Critical flux detection in a silica scaling RO system

Desalination of brackish water containing silica at high recovery levels leads to a rapid flux decline, due to the precipitation of a colloidal solution consisting of polymerized silica nano-particles. The critical flux (CF) phenomenon postulates that there is a certain permeate flux level below which the rate of flux decrease due to membrane fouling becomes negligible. The aim of this paper was to develop a simple technique for detecting the existence of a critical flux threshold limit under conditions at which the membrane surface is exposed to a constant precipitation potential. A series of experiments was carried out in a silica scaling tubular RO system. Experimental conditions were controlled such that the dissolved silica content on the membrane was held at the same supersaturation ratio level of 2.1–2.2 while varying the initial permeation flux value over the range of 17–45 l/h m2. The rate of flux decline was found to remain constant, irrespective of the permeate flux level examined.

The main contribution of this paper is in the development of a technique for investigating the critical flux phenomenon by a fundamental approach. A flux reduction by itself diminishes scale deposition by the reduced CP level. The CF phenomenon implies that flux reduction can cause a drastic, mitigating change, in the scaling mechanism. The proposed technique distinguishes between the effect of a reduction in flux decline rate generated by the reduced CP and the mitigating effect caused by a change in the deposition mechanism at a threshold flux limit. The above technique was applied in this study in an experimental system enabling well controlled runs in permeate fluxes between 15.7–43 l/h m2. No evidence for a CF could be detected over this flux range.