Pilot study on regulating the efficacy of bioleakage by adding sand bedding layer to bioactivated carbon filters in water plants

Ozone Bioactivated Carbon (OBAC) filters are widely used in the deep treatment section of urban water treatment plants in China. However, there is a general lack of effective control measures for microbial leakage of OBAC filters in the effluent water, especially during periods of high influent volume. It has a potential significant impact on the biosecurity of the water supply. In this study, a pilot study was conducted at Water Plant A to investigate the effect of adding quartz sand layers with different particle size at the bottom of the activated carbon layer on controlling microbial leakage under nine months operating conditions. It was found that at a filtration rate of 10 m/h and thicknesses of 1.8 m for the activated carbon layer and 0.4 m for the quartz sand layer, the average retention rate of microorganisms reached 92 %. The quartz sand layer could form biofilm and its biomass could reach about 50 % of the activated carbon layer in the same area. The use of a quartz sand layer with a wide range of particle sizes (0.5–1.1 mm) and an “air backwashing with intensity of 15 L/(m²·s) for 3 min with 1~2 min standing and exhausting with water backwashing with intensity of 8~9.5 L/(m²·s) for 8 min” backwashing method not only achieved a good backwashing effect but also keep the interface between the carbon and sand clear thus solving the problem of mixed carbon and sand layers.