Cell retention time

MCRT Mean Cell Retention Time - days. An expression of the average time that a microorganism will spend in an activated sludge process. 

Brown, M.J. Lester, J. N. (1982b). Role of bacterial extracellular polymers in metal uptake in pure bacterial culture and activated sludge-II effects of mean cell retention time. Water Research, 16, 1549-60. 

The sizing of aeration basins (for the activated sludge reactor, for example) are determined using the parameters of hydraulic detention time and mean cell retention time. Hydraulic detention time is the average time that the particles of water in an inflow to a basin are retained in the basin before outflow. Mean cell retention time, on the other hand, is the average time that cells of organisms (not water) are retained in the basin. 

Mean cell retention time—The average time that cells of organisms are retained in the basin. 

In the operation of the nitrification unit, the nitrifiers and the heterotrophs are coexisting. This means that there are two mean cell retention times that must be satisfied. The nitrifiers are slow growers compared to the heterotrophs, therefore, their mean cell retention time, is greater. Therefore, in the design of the reactor, G c must be made equal to Gc . In this case, the retention time for the heterotrophs is satisfied as well as the retention time for the nitrifiers. If the reverse is made equal, that is, if G is made equal to G the time will be very short and the nitrifiers wiU not have sufficient time to grow and get established and will simply be washed out into the effluent. In addition, a safety factor, SF, should be applied to obtain the design G G c = G ). A value for the safety factor of 2.0 may be used. In equation form, this is written as... 

Mean cell retention time (MCRT) for the denitrifiers... 

For a given waste stream, series of reactions Bjk, components A, and cell retention time 6c, the concentration of all components in the reactor effluent stream and gas discharge can be determined. This requires that the stoichiometric coefficients Vik be known, including the value for organism synthesis. Also required are the coefficients kk and Kk for each reaction. The method used for selection of appropriate values are discussed in the following. 

Figure 3. Computed concentration of various components as a function of cell retention time...

Figure 1. Pattern of continuous production of ethanol using column packed with immobilized yeast cells. Retention time, 1 hr gel, 20 mL flow rate, 20 mL/hr ...

In 1981, a novel flotation device known as the air-sparged hydrocyclone, shown in Figure 3, was developed (16). In this equipment, a thin film and swid flotation is accompHshed in a centrifugal field, where air sparges through a porous wall. Because of the enhanced hydrodynamic condition, separation of fine hydrophobic particles can be readily accompHshed. Also, retention times can be reduced to a matter of seconds. Thus, this device provides up to 200 times the throughput of conventional flotation cells at similar yields and product quaHties. 

Table 8.1 presents the results of the ICR retention time studies, sugar concentration (dual substrate) studies and cell density. The kinetic model for ICR was derived on the basis of a first order reaction, plug flow and steady-state behaviour. 

Fig. 8.11. Conversion versus retention time in the immobilised cell column. Reprinted from Najafpour et al. (2004).18 Copyright with permission from Elsevier.

Media flow rate, ml/h Retention time, t, h Cell Density, gd Substrate concentration (S), gd 1/5, 1/g - rA, Rate of substrate uptake, g/l.h l/-rA Ethanol concentration, gd... 

Kim et al. (1987) showed that the prolonged retention time of Ara-C in the peritoneal cavity after intraperitoneal administration of the drug in liposomal form as discussed above resulted in better therapeutic effects on intraperitoneally inoculated L1210 cells, as compared to the free drug. The activity of intraperitoneally administered cDDP on Ehrlich ascites carcinoma in mice was increased after encapsulation in neutral liposomes (Sur et al., 1983). The in vivo studies revealed improved antitumor activity and a lower toxicity sifter administration of cDDP liposomes compared to free drug. 

In the conventional activated sludge process with biological sludge recycled from the final sedimentation clarifier, shown in Figure 27.11, the mean cell residence time or sludge retention time is... 

It is important to know from Equation 27.7 that the performance of a complete mix with recycle system does not depend on hydraulic retention time. For a specific wastewater, a biological culture, and a particular set of environmental conditions, all coefficients Ks, b, Y, and km become constant. It is apparent from Equation 27.7 that the system performance is a function of mean cell residence time. 

Tc Mean cell residence time, or sludge retention time, time (s)... 

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