A zero discharge mode of operation at the South African fuel synthesis plants necessitates the infinite recycle of treated industrial waste water. The treatment process incorporates activated sludge units in which non-filamentous bulking and carry-over of solids often occurs. The use of ozone for the control of this non-filamentous binding was investigated on a 33 l/d small pilot-scale plant. An ozone dosage of 1 g per kg mixed liquor suspended solids per day could reduce the diluted sludge volume index from an average of 125 to about 70 ml/g mainly by preventing zoogleal growths, the main contributor to the non-filamentous bulking. The solids looses were reduced from 437 to 194 mg/l. Ozonation in the aeration compartment was more effective than in the sludge recycle.