## dilution rate, $$D$$

in biotechnology
https://doi.org/10.1351/goldbook.D01740
The ratio of the ingoing volume flow rate ($$\frac{\text{d}V}{\text{d}t}$$) 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 ($$\mathrm{HRT}$$).
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 152 (https://doi.org/10.1351/pac199264010143)