Coagulation optimization for low temperature and low turbidity source water using combined coagulants: a case study

Coagulation optimization using coagulants of ferric chloride (FeCl3), polymer–polyaluminum ferric chloride (PAFC) and their combinations (FeCl3/PAFC) was evaluated by performing jar tests for treating source water with low temperature (<10°C) and low turbidity (1.5–8 NTU) i,n a typical North-China water treatment plant (WTP), Tanggu WTP. The results indicated that compared to single coagulant, the combined coagulants showed superior coagulation performance in terms of turbidity, UV254, CODMn, iron, and aluminum removal. The optimal dosage was determined as 20 mg/L using the combined PAFC/FeCl3 (3:1 by mass) and dosing PAFC with a 5–20 s interval followed by FeCl3. Scanning electron micrographs further showed that the coagulations combination can form a more compact structure that increases the settleability of the flocculation flocs.