Effect of filter depth of biological activated carbon on fouling control of ultrafiltration: performance and mechanism

• Contaminants were mostly removed by the upper filter (0–10 cm), including organics, particles, and protein-like substances. • The upper layer (0–10 cm) could reduce both the number of particles and the average size. • Fouling control of BAC was mostly attributed to the upper filter (0∼10 cm), whose function was higher than the sum of the deeper filters (10–50 cm). Biological activated carbon (BAC) prefiltration is a common method to control fouling in ultrafiltration (UF) processes. BAC filter depth is a critical parameter for both design and operation of a prefiltration system. Here, the effect of filter depth on the performance and mechanism of contaminant removal and UF fouling characteristics was investigated. The results indicated that contaminants were mostly removed by the upper filter (0–10 cm), including organics, particles, and protein-like substances, while the deeper filter (10–50 cm) removed additional organics, which were mostly humic-like substances. The upper layer intercepted particles, which not only reduced the number of particles, but also significantly reduced the average particle size. The fouling control capability of BAC prefiltration was mostly attributed to the upper filter, whose function was greater than that of the deeper filter. However, the primary fouling mechanism did not vary with filter depth. In summary, during the combined process of BAC prefiltration and UF, when membrane fouling control is the main objective, the filter layer could be set as a thin layer (such as 10 cm). However, when the combined process needs to reduce the concentration of organic matter at the same time, a thicker layer is indispensable to achieve the desired performance.