Insights into the effects of acidification on sewage sludge dewaterability through pH repeated adjustment

Sludge acidification is a popular and efficient pretreatment method for improving sludge dewaterability. To explore the mechanisms of sludge acidification deeply and provide a theoretical basis for practical application, sewage sludge was repeatedly treated by acidification conditioning. The sludge pH was continuously adjusted from 7.0 to 3.0, then back to 7.0, and to 3.0 again in this study. When the sludge pH returned to a neutral condition, the sludge dewaterability was further deteriorated. However, after the second acidification conditioning, the sludge dewaterability was improved to the same level as that at the first conditioning. These experimental facts were due to two acidification effects. One was that acidification can efficiently destroy the structure of sludge flocs, thereby causing the degradation of extracellular polymeric substances (EPSs) and release of trapped water. Furthermore, some internal EPSs were released into the outer layer. The other was protonation effect, which can effectively compress the released EPSs, especially protein-like matters (PN) therein. These two combined effects caused a good sludge dewaterability. However, the former was irreversible, whereas the latter was reversible. When the pH of acidified sludge was readjusted to 7.0, the protonation effect weakened, thereby the original compressed EPS released into the outer layer of sludge and the PN content in soluble EPS fraction increased near five times, resulting in considerable deterioration of sludge dewaterability. The negative effects of released organic matters by acidification were often concealed and neglected due to protonation, thereby causing the reversal of dewatering performance in practice when the sludge pH was fluctuated.