This study was undertaken to evaluate the suitability of microfiltration (MF) as pre-treatment for reverse osmosis (RO) seawater desalination to treat the seawater suffering from red-tide contamination using long-term operation of pilot plant. The one and a half year pilot MF operation had two objectives: stable production of the flow rate of 5 m3/h and acceptable water quality (SDI less than 3). The pilot plant operation revealed that the MF system successfully produced the target flow rate despite red-tide contamination of the seawater. The average flow rate of 5.2 m3/h was obtained at the average operating pressure of 0.53 bar. However, the MF system failed to achieve the target flow rate at red-tide bloom. When red-tide bloom occurred the chlorophyll-a concentration became 136 mg/m3, the flow rate decreased to half of the target, and energy consumption became extremely high. Subsequently, the operation was stopped. According to the relationship between the flow rate of the MF system and chlorophyll-a concentration developed in this study, it would be desirable to stop the MF operation at chlorophyll-a concentration of 57 mg/m3. The MF system produced acceptable quality water for RO feeding. The SDI of the MF treated was consistently less than 3. The MF system consumed 0.5 KWh of energy to produce 1 m3/h of MF treated, if the data during the red-tide bloom were excluded. Extra equipment (intake pump, control system, monitoring system, air conditioner) caused higher energy consumption than expected.

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