Ultrafiltration enhanced by coagulation in an immersed membrane system

The aim of this study is to determinate optimum conditions, membrane configuration, air flow, coagulation rate, permeate flux, for the ultrafiltration of surface water. Results show how coagulation is a necessary pre-treatment to remove micro-particles in aqueous suspension (i) before a conventional sedimentation, (ii) before a membrane filtration. When membranes are immersed in a contactor, configuration allow to maintain the permeate flow rate at high value (200 L/h/m2/bar) mainly because coagulation induces large floc size. However, when the particle concentration of the suspension is very high (2 to 5 g/L), it is important to increase the shear stresses close to the immersed membrane to minimise a deposit set up on the surface. When no high cohesion forces exist between particles, the aeration, even under weak flow rate, allows to avoid this deposit making.

Experiments were carried on with and without coagulation. Results show that without coagulation small clay suspended particles stay dispersed and sedimentation phenomena were low and limited. In the same way, when a membrane filtration was operated on this suspension a rapid particle deposit appeared on the membrane surface which induced the permeate flow rate decrease. On the opposite side, the introduction of coagulation step was an excellent pre-treatment method for particle removal at the optimal dose around 0.1 gFeCl3/gClay. The turbidity removal reached 95–98% after settling step and then also provided a residual turbidity less than 5 NTU. A particle/floc size is measured before and after coagulation with ferric chloride, the mean size of clay particles was about 8–10 µm while the size of coagulated flocs was larger with the average 100 µm Fig. 10a.Particle size (2.0 g/L clay, 0.2 g/L FeCl3) before aeration.