Use of threshold flux concept to aid selection of sustainable operating flux: A multi-scale study from laboratory to full scale

The new sanitary standard in China, which the conventional treatment process (coagulation, sedimentation, sand filtration, disinfection) cannot satisfy, promotes opportunities for the application of ultrafiltration guided by the concept of sustainable flux. This concept aids the selection of an operational flux that balances the capital expenditure and operating costs. Laboratory, pilot and full scale experiments were conducted over short and long term filtration periods to study flux sustainability for a full scale drinking water application. Firstly two methods (flux cycling and an improved flux-step method which is more accurate due to higher relaxation at a fixed lower baseline flux) were used with raw feed water and with sand filtered water. For each the critical flux (i.e. flux for the onset of any fouling) and the flux at which irreversible fouling started were identified. From this 2 × 2 × 2 matrix eight values indicative of the threshold flux for full scale operation were obtained. Then long term tests at both laboratory and pilot scale were used to identify the most suitable flux. Finally the identified flux was successfully used in a full scale drinking water works in Shandong province for 21 months; it operated after sand filtration, at 30 L/m2 h, with very low frequency back flush but without chemical cleaning. This has proved the feasibility of using ultrafiltration in a Chinese drinking water works and verified the methodology adopted to identify the sustainable flux.