Denitrification by resting cells Resting cells were established in a 300-ml conical flask (with 200 ml liquid medium with a silicon stopper) using freshly prepared suspended cells in denitrifying medium so as to separately study the effects of PVA and boric acid on denitrification. The cell concentration was set at ODm=2.0 (1 .O g cells/l denitrifying medium) and 0, 5 and 10% PVA was added to each of three conical flasks (300m1, liquid volume 200 ml). These 3 flasks were incubated for 20d at 30°C under dark anaerobic conditions. Flasks were shaken vigorously by hand at 12-h intervals in order to maintain homogeneity.