Importance of the test volume on the lag phase in biodegradation studies

Increasing the total volume of test medium resulted in decreased lag times (T-L) in biodegradability shake flask batch tests conducted with either sur face water or with synthetic mineral medium inoculated with supernatant fro m settled activated sludge. Experiments were performed with test volumes ra nging from 1.8 mi to 100 L using two C-14-labeled model chemicals, 2,4-dich lorophenoxyacetic acid (2,4-D) and p-nitrophenol (PNP), both of which are k nown to be readily degradable after variable lag phases. The T-L ranged fro m 2.1 to 30.4 d for PNP and from 16 to 37 d for 2,4-D. Decreasing the test volume tended to increase the lag time, even when a single test batch was r edistributed into smaller flasks. With 5 ml supernatant added to different volumes of mineral medium, lag times for PNP were independent of the test v olume in a range from 10 to 1,000 ml. At small volumes of 10 mi or less, de gradation failed randomly. Our findings are partly explained by the hypothe ses that a sufficient total amount as well as a sufficient concentration of specifically degrading microorganisms or consortia of bacteria must be pre sent initially for biodegradation to get started, from which follows that w ith too small inoculations or with too small test volumes, biodegradation m ay fail randomly. A straightforward practical implication of the findings i s that the test volume in biodegradability tests can significantly influenc e the lag time and thus sometimes be decisive for the outcome in biodegrada tion studies.