Pilot-Scale Test On Drinking Water Treatment By Coagulation-microfiltration Hybrid Process

With the widespread pollution of water source and the stringent drinking water quality standards, the conventional drinking water treatment process can't meet the requirement. While the membrane process is well concerned in drinking water treatment for its great advantages such as superior water quality, less footprint, simple flow and high automatization. Two pilot-scale test systems of coagulation- mircofiltration process, one using cylinder-type membrane and the other using curtain-type membrane, were constructed in a water treatment plant in Tianjin. The aim was to develop the appropriate membrane technology for drinking water treatment, and offer the basic data and the operational experience for new water treatment plant.The quality of the treated water by coagulation-microfiltation process met the Water Quality Standards for Urban Water Supply (CJ/T 206-2005). Results showed that the turbidity was removed almost completely in this process. The treated water from the cylinder-type and the curtain-type membrane filtration system had the same average turbidity of 0.09 NTU, with 89.5% and 92% of turbidity less than 0.1 NTU respectively. The average CODMn of the treated water from two systems was 2.03 mg/L and 2.25 mg/L respectively. The quality of the treated water from two systems was excellent in microorganic indexes, with total count of bacteria of a few CFU/mL, and no coliform detected.The cylinder-type membrane system, with a flux of 53.3 L/h·m2, was backwashed after every 30 minutes'filtration. The membrane has been chemically cleaned five times during 154 days'steady operation. The membrane fouling rate and the quality of treated water were remarkedly influenced by the quality of the raw water, which was relatively well during low temperature and turbidity period when the dosage of FeCl3 was 6 mg/L. The dosage of polyaluminium chloride was 10~20 mg/L during normal temperature period, and a higher frequency of EFM was needed. The quality of the raw water was worst during high temperature and alga bloom period, so that further measures such as prechlorination and reduction of the flux were required.The curtain-type membrane system was operated under continuous aeration, with a 2 minutes'interval after every 8 minutes'filtration. The membrane flux was 16.7 L/h·m2 and the dosage of FeCl3 was 3~4 mg/L. The curtain-type membrane, whose fouling rate was much lower than that of the cylinder-type one, was never chemically cleaned during 134 days'steady operation. Higher coagulation dosage and prechlorination were also required to control the membrane fouling and improve treated water quality when the raw water quality was poor.The power consumption of cylinder-type and the curtain-type membrane system was 0.20 kW·h/m3 and 0.42 kW·h/m3 respectively. But the reagent cost of the curtain-type one was less than that of the cylinder-type one. As the total cost of the power and reagent was concerned, the curtain-type one was 0.247 yuan/m3,higher than that of the cylinder-type one during low temperature period and lower than that of the cylinder-type one during normal temperature period. Compared with the curtain-type membrane system's water productivity of 98.8%, the cylinder-type one's was only 90.9%, and it could reach 98.0% when the backwash drainage was reused.