Sludge reduction by ozone: Insights and modeling of the dose-response effects.

Applying ozone to the return flow in an activated sludge (AS) process is a way for reducing the residual solids production. To be able to extend the activated sludge models to the ozone-AS process, adequate prediction of the tri-atoms effects on the particulate COD fractions is needed. In this study, the biomass inactivation, COD mineralization, and solids dissolution were quantified in batch tests and dose-response models were developed as a function of the reacted ozone doses (ROD). Three kinds of model-sludge were used. S1 was a lab-cultivated synthetic sludge with two components (heterotrophs XH and XP). S2 was a digestate of S1 almost made by the endogenous residues, XP. S3 was from a municipal activated sludge plant. The specific ozone uptake rate (SO3UR, mgO3/gCOD.h) was determined as a tool for characterizing the reactivity of the sludges. SO3UR increased with the XH fraction and decreased with more XP. Biomass inactivation was exponential (e-ß.ROD) as a function of the ROD doses. The percentage of solids reduction was predictable through a linear model (CMiner + Ysol ROD), with a fixed part due to mineralization (CMiner) and a variable part from the solubilization process. The parameters of the models, i.e. the inactivation and the dissolution yields (ß, 0.008-0.029 (mgO3/mgCODini)-1 vs Ysol, 0.5-2.8 mg CODsol/mgO3) varied in magnitude, depending on the intensity of the scavenging reactions and potentially the compactness of the flocs for each sludge.

Prevalence of 'Candidatus Accumulibacter phosphatis' type II under phosphate limiting conditions.

P-limitation in enhanced biological phosphorus removal (EBPR) systems fed with acetate, has generally been considered as a condition leading to enrichment of organisms of the genotype' Candidatus Competibacter phosphatis' expressing the glycogen-accumulating organisms (GAO) phenotype. Recent studies have demonstrated in short-term experiments that organisms of the genotype 'Candidatus Accumulibacter phosphatis' clade I and II, known to express the polyphosphate-accumulating organisms (PAO) phenotype can switch to the GAO phenotype when poly-P is absent, but are performing the HAc-uptake at lower kinetic rates, where clade I showed the lowest rates. The objective of this study was to verify whether organisms of the genotype 'Candidatus Accumulibacter phosphatis' can also be enriched under P-limiting conditions while expressing a GAO phenotype and more specifically to see which specific clade prevails. A sequencing batch reactor was inoculated with activated sludge to enrich an EBPR culture for a cultivation period of 128 days (16 times the solids retention time) under P-limiting conditions. A mixed culture was obtained comprising of 49 % 'Candidatus Accumulibacter phosphatis' clade II and 46 % 'Candidatus Competibacter phosphatis'. The culture performed a full GAO metabolism for anaerobic HAc-uptake, but was still able to switch to a PAO metabolism, taking up excessive amounts of phosphate during the aerobic phase when it became available in the influent. These findings show that P-limitation, often used as strategy for enrichment of 'Candidatus Competibacter phosphatis', does not always lead to enrichment of only 'Candidatus Competibacter phosphatis'. Furthermore, it demonstrates that 'Candidatus Accumulibacter phosphatis' are able to proliferate in activated sludge systems for periods of up to 128 days or longer when the influent phosphate concentrations are just enough for assimilation purposes and no poly-P is formed. The 'Candidatus Accumulibacter phosphatis' retain the ability to switch to the PAO phenotype, taking up phosphate from the influent as soon as it becomes available.

Accumulibacter clades Type I and II performing kinetically different glycogen-accumulating organisms metabolisms for anaerobic substrate uptake.

The anaerobic acetate (HAc) uptake stoichiometry of phosphorus-accumulating organisms (PAO) in enhanced biological phosphorus removal (EBPR) systems has been an extensive subject of study due to the highly variable reported stoichiometric values (e.g. anaerobic P-release/HAc-uptake ratios ranging from 0.01 up to 0.93 P-mol/C-mol). Often, such differences have been explained by the different applied operating conditions (e.g. pH) or occurrence of glycogen-accumulating organisms (GAO). The present study investigated the ability of biomass highly enriched with specific PAO clades ('Candidatus Accumulibacter phosphatis' Clade I and II, hereafter PAO I and PAO II) to adopt a GAO metabolism. Based on long-term experiments, when Poly-P is not stoichiometrically limiting for the anaerobic VFA uptake, PAO I performed the typical PAO metabolism (with a P/HAc ratio of 0.64 P-mol/C-mol); whereas PAO II performed a mixed PAO-GAO metabolism (showing a P/HAc ratio of 0.22 P-mol/C-mol). In short-term batch tests, both PAO I and II gradually shifted their metabolism to a GAO metabolism when the Poly-P content decreased, but the HAc-uptake rate of PAO I was 4 times lower than that of PAO II, indicating that PAO II has a strong competitive advantage over PAO I when Poly-P is stoichiometrically limiting the VFA uptake. Thus, metabolic flexibility of PAO clades as well as their intrinsic differences are additional factors leading to the controversial anaerobic stoichiometry and kinetic rates observed in previous studies. From a practical perspective, the dominant type of PAO prevailing in full-scale EBPR systems may affect the P-release processes for biological or combined biological and chemical P-removal and recovery and consequently the process performance.

Decision support system for the provision of emergency sanitation.

Proper provision of sanitation in emergencies is considered a life-saving intervention. Without access to sanitation, refugees at emergency camps are at a high risk of contracting diseases. Even the most knowledgeable relief agencies have experienced difficulties providing sanitation alternatives in such challenging scenarios. This study developed a computer-based decision support system (DSS) to plan a sanitation response in emergencies. The sanitation alternatives suggested by the DSS are based on a sanitation chain concept that considers different steps in the faecal sludge management, from the toilet or latrine to the safe disposal of faecal matters. The DSS first screens individual sanitation technologies using the user's given input. Remaining sanitation options are then built into a feasible sanitation chain. Subsequently, each technology in the chain is evaluated on a scoring system. Different sanitation chains can later be ranked based on the total evaluation scores. The DSS addresses several deficiencies encountered in the provision of sanitation in emergencies including: the application of standard practices and intuition, the omission of site specific conditions, the limited knowledge exhibited by emergency planners, and the provision of sanitation focused exclusively on the collection step (i.e., just the provision of toilets).

Impact of salinity on the aerobic metabolism of phosphate-accumulating organisms.

The use of saline water in urban areas for non-potable purposes to cope with fresh water scarcity, intrusion of saline water, and disposal of industrial saline wastewater into the sewerage lead to elevated salinity levels in wastewaters. Consequently, saline wastewater is generated, which needs to be treated before its discharge into surface water bodies. The objective of this research was to study the effects of salinity on the aerobic metabolism of phosphate-accumulating organisms (PAO), which belong to the microbial populations responsible for enhanced biological phosphorus removal (EBPR) in activated sludge systems. In this study, the short-term impact (hours) of salinity (as NaCl) was assessed on the aerobic metabolism of a PAO culture, enriched in a sequencing batch reactor (SBR). All aerobic PAO metabolic processes were drastically affected by elevated salinity concentrations. The aerobic maintenance energy requirement increased, when the salinity concentration rose up to a threshold concentration of 2 % salinity (on a W/V basis as NaCl), while above this concentration, the maintenance energy requirements seemed to decrease. All initial rates were affected by salinity, with the NH4- and PO4-uptake rates being the most sensitive. A salinity increase from 0 to 0.18 % caused a 25, 46, and 63 % inhibition of the O2, PO4, and NH4-uptake rates. The stoichiometric ratios of the aerobic conversions confirmed that growth was the process with the highest inhibition, followed by poly-P and glycogen formation. The study indicates that shock loads of 0.18 % salt, which corresponds to the use or intrusion of about 5 % seawater may severely affect the EBPR process already in wastewater treatment plants not exposed regularly to high salinity concentrations.

Impact of salinity on the anaerobic metabolism of phosphate-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO).

The use of saline water as secondary quality water in urban environments for sanitation is a promising alternative towards mitigating fresh water scarcity. However, this alternative will increase the salinity in the wastewater generated that may affect the biological wastewater treatment processes, such as biological phosphorus removal. In addition to the production of saline wastewater by the direct use of saline water in urban environments, saline wastewater is also generated by some industries. Intrusion of saline water into the sewers is another source of salinity entering the wastewater treatment plant. In this study, the short-term effects of salinity on the anaerobic metabolism of phosphate-accumulating organisms (PAO) and glycogen-accumulating organisms (GAO) were investigated to assess the impact of salinity on enhanced biological phosphorus removal. Hereto, PAO and GAO cultures enriched at a relatively low salinity level (0.02 % W/V) were exposed to salinity concentrations of up to 6 % (as NaCl) in anaerobic batch tests. It was demonstrated that both PAO and GAO are affected by higher salinity levels, with PAO being the more sensitive organisms to the increasing salinity. The maximum acetate uptake rate of PAO decreased by 71 % when the salinity increased from 0 to 1 %, while that of GAO decreased by 41 % for the same salinity increase. Regarding the stoichiometry of PAO, a decrease in the P-release/HAc uptake ratio accompanied with an increase in the glycogen consumption/HAc uptake ratio was observed for PAO when the salinity increased from 0 to 2 % salinity, indicating a metabolic shift from a poly-P-dependent to a glycogen-dependent metabolism. The anaerobic maintenance requirements of PAO and GAO increased as the salinity concentrations risen up to 4 % salinity.

Initial-rate based method for estimating the maximum heterotrophic growth rate parameter (µHmax).

Currently, the method most used for measuring the maximum specific growth rate (µ(Hmax)) of heterotrophic biomass is by respirometry, using growth batch tests performed at high food/microorganism ratio. No other technique has been suggested, although the former approach was criticized for providing kinetic constants that could be unrepresentative of the original biomass. An alternative method (seed-increments) is proposed, which relies on measuring the initial rates of respiration (r(O2)(_ini)) at different seeding levels, in a single batch, and in the presence of excess readily biodegradable substrate (S(S)). The ASM1-based underlying equations were developed, which showed that µ(Hmax) could be estimated through the slope of the linear function of r(O2)(_ini)·(V(WW)+v(ML)) vs v(ML) (volume of mixed liquor inoculum); V(WW) represent the wastewater volume added. The procedure was tested, being easy to apply; the postulated linearity was constantly observed and the method is claimed to measure the characteristics of the biomass of interest.

Stepwise calibration of the activated sludge model no. 1 at a partially denitrifying large wastewater treatment plant.

Activated sludge modeling technology is maturing; however, currently, there exists a great need to increase its use in daily engineering practice worldwide. A good way for building the capacities of the practitioners is to promote good modeling practices and standardize the protocols. In this study, a systematic procedure was proposed to calibrate the Activated Sludge Model No. 1 (ASM1) at a large wastewater treatment plant, by which the model adequately predicted the quality of the effluent and the sludge quantities. A hydraulics model was set up and validated through a tracer test. The Vesilind settling constants were measured and combined with the default value of the flocculent zone settling parameter, to calibrate the clarifiers. A virtual anoxic tank was installed in the return activated sludge to mimic the denitrification occurring in the settlers. In ASM1, the calibrated parameters were only two influent chemical oxygen demand fractions and one kinetic constant (oxygen half-saturation coefficient).

Divergence between respirometry and physicochemical methods in the fractionation of the chemical oxygen demand in municipal wastewater.

For activated sludge modeling purposes, the methods used to evaluate the readily biodegradable chemical oxygen demand (RBCOD) in the influents are by biological or via physicochemical assays. However, there has not been sufficient wide comparison between these methods. The main goal of this study was to investigate the performance of the main chemical oxygen demand (COD) fractionation protocols, considering a representative wastewater in the context of tropical and developing countries. Different physicochemical characterization procedures, respirometric tests, and chemical analyses were performed. The fate of the soluble COD in the aeration tanks was studied. The results of the study showed that a marked difference may exist, in municipal wastewaters, between the estimates of the RBCOD fractions measured by respirometry and by any of the physicochemical methods. The evaluated influent showed a rather large fraction of COD that was passing the filters without being rapidly biodegradable, but which was removed quickly by enmeshment in the bioflocs. The consequences of such divergences and behavior are discussed.

Nitrogen mass balances for pilot-scale biofilm stabilization ponds under tropical conditions.

Nitrogen removal in biofilm waste stabilization ponds were modeled using nitrogen mass balance equations. Four pilot-scale biofilm maturation ponds were constructed in Uganda. Pond 1 was control; the others had 15 baffles in each of them. Two loading conditions were investigated (period 1, 18.2g and period 2, 26.8 g NH(4)-Nd(-1)). Total nitrogen and TKN mass balances were made. Bulk water and biofilm nitrification rates were determined and used in the TKN mass balance. Results for total nitrogen mass balance showed that for both periods, denitrification was the major removal mechanism. Nitrogen uptake by algae was more important during period 1 than in period 2. The TKN mass balance predicted well effluent TKN for period 2 than period 1. This could be due to fluctuations in algae density and ammonia uptake during period 1, no conclusions on reliability of mass balance model in period 1 was made.

Nitrification in bulk water and biofilms of algae wastewater stabilization ponds.

Nitrogen removal in wastewater stabilization ponds is poorly understood and effluent monitoring data show a wide range of differences in ammonium. For effluent discharge into the environment, low levels of nitrogen are recommended. Nitrification is limiting in facultative wastewater stabilization ponds. The reason why nitrification is considered to be limiting is attributed to low growth rate and wash out of the nitrifiers. Therefore to maintain a population, attached growth is required. The aim of this research is to study the relative contribution of bulk water and biofilms with respect to nitrification. The hypothesis is that nitrification can be enhanced in stabilization ponds by increasing the surface area for nitrifier attachment. In order to achieve this, transparent pond reactors representing water columns in algae WSP have been used. To discriminate between bulk and biofilm activity, 5-day batch activity tests were carried out with bulk water and biofilm sampled. The observed value for Rnitrbulk was 2.7 x 10(-1) mg-N L(-1) d(-1) and for Rbiofilm was 1,495 mg-N m(-2) d(-1). During the 5 days of experiment with the biofilm, ammonia reduction was rapid on the first day. Therefore, a short-term biofilm activity test was performed to confirm this rapid decrease. Results revealed a nitrification rate, Rbiofilm, of 2,125 mg-N m(-2) d(-1) for the first 5 hours of the test, which is higher than the 1,495 mg-N m(-2) d(-1), observed on the first day of the 7-day biofilm activity test. Rbiofilm and Rnitrbulk values obtained in the batch activity tests were used as parameters in a mass balance model equation. The model was calibrated by adjusting the fraction of the pond volume and biofilm area that is active (i.e. aerobic). When assuming a depth of 0.08 m active upper layer, the model could describe well the measured effluent values for the pond reactors. The calibrated model was validated by predicting effluent Kjeldahl nitrogen of algae ponds in Palestine and Colombia. The model equation predicted well the effluent concentrations of ponds in Palestine.

Nitrification activities in full-scale treatment plants with varying salt loads.

The effect of salt on the nitrification activity in full-scale wastewater treatment plants (WWTP) was investigated. Not only the activity of ammonia and nitrite oxidisers was measured, but also the nitrifying population was assessed (by Fluorescent In Situ Hybridisation)--in full-scale domestic and industrial WWTPs, operated under various salt levels. The results demonstrate a decline in the activity of ammonia and nitrite oxidisers with an increase in salt content: the domestic WWTP with the lowest salt level (0.13g Cl- l(-1)) had the highest specific activity of ammonia and nitrite oxidisers (4.3 and 2.4 mg N (gVSS)(-1) h(-1), respectively), while the lowest specific activities of ammonia and nitrite oxidisers (1.1 and 0.5 mg N (gVSS)(-1) h(-1)) were measured at the highest NaCl concentration (16g Cl- l(-1)). However, comparing the nitrification activity of different types of sludge developed under different operational conditions with the reported values was not directly possible. So we have used the activated sludge model (ASM) to translate the routine operational data into parameters to enable the calculation of the actual fraction of nitrifiers and consequently the actual specific activity of ammonia and nitrite oxidisers. Expressing the activity of ammonia oxidisers in terms of actual specific activity makes the results from pure cultures, enriched cultures, pilot scale and full scale WWTPs comparable. Moreover, these results confirm the behaviour of nitrifiers under salt stress and validate the results obtained from pure and enriched cultures to be extrapolated to full scale.

Long term effects of salt on activity, population structure and floc characteristics in enriched bacterial cultures of nitrifiers.

The effect of salinity on the activity, the composition of nitrifiers and floc characteristics of nitrifying sludge was studied. Non-adapted and adapted (to 10g NaCl-Cl(-)/L for one year) enriched cultures of nitrifiers were tested in three sequencing batch reactors. Salt was increased gradually with 5 up to 40 g Cl(-)/L. No difference in steady state activity was observed between the adapted and non-adapted sludge. The activities of ammonia and nitrite oxidizers dropped 36% and 11%, respectively, at salt concentrations of 10 g Cl(-)/L. At 40 g Cl(-)/L inhibition reached 95% of salt free activity for ammonia and nitrite oxidizers in both adapted and non-adapted reactors. Nitrosomonas europaea and Nitrobacter sp. (fluorescent in situ hybridization) were the only nitrifiers present at high salt levels. Increased salt concentrations resulted in better settling characteristics of the nitrifying sludge. After 118 days the sludge was brought back to the initial conditions (0 g Cl(-)/L for non-adapted and 10 g Cl(-)/L for adapted). Despite the change in population composition similar kinetics as before the salt stress were observed.

Modelling nitrification, heterotrophic growth and predation in activated sludge.

A mathematical model describing the interaction between nitrifiers, heterotrophs and predators in wastewater treatment has been developed. The inclusion of a predation mechanism is a new addition to the existing activated sludge models. The developed model considered multi-substrate consumption and multi-species growth, maintenance and decay in a culture where nitrifiers, heterotrophs and predators (protozoa and metazoa) are coexisting. Two laboratory-scale sequenced batch reactors (SBRs) operated at different sludge retention time (SRT) of 30 and 100 days for a period of 4 years were used to calibrate and validate the model. Moreover, to assess the predator activity, a simple procedure was developed, based on measuring the respiration rate with and without the presence of the predators. The model successfully described the performance of two SBRs systems. The fraction of active biomass (ammonia oxidisers, nitrite oxidisers and heterotrophs) predicted by the proposed model was only 33% and 14% at SRT of 30 and 100 days, respectively. The high fraction of inert biomass predicted by the model was in accordance with the microscopic investigations of biomass viability in both reactors. The presented model was used to investigate the effect of increasing sludge age and the role of predators on the biomass composition of the tested SBR system.

Model-based evaluation of nitrogen removal in a tannery wastewater treatment plant.

Computer modelling has been used in the last 15 years as a powerful tool for understanding the behaviour of activated sludge wastewater treatment systems. However, computer models are mainly applied for domestic wastewater treatment plants (WWTPs). Application of these types of models to industrial wastewater treatment plants requires a different model structure and an accurate estimation of the kinetics and stoichiometry of the model parameters, which may be different from the ones used for domestic wastewater. Most of these parameters are strongly dependent on the wastewater composition. In this study a modified version of the activated sludge model No. 1 (ASM 1) was used to describe a tannery WWTP. Several biological tests and complementary physical-chemical analyses were performed to characterise the wastewater and sludge composition in the context of activated sludge modelling. The proposed model was calibrated under steady-state conditions and validated under dynamic flow conditions. The model was successfully used to obtain insight into the existing plant performance, possible extension and options for process optimisation. The model illustrated the potential capacity of the plant to achieve full denitrification and to handle a higher hydraulic load. Moreover, the use of a mathematical model as an effective tool in decision making was demonstrated.

Improved method for determination of ammonia and nitrite oxidation activities in mixed bacterial cultures.

A simple and reliable method to measure the activity of ammonia and nitrite oxidisers in mixed bacterial cultures was developed. The developed method differentiates between the ammonia and nitrite oxidisers by consecutive injection of NO2- and NH4+. The main advantage of this method is that it avoids the use of metabolic inhibitors for ammonia or nitrite oxidisers, as used by other methods. Moreover, it allows measuring of the short-term effect of an inhibitor on both the ammonia and nitrite oxidisers in one test under controlled environmental conditions (pH, temperature). The developed method was applied to determine the inhibitory effects of salt (NaCl up to 15 g Cl/l) on an enriched culture of nitrifying bacteria. The results of the method demonstrate its potential to accurately determine the individual activities of nitrite and ammonia oxidisers.

Comparison of two experimental methods for the determination of Michaelis-Menten kinetics of an immobilized enzyme.

For the application of immobilized enzymes, the influence of immobilization on the activity of the enzyme should be Known. This influence can be obtained by determining the intrinsic kinetic parameters of the immobilized enzyme, and by comparing them with the kinetic parameters of the suspended enzyme. This article deals with the determination of the intrinsic kinetic parameters of an agarose-gel bead immobilized oxygen-consuming enzyme: L-lactate 2-monooxygenase. The reaction rate of the enzyme can be described by Michaelis-Menten kinetics. Batch conversion experiments using a biological oxygen monitor, as well as steady-state profile measurements within the biocatalyst particles using an oxygen microsensor, were performed. Two different mathematical methods were used for the batch conversion experiments, both assuming a pseudosteady-state situation with respect to the shape of the profile inside the bead. One of the methods used an approximate relation for the effectiveness factor for Michaelis-Menten kinetics which interpolates between the analytical solutions for zero- and first-order kinetics. The other mathematical method was based on a numerical solution and combined a mass balance over the reactor with a mass balance over the bead. The main difference in the application of the two methods is the computer calculation time; the completely numerical calculation procedure was about 20 times slower than the other calculation procedure.The intrinsic kinetic parameters resulting from both experimental methods were compared to check the reliability of the methods. There was no significant difference in the intrinsic kinetic parameters obtained from the two experimental methods. By comparison of the kinetic parameters for the suspended enzyme with the intrinsic kinetic parameters for the immobilized enzyme, it appeared that immobilization caused a decrease in the value of V(m) by a factor of 2, but there was no significant difference in the values obtained for K(m).

The use of a metabolically structured model in the study of growth, nitrification, and denitrification by Thiosphaera pantotropha.

Thiosphaera pantotropha is capable of aerobic heterotrophic nitrification and both aerobic and anaerobic denitrification. These phenomena have been studied in acetate-limited aerobic and anaerobic continuous cultures supplied with ammonia and nitrate. The internal reaction rates were defined, based on biochemical knowledge. The observable external conversion rates are related through a linear equation on the basis of the specified internal reaction rates. The linear equation is a Pirt relation extended for microbial systems with multiple electron donors (acetate and ammonia) and electron acceptors (oxygen and nitrate). The coefficients in this equation were estimated from the continuous culture measurements, and are composed of parameters involved in ATP production and consumption by the microorganism. It is shown that with realistic values for these parameters, the metabolically structured model describes the aerobic as well as the anaerobic experiments.

Determination of growth and coupled nitrification/denitrification by immobilized Thiosphaera pantotropha using measurement and modeling of oxygen profiles.

An oxygen microsensor in combination with mathematical modeling was used to determine the behavior of immobilized Thiosphaera pantotropha. This organism can convert ammonia completely to nitrogen gas under aerobic conditions (coupled nitrification/denitrification) and denitrifies nitrate at highest rates under anaerobic conditions. Immobilization of T. pantotropha can result in aerobic and anaerobic zones inside the biocatalyst particle which will be advantageous for the conversion of ammonia and nitrate from wastewater. However, information of the effects of immobilization on the physiology of T. pantotropha is necessary for the development of such a system. This article gives the extension of a model developed to describe the behavior of chemostat cultures of T. pantotropha so that it can be used for immobilized cells. The original model was based on metabolic reaction equations. Kinetic and diffusion equations have now been added. Experimental verification was carried out using a stirred tank reactor and a Kluyver flask. After immobilization in agarose, the cells were grown in the particles under continuous culture conditions for 3 days. After 24 h the oxygen penetration depth showed a constant value of 100 microm, indicating that a steady state was reached. Scanning electron micrographs showed that large colonies of cells were present in this 100-microm aerobic layer.From the dynamics of the start-up phase, several parameters were determined from measurements of the oxygen concentration profiles made every few hours. The profiles simulated by the model were fitted to the measured data. The average value for the maximum specific growth rate was 0.52 h(-1), and the maximum oxygen conversion rate was 1.0 mol Cmol(-1) h(-1). The maximum specific acetate uptake rate was 2.0 mol Cmol(-1) h(-1), and the Monod constant for acetate was 2.9 x 10(-2) mol m(-3). The maximum specific nitrification rate was 0.58 x 10(-1) mol Cmol(-1) h(-1), and the amount of oxygen necessary for nitrification was 11% of the total oxygen uptake rate. Most of the kinetic parameters determined for the immobilized cells were in good agreement with those for the suspended cells. Only the maximum specific growth rate was significantly higher, and the maximum specific nitrification rate was some what lower than for suspended cells. The experimental results clearly show that an oxygen microsensor, in combination with mathematical modeling, can successfully be used to elucidate the kinetic behavior of immobilized, oxygen-consuming, cells.

Measurement of oxygen concentration gradients in gel-immobilized recombinant Escherichia coli.

In this study, an oxygen microsensor was used to measure oxygen concentration profiles in carrageenan gel particles containing growing, immobilized Escherichia coli B (pTG201). Profiles, which were measured at intervals during continuous culture of gel slabs and beads, became increasingly steep with time. The oxygen penetration depth in the gel decreased with time, eventually reaching a steady state value of approximately 100 microns for both gel beads and slabs. A reaction-diffusion model employing zero-order cell growth kinetics was found to provide an excellent fit to the experimental concentration data. Growth rates estimated from profiles obtained during the first few hours of culture were 0.24h-1 (gel slabs) and 0.18h-1 (beads), compared to a value of 0.30 h-1 measured in free-cell suspensions at 25 degrees C.