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Applied Microbiology and Biotechnology (v.67, #1)

Polycarnitine—a new biomaterial by B. Kamm; M. Kamm; A. Kiener; H.-P. Meyer (pp. 1-7).

The natural product l-carnitine is—due to its biotechnological accessibility and specific properties—on the way to becoming an attractive biobased bulk product. l-Carnitine is a natural betaine with vitamin properties. Carnitine is an essential part of the fatty acid metabolism of human beings and animals. Carnitine was first isolated in 1905 from meat extract and important recent developments include the biosyntheses of l-carnitine from l-lysine or γ-butyrobetaine. Our synthesis routes are designed to maintain the primary structure and specific properties of carnitine, such as hydrophilicity and “stiffening” effects for polymeric structures and applications. l-Carnitine is converted via lactonization or olefinization into polymerizable basic molecules. The properties and the applications of carnitine polymers are described.

Bioactive berry compounds—novel tools against human pathogens by Riitta Puupponen-Pimiä; Liisa Nohynek; Hanna-Leena Alakomi; Kirsi-Marja Oksman-Caldentey (pp. 8-18).

Berry fruits are rich sources of bioactive compounds, such as phenolics and organic acids, which have antimicrobial activities against human pathogens. Among different berries and berry phenolics, cranberry, cloudberry, raspberry, strawberry and bilberry especially possess clear antimicrobial effects against, e.g. Salmonella and Staphylococcus. Complex phenolic polymers, like ellagitannins, are strong antibacterial agents present in cloudberry and raspberry. Several mechanisms of action in the growth inhibition of bacteria are involved, such as destabilisation of cytoplasmic membrane, permeabilisation of plasma membrane, inhibition of extracellular microbial enzymes, direct actions on microbial metabolism and deprivation of the substrates required for microbial growth. Antimicrobial activity of berries may also be related to antiadherence of bacteria to epithelial cells, which is a prerequisite for colonisation and infection of many pathogens. Antimicrobial berry compounds may have important applications in the future as natural antimicrobial agents for food industry as well as for medicine. Some of the novel approaches are discussed.

Biotechnological processes for conversion of corn into ethanol by R. J. Bothast; M. A. Schlicher (pp. 19-25).

Ethanol has been utilized as a fuel source in the United States since the turn of the century. However, it has repeatedly faced significant commercial viability obstacles relative to petroleum. Renewed interest exists in ethanol as a fuel source today owing to its positive impact on rural America, the environment and United States energy security. Today, most fuel ethanol is produced by either the dry grind or the wet mill process. Current technologies allow for 2.5 gallons (wet mill process) to 2.8 gallons (dry grind process) of ethanol (1 gallon = 3.785 l) per bushel of corn. Valuable co-products, distillers dried grains with solubles (dry grind) and corn gluten meal and feed (wet mill), are also generated in the production of ethanol. While current supplies are generated from both processes, the majority of the growth in the industry is from dry grind plant construction in rural communities across the corn belt. While fuel ethanol production is an energy-efficient process today, additional research is occurring to improve its long-term economic viability. Three of the most significant areas of research are in the production of hybrids with a higher starch content or a higher extractable starch content, in the conversion of the corn kernel fiber fraction to ethanol, and in the identification and development of new and higher-value co-products.

Selection pressure-driven aerobic granulation in a sequencing batch reactor by Yu Liu; Zhi-Wu Wang; Lei Qin; Yong-Qiang Liu; Joo-Hwa Tay (pp. 26-32).

In recent years, the research on aerobic granulation has been intensive. So far, almost all aerobic granules can form only in sequencing batch reactors (SBR), while the reason is not yet understood. This paper attempts to review the factors involved in aerobic granulation in SBR, including substrate composition, organic loading rate, hydrodynamic shear force, feast-famine regime, feeding strategy, dissolved oxygen, reactor configuration, solids retention time, cycle time, settling time and exchange ratio. The major selection pressures responsible for aerobic granulation are identified as the settling time and exchange ratio. A concept of the minimal settling velocity of bioparticles is proposed; and it is quantitatively demonstrated that the effects of settling time and exchange ratio on aerobic granulation in SBR can be interpreted and unified on the basis of this concept very well. It appears that the formation and characteristics of aerobic granules can be manipulated through properly adjusting either the settling time or the exchange ratio in SBR. Consequently, theoretical and experimental evidence point to the fact that aerobic granulation is a selection pressure-driven cell-to-cell immobilization process.

Robust NADH-regenerator: improved α-haloketone-resistant formate dehydrogenase by H. Yamamoto; K. Mitsuhashi; N. Kimoto; Y. Kobayashi; N. Esaki (pp. 33-39).

Formate dehydrogenases (FDH) are useful for the regeneration of NADH, which is required for asymmetric reduction by several dehydrogenases and reductases. FDHs have relatively low activity and are labile, especially to α-haloketones, thus FDH cannot be applied to the industrial manufacture of optically active α-haloalcohols. To stabilize a FDH from Mycobacterium vaccae (McFDH) against the α-haloketone ethyl 4-chloroacetoacetate (ECAA), a set of cysteine-mutant enzymes was constructed. Sensitivity to ECAA of mutant C6S was similar to that of the wild-type enzyme, and mutants C249S and C355S showed little activity. In contrast, mutant C256S exhibited remarkable tolerance to ECAA. Surprisingly, mutant C146S was activated by several organic compounds such as ethyl acetate. An optimized mutant, C6A/C146S/C256V (McFDH-26), was obtained by combining several effective mutations. Ethyl (S)-4-chloro-3-hydroxybutanoate [(S)-ECHB] was synthesized from ECAA to 49.9 g/l with an optical purity of more than 99% e.e. using recombinant Escherichia coli cells coexpressing McFDH-26 and a carbonyl reductase (KaCR1) from Kluyveromyces aestuarii.

Elicitor-induced nitric oxide burst is essential for triggering catharanthine synthesis in Catharanthus roseus suspension cells by Maojun Xu; Jufang Dong (pp. 40-44).

Elicitor prepared from the cell walls of Penicillium citrinum induced multiple responses in Catharanthus roseus suspension cells, including rapid generation of nitric oxide (NO), sequentially followed by enhancement of catharanthine production by C. roseus cells. Elicitor-induced catharanthine biosynthesis was blocked by NO-specific scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and nitric oxide synthase (NOS) inhibitor S,S′-1,3-phenylene-bis(1,2-ethanediyl)-bis-isothiourea (PBITU). PBITU also strongly inhibited elicitor-induced NO generation by C. roseus suspension cells. The inhibiting effect of PBITU on elicitor-induced catharanthine production was reversed by external application of NO via the NO-donor sodium nitroprusside. The results strongly suggested that NO, generated by NOS or NOS-like enzymes in C. roseus suspension cells when treated with the fungal elicitor, was essential for triggering catharanthine synthesis.

Production of bacterial cellulose by Acetobacter xylinum BPR2001 using molasses medium in a jar fermentor by S. O. Bae; M. Shoda (pp. 45-51).

Bacterial cellulose (BC) production by Acetobacter xylinum subsp. sucrofermentans BPR2001 using molasses medium was carried out in a jar fermentor. When molasses was subjected to H₂SO₄-heat treatment, the maximum BC concentration increased to 76% more than that achieved using untreated molasses, and the specific growth rate increased 2-fold. When the initial sugar concentrations in the H₂SO₄-heat treated molasses were varied from 23 g/l to 72 g/l, BC concentration, production rate, and yield were maximum at sugar concentrations of 23 g/l and 37 g/l, and production of by-products, such as polysaccharides and CO₂, was lower than at sugar concentrations of 48 g/l and 72 g/l, indicating that maintaining a lower molasses concentration is essential for efficient BC production in jar fermentors, this being due mainly to the complex nature of molasses. Molasses has a clear advantage over pure sugars as a carbon source from an economic viewpoint.

In vitro gas production as a surrogate measure of the fermentability of cellulosic biomass to ethanol by P. J. Weimer; B. S. Dien; T. L. Springer; K. P. Vogel (pp. 52-58).

Current methods for measuring ethanol yields from lignocellulosic biomass are relatively slow and are not well geared for analyzing large numbers of samples generated by feedstock management and breeding research. The objective of this study was to determine if an in vitro ruminal fermentation assay used in forage quality research was predictive of results obtained using a conventional biomass-to-ethanol conversion assay. In the conventional assay, herbaceous biomass samples were converted to ethanol by Saccharomyces cerevisiae cultures in the presence of cellulase enzymes. Cultures were grown in sealed serum bottles and gas production monitored by measuring increasing head space pressure. Gas accumulation as calculated from the pressure measurements was highly correlated (r²>0.9) with ethanol production measured by gas chromatography at 24 h or 7 days. The same feedstocks were also analyzed by in vitro ruminal digestion, as also measured by gas accumulation. Good correlations (r²∼0.63–0.82) were observed between ethanol production during simultaneous saccharification and fermentation and gas accumulation in parallel in vitro ruminal fermentations. Because the in vitro ruminal fermentation assay can be performed without sterilization of the medium and does not require aseptic conditions, this assay may be useful for biomass feedstock agronomic and breeding research.

Cloning, recombinant expression and biochemical characterisation of novel esterases from Bacillus sp. associated with the marine sponge Aplysina aerophoba by A. Karpushova; F. Brümmer; S. Barth; S. Lange; R. D. Schmid (pp. 59-69).

Two novel esterases (EstB1 and EstB2) were isolated from a genomic library of Bacillus sp. associated with the marine sponge Aplysina aerophoba. EstB1 shows low identity (26–44%) with the published hydrolases of the genus Bacillus, whereas EstB2 shows high identity (73–74%) with the carboxylesterases from B. cereus and B. anthracis. Both esterases were efficiently expressed in Escherichia coli under the control of T7 promoter using the vector pET-22b(+). Recombinant EstB1 was purified in a single step to electrophoretic homogeneity by IMAC. A method for the refolding of inclusion bodies formed by the recombinant EstB2 was established to obtain active enzyme. Substrate specificity of the two enzymes towards p-nitrophenyl and methyl esters and the respective kinetic parameters K_m and V_max were determined. The temperature optima of EstB1 and EstB2 were determined to be in the range of 30–50°C and 20–35°C, respectively. The pH optima were found to be in the range of 6.5–7.5 and 6.5–8.0, respectively. Both enzymes showed the highest stability in up to 50% (v/v) DMSO followed by methanol, ethanol and 2-propanol. The influence of high NaCl and KCl concentrations was tested. The inhibition effect of 10–50 mM Zn²⁺ and 50 mM Mg²⁺ and Ca²⁺ ions was observed for both esterases. One to five millimolar PMSF deactivated the enzymes, whereas β-mercaptoethanol, DTT and EDTA had no effect on the enzymes activity.

Quantification of supplemental enzymes in animal feedingstuffs by radial enzyme diffusion by G. Walsh; R. A. Murphy; G. F. Killeen; R. F. Power (pp. 70-74).

Methods are described which facilitate quantification of supplemental cellulase, protease and α-amylase when added to animal feedingstuffs at normal industrial inclusion levels. The methods entail extraction of the enzymes from the feedingstuffs by agitation in buffer followed by quantification of extract activity using radial diffusion techniques. A linear relationship between the diameter of the zone of hydrolyzed substrate and the log of the enzyme activity applied is observed over a broad activity range. Assay of a feedingstuff supplemented with 1 kg t⁻¹ cellulase, protease and α-amylase yielded net supplemental activity recoveries of 104±11.7%, 91.3±6.74% and 126±29.5%, respectively. A similar assay method did not prove sufficiently sensitive to facilitate detection of xylanase at typical in-feed inclusion levels. The levels of endogenous cellulase, protease and α-amylase activity detected in the unsupplemented feedingstuffs were equivalent to 6.4±0.47%, 6.6±0.82% and 29.0±14.1%, respectively, of a 1 kg t⁻¹ supplement. The methods are technically straightforward and will facilitate determination of enzyme stabilities during processes such as high-temperature pelleting of feedingstuffs, as well as allowing more rigorous quality control related to enzyme-supplemented animal feedingstuffs.

Different control mechanisms regulate glucoamylase and protease gene transcription in Aspergillus oryzae in solid-state and submerged fermentation by R. te Biesebeke; N. van Biezen; W. M. de Vos; C. A. M. J. J. van den Hondel; P. J. Punt (pp. 75-82).

Solid-state fermentation (SSF) with Aspergillus oryzae results in high levels of secreted protein. However, control mechanisms of gene expression in SSF have been only poorly studied. In this study we show that both glucoamylase (glaB) and protease (alpA, nptB) genes are highly expressed during surface cultivation on wheat-based solid medium, and even higher during cultivation on wheat kernels. In wheat-based liquid medium, low levels of gene expression are observed. Typical SSF cultivation conditions, such as low water activity and the formation of aerial hyphae, did not contribute to the high-level gene expression on wheat-based solid medium. Analysis of wheat-based solid and liquid cultivations showed differences in carbon and nitrogen utilisation and external pH. The results presented show that the difference in regulation of transcription of the alpA and nptB genes in wheat-based liquid and solid medium could be pH dependent, involving a pH-dependent transcription regulator. The results obtained suggest that the difference in regulation of transcription of the glaB gene in wheat-based liquid and solid medium is caused by a difference in carbohydrate degradation and consumption under the different culture conditions.

Using Lactococcus lactis for glutathione overproduction by Yin Li; Jeroen Hugenholtz; Wilbert Sybesma; Tjakko Abee; Douwe Molenaar (pp. 83-90).

Glutathione and γ-glutamylcysteine were produced in Lactococcus lactis using a controlled expression system and the genes gshA and gshB from Escherichia coli encoding the enzymes γ-glutamylcysteine synthetase and glutathione synthetase. High levels of γ-glutamylcysteine were found in strains growing on chemically defined medium and expressing either gshA alone or both gshA and gshB. As anticipated, glutathione was found in a strain expressing gshA and gshB. The level of glutathione production could be increased by addition of the precursor amino acid cysteine to the medium. The addition of cysteine led to an increased activity of glutathione synthetase, which is remarkable because the amino acid is not a substrate of this enzyme. The final intracellular glutathione concentration attained was 358 nmol mg⁻¹ protein, which is the highest concentration reported for a bacterium, demonstrating the suitability of engineered L. lactis for fine-chemical production and as a model for studies of the impact of glutathione on flavour formation and other properties of food.

Purification, cloning, and properties of α-galactosidase from Saccharopolyspora erythraea and its use as a reporter system by David A. Post; Vicki E. Luebke (pp. 91-96).

An α-galactosidase from the erythromycin-producing bacterium Saccharopolyspora erythraea was purified to near homogeneity. The enzyme has an apparent molecular mass of 45 kDa as determined by SDS-PAGE. The pH optimum, K_m for p-nitrophenyl-α-d-glucopyranoside (pNPαG), K_m for melibiose and the V_max are similar to those of other studied α-galactosidase enzymes. The N-terminal amino-acid sequence of this protein was determined. PCR amplification was used to generate a 640-bp product using oligonucleotide primers based on the N-terminal amino-acid sequence and a downstream region that is conserved in other related α-galactosidase enzymes. This fragment was used as a probe to clone the α-galactosidase gene, designated melA, from a S. erythraea lambda phage chromosomal library. S. erythraea appears to possess an unique α-galactosidase enzyme, encoded by melA, that can utilize galactopyranosides as carbon sources. Furthermore, the ability to use the product of melA as a reporter enzyme in S. erythraea has been demonstrated. The α-galactosidase uses the substrates 5-bromo-4-chloro-3-indoyl-α-d-galactosidase (X-α-gal) on agar media and pNPαG in liquid media.

Production of 3-hydroxy-n-phenylalkanoic acids by a genetically engineered strain of Pseudomonas putida by Ángel Sandoval; Elsa Arias-Barrau; Francisco Bermejo; Librada Cañedo; Germán Naharro; Elías R. Olivera; José M. Luengo (pp. 97-105).

Overexpression of the gene encoding the poly-3-hydroxy-n-phenylalkanoate (PHPhA) depolymerase (phaZ) in Pseudomonas putida U avoids the accumulation of these polymers as storage granules. In this recombinant strain, the 3-OH-acyl-CoA derivatives released from the different aliphatic or aromatic poly-3-hydroxyalkanoates (PHAs) are catabolized through the β-oxidation pathway and transformed into general metabolites (acetyl-CoA, succinyl-CoA, phenylacetyl-CoA) or into non-metabolizable end-products (cinnamoyl-CoA). Taking into account the biochemical, pharmaceutical and industrial interest of some PHA catabolites (i.e., 3-OH-PhAs), we designed a genetically engineered strain of P. putida U (P. putida U ΔfadBA-phaZ) that efficiently bioconverts (more than 80%) different n-phenylalkanoic acids into their 3-hydroxyderivatives and excretes these compounds into the culture broth.

Degradation of estradiol and ethinyl estradiol by activated sludge and by a defined mixed culture by Stefanie Weber; Prisca Leuschner; Peter Kämpfer; Wolfgang Dott; Juliane Hollender (pp. 106-112).

The aerobic degradation of the natural hormone 17-β-estradiol (E2) and the synthetic hormone 17-α-ethinyl estradiol (EE2) was investigated in batch experiments with activated sludge from a conventional and a membrane sewage treatment plant. E2 was converted to estrone (E1), the well known metabolite, and further completely transformed within 3 days. The turnover rates of E2 did not differ greatly between conventional and membrane activated sludge. EE2 was persistent in both sludges. By several transfers into fresh E2-medium an enrichment culture could be selected that used E2 as growth substrate. Further enrichment and isolation led to a defined mixed culture consisting of two strains, which were identified by a polyphasic approach as Achromobacter xylosoxidans and Ralstonia sp., respectively. The culture used E2 and E1 as growth substrates and transformed estriol (E3) and 16-α-hydroxyestrone but not the xenoestrogens bisphenol A, α-zearalenol, mestranol or EE2. The turnover rates of E2 were 0.025–0.1 μg h⁻¹ cfu⁻¹ and did not depend on the steroid concentration.

The effect of decreasing oxygen feed rates on growth and metabolism of Torulaspora delbrueckii by Lars Hanl; Peter Sommer; Nils Arneborg (pp. 113-118).

The effect of decreasing oxygen feed rates on the growth and metabolism of Torulaspora delbrueckii and Saccharomyces cerevisiae in chemostat cultures was investigated. The biosynthetic oxygen requirement, i.e. the minimum specific oxygen consumption rate required for steady-state growth at a dilution rate of 0.10 h⁻¹, of T. delbrueckii was quantified to be less than 0.1 mmol O₂ g⁻¹ h⁻¹. Under strict anaerobiosis, washout of T. delbrueckii occurred, whereas for S. cerevisiae it did not. Under oxygen-limited conditions, the increase in fermentative ability of T. delbrueckii with diminishing oxygen supply was less pronounced than that of S. cerevisiae. These results indicate that T. delbrueckii was more disturbed in its energy balance than S. cerevisiae under strict anaerobiosis, and they may explain why T. delbrueckii exhibits poorer growth than S. cerevisiae under this condition.

Analysis of acyl CoA ester intermediates of the mevalonate pathway in Saccharomyces cerevisiae by Tamay Seker; Kasper Møller; Jens Nielsen (pp. 119-124).

The mevalonate pathway plays an important role in providing the cell with a number of essential precursors for the synthesis of biomass constituents. With respect to their chemical structure, the metabolites of this pathway can be divided into two groups: acyl esters [acetoacetyl CoA, acetyl CoA, hydroxymethylglutaryl (HMG) CoA] and phosphorylated metabolites (isopentenyl pyrophosphate, dimethylallyl pyrophosphate, geranyl pyrophosphate, farnesyl pyrophosphate). In this study, we developed a method for the precise analysis of the intracellular concentration of acetoacetyl CoA, acetyl CoA and HMG CoA; and we used this method for quantification of these metabolites in Saccharomyces cerevisiae, both during batch growth on glucose and on galactose and in glucose-limited chemostat cultures operated at three different dilution rates. The level of the metabolites changed depending on the growth phase/specific growth rate and the carbon source, in a way which indicated that the synthesis of acetoacetyl CoA and HMG CoA is subject to glucose repression. In the glucose batch, acetyl CoA accumulated during the growth on glucose and, just after glucose depletion, HMG CoA and acetoacetyl CoA started to accumulate during the growth on ethanol. In the galactose batch, HMG CoA accumulated during the growth on galactose and a high level was maintained into the ethanol growth phase; and the levels of acetyl CoA and HMG CoA were more than two-fold higher in the galactose batch than in the glucose batch.

Extraction of extracellular polymeric substances from the photosynthetic bacterium Rhodopseudomonas acidophila by Guo-Ping Sheng; Han-Qing Yu; Zhou Yu (pp. 125-130).

Among the four methods for extracting extracellular polymeric substances (EPS) from Rhodopseudomonas acidophila (EDTA, NaOH, H₂SO₄, heating/centrifugation), EDTA extraction was found to be the most effective. The contents of the major components of EPS from R. acidophila, i.e., carbohydrate, protein and nucleic acid, were 6.5, 58.4 and 5.4 mg g⁻¹ dry cells, respectively. The optimum extraction time was 1–3 h and the EDTA dosage was approximately 2.8 g g⁻¹ dry cells. Under these conditions, no cell lysis was observed. The EPS content and the percentage of the three main components were greatly dependent on the extraction method. The intensity of absorption peaks for photosynthetic pigments in the UV–visible spectrum of bacteria remained unchanged prior to and after EDTA extraction; and no pigment peaks appeared in the EPS spectrum. This suggests that few cells were destroyed and lysis did not occur. UV–visible spectrum analysis, an easy and rapid technique, could be used to monitor cell lysis during EPS extraction from R. acidophila.

Phosphonium ionic liquids for degradation of phenol in a two-phase partitioning bioreactor by M. D. Baumann; A. J. Daugulis; P. G. Jessop (pp. 131-137).

Six ionic liquids (ILs), which are organic salts that are liquid at room temperature, were tested for their biocompatibility with three xenobiotic-degrading bacteria, Pseudomonas putida, Achromobacter xylosoxidans, and Sphingomonas aromaticivorans. Of the 18 pairings, seven were found to demonstrate biocompatibility, with one IL (trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl) amide) being biocompatible with all three organisms. This IL was then used in a two-phase partitioning bioreactor (TPPB), consisting of 1 l aqueous phase loaded with 1,580 mg phenol and 0.25 l IL, inoculated with the phenol degrader P. putida. This initially toxic aqueous level of phenol was substantially reduced by phenol partitioning into the IL phase, allowing the cells to utilize the reduced phenol concentration. The partitioning of phenol from the IL to the aqueous phase was driven by cellular demand and thermodynamic equilibrium. All of the phenol was consumed at a rate comparable to that of previously used organic-aqueous TPPB systems, demonstrating the first successful use of an IL with a cell-based system. A quantitative ³¹P NMR spectroscopic assay for estimating the log P values of ILs is under development.

Transgenic tobacco expressing fungal laccase promotes the detoxification of environmental pollutants by Tomonori Sonoki; Shinya Kajita; Seiichiro Ikeda; Mikiko Uesugi; Kenji Tatsumi; Yoshihiro Katayama; Yosuke Iimura (pp. 138-142).

The phytoremediation of soils contaminated with organic pollutants offers a low-cost method for removal of such pollutants. We have attempted to enhance the environmental decontamination functions of plants by introducing appropriate enzymatic activities from microorganisms. In the present study, we introduced an extracellular fungal enzyme, the laccase of Coriolus versicolor, into tobacco plants. One transgenic plant, designated FL4, produced laccase that was secreted into the rhizosphere. FL4 was able to remove 20 μmol bisphenol A or pentachlorophenol per gram dry weight. The efficiency of this removal was apparently greater than that of control lines. Our results should stimulate efforts to develop plant-based technologies for the removal of environmental pollutants from contaminated environments.

Treatment of H₂S using a horizontal biotrickling filter based on biological activated carbon: reactor setup and performance evaluation by Huiqi Duan; Lawrence Choon Chiaw Koe; Rong Yan (pp. 143-149).

Biological treatment is an emerging and prevalent technology for treating off-gases from wastewater treatment plants. The most commonly reported odorous compound in off-gases is hydrogen sulfide (H₂S), which has a very low odor threshold. A self-designed, bench-scale, cross-flow horizontal biotrickling filter (HBF) operated with bacteria immobilized activated carbon (termed biological activated carbon—BAC), was applied for the treatment of H₂S. A mixed culture of sulfide-oxidizing bacteria dominated by Acidithiobacillus thiooxidans acclimated from activated sludge was used as bacterial seed and the biofilm was developed by culturing the bacteria in the presence of carbon pellets in mineral medium. HBF performance was evaluated systematically over ∼120 days, depending on a series of changing factors including inlet H₂S concentration, gas retention time (GRT), pH of recirculation solution, upset and recovery, sulfate accumulation, pressure drop, gas-liquid ratio, and shock loading. The biotrickling filter system can operate at high efficiency from the first day of operation. At a volumetric loading of 900 m³ m⁻³ h⁻¹ (at 92 ppmv H₂S inlet concentration), the BAC exhibited maximum elimination capacity (113 g H₂S/m⁻³ h⁻¹) and a removal efficiency of 96% was observed. If the inlet concentration was kept at around 20 ppmv, high H₂S removal (over 98%) was achieved at a GRT of 4 s, a value comparable with those currently reported for biotrickling filters. The bacterial population in the acidic biofilter demonstrated capacity for removal of H₂S over a broad pH range (pH 1–7). A preliminary investigation into the different effects of bacterial biodegradation and carbon adsorption on system performance was also conducted. This study shows the HBF to be a feasible and economic alternative to physical and chemical treatments for the removal of H₂S.

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Applied Microbiology and Biotechnology (v.67, #1)