Removal efficiency of invertebrates in the filtrate of biologically activated carbon filter with sand bed

In view of the aesthetic problem and potential threat to safe drinking water caused by invertebrates, a series of different depth sand beds were located under granular activated carbon (GAC) media in five pilot-GAC filtration (GAC_F) columns to restrict invertebrates' access into the distribution system. During the study period May 2010 to March 2011, seven groups of invertebrates (rotifers and crustaceans as the predominant species) were detected in the filtrates of the five GAC_F columns. The experimental results indicated that invertebrates could be removed effectively with the added sand beds compared with the sand bed-free GAC_F column. The mean abundances of invertebrates decreased significantly with the increase in the depth of sand beds, while there were different removal ratios between rotifers (29.8–46.6%) and larger invertebrates (size >200 μm) (41.7–85.5%). Sand sizes had a greater impact on rotifer removal than on larger invertebrates. Also, increasing removal ratios of particle matter were detected with the sand beds added. Further data analysis showed that there was significant correlation between the mean values of particle counts and abundances of invertebrates in the filtrates. The mixed-layer phenomenon between the GAC and the sand media could be controlled effectively under the optimized backwashing procedures.