Quaternization-based Graft Modification of Straw Fibers for Conditioning the Sludge Dewatering Performance

Abstract

Abstract Extracellular polymeric substances (EPS) are a critical influencing factor in sludge dewatering. Disrupting such EPS contributes to the release of bound water in sludge, enhancing the sludge dewatering performance. This study quaternized straw fibers destructive for the EPS structure and components in active sludge were prepared with heterogeneous free radical graft polymerization. Straw fibers, dimethyl diallyl ammonium chloride (DMDAAC), ammonium persulfate (APS), and acrylamide (AM) were taken as the substrate, grafting monomer, catalyst, and cross-linking agent, respectively. The impacts of reaction temperature, reaction time, catalyst dosage, and monomer dosage on the grafting efficiency of straw fibers were explored. Further, the morphology, elemental composition, and structure of the modified fibers were characterized employing scanning electron microscopy (SEM), elemental analysis (EA), and Fourier transform infrared spectroscopy (FTIR), respectively. The optimal processing conditions determined for the DMDAAC-based quaternization and graft modification of straw fibers were as follows: reaction temperature of 60°C, reaction time of 5 h, 0.1 g of catalyst APS dosage per gram of straw, and 3 ml of DMDAAC dosage per gram of straw. The optimal processing conditions yielded 1.335 g of modified straw fibers per gram of straw, 33.5% grafting rate, and 31.70% substitution of the quaternary ammonium groups. The modified straw fibers disrupted the sludge EPS, boosting sludge dewatering. They improved the filtering performance of the sludge. The capillary suction time (CST) was conditioned from 243.3 ± 22.6 s in the original sludge to 134.5 ± 34.45 s. The specific resistance to filtration (SRF) was reduced from 8.82 ± 0.51 × 1012 m/kg in the original sludge to 4.59 ± 0.23 × 1012 m/kg. The modified straw fibers disruptively affected the structure of sludge EPS, significantly changing its content and composition. The proportion of tightly bound extracellular polymeric substances (T-EPS) in the sludge declined from 77–62% due to such modified fibers. Moreover, the protein and polysaccharide contents in soluble extracellular polymeric substances (S-EPS) and loosely bound extracellular polymeric substances (L-EPS) showed an increasing trend.