Alejandro Vargas

Rapid quantification of intracellular PHA using infrared spectroscopy: An application in mixed cultures

Fourier transform infrared (FT-IR) spectroscopy is proposed for a method for rapid quantification of polyhydroxyalkanoates (PHA) in mixed culture bacterial systems. Spectra from 122 samples from a wide range of PHA production systems were studied. The spectra were collected in a library that was used to calibrate a partial least squares (PLS) model linking FT-IR spectra with PHA content in the biomass. The library of spectra contained samples with a range of total PHA content (0.03-0.58 w/w) as well as varying compositions (poly-(3-hydroxyvalerate) (3HV) content of 0-63% in Cmol basis). A robust PLS model was developed using calibration data from a diverse range of systems and PHA content. Coupling this model with FT-IR spectra has been shown to be applicable for predicting PHA content in mixed culture production systems. The method was used to reliably determine PHA content in biomass from a new, independent PHA production system with a standard error of prediction (RMSEP) value of 0.023 w/w, despite the complexity of the matrices. This method reduces the analytical time for PHA quantification down to under 30 min (5 min handling time was achieved when FT-IR equipment was immediately available), and eliminates hazardous waste by-products. The work has demonstrated a level of accuracy and reproducibility in quantifying PHA in mixed culture systems similar to that obtained from the GC analytical technique. Further work is required to enable the use of the method to analyze crystallinity related factors that may be useful towards quantifying poly-(3-hydroxybutyrate) and poly-(3-hydroxyvalerate) (3HB/3HV) composition. The method has been shown to be suitable for rapid quantification in large scale applications and in its present form is reliable for routine process monitoring.

Instrumentation, control and automation in wastewater - From London 1973 to Narbonne 2013

Key developments of instrumentation, control and automation (ICA) applications in wastewater systems during the past 40 years are highlighted in this paper. From the first ICA conference in 1973 through to today there has been a tremendous increase in the understanding of the processes, instrumentation, computer systems and control theory. However, many developments have not been addressed here, such as sewer control, drinking water treatment and water distribution control. It is hoped that this review can stimulate new attempts to more effectively apply control and automation in water systems in the coming years.

Design and test of a low-cost RGB sensor for online measurement of microalgae concentration within a photo-bioreactor.

In this study, a low-cost RGB sensor is developed to measure online the microalgae concentration within a photo-bioreactor. Two commercially available devices, i.e., a spectrophotometer for offline measurements and an immersed probe for online measurements, are used for calibration and comparison purposes. Furthermore, the potential of such a sensor for estimating other variables is illustrated with the design of an extended Luenberger observer.

Enhanced polyhydroxyalkanoate production from organic wastes via process control.

This work explores the use of a model-based control scheme to enhance the productivity of polyhroxyalkanoate (PHA) production in a mixed culture two-stage system fed with synthetic wastewater. The controller supplies pulses of substrate while regulating the dissolved oxygen (DO) concentration and uses the data to fit a dynamic mathematical model, which in turn is used to predict the time until the next pulse addition. Experiments in a bench scale system first determined the optimal DO set-point and initial substrate concentration. Then the proposed feedback control strategy was compared with a simpler empiric algorithm. The results show that a substrate conversion rate of 1.370±0.598mgPHA/mgCOD/d was achieved. The proposed strategy can also indicate when to stop the accumulation of PHA upon saturation, which occurred with a PHA content of 71.0±7.2wt.%.

Approximate high-gain observers for non-Lipschitz observability forms

A methodology for the design of continuous practical high-gain observers for non-linear observable systems is presented. Only two conditions are required: injectivity of the observability map of order n and uniform continuity of its inverse. Using this map, the system is transformed to observability normal form, which could have a non-Lipschitz continuous right-hand side and eventually multiplicity of solutions. A so-called ε-approximate high gain observer is designed for this form, constituting the dynamic part of the observer. A uniformly continuous extension of the inverse of the transformation is used as the algebraic part. Convergence of the error to a neighbourhood of the origin is guaranteed.

A simple output-feedback controller for fed-batch cultures of microbial strains with overflow metabolism

Abstract An output-feedback controller is proposed for fed-batch cultures of microbial strains exhibiting overflow metabolism, i.e., cultures where cell strains may produce inhibiting metabolites when excess substrate is fed. The control objective is therefore to control the substrate feed rate in order to remain near the optimal substrate concentration during the whole culture. The proposed controller uses measurements of an output signal that can be calculated from on-line process data and determines the necessary dilution rate to maintain the output near its local, time-varying, maximum at all times. The controller is hybrid, combining a proportional plus integral controller with antiwindup, modified so that the error signal used in the feedback may change its sign according to a state machine which keeps track of whether the control action should be positive or negative. The transition is made smoothly to avoid chattering and based on an auxiliary variable which follows the local output maximum. Partial proof of convergence is given and simulations of the model of a yeast production system show the applicability of the proposed strategy.

Virtual output estimation in a bioreactor using a generalized super-twisting algorithm

Abstract This paper presents the application of a generalized super-twisting algorithm to estimate a critical variable needed for implementing real time optimization feedback controllers in a certain class of bioreactors. A bank of redundant super-twisting observers designed for equivalent systems under a state transformation is used. The procedure is motivated by the fed-batch cultivation of a strain of E. coli that has overflow metabolism, where a useful variable to estimate is the difference between two reaction rates. The results show the applicability of the procedure and illustrates some compromises between the accuracy and the measurement noise.

Global observability and detectability analysis for a class of nonlinear models of biological processes with bad inputs

This paper proposes a new general approach to the global analysis of observability and detectability for nonlinear systems. Based on the definition of indistinguishability it is possible to derive the dynamics of the non-observable part of the system and thus to study its stability properties using methods of nonlinear systems theory. The method is first introduced in general and then applied to a class of nonlinear models for biological processes as e.g. in waste water treatment. Finally detectability conditions for the reactor model are deduced.

Controlled operation of a membrane SBR for inhibitory wastewater treatment.

This work presents the design and practical implementation of a control strategy for the operation of a sequencing batch reactor with a submerged membrane. The bioreactor is used for treating wastewater that is inhibitory for sufficiently high concentrations. The strategy combines the use of a robust feeding strategy to ensure biodegradation near an optimal unknown concentration value, using only the dissolved oxygen concentration as feedback signal, and a strategy to minimize membrane fouling based on flux and transmembrane pressure measurements during filtration. The strategy was tested on a 10 L laboratory bioreactor treating synthetic wastewater containing 4-chlorophenol as inhibitory model compound and sole source of carbon. Experimental results show the applicability of the proposed strategy.

Nonlinear Observer Design Strategies Via Observability Forms

Abstract The design of nonlinear observers is considered for nonlinear SISO systems with reduced observability such that some care is necessary with todays constructive design methods. The reduction of observability concerns the fact that the nonlinear observability map is not diffeomorphic or not injective for some states and inputs. The coordinates of the observability form are used to develop appropriate design strategies for nonlinear observers bypassing the reduced observability properties. The design of continuous observers, approximate high gain observers, and observers for systems with bad input points are treated and illustrated with examples.