Effects of Phenol Feeding Pattern on Microbial Community Structure and Cometabolism of Trichloroethylene

Cometabolism of trichloroethylene (TCE) by phenol-fed enrichments was evaluated in four reactors with distinct phenol feeding patterns. The reactors were inoculated from the same source, operated at the same average dilution rate, and received the same mass of phenol over time. Only the timing of phenol addition differed. Reactor C received phenol continuously; reactor SC5 received phenol semicontinuously--alternating between 5 h of feed and 3 h without feed; reactor SC2 alternated between 2 h of feed and 6 h without feed; and reactor P received a single pulse every 24 h. The structure of the enrichments and their capacity for TCE transformation were analyzed. In long-term operation, reactors C and SC5 were dominated by fungi, had higher levels of predators, were more susceptible to biomass fluctuations, and exhibited reduced capacity for TCE transformation. Reactors P and SC2 were characterized by lower levels of fungi, higher bacterial biomass, higher concentrations of TCE-degrading organisms, and higher rates of TCE transformation. After 200 days of operation, rates of TCE transformation increased 10-fold in reactor P, resulting in TCE transformation rates that were 20 to 100 times higher than the rates of the other reactor communities. The cause of this shift is unknown. Isolates capable of the highest rates of TCE transformation were obtained from reactor P. We conclude that cometabolic activity depends upon microbial community structure and that the community structure can be manipulated by altering the growth substrate feeding pattern.     

Biodegradation of phenol and m-cresol in a batch and fed batch operated internal loop airlift bioreactor by indigenous mixed microbial culture predominantly Pseudomonas sp

An internal loop airlift reactor (ILALR) is developed and studied for biodegradation of phenol/m-cresol as single and dual substrate systems under batch and fed batch operation using an indigenous mixed microbial strain, predominantly Pseudomonas sp. The results showed that the culture could degrade phenol/m-cresol completely at a maximum concentration of 600mgl(-1) and 400mgl(-1), respectively. Batch ILALR study has revealed that phenol has been preferentially degraded by the microbial culture rather than m-cresol probably owing to the toxic effect of the later. Sum kinetic model evaluated the interaction between the phenol/m-cresol in dual substrate system, which resulted in a high coefficient of determination (R(2)) value >0.98). The fed batch results showed that the strain was able to degrade phenol/m-cresol with maximum individual concentrations 600mgl(-1) each in 26h and 37h, respectively. Moreover for fed batch operation, degradation rates increased with increase in feed concentration without any lag in the degradation profile.     

Immobilized white-rot fungal biodegradation of phenol and chlorinated phenol in trickling packed-bed reactors by employing sequencing batch operation

The biodegradation of phenol and 2,4,6-trichlorophenol (2,4,6-TCP) by immobilized white-rot fungal cultures was studied in pinewood chip and foam glass bead-packed trickling reactors. The reactors were operated in sequencing batch format. Removal efficiency increased over time and elevated influent phenol and 2,4,6-TCP (800 and 85 mg l(-1)) concentrations were removed by greater than 98% in 24-30 h batch cycles. Comparable performance between the packing materials was shown. Increased lignin peroxidase (LiP) activity was detected with the introduction of the compounds and optimum activity corresponded to optimum removal periods. Higher LiP activity (16.7-19 Ul(-1)) was detected in glass bead-packed reactor compared to wood chip reactor (0.2-5 Ul(-1)). The presence of Mn(2+) in the wood material possibly effected elevated manganese peroxidase (MnP) activity (0.3-5.8 Ul(-1)) compared to low to negligible activity in the glass bead reactor. Reactor performances are discussed in relation to sequencing batch operation and nutrient requirements necessary to induce and sustain fungal enzyme activity in inert vs. organic material packed systems.     

Empirical model for the autotrophic biodegradation of thiocyanate in an activated sludge reactor

AIMS: The aim of this investigation was to develop an empirical model for the autotrophic biodegradation of thiocyanate using an activated sludge reactor. METHODS AND RESULTS: The methods used for this purpose included the use of a laboratory scale activated sludge reactor unit using thiocyante feed concentrations from 200 to 550 mg x l(-1). Reactor effluent concentrations of <1 mg x l(-1) thiocyanate were consistently achieved for the entire duration of the investigation at a hydraulic retention time of 8 h, solids (biomass) retention of 18 h and biomass (dry weight) concentrations ranging from 2 to 4 g x l(-1). A biomass specific degradation rate factor was used to relate thiocyanate degradation in the reactor to the prevailing biomass and thiocyanate feed concentrations. A maximum biomass specific degradation rate of 16 mg(-1) x g(-1) x h(-1) (mg thiocyanate consumed per gram biomass per hour) was achieved at a thiocyanate feed concentration of 550 mg x l(-1). The overall yield coefficient was found to be 0.086 (biomass dry weight produced per mass of thiocyanate consumed). CONCLUSION: Using the results generated by this investigation, an empirical model was developed, based on thiocyanate feed concentration and reactor biomass concentration, to calculate the required absolute hydraulic retention time at which a single-stage continuously stirred tank activated sludge reactor could be operated in order to achieve an effluent concentration of <1 mg x l(-1). The use of an empirical model rather than a mechanistic-based kinetic model was proposed due to the low prevailing thiocyanate concentrations in the reactor. SIGNIFICANCE AND IMPACT OF THE STUDY: These results represent the first empirical model, based on a comprehensive data set, that could be used for the design of thiocyanate-degrading activated sludge systems.     

An immobilized cell reactor with simultaneous product separation. II. Experimental reactor performance

The simultaneous separation of volatile fermentation products from product-inhibited fermentations can greatly increase the productivity of a bioreactor by reducing the product concentration in the bioreactor, as well as concentrating the product in an output stream free of cells, substrate, or other feed impurities. The Immobilized Cell Reactor-Separator (ICRS) consists of two column reactors: a cocurrent gas-liquid "enricher" followed by a countercurrent "stripper" The columns are four-phase tubular reactors consisting of (1) an inert gas phase, (2) the liquid fermentation broth, (3) the solid column internal packing, and (4) the immobilized biological catalyst or cells. The application of the ICRS to the ethanol-from-whey-lactose fermentation system has been investigated. Operation in the liquid continuous or bubble flow regime allows a high liquid holdup in the reactor and consequent long and controllable liquid residence time but results in a high gas phase pressure drop over the length of the reactor and low gas flow rates. Operation in the gas continuous regime gives high gas flow rates and low pressure drop but also results in short liquid residence time and incomplete column wetting at low liquid loading rates using conventional gas-liquid column packings. Using cells absorbed to conventional ceramic column packing (0.25-in. Intalox saddles), it was found that a good reaction could be obtained in the liquid continuous mode, but little separation, while in the gas continuous mode there was little reaction but good separation. Using cells sorbed to an absorbant matrix allowed operation in the gas continuous regime with a liquid holdup of up to 30% of the total reactor volume. Good reaction rates and product separation were obtained using this matrix. High reaction rates were obtained due to high density cell loading in the reactor. A dry cell density of up to 92 g/L reactor was obtained in the enricher. The enricher ethanol productivity ranged from 50 to 160 g/L h while the stripper productivity varied from 0 to 32 g/L h at different feed rates and concentrations. A separation efficiency of as high as 98% was obtained from the system.     

Kinetic parameters of continuous cultures of Bacillus thermoleovorans sp. A2 degrading phenol at 65 degrees C

In this paper, we report on the kinetics of phenol degradation and cell growth in continuous cultures of suspended cells of Bacillus thermoleovorans sp. A2 at 65 degrees C. A high yield coefficient of Y(x/s)=0.84 g cell dry weight g(-1) phenol was measured at a dilution rate of 0.5 h(-1). At the same dilution rate the coefficient for maintenance metabolism (m(s)) was determined to be 0.045 g phenol g(-1) cell dry weight h(-1). The maximal growth rate (wash-out) determined at a phenol inlet concentration of 188 mg l(-1) was 0.9 h(-1). Up to 7 g phenol l(-1) per day were degraded in a continuously operated 2-l stirred tank reactor with suspended cells (feed concentration 660 mg l(-1)). Additionally, yield coefficients for oxygen and ammonium are reported.     

Growth kinetics of Pseudomonas putida in cometabolism of phenol and 4-chlorophenol in the presence of a conventional carbon source

Growth kinetics of Pseudomonas putida (ATCC 49451) in cometabolism of phenol and 4-chlorophenol (4-cp) in the presence of sodium glutamate (SG) were studied. In the ternary substrate mixture, phenol and SG are growth substrates while 4-cp is a nongrowth substrate. Cell growth on phenol was found to follow Andrews kinetics and cells displayed substrate inhibition pattern on sodium glutamate in the range of 0-4 g L(-1) as well. A cell growth model for the ternary substrate system was established based on a simplified cell growth mechanism and subsequently modified by experimental results. Model analysis over a wide range of substrate concentrations shows that the inhibition of SG is much larger than phenol at low phenol concentrations (/=600 mg L(-1)). The nongrowth substrate, 4-cp, inhibits cell growth mainly through inactivation of cells (cell decay) and competitive inhibition to cell growth on phenol. In the absence of SG, 4-cp retards cell growth severely and cells cannot grow at 250 mg L(-1) 4-cp. Addition of sodium glutamate, however, greatly attenuates the toxicity of 4-cp and supports cell growth at 4-cp concentration higher than 250 mg L(-1). By using the proposed cell growth model, we were able to optimize the amount of SG needed to enhance cell growth rate and validate model predictions against experimental data.     

Biosorption of copper(II) ions onto powdered waste sludge in a completely mixed fed-batch reactor: estimation of design parameters

Biosorption of Cu(II) ions onto pre-treated powdered waste sludge (PWS) was investigated using a fed-batch operated completely mixed reactor. Fed-batch adsorption experiments were performed by varying the feed flow rate ( 0.075-0.325 l h(-1)), feed copper (II) ion concentrations (50-300 mg l(-1)) and the amount of adsorbent (1-6 g PWS) using fed-batch operation. Breakthrough curves describing the variations of effluent copper ion concentrations with time were determined for different operating conditions. Percent copper ion removals from the aqueous phase decreased, but the biosorbed (solid phase) copper ion concentrations increased with increasing the feed flow rate and Cu(II) concentration. A modified Bohart-Adams equation was used to determine the biosorption capacity of PWS and the rate constant for Cu(II) ion biosorption. Adsorption rate constant in fed-batch operation was an order of magnitude larger than those obtained in adsorption columns because of elimination of mass transfer limitations encountered in the column operations while the biosorption capacity of PWS was comparable with powdered activated (PAC) in column operations. Therefore, a completely mixed reactor operated in fed-batch mode was proven to be more advantageous as compared to adsorption columns due to better contact between the phases yielding faster adsorption rates.     

Phenol removal in packed bed reactor under denitrifying conditions

Detailed studies on the efficiency of phenol degradation by a biofilm in an anaerobic packed bed reactor were carried out. The efficiency of phenol degradation depended on both the concentration of phenol in the medium and the phenol load in anaerobic packed bed reactor. Increasing phenol concentrations from 200 to 1,250 mg l(-1) and retention time (Tr)= 12 h were paralleled by increasing efficiency of the process, which reached a maximum value of 1,390 mg l(-1) day(-1) at 700 mg phenol l(-1). The highest concentration of phenol used inhibited growth by approximately 95%. When the phenol load in medium containing 200, 300, 400 and 500 mg l(-1) was increased through a shortening of the retention time (Tr from 24 to 2 h) a maximum efficiency of phenol degradation of 2,200 mg l(-1) day(-1) was obtained at Tr=4 h and phenol concentrations in the medium of 200 mg l(-1). Phenol in concentrations from 300 to 500 mg l(-1) was fully degraded at Tr>9 h and phenol load reaching 530-1330 mg l(-1) day(-1) for the individual concentrations. The post-denitrification effluent leaving packed bed reactor in spite of the absence or even trace amounts of phenol in it requires further purification.     

Hydrolysis of liquefied corn starch in a membrane reactor

The objective of this study was to develop a continuous hydrolysis process for the enzymatic saccharification of liquefied corn starch using a membrane reactor. A residence time distribution study confirmed that the membrane reactor could be modeled as a simple continuous stirred tank reactor (CSTR). Kinetic studies indicated that the continuous reactor operated in the first-order region with respect to substrate concentration at substrate concentrations greater than 200 g/L. At a residence time of 1 h and an enzyme concentration of 1 g/L, the maximum reaction velocity (V(m)) was 3.86 g glucose/L min and the apparent Michaelis constant (K(m) (')) was 562 g/L. The K(m) (') value for the continuous reactor was 2-7 times greater than that obtained in a batch reactor.Kinetic data were fit to a model based on the Michaelis-Menten rate expression and the design equation for a CSTR. Application of the model at low reactor space times was successful. At space times of 6 min or less, the model predicted the reactor's performance reasonably well. Additional work on the detection and quantitation of reversion products formed by glucoamylase is required. Isolation, detection, and quantitation of reversion products by HPLC was difficult. Detailed analysis on the formation of these reversion products could lead to better reactor designs in the future.     

Salt accumulation resulting from base added for pH control, and not ethanol, limits growth of Thermoanaerobacteriumthermosaccharolyticum HG-8 at elevated feed xylose concentrations in continuous culture

Thermoanaerobacter thermosaccharolyticum HG-8 was grown in continuous culture to characterize growth limitation at high feed substrate and product concentrations. Continuous fermentation of 50 and 73 g/L xylose at a dilution rate based on the feed flow, D(f), of 0.053 h(-)(1) and with the pH controlled at 7.0 by addition of KOH resulted in steady state utilization of >99% of the xylose fed and production of ethanol and acetic acid at a mass ratio of about 2:1. Continuous cultures of T. thermosaccharolyticum growing at D(f) = 0.053 h(-)(1) achieved complete utilization of 75 g/L xylose in the presence of 19.1 g/L K(+) (0.49 M) and an ethanol concentration of 22.4 g/L ethanol. When the feed to a culture initially at steady state with a 75 g/L xylose feed and D(f) = 0.053 h(-)(1) was increased to 87.5 g/L xylose, limitation of growth and xylose utilization was observed. This limitation was not relieved by repeating this feed upshift experiment in the presence of increased nutrient levels and was not reproduced by addition of ethanol to a steady-state culture fed with 75 g/L xylose. By contrast, addition of KCl to a steady-state culture fed with 75 g/L xylose reproduced the K(+) concentration, limitation of growth and xylose utilization, and product concentration profiles observed in the feed upshift experiment. The maximum concentration at which growth of batch cultures was observed was 0.43 M for KCl, NaCl, and equimolar mixtures of these salts, suggesting that the observed limitation is not ion-specific. These data support the interpretation that inhibition salt accumulation resulting from addition of KOH for pH control is the limiting factor manifested in the feed upshift experiment and that both nutrient limitation and ethanol inhibition played little or no role as limiting factors. More generally, salt inhibition would appear to be a possible explanation for the discrepancy between the tolerance to added ethanol and the maximum concentration of produced ethanol reported in the literature for fermentation studies involving thermophilic bacteria.     

NADH fluorscence in a carbon-limited fermentation

The continuous flow acetone-butanol fermentation conducted at lowered inlet feed sugar concentrations and at a constant dilution rate D =0.075 h(-1) demonstrated a significant decreases in the availability of the cell population "reduction energy" (F/X), resulting in an exclusive accumulation of intermediate acids under those conditions. The cultures resumed its solvent production activity when the inlet sugar concentration in the feed stream was increased from 20 to 40 g/L at the same low growth/dilution rates. A linear correlation between the culture reduction content (F/X) and the specific butanol rate (q(B)) was observed under the present conditions, indicating the necessity of the NADH availability for the increased solvent production.     

Modeling of local dynamic behavior of phenol degradation in an internal loop airlift bioreactor by yeast Candida tropicalis

A coupled computational fluid dynamic (CFD) model, combining hydrodynamics with biochemical reactions, was developed to simulate the local transient flow patterns and the dynamic behaviors of cell growth and phenol biodegradation by yeast Candida tropicalis in an internal loop airlift reactor (ILALR). To validate this proposed model effectively, the simulated local hydrodynamic characteristics of the gas-mineral salt medium solution (gas-liquid) two-phase system, at a phenol concentration of 1,200 mg L(-1) and no presence of cells, was experimentally investigated in the ILALR using laser Doppler anemometer (LDA) measurements and conductivity probe. Furthermore, the validation of the simulated phenol biodegradation behavior by C. tropicalis at different initial concentrations of phenol and cell was also carried out in the ILALR. The time-averaged and transient results of the model simulations illustrated a satisfactory agreement with the experimental data. Finally, the local instantaneous flow and phenol biodegradation features, including gas holdup, gas velocity, liquid velocity, cell concentration, and phenol concentration inside the ILALR were successfully predicted by the proposed model.     

氧化葡萄糖酸杆菌生物催化1,3-丙二醇合成3-羟基丙酸

3-羟基丙酸是一种潜在的重要化工产品,可作为中间体合成多种有经济价值的工业用化合物。文中利用氧化葡萄糖酸杆菌生物催化1,3-丙二醇合成3-羟基丙酸。首先在50 mL摇瓶中(转化体系为10 mL)考察细胞加入量、底物和产物浓度等对催化反应的影响。在此基础上,在2 L鼓泡塔中(转化体系为1 L),采取适当的补料方式和生物转化与分离相耦合的手段解除抑制,以提高目标产物终浓度。结果表明:高底物和产物浓度通过降低反应初速度抑制转化的进行,并确定了最佳催化反应条件为6 g/L菌体量,pH 5.5。利用流加补料方式维持反应体系中底物浓度在15~20 g/L,经过60 h的反应,3-羟基丙酸的浓度达到60.8 g/L,生产强度为1.0g/(L.h),转化率为84.3%。采用生物转化与分离相耦合的方法,经过50 h的转化反应,3-羟基丙酸的总产量达76.3 g/L,生产强度为1.5 g/(L.h),转化率83.7%。研究结果对利用氧化葡萄糖酸杆菌的不完全氧化醇类化合物特性实现其在工业生物催化中的应用具有一定的指导意义。     

An immobilized cell reactor with simultaneous product separation. I. Reactor design and analysis

A four-phase reactor-separator (gas, liquid, solid, and immobilized catalyst) is proposed for fermentations characterized by a volatile product and nonvolatile substrate.In this reactor, the biological catalyst is immobilized onto a solid column packing and contacted by the liquid containing the substrate.A gas phase is also moved through the column to strip the volatile product into the gas phase. The Immobilized Cell Reactor-Separator (ICRS) consists of two basic gas-liquid flow sections: a cocurrent "enricher" followed by a countercurrent-"stripper".In this article, an equilibrium stage model of the reactor is developed to determine the feasibility and important operational variables of such a reactor-separator. The ICRS concept is applied to the ethanol from whey lactose fermentation using some preliminary immobilized cell reactor performance data. A mathematical model for a steady-state population based on an adsorbed monolayer of cells is also developed for the reactor. The ICRS model demonstrated that the ICRS should give a significant increase in reactor productivity as compared to an identically sized Immobilized Cell Reactor (ICR) with no separation. The gas-phase separation of the product also allows fermentation of high inlet substrate concentrations. The model is used to determine the effects of reactor parameters on ICRS performance including temperature, pressure, gas flow rates, inlet substrate concentration, and degree of microbial product inhibition.     

Packed- and fluidized-bed biofilm reactor performance for anaerobic wastewater treatment

Anaerobic degradation performance of a laboratory-scale packed-bed reactor (PBR) was compared with two fluidized-bed biofilm reactors (FBRs) on molasses and whey feeds. The reactors were operated under constant pH (7) and temperature (35 degrees C) conditions and were well mixed with high recirculation rates. The measured variables were chemical oxygen demand (COD), individual organic acids, gas composition, and gas rates. As carrier, sand of 0.3-0.5 mm diameter was used in the FBR, and porous clay spheres of 6 mm diameter were used in the PBR. Startup of the PBR was achieved with 1-5 day residence times. Start-up of the FBR was only successful if liquid residence times were held low at 2-3 h. COD degradations of 86% with molasses (90% was biodegradable) were reached in both the FBR and PBR at 6 h residence time and loadings of 10 g COD/L day. At higher loadings the FBR gave the best performance; even at 40-45 g COD/L day, with 6 h residence times, 70% COD was degraded. The PBR could not be operated above 20 g COD/L day without clogging. A comparison of the reaction rates show that the PBR and FBR per formed similarly at low concentrations in the reactors up to 1 g COD/L, while above 3 g COD/L the rates were 17.4 g COD/L day for the PBR and 38.4 g COD/L day for the FBR. This difference is probably due to diffusion limitations and a less active biomass content of the PBR compared with the fluidized bed.The results of dynamic step change experiments, in which residence times and feed concentrations were changed hanged at constant loading, demonstrated the rapid response of the reactors. Thus, the response times for an increase in gas rate or an increase in organic acids due to an increase in feed concentration were less than 1 day and could be explained by substrate limitation. Other slower responses were observed in which the reactor culture adapted over periods of 5-10 days; these were apparently growth related. An increase in loading of over 100% always resulted in large increases inorganic acids, especially acetic and propionic, as well as large increases in the CO(2) gas content. In general, the CO(2) content of the gas was very low, due to the large amount of dissolved CO(2) that exited with the liquid phase at low residence times. The performance of the FBR with whey was comparable to its performance with molasses, and switching of molasses to whey feed resulted in immediate good performance without adaptation.     

Effluent recirculation to improve perchlorate reduction in a fixed biofilm reactor

The effect of effluent recirculation on perchlorate reduction in a nominally plug-flow fixed biofilm reactor was studied in two cases: influent concentrations of 10 and 400 microg/L at low hydraulic loading rates (1.9 and 37.5 m(3)/m(2)/day without and with recirculation, respectively) and after a step increase in perchlorate concentration to 1,000 microg/L at the higher hydraulic loading rate (5 and 100 m(3)/m(2)/day without and with recirculation, respectively). Complete perchlorate reduction was sustained for influent concentrations of 400 and 10 microg/L in both flow regimes at the lower hydraulic loading rates. Reactor tracer profiles showed that biofilm diffusion had a more significant effect on mass transfer in the plug flow reactor compared with recirculation. The recirculation bioreactor acclimated more rapidly to increased hydraulic and perchlorate mass loading rates with significantly lower effluent perchlorate compared to the plug flow reactor: 16 microg/L versus 46 microg/L, respectively, although complete perchlorate removal was not achieved in either flow regime after 21 days acclimation to the higher loading. Total biofilm mass was more uniformly distributed in the recirculation reactor which may have contributed to better performance under increased perchlorate loading.     

Experimental sensitivity analysis of a trickle bed bioreactor for lignin peroxidases production by P. chrysosporium

A systematic experimental analysis of the parameters affecting the behaviour of a Trickle Bed Reactor (TBR) was performed. Lignin peroxidases (LiPs) were produced in the reactor by Phanerochaete chrysosporium entrapped in a Ca-alginate film covering ceramic supports (Berl saddles). Gas and liquid velocity, liquid flow regime, and thickness of encapsulating matrix were experimentally investigated. Reactor productivity and biological activity were considered as representative of reactor performance and evaluated by six probe parameters to take into account different process system configurations, i.e. single LiP extraction, multiple LiP extraction every 24 h and in vivo applications. Liquid fluid dynamics were the critical phenomena affecting LiPs production; the ranges of gas and liquid superficial velocity influencing the reactor performance were also identified. Alginate film thickness was shown to influence specific biomass activity but a limited impact on reactor productivity was verified.     

Biological treatment of 2,4-dichlorophenol containing synthetic wastewater using a rotating brush biofilm reactor

A newly developed rotating brush biofilm reactor was used for DCP, COD and toxicity removal from 2,4-dichlorophenol (DCP) containing synthetic wastewater at different feed COD, TCP concentrations and A/Q (biofilm surface area/feed flow rate) ratios. A Box-Wilson statistical experiment design was used by considering the feed DCP (50-500 mg l(-1)), COD (2000-6000 mg l(-1)) and A/Q ratio (73-293 m2 d m(-3)) as the independent variables while percent DCP, COD, and toxicity removals were the objective functions. The experimental data were correlated by a quadratic response function and the coefficients were determined by regression analysis. Percent DCP, COD and toxicity removals calculated from the response functions were in good agreement with the experimental data. DCP, COD and toxicity removals increased with increasing A/Q ratio and decreasing feed DCP concentrations. The optimum A/Q ratio resulting in the highest COD (90%), DCP (100%) and toxicity (100%) removals with the highest feed COD (6000 mg l(-1)) and DCP (500 mg l(-1)) contents was nearly 210 m2 d m(-3).     

Effects of hydraulic retention time and sulfide toxicity on ethanol and acetate oxidation in sulfate-reducing metal-precipitating fluidized-bed reactor

The effects of hydraulic retention time (HRT) and sulfide toxicity on ethanol and acetate utilization were studied in a sulfate-reducing fluidized-bed reactor (FBR) treating acidic metal-containing wastewater. The effects of HRT were determined with continuous flow FBR experiments. The percentage of ethanol oxidation was 99.9% even at a HRT of 6.5 h (loading of 2.6 g ethanol L(-1) d(-1)), while acetate accumulated in the FBR with HRTs below 12 h (loading of 1.4 g ethanol L(-1) d(-1)). Partial acetate utilization was accompanied by decreased concentrations of dissolved sulfide (DS) and alkalinity in the effluent, and eventually resulted in process failure when HRT was decreased to 6.1 h (loading of 2.7 g ethanol L(-1) d(-1)). Zinc and iron precipitation rates increased to over 600 mg L(-1) d(-1) and 300 mg L(-1) d(-1), respectively, with decreasing HRT. At HRT of 6.5 h, percent metal precipitation was over 99.9%, and effluent metal concentrations remained below 0.08 mg L(-1). Under these conditions, the alkalinity produced by substrate utilization increased the wastewater pH from 3 to 7.9-8.0. The percentage of electron flow from ethanol to sulfate reduction averaged 76 +/- 10% and was not affected by the HRT. The lowest HRT did not result in significant biomass washout from the FBR. The effect of sulfide toxicity on the sulfate-reducing culture was studied with batch kinetic experiments in the FBR. Noncompetitive inhibition model described well the sulfide inhibition of the sulfate-reducing culture. (DS) inhibition constants (K(i)) for ethanol and acetate oxidation were 248 mg S L(-1) and 356 mg S L(-1), respectively, and the corresponding K(i) values for H(2)S were 84 mg S L(-1) and 124 mg S L(-1). In conclusion, ethanol oxidation was more inhibited by sulfide toxicity than the acetate oxidation.