Filtration followed by expression characteristics of the alum- coagulated clay slurries are first reported in this paper. A strong correlation between alum chemistry and the dewatering efficiency exists. At pH 3, where the positively charged ions are the dominating species, the alum addition has only a minor improvement an dewatering efficiency. At pH 7 and pH 10, where sweep flocs enmeshment play the major role, dewatering performance improves continuously as alum dose increases. At pH 5.2, where a charge reversal regime exists, the dewatering efficiency exhibits a reversal phenomenon as well. The overall dewaterability follows: pH 5.2 > pH 3 > pH 7 > pH 10. A complete test consists of the filtration and expression stages, which are discussed separately. The filtration stage is discussed based on the average specific resistance and the residual moisture in the filtration cake. The expression stage is discussed based on the combined Terzaghi-Voigt rheological model, from which the fraction contributed by the secondary consolidation (B) and the particle creeping factor (η) are evaluated. For the dewatering sake, a lower B and a higher η are desired. Both parameters B and η increase at elevated alum dose and/or lower pH value, while exhibiting a local minimum in charge reversal regime at pH 5.2. The optimal alum dose considering sludge dewaterability is discussed.