Ultrafiltration for the reuse of spent filter backwash water from drinking water treatment

During most water treatment processes, spent filter backwash water (SFBW) is generated. Reuse of SFBW is of concern because of the possible recycling of heavy metals, precursors for disinfection by-products and microorganisms. Innovations in membrane technology, especially in micro- and ultrafiltration processes, offer a suitable treatment for SFBW in order to guarantee a water quality necessary for reuse. Results from a pilot-scale ultrafiltration plant with submerged membranes are presented. Experiments were performed with SFBW from a full-scale water treatment plant. The plant was operated with high fluxes of more than 40 L/(m2·h) using clarified and non-clarified SFBW. Best membrane performance was obtained using non-clarified SFBW. As a result, no space- and timeconsuming sedimentation processes are necessary. The presence of powdered activated carbon in the SFBW did not have a negative impact on flux and TMP. Results confirmed that the filtrate can be used as an additional and safe water source. When a continuous maintenance disinfection was provided, filtrate was free of microbial contamination and could be reused without any safety concerns.

In order to obtain information on the application of UF technology for the reuse of SFBW, several experiments with a submerged UF pilot plant were performed with clarified and nonclarified SFBW. For both experiments, high fluxes of more than 40 L/(m2·h) and maximum TSS concentrations of 6 g/L in the concentrate were achieved. As a consequence, recoveries of 90% were possible. However, the use of clarified SFBW after a sedimentation process did not show any operational advantage compared to non- clarified SFBW. Thus, for the submerged membrane system, no time- and space-consuming sedimentation process prior to the UF treatment is necessary. As a result, the volume of storage tanks can be reduced significantly. The presence of PAC in the SFBW resulting in an increased influent TSS concentration did not lead to a deterioration of membrane performance (flux, TMP, permeability). Consequently, the required membrane area for the operation with and without activated carbon does not need to be increased. In order to reduce energy consumption, best results using intermittent membrane aeration were obtained when a 2-s on/off aeration interval was applied. With this configuration, the membrane area served with compressed air for membrane aeration can be increased. Focusing on water quality to be reused, major water quality indicators were always below corresponding drinking water limits as well as raw water concentrations. As a result, the reuse of UF treated SFBW should not lead to an influence of drinking water quality.