be decomposed into elemental sulfur and therefore achieved desulfurization. However, this process was relatively weak as more than 60% thiosulfate was converted into soluble tetrathionate. The whole genome sequencing and functional annotation identified the genes in PSP1 associated with sulfur metabolism and the RT-qPCR analysis quantified the expression level of corresponding genes. The expression level of tsdA, which was responsible for tetrathionate production, was around 60 folds higher than genes for sulfane production (e.g. TST1, cysI). Further research would focus on the enhancement of sulfane production pathway through metabolic engineering tools.

A waste to energy technology for Enrichment of biomethane generation: A review on operating parameters, types of biodigesters, solar assisted heating systems, socio economic benefits and challenges

Anaerobic Digestion (AD) is one of the promising wastestoenergy (WtE) technologies that convert organic wastes to useful gaseous fuel (biogas). In this process methane is produced in the presence of methanogens (bacteria). The survival and activities of methanogens are based on several parameters such as pH, temperature, organic loading rate, types of biodigester. Moreover, these parameters influence the production of biogas in terms of yield and composition. Maintaining an appropriate temperaturefor AD is highly critical and energy intensive. This study reviews the various hybrid technologies assistedbio gas production schemes particularly from renewable energy sources. Also discuss the direct and indirect solar assisted bio-digester impacts and recommendation to improve its performance. In addition, the performance analysis Solar Photovoltaic (PV) and thermal collector assisted bio gas plants; besides their impact on the performance of anaerobic digesters. Since opportunities of solar energy are attractive, the effective utilization of the same is selected for the discussion. Besides, the various constraints that affect the yield and composition of biogas are also evaluated along with the current biogas technologies and the biodigesters. The environmental benefits, challenges and socio-economic factors are also discussed for the successful implementation of various technologies.

Development and validation of a sampling and analysis method to determine biogenic sulfur in a desulfurization bioreactor by gas chromatography coupled with a pulsed flame photometric detector (GC-PFPD)

Suspended biomass bioreactors can be operated to remove H₂S from biogas under anoxic conditions and produce elemental sulfur, the commercial value of which has been demonstrated. In the present paper, a novel methodology comprising the optimization of a determination method performed in a gas chromatograph equipped with a pulsed flame photometric detector (GC-PFPD), combined with a simple preparation based on filtration and extraction with toluene, is proposed. The injector temperature and carrier gas flow rate (Q_He) values were optimized using a response surface methodology based on a face-centred composite central design. This optimization revealed that the optimum conditions were an injector temperature and carrier gas flow rate of 222 °C and 7 mL min^-1, respectively. The chromatographic method shows an analysis time of 48 min, a detection limit of more than 5.9 mg L^-1, a relative standard deviation of less than 3.71%, and a sulfur recovery percentage of more than 98%. These values provide excellent linearity and a reasonable concentration range (10-200 mg L^-1). Finally, a measurement error of 4.45% was obtained when using the present method in a selectivity test.

Effect of sulfide on morphology and particle size of biologically produced elemental sulfur from industrial desulfurization reactors

We investigated the effect of polysulfide formation on properties of biologically produced elemental sulfur (S₈) crystals, which are produced during biological desulfurization (BD) of gas. The recent addition of an anoxic-sulfidic reactor (AnSuR) to the BD process resulted in agglomerated particles with better settleability for S₈ separation. In the AnSuR, polysulfides are formed by the reaction of bisulfide (HS^-) with S₈ and are subsequently oxidized to S₈ in a gas-lift reactor. Therefore, sulfur particles from the BD are shaped (i.e. morphology and particle size) both by formation and dissolution. We assessed the reaction of HS^- with S₈ particles in anoxic, abiotic experiments in a batch reactor using two S₈ samples from industrial BD reactors. Under these conditions, the sulfur particle surface became coarser and more porous, and in addition the smallest particles disappeared. Agglomerates initially fell apart but were reformed at a later stage. Moreover, we found different observed polysulfide formation rates for each S₈ sample, which was related to the initial morphology and size. Our findings show that particle properties can be controlled abiotically and that settleability of S₈ is increased by increasing both the HS^--S₈ ratio and retention time.

Novel biotechnologies for nitrogen removal and their coupling with gas emissions abatement in wastewater treatment facilities

Wastewaters contaminated with nitrogenous pollutants, derived from anthropogenic activities, have exacerbated our ecosystems sparking environmental problems, such as eutrophication and acidification of water reservoirs, emission of greenhouse gases, death of aquatic organisms, among others. Wastewater treatment facilities (WWTF) combining nitrification and denitrification, and lately partial nitrification coupled to anaerobic ammonium oxidation (anammox), have traditionally been applied for the removal of nitrogen from wastewaters. The present work provides a comprehensive review of the recent biotechnologies developed in which nitrogen-removing processes are relevant for the treatment of both wastewaters and gas emissions. These novel processes include the anammox process with alternative electron acceptors, such as sulfate (sulfammox), ferric iron (feammox), and anodes in microbial electrolysis cells (anodic anammox). New technologies that couple nitrate/nitrite reduction with the oxidation of methane, H₂S, volatile methyl siloxanes, and other volatile organic compounds are also described. The potential of these processes for (i) minimizing greenhouse gas emissions from WWTF, (ii) biogas purification, and (iii) air pollution control is critically discussed considering the factors that might trigger N₂O release during nitrate/nitrite reduction. Moreover, this review provides a discussion on the main challenges to tackle towards the consolidation of these novel biotechnologies.

Microbial interaction promotes desulfurization efficiency under high pH condition

The existence of H₂S in biogas may cause equipment corrosion and considerable SO₂ emission. Commonly used biotrickling filters may cause biogas dilution or generation of explosive mixtures. Compared with biotrickling filters, two-step process such as bioscrubber filters can overcome these shortages. However, its removal efficiency was still limited due to low microbial activity under high pH condition. Here, a bioreactor filter was carried out under pH 9.0. Removal efficiency higher than 99% was achieved under sulfide loading rate reaching 4.24 kg S m^-3d^-1. Results of network and high throughput sequencing showed that Thiobacillus acted as both dominant species (accounting for 75%) and unique kinless hub in this bioreactor. Other bacteria (accounting for 25%) contributed 75% to the network, which implied the intensive interaction between Thiobacillus and others. Sulfide removal ability and pH tolerance of pure bacteria and mixed culture were considered to verify how microbial interaction influenced them. Compared with pure bacteria, mixed culture had better performance under high pH condition, which confirmed that microbial interaction promoted desulfurization efficiency under high pH condition. These results showed that intensive microbial interaction might be the key to enhance sulfide removal efficiency under high pH condition.