As the minimum concentration required to achieve reduction in
sludge disposal could be in the range from 24 to 50 mg/L, the
biosurfactant concentration was reduced to 40 mg/L. In operation
period with 40 mg/L biosurfactant, average VSS concentrations of
2679 205 mg/L and 1989 312 mg/L were found for Control and
Test bioreactors, respectively. The sludge disposal in the Test
bioreactor (58 8 mg TSS/day) was lower than in the Control
bioreactor (78 5 mg TSS/day), but with a difference between
bioreactors of only 26%. Although this value has been shown to be
statistically significant by the Student’s t test (95% confidence
level), in Fig. 1B it is possible to observe the increase in VSS
concentration in the Test bioreactor under this condition,
demonstrating that the biosurfactant has lost effect. Lower values
at the beginning of the period, resulting from the previous
condition (50 mg/L) have led to lower average value. Within the
same period, the VSS concentration in the Control bioreactor was
kept constant (2679 205 mg/L) while its concentration in the Test
bioreactor increased at a rate of 32 mg VSS/L d (R2 0.7355)
increasing from 1592 to 2184 mg VSS/L.