The study aims to evaluate the role of a polymeric substrate (starch) on sludge settleability. Despite being an important COD component of the wastewater, the relationship between polymeric substrates and bulking sludge has been hardly studied. The polymers are hydrolysed at a rate smaller than the consumption rate of monomers. This means that the soluble substrate resulting from hydrolysis is likely to be present at growth rate limiting concentrations. According to the kinetic selection theory this leads to bulking sludge. However, a recently postulated theory suggests that, strong diffusion limited micro-gradients of substrate concentration inside flocs lead to bulking sludge, and not a low substrate concentration as such. If the polymeric COD is first incorporated in the sludge floc and afterwards hydrolysed in the sludge floc then there is essentially no substrate gradient inside the biological flocs. The experiments showed that conditions leading to bulking sludge with monomers (glucose) did not lead to bulking when starch was used. A bulking sludge event was even cured just by substituting the monomer with starch. These results are clearly in line with a diffusion gradient based theory for bulking sludge. Nevertheless, flocs growing on starch are more open, fluffy and porous than flocs formed on maltose or glucose, most likely because the starch needs to be hydrolysed at the surface of the micro-colonies forming the flocculated sludge. Some additional observations on occurrence of filamentous bacteria in oxygen diffusion limited systems are also discussed in this manuscript. (c) 2010 Elsevier Ltd. All rights reserved.