Enhancement of sand filtration performance by titanium-chitosan micro-flocculation coupled with ferrate(VI) pre-oxidation

Due to seasonal algal outbreaks in source water, the sand filtration (SF) suffering high load exhibited poor performance in turbidity and organic matter (OM) removal. However, the filtration performance is highly dependent on the state of contaminants. This study aims to improve removal efficiency of turbidity, algae and DOM by coupling a novel green titanium sulfate-chitosan (TS-CTS) compound filter aids with/without ferrate (VI) pre-oxidation. The results indicated that the turbidity decreased to 0.25 NTU and chlorophyll a removal up to 73.5% with TS/CTS (0.8/0.3 mg/L) filter aids were utilized, while the filtration cycle was not significantly shortened. The introduced Fe(VI) (5 mg/L, 10 min) before micro-flocculation realized moderate pre-oxidation and formed in-situ Fe(III), improved algae cells removal and effectively controlled of 2-methylisoborneol (< 10 ng/L) as well as the disinfection by-product precursors as polysaccharides and amino acid-like proteins. Positively charged in-situ Fe(III) as a dual-coagulant facilitated Ti hydrolysis and promoted the clustering, cross linking, formed Ti-AOM/NOM-Fe complexes. Under the adsorption bridging and adsorption electroneutralization of TS-CTS and in-situ Fe(III), the microflocs were formed, migrated, adhered and ultimately retained in filter layer media. This study can provide guidance for technological and agent selection in algae laden surface water treatment.