The SGRs were operated by removing (wasting) a
pre-determined amount of reactor contents (mixed
liquor) from the wasting line at the bottom of the SGR
(Fig. 1) each day. After the appropriate mixed liquor
volume was wasted, the toluene and air ¯ow were tem-
porarily turned o to re®ll the removed volume with a
nutrient/distilled water mixture. At this time, the sparge
stone was cleaned of accumulated biomass growth. The
nutrient solution shown in Table 2 was added to assure
adequate nutrients for the microbial activity in the
SGRs. The monobasic (KH2PO4) and dibasic
(K2HPO4) potassium phosphate (Table 2) used also
provided buering capacity for the SGR systems. All
gas sampling was performed with the SGRs ®lled to 2-l.
Dissolved oxygen concentrations of the SGR contents
were checked periodically to assure that oxygen was not
limiting aerobic degradation of the toluene. At all times,
the dissolved oxygen concentration was found to be
above the desired bulk liquid concentration of 2 mg/l. In
addition, the nutrient content of the SGR mixed
liquor was checked at the beginning and end of an
experiment and during poor reactor performance to
ensure that the SGRs were not nutrient-limited. At no
time were the SGRs determined to be operating under
nutrient-limited conditions.