The microbial population in an activated-sludge reactor can grow and f
unction within a pH range around neutrality. Upset of the activated-sl
udge system can occur when the reactor pH falls outside this narrow ra
nge. An alkaline-tolerating culture was used in this study for augment
ation of a laboratory-scale activated-sludge system that suffered abno
rmal pH. During pulse shocks of pH 12 in the influent, treatment effic
iencies of both an augmented reactor and an unaugmented control were f
ound to decline when the pH value of the reactor went above 9. During
a prolonged period of elevated influent pH, the augmented reactor had
some chance of performing slightly better than the unaugmented control
. Both reactors failed because the activated sludge was washed out aft
er about one mean cell residence time. (C) 1997 Elsevier Science Ltd.