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.

纳米胶体二氧化硅（nanoscale colloidal silica）的粒径介于2~5 nm之间，经表面改性可使颗粒携带阴离子或阳离子电荷。该材料可被聚集为最多包含25个颗粒的结构化聚集体，此类聚集体的尺寸范围为5~150 nm，比表面积达50~800 m²/g。纳米二氧化硅（nano-silica）已成功应用于造纸双保留体系（papermaking dual-retention system），用于提升细料与填料的留着率，但目前尚无其应用于污泥脱水领域的相关研究报道。本研究联合台湾中部一家生产文化用纸及工业用纸的造纸厂开展实验，该厂采用沉降工艺与单级活性污泥（activated sludge, AC）工艺处理造纸废水。研究人员收集了沉降工序产生的初沉污泥以及活性污泥工序产生的剩余生物污泥用于实验。分别以常规阳离子聚合物药剂与纳米二氧化硅制剂作为脱水剂与助凝剂，探究二者协同提升污泥脱水效率的可行性。采用过滤比阻抗（specific resistance to filtration, SRF）与毛细吸水时间（capillary suction time, CST）两种指标量化污泥脱水性能。通过2³析因实验设计（2³ factorial experimental design），分析投加顺序与投加量对脱水效果的影响及交互作用。对CST与SRF测试结果的析因设计分析表明，初沉污泥与生物污泥的处理行为相似。所考察的三项变量均对脱水效果存在显著影响，但三者之间无显著交互作用。在CST与SRF测试中，生物污泥的脱水效率均低于初沉污泥。实验结果显示，应先投加阳离子聚合物，再投加阴离子型纳米二氧化硅；反之的投加顺序则会显著损害初沉污泥的脱水效果。阳离子聚合物与纳米二氧化硅对脱水效率的权重因子相近，该发现为后续工厂中试提供了可行方向。后续研究将进一步优化工艺变量，并阐明该复合调理体系的作用机制。