Removal of Escherichia coli in wastewater by activated sludge

Removal of bacteria from wastewater treated with activated sludge was studied by the use of a streptomycin-resistant Escherichia coli strain. The removal appeared to be a biphasic process. A rapid sorption of bacteria to the sludge flocs took place in the first hour after seeding mixed liquor with E. coli. Thereafter, slower elimination of E. coli was observed. The latter process was due to predation on E. coli by ciliated protozoa. This was shown by: (i) appearance of fluorescent food vacuoles of ciliates when fluorescent E. coli cells were added to mixed liquor; (ii) inhibition of predation either in the presence of cycloheximide or under anaerobic conditions; and (iii) absence of predation in bulking and washed sludge.