Effect of S/N Ratio on the Removal of Hydrogen Sulfide from Biogas in Anoxic Bioreactors

Approaches to mitigation of hydrogen sulfide during anaerobic digestion process - A review

Anaerobic digestion (AD) is the primary technology for energy production from wet biomass under a limited oxygen supply. Various wastes rich in organic content have been renowned for enhancing the process of biogas production. However, several other intermediate unwanted products such as hydrogen sulfide, ammonia, carbon dioxide, siloxanes and halogens have been generated during the process, which tends to lower the quality and quantity of the harvested biogas. The removal of hydrogen sulfide from wastewater, a potential substrate for anaerobic digestion, using various technologies is covered in this study. It is recommended that microaeration would increase the higher removal efficiency of hydrogen sulfide based on a number of benefits for the specific method. The process is primarily accomplished by dosing smaller amounts of oxygen in the digester, which increases the system's oxidizing capacity by rendering the sulfate reducing bacteria responsible for converting sulfate ions to hydrogen sulfide inactive. This paper reviews physicochemical and biological methods that have been in place to eliminate the effects of hydrogen sulfide from wastewater treated anaerobically and future direction to remove hydrogen sulfide from biogas produced.

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.

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.

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.

Markers for the Comparison of the Performances of Anoxic Biotrickling Filters in Biogas Desulphurisation: A Critical Review

The agriculture and livestock industry generate waste used in anaerobic digestion to produce biogas containing methane (CH4), useful in the generation of electricity and heat. However, although biogas is mainly composed of CH4 (~65%) and CO2 (~34%), among the 1% of other compounds present is hydrogen sulphide (H2S) which deteriorates engines and power generation fuel cells that use biogas, generating a foul smell and contaminating the environment. As a solution to this, anoxic biofiltration, specifically with biotrickling filters (BTFs), stands out in terms of the elimination of H2S as it is cost-effective, efficient, and more environmentally friendly than chemical solutions. Research on the topic is uneven in terms of presenting performance markers, underestimating many microbiological indicators. Research from the last decade was analyzed (2010–2020), demonstrating that only 56% of the reviewed publications did not report microbiological analysis related to sulphur oxidising bacteria (SOB), the most important microbial group in desulphurisation BTFs. This exposes fundamental deficiencies within this type of research and difficulties in comparing performance between research works. In this review, traditional and microbiological performance markers of anoxic biofiltration to remove H2S are described. Additionally, an analysis to assess the efficiency of anoxic BTFs for biogas desulphurisation is proposed in order to have a complete and uniform assessment for research in this field.

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

Biological desulfurization of biogas has been extensively studied using biotrickling filters (BTFs). However, the accumulation of elemental sulfur (S°) on the packing material limits the use of this technology. To overcome this issue, the use of a continuous stirred tank bioreactor (CSTBR) under anoxic conditions for biogas desulfurization and S° production is proposed in the present study. The effect of the main parameters (stirring speed, N/S molar ratio, hydraulic residence time (HRT) and gas residence time (GRT)) on the bioreactor performance was studied. Under an inlet load (IL) of 100 g S-H₂S m^-3 h^-1 and a GRT of 119 s, the CSTBR optimal operating conditions were 60 rpm, N/S molar ratio of 1.1 and a HRT of 42 h, in which a removal efficiency (RE) and S° production of 98.6 ± 0.4 % and 88 % were obtained, respectively. Under a GRT of 41 s and an IL of 232 g S-H₂S m^-3 h^-1 the maximum elimination capacity (EC) of 166.0 ± 7.2 g S-H₂S m^-3 h^-1 (RE = 71.7 ± 3.1 %) was obtained. A proportional-integral feedback control strategy was successfully applied to the bioreactor operated under a stepped variable IL.

Simultaneous removal of ammonium from landfill leachate and hydrogen sulfide from biogas using a novel two-stage oxic-anoxic system

Anoxic biodesulfurization has been achieved in several bioreactor systems that have shown robustness and high elimination capacities (ECs). However, the high operating costs of this technology, which are mainly caused by the high requirements of nitrite or nitrate, make its full-scale application difficult. In the present study, the use of biologically produced nitrate/nitrite by nitrification of two different ammonium substrates, namely synthetic medium and landfill leachate, is proposed as a novel alternative. The results demonstrate the feasibility of using both ammonium substrates as nutrient solutions. A maximum elemental sulfur production of 95 ± 1% and a maximum H₂S EC of 141.18 g S-H₂S m^-3 h^-1 (RE = 95.0%) was obtained using landfill leachate as the ammonium source. Next Generation Sequencing (NGS) analysis of the microbial community revealed that the most common genera present in the desulfurizing bioreactor were Sulfurimonas (91.8-50.9%) followed by Thauera (1.1-24.2%) and Lentimicrobium (2.0-9.7%).

Characterization of eubacterial communities by Denaturing Gradient Gel Electrophoresis (DGGE) and Next Generation Sequencing (NGS) in a desulfurization biotrickling filter using progressive changes of nitrate and nitrite as final electron acceptors

Anoxic biotrickling filters (BTFs) represent a technology with high H₂S elimination capacity and removal efficiencies widely studied for biogas desulfurization. Three changes in the final electron acceptors were made using nitrate and nitrite during an operating period of 520 days. The stability and performance of the anoxic BTF were maintained when a significant perturbation was applied to the system that involved the progressive change of nitrate to nitrite and vice versa. Here the impact of electron acceptor changes on the microbial community was characterized by denaturing gel gradient electrophoresis (DGGE) and next generation sequencing (NGS). Both platforms revealed that the community underwent changes during the perturbations but was resilient because the removal capacity did not significantly change. Proteobacteria and Bacteroidetes were the main Phyla and Sulfurimonas and Thiobacillus the main nitrate-reducing sulfide-oxidizing bacteria (NR-SOB) genera involved in the biodesulfurization process.

Removal of dimethyl sulphide via a bio-scrubber under anoxic conditions

The removal performance of dimethyl sulphide (DMS) by anoxic laboratory-scale bio-scrubber was studied under different operation conditions for 315 days. DMS removal in bio-scrubber system was performed by controlling and changing the operation parameters, including inlet concentration, empty bed residence time (EBRT) and spraying density (SD) of irrigation. Best conditions in the system were achieved for SD of 0.18 m³/m² h within EBRT of 40 s at an inlet gas concentration of 150 mg/m³ in which 93% of waste gas stream was removed in the bio-scrubber column and bio-degradation in the bio-reactor tank led to 89% of DMS removal from the transferred bio-reactor, while 91.5% of input chemical oxygen demand (COD) was successfully removed. The use of closer values of the average experimental yield to the theoretical value (Y_NO3/NO3 -) of 0.74 led to the production of elemental sulphur (S°) and other sulphur forms rather than sulphate (SO₄^2-) , which was also was recognized as a pale-yellow coloured substance of S° that appeared within the biomass.

Removal of ethanethiol using a biotrickling filter with nitrate as an electron acceptor

Many studies have discussed the biotreatment of ethanethiol (ET) under aerobic conditions. However, O₂ free conditions offer bio-conversion of ET gas into elemental sulphur and/or sulphate using [Formula: see text] as electron acceptor, and this has been not studied. In this study, an anoxic biotrickling filter was tested in lab-scale conditions with ET/[Formula: see text] ratio 0.74 and 0.34 mole/mole to remove malodorous ET waste gas. The study examined the effect of three operational parameters: ET inlet concentrations (150, 300, 800, and 1500 mg/m³), trickling velocities (0.12, 0.18, 0.24, 0.3, and 0.45 m/h), and empty bed residence times (30, 60, 90, and 120 s). It found that the effect of trickling velocity on removal efficiency depended on inlet concentrations; 0.24 m/h trickling velocity resulted in efficient ET removal (higher than 90.8% for 150 mg/m³ of inlet concentration) while 0.45 m/h trickling velocity could only achieve a removal of 80.6% for 1500 mg/m³ of inlet concentration at fixed EBRT 60 s. Increasing the EBRT up to 60 s was adequate to achieve removal efficiency, i.e. 92 and 80% for ET inlet concentrations 150 and 1500 mg/m³ respectively, and the maximum elimination capacity was 75.18 g/m³/h at 0.45 m/h. Overall, the anoxic conditions enhanced the low oxidation rates of ET in an anoxic biotrickling filter despite mass transfer limitations and poor solubility of ET.

H2S removal and microbial community composition in an anoxic biotrickling filter under autotrophic and mixotrophic conditions

Removal of H₂S from gas streams using NO₃^--containing synthetic wastewater was investigated in an anoxic biotrickling filter (BTF) at feed N/S ratios of 1.2-1.7 mol mol^-1 with an empty bed residence time of 3.5 min and a hydraulic retention time of 115 min. During 108 days of operation under autotrophic conditions, the BTF showed a maximum elimination capacity (EC) of 19.2 g S m^-3 h^-1 and H₂S removal efficiency (RE) >99%. When the BTF was operated under mixotrophic conditions by adding organic carbon (10.2 g acetate m^-3 h^-1) to the synthetic wastewater, the H₂S EC decreased from 16.4 to 13.1 g S m^-3 h^-1, while the NO₃^- EC increased from 9.9 to 11.1 g NO₃^--N m^-3 h^-1, respectively. Thiobacillus sp. (98-100% similarity) was the only sulfur-oxidizing nitrate-reducing bacterium detected in the BTF biofilm, while the increased abundance of heterotrophic denitrifiers, i.e. Brevundimonas sp. and Rhodocyclales, increased the N/S ratio during BTF operation. Residence time distribution tests showed that biomass accumulation during BTF operation reduced gas and liquid retention times by 17.1% and 83.5%, respectively.

Ethanethiol gas removal in an anoxic bio-scrubber

The performance of ethanethiol removal in an anoxic lab-scale bio-scrubber was investigated under different operating parameters and conditions for 300 days. The removal efficiency (RE) of ethanethiol was examined as a function of inlet concentration, empty bed residence time (EBRT) and spray density of irrigation. The results showed the best operation conditions and operation characteristics of the bio-scrubber for this study were at an inlet concentration of 150 mg/m³, a spray density of 0.23 m³/m² h and an EBRT of 90 s. An average RE of 91% and elimination capacity (EC) of 24.74 g/m³ h was found for all inlet ethanethiol concentrations. Variations in spray density higher than 0.23 m³/m² h had no effect on ethanethiol RE at different ethanethiol concentrations. The average experimental yield values were closer to the Y_ET/NO3^- theoretical value of 0.74 when the main product was elemental sulphur (S^o). This indicates that S^o and other forms of sulphur were formed rather than sulphate (SO₄^2-) as the end product. Furthermore, growth kinetics for bio-degradation were evaluated in batch culture experiments using the Monod model, and bio-kinetic parameters of μ_max, K_s, Y_xs and q_max were obtained as 0.14 1/h, 1.17 mg/L, 0.52 g_x/g_s and 0.26 g_s/g_x h, respectively.

Mitigating adverse impacts of varying sulfide/nitrate ratios on denitrifying sulfide removal process performance

Complete removal of nitrogen, sulfur and carbon in wastewaters by denitrifying sulfide removal (DSR) process can be achieved at stoichiometry sulfide to nitrate ratio (S/N) of 1:1 in expanded granular sludge bed reactor. Wastewaters with varying S/N ratios can adversely impact the DSR performances with deterioration of synergetic cooperation between autotrophic and heterotrophic denitrifiers. DO (dissolved oxygen) serves effectively as supplementary electron receiver for sulfide oxidation, leaving more nitrate for heterotrophic denitrifiers to utilize acetate. The optimal oxygen to sulfide molar ratio (DO/S) is 0.5:1 for complete removal of sulfide, nitrate and acetate at different S/N ratios. The heterotrophic denitrification rate was decreased to 0.03 ± 0.002, 0.24 ± 0.011 and 0.35 ± 0.027 NO₃^--N·h^-1·gVSS^-1 at S/N ratio of 5:2, 5:5 and 5:8, respectively, when DO/S of 3:1 was performed. This optimal condition was proposed as an easy-to-implement control criterion for subsiding the adverse impact by varying S/N ratios in handling real wastewaters.