Combination of coagulation and ultrafiltration for drinking water production: impact of process configuration and module design

The in-line coagulation/ultrafiltration (UF) combined process has been studied in order to improve membrane performance for the treatment of surface water with high organic content. Ultrafiltration experiments were performed using cellulose derivative hollow-fibre membranes with ferric chloride as coagulant. The efficacy of adding coagulant just before the membrane module depends on the filtration mode, the process configuration and the module design. In dead-end mode, permeation flux of 70 l/hm² can be maintained, but high backwash frequency (low recovery) is necessary to prevent plugging. Cross-flow filtration was effective in reducing fouling and allows operation at higher permeation flux. In cross-flow filtration, the feed-and-bleed configuration (single stage continuous process) appears promising to reduce backwash frequencies: for a recovery of 95% and at a permeation flux of 120 l/h.m², quasi-steady transmembrane pressure can be maintained with a low fouling rate (10 kPa in 400 min.). In cross-flow filtration, the use of curved modules with Dean vortices reduces the fouling rate by a factor of 2 compared with a conventional straight module.