Dechlorination of trichloroethene in a continuous-flow bioelectrochemical reactor: effect of cathode potential on rate, selectivity, and electron transfer mechanisms

The exciting discovery that dechlorinating bacteria can use polarized graphite cathodes as direct electron donors in the reductive dechlorination has prompted investigations on the development of novel bioelectrochemical remediation approaches. In this work, we investigated the performance of a bioelectrochemical reactor for the treatment of trichloroethene (TCE). The reactor was continuously operated for about 570 days, at different potentiostatically controlled cathode potentials, ranging from -250 mV to -750 mV vs standard hydrogen electrode. The rate and extent of TCE dechlorination, as well as the competition for the available electrons, were highly dependent on the set cathode potential. When the cathode was controlled at -250 mV, no abiotic hydrogen production occurred and TCE dechlorination (predominantly to cis-DCE and VC), most probably sustained via direct extracellular electron transfer, proceeded at an average rate of 15.5 ± 1.2 μmol e(-)/L d. At this cathode, potential methanogenesis was almost completely suppressed and dechlorination accounted for 94.7 ± 0.1% of the electric current (15.0 ± 0.8 μA) flowing in the system. A higher rate of TCE dechlorination (up to 64 ± 2 μmol e(-)/L d) was achieved at cathode potentials lower than -450 mV, though in the presence of a very active methanogenesis which accounted for over 60% of the electric current. Remarkably, the bioelectrochemical reactor displayed a stable and reproducible performance even without the supply of organic carbon sources with the feed, confirming long-term viability.     

Trichloroethylene degradation in a coupled anaerobic/aerobic reactor oxygenated using hydrogen peroxide

In this work, trichloroethylene (TCE) degradation under combined anaerobic-aerobic conditions was studied in an ethanol-fed biofilm reactor oxygenated using hydrogen peroxide. The reactor was inoculated with a biomass originating from an anaerobic digestor. Granulated peat was added to the reactor as a substratum for biofilm development. Extensive characterization of reactor populations using activity tests and PCR analysis revealed the development of a mutualistic consortium, particularly methanotrophic and methanogenic microorganisms. This consortium was shown to degrade TCE by a combination of reductive and oxidative pathways. A near complete degradation of TCE at a load of 18 mg L(R)(-1) day(-1) was evidenced by a stoichiometric release of inorganic chloride.     

Combined removal of chlorinated ethenes and heavy metals by zerovalent iron in batch and continuous flow column systems

The combined removal of chlorinated ethenes and heavy metals from a simulated groundwater matrix by zerovalent iron (ZVI) was investigated. In batch, Ni (5-100 mg L(-1)) enhanced trichloroethene (TCE, 10 mg L(-1)) reduction by ZVI (100 g L(-1)) due to catalytic hydrodechlorination by bimetallic Fe0/Ni0. Cr(VI) or Zn (5-100 mg L(-1)) lowered TCE degradation rates by a factor of 2 to 13. Cr(VI) (100 mg L(-1)) in combination with Zn or Ni (50-100 mg L(-1)) inhibited TCE degradation. Addition of 20% H2(g) in the headspace, or of Zn (50-100 mg L(-1)), enhanced TCE removal in the presence of Ni and Cr(VI). Sorption of Zn to ZVI alleviated the Cr(VI) induced inhibition of bimetallic Fe0/Ni0 apparently due to release of protons necessary for TCE hydrodechlorination. In continuous ZVI columns treating tetrachloroethene (PCE, 1-2 mg L(-1)) and TCE (10 mg L(-1)), and a mixture of the metals Cr(VI), Zn(II), and Ni(II) (5 mg (L-1)), the PCE removal efficiency decreased from 100% to 90% in columns operated without heavy metals. The PCE degradation efficiency remained above 99% in columns receiving heavy metals as long as Ni was present. The findings of this study indicate the feasibility and limitations of the combined treatment of mixtures of organic and inorganic pollutants by ZVI.     

Process Characteristics of Hydrolysis of Chitosan in a Continuous Enzymatic Membrane Reactor

ABSTRACT: Crude enzyme from Bacillus cereus NTU-FC-4 was used to hydrolyze chitosan of 66% deacetylation in a membrane reactor, operated at 45 °C and pH 5, to continuously produce chitooligosaccharides. Major oligomers in the product from the reactor were chitobiose, chitotriose, chitotetraose, chitopentaose, and chitohexaose. When the membrane reactor was operated at an enzyme/substrate ratio of 0.2 (unit/mg) and residence time of 100 min, it reached steady state in 2.5 h. The system could be operated for 15 h and still maintained a stable product composition. When the volume replacement exceeded 2.5, the productivity of the membrane reactor became higher than that of the batch reactor, and the difference between them became even greater when the volume replacement was further increased. The apparent Michaelis constant (K_m) for the enzyme in the membrane reactor was 18.8 mg/mL, but the apparent K_m was 5.4 mg/mL for the batch reactor, suggesting that the affinity of the enzyme for chitosan was lower in the membrane reactor compared with the enzyme in the batch reactor. The estimated values of apparent V_max were 0.18 and 0.20 mg reducing sugar/mL/min for the enzyme in the membrane reactor and in the batch reactor, respectively, indicating that the enzyme activity was not greatly altered when used in the membrane reactor.     

The anaerobic conversion of methanol under thermophilic conditions: pH and bicarbonate dependence

The thermophilic (55 degrees C) anaerobic conversion of methanol was studied in an unbuffered medium (pH 4+/-0.2) and in a phosphate buffered medium (pH 6.4+/-0.1), in both cases without bicarbonate addition. Our cultivated sludge consortium was unable to degrade methanol under acidic conditions. During the 160 d of continuous operation of an up-flow anaerobic sludge blanket (UASB) reactor (R1), at an organic loading rate (ORL) of 6 gCOD/(l.d) and pH around 4, only 5% of the applied methanol load was consumed and no methane (CH4) was detected. However, hydrogenotrophic methanogens were found to be resistant to exposure to such conditions. At the end of the trial, the hydrogenotrophic methanogenic activity of the sludge was 1.23+/-0.16 gCOD/(gVSS.d) at neutral pH. With methanol as the test substrate, the addition of bicarbonate led to acetate accumulation. A second reactor (R2) was operated for 303 d at OLRs ranging from 5.5 to 25.4 gCOD/(l.d) in order to assess the conversion of methanol at neutral pH (phosphate buffered) in a bicarbonate deprived medium. The reactor performance was poor with a methanol-COD removal capacity limited to about 9.5 gCOD/(l.d). The system appeared to be quite susceptible to any type of disturbance, even at low OLR. The fraction of methanol-COD converted to CH4 and acetate was found to be unaffected by the OLR applied. At the end of the trial, the outcome of the competition was about 50% methanogenesis and 50% homoacetogenesis.     

Cometabolic degradation of TCE vapors in a foamed emulsion bioreactor

Effective cometabolic biodegradation of trichloroethylene (TCE) vapors in a novel gas-phase bioreactor called the foamed emulsion bioreactor (FEBR) was demonstrated. Toluene vapors were used as the primary growth substrate for Burkholderia cepacia G4 which cometabolically biodegraded TCE. Batch operation of the reactor with respect to the liquid feed showed a drastic decrease of TCE and toluene removal over time, consistent with a loss of metabolic activity caused by the exposure to TCE metabolites. Sustained TCE removal could be achieved when continuous feeding of mineral medium was implemented, which supported cell growth and compensated for the deactivation of cells. The FEBR exhibited its highest TCE removal efficiencies (82-96%) and elimination capacities (up to 28 gTCE m(-3) h(-1)) when TCE and toluene vapors were fed sequentially to circumvent the competitive inhibition by toluene. The TCE elimination capacity was 2-1000 times higher than reported in other gas-phase biotreatment reports. During the experiments, 85-101% of the degraded TCE chlorine was recovered as chloride. Overall, the results suggestthatthe FEBR can be a very effective system to treat TCE vapors cometabolically.     

Characterization of efficient aerobic denitrifiers isolated from two different sequencing batch reactors by 16S-rRNA analysis

By adopting two sequencing batch reactors (SBRs) A and B, nitrate as the substrate, and the intermittent aeration mode, activated sludge was domesticated to enrich aerobic denitrifiers. The pHs of reactor A were approximately 6.3 at DOs 2.2-6.1 mg/l for a carbon source of 720 mg/l COD; the pHs of reactor B were 6.8-7.8 at DOs 2.2-3.0 mg/l for a carbon source of 1500 mg/l COD. Both reactors maintained an influent nitrate concentration of 80 mg/l NO3- -N. When the total inorganic nitrogen (TIN) removal efficiency of both reactors reached 60%, aerobic denitrifier accumulation was regarded completed. By bromthymol blue (BTB) medium, 20 bacteria were isolated from the two SBRs and DNA samples of 8 of these 20 strains were amplified by PCR and processed for 16SrRNA sequencing. The obtained results were analysed by a Blast similarity search of the GenBank database, and constructing a phylogenetic tree for identification by comparison. The 8 bacteria were found to belong to the genera Pseudomonas, Delftia, Herbaspirillum and Comamonas. At present, no Delftia has been reported to be an aerobic denitrifier.     

剩余污泥高固态卧式厌氧消化的中试启动研究

针对含固率20%的剩余污泥,采用卧式反应器进行厌氧消化的中试启动研究.采取连续进料的方式,通过太阳能保温系统进行反应器的启动,经过3个月的运行,污泥产沼气率达到274 mL/gVS,甲烷体积分数为58%,VS转化率达到46%,蛋白质和多糖降解率分别为36.7%和30.1%,消化过程中VFA质量分数最大值为2 395 mg/kg,最大值出现在第42天。消化过程中氨氮质量分数一直缓慢增加,最大值约124.3 mg/kg。反应器沿程4个取样口污泥的VS,蛋白质和多糖降解率呈现出明显的差异性,反应器内污泥的降解率与其所处的沿程距离成正相关。经过73 d厌氧消化后,污泥的产甲烷潜力值达到273 mL/gVS,说明其已具备了良好的产甲烷潜力,反应器中的剩余污泥已转变为具有厌氧活性的污泥。     

Enhancement of cometabolic biodegradation of trichloroethylene (TCE) gas in biofiltration

A biofilter column inoculated with Pseudomonas putida F1 was operated to study cometabolic biodegradation of trichloroethylene (TCE) gas using toluene as a primary substrate. Variations in the efficiency and capacity of TCE elimination with different inlet concentrations of toluene and TCE were investigated in order to understand the competitive inhibition between toluene and TCE. Two toluene feeding methods, stage feeding along the column and cyclic feeding, were examined as strategies to enhance TCE cometabolic biodegradation by avoiding the toluene inhibition of TCE biodegradation and the toxic effect of TCE on cells and toluene dioxygenase enzymes. It was concluded that both methods are promising and that the determination of a suitable feeding frequency, recovery period, and inlet toluene concentration was required to optimize cyclic feeding in the cometabolic biodegradation of TCE.     

Bioremediation of 2,4,6-trinitrotoluene contaminated soil in slurry and column reactors

The bioremediation of 2,4,6-trinitrotoluene (TNT) contaminated soil was performed on a laboratory scale. To compare bioremediation methods, a soil slurry reactor and a soil column reactor were operated and the effects of supplemental sources were investigated. Optimal conditions for the two bioremediation systems for the removal of TNT were obtained. In the soil slurry reactor, about 60% of the 1000 mg/kg TNT contaminated soil was degraded after 10 d, nearly complete biodegradation (>99%) was achieved within 25 d, and the microorganisms grew and reached a maximum of 9.5 x 10(9) CFU/ml at 15 d. In the soil column reactor, about 50% of the 1000 mg/kg TNT contaminated soil was degraded after 25 d and nearly complete biodegradation (>99%) was achieved within 60 d. Microorganisms grew and reached a maximum of 9.8 x 10(10) CFU/g soil at 40 d. These results should help in determining the best bioremediation method and improving the design and operation of large scale clean up of contaminated sites by bioremediation systems.     

Technical note: Application of immobilized cells of Kluyveromyces marxianus for continuous hydrolysis to fructose of fructans in Jerusalem artichoke extracts

Dead cells of Kluyveromyces marxianus having inulase (β-D-fructo fructanohydrolase E.C.3.2.1.7) activity were immobilized in alginate beads and used as a biocatalyst in a packed bed reactor and a stirred batch reactor. Fructosans of Jerusalem artichoke tubers, after extraction, were utilized for continuous or semi-continuous production of fructose. In a bed reactor packed with 100 ml of beads, a volumetric productivity of 36 g/l/hr total reducing sugars was obtained with 98% substrate conversion. When operated continuously for 30 days, a half life of 28 days was observed for the biocatalyst. Using artichoke extract containing 20% fructan solution, 98% conversion could be achieved in a batch reactor in 20 hr. Repeated cycling of beads resulted in considerable loss of catalyst from the reactor and subsequent loss in catalytic activity, thus giving a half life of only 14 days.     

Continuous acetic acid production by a packed bed bioreactor employing charcoal pellets derived from waste mushroom medium

A packed bed bioreactor using charcoal pellets produced from waste mushroom medium by thermal carbonization was developed and applied to continuous acetic acid production. The pellets were characterized by their high specific surface area (200 m2/g) with numerous micropores (2-10 microm). The continuous acetic acid fermentation started up smoothly after seeding and was successfully operated for about 180 d under various retention times. The maximum acetic acid productivity was about 3.9 g/l/h using normal aeration and 6.5 g/l/h using air enriched with 40% O2. The pellets are expected to prove useful as a new packing material for bioreactor in terms of their bacterial affinity, high specific surface area with appropriate pore sizes for bacteria, as well as the operational stability of the system and the low production cost.     

Continuous plant cell perfusion culture: bioreactor characterization and secreted enzyme production

Culture perfusion is widely practiced in mammalian cell processes to enhance secreted antibody production. Here, we report the development of an efficient continuous perfusion process for the cultivation of plant cell suspensions. The key to this process is a perfusion bioreactor that incorporates an annular settling zone into a stirred-tank bioreactor to achieve continuous cell/medium separation via gravitational sedimentation. From washout experiments, we found that under typical operating conditions (e.g., 200 rpm and 0.3 vvm) the liquid phase in the entire perfusion bioreactor was homogeneous despite the presence of the cylindrical baffle. Using secreted acid phosphatase (APase) produced in Anchusa officinalis cell culture as a model we have studied the perfusion cultures under complete or partial cell retention. The perfusion culture was operated under phosphate limitation to stimulate APase production. Successful operation of the perfusion process over four weeks has been achieved in this work. When A. officinalis cells were grown in the perfusion reactor and perfused at up to 0.4 vvd with complete cell retention, a cell dry weight exceeding 20 g/l could be achieved while secreted APase productivity leveled off at approximately 300 units/l/d. The culture became extremely dense with the maximum packed cell volume (PCV) surpassing 70%. In comparison, the maximum cell dry weight and overall secreted APase productivity in a typical batch culture were 10-12 g/l and 100-150 units/l/d, respectively. Operation of the perfusion culture under extremely high PCV for a prolonged period, however, led to declined oxygen uptake and reduced viability. Subsequently, cell removal via a bleed stream at up to 0.11 vvd was tested and shown to stabilize the culture at a PCV below 60%. With culture bleeding, both specific oxygen uptake rate and viability were shown to increase. This also led to a higher cell dry weight exceeding 25 g/l, and further improvement of secreted APase productivity that reached a plateau fluctuating around 490 units/l/d.     

Effective onion vinegar production by a two-step fermentation system

A two-step fermentation system combining a repeated batch process using a flocculating yeast with a charcoal pellet bioreactor was developed for onion vinegar production. Juice from the red onion R-3, which contained 67.3 g/l total sugar, was smoothly converted to onion alcohol containing 30.6 g/l ethanol by repeated batch operation using the flocculating yeast Saccharomyces cerevisiae strain IR-2. Stable operation was possible and the maximum productivity was about 8.0 g/l/h. A packed bed bioreactor containing charcoal pellets produced from waste mushroom medium was then applied to continuous onion vinegar production from the onion alcohol. Onion vinegar was successfully produced, with a maximum productivity and acetic acid concentration of about 3.3 g/l/h and 37.9 g/l, respectively. The total acetic acid yield calculated from the amount of sugar consumed was 0.86. The two-step system was operated for 50 d and proved to be competitive with other systems in terms of its high productivity, high acetic acid yield, operational stability and low production costs.     

Comparison of thermophilic anaerobic digestion characteristics between single-phase and two-phase systems for kitchen garbage treatment

Lab-scale single-phase and two-phase thermophilic methane fermentation systems (SPS and TPS, respectively) were operated and fed with artificial kitchen waste. In both SPS and TPS, the highest methane recovery ratio of 90%, in terms of chemical oxygen demand by dichromate (CODcr), was observed at an organic loading rate (OLR) of 15 gCODcr/(l.d). The ratio of particle CODcr remaining to total CODcr in the influent was 0.1 and the ratio of NH(4)-N concentration to the input total nitrogen concentration was 0.5 in both SPS and TPS. However, the propionate concentration in the SPS reactor fluctuated largely and was 2 gCODcr/l higher than that in TPS, indicating less stable digestion. Regardless, efficient kitchen waste degradation can be accomplished in both SPS and TPS at an OLR of <20 gCODcr/(l.d), even though TPS may be more stable and easier to maintain. Bacillus coagulans predominated with an occupied ratio of approximately 90% in the acid fermentation reactor of TPS, and then a richer microbial community with a higher Shannon index value was maintained in the methane fermentation reactor of TPS than in the SPS reactor.     

Production of high degree polymerized chitooligosaccharides in a membrane reactor using purified chitosanase from Bacillus cereus

Crude chitosanase from Bacillus cereus NTU-FC-4 was separated by a cation exchanger to three fractions named CBCI, CBCII, and CBCIII. The CBCI hydrolyzed chitosan to yield dimers. The primary hydrolytic products of CBCII were low degree polymerized (DP) chitooligosaccharides. The CBCIII had the fastest reaction rate and yielded high DP chitooligosaccharides (heptamer and higher DP oligomers). When CBCIII was used in the ultrafiltration membrane reactor with enzyme/substrate ratio 0.06 unit/mg and 100 min of residence time (RT), the concentration of high DP oligomers was 9.78 mg/mL which occupied ca. 48% of total oligomers in the final product as compared to ca. 29% resulted from the crude enzyme. Decrease of RT to 50 min and 33 min, the high DP oligomers in the products were ca. 61% and 69%, respectively. This system could be operated for at least 24 h and kept a constant permeate flux and product output rate.     

Trichloroethylene transformation by natural mineral pyrite: the deciding role of oxygen

The transformation of trichloroethylene (TCE) in natural mineral iron disulfide (pyrite) aqueous suspension under different oxygen conditions was investigated in laboratory batch experiments. TCE transformation was pursued by monitoring its disappearance and products released with time. The effect of oxygen was studied by varying the initial dissolved oxygen concentration (DO(i)) inside each reactor. Transformation rates depended strongly on DO(i) in the system. In anaerobic pyrite suspension, TCE did not transform as it did under aerobic conditions. The transformation rate increased with an increase in DO(i). The TCE transformation kinetics was fitted to a pseudo-first-order reaction with a rate constant k (h(-1)) varying from 0.004 to 0.013 for closed systems with DO(i) varying from 0.017 to 0.268 mmol/L under the experimental conditions. In the aerobic systems, TCE transformed to several organic acids including dichloroacetic acid, glyoxylic acid, oxalic acid, formic acid, and finally to CO2 and chloride ion. Dichloroacetic acid was the only chlorinated intermediate found. Both TCE and the pyrite surface were oxidized in the presence of O2. Oxygen consumption profiles showed O2 was the common oxidant in both TCE and pyrite oxidation reactions. Ferric ion cannot be used as an alternative oxidant to oxygen for TCE transformation.     

Continuous ethanol fermentation from non-sulfuric acid-washed molasses using traditional stirred tank reactors and the flocculating yeast strain KF-7

Waste molasses is one of the most important feedstock for ethanol production in Brazil as well as in many Southeast Asian countries, including China. Sulfuric acid pretreatment is employed in most ethanol distilleries in China to control bacterial contamination, which results in difficulties in the treatment of wastewater containing high levels of sulfate ions. In this study, a high efficiency, non-sterilized, continuous ethanol fermentation process without sulfuric acid pretreatment was developed using the flocculating yeast strain KF-7 and the widely utilized, traditional, stirred tank reactors. An alternative molasses medium feeding method, which differs from traditional methods, is proposed that effectively controls bacterial contamination. Separate feeding of 1.2-fold diluted molasses and tap water into the reactor proved to be effective against bacterial contamination during long-term continuous fermentation. By feeding yeast cells with high metabolic activity to the second reactor, a two-stage continuous fermentation process that yielded a high ethanol concentration of 80 g/l as well as high ethanol productivity of 6.6 g/l/h was successfully operated for more than one month. This fermentation process can be applied to ethanol distilleries in which traditional tank reactors are used.     

Stimulating accumulation of nitrifying bacteria in porous carrier by addition of inorganic carbon in a continuous-flow fluidized bed wastewater treatment reactor

Porous polyurethane carrier particles have been successfully applied for microbial immobilization to simultaneously remove carbonaceous and nitrogenous substances from wastewater by a fill-and-draw operation. This reactor system was extended to a continuous-flow operation mode, by which inorganic carbon (IC) was supplemented in order to stimulate the growth of autotrophic nitrifying bacteria. By addition of sodium bicarbonate, the ammonia oxidation reaction proceeded remarkably in the porous particle fluidized bed reactor, while a small increase in the nitrification was observed in a reactor with suspended microbes. Dissolved oxygen profile was obtained using an oxygen microelectrode to measure the microbial consumption of oxygen in the porous carrier. The size of ammonia-oxidizing bacterial populations in the carrier was proportional to the volume of the aerobic region of the carrier. The aerobic region decreased with the increase in sodium bicarbonate concentration, which improved the ammonia-oxidizing activity of retained nitrifiers in the carrier. The maximum ammonia oxidation rate was up to 55.6 gN/m3/h within the aerobic region of the carrier under the following feed conditions: 100 mg/l of total organic compound, 55 mg/l of ammonium concentration and 48 mg/l of inorganic carbon.     

High nitrogen removal performance at moderately low temperature utilizing anaerobic ammonium oxidation reactions

High rates of nitrogen removal from wastewater have been reported using anammox bacteria at temperatures around 37 degrees C, but not at moderately low temperatures. In this study, nitrogen removal performance of an anaerobic biological filtrated (ABF) reactor, filled with porous polyester nonwoven fabric carriers as a fixed bed for anammox bacteria, was tested at 37 degrees C and at moderately low temperature (20-22 degrees C). To attain higher nitrogen removal performance, effects of influent nitrogen concentrations and hydraulic retention time (HRT) on nitrogen removal rates were investigated. Nitrogen removal rate increased with influent ammonium and nitrite concentrations, resulting in a removal rate of 3.3 kg-N/m(3)/d on day 32 for an HRT of 180 min at 37 degrees C. However, influent nitrite concentrations greater than 280 mg/l inhibited anammox activity. Therefore, the influent nitrite concentration was adjusted to be below 280 mg/l, and high-loading tests were performed for a shorter HRT. As a result, a nitrogen conversion rate of 11.5 kg-N/m(3)/d was achieved. Moreover, to evaluate long-term anammox activity at moderately low temperatures, ABF reactors were operated for 446 d. Anammox activity could be maintained at 20-22 degrees C, and stable nitrogen removal performance was observed. Furthermore, high nitrogen conversion rate of 8.1 kg-N/m(3)/d was attained. These results clearly show that an appropriate nitrite concentration in the influent and a shorter HRT resulted in high nitrogen conversion rates. The nitrogen removal performance we obtained at moderately low temperatures will open the door for application of anammox processes to many types of industrial wastewater treatment.