Recycle page 1

Recycle of cells from the effluent back to the bioreactor has long been practiced for waste treatment because the low cell populations without recycle establish slow rates. Separation and recycle of cells results in much longer residence times for the cells than for the fluid. More rapid processing is attained with higher populations through cell recycle, although a higher percentage of the cells may be dead if all are in a starved state.

This schematic figure shows the typical arrangement of a process for biological waste treatment.

The notation is not the same in all textbooks. The Greek symbol, alpha, is often used for the recycle ratio instead of r used here. The separator in the sketch is labeled centrifuge, but sedimentation is almost always used for waste treatment because centrifugation is relatively much more expensive.

The assumption is usually made that clarification is so good that cells in the effluent, Xe, can be neglected. Note that some of the solids that are collected are wasted. Total recycle of solids is bad practice because there would be no way for inert solids to leave the system. In biological waste treatment, the excess solids are sent for sludge digestion to reduce their volume and to condition them for easier dewatering later.

Derivations and equations
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