Characterization of sulfide-oxidizing microbial mats developed inside a full-scale anaerobic digester employing biological desulfurization

Biogenic sulfur recovery from sulfate-laden antibiotic production wastewater using a single-chamber up-flow bioelectrochemical reactor

The high-concentration sulfate (SO42-) in the antibiotic production wastewater hinders the anerobic methanogenic process and also proposes possible environmental risk. In this study, a novel single-chamber up-flow anaerobic bioelectrochemical reactor (UBER) was designed to realize simultaneous SO42- removal and elemental sulfur (S0) recovery. With the carbon felt, the cathode was installed underneath and the anode above to meet the different biological niches for sulfate reducing bacteria (SRB) and sulfur oxidizing bacteria (SOB). The bio-anode UBER (B-UBER) demonstrated a much higher average SO42- removal rate (SRR) of 113.2 ± 5.7 mg SO42--S L-1 d-1 coupled with a S0 production rate (SPR) of 54.4 ± 5.8 mg S0-S L-1 d-1 at the optimal voltage of 0.8 V than that in the abio-anode UBER (control reactor) (SRR = 86.6 ± 13.4 mg SO42--S L-1 d-1; SPR = 25.5 ± 9.7 mg S0-S L-1 d-1) under long-term operation. A large amount of biogenic S0 (about 72.2 mg g-1 VSS) was recovered in the B-UBER. The bio-anode, dominated by Thiovirga (SOB genus) and Acinetobacter (electrochemically active bacteria genus), exhibited a higher current density, lower overpotential, and lower internal resistance. C-type cytochromes mainly served as the crucial electron transfer mediator for both direct and indirect electron transfer, so that significantly increasing electron transfer capacity and biogenic S0 recovery. The reaction pathways of the sulfur transformation in the B-UBER were hypothesized that SRB utilized acetate as the main electron donor for SO42- reduction in the cathode zone and SOB transferred electrons to the anode or oxygen to produce biogenic S0 in the anode zone. This study proved a new pathway for biogenic S0 recovery and sulfate removal from sulfate-laden antibiotic production wastewater using a well-designed single-chamber bioelectrochemical reactor.

Evaluation of microaeration strategies in the digestion zone of UASB reactors as an alternative for biogas desulfurization

This study aimed at evaluating the microaeration as an alternative for hydrogen sulfide removal from biogas of UASB reactors treating sewage. The set-up consisted of two pilot-scale UASB reactors, including a conventional anaerobic and a modified UASB reactor, operated under microaerated conditions. Air was supplied in the digestion zone, at 1 and 3 m from the bottom of the reactor, and three different air flows were investigated: 10, 20, and 30 mL.min^-1, corresponding to 0.003, 0.005 and 0.005 LO₂/L_influent, respectively. The main results showed that the microaeration provided a substantial decrease in hydrogen sulfide concentrations when compared to the concentrations observed in the biogas of the anaerobic UASB reactor. Hydrogen sulfide concentrations remained below 70 ppm_v throughout the experimental period, corresponding to an average removal efficiency of 98%. Although a decrease in methane concentrations in biogas was observed, the feasibility of energy use would not be affected. The effect of microaeration on the overall performance of the reactor was evaluated, however, no significant differences were observed. The feasibility of limiting aeration conditions in the reactor digestion zone as an efficient alternative for hydrogen sulfide removal from biogas was demonstrated.

Intensification of Acidogenic Fermentation for the Production of Biohydrogen and Volatile Fatty Acids—A Perspective

Utilising ‘wastes’ as ‘resources’ is key to a circular economy. While there are multiple routes to waste valorisation, anaerobic digestion (AD)—a biochemical means to breakdown organic wastes in the absence of oxygen—is favoured due to its capacity to handle a variety of feedstocks. Traditional AD focuses on the production of biogas and fertiliser as products; however, such low-value products combined with longer residence times and slow kinetics have paved the way to explore alternative product platforms. The intermediate steps in conventional AD—acidogenesis and acetogenesis—have the capability to produce biohydrogen and volatile fatty acids (VFA) which are gaining increased attention due to the higher energy density (than biogas) and higher market value, respectively. This review hence focusses specifically on the production of biohydrogen and VFAs from organic wastes. With the revived interest in these products, a critical analysis of recent literature is needed to establish the current status. Therefore, intensification strategies in this area involving three main streams: substrate pre-treatment, digestion parameters and product recovery are discussed in detail based on literature reported in the last decade. The techno-economic aspects and future pointers are clearly highlighted to drive research forward in relevant areas.

Comparative Metagenomics of Anaerobic Digester Communities Reveals Sulfidogenic and Methanogenic Microbial Subgroups in Conventional and Plug Flow Residential Septic Tank Systems

On-site wastewater treatment systems (OWTS) are primarily monitored using physiochemical factors, including chemical oxygen demand (COD) and residual total suspended solids (TSS), which are indirect measures of the microbial action during the anaerobic digestion process. Changes in anaerobic digester microbial communities can alter the digester performance, but this information cannot be directly obtained from traditional physicochemical indicators. The potential of metagenomic DNA sequencing as a tool for taxonomic and functional profiling of microbial communities was examined in both common conventional and plug flow-type anaerobic digesters (single-pass and recirculating). Compared to conventional digesters, plug flow-type digesters had higher relative levels of sulfate-reducing bacteria (Desulfovibrio spp.) and hydrogenotrophic methanogens (Methanospirillum spp.). In contrast, recirculating anaerobic digesters were enriched with denitrifier bacteria and hydrogenotrophic methanogens, and both were significantly correlated with physicochemical factors such as COD and TSS. Stratification of microbial communities was observed along the digester treatment process according to hydrolytic, acidogenic, acetogenic, and methanogenic subgroups. These results indicate that the high-throughput DNA sequencing may be useful as a monitoring tool to characterize the changes in bacterial communities and the functional profile due to differences in digester design in on-site systems.

Achieving superior nitrogen removal performance in low-strength ammonium wastewater treatment by cultivating concentrated, highly dispersive, and easily settleable granule sludge in a one-stage partial nitritation/anammox-HAP reactor

In low-strength ammonium wastewater (LSAWW) treatment, the application of anammox-based process is still limited due to extreme instability and the poor nitrogen removal rate (NRR). In this work, granule sludge, comprised of functional microbes and hydroxyapatite (HAP), was inoculated and cultivated in a one-stage partial nitritation/anammox (PNA) reactor for LSAWW treatment. The results showed that at the hydraulic retention time (HRT) of about 1.0 h and the influent ammonium concentration of 63.0 mg/L, an average NRR of 1.28 kg/m³/d was achieved, which far exceeds that reported in similar studies. The main inorganic matter in sludge was identified as HAP through the X-ray diffractometer and Raman spectrum analysis. The tomographic images of wet granule created through computed tomography revealed that the interior density of the granules was uneven and many hollow structures existed in the granule interior. Combined with the Scanning Electron Microscope images of dry granules, it was found that the granules were comprised of hollow sub-granules. Since the biomass in the reactor increased with no obvious increase in the granule size, it was inferred that the hollow sub-granules had fragile connections with each other and that granules division occurred easily, resulting in the high dispersity of sludge. Florescence in situ hybridization results also showed that the ammonium-oxidizing bacteria and anammox bacteria were mainly distributed in the two sides of the sub-granule shells and the HAP in the middle. This kind of structure raised the density of granules and improved the settleability of sludge, which made it possible to achieve a high biomass in the reactor at a short HRT. Therefore, the sludge formed in the reactor was concentrated, highly dispersive and easily settleable. These factors appear to be crucial for achieving the desired nitrogen removal performance. This study marks a big leap in LSAWW treatment through the one-stage PNA process and has great potential in actual applications.

Fungi and viruses as important players in microbial mats

Microbial mats are compacted, surface-associated microbial ecosystems reminiscent of the first living communities on early Earth. While often considered predominantly prokaryotic, recent findings show that both fungi and viruses are ubiquitous in microbial mats, albeit their functional roles remain unknown. Fungal research has mostly focused on terrestrial and freshwater ecosystems where fungi are known as important recyclers of organic matter, whereas viruses are exceptionally abundant and important in aquatic ecosystems. Here, viruses have shown to affect organic matter cycling and the diversity of microbial communities by facilitating horizontal gene transfer and cell lysis. We hypothesise fungi and viruses to have similar roles in microbial mats. Based on the analysis of previous research in terrestrial and aquatic ecosystems, we outline novel hypotheses proposing strong impacts of fungi and viruses on element cycling, food web structure and function in microbial mats, and outline experimental approaches for studies needed to understand these interactions.

Microbiological Sulfide Removal-From Microorganism Isolation to Treatment of Industrial Effluent

Management of excessive aqueous sulfide is one of the most significant challenges of treating effluent after biological sulfate reduction for metal recovery from hydrometallurgical leachate. The main objective of this study was to characterize and verify the effectiveness of a sulfide-oxidizing bacterial (SOB) consortium isolated from post-mining wastes for sulfide removal from industrial leachate through elemental sulfur production. The isolated SOB has a complete sulfur-oxidizing metabolic system encoded by sox genes and is dominated by the Arcobacter genus. XRD analysis confirmed the presence of elemental sulfur in the collected sediment during cultivation of the SOB in synthetic medium under controlled physicochemical conditions. The growth yield after three days of cultivation reached ~2.34 g_protein/mol_sulfid, while approximately 84% of sulfide was transformed into elemental sulfur after 5 days of incubation. Verification of isolated SOB on the industrial effluent confirmed that it can be used for effective sulfide concentration reduction (~100% reduced from the initial 75.3 mg/L), but for complete leachate treatment (acceptable for discharged limits), bioaugmentation with other bacteria is required to ensure adequate reduction of chemical oxygen demand (COD).

Hydroxyapatite crystallization-based phosphorus recovery coupling with the nitrogen removal through partial nitritation/anammox in a single reactor

For digestion effluent treatment, while the anammox-based process has been successfully applied for nitrogen removal, in most cases, phosphorus (P) represents another major concern. In this study, a novel process, integrating the partial nitritation/anammox and hydroxyapatite crystallization (PNA-HAP) in a single airlift reactor, was developed for the simultaneous nitrogen removal and P recovery from synthetic digestion effluent. With a stable influent P concentration of 20.0 mg/L, an HRT of 6 h, and alternating increases of influent calcium and ammonium, the final achieved nitrogen removal rate was 1.2 kg/m³/d and the P removal efficiency was 83.0%. The settleability of sludge was desirably enhanced with the calcium addition and a high biomass concentration was achieved in reactor. Quantitative and qualitative analyses confirmed that HAP was the main inorganic content in sludge, which could be harvested for P recovery. According to the Scanning Electron Microscope observation and the Energy Dispersive X-ray spectrometry analysis, the microbes were mainly distributed on the outer layer of the sludge aggregate, while the HAP mainly in the interior. The relevant theoretical calculation also revealed that the sludge discharge manipulation has direct effect on the sludge composition and aggregate structure. In sum, the results are evidence of the feasibility of simultaneous nitrogen removal and P recovery through one-stage PNA-HAP process for digestion effluent.

Combining thermophilic aerobic reactor (TAR) with mesophilic anaerobic digestion (MAD) improves the degradation of pharmaceutical compounds

The removal efficiency of nine pharmaceutical compounds from primary sludge was evaluated in two different operating conditions: (i) in conventional Mesophilic Anaerobic Digestion (MAD) alone and (ii) in a co-treatment process combining Mesophilic Anaerobic Digestion and a Thermophilic Aerobic Reactor (MAD-TAR). The pilot scale reactors were fed with primary sludge obtained after decantation of urban wastewater. Concerning the biodegradation of organic matter, thermophilic aeration increased solubilization and hydrolysis yields of digestion, resulting in a further 26% supplementary removal of chemical oxygen demand (COD) in MAD-TAR process compared to the conventional mesophilic anaerobic digestion. The highest removal rate of target micropollutants were observed for caffeine (CAF) and sulfamethoxazole (SMX) (>89%) with no substantial differences between both processes. Furthermore, MAD-TAR process showed a significant increase of removal efficiency for oxazepam (OXA) (73%), propranolol (PRO) (61%) and ofloxacine (OFL) (41%) and a slight increase for diclofenac (DIC) (4%) and 2 hydroxy-ibuprofen (2OH-IBP) (5%). However, ibuprofen (IBP) and carbamazepine (CBZ) were not degraded during both processes. Anaerobic digestion affected the liquid-solid partition of most target compounds. Sorbed fraction of pharmaceutical compounds on the sludge tend to decrease after digestion, this tendency being more pronounced in the case of the MAD-TAR process due to much lower concentration of solids.

Application of activated sludge for odor control in wastewater treatment plants: Approaches, advances and outlooks

Odors from wastewater treatment plants (WWTPs) have attracted extensive attention and stringent environmental standards are more widely adopted to reduce odor emissions. Biological odor treatment methods have broader applications than the physical and chemical counterparts as they are environment-friendly, cost-effective and generate low secondary wastes. The aqueous activated sludge (AS) processes are among the most promising approaches for the prevention or end-of-pipe removal of odor emissions and have the potential to simultaneously treat odor and wastewater. However, AS deodorization biotechnologies in WWTPs still need to be further systematically summarized and categorized while in-depth discussions on the characteristics and underlying mechanisms of AS deodorization process are still lacking. Recently, considerable studies have been reported to elucidate the microbial metabolisms in odor control and wastewater treatment. This paper reviews the fundamentals, characteristics, advances and field experiences of three AS biotechnologies for odor treatment in WWTPs, i.e., AS recycling, microaeration in AS digester and AS diffusion. The underlying deodorization mechanisms of typical odors have been revealed through the summary of recent advances on multi-element conversions, metabolic interactions of bacteria, microscopic characterization and identification of functional microorganisms. Future research aspects to advance the emerging deodorization AS process, such as deodorization mechanisms, simultaneous odor and water treatment, synergistic treatment with other air emissions, are discussed.

The performance and microbial community in a slightly alkaline biotrickling filter for the removal of high concentration H2S from biogas

In this study, high concentration of H₂S (i.e., 5000 ppmv) in biogas was effectively removed by a slightly alkaline biotricking filter (BTF) with Polypropylene rings as packing material and oxygen from air as the electron acceptor. The results showed that when the inlet loading of H₂S increased from 101.7 to 422.0 g/m³/h, the removal efficiency of H₂S decreased from 100.0% to 91.4%, and the maximum elimination capacity (EC) was 386.0 ± 10.5 gH₂S/m³/h when empty bed retention time (EBRT) was 1.0 min. The slightly alkaline condition could increase the mass transfer of H₂S from gas to liquid phase and avoid the toxic effect of high concentration of H₂S, resulting in high removal performance of H₂S in the system. With the increase of H₂S inlet loading, the ratio of SO₄^2- in bio-desulfurization products gradually decreased, while that of S⁰ increased. At 101.7-210.7 gH₂S/m³/h of inlet loading, SO₄^2- was the dominant product with the ratio of above 50.00%, while S⁰ became the dominant product with 62.96% at 422.0 gH₂S/m³/h of inlet loading. The 16S rDNA sequencing results showed that the dominant genus in the BTF was sulfide-oxidizing bacteria (SOB), with the abundance of SOB decreased with the increase of inlet loading. The dominant genus were Pseudomonas, Halothiobacillus and Sulfurimonas in the BTF at 101.7, 139.8 and 210.7 gH₂S/m³/h of inlet loading, respectively. The SOB Sulfurimonas might play an important role for bio-desulfurization of high concentration of H₂S in a slightly alkaline BTF under high inlet loading of H₂S.

Review on microaeration-based anaerobic digestion: State of the art, challenges, and prospectives

Microaeration (dosing small quantities of air or oxygen) is an effective approach to facilitate anaerobic digestion (AD) process and has gained increased attention in recent years. The underlying mechanisms of the facilitation effect of microaeration on AD process were reviewed in terms of accelerating hydrolysis, scavenging hydrogen sulfide, and affecting microbial diversity. Process parameters and control strategies were summarized to reveal considerable factors in implementing microaeration-based AD process. In addition, current applications, including lab-, pilot- and full-scale level cases, were summarized to provide guidance for further improvement in large-scale applications. The challenges and future perspectives were also highlighted to promote the development of AD process associated with microaeration.

Enhancing the removal of H2S from biogas through refluxing of outlet gas in biological bubble-column

Biological bubble-column (BBC) is beneficial for elemental sulfur recycle from H₂S, but it's difficult to remove high concentration of H₂S in biogas efficiently due to the mass transfer limitation of H₂S from gas to liquid. In this study, a novel method with refluxing outlet gas in BBC was investigated. The results showed that gas reflux greatly enhanced the removal of high concentration of H₂S (about 5000 ppmv) from biogas. The removal efficiency of H₂S was 88.0 ± 4.1% with the reflux ratio at 1.0, which was higher than those without gas reflux (58.4 ± 1.0%), when the inlet H₂S loading was 143.1 ± 4.5 g/(m³·h). Moreover, the removal capacity of H₂S improved significantly with the increase of the reflux ratios from 1.0 to 4.0 and achieved the maximum at 271.8 ± 2.4 g/(m³·h). This might mainly be attributed to longer residence time and enhanced the mass transfer of O₂ and H₂S from gas to liquid through gas reflux.

Evaluation of different air dosing strategies to enhance H2S removal in microaerobic systems treating low-strength wastewaters

This study aimed to evaluate different air dosing strategies such as microaeration flow rates and air dosing points to enhance H₂S removal in microaerobic systems treating low-strength wastewaters. Efficiency and stability of the reactors, as well as biogas quality, were assessed, and microbial community changes were evaluated using the PCR-DGGE technique. The results showed that the air dosing point affected the H₂S concentration and that air dosing at the headspace promoted the highest H₂S removal efficiency. The airflow rate also affected the process, since H₂S concentration in the biogas was higher at 0.1 mL air.min^-1 than at 0.3 mL air.min^-1. The methane concentration in the biogas was also affected by both air dosing point and flow rate, since the lowest value was observed at the highest airflow rate of the headspace dosing point, due to dilution by the N₂ influx applied to the system. The highest productivity and operational efficiency were observed at this air dosing point, with this airflow (HD_0.3), which corroborates with the operational results and the ecological parameters, since the microaeration at this stage promoted high bacterial and archaeal species richness and diversity, optimum functional organization, high COD and H₂S removal efficiencies.

Mathematical modelling of in-situ microaerobic desulfurization of biogas from sewage sludge digestion

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An extension of the ADM1 model for the microaeration process is proposed.

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The model was tested with data from pilot scale digester operated for 200 d.

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Results indicate that the model can be used to predict the digester behavior.

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The addition of a retention parameter for the SOB improved the model performance.

Microaeration can be used to cost-effectively remove in-situ H₂S from the biogas generated in anaerobic digesters. This study is aimed at developing and validating an extension of the Anaerobic Digestion Model n°1 capable of incorporating the main phenomena which occurs during microaeration. This innovative model was implemented and tested with data from a pilot scale digester microaerated for ∼ 200 d. The results showed that despite the model’s initial ability to predict the digester’s behavior, its predicted performance was improved by calibrating the most influential parameters. The model’s prediction potential was largely enhanced by adding retention parameters that account for the activity of sulfide oxidizing bacteria retained inside the anaerobic digester, which have been consistently shown to be responsible for a large share of the H₂S removed.

A new process for simultaneous nitrogen removal and phosphorus recovery using an anammox expanded bed reactor

Phosphorus recovery from wastewater is an important approach for sustainable phosphorus use. In this work, a process combining anammox and hydroxyapatite (HAP) precipitation in an expanded bed reactor for simultaneous nitrogen removal and phosphorus recovery was developed by applying specific Ca/P ratio and pH control. A high phosphorus removal rate (0.14 ± 0.01 kg-P/m³/d) was obtained while a stable nitrogen removal efficiency (87.4 ± 2.9%) maintained with an effluent recirculation system applied. Average 13.4% phosphorus (30.7% in P₂O₅) accumulation in the dry sludge and a Ca/P ratio very close to HAP was observed by quantitative elemental analysis. In this work, different analysis revealed the two layers structure with anammox biofilm attached to inorganic core of the granules. Different spectral analysis determined the major phase of the inorganic content as hydroxyapatite. With proper Ca/P ratio and pH control, anammox expanded bed reactor was transformed into an efficient process to simultaneously remove nitrogen and recover phosphorus.

Technical, Economical, and Microbiological Aspects of the Microaerobic Process on H2S Removal for Low Sulfate Concentration Wastewaters

We studied the feasibility of the microaerobic process, in comparison with the traditional chemical absorption process (NaOH), on H₂S removal in order to improve the biogas quality. The experiment consisted of two systems: R1, biogas from an anaerobic reactor was washed in a NaOH solution, and R2, headspace microaeration with atmospheric air in a former anaerobic reactor. The microaeration used for low sulfate concentration wastewater did not affect the anaerobic digestion, but even increased system stability. Methane production in the R2 was 14 % lower compared to R1, due to biogas dilution by the atmospheric air used. The presence of oxygen in the biogas reveals that not all the oxygen was consumed for sulfide oxidation in the liquid phase indicating mass transfer limitations. The reactor was able to rapidly recover its capacity on H₂S removal after an operational failure. Bacterial and archaeal richness shifted due to changes in operational parameters, which match with the system functioning. Finally, the microaerobic system seems to be more advantageous for both technical and economical reasons, in which the payback of microaerobic process for H₂S removal was 4.7 months.

Sulphate production by Paracoccus pantotrophus ATCC 35512 from different sulphur substrates: sodium thiosulphate, sulphite and sulphide

One of the problems in waste water treatment plants (WWTPs) is the increase in emissions of hydrogen sulphide (H2S), which can cause damage to the health of human populations and ecosystems. To control emissions of this gas, sulphur-oxidizing bacteria can be used to convert H2S to sulphate. In this work, sulphate detection was performed by spectrophotometry, ion chromatography and atomic absorption spectrometry, using Paracoccus pantotrophus ATCC 35512 as a reference strain growing in an inorganic broth supplemented with sodium thiosulphate (Na2S2O3·5H2O), sodium sulphide (Na2S) or sodium sulphite (Na2SO3), separately. The strain was metabolically competent in sulphate production. However, it was only possible to observe significant differences in sulphate production compared to abiotic control when the inorganic medium was supplemented with sodium thiosulphate. The three methods for sulphate detection showed similar patterns, although the chromatographic method was the most sensitive for this study. This strain can be used as a reference for sulphate production in studies with sulphur-oxidizing bacteria originating from environmental samples of WWTPs.

Understanding methane bioelectrosynthesis from carbon dioxide in a two-chamber microbial electrolysis cells (MECs) containing a carbon biocathode

To better understand the underlying mechanisms for methane bioelectrosynthesis, a two-chamber MECs containing a carbon biocathode was developed and studied. Methane production substantially increased with increasing cathode potential. Considerable methane yield was achieved at a poised potential of -0.9 V (vs. Ag/AgCl), reaching 2.30±0.34 mL after 5 h of operation with a faradaic efficiency of 24.2±4.7%. Confirmatory tests done at 0.9 V by switching the type of flushed substrates (CO2/N2) or the electrical exposure modes (ON/OFF) demonstrated that cathode serving as an electron donor was the vital driving force for methanogenesis occurring at microbe-electrode surface. Fluorescence in situ hybridization reveled Methanobacteriaceae (particularly Methanobacterium) was the predominant methanogens, supporting the mechanisms of direct electron transfer between cell-electrode. Additionally, the analysis of scanning electron microscope confirmed that the multiple pathways of electron transfer, including direct cathode-to-cell, interspecies exchange and semi-conductive conduits all together ensured the successful electromethanogenesis process.

Where does the removal of H₂S from biogas occur in microaerobic reactors?

In order to maximise the efficiency of biogas desulphurisation and reduce the oxygen cost during microaerobic digestion, it is essential to know how the process occurs. For this purpose, a reactor with a total volume of 266 L, treating 10 L/d of sewage sludge, was operated with 25.0 L and without headspace. Under anaerobic conditions, the H2S concentration in the biogas varied between 0.21 and 0.38%v/v. Next, O2 was supplied from the bottom of the reactor. At 0.25-0.30 NLO₂/Lfed, the biogas was entirely desulphurised, and its O₂ content remained below 1.03%v/v, when the digester had 25.0 L of gas space. However, with almost no headspace, the H2S content in the biogas fluctuated from 0.08 to 0.21%v/v, while the average O2 concentration was 1.66%v/v. The removed H2S accumulated in the outlet pipe of the biogas in the form of S(0) due to the insufficient headspace.

Microbial community functional structure in response to micro-aerobic conditions in sulfate-reducing sulfur-producing bioreactor

Limited oxygen supply to anaerobic wastewater treatment systems had been demonstrated as an effective strategy to improve elemental sulfur (S(0)) recovery, coupling sulfate reduction and sulfide oxidation. However, little is known about the impact of dissolved oxygen (DO) on the microbial functional structures in these systems. We used a high throughput tool (GeoChip) to evaluate the microbial community structures in a biological desulfurization reactor under micro-aerobic conditions (DO: 0.02-0.33 mg/L). The results indicated that the microbial community functional compositions and structures were dramatically altered with elevated DO levels. The abundances of dsrA/B genes involved in sulfate reduction processes significantly decreased (p < 0.05, LSD test) at relatively high DO concentration (DO: 0.33 mg/L). The abundances of sox and fccA/B genes involved in sulfur/sulfide oxidation processes significantly increased (p < 0.05, LSD test) in low DO concentration conditions (DO: 0.09 mg/L) and then gradually decreased with continuously elevated DO levels. Their abundances coincided with the change of sulfate removal efficiencies and elemental sulfur (S(0)) conversion efficiencies in the bioreactor. In addition, the abundance of carbon degradation genes increased with the raising of DO levels, showing that the heterotrophic microorganisms (e.g., fermentative microorganisms) were thriving under micro-aerobic condition. This study provides new insights into the impacts of micro-aerobic conditions on the microbial functional structure of sulfate-reducing sulfur-producing bioreactors, and revealed the potential linkage between functional microbial communities and reactor performance.

The headspace of microaerobic reactors: sulphide-oxidising population and the impact of cleaning on the efficiency of biogas desulphurisation

O2-limiting/microaerobic conditions were applied in order to control the H2S content of biogas. The S(0)-rich deposits found all over the headspace of two pilot reactors (R1 and R2) as a result of operating under such conditions for 7 and 15 months (respectively) were sampled and removed. After restarting micro-oxygenation, H2S-free biogas was rapidly obtained, and the O2 demand of R2 decreased. This highlighted the need for a cleaning interval of less than 14 months in order to minimise the micro-oxygenation cost. The H2S removed from R2 after approximately 1 month was recovered from its headspace as S(0), thus indicating that the biogas desulphurisation did not take place at the liquid interface. Denaturing gradient gel electrophoresis indicated that the composition, species richness and size of the sulphide-oxidising bacteria population depended on the location, and, more specifically, moisture availability, and indicated increasing species richness over time. Additionally, a possible succession was estimated.

Microbial community composition of a down-flow hanging sponge (DHS) reactor combined with an up-flow anaerobic sludge blanket (UASB) reactor for the treatment of municipal sewage

The microbial community composition of a down-flow hanging sponge (DHS) reactor in an up-flow anaerobic sludge blanket (UASB)-DHS system used for the treatment of municipal sewage was investigated. The clone libraries showed marked differences in microbial community composition at different reactor heights and in different seasons. The dominant phylotypes residing in the upper part of the reactor were likely responsible for removing organic matters because a significant reduction in organic matter in the upper part was observed. Quantification of the amoA genes revealed that the proportions of ammonia oxidizing bacteria (AOB) varied along the vertical length of the reactor, with more AOB colonizing the middle and lower parts of the reactor than the top of the reactor. The findings indicated that sewage treatment was achieved by a separation of microbial habitats responsible for organic matter removal and nitrification in the DHS reactor.

Microaerobic desulphurisation unit: a new biological system for the removal of H₂S from biogas

A new biotechnology for the removal of H2S from biogas was devised. The desulphurisation conditions present in microaerobic digesters were reproduced inside an external chamber called a microaerobic desulphurisation unit (MDU). A 10 L-unit was inoculated with 1L of digested sludge in order to treat the biogas produced in a pilot digester. During the 128 d of research under such conditions, the average removal efficiency was 94%. The MDU proved to be robust against fluctuations in biogas residence time (57-107 min), inlet H2S concentration (0.17-0.39% v/v), O2/H2S supplied ratio (17.3-1.4 v/v), and temperature (20-35°C). Microbiological analysis confirmed the presence of at least three genera of sulphide-oxidising bacteria. Approximately 60% of all the H2S oxidised was recovered from the bottom of the system in the form of large solid S(0) sheets with 98% w/w of purity. Therefore, this system could become a cost-effective alternative to the conventional biotechniques for biogas desulphurisation.

Shifts in nitrification kinetics and microbial community during bioaugmentation of activated sludge with nitrifiers enriched on sludge reject water

This study used two laboratory-scale sequencing batch reactors (SBRs) to evaluate the shifts in nitrification kinetics and microbial communities of an activated sludge sewage treatment system (main stream) during bioaugmentation with nitrifiers cultivated on real sludge reject water (side stream). Although bioaugmentation exerted a strong influence on the microbial community and the nitrification kinetics in the main stream, there was 58% of maximum ammonia uptake rate (AUR) and 80% of maximum nitrite uptake rate (NUR) loss of the seed source after bioaugmentation. In addition, nitrite accumulation occurred during bioaugmentation due to the unequal and asynchronous increase of the AUR (from 2.88 to 13.36 mg N/L·h) and NUR (from 0.76 to 4.34 mg N/L·h). FISH results showed that ammonia oxidizing bacteria (AOB) was inclined to be washed out with effluent in contrast to nitrite oxidizing bacteria (NOB), and Nitrosococcus mobilis lineage was the dominant AOB, while the dominant NOB in the main stream gradually transferred from Nitrospira to Nitrobacter. Nitrospina and Nitrococcus which existed in the seed source could not be detected in the main stream. It can be inferred that nitrite accumulation occurred due to the mismatch of NOB structure but washed out with effluent.

Molecular analysis of the biomass of a fluidized bed reactor treating synthetic vinasse at anaerobic and micro-aerobic conditions

The microbial communities (Bacteria and Archaea) established in an anaerobic fluidized bed reactor used to treat synthetic vinasse (betaine, glucose, acetate, propionate, and butyrate) were characterized by denaturing gradient gel electrophoresis (DGGE) and phylogenetic analysis. This study was focused on the competitive and syntrophic interactions between the different microbial groups at varying influent substrate to sulfate ratios of 8, 4, and 2 and anaerobic or micro-aerobic conditions. Acetogens detected along the anaerobic phases at substrate to sulfate ratios of 8 and 4 seemed to be mainly involved in the fermentation of glucose and betaine, but they were substituted by other sugar or betaine degraders after oxygen application. Typical fatty acid degraders that grow in syntrophy with methanogens were not detected during the entire reactor run. Likely, sugar and betaine degraders outnumbered them in the DGGE analysis. The detected sulfate-reducing bacteria (SRB) belonged to the hydrogen-utilizing Desulfovibrio. The introduction of oxygen led to the formation of elemental sulfur (S(0)) and probably other sulfur compounds by sulfide-oxidizing bacteria (γ-Proteobacteria). It is likely that the sulfur intermediates produced from sulfide oxidation were used by SRB and other microorganisms as electron acceptors, as was supported by the detection of the sulfur respiring Wolinella succinogenes. Within the Archaea population, members of Methanomethylovorans and Methanosaeta were detected throughout the entire reactor operation. Hydrogenotrophic methanogens mainly belonging to the genus Methanobacterium were detected at the highest substrate to sulfate ratio but rapidly disappeared by increasing the sulfate concentration.