Insight into the effect of calcium carbonate filler on the dewatering performance of simulated pulp & paper mill sludge

The dewatering of large amount of industry sludge is one of the significant environmental problems in the pulp and paper industry. Calcium carbonate fillers are an important component in primary sludge from paper mills. In this paper, the role of precipitated calcium carbonate (PCC) on the dewaterability of simulated mixed paper mill sludge (the mixture of PCC and secondary sludge) without and with FeCl3 conditioning was studied. The results showed that when the PCC content increased from 0 to 600 mg/g dry sludge (DS) without FeCl3 conditioning, the moisture content (MC) and the specific resistance to filtration (SRF) decreased from 89.4% and 3.1 × 1013 m/kg to 66.7% and 2.4 × 1012 m/kg, respectively. However, when the PCC content was high enough (> 444 mg/g DS), the effective moisture content (MC*) changed little and the net sludge solid yield (YN) decreased, indicating that the non-conditioning effect of PCC dominated at high PCC content. Furthermore, increasing PCC content reduced the compressibility coefficient, although the change was insignificant from 1.06 to 0.91. When FeCl3 was used as the chemical conditioner, FeCl3 and PCC had additive effects on improving the sludge dewaterability (such as MC, SRF and compressibility). The improving mechanism of PCC on the sludge dewaterability can be explained by the release of Ca2+ from PCC caused by lower pH, the bridging effect of multivalent cation and the formation of more rigid sludge flocs.