Effective removal of diatoms (Synedra sp.) by pilot-scale UV/chlorine-flocculation process

Diatom blooms pose serious threats to drinking water production and safety. However, there is limited information regarding diatom removal. Here we established a pilot-scale system applying UV/chlorine pre-treatment with subsequent flocculation and sedimentation for treating real diatom-laden water. Results show that UV/chlorine-flocculation displayed better treatment efficiency compared to chlorine-flocculation. The reductions of algae, turbidity and dissolved organic matter (DOC) in UV/chlorine-flocculation reached 88.1%, 89.2% and 47.7%, respectively, at a UV dose of 130 mJ/cm², a chlorine dose of 1 mg/L and a polyaluminum chloride flocculant dose of 10 mg/L. The high performance of this process was associated with the formation of large and dense algae-flocs, as shown by an in-situ image analysis. The performance of the process could be further enhanced by increasing chlorine and UV doses. UV/chlorine-flocculation also decreased disinfection by-products formation. Reactive radicals (HO^• and RCS) generated in UV/chlorine effectively reduced zeta potential and enhanced charge neutralization of the cell fragments. These reactive radicals also reduced steric hindrance via breaking long and straight diatom cells down to smaller fragments. Overall, UV/chlorine-flocculation is an efficient and safe strategy to treat diatom-laden water, with a bright application prospect.