Turbidostat operation

In the turbidostat, P and F are kept equal but the dilution rate D is automatically adjusted to a preset cell concentration in the product by continuously measuring its turbidity. Compared to chemostat, turbidostat operation can be more stable in the region near the washout point, but requires more expensive instruments and automatic control systems. 

There are two different ways of operating a continuous stirred-tank fermentor, namely chemostat and turbidostat. In the chemostat, the flow rate of the feed medium and the liquid volume in the fermentor are kept constant. The rate of cell growth will then adjusts itself to the substrate concentration, which depends on the feed rate and substrate consumption by the growing cells. In the turbidostat the liquid volume in the fermentor and the liquid turbidity, which varies with the cell concentration, are kept constant by adjusting the liquid flow rate. Whereas, turbidostat operation requires a device to monitor the cell concentration (e.g., an optical sensor) and a control system for the flow rate, chemostat is much simpler to operate and hence is far more commonly used for continuous fermentation. The characteristics of the continuous stirred-tank fermentor (CSTF), when operated as a chemostat, is discussed in Chapter 12. 

Chemostat and Turbidostat Operation Under Continuous Light... 

Despite the fact that productivity of chemostat operation is similar to turbidostat operation the turbidostat is preferred especially at locations with more day-to-day variation in irradiance. Under chemostat operation biomass concentration will slowly decrease on consecutive days with cloud cover. If followed by a clear-sky day, the culture is vulnerable to photoinhibition because of its low biomass concentration. Turbidostat operation allows for a direct control of biomass concentration. Washout of a microalgae culture in a turbidostat is by definition not possible. In a chemostat this could occur in situations the dilution rate is not adjusted in time. Turbidostat operation is thus more robust and allows for rehable automation of outdoor microalgae production (Bosma et al, 2014). 

Most of the experiments have been done in batch, using bottles, flat flasks, tubes or columns. Some references to semi-continuous and continuous operations are also included and, in the last case, the dilution rate (D) is presented, when available. In continuous operation, the one stage chemostat is the most often used but there are also references to more than one (two or three) stage chemostat and to turbidostat. 

Stirred tank reactor systems can also operate in a continuous mode. In this configuration, fresh medium is continually supplied to the reactor and the desired products are continuously removed in the course of production. A continuous system is referred to as a chemostat when the flow rate is set to a constant value. It is further known as a turbidostat when the flow rate is set to maintain a constant turbidity or cellular concentration.f Continuous reactor systems are commonly abbreviated as CSTR or CSTF and they refer to continuous stirred tank reactor and continuous stirred tank fermenter, respectively. 

The continuous operations of Table 2 are elaborated in Table 3 as three types of operations. In a chemostat without feedback control, the feed medium containing all the nutrients is continuously fed at a constant rate (dilution rate) and the cultured broth is simultaneously removed from the fermenter at the same rate. A typical chemostat is shown in Fig. 1. The chemostat is quite useful in the optimization of media formulation and to investigate the physiological state of the microorganism. A turbidostat with feedback control is a continuous process to maintain the cell concentration at a constant level by controlling the medium feeding rate. A nutristat with feedback control is a cultivation technique to maintain a nutrient concentration at a constant level. A phauxostat is an extended nutristat which maintains the pH value of the medium in the fermenter at a preset value. 

Microbial populations can be maintained in a state of exponential growth for extended time by using a system of continuous culture, operated either as chemostat or as a turbidostat. In a chemostat, the flow rate is set at a particular value and the rate of growth of the culture adjusts to this flow rate. In a turbidostat, the turbidity is set at a constant level by adjusting the flow rate. It is easier to operate a chemostat than a turbidostat, because the former is at a constant flow rate, whereas... 

Continuous cultivation under turbidostat conditions can be performed with the same productivity for low biomass concentration and high dilution rate or vice versa. Pruvost et al (2011) give a reactor model-based calculation of the best window of operation. Furthermore, this results... 

Photobioreactors usually are operated according to a chemostat mode (constant dilution) or turbidostat mode (constant turbidity) of operation. Details will be explained later in this chapter but below the hiomass balance over a microalgal cultivation system is aheady presented ... 

In order to assess the volumetric productivity of the system the micro-algal biomass concentration and average specific growth rate need to be known. For the moment it will be assumed that the biomass concentration can be set at any desired level by means of a turbidostat mode of operation. Consequently, only the average specific growth rate of the microalgae in the photobioreactor needs to be calculated. 

Microalgae can be grown batchwise in photobioreactors. In case biomass concentrations become too high the culture is diluted, and biomass is harvested, after which growth is allowed to continue again. Microalgae production systems can also be operated in a continuous manner based on a chemostat or turbidostat mode of operation. Flere these three different... 

A turbidostat mode of operation relies on the use of a sensor to measure the biomass concentration in the photobioreactor. Such a sensor can be based on the online measurement of culture turbidity, for example, but any other biomass sensor can be used as well. Based on such a continuous biomass measurement the flow rate through the system then is automatically adjusted such that the biomass concentration is maintained at a preselected value (Fig. 29). 

Figure 29 Schematic of turbidostat mode of operation of a microalgae cultivation system.

Under outdoor conditions under day/night cycles biomass accumulation inside the photobioreactor cannot be neglected. Especially in case of chemostat operation considerable changes in biomass concentration will occur. Even in a turbidostat in the ri ht biomass concentrations will decrease because of respiration. The biomass balance then must be written as follows ... 

In the nonlimited growth range, the steady state is nonstable If the cell concentration, X, exceeds the steady state cell concentration, X, X increases until it reaches a subs ate or oxygen transfer limited state. If X drops below X,the cells are washed out. A continuous operation is only possible if the dilution rate, D, is varied j o keep the cell concentration constg it if X exceeds X, D is increased, if X drops below a, D is diminished. A reactor operated in this manner is called turbidostat. 

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