Dissolved air flotation of surface water for spiral-wound module nanofiltration pre-treatment

Bench-scale dissolved air flotation (DAF) of Tagus River surface water (Valadas, Portugal) was investigated as pre-treatment for spiral-wound module nanofiltration (SWNF). Before the DAF, a coagulation/flocculation of the surface water was performed using aluminium sulphate, ferric chloride and chitosan coagulants and several commercial coagulant aids. The coagulation/flocculation experiments were carried with 2 L of surface water in a jar-test equipment at room temperature. The DAF performance to remove colloidal matter and suspended solids was evaluated through the measurement of the silt density index (SDI) and the modified fouling index (MFI) of the treated water. The air saturation pressure was varied between 300 and 600 kPa and the recycle ratio between 5 and 50%. The DAF without coagulation/flocculation was not effective in the reduction of both the SDI and MFI, even at a high DAF recycle ratio of 50%. The use of coagulants improved the DAF efficacy in reducing the fouling indexes and, for the optimal operating conditions, the MFI of the treated water was reduced by more than one order of magnitude. However, it was not possible to obtain treated water with SDI and MFI values below the recommended ones for SWNF. Further filtration of the DAF treated water with a 5 micron capsule filter was necessary to reduce the SDI below 5%/min and the MFI below 10 s/l2. The integration of DAF with another process that removes the residual suspended solids and colloidal matter is, therefore, necessary to provide a good pretreatment for the SWNF of Tagus River surface water.