Azo dye reduction by thermophilic anaerobic granular sludge,and the impact of the redox mediator anthraquinone-2,6-disulfonate (AQDS) on the reductive biochemical transformation

Azo dye reduction at 55°C by thermophilic anaerobic granular sludge was investigated distinguishing between the biotic and abiotic mechanisms. The impact of the redox mediator anthraquinone-2,6-disulfonate (AQDS) on colour removal and co-substrate oxidation was also investigated. Metabolic activities of the thermophilic inoculum induced a fast azo dye reduction and indicated a biotic predominance in the process. The addition of co-substrate enhanced the decolourisation rates 1.7-fold compared with the bottles free of co-substrate. Addition of AQDS together with co-substrate enhanced the k value 1.5-fold, compared with the incubation containing co-substrate in the absence of AQDS. During a comparative study between sludge samples incubated under mesophilic (30°C) and thermophilic (55°C) conditions, the decolourisation rate at 55°C reached values up to sixfold higher than at 30°C. Biological treatment at 55°C showed a fast initial generation of reducing compounds via co-substrate oxidation, with AQDS increasing the azo dye reduction rate in all the incubations tested. Nevertheless, high concentrations of AQDS showed severe inhibition of thermophilic acetate and propionate oxidation and methane production rates. These promising results indicate that there may be good prospects for thermophilic anaerobic treatment of other reductive transformations such as reduction of nitroaromatics and dehalogenation.     

Application of redox mediators to accelerate the transformation of reactive azo dyes in anaerobic bioreactors.

Azo dyes are nonspecifically reduced under anaerobic conditions but the slow rates at which reactive azo dyes are converted presents a serious problem for the application of anaerobic technology as a first stage in the complete biodegradation of these compounds. As quinones have been found to catalyze reductive transfers by acting as redox mediators, the application of anthraquinone-2,6-disulfonic acid (AQDS) during continuous anaerobic treatment of the reactive azo dye, Reactive Red 2 (RR2), was evaluated. A mixture of volatile fatty acids was used as the electron-donating primary substrate. Batch experiments demonstrated that AQDS could increase the first-order rate constant of RR2 reductive cleavage by one order of magnitude. In the continuous experiment, treatment of RR2 containing synthetic wastewater in a lab-scale upflow anaerobic sludge blanket (UASB) reactor yielded low dye removal efficiencies (<30%). Consequently, severe toxicity problems occurred, eventually resulting in almost complete inhibition of the methanogenic activity. Addition of catalytic concentrations of AQDS (19 microM) to the reactor influent caused an immediate increase in the dye removal efficiency and recovery of biological activity. Ultimately, RR2 removal efficiency stabilized at 88%, and higher AQDS loads resulted in higher RR2 removal efficiencies (up to 98% at 155 microM AQDS). Examination of the RR2 decolorizing properties of dye-adapted reactor sludge and of nonadapted reactor seed sludge revealed that RR2 decolorization was principally a biologically driven transfer of reducing equivalents from endogenous and added substrates to the dye. Hydrogen, added in bulk, was clearly the preferred electron donor. Bacteria that couple dye decolorization to hydrogen oxidation were naturally present in seed sludge. However, enrichment was required for the utilization of electrons from volatile fatty acids for dye reduction. The stimulatory effect of AQDS on RR2 decolorization by AQDS-unadapted sludge was mainly due to assisting the electron transfer from endogenous substrates in the sludge to the dye. The stimulatory effect of AQDS on RR2 decolorization by sludge from the AQDS-exposed reactor was, in addition, strongly associated with the transfer of electrons from hydrogen and acetate to the dye, probably due to enrichment of specialized AQDS-reducing bacteria.     

Enhancing the electron transfer capacity and subsequent color removal in bioreactors by applying thermophilic anaerobic treatment and redox mediators

The effect of temperature, hydraulic retention time (HRT) and the redox mediator anthraquinone-2,6-disulfonate (AQDS), on electron transfer and subsequent color removal from textile wastewater was assessed in mesophilic and thermophilic anaerobic bioreactors. The results clearly show that compared with mesophilic anaerobic treatment, thermophilic treatment at 55 degrees C is an effective approach for increasing the electron transfer capacity in bioreactors, and thus improving the decolorization rates. Furthermore, similar color removals were found at 55 degrees C between the AQDS-free and AQDS-supplemented reactors, whereas a significant difference (up to 3.6-fold) on decolorization rates occurred at 30 degrees C. For instance, at an HRT of 2.5 h and in the absence of AQDS, the color removal was 5.3-fold higher at 55 degrees C compared with 30 degrees C. The impact of a mix of mediators with different redox potentials on the decolorization rate was investigated with both industrial textile wastewater and the azo dye Reactive Red 2 (RR2). Color removal of RR2 in the presence of anthraquinone-2-sulfonate (AQS) (standard redox potential E(0)' of -225 mV) was 3.8-fold and 2.3-fold higher at 30 degrees C and 55 degrees C, respectively, than the values found in the absence of AQS. Furthermore, when the mediators 1,4-benzoquinone (BQ) (E(0)' of +280 mV), and AQS were incubated together, there was no improvement on the decolorization rates compared with the bottles solely supplemented with AQS. Results imply that the use of mixed redox mediators with positive and negative E(0)' under anaerobic conditions is not an efficient approach to improve color removal in textile wastewaters.     

Biological sulphate reduction and redox mediator effects on azo dye decolourisation in anaerobic–aerobic sequencing batch reactors

In this work, the anaerobic period of an anaerobic–aerobic sequencing batch reactor was found to allow the reductive decolourisation of azo dyes. 1-l reactors were operated in 24-h cycles comprising anaerobic and aerobic reaction phases, fed with a simulated textile effluent including a reactive type (Remazol Brilliant Violet 5R) or an acid type (Acid Orange 7) azo dye. The aim was to assess the role of different redox phenomena in the anaerobic decolourisation process. Selective inhibition of sulphate reducing bacteria was carried out in the sulphate-containing, reactive dye fed reactor, resulting in nearly complete, though reversible and inhibition of decolourisation. The acid dye fed reactor's supplementation with sulphate, though resulting in sulphate reduction, did not improve decolourisation. Other redox mediators, namely quinones, were more effective in promoting electron transfer to the azo bond. Bio-augmentation of the acid dye fed reactor with a pure sulphate reducer strain known to decolourise azo dyes, Desulfovibrio alaskensis, was also carried out. Decolourisation was improved, but apparently as a result of the carbon source change required to support D. alaskensis growth. A chemically mediated reduction of the azo bond coupled to biological sulphate reduction, thus seemed to account for the high decolourisation yields of both dyes.     

Batch tests for assessing decolourisation of azo dyes by methanogenic and mixed cultures

Most of the published studies on azo dye colour removal involve anaerobic mixed cultures and there is some interest in the knowledge of how dye reduction occurs, if by facultative, strictly anaerobic or both bacterial trophic groups present in classic anaerobic digestors. This paper describes the behaviour of methanogenic and mixed bacteria cultures on the colour removal in batch systems, of a commercial azo dye, C.I. Acid Orange 7, used in paper and textile industries. The aim of this study is to demonstrate, by analysing dye decolourisation, that it occurs with mixed cultures as well as with strictly anaerobic (methanogenic) cultures. Tests were performed with a range of dye concentrations between 60 and 300 mg l⁻¹. The influence of dye concentration on the carbon source removal and decolourisation processes was studied. The effect of carbon source concentration on colour removal was also analysed for both cultures. The degradation rates in mixed and methanogenic cultures were compared. The consumption of carbon source was monitored by COD analysis and dye degradation by ultraviolet-visible spectrophotometry and thin layer chromatography.     

Accelerated removal of Sudan dye by Shewanella oneidensis MR-1 in the presence of quinones and humic acids

Although there have been many studies on bacterial removal of soluble azo dyes, much less information is available for biological treatment of water-insoluble azo dyes. The few bacterial species capable of removing Sudan dye generally require a long time to remove low concentrations of insoluble dye particles. The present work examined the efficient removal of Sudan I by Shewanella oneidensis MR-1 in the presence of redox mediator. It was found that the microbially reduced anthraquinone-2,6-disulfonate (AQDS) could abiotically reduce Sudan I, indicating the feasibility of microbially-mediated reduction. The addition of 100 μM AQDS and other different quinone compounds led to 4.3–54.7 % increase in removal efficiencies in 22 h. However, adding 5-hydroxy-1,4-naphthoquinone into the system inhibited Sudan I removal. The presence of 10, 50 and 100 μM AQDS stimulated the removal efficiency in 10 h from 26.4 to 42.8, 54.9 and 64.0 %, respectively. The presence of 300 μM AQDS resulted in an eightfold increase in initial removal rate from 0.19 to 1.52 mg h^?1 g^?1 cell biomass. A linear relationship was observed between the initial removal rates and AQDS concentrations (0–100 μM). Comparison of Michaelis–Menten kinetic constants revealed the advantage of AQDS-mediated removal over direct reduction. Different species of humic acid could also stimulate the removal of Sudan I. Scanning electronic microscopy analysis confirmed the accelerated removal performance in the presence of AQDS. These results provide a potential method for the efficient removal of insoluble Sudan dye.     

Monoazo and diazo dye decolourisation studies in a methanogenic UASB reactor

Mixed anaerobic bacterial consortia have been show to reduce azo dyes and batch decolourisation tests have also demonstrated that predominantly methanogenic cultures also perform azo bond cleavage. The anaerobic treatment of wool dyeing effluents, which contain acetic acid, could thus be improved with a better knowledge of methanogenic dye degradation. Therefore, the decolourisation of two azo textile dyes, a monoazo dye (Acid Orange 7, AO7) and a diazo dye (Direct Red 254, DR254), was investigated in a methanogenic laboratory-scale Upflow Anaerobic Sludge Blanket (UASB), fed with acetate as primary carbon source. As dye concentration was increased a decrease in total COD removal was observed, but the acetate load removal (90%) remained almost constant. A colour removal level higher than 88% was achieved for both dyes at a HRT of 24h. The identification by HPLC analysis of sulfanilic acid, a dye reduction metabolite, in the treated effluent, confirmed that the decolourisation process was due mainly to azo bond reduction. Although, HPLC chromatograms showed that 1-amino-2-naphthol, the other AO7 cleavage metabolite, was removed, aeration batch assays demonstrated that this could be due to auto-oxidation and not biological mineralization. At a HRT of 8h, a more extensive reductive biotransformation was observed for DR254 (82%) than for AO7 (56%). In order to explain this behaviour, the influence of the dye aggregation process and chemical structure of the dye molecules are discussed in the present work.     

Effects of alkalinity and co-substrate on the performance of an upflow anaerobic sludge blanket (UASB) reactor through decolorization of Congo Red azo dye

The effect of substrate (glucose) concentrations and alkalinitiy (NaHCO3) on the decolorization of a synthetic wastewater containing Congo Red (CR) azo dye was performed in an upflow anaerobic sludge blanket (UASB). Color removal efficiencies approaching 100% were obtained at glucose-COD concentrations varying between 0 and 3000 mg/l. The methane production rate and total aromatic amine (TAA) removal efficiencies were found to be 120 ml per day and 43%, respectively, while the color was completely removed during glucose-COD free operation of the UASB reactor. The complete decolorization of CR dye under co-substrate free operation could be attributed to TAA metabolism which may provide the electrons required for the cleavage of azo bond in CR dye exist in the UASB reactor. No significant differences in pH levels (6.6-7.4), methane production rates (2000-2700 ml/day) and COD removal efficiencies (82-90%) were obtained for NAHCO3 concentrations ranging between 550 and 3000 mg/l. However, decolorization efficiency remained at 100% with decreasing NaHCO3 concentrations as low as 250 mg/l in the feed. An alkalinity/COD ratio of 0.163 in the feed was suggested for simultaneous optimum COD and color removal.     

Kinetics during the redox biotransformation of pollutants mediated by immobilized and soluble humic acids

The aim of this study was to elucidate the kinetic constraints during the redox biotransformation of the azo dye, Reactive Red 2 (RR2), and carbon tetrachloride (CT) mediated by soluble humic acids (HA_s) and immobilized humic acids (HA_i), as well as by the quinoid model compounds, anthraquinone-2,6-disulfonate (AQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). The microbial reduction of both HA_s and HA_i by anaerobic granular sludge (AGS) was the rate-limiting step during decolorization of RR2 since the reduction of RR2 by reduced HA_i proceeded at more than three orders of magnitute faster than the electron-transferring rate observed during the microbial reduction of HA_i by AGS. Similarly, the reduction of RR2 by reduced AQDS proceeded 1.6- and 1.9-fold faster than the microbial reduction of AQDS by AGS when this redox mediator (RM) was supplied in soluble and immobilized form, respectively. In contrast, the reduction of NQS by AGS occurred 1.6- and 19.2-fold faster than the chemical reduction of RR2 by reduced NQS when this RM was supplied in soluble and immobilized form, respectively. The microbial reduction of HA_s and HA_i by a humus-reducing consortium proceeded 1,400- and 790-fold faster than the transfer of electrons from reduced HA_s and HA_i, respectively, to achieve the reductive dechlorination of CT to chloroform. Overall, the present study provides elucidation on the rate-limiting steps involved in the redox biotransformation of priority pollutants mediated by both HA_s and HA_i and offers technical suggestions to overcome the kinetic restrictions identified in the redox reactions evaluated.     

Anaerobic degradation of azo dye Drimaren blue HFRL in UASB reactor in the presence of yeast extract a source of carbon and redox mediator

This paper presents results on anaerobic degradation of the azo dye blue HFRL in a bench scale Upflow anaerobic sludge blanket (UASB) reactor operated at ambient temperature. The results show that the addition of yeast extract (500 mg/L) increased color removal (P < 0.05) from 62 to 93% despite the low chemical oxygen demand (COD) removal (~35%) which happened due to volatile fatty acids (VFA) accumulation. There were no differences in color removal (~91%) when yeast extract (500 mg/L) was used in the presence or absence of glucose, suggesting that yeast extract acted as source of redox mediator (riboflavin) and carbon. The specific rate of dye removal increased along the operational phases and depended on the presence of yeast extract, suggesting progressive biomass acclimatization. Analysis of bacterial diversity by Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR–DGGE) method showed there was biomass selection along the bioreactor operation and no evidence of azo dye degrading bacteria predominance. This strengthens the hypothesis that color removal happens extracellularly by the reduction of azo bond by reduced redox mediators, such as riboflavin, which is present in high amount in the yeast extract.     

Comparison of the performance of one stage and two stage sequential anaerobic-aerobic biological processes for the treatment of reactive-azo-dye-containing synthetic wastewaters

In the present study the performance of anaerobic-aerobic one and two stage processes for the biological treatment of synthetic wastewaters containing Reactive Black 5 (RB5) were studied and compared with each other. In both processes the majority of colour removal by biodegradation occurred under anaerobic environment. The colour change under aerobic conditions was correlated with extent of anaerobic decolourisation in the preceding phase/stage of the process. Partial mineralisation of the anaerobic dye metabolites, roughly to the same extent, was achieved aerobically in both one stage and two stage processes. The majority of COD was removed in the anaerobic stage for two stage processes and aerobic stage in one stage processes. In one stage processes, the exposure of anaerobic sludge to alternating anaerobic-aerobic environment decreased anaerobic decolourisation efficiency and COD removal; when employing activated sludge, the same exposure enhanced anaerobic substrate utilisation whereas the effect on the anaerobic decolourisation efficiency depended on RB5 concentration. The comparative performance of one and two stage processes in terms of overall dye decolourisation depended on RB5 concentration. Both types of processes brought about similar overall COD removal. Increase in RB5 concentration, in the range studied, resulted in decrease in overall COD removal for both processes.     

Assessment of the biodegradability of a monosulfonated azo dye and aromatic amines

Biodecolourisation of an azo dye by anaerobic cultures using a liposomal textile levelling agent as primary substrate was assessed. Liposomes seem to facilitate the uptake of the dye (Acid Orange 7) by anaerobic biomass, leading to a fast decolourisation (colour removal of 96% was achieved in the first sample port of the reactor profiles). On the other hand, the presence of dye (60–300 mg l⁻¹) caused a decrease in the chemical oxygen demand (COD) degradation rate (4.1–2.5 g COD removed l⁻¹ d⁻¹ for 60 and 300 mg l⁻¹ of dye, respectively), suggesting inhibitory effects.Aerobic degradation of aromatic amines was investigated in aerobic respirometric assays with different types of inocula. Sulfanilic acid and aniline were mineralised by inocula with a significant microbiological diversity, even with domestic effluent. These results were confirmed by a significant reduction of COD, total organic carbon (TOC) and a high oxygen consumption (biochemical oxygen demand/theoretical oxygen demand), 92±4%. Kinetic analysis showed that a sigmoid function describes quite well the experimental data, even better than the exponential model. Orthanilic and metanilic acids and 1-amino-2-naphtol were persistent under the tested conditions.     

Effect of nitrate on anaerobic azo dye reduction

The aim of the study was to investigate the effect of nitrate on anaerobic color removal efficiencies. For this aim, anaerobic–aerobic sequencing batch reactor (SBR) fed with a simulated textile effluent including Remazol Brilliant Violet 5R azo dye was operated with a total cycle time of 12 h, including anaerobic (6 h) and aerobic cycles (6 h). Microorganism grown under anaerobic phase of the reactor was exposed to different amounts of competitive electron acceptor (nitrate) and performance of the system was determined by monitoring color removal efficiency, nitrate removal, nitrite formation and removal, oxidation reduction potential, color removal rate, chemical oxygen demand (COD), specific anaerobic enzyme (azo reductase) and aerobic enzyme (catechol 1,2 dioxygenase), and formation and removal of aromatic amines. Variations of population dynamics of microorganisms exposed to various amount of nitrate were identified by denaturing gradient gel electrophoresis (DGGE). It was found that nitrate has adverse effect on anaerobic color removal efficiency and color removal was achieved after denitrification process was completed. It was found that nitrate stimulates the COD removal efficiency and accelerates the COD removal in the first hour of anaerobic phase. About 90 % total COD removal efficiencies were achieved in which microorganism exposed to increasing amount of nitrate. Population dynamics of microorganisms exposed to various amount of nitrate were changed and diversity was increased.     

Different bioreactor configurations for the decolourisation of the azo dye reactive black 5 by Geotrichum sp. CCMI 1019

Decolourisation of the azo dye Reactive Black 5 by Geotrichum sp. CCMI 1019 was studied using stirred tank reactors (STR) and two types of bubble columns (porous plate (PP) bubble column and aeration tube (AT) bubble column). For the bubble columns, the k_La increased with the gas fractional hold-up (ε_G) and the aeration rate. A linear relationship between ε_G and superficial gas velocity was obtained for all reactors. At same aeration rates, the PP bubble columns showed higher k_La and hold-up values than the AT bubble column. In the STRs, large and dense aggregates were formed which adhered to surfaces whereas bubble columns gave smaller and less compact pellets.

Manganese peroxidase and laccase were detected in the extracellular media in all reactors. However, laccase was only detected after the onset of decolourisation, suggesting that additional enzymes may be involved. Mn peroxidase activity was detected (about 46 U/ml) in both the STRs and AT bubble columns but higher values (110 U/ml) were obtained with the PP bubble columns.

Out of the three reactor systems studied, the AT bubble columns gave the most favourable results for Reactive Black 5 decolourisation. Rapid and complete colour removal was obtained throughout the visible spectrum. Bubble columns are simple in design as well as operation and may be useful for the bioremediation of textile wastewater.     

Enhanced bio-decolorization of azo dyes by quinone-functionalized ceramsites under saline conditions

Anthraquinone-2-sulfonate was immobilized on ceramsites (AQS-ceramsites) using a novel adsorption/covalence coupling method and their effects on the anaerobic bio-decolorization rates of azo dyes by salt-tolerant AQS-reducing (STAR) community were investigated. The results showed that AQS-ceramsites mediated specific bio-decolorization rates of four azo dyes Acid Yellow 36, Reactive Red 2, Acid Red 27 and Acid Orange 7 increase 2.3–6.4 fold than those lacking ceramsites in the presence of 50 g/L NaCl. Moreover, repeated experiments with AQS-ceramsites showed that the decolorization efficiencies of azo dyes could remain over 98% of their original value. These results indicated that AQS-ceramsites functioning as redox mediators exhibited good catalytic activity and stability under saline conditions. The dynamics of the STAR community structure revealed by PCR-DGGE also showed that the presence of AQS-ceramsites made STAR bacteria keeping predominant in the catalytic system. Therefore, it can be concluded that this novel solid redox mediator is potentially useful for the treatment of saline dye wastewater.     

Biological treatment of acid dyeing wastewater using a sequential anaerobic/aerobic reactor system

The treatment of the wastewater taken from a wool dyeing processing in a wool manufacturing plant was investigated using an anaerobic/aerobic sequential system. The process units consisted of an anaerobic UASB reactor and an aerobic CSTR reactor. Glucose, alkalinity and azo dyes were added to the raw acid dyeing wastewater in order to simulate the dye industry wastewater since the raw wastewater contained low levels of carbon, NaHCO₃ and color through anaerobic/aerobic sequential treatment. The UASB reactor gave COD and color removals of 51–84% and 81–96%, respectively, at a HRT of 17 h. The COD and color removal efficiencies of the UASB/CSTR sequential reactor system were 97–83% and 87–80%, respectively, at a hydraulic retention time (HRTs) of 3.3 days. The aromatic amines (TAA) formed in the anaerobic stage were effectively removed in the aerobic stage.     

Detoxification and partial mineralization of the azo dye mordant orange 1 in a continuous upflow anaerobic sludge-blanket reactor

In batch toxicity assays, azo dye compounds were found to be many times more toxic than their cleavage products (aromatic amines) towards methanogenic activity in anaerobic granular sludge. Considering the ability of anaerobic microorganisms to reduce azo groups, detoxification of azo compounds towards methanogens can be expected to occur during anaerobic wastewater treatment. In order to test this hypothesis, the anaerobic degradation of one azo dye compound, Mordant orange 1 (MO1), by granular sludge was investigated in three separate continuous upflow anaerobic sludge-blanket reactors. One reactor, receiving no cosubstrate, failed after 50 days presumably because of a lack of reducing equivalents. However, the two reactors receiving either glucose or a volatile fatty acids (acetate, propionate, butyrate) mixture, could eliminate the dye during operation for 217 days. The azo dye was reductively cleaved to less toxic aromatic amines (1,4-phenylenediamine and 5-aminosalicylic acid) making the treatment of MO1 feasible at influent concentrations that were over 25 times higher than their 50% inhibitory concentrations. In the reactor receiving glucose as cosubstrate, 5-aminosalicylic acid could only be detected at trace levels in the effluent after day 189 of operation. Batch biodegradability assays with the sludge sampled from this reactor confirmed the mineralization of 5-aminosalicylic acid to methane. Received: 11 July 1996 / Received revision: 18 September 1996 / Accepted: 18 September 1996     

Simultaneous decolorization of azo dye and bioelectricity generation using a microfiltration membrane air-cathode single-chamber microbial fuel cell

Electricity generation from readily biodegradable organic substrates accompanied by decolorization of azo dye was investigated using a microfiltration membrane air-cathode single-chamber microbial fuel cell (MFC). Batch experiment results showed that accelerated decolorization of active brilliant red X-3B (ABRX3) was achieved in the MFC as compared to traditional anaerobic technology. Biodegradation was the dominant mechanism of the dye removal, and glucose was the optimal co-substrate for ABRX3 decolorization, while acetate was the worst one. Confectionery wastewater (CW) was also shown to be a good co-substrate for ABRX3 decolorization and a cheap fuel source for electricity generation in the MFC. Low resistance was more favorable for dye decolorization than high resistance. Suspended sludge (SS) should be retained in the MFC for accelerated decolorization of ABRX3. Electricity generation was not significantly affected by the ABRX3 at 300 mg/L, while higher concentrations inhibited electricity generation. However, voltage can be recovered to the original level after replacement with anodic medium not containing azo dye.     

Assessment of the addition of Thiobacillus denitrificans and Thiomicrospira denitrificans to chemolithoautotrophic denitrifying bioreactors

The aim of the present study was to assess the impact of adding cultures of Thiobacillus denitrificans and Thiomicrospira denitrificans to two upflow anaerobic sludge bed (UASB) reactors: one inoculated with granular sludge and the other filled only with activated carbon (AC). The performances of the bioreactors and the changes in biomass were compared with a non-bioaugmented control UASB reactor inoculated with granular sludge. The reactors inoculated with granular sludge achieved efficiencies close to 90% in nitrate and thiosulfate removal for loading rates as high as 107 mmol-NO3 -/l per day and 68 mmol-S2O3 2-/l per day. Bioaugmentation with Tb. denitrificans and Tm. denitrificans did not enhance the efficiency compared to that achieved with non-bioaugmented granular sludge. The loading rates and efficiencies were 30-40% lower in the AC reactor. In all the reactors tested, Tb. denitrificans became the predominant species. The results strongly suggest that this bacterium was responsible for denitrification and sulfoxidation within the reactors. We additionally observed that granules partially lost their integrity during operation under chemolithoautotrophic conditions, suggesting limitations for long-term operation if bioaugmentation is applied in practice.     

Enhancement of nitroaromatic compounds anaerobic biotransformation using a novel immobilized redox mediator prepared by electropolymerization

Functionalized polypyrrole (PPy) composites were prepared by incorporation of a model redox mediator, anthraquinonedisulphonate (AQDS), as doping anion during the electropolymerization of pyrrole (Py) monomer on active carbon felt (ACF) electrode. Then, the resulting composite, ACF/PPy/AQDS as a novel immobilized redox mediator for catalyzing anaerobic biotransformation of the model nitroaromatic compounds (NACs), such as nitrobenzene (NB), 2,4- and 2,6-dinitrotoluene (DNT), were investigated in detail. The results showed that ACF/PPy/AQDS exhibited good catalytic activity and stability, and its addition effectively accelerated the NACs anaerobic reduction to the corresponding amino compounds. In order to estimate the relationship between community dynamics and the function of immobilized redox mediator, a combined method based on fingerprints (ribosomal intergenic spacer analysis, RISA) and 16S rRNA gene sequencing was used. The results indicated that the existence of ACF/PPy/AQDS made the potent AQDS-reducing bacteria keeping predominant in the catalytic systems. Based on the results above, it can be concluded that this novel immobilized redox mediator is feasible and potentially useful to enhance NACs anaerobic reduction.