Invertebrate removal by traditional biological activated carbon (tra-BAC) and pre-BAC treatment processes was investigated in a full-scale water treatment plant. The results showed that invertebrate reproduction occurred in both BAC filters, but the invertebrate abundance in the finished water processed by tra-BAC was about 15 times greater than that processed using the pre-BAC process. In the pre-BAC process, the sand filter was placed after the BAC filter, and sand filtration removed most of the invertebrates, with an average removal efficiency of 91.1%. However, the pre-BAC filter, which was positioned behind the sedimentation tank, needed to be backwashed more frequently than the tra-BAC filter because of the high turbidity of the inlet water. The frequent backwashing reduced the biomass on the activated carbon and decreased the invertebrate reproductive rate. The results of this study are helpful for evaluating the pre-BAC treatment process in drinking water treatment plants.

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a b s t r a c tThe problem of excessive propagation and leakage of invertebrates in the biological activated carbon (BAC) treatment process have received increasing attention in recent years. Herein, the potential of a combined chlorination and sand filtration process for reducing invertebrate leakage is investigated, based on pilot-scale studies. In this developed design, the chlorine dosage and filtration velocity of the sand filter were optimized. Meanwhile, the abundance of invertebrates, the turbidity, the chlorine concentrations in the inflow water and the effluent were also recorded. It was found that the addition of chlorine could improve the efficiency of invertebrate removal in a sand filtration system. Within the parameters of the filtration velocity (8 m/h) and the operation time (144 h), the average invertebrate removal efficiency of the sand column (particle size: 0.3-0.5 mm) increased from 61.9% (without chlorine added) to 89.9% (with 0.5 mg/L chlorine added). When the filtration velocity increased to 12 m/h, a dose of 1.5 mg/L chlorine was required to obtain a relatively high average removal efficiency (83.0%), as the removal efficiency decreased with the increase of filtration velocity. The invertebrate survival status was confirmed to be the key factor that affected the removal efficiency. Chlorine was effective for inactivating invertebrates, and for further inhibiting their movement and reproduction, which finally resulted in an improved interception function for the sand filtration. Additionally, during the filtration, the head loss and chlorine consumption were too minimal to be of concern.