Application of Nano-silica to Paper Mill Sludge Dewatering

Nanoscale colloidal silica has particle diameters between 2 and 5 nm, and surface modification allows the particles to bear either anionic or cationic charges. They can also be aggregated to provide structured entities with a maximum of 25 particles. The aggregates have dimensions of 5~150 nm and surface areas of 50~800 m2/g. Nano- silica has been successfully used in a papermaking dual-retention system to help increase retention of fines and fillers. There is yet any study on their application to sludge dewatering purpose, however. We engaged the help of a central Taiwan paper mill producing cultural and industrial paper products that applies sedimentation and a single stage activated sludge (AC) process to treat the mill effluent. The primary sludge from the sedimentation and the waste bio-sludge from the AC stage were collected for the experiment. A conventional cationic polymer chemical and a nano-silica preparation were used as dewatering agent and co-agent to see whether dewatering efficiency could be enhanced. Sludge dewatering efficiencies were quantified using specific resistance to filtration (SRF) and capillary suction time (CST). A 23 factorial experimental design was used to delineate the effects and interactions of addition sequences and dosages. Analysis of the factorial design on CST and SRF tests showed that both primary sludge and bio-sludge had similar treatment behaviors. All 3 variables under investigation were significant but without interactions between each others. The bio-sludge had poorer dewatering efficiency than did the primary sludge on the CST and SRF tests. The cationic polymer should be added first and then followed by the anionic nano-silica. The reverse sequence of addition was largely harmful to the dewatering of primary sludge. Both cationic polymer and nano-silica showed close weighting factor on dewatering efficiency. It opens opportunities for further mill trials. Further optimization of the variables and establishing a plausible mechanism of the co-conditioners system will be the next step of work..