Dynamics and control of continuous microbial propagators to subject substrate inhibition

Abstract

SummaryInhibitory substrate levels are common in industrial fermentations and in biological waste-water treatment of many industrial wastes. Continuous microbial cultures are unstable to certain disturbances, such as shock loading by inhibitory substrates. Two feedback proportional control strategies are analyzed and compared for a simple model culture assumed representable by the culture concentrations of biomass and a single rate-limiting and growth-limiting nutrient (substrate)). One control strategy, the well known turbidostat, consists of adjusting culture holding time (e.g., by flow rate adjustment) in response to deviations in turbidity or some other measure of culture biomass concentration. The other control strategy is to adjust holding time in response to deviations in limiting nutrient concentrations in the culture. This second control. strategy, termed the nutristat, can be superior to the turbidosJat i? many applications. The sign and magnitude of the dimensionless group { ( X / I ; D ) ( d p / d S ] ; ), is shown to be an important determinant in the behavior of the open loop and the two closed loop processes. This characteristic group is positive when the specific growth rate is increased by increases in the nutrient concentration, zero when the growth rate is unaffected by the nutrient concentration, and negative in the presence of nutrient or substrate inhibition.The effects of process modifications and of modeling assumptions on the control of the process are discussed and more sophisticated control schemes are also proposed.

Multiplicity and stability analysis of microorganisms in continuous culture: Effects of metabolic overflow and growth inhibition

“…In Figures 5-7 where the dilution rate is varied, no multiplicity can be observed if the metabolic overflow is not considered. These results are in accordance with those of Yano and Koga (1969) and Edwards et al (1972), who studied the influence of substrate inhibition on the multiplicity and dynamic behavior of continuous culture.…”

“…Using the parameters of Table II, the results obtained (not shown) are qualitatively similar to those of Yano and Koga (1969) and Edwards et al (1972). In general, two nonwashout steady state solutions can be obtained for a given dilution rate due to substrate inhibition, but only one is stable.…”

The permittistat: a novel type of turbidostat

“…In a turbidostat the biomass level in the fermenter is determined continuously, the nutrient pump being turned on when it exceeds a set point. The usual method of biomass estimation is via the optical turbidity of the cell suspension (Myers & Clark, 1944;Bryson & Szybalski, 1952;Anderson, 1953Anderson, , 1956Northrop, 1954;Moss, 1956;Cooper et al, 1959;Schlecht et al, 1958;Herbert et al, 1965;Dean, 1967;Moss & Bush, 1967;Blachere & Jamart, 1969;Munson, 1970;Edwards et al, 1972;Agrawal, 1987). However, turbidity measurements have two major drawbacks: (i) they are only linear (in * Author for correspondence.…”

Development of Bioreaction Engineering

“…Continuous cultivation of microorganisms became popular. Mass-balance relationships for steady state and substrate limited cultivation (Chemostat) were published [92][93][94][95][96][97][98][99][100][101][102]. These relationships were used for macroscopic material balances in cultures [54c-80].…”