Different types of sludge granules in UASB reactors treating acidified wastewaters

The influence of a high energy substrate, i.e. sucrose, on the granular sludge yield and the development of different types of granular sludge was investigated by using Upflow Anaerobic Sludge Bed (UASB) reactors fed with synthetic wastewater. The feed COD was a mixture of volatile fatty acids (VFA) i.e., 20, 40, and 40% of the COD as C₂-, C₃-, and C₄-VFA, respectively. Furthermore, experiments were carried out in which 10 and 30% of the VFA COD was substituted with sucrose. The following distinctly different types of granules were observed in each testrun: in the reactor fed with solely VFA, black (B) and white (W) granules developed; in the reactor fed with a mixture of 90% VFA and 10% sucrose, three types of granules i.e., B, W, and grey (G) granules could be seen; in the reactor fed with 70% VFA and 30% sucrose, only W and G granules were found. The granular sludge yield increased proportional to the amount of sucrose COD. At steady-state performance of the reactors, specific acidogenic (SAA) and methanogenic (SMA) activity tests on these granules revealed that B granules had the highest SMA with low SAA. The W granules had very high SMA with low SAA. G granules gave the highest SAA with a considerable SMA. Measurement of coenzyme F₄₂₀ revealed that B granules consist mainly of acetoclastic methanogens. The fore-mentioned tests were supplemented with analyses of the wash-out cells present in the reactor effluent and the results suggested that acidogens, if present, prevail at the granule surface. The B granules were particularly rich in Ca, Mn, and Zn minerals. The size distribution analysis showed that the granule diameter increased in the following order: B相似文献   

Performance of staged and non-staged up-flow anaerobic sludge bed (USSB and UASB) reactors treating low strength complex wastewater

The use of anaerobic processes to treat low-strength wastewater has been increasing in recent years due to their favourable performance-costs balance. For optimal results, it is necessary to identify reactor configurations that are best suited for this kind of application. This paper reports on the comparative study carried out with two high-rate anaerobic reactor systems with the objective of evaluating their performances when used for the treatment of low-strength, complex wastewater. One of the systems is the commonly used up-flow anaerobic sludge blanket (UASB) reactor. The other is the up-flow staged sludge bed (USSB) system in which the reactor was divided longitudinally into 3, 5 and 7 compartments by the use of baffles. The reactors (9 l) were fed with a synthetic, soluble and colloidal waste (chemical oxygen demand (COD) < 1000 mg/l) and operated at 28°C and 24 h hydraulic retention time. Intermediate flow hydraulics, between plug-flow and completely-mixed, in the UASB and 7 stages USSB reactors allowed efficient degradation of substrates with minimum effluent concentrations. Low number of compartments in the USSB reactors increased the levels of short-circuiting thus reducing substrate removal efficiencies. All reactors showed high COD removal efficiencies (93–98%) and thus can be regarded as suitable for the treatment of low strength, complex wastewater. Staged anaerobic reactors can be a good alternative for this kind of application provided they are fitted with a large enough (≥7) number of compartments to fully take advantage of their strengths. Scale factors seem to have influenced importantly on the comparison between one and multi staged sludge-bed reactors and, therefore, observations made here could change at larger reactor volumes.     

Continuous detoxification, transformation, and degradation of nitrophenols in upflow anaerobic sludge blanket (UASB) reactors

The anaerobic transformation and degradation of nitrophenols by granular sludge was investigated in upflow anaerobic sludge blanket (UASB) reactors continuously fed with a volatile fatty acid (VFA) mixture as the primary substrate. During the start-up, subtoxic concentrations of 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), and 2, 4-dinitrophenol (2, 4-DNP) were utilized. 4-NP and 2, 4-DNP were readily converted to the corresponding aromatic amine; whereas 2-NP was converted to nonaromatic products via intermediate formation of 2-aminophenol (2-AP). These conversions led to a dramatic detoxification of the mononitrophenols because the reactors treated the nitrophenolics at the concentrations which were over 25 times higher than those that caused severe inhibition. VFA removal efficiencies greater than 99% were achieved in both reactors at loading rates greater than 11.4 g COD per liter of reactor volume per day even at volumetric loading of mononitrophenols up to 910 mg/L . d.The sludges obtained from each of the reactors at the end of the continuous experiments were assayed for their specific nitrophenol reducing activity in the presence of different primary substrates. Reduction rates of 45 and 26 mg/g volatile suspended solids per day were observed for 2-NP and 4-NP, respectively, when utilizing the VFA mixture as primary substrate. Hydrogen, an interspecies-reduced compound, and substrates that provide interspecies-reducing equivalents-such as butyrate, propionate, and ethanol stimulated nitrophenol reduction, whereas acetate and methanol did not. Anaerobic batch biodegradability tests with the 2-NP-adapted sludge revealed that its corresponding aromatic amine, 2-AP, was degraded to methane at a specific rate of 14.5 mg/g VSS . d. Acetate was observed to be the major intermediate during 2-AP degradation in the presence of a specific methanogenic inhibitor 2-bromoethanesulfonate. The results of this study indicate that UASB reactors can be applied to rapidly detoxify and, under certain circumstances, degrade nitroaromatic compounds. (c) 1996 John Wiley & Sons, Inc.     

The role of HRT and low temperature on the acid-phase anaerobic digestion of municipal and industrial wastewaters

Two identical 31 completely mixed reactors with solids recycling capabilities were used to investigate the effects of hydraulic retention time (HRT) and low temperatures on volatile fatty acid (VFA) production. One reactor was fed with a 1:1 ratio of diluted primary sludge and a starch-rich industrial wastewater, while the other was fed with diluted primary sludge alone. The VFA and soluble COD concentrations and specific production rates reached their highest values at 30 h HRT and at 25 degrees C. Further increase in HRT (at 25 degrees C) or decrease in temperature (at an HRT of 30 h) resulted in lower amounts of VFA and COD produced. All parameters related to VFA and COD production were significantly higher in the industrial-municipal reactor than in the municipal-only reactor. The VFA:COD ratios were very high, with values ranging from about 0.8 to 1.0 indicating that hydrolysis was the rate-limiting step. Degradation of proteins (measured by ammonia production) was inhibited by the starch-rich wastewater in the industrial-municipal reactor, while no evidence of inhibition was found in the municipal-only reactor. This study revealed that VFA production was feasible at low temperatures (down to 8 degrees C), particularly in the presence of the industrial waste. Ultimately, the amount VFA produced was adequate, in most cases, to support subsequent biological nutrient removal (BNR) processes.     

High-rate anaerobic treatment of wastewater at low temperatures

Anaerobic treatment of a volatile fatty acid (VFA) mixture was investigated under psychrophilic (3 to 8 degrees C) conditions in two laboratory-scale expanded granular sludge bed reactor stages in series. The reactor system was seeded with mesophilic methanogenic granular sludge and fed with a mixture of VFAs. Good removal of fatty acids was achieved in the two-stage system. Relative high levels of propionate were present in the effluent of the first stage, but propionate was efficiently removed in the second stage, where a low hydrogen partial pressure and a low acetate concentration were advantageous for propionate oxidation. The specific VFA-degrading activities of the sludge in each of the modules doubled during system operation for 150 days, indicating a good enrichment of methanogens and proton-reducing acetogenic bacteria at such low temperatures. The specific degradation rates of butyrate, propionate, and the VFA mixture amounted to 0.139, 0.110, and 0.214 g of chemical oxygen demand g of volatile suspended solids-1 day-1, respectively. The biomass which was obtained after 1.5 years still had a temperature optimum of between 30 and 40 degrees C.     

Influence of pH shocks on trace metal dynamics and performance of methanol fed granular sludge bioreactors

The influence of pH shocks on the trace metal dynamics and performance of methanol fed upflow anaerobic granular sludge bed (UASB) reactors was investigated. For this purpose, two UASB reactors were operated with metal pre-loaded granular sludge (1mM Co, Ni and Fe; 30°C; 96h) at an organic loading rate (OLR) of 5gCOD l reactor^–1d^–1. One UASB reactor (R1) was inoculated with sludge that originated from a full scale reactor treating alcohol distillery wastewater, while the other reactor (R2) was inoculated with sludge from a full scale reactor treating paper mill wastewater. A 30h pH shock (pH 5) strongly affected the metal retention dynamics within the granular sludge bed in both reactors. Iron losses in soluble form with the effluent were considerable: 2.3 and 2.9% for R1 and R2, respectively, based on initial iron content in the reactors, while losses of cobalt and nickel in soluble form were limited. Sequential extraction of the metals from the sludge showed that cobalt, nickel, iron and sulfur were translocated from the residual to the organic/sulfide fraction during the pH shock in R2, increasing 34, 47, 109 and 41% in the organic/sulfide fraction, respectively. This is likely due to the modification of the iron sulfide precipitate stability, which influences the extractability of iron and trace metals. Such a translocation was not observed for the R1 sludge during the first 30h pH shock, but a second 4day pH shock induced significant losses of cobalt (18%), iron (29%) and sulfur (29%) from the organic/sulfide fraction, likely due to iron sulfide dissolution and concomitant release of cobalt. After the 30h pH shock, VFA accumulated in the R2 effluent, whereas both VFA and methanol accumulated in R1 after the 4day pH shock. The formed VFA, mainly acetate, were not converted to methane due to the loss of methanogenic activity of the sludge on acetate. The VFA accumulation gradually disappeared, which is likely to be related to out-competition of acetogens by methanogens. Zinc, copper and manganese supply did not have a clear effect on the acetate removal and methanol conversion, but zinc may have induced the onset of methanol degradation after day 152 in R1.     

High-Rate Anaerobic Treatment of Wastewater at Low Temperatures

Anaerobic treatment of a volatile fatty acid (VFA) mixture was investigated under psychrophilic (3 to 8°C) conditions in two laboratory-scale expanded granular sludge bed reactor stages in series. The reactor system was seeded with mesophilic methanogenic granular sludge and fed with a mixture of VFAs. Good removal of fatty acids was achieved in the two-stage system. Relative high levels of propionate were present in the effluent of the first stage, but propionate was efficiently removed in the second stage, where a low hydrogen partial pressure and a low acetate concentration were advantageous for propionate oxidation. The specific VFA-degrading activities of the sludge in each of the modules doubled during system operation for 150 days, indicating a good enrichment of methanogens and proton-reducing acetogenic bacteria at such low temperatures. The specific degradation rates of butyrate, propionate, and the VFA mixture amounted to 0.139, 0.110, and 0.214 g of chemical oxygen demand g of volatile suspended solids⁻¹ day⁻¹, respectively. The biomass which was obtained after 1.5 years still had a temperature optimum of between 30 and 40°C.     

Effects of an electric field and zero valent iron on anaerobic treatment of azo dye wastewater and microbial community structures

A zero valent iron (ZVI) bed with a pair of electrodes was packed in an anaerobic reactor aiming at enhancing treatment of azo dye wastewater. The experiments were carried out in three reactors operated in parallel: an electric field enhanced ZVI-anaerobic reactor (R1), a ZVI-anaerobic reactor (R2) and a common anaerobic reactor (R3). R1 presented the highest performance in removal of COD and color. Raising voltage in R1 further improved its performance. Scanning electron microscopy images displayed that the structure of granular sludge from R1 was intact after being fed with the high dye concentration, while that of R3 was broken. Fluorescence in situ hybridization analysis indicated that the abundance of methanogens in R1 was significantly greater than that in the other two reactors. Denaturing gradient gel electrophoresis showed that the coupling of electric field and ZVI increased the diversity of microbial community and especially enhanced bacterial strains responsible for decolorization.     

Effect of redox mediator, AQDS, on the decolourisation of a reactive azo dye containing triazine group in a thermophilic anaerobic EGSB reactor

The feasibility of thermophilic (55 °C) anaerobic treatment applied to colour removal of a triazine contained reactive azo dye was investigated in two 0.53 l expanded granular sludge blanket (EGSB) reactors in parallel at a hydraulic retention time (HRT) of 10 h. Generally, this group of azo dyes shows the lowest decolourisation rates during mesophilic anaerobic treatment. The impact of the redox mediator addition on colour removal rates was also evaluated. Reactive Red 2 (RR2) and anthraquinone-2,6-disulfonate (AQDS) were selected as model compounds for azo dye and redox mediator, respectively. The reactors achieved excellent colour removal efficiencies with a high stability, even when high loading rates of RR2 were applied (2.7 g RR2 l⁻¹ per day). Although AQDS addition at catalytic concentrations improved the decolourisation rates, the impact of AQDS on colour removal was less apparent than expected. Results show that the AQDS-free reactor R2 achieved excellent colour removal rates with efficiencies around 91%, compared with the efficiencies around 95% for the AQDS-supplied reactor R1. Batch experiments confirmed that the decolourisation rates were co-substrate dependent, in which the volatile fatty acids (VFA) mixture was the least efficient co-substrate. The highest decolourisation rate was achieved in the presence of either hydrogen or formate, although the presence of glucose had a significant impact on the colour removal rates.     

Long-term competition between sulfate reducing and methanogenic bacteria in UASB reactors treating volatile fatty acids

The competition between acetate utilizing methane-producing bacteria (MB) and sulfate-reducing bacteria (SRB) was studied in mesophilic (30 degrees C) upflow anaerobic sludge bed (UASB) reactors (upward velocity 1 m h-1; pH 8) treating volatile fatty acids and sulfate. The UASB reactors treated a VFA mixture (with an acetate:propionate:butyrate ratio of 5:3:2 on COD basis) or acetate as the sole substrate at different COD:sulfate ratios. The outcome of the competition was evaluated in terms of conversion rates and specific methanogenic and sulfidogenic activities. The COD:sulfate ratio was a key factor in the partitioning of acetate utilization between MB and SRB. In excess of sulfate (COD:sulfate ratio lower than 0.67), SRB became predominant over MB after prolonged reactor operation: 250 and 400 days were required to increase the amount of acetate used by SRB from 50 to 90% in the reactor treating, respectively, the VFA mixture or acetate as the sole substrate. The competition for acetate was further studied by dynamic simulations using a mathematical model based on the Monod kinetic parameters of acetate utilizing SRB and MB. The simulations confirmed the long term nature of the competition between these acetotrophs. A high reactor pH (+/-8), a short solid retention time (<150 days), and the presence of a substantial SRB population in the inoculum may considerably reduce the time required for acetate-utilising SRB to outcompete MB.     

Monitoring granule formation in anaerobic upflow bioreactors using oligonucleotide hybridization probes

The process of granule formation in upflow anaerobic sludge blanket (UASB) reactors was studied using oligonucleotide hybridization probes. Two laboratory-scale UASB reactors were inoculated with sieved primary anaerobic digester sludge from a municipal wastewater treatment plant and operated similarly except that reactor G was fed glucose, while reactor GP was fed glucose and propionate. Size measurements of cell aggregates and quantification of different populations of methanogens with membrane hybridization targeting the small-subunit ribosomal RNA demonstrated that the increase in aggregate size was associated with an increase in the abundance of Methanosaeta concilii in both reactors. In addition, fluorescence in situ hybridization showed that the major cell components of small aggregates collected during the early stages of reactor startup were M. concilii cells. These results indicate that M. concilii filaments act as nuclei for granular development. The increase in aggregate size was greater in reactor GP than in reactor G during the early stages of startup, suggesting that the presence of propionate-oxidizing syntrophic consortia assisted the formation of granules. The mature granules formed in both reactors exhibited a layered structure with M. concilii dominant in the core, syntrophic consortia adjacent to the core, and filamentous bacteria in the surface layer. The excess of filamentous bacteria caused delay of granulation, which was corrected by increasing shear through an increase of the recycling rate.     

Stimulation of methanol degradation in UASB reactors: in situ versus pre-loading cobalt on anaerobic granular sludge

The effect of pre-loading and in situ loading of cobalt onto a cobalt-limited granular sludge on the performance of methanol fed bioreactors was investigated. One upflow anaerobic sludge bed (UASB) reactor was inoculated with cobalt pre-loaded sludge (24h; 30 degrees C; 1 mM CoCl2) and a second UASB with unloaded sludge. The UASB reactors (30 degrees C; pH 7) were operated for 77 days at 8 h hydraulic retention time and organic loading rates ranging from 5 to 20 g COD.L reactor(-1).d(-1). Cobalt pre-loading clearly stimulated the methanogenic activity of the sludge with methanol as the substrate, e.g., after 30 days of reactor operation this activity was 5.8 times higher than that of the cobalt unloaded sludge. During the experiment, part of the cobalt leached from the pre-loaded sludge, i.e., 54% of the cobalt content was lost during the 77 days of reactor operation. Sequential metal extraction showed that losses mainly occurred from the exchangeable and carbonate fraction and in the sludge remaining cobalt was mainly present in the organic/sulfide fraction of the sludge. In situ loading of cobalt in the unloaded UASB reactor on day 57 by adding 31 microM cobalt to the influent for a 24-h period (16% of the cobalt present in the loaded sludge at day 11) resulted in a 4 time increase of the methanogenic activity of the sludge with methanol as the substrate at the end of the reactor experiment, while the accumulated amount of cobalt in the sludge only amounted to 6% of the cobalt accumulated in the loaded sludge (on day 11). This study showed that both pre-loading sludge and in situ loading are adequate for achieving an increased reactor performance of methanol fed UASB reactors operating under cobalt limitation. However, the in situ dosing procedure needs substantially lower amounts of cobalt, while it also gives significantly smaller losses of cobalt with the effluent.     

Responses of the biogas process to pulses of oleate in reactors treating mixtures of cattle and pig manure

The effect of oleate on the anaerobic digestion process was investigated. Two thermophilic continuously stirred tank reactors (CSTR) were fed with mixtures of cattle and pig manure with different total solid (TS) and volatile solid (VS) content. The reactors were subjected to increasing pulses of oleate. Following pulses of 0.5 and 1.0 g oleate/L, the most distinct increase in volatile fatty acid (VFA) concentrations were observed in the reactor with the lowest TS/VS content. This suggests a higher adsorption of oleate on the surfaces of biofibers in the reactor with the highest TS/VS and a less pronounced inhibition of the anaerobic digestion process. On the other hand, addition of 2.0 g oleate/L severely inhibited the process in both reactors, and a significant increase in all VFA concentrations combined with an immediate drop in methane production was noticed. However, 20 days after the reactors had been exposed to oleate both reactors showed a lower VFA concentration along with a higher methane production than before the pulses. This indicates that oleate had a stimulating effect on the overall process. The improved acetogenic and methanogenic activity in the reactors was confirmed in batch activity tests. In addition to this, toxicity tests revealed that the oleate pulses induced an increase in the tolerance level of acetotrophic methanogens towards oleate. When evaluating the usability of different process parameters (i.e., VFA and methane production) as indicators of process recovery, following the inhibition by oleate, propionate was found to be most suitable.     

The inhibitory effects and removal of dieldrin in continuous upflow anaerobic sludge blanket reactors

The inhibitory effects and removal efficiency of dieldrin (DLD) in anaerobic reactors were investigated. Anaerobic toxicity assay (ATA) experiments conducted in batch reactors revealed that 30 mg/l DLD had inhibitory effects on the unacclimated mixed anaerobic cultures. Continuous reactor experiments performed in a lab-scale two-stage upflow anaerobic sludge blanket (UASB) reactor system which was fed with ethanol as the sole carbon source, indicated that anaerobic granular cultures could be successfully acclimated to DLD. Chemical oxygen demand (COD) removal efficiencies were 88-92% for the two-stage system. The influent DLD concentration of 10 mg/l was removed by 44-86% and 86-94% in the second stage and overall UASB system, respectively. Biosorption of DLD on granular anaerobic biomass was found to be a significant mechanism for DLD removal in the UASB system. The maximum DLD loading rate and minimum HRT achievable for the first stage UASB reactor were 0.5 mg/lday (76 microg DLD/g VSS.day) and 10 h, respectively, which resulted in the overall COD removal efficiency of 85%.     

Granule development and performance in sucrose fed anaerobic baffled reactors

Two 90 L anaerobic baffled reactors were used to study the granulation of sludge and the effect of the organic loading rate and NaHCO3/COD ratios on reactor performance. Furthermore, it was determined whether an anaerobic baffled reactor would promote phase separation and if additive of bentonite or granular active carbon was capable of enhancing granule formation. In order to minimize feed variations, and have a totally biodegradable substrate, a synthetic sucrose substrate was used. Granulation was achieved in both reactors within 75 days. However, the granules from the granular active carbon amended reactor appeared earlier and were larger and more compact. The reactors were maintained at a hydraulic retention time of 20 h during performance study stage. The results showed that when organic loading rate were changed from 2.15 to 6.29 kg COD m(-3)day(-1), chemical oxygen demand (COD) removal was not decreased (91-93%), but a slight increase in effluent COD was observed. It was found that the COD removals were generally good (87-92%) and had not obviously change with the decreasing NaHCO3/COD ratios. From the bacterial distribution and the concentration of volatile fatty acids in four compartments, it was concluded that a separation of phases occurred within the anaerobic baffled reactors.     

Granular sludge growth under different reactor liquid surface tensions in lab-scale upflow anaerobic sludge blanket reactors treating wastewater from sugar-beet processing

The formation of anaerobic granular sludge on wastewater from sugar-beet processing was examined in upflow anaerobic sludge blanket reactors. Two strategies were investigated: addition of high-energy substrate, i.e. sugars, and varying the reactor liquid surface tension. When there were insufficient amounts of sugars i.e. less than 7% of the chemical O₂ demand of the influent, no granulation was observed; moreover lowering the reactor liquid surface tension below 48 mN/m was found to increase biomass wash-out. On the other hand, when there were sufficient sugars, granular sludge growth occurred; moreover operating the reactor at a low reactor liquid surface tension reduced biomass wash-out and increased granular yield.     

Long-term impact of dissolved O(2) on the activity of anaerobic granules

The impact of influent dissolved O(2) on the characteristics of anaerobic granular sludge was investigated at various dissolved O(2) concentrations (0.5-8.1 ppm) in 1- and 5-L laboratory-scale upflow anaerobic sludge bed (UASB)-like anaerobic/aerobic coupled reactors with a synthetic wastewater (carbon sources containing 75% sucrose and 25% acetate). The rate of dissolved O(2) supplied to the coupled reactor was as high as 0.40 g O(2)/L(rx).d, and the anaerobic/aerobic coupled reactors maintained excellent methanogenic performances at a COD loading rate of 3 g COD/L(rx).d even after the reactors had been operated with dissolved O(2) for 3 months. The activities of granular sludge on various substrates (glucose, propionate, and hydrogen) were not impaired, and acetate activity was even improved over a short term. However, after 3 months of operation, slight declines on the acetoclastic activities of granules were observed in the coupled reactor receiving the recirculated fluid containing 8.1 ppm dissolved O(2).Methane yield in the anaerobic control reactor and anaerobic/aerobic coupled reactors revealed that a significant aerobic elimination (up to 30%) of substrate occurred in the coupled reactors, as expected. The presence of dissolved O(2) in the recirculated fluid resulted in the development of fluffy biolayers on the granule surface, which imposed a negative impact on the settleability of granular sludge and caused a slightly higher sludge washout. This research shows that the anaerobic/aerobic coupled reactor can be successfully operated under O(2)-limited conditions and is an ideal engineered ecosystem integrating oxic and anaerobic niches. (c) 1996 John Wiley & Sons, Inc.     

Method for estimating the kinetics of substrate degradation in horizontal-flow anaerobic immobilized sludge reactors

Apparent and intrinsic kinetic parameters of substrate decomposition were estimated from the profiles of chemical oxygen demand (COD) and volatile fatty acids (VFA) concentrations along the length of the horizontal-flow anaerobic immobilized sludge (HAIS) reactor. The first order substrate utilization kinetic model describes well the experimental data.     

Effects of the acidogenic biomass on the performance of an anaerobic membrane bioreactor for wastewater treatment

Continuous flow experiments were performed to study the effects of acidogenic biomass development, induced by feeding with non-acidified substrate, on the operation and performance of an anaerobic membrane bioreactor (AnMBR). The AnMBR was operated at cross-flow velocities up to 1.5 m/s and fed with a gelatine–starch–ethanol mixture. A significant fraction of acidogenic biomass developed during reactor operation, which fully determined the sludge rheology, and influenced the particle size distribution. As a result, flux levels of only 6.5 l/m² h were achieved, at a liquid superficial velocity of 1.5 m/s. Even though the soluble microbial products levels in the AMBR were as high as 14 g COD/l, the observed hydraulic flux was not limited by irreversible pore fouling, but by reversible cake layer formation. Propionate oxidation was the limiting step for the applied organic loading rate. The assessed specific methanogenic activity (SMA) with propionate as substrate was, however, similar to the values found by others during thermophilic treatment of non or partially acidified substrates in granular sludge bed reactors, indicating an appropriate level of the propionate oxidation capacity.