Higher Oxygen Uptake Rate Research Articles

Low intensity ultrasound stimulates biological activity of aerobic activated sludge

This work aims to explore a procedure to improve biological wastewater treatment efficiency using low intensity ultrasound. The aerobic activated sludge from a municipal wastewater treatment plant was used as the experimental material. Oxygen uptake rate (OUR) of the activated sludge (AS) was determined to indicate the changes of AS activity stimulated by ultrasound at 35 kHz for 0–40 min with ultrasonic intensities of 0–1.2 W/cm2. The highest OUR was observed at the ultrasonic intensity of 0.3 W/cm2 and an irradiation period of 10 min; more than 15% increase was achieved immediately after sonication. More significantly, the AS activity stimulated by ultrasound could last 24 h after sonication, and the AS activity achieved its peak value within 8 h after sonication, or nearly 100% higher than the initial level after sonication. Therefore, to improve the wastewater treatment efficiency of bioreactors, ultrasound with an intensity of 0.3 W/cm2 could be employed to irradiate a part of the AS in the bioreactor for 10 min every 8 h.

Oxygen uptake rates and liver‐specific functions of hepatocyte and 3T3 fibroblast co‐cultures

Bioartificial liver (BAL) devices have been developed to treat patients undergoing acute liver failure. One of the most important parameters to consider in designing these devices is the oxygen consumption rate of the seeded hepatocytes which are known to have oxygen consumption rates 10 times higher than most other cell types. Hepatocytes in various culture configurations have been tested in BAL devices including those formats that involve co-culture of hepatocytes with other cell types. In this study, we investigated, for the first time, oxygen uptake rates (OUR)s of hepatocytes co-cultured with 3T3-J2 fibroblasts at various hepatocyte to fibroblast seeding ratios. OURs were determined by measuring the rate of oxygen disappearance using a ruthenium-coated optical probe after closing and sealing the culture dish. Albumin and urea production rates were measured to assess hepatocyte function. Lower hepatocyte density co-cultures demonstrated significantly higher OURs (2 to 3.5-fold) and liver- specific functions (1.6-fold for albumin and 4.5-fold for urea production) on a per cell basis than those seeded at higher densities. Increases in OUR correlated well with increased liver-specific functions. OURs (V(m)) were modeled by fitting Michaelis-Menten kinetics and the model predictions closely correlated with the experimental data. This study provides useful information for predicting BAL design parameters that will avoid oxygen limitations, as well as maximize metabolic functions.

Microbial destruction of organic matter in the bottom sediments of Lithuanian lakes

The rates of the processes of bacterial sulfate reduction (SR) and total destruction of organic matter (Dtotal) were studied in the bottom sediments (BS) of 14 lakes in Lithuanian national and regional parks in the summers of 1998–2002. Anaerobic processes accounted for an average of 92% of Dtotal in the depressions of deep-water lakes; for the sediments of shallow lakes, high rates of oxygen uptake were noted. The SR rate in different lakes varied from 0.09 to 2.60 mg S2−/(dm3 day). At low sulfate concentrations (13.3–70.6 mg S-SO 4 2− /dm3), characteristic of the BS of freshwater ecosystems, the main factor that affected the SR rate in the BS of the lakes studied was the content of readily available organic matter; only in special cases, was it affected by a change in the sulfate ion concentration. In shallow lakes, temperature-dependent activation of sulfate-reducing bacteria and their inhibition by acidification of the environment were recorded. The contribution of SR to Dtotal was 0.2 to 11.0%.

Biodegradation of tannery wastewater using sequencing batch reactor—Respirometric assessment

This investigation proved that respirometry combined with sequencing batch reactor (SBR) could be an effective way for the removal of COD in tannery wastewater. Measurement of oxygen uptake rates (OUR) and corresponding COD uptake rates showed that a 12-h operating cycle was optimum for tannery wastewater. The removal of COD by degradation was stoichiometric with oxygen usage. A plot of OUR values provided a good indication of the biological activity in the reactor. A high OUR value corresponded to the feed period; at the end of the cycle, when the substrate was depleted, the OUR value was low. At a 12-h SBR cycle with a loading rate of 1.9–2.1 kg m −3 d −1, removal of 80–82% COD, 78–80% TKN and 83–99% NH 3-N were achieved. These removal efficiencies were much higher than the conventional aerobic systems. A simple method of COD fractionation was performed from the OUR and COD uptake rate data of the SBR cycle. About 66–70% of the influent COD was found to be readily biodegradable, 10–14% was slowly degradable and 17–21% was non-biodegradable. The oxygen mass transfer coefficient, K L a (19 ± 1.7 h −1) was derived from respirometry. It was observed that with the exception of high organic load at the initial feed the oxygen transfer capacity was in excess of the OUR, and aerobic condition was generally maintained. Simultaneous nitrification–denitrification was observed in the SBR during the feed period as proved by mass balance.

Significance of bed porosity, bran and specific surface area in solid-state cultivation of Aspergillus oryzae

In this paper, the effects of bed porosity, bran and specific surface area on the oxygen uptake rate and α-amylase production during growth of Aspergillus oryzae on wheat grain and wheat–flour substrate are reported. The high oxygen uptake rate found during cultivation of A. oryzae on wheat–flour substrate was not reached on wheat grain. This is mainly due to the bran of the wheat grain. Using wheat–flour substrates, it was shown that extra bed porosity increased the α-amylase production and oxygen uptake rates. Furthermore, the peak oxygen uptake rate decreased with increasing surface area–volume ratio of the substrate particles, while the α-amylase production and the cumulative oxygen uptake per gram of initial substrate dry matter increased. The present work does not support a direct correlation between aerial mycelia and enzyme production. There is, however, a correlation between the α-amylase yield and the cumulative oxygen uptake (not the uptake rate). This implies that aerial mycelia could accelerate α-amylase production even if they do not increase the yield.

A nitrate biosensor based methodology for monitoring anoxic activated sludge activity

An improved methodology based on a nitrate biosensor is developed and applied successfully for in-depth monitoring and study of anoxic activated sludge activities. The major advantages of the methodology are its simplicity, reliability and high data quality. The resulting data allowed for the first time to monitor anoxic respiration rate of activated sludge (nitrate uptake rate (NUR)) at a high time resolution making it clearly comparable with high frequency oxygen uptake rate (OUR) measurements obtained under aerobic conditions. Further, the anoxic respiration data resulting from a pulse addition of carbon source to endogenously respiring anoxic activated sludge shows a clear start-up phenomenon and storage tail that is usually also observed in high-frequency OUR measurements. Finally, the improved methodology can be expected to serve as an anoxic respirometer for activated sludge treatment plants where denitrification process occurs in single-step. Further, it can be used for a variety of purposes e.g. for toxicity and activity monitoring, process control and parameter estimation of the activated sludge process, similar to the aerobic respirometers.

Mass transfer impacts in flocculent and granular biomass from SBR systems

An experimental study was conducted to describe mass transfer impacts within nitrifying aggregates sourced from sequencing batch reactor (SBR) activated sludge systems. Flocculent and granular sludge with high nitrification activity was obtained in two laboratory SBR systems, supplied with a synthetic, ammonium-based feed. The flocculent biomass was fractionated using a sieving procedure, in order to obtain biomass fractions with different particle size distributions. The oxygen uptake rate (OUR) response to changes in dissolved oxygen concentration was measured under highly controlled conditions in a titrimetric and off-gas analysis (TOGA) sensor, and the results used to assess mass transfer effects. As the average particle size of the biomass increased, mass transfer limitations were found to increase significantly. Empirically fitted, apparent K(S,O2) values were demonstrated to be highly dependent on particle size, and reflect the mass transfer limitations occurring in the aggregates within a given system. Such parameters thus have little to do with the actual biokinetic parameter from which they are derived. The results obtained from the TOGA sensor study were consistent with those obtained from a microelectrode study on the same nitrifying granules. Together, these studies add considerable weight to the conclusion that consideration of external and internal mass transfer limitations is vital to the accurate description of activated sludge treatment processes, particularly those with a high oxygen uptake rate.

High rate treatment by aerobic upflow sludge blanket (AUSB) with external oxygenation

A 3-year study was conducted using an aerobic upflow sludge blanket (AUSB) reactor to achieve high-rate biological treatment through maintenance of a high biomass concentration (7-10 g VSS/L) together with a high oxygen flux. The AUSB reactor was not aerated directly; instead, oxygenation was provided in an external chamber with flow recirculation. The oxygenation was provided at four different pressurizations: 0, 15, 20 and 25 psig. The AUSB reactor was also pressurized to avoid the flotation of biomass. The flow recirculation rate was varied from 400% to 1,500% in order to ensure adequate oxygen supply. It was found that the AUSB system was able to handle a volumetric loading of as high as 10 kg COD/m3-day with a removal efficiency of 92%. Despite a high upflow velocity through AUSB, the effluent suspended solid concentration was mostly below 60 mg/L. The active fraction of biomass in the AUSB sludge was about 3 times higher than that of the regular activated sludge. This was indicated by a very high specific oxygen uptake rate (SOUR), up to 180-250 mg O2/g VSS-hr. The sludge yield in the entire system was only 0.09 to 0.13 g VSS/g COD removed. This was mainly caused by additional auto-oxidation of biosolids in the oxygenation chamber due to flow recirculation.

Oxygen transfer effects on recombinant benzaldehyde lyase production

Oxygen transfer (OT) effects on benzaldehyde lyase (BAL) production by recombinant Escherichia coli carrying pUC18:: bal gene were investigated on a defined medium with glucose and (NH 4 ) 2 HPO 4 as the sole carbon and nitrogen sources respectively, in 3.0 dm 3 bioreactor systems at six different conditions with the parameters, air inlet rate of Q O / V R = 0.5 vvm, and agitation rates of N = 250 , 375, 500, 625, 750 min - 1 and at Q O / V R = 0.7 vvm , N = 750 min - 1 . Cell concentration ( C X ), BAL volumetric and specific activities, organic acid concentrations, yield values, maintenance coefficients, oxygen transfer coefficients ( K L a ) and oxygen uptake rates (OUR) were determined throughout the bioprocess. Increase in the oxygen transfer rate increased the cell formation up to 0.5 vvm, 500 min - 1 ; however, further increase inhibited cell formation. The highest BAL volumetric activity was obtained at 0.5 vvm, 500 min - 1 as 860 U cm - 3 . K L a , and enhancement factor E ( K L a / K L a o ) increased with the cultivation time and agitation rate, their values varied between 0.008–0.046 s - 1 and 1.00–3.07, respectively. OUR first increased, and then decreased with the cultivation time at all OT conditions, the highest OUR was observed at 0.5 vvm, 500 min - 1 as 3.6 mmol m - 3 s - 1 at t = 8 h ; while the highest specific OUR was obtained at 0.5 vvm, 250 min - 1 as 0.74 kg kg - 1 h - 1 at t = 1 h . With the increase in Q O / V R and/or N, Damköhler number ( Da ) , that is the OT limitation, decreased. The effectiveness factor ( η ) values were close to 1.0 at medium and high OT conditions at the beginning of the processes as cells consumed oxygen with such a high rate that maximum possible oxygen utilization rate (oxygen demand) value was approached; thereafter, η decreased with the increase in cultivation time and Da, and the decrease rate was higher at lower agitation rates due to the lower K L a . Oxygen consumption for maintenance ( m O ) was the highest at 0.5 vvm, 500 min - 1 and the value was 0.20 kg kg - 1 h - 1 .

The gut microenvironment of helicid snails (Gastropoda: Pulmonata): in-situ profiles of pH, oxygen, and hydrogen determined by microsensors

In-situ profiles of pH, oxygen, and hydrogen were measured in isolated guts of starved terrestrial gastropods belonging to four species, Cornu aspersum (syn. Helix aspersa), Elona quimperiana, Helix pomatia, and Helix lucorum (excepted pH), using Clark-type oxygen and hydrogen microsensors and liquid-ion-exchanger pH microelectrodes. The pH profiles in the two phyllophagous species, H. pomatia and C. aspersum, increased by 0.9 and 1.4 from the crop to the distal intestine (pH 6.4 and 7.4, respectively). In the saprophagous E. quimperiana, as in H. pomatia, the pH along the gut axis remained acidic (5.1–6.6), suggesting saprophagous habits in the latter. In all four species, no oxygen was detected in the gut lumen. Nevertheless, steep oxygen gradients around the gut epithelium indicated high oxygen-uptake rates. The estimated respiratory activity of the intestine ranged between 6.5 (E. quimperiana) and 13.1 (H. lucorum) μmol O2·g fresh mass–1·h–1. Hydrogen accumulated in the intestine and digestive gland of all snails tested, with the highest values in E. quimperiana and H. pomatia (58 and 78 μM, respectively). These results provide the basis for a better understanding of the microbial and biochemical processes involved in digestion.

Two mechanisms for oxidation of cytosolic NADPH by Kluyveromyces lactis mitochondria.

Null mutations in the structural gene encoding phosphoglucose isomerase completely abolish activity of this glycolytic enzyme in Kluyveromyces lactis and Saccharomyces cerevisiae. In S. cerevisiae, the pgi1 null mutation abolishes growth on glucose, whereas K.lactis rag2 null mutants still grow on glucose. It has been proposed that, in the latter case, growth on glucose is made possible by an ability of K. lactis mitochondria to oxidize cytosolic NADPH. This would allow for a re-routing of glucose dissimilation via the pentose-phosphate pathway. Consistent with this hypothesis, mitochondria of S. cerevisiae cannot oxidize NADPH. In the present study, the ability of K. lactis mitochondria to oxidize cytosolic NADPH was experimentally investigated. Respiration-competent mitochondria were isolated from aerobic, glucose-limited chemostat cultures of the wild-type K. lactis strain CBS 2359 and from an isogenic rag2Delta strain. Oxygen-uptake experiments confirmed the presence of a mitochondrial NADPH dehydrogenase in K.lactis. This activity was ca. 2.5-fold higher in the rag2Delta mutant than in the wild-type strain. In contrast to mitochondria from wild-type K. lactis, mitochondria from the rag2Delta mutant exhibited high rates of ethanol-dependent oxygen uptake. Subcellular fractionation studies demonstrated that, in the rag2Delta mutant, a mitochondrial alcohol dehydrogenase was present and that activity of a cytosolic NADPH-dependent 'acetaldehyde reductase' was also increased. These observations indicate that two mechanisms may participate in mitochondrial oxidation of cytosolic NADPH by K. lactis mitochondria: (a) direct oxidation of cytosolic NADPH by a mitochondrial NADPH dehydrogenase; and (b) a two-compartment transhydrogenase cycle involving NADP(+)- and NAD(+)-dependent alcohol dehydrogenases.

Automated fed-batch culture of Kluyveromyces fragilis based on a novel method for on-line estimation of cell specific growth rate

A corrected feed-forward control strategy was developed for automated substrate feeding in aerobic fed-batch cultures. The control strategy incorporates a novel method for on-line estimation of specific growth rate as a performance indicator of the fed-batch culture. The estimation is based on the measurement of the maximum substrate uptake rate (MSUR) using on-line dissolved oxygen (DO) concentration data and mass balances. It allows the controller to track changes in the specific growth rate to compensate for process disturbances. The control strategy was applied to fed-batch culture of Kluyveromyces fragilis to maximize the volumetric productivity of lactase. A maximum volumetric lactase productivity of 2.98 U/ml h was achieved. The control was shown to be stable throughout the culture period even at the highest cell density of 69 g/l. This technique is applicable to various aerobic systems, especially for microbial cultures showing high specific oxygen uptake rates, since it only requires on-line measurement of DO concentration without the need for off-gas analysis. In a comparison carried out with three other fed-batch control strategies (DO-stat, exponential feeding and exponential feeding with manual feedback control), the corrected feed-forward control strategy exhibited the best performance by achieving the highest volumetric lactase and biomass productivity.

Reduction of superoxide production by mitochondria oxidizing NADH in the presence of organic acids

Effects of multiple substrates on oxygen uptake and superoxide production by mitochondria isolated from the pericarp tissue of green bell pepper ( Capsicum annuum L.) were studied. Mitochondria isolated from peppers stored at 4 °C for 5 and 6 days had higher rates of oxygen uptake and were less sensitive to cyanide than mitochondria isolated from freshly harvested peppers. Succinate enhanced state 2 and state 4 rates of oxygen uptake with exogenous NADH in the absence of cytochrome path inhibitors, but not state 3 rates by mitochondria isolated from either freshly harvested or cold-stored bell peppers. The sensitivity of NADH oxidation to cyanide was reduced by both malate and succinate in mitochondria from cold-stored bell peppers, whereas only succinate was effective in mitochondria from freshly harvested peppers. Mitochondria isolated from both freshly harvested peppers and those stored at 4 °C for 5 and 6 days produced superoxide in the absence of exogenous substrates. Superoxide production by mitochondria from freshly harvested bell peppers increased when the mitochondria were supplied with malate, succinate or NADH, but only NADH enhanced superoxide production by mitochondria from cold-stored peppers. Both succinate and malate reduced the production of superoxide by mitochondria isolated from cold-stored bell peppers. Succinate and malate as second substrates also reduced the production of superoxide with NADH by mitochondria from both freshly harvested and cold-stored bell peppers. Malonate, a competitive inhibitor of succinate dehydrogenase, was inhibitory to oxygen uptake and to superoxide production. Mitochondria isolated from cold-stored bell peppers converted succinate to pyruvate at 25 °C at considerably higher rates than those of mitochondria from freshly harvested bell peppers. Since pyruvate has been shown to activate the alternative oxidase and the presence of pyruvate is essential for continued alternative oxidase activity, we suggest that pyruvate limits superoxide production by enhancing the flow of electrons through the alternative path. A direct scavenging of superoxide by succinate, malate and pyruvate, however, cannot be ruled out.

Flexibility in basal metabolic rate and evaporative water loss among hoopoe larks exposed to different environmental temperatures.

The 'energy demand' hypothesis for short-term adjustments in basal metabolic rate (BMR) posits that birds adjust the size of their internal organs relative to food intake, a correlate of energy demand. We tested this hypothesis on hoopoe larks (Alaemon alaudipes), inhabitants of the Arabian desert, by acclimating birds for 3 weeks at 15 degrees C and at 36 degrees C, then measuring their BMR and total evaporative water loss (TEWL). Thereafter, we determined the dry masses of their brain, heart, liver, kidney, stomach, intestine and muscles of the pectoral region. Although mean body mass did not differ initially between the two groups, after 3 weeks, birds in the 15 degrees C group had gained mass (44.1+/-6.5 g), whereas larks in the 36 degrees C group had maintained a constant mass (36.6+/-3.6 g; means +/- s.d., N=6). Birds in the 15 degrees C group had a mean BMR of 46.8+/-6.9 kJ day(-1), whereas birds in the 36 degrees C group had a BMR of 32.9+/-6.3 kJ day(-1), values that were significantly different when we controlled for differences in body mass. When measured at 35 degrees C, larks in the cold-exposure group had a TEWL of 3.55+/-0.60 g H(2)O day(-)(1), whereas TEWL for birds in the 36 degrees C group averaged 2.23+/-0.28 g H(2)O day(-1), a difference of 59.2%. Mass-independent TEWL differed significantly between groups. Larks in the 15 degrees C group had a significantly larger liver, kidney and intestine than larks in the 36 degrees C group. The total increase in organ mass contributed 14.3% towards the total mass increment in the cold exposure group. Increased food intake among larks in the cold group apparently resulted in enlargement of some of the internal organs, and the increase in mass of these organs required a higher rate of oxygen uptake to support them. As oxygen demands increased, larks apparently lost more evaporative water, but the relationship between increases in BMR and TEWL remains unresolved.

In situ respirometry in an SBR treating wastewater with high phenol concentrations

This investigation demonstrates that in situ respirometry can be an effective tool to manage the removal of an inhibitory substrate in a sequencing batch reactor (SBR). Data collected enabled the determination of an optimum operating cycle for the effective treatment of a synthetic wastewater containing up to 1300 mg/l phenol as the sole carbon source. Oxygen uptake rates were monitored in situ at various stages of a bench-scale sequencing batch reactor. Respirometry was used as an indicator of microbial activity and substrate utilisation. Although the profile of the Oxygen Uptake Rate (OUR) followed closely that of phenol substrate removal any correlation between the OUR and soluble residual substrate COD was specific to that system. A high OUR value corresponded to the feed period; at the end of the cycle, when the substrate was depleted, the OUR value was low. A plot of OUR values provided a good indication of the biological activity in the reactior. It was possible to select an optimum operating cycle using the oxygen uptake rates as an indicator for the removal of phenol. The initial operating cycle was 24 h, which included phases for filling, reaction, quiescence and decanting. From oxygen uptake rates and corresponding soluble COD values of the remaining substrate, data showed that the 4 h operating cycle was able to achieve similar substrate removal efficiencies. At a SBR loading rate of 3.12 kg phenol m −3d −1, removal of 97% COD was achieved at the end of the cycle. The reactor was operated at a SRT of 4–27 days with biomass concentrations ranging from 3500–3900 mg/l.

Tolerance of pupae and pharate adults of Callosobruchus subinnotatus Pic (Coleoptera: Bruchidae) to modified atmospheres: a function of metabolic rate

Three developmental stages (pupae, early pharate and late pharate adults) of Callosobruchus subinnotatus (Pic.) were investigated for their tolerance or susceptibility to four modified atmospheres. Two of these atmospheres were hypercarbic and two were hypoxic. The hypercarbic atmospheres were found to cause mortality earlier than hypoxic atmospheres. Late pharate adults died earlier than pupae or early pharate adults. Late pharate adults that survived the exposure took a longer time to eclose than the pupae or early pharate adult. Using high resolution microrespirometric techniques, it was possible to record the oxygen consumption rate and CO 2 output of different developmental stages in air. The metabolic rate was determined manometrically as the oxygen uptake rate at an ambient temperature of 25°C. The oxygen uptake rate differed significantly between groups of 20 individuals of different stages ( p<0.01; t-test). The lowest rate of oxygen uptake (510.6±52.2 μl g −1 h −1) was recorded in pupae. Higher oxygen uptake rates were found in early pharate adults (668.4±45.6 μl g −1 h −1) and late pharate adults (1171.2±45.0 μl g −1 h −1), and adult beetles (1310.4±53.4 μl g −1 h −1). The patterns of CO 2 release were similar to those of oxygen uptake. CO 2 release was highest in eclosed adults and late pharate adults followed by early pharate adults, and lowest in pupae. The mode of CO 2 release ranged from continuous CO 2 release in pupae to discontinuous CO 2 release in late pharate and eclosed adults. Thus, high metabolic rates, and perhaps, in conjunction with discontinuous CO 2 of late pharate adults are responsible for their higher susceptibility to modified atmospheres than pupae and early pharate adults.

Kinetics of growth, oxygen uptake, and substrate utilization of wild type and genetically engineeredBurkholderia sp

The gene (vgb) encoding the hemoglobin (VHb) ofVitreoscilla sp. was cloned intoBurkholderia sp. and the effect of VHb on the growth characteristics of genetically engineeredBurkholderia (YV1) were compared with wild typeBurkholderia (R34) using continuous flow reactors (chemostat) at various dilution rates under aerobic conditions. Batch oxygen uptake rate showed that YV1 has much higher oxygen uptake rate than R34 (i.e. 0.63 mg O2/g biomass/min vs. 1.43 mg O2/g biomass/min for R34 and YV1 respectively at a dilution rate of 1.2 day−1). Monod parameters, maximum growth rate (μmax) and half saturation coefficient (Ks) were found to be 7.03 day−1 and 691 mg/L for R34 respectively, compared to 5.49 day−1 and 404 mg/L for YV1 respectively. At low dilution rates (<2.5 day−1), when the substrate is present in low concentrations, the growth yield was much higher in YV1 (0.52) than in R34 (0.37). Although substrate utilization rates were similar between R34 and YV1, the latter showed much higher oxygen uptake rate than did R34 at all dilution rates. When the stability of VHb was tested on agar plates containing 40 μg/L of kanamycin and 100 μg/L of ampicillin,vgb gene containing VHb plasmid in YV1 was stable over 82 days. When survivability under oxygen limited conditions was tested, R34 survived only for 11 days whereas YV1 survived over 25 days in liquid media; in agar plate experiments, R34 did not survive more than 40 days whereas more than 75% of YV1 survived over 110 days.

The influence of impeller type in pilot scale Xanthan fermentations

The rheological complexity of Xanthan fermentations presents an interesting problem from a mixing viewpoint, because the phenomena of poor bulk blending and low oxygen mass transfer rates inherent in highly viscous fermentations (and their consequences) can be systematically investigated, even at the pilot plant scale. This study in a 150 L fermentor compares the physical and biological performance of four pairs of impellers: a standard Rushton turbine, a large diameter Rushton turbine, a Prochem Maxflo T, and a Scaba 6SRGT. Accurate in-fermentor power measurements, essential for the comparison of impellers in relation to operating costs are also reported. It is demonstrated that the agitator performance in Xanthan fermentations is very specific and the choice of which impeller to use in bioreactors to obtain enhanced performance is dependant on the applied criterion. None of the criterion favored the use of the standard Rushton turbine, therefore suggesting that there are strong grounds for retrofitting these impellers with either large diameter impellers of similar design or with novel agitators. In addition, fluid dynamic modeling of cavern formation has clearly highlighted the importance of a well mixed and oxygenated region for providing the capacity for high microbial oxygen uptake rates which govern Xanthan productivity and quality. Copyright 1998 John Wiley & Sons, Inc.

Effect of Dissolved Oxygen on Sludge Acidification during the SSDML-process

There is a distinct possibility that the simultaneous sludge digestion and metal leaching (SSDML) process will encounter oxygen limitation, at least for some duration of the process. This is because of the high oxygen requirement, for sulfur oxidation as well as for sludge solids degradation, combined with the low solubility of oxygen. This can alter the system kinetics and downgrade the process performance. The present work focuses on the effect of dissolved oxygen concentration and oxygen transfer rates on the process of sludge acidification. SSDML experiments were carried out in 20 L capacity laboratory reactors as well as a 4000 L capacity pilot plant. It was found that the duration of oxygen limited kinetics is directly proportional to the concentration of biodegradable sludge volatile solids and inversely proportional to the surface area of sulfur and initial concentration of less acidophilic bacteria in the sludge. During this period, the specific growth rates and sulfate production rates of the sulfur oxidizing bacteria are reduced to a fraction of their actual values due to the limitation in oxygen. Based on this information, empirical relations were developed and these were used to simulate oxygen limited SSDML process sequences. The oxygen uptake rate (OUR) and the volumetric oxygen transfer coefficient (kLa) values were calculated for 12 runs carried out in the pilot plant. Increase in kLa obtained by increasing the aeration rates were marginal at higher aeration rates. Also, very high OUR values observed in the complete absence of sulfur oxidizing bacterial activity indicate aerobic digestion to be the predominant oxygen consuming step.

Ozonation of wastewater containing N-methylmorpholine-N-oxide

Ozonation of synthetic wastewater containing N-methylmorpholine-N-oxide (NMMO) was investigated in batch experiments at different pH-values. Increasing pH caused higher initial elimination rates of NMMO as well as higher formation rates of nitrate. In mass transfer limited experiments, the shortest time needed for NMMO removal was observed at pH 7. This could be explained on the one hand through the importance of ·OH-radicals for the oxidation of NMMO and on the other hand through ozone decomposition reactions by ·OH-radicals at high pH-values. Dissolved ozone increased rapidly after elimination of NMMO at pH 7 and 8.5. High oxygen uptake rates of biomass with ozonation products showed the increase of the biodegradability compared to the initial synthetic wastewater. An improvement of ozone efficiency for elimination of NMMO and organic carbon can be expected in a process using a bioreactor in a recycle system.