C. P. Leslie Grady

Variability in kinetic parameter estimates: A review of possible causes and a proposed terminology

The measured kinetics of a bacterial culture degrading a single organic compound as sole carbon source in a batch reactor depend on the history of the culture, the identifiability of the parameters, and the manner in which the experiment to measure them is run. The initial substrate to biomass ratio used in the experiment is particularly important because it influences both parameter identifiability and the expression of the culture history. Reasons for these effects are reviewed using information from the literature and a nomenclature is proposed whereby investigators can make the nature of reported kinetic parameters clear.

Biodegradation kinetics of substituted phenolics: demonstration of a protocol based on electrolytic respirometry

Abstract Data from electrolytic respirometers were used to evaluate the kinetics of biodegradation of phenol, 4-chlorophenol, m -cresol, 2,4-dimethylphenol and 2,4-dinitrophenol through use of spreadsheets and nonlinear curve-fitting. The protocol is explained in detail. 4-Chlorophenol exhibited substrate inhibition kinetics and 2,4-dinitrophenol exhibited product inhibition kinetics. The others could be characterized by Monod kinetics. Values of the kinetic parameters describing biodegradation of each compound are presented.

Factors influencing deterioration of denitrification by oxygen entering an anoxic reactor through the surface.

The purpose of the paper is to examine the factors that influence the deterioration of denitrification in open anoxic reactors. For this investigation an ASM 1-based simulation model was developed and successfully applied to fit data from batch experiments carried out in lab-scale reactor vessels (uncovered and covered) using both clarified domestic wastewater and synthetic wastewater. Applying the verified model, simulation studies were performed to investigate the effects of available denitrifiable substrate, biomass concentration, oxygen transfer rate, and temperature on deterioration of denitrification in open anoxic reactors. It has been shown that oxygen entering an anoxic reactor through the surface may not just affect denitrification metabolically, but also kinetically, due to increased dissolved oxygen (DO) concentration exerting an inhibitory effect on the denitrification rate. When the exogenous substrate concentration in the reactor vessel is high enough for a high consumption rate, the DO concentration is kept low. The higher the biomass concentration, and thereby the consumption rate of endogenous substrate, the lower the DO concentration during the low-rate denitrification phase. At low substrate removal rates, decreasing temperature will cause the DO concentration in anoxic vessels to increase. The results suggest that assuring removal of available exogenous carbon source at high rate by staging of open anoxic bioreactors may significantly improve denitrification efficiency.

Respiration inhibition kinetic analysis

Abstract A protocol was developed for measuring the inhibitory effects of synthetic organic compounds on the biodegradation of naturally occurring (biogenic) organic matter. A tiered approach based on respirometry was employed. In the first tier, compounds were screened to determine if their effects were sufficiently inhibitory to warrant further testing. This was done at an inhibitor concentration of 1000 mg 1 −1 , or the solubility limit if lower, and at both high and low biogenic substrate concentrations. If a compound caused more than 50% inhibition in a screening test, the stability of its effect was determined next. Finally, compounds causing stable responses were subjected to the respiration inhibition kinetic analysis in which the Monod kinetic parameters describing the biodegradation of butyric acid were measured in the presence of several inhibitor concentrations by using a technique based on that of Cech et al . ( Wat. Sci. Technol . 17 (2/3), 259–272, 1985), thereby allowing the effects of inhibitor concentration on biodegradation kinetics to be quantified. Examples of application of the protocol are given.

Automatic control of the activated sludge process—II. Efficacy of control strategies

Abstract Through the use of computer simulation, the efficacies of the following strategies for control of the activated sludge process were evaluated: single loop feedback control; multiple loop feedback control; single loop feedforward control; feedforward/feedback control; and multiple loop feedforward control. The best improvement in system performance was found with multiple loop feedforward control although the full benefits of the technique could not be achieved due to saturation of the manipulated variables. During the research two types of plants were simulated: Case I in which the concentration of suspended solids in the final effluent decreased as the MLSS concentration entering the settler increased, and Case II in which the effluent suspended solid increased. The best choices for the measured and controlled variables in the multiple loop feedforward control systems depended heavily upon the type of behavior exhibited by the final settler.

Comparison of biosorption isotherms for di-n-butyl phthalate by live and dead bacteria

Abstract Isotherms were determined for the sorption of di-n-butyl phthalate (DBP) on live and dead suspensions of Pseudomonas fluorescens IFO 12055. When isotherms were expressed in terms of the suspended solids concentration actually present in the sorption flasks, the dead biomass had an apparently higher affinity for DBP. However, when the loss of biomass due to cell lysis during a autoclaving was considered, the isotherms on the two types of biomass were the same.

Optimization of activated sludge reactor configuration:: kinetic considerations

To evaluate and design staged activated sludge systems it is necessary to determine the biomass requirement for a given configuration. This depends on both kinetics and treatment requirements. We present a procedure to determine the optimum reactor configuration for a range of influent and effluent substrate concentrations, half saturation coefficients, and number of tanks in series for both inhibitory and non-inhibitory substrates. Dimensionless plots of the results show the minimum biomass requirement of the series relative to that for a single CSTR and the optimal relative sizes of the tanks. The plots may be used directly for staged system design and lead to the following conclusions: three tanks in series is generally best, high influent substrate concentrations and stringent discharge requirements increase the benefit of staging, and optimal tank sizing is significantly better than using equal sized tanks.

Aerobic and anoxic biodegradation of benzoate: stability of biodegradative capability under endogenous conditions

Aromatic organic compounds are degraded by different enzyme systems under aerobic and anoxic conditions. This raises the question of how bacteria in biological nitrogen removal processes, which cycle bacteria between aerobic and anoxic environments, regulate their enzyme systems for degrading aromatic compounds. As a first step in answering that question, mixed microbial communities were grown on benzoate as sole carbon source in chemostats under fully aerobic and fully anoxic (nitrate as the electron acceptor) conditions and tested for their ability to degrade benzoate in batch reactors after exposure to aerobic or anoxic conditions in the absence of substrate. Aerobically grown biomass retained its ability to degrade benzoate without loss of activity after endogenous exposure to aerobic conditions for up to 8 h. However, when exposed to anoxic conditions, the biomass rapidly lost its aerobic benzoate degrading activity, retaining less than 20% of the initial activity after 8 h. Similarly, anoxically grown biomass retained its ability to degrade benzoate without loss of activity after endogenous exposure to anoxic conditions for up to 8 h. However, when anoxically grown biomass was exposed to aerobic conditions, only 20% of its initial activity was lost in the first 2 h, after which the remaining activity was retained for up to 8 h. Similar experiments with pyruvate showed that the 20% loss of activity was not due to loss of denitrifying enzymes, suggesting that it was due to loss of catabolic enzymes.

Automatic control of the activated sludge process—I. Development of a simplified dynamic model

Abstract A set of simplified dynamic models was developed for the activated sludge process (primary settler, biological reactor, and final settler). The models incorporate variable gains, variable time constants, and fixed dead times. Because the parameters may be easily evaluated on site, the models will be particularly useful for process analysis, control system design, and automatic control of a particular plant. Two different cases were studied; one for a plant in which the steady state concentration of suspended solids in the final effluent is directly proportional to the mixed liquor suspended solids, and one in which it is inversely proportional. Due to this difference in steady state gains the final performance of the system is quite different for the two cases although the dynamics are very similar. Therefore, care should be taken in characterizing the final settler.

A technique for obtaining representative biokinetic parameter values from replicate sets of parameter estimates

The rational design and simulation of biotreatment systems requires sound mathematical models and reliable parameter estimates. Biodegradation kinetics are usually modeled using the Monod or Andrews equation, and model parameters are usually obtained by fitting an observed concentration-time profile to the model. Since the nature of the models and the method of parameter estimation result in correlation between the parameters, the question arose as to how to obtain a single set of Parameters Representing the Average μ-S response (referred to as PRAMUS values) from replicate sets of experimentally derived parameter estimates. The common approach of using the arithmetic mean parameter values was adequate only for data conforming to the Monod model, particularly when the variability among replicate parameter estimates was small. For the Andrews model, nonlinear regression was used to obtain PRAMUS values. The ability of the PRAMUS and mean parameter values to reproduce the average μ-S response and to simulate experimentally observed oxygen uptake was evaluated. On this basis, the use of PRAMUS values is recommended, particularly when dealing with systems modeled by the Andrews equation, with experimental data characterized by a high degree of experimental variability, and/or with process situations where high substrate concentrations are likely to be encountered, e.g. shock loads.