EXPRESSION DEWATERING OF ALUM-COAGULATED CLAY SLURRIES

Filtration followed by expression characteristics of the alum-coagulat ed clay slurries are first reported in this paper. A strong correlatio n between alum chemistry and the dewatering efficiency exists. At pH 3 , where the positively charged ions are the dominating species, the al um addition has only a minor improvement on dewatering efficiency. At pH 7 and pH 10, where sweep flocs enmeshment play the major role, dewa tering 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 fo llows: pH 5.2 > pH 3 > pH 7 > pH 10. A complete test consists of the f iltration and expression stages, which are discussed separately. The f iltration 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 (eta) are evaluated. For the dewater ing sake, a lower B and a higher eta are desired. Both parameters B an d eta increase at elevated alum dose and/or lower pH value, while exhi biting a local minimum in charge reversal regime at pH 5.2. The optima l alum dose considering sludge dewaterability is discussed.