Foam formation in a biotechnological process for the removal of hydrogen sulfide from gas streams

Continuous single-stage elemental sulfur reduction and copper sulfide precipitation under thermoacidophilic conditions

Metal sulfide precipitation is a viable technology for high-yield metal recovery from hydrometallurgical streams, with the potential to streamline the process design. A single-stage elemental sulfur (S⁰)-reducing and metal sulfide precipitating process can optimize the operational and capital costs associated with this technology, boosting the competitiveness of this technology for wider industrial application. However, limited research is available on biological sulfur reduction at high temperature and low pH, frequent conditions of hydrometallurgical process waters. Here we assessed the sulfidogenic activity of an industrial granular sludge previously shown to reduce S⁰ under hot (60-80 °C) and acidic conditions (pH 3.6). A 4 L gas-lift reactor was operated for 206 days and fed continuously with culture medium and copper. During the reactor operation, we explored the effect of the hydraulic retention time, copper loading rates, temperature, and H₂ and CO₂ flow rates on the volumetric sulfide production rates (VSPR). A maximum VSPR of 274 ± 6 mg·L^-1·d^-1 was reached, a 3.9-fold increase of the VSPR previously reported with this inoculum in batch operation. Interestingly, the maximum VSPR was achieved at the highest copper loading rates. At the maximum copper loading rate (509 mg·L^-1·d^-1), a 99.96% copper removal efficiency was observed. 16 s rRNA gene amplicon sequencing revealed an increased abundance of reads assigned to Desulfurella and Thermoanaerobacterium in periods of higher sulfidogenic activity.

Removal of small elemental sulfur particles by polysulfide formation in a sulfidic reactor

For over 30 years, biological gas desulfurization under halo-alkaline conditions has been studied and optimized. This technology is currently applied in already 270 commercial installations worldwide. Sulfur particle separation, however, remains a challenge; a fraction of sulfur particles is often too small for liquid-solid separation with conventional separation technology. In this article, we report the effects of a novel sulfidic reactor, inserted in the conventional process set-up, on sulfur particle size and morphology. In the sulfidic reactor polysulfide is produced by the reaction of elemental sulfur particles and sulfide, which is again converted to elemental sulfur in a gas-lift reactor. We analyzed sulfur particles produced in continuous, long term lab-scale reactor experiments under various sulfide concentrations and sulfidic retention times. The analyses were performed with laser diffraction particle size analysis and light microscopy. These show that the smallest particles (< 1 µm) have mostly disappeared under the highest sulfide concentration (4.1 mM) and sulfidic retention time (45 min). Under these conditions also agglomeration of sulfur particles was promoted. Model calculations with thermodynamic and previously derived kinetic data on polysulfide formation confirm the experimental data on the removal of the smallest particles. Under the 'highest sulfidic pressure', the model predicts that equilibrium conditions are reached between sulfur, sulfide and polysulfide and that 100% of the sulfur particles <1 µm are dissolved by the (autocatalytic) formation of polysulfides. These experiments and modeling results demonstrate that the insertion of a novel sulfidic reactor in the conventional process set-up promotes the removal of the smallest individual sulfur particles and promotes the production of sulfur agglomerates. The novel sulfidic reactor is therefore a promising process addition with the potential to improve process operation, sulfur separation and sulfur recovery.

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.

Recent advances in microbial capture of hydrogen sulfide from sour gas via sulfur-oxidizing bacteria

Biological desulfurization offers several remarkably environmental advantages of operation at ambient temperature and atmospheric pressure, no demand of toxic chemicals as well as the formation of biologically re-usable sulfur (S0), which has attracted increasing attention compared to conventionally physicochemical approaches in removing hydrogen sulfide from sour gas. However, the low biomass of SOB, the acidification of process solution, the recovery of SOB, and the selectivity of bio-S0 limit its industrial application. Therefore, more efforts should be made in the improvement of the BDS process for its industrial application via different research perspectives. This review summarized the recent research advances in the microbial capture of hydrogen sulfide from sour gas based on strain modification, absorption enhancement, and bioreactor modification. Several efficient solutions to limitations for the BDS process were proposed, which paved the way for the future development of BDS industrialization.

Properties of Sulfur Particles Formed in Biodesulfurization of Biogas

In the biodesulfurization (BD) process under halo-alkaline conditions, toxic hydrogen sulfide is oxidized to elemental sulfur by a mixed culture of sulfide oxidizing bacteria to clean biogas. The resulting sulfur is recovered by gravitational settling and can be used as raw material in various industries. However, if the sulfur particles do not settle, it will lead to operational difficulties. In this study, we investigated the properties of sulfur formed in five industrial BD facilities. Sulfur particles from all samples showed large differences in terms of shape, size, and settleability. Both single crystals (often bipyramidal) and aggregates thereof were observed with light and scanning electron microscopy. The small, non-settled particles account for at least 13.6% of the total number of particles and consists of small individual particles with a median of 0.3 µm. This is undesirable, because those particles cannot be removed from the BD facility by gravitational settling and lead to operational interruption. The particles with good settling properties are aggregates (5–20 µm) or large single crystals (20 µm). We provide hypotheses as to how the differences in sulfur particle properties might have occurred. These findings provide a basis for understanding the relation between sulfur particle properties and formation mechanisms.

Effect of biologically produced sulfur on gas absorption in a biotechnological hydrogen sulfide removal process

Absorption of hydrogen sulfide in aqueous suspensions of biologically produced sulfur particles was studied in a batch stirred cell reactor, and in a continuous set-up, consisting of a lab-scale gas absorber column and a bioreactor. Presence of biosulfur particles was found to enhance the absorption rate of H(2)S gas in the mildly alkaline liquid. The mechanism for this enhancement was however found to depend on the type of particles used. In the gently stirred cell reactor only small hydrophilic particles were present (d(p) < 3 microm) and the enhancement of the H(2)S absorption rate can be explained from the heterogeneous reaction between dissolved H(2)S and solid elemental sulfur to polysulfide ions, S(x) (2-). Conditions favoring enhanced H(2)S absorption for these hydrophilic particles are: low liquid side mass transfer (k(L)), high sulfur content, and presence of polysulfide ions. In the set-up of gas absorber column and bioreactor, both small hydrophilic particles and larger, more hydrophobic particles were continuously produced (d(p) up to 20 microm). Here, observed enhancement could not be explained by the heterogeneous reaction between sulfide and sulfur, due to the relatively low specific particle surface area, high k(L), and low [S(x) (2-)]. A more likely explanation for enhancement here is the more hydrophobic behavior of the larger particles. A local increase of the hydrophobic sulfur particle concentration near the gas/liquid interface and specific adsorption of H(2)S at the particle surface can result in an increase in the H(2)S absorption rate.