dilution rate, D in biotechnology

The ratio of the ingoing volume flow rate (d V d t) and the culture volume (V). In continuous fermentation, a measure of a rate at which the existing medium is replaced with fresh medium, D is the reciprocal of the hydraulic retention time (HRT).
PAC, 1992, 64, 143 (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) on page 152