Oxygen-limited continuous cultures

Application of calorimetry to oxygen-limited continuous cultures has been presented in detail in references [67,71,72]. In fact, reductive metabolism is almost athermic as compared to aerobic metabolism. In addition, it is possible to set the oxygen limitation to the desired value by changing the oxygen gas feed composition [67,73]. The level of oxygen limitation is quantified by the ratio PoiKi (called aerobicity and also denoted Q [74]) ... 

Under conditions of oxygen limitation in continuous culture, mixed populations of aerobic bacteria and the strictly anaerobic Methanobacterium formicicum and Methanosarcina barkeri could be maintained (Gerritse and Gottschal 1993a). 

Odour will return in treated slurry as a result of post treatment fermentation. The concentration of readily fermentable substrates, measured as BOD5, provide an indicator of this problem. In continuous culture without oxygen limitation the BOD5 can be described by a model derived from the Monod (13) model of microbial growth (14). The supernatant BOD5 (g/1) from treatment at 15 to 45°C, was described by equation 3 and the whole BOD5 by equations 4 and 5(15). 

Senior PJ, Beech GA, Ritchie GAP, Dawes EA (1972) The role of oxygen limitation in the formation of poly-P-hydroxybutyrate during batch and continuous culture of Azotobacter beijerinckii. Biochem J 128 1193-1201... 

This simple approach is illustrated in Figure 19 for a continuous culture of K. marxianus. The evolution of the biomass yield on glucose is linear for the whole range of oxygen limitation. 

Autotrophic activity. Because of the low C N ratio and its declining value as carbonaceous residues are degraded there is substantial ammonification. With all mean treatment times greater than the doubling time of Nitrobacter sp. nitrification will occur provided that oxygen is not limiting. Smith and Evans (19) found that with DO levels above 15% of saturation, nitrification continued until the culture was limited by a fall in pH level. Up to 40% of the slurry ammonia was oxidised. The autotrophic activity never achieved steady state and cycled between periods of activity when the pH value was above about 5.5 and periods of inactivity when the pH value fell below 5.5. Complete nitrification of all ammonia only occurred if the pH value was controlled at about 7 by the addition of alkali. When the DO level was held within the range of 1 to 15% of saturation a system of simultaneous nitrification and denitrification was established. The reduction of nitrate allowed the pH value to remain above 6 and nitrification to continue. Thus more than 70% of the ammonia was oxidised. If the DO level was held below 0.1% of saturation, nitrification was inhibited (unpublished). 

Culture techniques can be classified into batch, fed-batch, and continuous operation (Table 2). In batch processes, all the nutrients required for cell growth and product formation are present in the medium prior to cultivation. Oxygen is supplied by aeration. The cessation of growth reflects the exhaustion of the limiting substrate in the medium. For fed-batch processes, the usual fed-batch and the repeated fed-batch operations are listed in Table 2. 

For each parameter, the pH, DO (dissolved oxygen), ORP (oxidation-reduction potential), temperature, agitation speed, culture volume and pressure can be measured with sensors located in the fermenter. The output of the individual sensors is accepted by the computer for the on-line, continuous and real-time data analysis. Information stored in the computer control system then regulates the gas flow valves and the motors to the feed pumps. A model of a computer control system is shown in Fig. 11. The computer control systems, like the batch systems for mammalian cell culture, seem to level out at a maximum cell density of 10 cells/ml. It may be impossible for the batch culture method to solve the several limiting factors (Table 10) that set into high density culture where the levels are less than 10 cells/ml. 

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