in which eP is the volume fraction of particles and k is an empirical function of eP. For dilute suspensions, eP < 0.15, whereas in slurries 0.15 < eP < 0.50.

The limiting settling velocity for a system has a strong influence on equipment design and operation. Consider the case of a continuous sedimentation tank with volumetric throughput (F) and constant cross-sectional area (A). The sedimentation tank performance can then be described by equation 42, in which throughput is directly proportional to A and independent of tank depth:

in which dP is the particle diameter, qP is the particle density (the specific gravity of a typical cell is 1.05), qF is the

The throughput and limiting settling time will thus dictate equipment size and costs. Another design consideration is the residence time of cells in the settling tank, which must be considered in the context of nutrient depletion (particularly for oxygen) and its potential effects on performance.

Sedimentation at laboratory scale may be implemented with an external settling column (47,50,51). Similar devices may be used at bioprocessing scales, whereas large open-air tanks must be used in high-volume wastewater treatment. Internal sedimentation has been implemented u h u0 =

Table 4. Ethanol Productivity Enhancements for S. cerevisiae and Z. mobilis

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