Various measures were tested at a full-scale wastewater treatment plant to control sludge bulking by type 0041 and 0675 filamentous microorganisms, instigated by highly variable wastewater loadings from a food-processing facility. Intermittent aeration on a 1-h on 1-h off basis was found to effect a marginal improvement in sludge settling characteristics, as reflected by about an 11-36% reduction in the Sludge Volume Index (SVI) to 118cm (3)g (1). At BOD loadings of 1500 kg d (1) which marginally exceeded the design capacity of the plant of 1200kg d (1), SVI rose sharply to 230cm (3)g (1) in less than a week. The anoxic selector effected a reduction in SVI to 170cm (3)g (1) within 3 weeks of operation at temperatures of 8-12 degreesC. The aerobic selector was most effective, reducing SVIs further to 79 cm(-3)g(-1) in 2 weeks. Sludge settleablity was found to be inversely proportional to the aerobic selector food-to-microorganism ratio. The optimum aerobic selector loading was found to be 1.8-2.7 kgBOD(5) kgMLVSS d(-1), with corresponding SVIs in the range of 80-120cm (3)g (1). (C) 2003 Society of Chemical Industry.