High-level deliquoring of excess municipal activated sludge was accomplished by use of an inorganic flocculant, polyaluminum chloride (PACl), and an organic polymer flocculant, Kurifix. The deliquoring process consisted of filtration of flocculated sludge followed by ultrahigh-pressure expression combined with water permeation through the filter cake, resulting in the re-distribution of flocs in the filter cake. Whereas the use of Kurifix increased the filtration rate significantly more than PACl, it decreased the expression rate, particularly at low expression pressure. As the expression pressure increased, the cake moisture content decreased more remarkably in the case of PACl compared with Kurifix. Of particular interest is that an ultrahigh pressure of 50 MPa decreased the cake moisture content to 23.7 wt% at the end of expression. The kinetics of ultrahigh-pressure expression such as the variations with time of the average consolidation ratio and average cake moisture content was well elucidated by considering a Terzaghi type of primary consolidation followed by a mainly two-stage creep phenomenon. Consequently, the creep effect was marked in the case of Kurifix, particularly at low expression pressure. Moreover, the modified consolidation coefficient increased with the expression pressure and remained constant above a critical expression pressure, which depended on the flocculant: ca. 5 MPa for PACl and ca. 10 MPa for Kurifix.
