The pHauxostat

Abstract

The pHauxostat technique for process control was proposed in the late nineteen fifties with a theoretical explanation done by Martin and Hempfling in 1976. The theory was extended in 1985 (Rice&Hempfling), but concluded to be incomplete. The objective of this study was to develop a theory for the pHauxostat and to investigate and explain the principles involved. This was done by investigating the pH, as the controlled output variable, and the control methodology with the feed system the manipulated input variable. Laboratory test work was conducted to verify a proposed theory by using a chemically defined substrate. The technique was thereafter applied in treating a petrochemical effluent in a demonstration plant, demonstrating the generality and applicability of the theory and the pHauxostat technique. The controlled pH of the reactor solution was found to be a function of the weak acids and bases in the reactor solution and the strong acids and bases added to the substrate, in combination with the chemical species removed from the substrate during biodegradation. A method proposed by Loewenthal et al. (1991) that was developed for chemical conditioning, utilising solution and subsystem alkalinities, proved to be successful in characterising the reactor solution in combination with traditional equilibrium chemistry. The pHauxostat control system was shown to keep the alkalinity constant, resulting in a controlled and constant difference in solution alkalinity between the reactor and the substrate solutions. The feed rate is controlled by this difference in combination with the alkalinity generation rate. The alkalinity generation rate is defined with a proposed alkalinity yield coefficient, linking water chemistry and growth kinetics. The alkalinity yield coefficient indicates the amount of alkalinity generated per substrate removed, similarly to the conventional growth yield. The alkalinity yield coefficient was successfully modelled by a theoretical alkalinity yield coefficient, based on oxidation-reduction half reactions as developed by McCarty (1975). This was shown to be true when the change in alkalinity is mainly due to substrate removal. The developed theory is based on alkalinity, modelling the pHauxostat technique by completing a mass balance on solution alkalinity. The model proved to accurately predict the results for the laboratory and the demonstration plant test work. The model is represented by the following formula, respectively for layouts of a chemostat and a CSTR with biomass separation: isXCOD YALK / Yobs = S ALK-SALK0 and isXCOD YALK (<font face="symbol">t/q</font>c) / Yobs S ALK-SALK0 The growth limiting nutrient (S) may be a part of a weak acid/base subsystem or not, implicating two methods of control. pHauxostats were categorised on this basis, giving Category A pHauxostats with S = f(pH)and Category B pHauxostats with S <font face="symbol">¹</font> f(pH). The process for Category A pHauxostats is controlled by the concentration of the growth limiting nutrient (determined by the set point pH and the substrate composition), in combination with the difference in the solution alkalinities between the substrate and reactor solutions. The growth limiting nutrient concentration for Category B pHauxostats, is not controlled but is a result of the control system which is determined by the feed rate of the growth limiting nutrient and the difference in the solution alkalinities. The main contribution of this study is the analysis of the pHauxostat on an alkalinity basis and the subsequent proposed theory with inclusion of an alkalinity yield coefficient. The alkalinity yield coefficient is universal for biological processes in general. Calculation methods for chemical characterisation of the reactor solution were determined together with a method to predict the alkalinity yield coefficient by a theoretical alkalinity yield coefficient. The control methodology was disclosed and pHauxostats were categorised. This study makes the modelling of the pHauxostat technique possible and the implementation thereof, available to the water industry.