Nonlinear control of bioreactors with input multiplicities—an experimental work

The phrase input multiplicities means that an input variable with more than one value produces the same output value as if there were a single input–single output process. With input multiplicities, the value of the process gain changes as the manipulated variable changes, and beyond a certain input value, the sign of the gain also changes. A conventional PI controller for processes with input multiplicities may give unstable, less economical, or oscillatory responses. In the present work, control problems of a continuous bioreactor exhibiting two input multiplicities in the dilution rate on productivity were experimentally analyzed. A regulatory problem for the evaluation of controllers was taken up, i.e. a step change was made in the feed substrate concentration from 20 to 25 g/l at steady state conduction at lower (0.09386 h⁻¹) and higher (0.2278 h⁻¹) dilution rates for the same productivity of 2.9 g/l h. The nonlinear PI controller gave a more stable and fast response at both input dilution rates. The linear PI controller designed for a lower input dilution rate was stable, with some oscillations at the lower dilution rate, but the response was unstable at a higher dilution rate due to the input multiplicity behaviour of the process. Thus, nonlinear PI controller performance was found to be superior to that of the linear controller, and earlier reported theoretical results have been validated by the present experimental work.     

Improvement of nitrogen removal and reduction of operating costs in an activated sludge process with feedforward–cascade control strategy

In this paper, a feedforward–cascade controller for dissolved oxygen concentration in an activated sludge process is designed in order to meet stricter effluent quality standards at a minimum cost. Conventional proportional and integral (PI) constant dissolved oxygen set-point control and feedforward–cascade dissolved oxygen set-point control are evaluated using the reduced model of activated sludge model no. 1 and reduced IWA simulation benchmark. The feedforward–cascade control has been based on a hierarchical structure where a high level or cascade control selects the set-point of the low level or conventional controller and low level directly control dissolved oxygen concentration. And feedforward control is introduced in the control system for preventing the influent loading from influencing the system. Simulation results show that feedforward–cascade control of the activated sludge process is more successful than conventional PI control in meeting the effluent standards and reducing operational costs. This control strategy can be expected to be accepted by the operating personnel in wastewater treatment plants.     

Cybernetic model predictive control of a continuous bioreactor with cell recycle

The control of poly-beta-hydroxybutyrate (PHB) productivity in a continuous bioreactor with cell recycle is studied by simulation. A cybernetic model of PHB synthesis in Alcaligenes eutrophus is developed. Model parameters are identified using experimental data, and simulation results are presented. The model is interfaced to a multirate model predictive control (MPC) algorithm. PHB productivity and concentration are controlled by manipulating dilution rate and recycle ratio. Unmeasured time varying disturbances are imposed to study regulatory control performance, including unreachable setpoints. With proper controller tuning, the nonlinear MPC algorithm can track productivity and concentration setpoints despite a change in the sign of PHB productivity gain with respect to dilution rate. It is shown that the nonlinear MPC algorithm is able to track the maximum achievable productivity for unreachable setpoints under significant process/model mismatch. The impact of model uncertainty upon controller performance is explored. The multirate MPC algorithm is tested using three controllers employing models that vary in complexity of regulation. It is shown that controller performance deteriorates as a function of decreasing biological complexity.     

Feedforward-feedback control of dissolved oxygen concentration in a predenitrification system

As the largest single energy-consuming component in most biological wastewater treatment systems, aeration control is of great interest from the point of view of saving energy and improving wastewater treatment plant efficiency. In this paper, three different strategies, including conventional constant dissolved oxygen (DO) set-point control, cascade DO set-point control, and feedforward-feedback DO set-point control were evaluated using the denitrification layout of the IWA simulation benchmark. Simulation studies showed that the feedforward-feedback DO set-point control strategy was better than the other control strategies at meeting the effluent standards and reducing operational costs. The control strategy works primarily by feedforward control based on an ammonium sensor located at the head of the aerobic process. It has an important advantage over effluent measurements in that there is no (or only a very short) time delay for information; feedforward control was combined with slow feedback control to compensate for model approximations. The feedforward-feedback DO control was implemented in a lab-scale wastewater treatment plant for a period of 60 days. Compared to operation with constant DO concentration, the required airflow could be reduced by up to 8–15% by employing the feedforward-feedback DO-control strategy, and the effluent ammonia concentration could be reduced by up to 15–25%. This control strategy can be expected to be accepted by the operating personnel in wastewater treatment plants.     

Modification of A-stat for the characterization of microorganisms

Two novel modifications of continuous culture with gradual change of dilution rate (A-stat): D-stat and auxo-accelerostat were evaluated in the studies of the effect of changing individual environmental parameters (T, pH, pO(2), substrate concentration, etc.) on growth characteristics of different microorganisms. Common for those cultivation methods is that one environmental parameter is programmed to change with constant change rate (change-stat) while the others are kept constant or in the range not affecting the growth characteristics. The environment response growth curves were obtained starting with chemostat (in A-stat and D-stat) or auxostat (in auxo-accelerostat) steady-state cultures followed by change of set-point value of the desired cultivation parameter. Physiological studies of Saccharomyces sp. and Lactococcus lactis were combined with validation of the different modifications of the A-stat method based on well-known cultivation techniques: chemostat, pH-auxostat, pO(2)-auxostat CO(2)-auxostat and fed-batch. The auxo-accelerostat was shown to be very efficient for cell characterization and dynamic studies in growth environments with excess of essential substrates. Choosing the rate of change of environmental parameters was shown to be critical in comparative physiological studies of microorganisms.     

Regulation of the organic pollution level in anaerobic digesters by using off-line COD measurements

A sampled delayed scheme is proposed to regulate the organic pollution level in anaerobic digestion processes by using off-line COD measurements. The proposed scheme is obtained by combining an error feedback control with a steady state estimator to track constant references and attenuate process load disturbances. The controller performance is tested experimentally for the treatment of tequila vinasses over a period of 68 days under different set-point values and several uncertain scenarios which include badly known kinetic parameters and load disturbances. Experimental results show that the COD concentration can be effectively regulated under the influence of set-point changes and high load disturbances by using only a daily off-line COD measurement, which makes the industrial application of the proposed control scheme feasible.     

The study of neural network-based controller for controlling dissolved oxygen concentration in a sequencing batch reactor

The design and development of the neural network (NN)-based controller performance for the activated sludge process in sequencing batch reactor (SBR) is presented in this paper. Here we give a comparative study of various neural network (NN)-based controllers such as the direct inverse control, internal model control (IMC) and hybrid NN control strategies to maintain the dissolved oxygen (DO) level of an activated sludge system by manipulating the air flow rate. The NN inverse model-based controller with the model-based scheme represents the controller, which relies solely upon the simple NN inverse model. In the IMC, both the forward and inverse models are used directly as elements within the feedback loop. The hybrid NN control consists of a basic NN controller in parallel with a proportional integral (PI) controller. Various simulation tests involving multiple set-point changes, disturbances rejection and noise effects were performed to review the performances of these various controllers. From the results it can be seen that hybrid controller gives the best results in tracking set-point changes under disturbances and noise effects.     

Baroreceptor control of pressure-flow relationships during hypoxemia

In the absence of peripheral chemoreceptors, the effects of graded hypoxemia on the carotid sinus control of central and regional hemodynamics were studied in anesthetized mongrel dogs. Baroreceptor stimulation was effected by carotid sinus isolation and perfusion under controlled pressure. Blood flows were measured in the aorta and the celiac, mesenteric, left renal, and right iliac arteries. Carotid sinus reflex set-point pressures were well maintained until hypoxemia was severe. Carotid sinus reflex set-point gain was maximal during mild hypoxemia. Reflex operating point regional flows were unaffected by hypoxemia. A factorial analysis of overall reflex increases in mean aortic pressure, flow, and power during reduced baroreceptor stimulation showed potentiation by increasing hypoxemia. Corresponding effects of baroreceptor stimulation and hypoxemia on aortic resistance and heart rate were additive. Celiac, renal, and iliac blood flows increased during both hypoxemia and reduced baroreceptor stimulation. Only in the celiac bed were blood flow changes independent of concomitant changes in cardiac output. Thus, at maximum sympathetic stimulation (low carotid sinus pressure) during hypoxemia, the cardiovascular system maintained both central and regional blood flows at high systemic blood pressures independent of the peripheral chemoreceptors.     

Thermodynamics of the binding of human apolipoprotein A-I to dimyristoylphosphatidylglycerol

The interaction of human serum apolipoprotein A-I with dimyristoylphosphatidylglycerol was analyzed by isothermal titration calorimetry. Binding of the apolipoprotein A-I to large unilamellar vesicles of dimyristoylphosphatidylglycerol, a negatively charged phospholipid, is characterized by thermodynamic parameters which are invariant over the 30-40 degrees C temperature range. The enthalpy change resulting from the first additions of lipid are positive and decline in magnitude with subsequent additions of lipid. After several additions of lipid, the sign of the enthalpy changes to negative and then reaches a constant value/injection. This exothermic process is larger and opposite in sign to the heat of dilution. Similar behavior is also observed when the lipid is in the form of a dispersion in distilled water. Only a non-saturable exothermic process is observed at 30 degrees C with large unilamellar vesicles of the zwitterionic lipid, dimyristoylphosphatidylcholine. The beginning of an exothermic process can also be observed prior to the larger endotherm in the first injections of large unilamellar vesicles of dimyristoylphosphatidylglycerol into the protein. We analyze the enthalpy changes for the reaction of dimyristoylphosphatidylglycerol with the protein as arising from two distinct processes, one endothermic and the other exothermic. The binding isotherms for the high affinity binding of the apolipoprotein A-I to large unilammelar vesicles of dimyristoylphosphatidylglycerol, over the temperature range 30-40 degrees C, gave an enthalpy change of 1.43 +/- 0.07 kcal/mol of protein and a free energy change of -5.91 +/- 0.04 kcal/mol of protein for the binding of the protein to a cluster of 25 +/- 2 lipid molecules. Thus this reaction is entropically driven.     

Response pattern of nitrifying biofilm reactor to shock loading

The effects of shock loading on the performance of a nitrifying biofilm reactor were investigated over a wide range of conditions by varying the feed concentration of ammonium (S_O) and dilution rate (D) respectively. It was found that variation in the effluent ammonium concentration as S_O or D changes was subject to a semi-U shaped curve. The response patterns of the nitrifying biofilm reactor to one-step change in S_O at a constant dilution rate, and to one-step change in D with keeping constant S_O have no significant difference.     

Nonlinear control of bioreactors with input multiplicities in dilution rate

Control problems of continuous bioreactors having two input multiplicities in dilution rate on the productivity are analyzed. The nonlinear system is represented by a unity gain linear subsystem cascaded with a nonlinear gain subsystem. A conventional PI controller designed for the linear subsystem followed by the solution of the nonlinear gain equation gives a nonlinear controller. The performance of the nonlinear controller is compared with that of the conventional PI controller and also of the nonlinear controller [1] designed based on the output equation. The present nonlinear PI controller gives a superior performance. A single set of controller settings can be used for both the operating points. Whereas the linear PI controller and the nonlinear controller proposed by Henson and Seborg [1] destabilize the system.     

Time-resolved fluorescence study during denaturation and renaturation of curcumin-myoglobin complex

Curcumin influences the transition point, the concentration of denaturant required to effect 50% of the total change, of myoglobin denaturation. Curcumin enhances absorbance of myoglobin at 280 nm with a binding constant K=3.0×10(4) M(-1) whereas fluorescence of curcumin is quenched by myoglobin with a Stern-Volmer association constant of 2.5×10(5) M(-1). Unfolding process of myoglobin-curcumin induces a recovery in fluorescence lifetime loss. The gain in time-resolved fluorescence lifetime during unfolding has been again lost during refolding of curcumin-myoglobin complex by dilution process suggesting partial reversibility of unfolding process for both myoglobin and curcumin-myoglobin complex.     

Control of the specific growth rate of Bacillus subtilis for the production of biosurfactant lipopeptides in bioreactors with foam overflow

This paper formulates a feeding law for a bioprocess dedicated to the production of an antibiotic surfactant using Bacillus subtilis. The specificity of the process relies on the use of the surface active property of the product to extract it by foaming. The control law is designed to maintain a constant specific biomass growth rate while taking into account the particularity of the process. This law can be regarded as a generalization of the conventional exponential feeding strategy and is generic enough to encompass the case of continuous processes with partial recycling. Conventional exponential feeding strategies indeed fail to account for the loss of biomass induced by the foaming. Previous experiments have provided a model of the process and values for its parameters. From this information, a feeding rate law was computed using the feeding strategy proposed in this paper and applied to an experimental culture. This experiment allows discussion of the modeling of the biomass extraction method used in this study. The results on the estimated specific growth rate highlight the complete agreement between the expected and experimental features. Further process optimization studies can now be performed on the basis of the constant specific biomass growth rate.     

An optimizing start-up strategy for a bio-methanator

This paper presents an optimizing start-up strategy for a bio-methanator. The goal of the control strategy is to maximize the outflow rate of methane in anaerobic digestion processes, which can be described by a two-population model. The methodology relies on a thorough analysis of the system dynamics and involves the solution of two optimization problems: steady-state optimization for determining the optimal operating point and transient optimization. The latter is a classical optimal control problem, which can be solved using the maximum principle of Pontryagin. The proposed control law is of the bang–bang type. The process is driven from an initial state to a small neighborhood of the optimal steady state by switching the manipulated variable (dilution rate) from the minimum to the maximum value at a certain time instant. Then the dilution rate is set to the optimal value and the system settles down in the optimal steady state. This control law ensures the convergence of the system to the optimal steady state and substantially increases its stability region. The region of attraction of the steady state corresponding to maximum production of methane is considerably enlarged. In some cases, which are related to the possibility of selecting the minimum dilution rate below a certain level, the stability region of the optimal steady state equals the interior of the state space. Aside its efficiency, which is evaluated not only in terms of biogas production but also from the perspective of treatment of the organic load, the strategy is also characterized by simplicity, being thus appropriate for implementation in real-life systems. Another important advantage is its generality: this technique may be applied to any anaerobic digestion process, for which the acidogenesis and methanogenesis are, respectively, characterized by Monod and Haldane kinetics.     

Steady state and transient growth of autotrophic nitrifying bacteria

Growth of the autotrophic nitrifying bacteria Nitrosomonas europaea and Nitrobacter sp. was studied in continuous culture. Steady state growth kinetics of both organisms conformed with that predicted by chemostat theory, modified to account for maintenance energy requirement. Steady state data were used to calculate the maximum specific growth rate, the saturation constant for growth, the true growth yield and the maintenance coefficient. Transient growth was studied by imposing step changes in dilution rate. Step increases resulted in overshoots and oscillations in substrate concentration before establishment of a new steady state while step decreases in dilution rate were followed by monotonic changes in substrate concentration. The size of overshoots in substrate concentration following step increases in dilution rate was dependent on both the magnitude of the increase and of the dilution rate prior to the change.     

Description of a redox-controlled sulfidostat for the growth of sulfide-oxidizing phototrophs

This paper describes a novel type of continuous culture for the growth of phototrophic sulfur oxidizers under constant concentrations of hydrogen sulfide. The culture maintains a constant concentration of sulfide despite possible variations in external factors likely to affect photosynthetic activity. Variations in biological activity lead to small departures from the steady-state concentration of hydrogen sulfide which result in variations of the redox potential. These changes in redox, monitored through a redox controller, modulate the rate at which the medium is pumped into the culture and therefore govern the dilution rate. As a result, when changes in external factors such as the light supply occur, the dilution rate of the culture adjusts to the new rate of sulfide oxidation, while maintaining a virtually constant concentration of hydrogen sulfide. The system has been successfully tested for an extended period of several weeks and under conditions of shifting illumination (868 to 113, 113 to 23, and 23 to 7 (mu)E(middot)m(sup-2)(middot)s(sup-1)). After changes in illumination, a transition to a new dilution rate started immediately, reaching a new equilibrium in less than 3 h.     

Set-point: Is it a distinct structural entity in biological control?

Interest in biological control mechanisms focusses largely on quantities that are held relatively constant, in contrast to those which adapt to varying conditions. Studies of the former, via systems engineering concepts, often utilize the notion of set-point. Although set-point serves a useful purpose to configure control systems, there is no basis to conclude the existence of separate structural entities which provide such reference in biological systems.     

Optimization of substrate feed for continuous production of lactic acid by Lactococcus lactis IO-1

Summary Optimization of substrate feed for continuous production of lactic acid by the homofermentative bacterium, Lactococcus lactis IO-1, in glucose medium was investigated. A pH-dependent feed with two pH set-points, a lower set-point for neutralization with alkali and an upper set-point for substrate feed, proved better than continuous substrate feed with one pH set-point for neutralization with alkali only. Built-in electrodialysis with a cell-recycling system was tested and high cell density was achieved as a result of the use of enriched medium. However, specific lactate productivity in this system was not satisfactorily high. pH-dependent feed was combined with turbidity control and a cell recycling. With this system, we achieved high specific lactate productivity of 2 g (g-cell)^-1 h^-1 at a dilution rate of 0.5 h^-1, a dry cell weight of 5 g l ^-1, a level of lactate in the broth of 20 g l ^-1, and a concentration of glucose in the spent medium of about 5 gl ^-1.     

Computer-aided control of water activity for lipase-catalyzed esterification in solvent-free systems

A computer system for on-line monitoring and control of the water activity (a(w)) in solvent-free media has been developed. The performance of this system was investigated by carrying out the lipase-catalyzed esterification of n-capric acid with n-decyl alcohol. A humidity sensor measured the relative humidity in the reactor headspace, which was then transmitted electrically to a digital computer that was used as a feedback controller. The water activity control was achieved by sparging either humidified air or dried air through the reaction medium at a flow rate determined by the digital feedback controller. The use of humid air and dry air for a(w) control made it possible to induce a larger a(w) gradient and thereby higher water transfer rate. As a result, the water activity quickly reached the desired a(w) values. We tested whether water activity in the reaction medium can be monitored by measuring relative humidity in the headspace. When the water activity in the liquid phase was determined from measurements of water content in the medium and compared to that measured directly with the humidity sensor, the a(w) in the reaction medium did not differ significantly from that in the headspace. This indicates that there is a near-equilibrium between the liquid medium and the exit air stream. Water activity was also successfully maintained close to the set point despite the massive production of water during the esterification process. Thus, the control system developed in this study is particularly useful for systems where large amounts of water are produced and where conventional methods make it difficult to control water activity as a result of a low water transfer rate. The effects that computer control of the water activity had on the reaction rate and yield were also examined. The reaction yield was significantly improved with water activity control. The conversions obtained at 28 h without and those with water activity control were 70% and 96%, respectively. In addition, from the fact that the final yields increased with decreasing a(w), computer-aided water activity control was performed with a set-point change. By controlling a(w) at 0.55 during initial reaction phase, followed by a step change of a(w) from 0.55 to 0 after 11 h of reaction, it was possible to enhance the final conversion to 100%.     

Nonlinear control of bioreactors with input multiplicities

Control of bioreactors exhibiting two input multiplicities, i.e., steady-state gains having opposite sign, is theoretically analyzed. The nonlinear system is represented by a unity gain linear system cascaded with a nonlinear gain. A conventional PI controller designed for the linear portion of the system followed by the solution of the nonlinear gain equation gives a nonlinear controller. The performance of the nonlinear controller is compared with that of the linear PI controller designed for the overall linear system. The nonlinear PI controller performance is superior to that of the linear PI controller.