Study On Coagulation-microfiltration Process As A Pretreatment Of Seawater Reverse Osmosis (SWRO)

Reverse osmosis (RO) has been widely used over the last 30 years for desalination ofbrackish water and seawater due to the improvements of membrane technology.Controlling membrane fouling has always been a major problem in the process. Butthe conventional pretreatment, which is space consuming and costly can't ensure thequality of effluent fed to the RO membranes. Long-term operating experience at homeand abroad proves microfiltration a viable pretreatment for RO systems.Coagulation-microfiltration process, a combination of coagulation and microfiltion(MF), is widely used in water purification because of the steady filtrate quality, easyautomation and low operating pressure. In this study, FeCl₃ was used as the coagulant.Powdered activated carbon (PAC) was also injected into the reactor to remove organicmatter and retard membrane fouling.The main purpose of this study is to investigate the feasibility ofcoagulation-microfiltration process to provide eligible influent to RO systems withsuitable MF membrane by adjusting the operational parameters and the dosage ofchemicals injected.In the first part of the experiment, a MF module with pore size of 0.1μm made inJapan was used. According to the experimental findings the quality of the filtrateproduced by coagulation-microfiltration process is consistent and the 0.1μm MFmembrane can provide effluent with low SDI constantly below 3.Coagulation-microfiltration process with 0.1μm MF membrane is capable of treatingseawater and producing suitable effluent to feed RO system without any furthertreatments.The major problem of this part of experiment is MF module's susceptibility tomembrane fouling. The permeability decreases fast under the four conditions evenwhen PAC is injected. Chemical cleaning must be performed frequently.In the second part of the experiment, a domestic MF module with pore size of 0.22μmwas used. Result shows that the filtrate with good quality is obtained when seawater istreated with 2.5 mg/L of FeCl₃ (calculated as Fe³⁺) and when the PAC consumption is40mg/L. The turbidity of the filtrate is below 0.1NTU, SDI value is less than 1.5, theconcentration of Fe in the filtrate is no more than 0.1mg/L and the average CODMnremoval efficiency is 24%. The experimental findings reveal thatcoagulation-microfiltration process with 0.22μm MF membrane is capable ofproducing suitable effluent to feed RO system and the domestic MF membrane hasbetter antifouling ability than the Japanese MF membrane.Continuous experiment with 0.22μm MF module was conducted under the flux of 20L/h·m2 when 2.5 mg/L of FeCl3 (calculated as Fe3+) was injected and the PACconsumption was 40mg/L. The membrane module was cleaned with aeration whenthe maximum flow rate can't meet the designed requirement. The volume of thetreated water adds up to 1800L/m2.Coagulation and the injection of PAC have effects not only on the filtrate waterquality but also on the permeability of the membrane. Better water quality wasobtained with coagulation and intermittent PAC injection. In addition, the particleswhose sizes are below pore size are enlarged after FeCl3 coagulation and PACadsorption and easily rejected by microfiltration, which confirms that coagulation andPAC adsorption effectively guarantee the filtrate quality.A Full-scale pretreatment plant was designed and the costs of capital and operationwere evaluated. Compared with conventional pretreatment, this process reduces theinitial investment and the running cost slightly.