The influence of substrate acidification on sludge filtration characteristics was systematically investigated by using short term filtration tests. Four reactors were operated with raw and acidified whey permeate in order to evaluate the effect of acidogens on sludge filterability. The results showed that feeding non-acidified substrate promoted the growth of acidogens which in return decreased the median particle size of the sludge and adversely influenced specific resistance to filtration (SRF). In addition to the presence of acidogens, the food to mass (F:M) ratio was found as an important operation parameter on sludge filterability. Various filterability indicators, such as capillary suction time (CST), SRF and supernatant filterability, tended to became worse at increased F:M ratios. The decreased filterability at high F:M ratio was attributed to the accumulation of soluble microbial products (SMP) in the reactors. Interestingly, impact of acidogens on short term critical flux tests was not significant, but this may be a consequence of the experimental set-up. (C) 2015 Elsevier Ltd. All rights reserved.