Production of Cationic Starch-Based Flocculants and Their Application in Thickening and Dewatering of the Municipal Sewage Sludge.

Polymer flocculants are used to promote solid-liquid separation processes in wastewater treatment technologies, and bio-based flocculants possess many advantages over conventional synthetic polymers. Potato starch microgranules were chemically modified and mechanically sheared to produce modified starch flocculants. The effectiveness of produced cationic starch (CS) and cross-linked cationic starch (CCS) flocculants in the thickening and dewatering of surplus activated sewage sludge was evaluated and compared with that of synthetic cationic flocculants (SCFs) The flocculation efficiency of SCF, CS, and CCS in sludge thickening was determined by measuring the filtration rate of treated surplus activated sludge. Comparing the optimal dose of SCFs and CCS flocculants needed for thickening, the CCS dose was more than 10 times higher, but a wide flocculation window was determined. The impact of used flocculants on the dewatering performance of surplus activated sludge at optimal dose conditions was investigated by measuring capillary suction time. The filtration efficiencies (dewaterability) of surplus activated sludge using SCF, CS, and CCS were 69, 67, and 72%, respectively. The study results imply that mechanically processed cross-linked cationic starch has a great potential to be used as an alternative green flocculant in surplus activated sludge thickening and dewatering operations in municipal sewage sludge treatment processes.

Biodegradation of cross-linked and cationic starches.

The biodegradability and the influence of the degree of substitution of cationic groups or cross-linking level of starch were studied by using enzymatic hydrolysis and two aerobic degradation methods. Cationic starches with a degree of substitution varying from 0 to 0.54 were obtained by modifying native potato starch with 2,3-epoxypropyltrimethylammonium chloride, while cross-linked starches with a degree of cross-linking varying from 0 to 92.5% were obtained by reaction of native potato starch with epichlorohydrin. Enzymatic hydrolysis experiment was performed using α-amylase preparation, and aerobic degradation studies were carried out in liquid and solid media by using ISO 14855-2 and 14851 standards methods. The dextrose equivalent, molecular weight, viscosity and biodegradability parameters were used to assess biodegradation process. Biodegradability of modified starches decreased with increasing degree of modification. The addition of cationic groups to starch to the extent >0.1 mol/molAGU reduced the biodegradability of starch derivatives, and CS became non-biodegradable when DS ≥ 0.54. The cross-linking of starch by building the alkyl chain cross-links between the polysaccharide macromolecules reduced ultimate biodegradability of starch derivatives, when the degree of cross-linking was higher than 92.5%.