Fiber filter as an alternative to the process of flocculation–sedimentation for water treatment

The conventional treatment for production of drinking water from surface water generally consists of coagulation, flocculation, sedimentation and sand filtration. One of the main disadvantages of the conventional process combining sedimentation and filtration is the rather long residence time. This is mostly due to the flocculation and sedimentation phases (typically 2 h). The fiber filter recently developed has been very successful for the tertiary treatment of biologically treated sewage effluents (more than 2 millions m3/d treated by this technology in Korea). In this paper, a different application of this filter to drinking water production was tested. In this case, the fiber filter (with in-line coagulation) was used as an alternative to the process of flocculation and sedimentation. Even though the first design of the fiber filter showed good removal efficiency, the filter was modified in order to improve its performance commensurate with the tightening of regulation on the water quality regulation. In this study, the filtration efficiency of the newly designed filter was estimated using a range of filtration velocities from 60 to 100 m/h and a small dosage of coagulant (1–3 mg/L) injected in-line. The polymeric coagulant (PAC: 11% as Al2O3) was used during this experiment. The optimal operation conditions were determined based on the performance and filtration time. Through these experiments, it was shown that the new fiber filter design was very efficient for particle removal at a filtration velocity of 60 m/h (1500 m3/d) and 1 mg/L coagulant dosage, and these were considered as the optimal operating conditions. Through the direct suspended solids (SS) measurement it was verified that backwash efficiencies were also very good.

The following conclusions were drawn for a better adaptation of the filter to the production of drinking water from this study on the potential use of the fiber filter as an alternative to the conventional process of flocculation–sedimentation: 1) With an in-line addition of 1 mg/L PAC, the fiber filter could meet the standard of coagulation, flocculation–sedimentation of 1 NTU. The filtration operation could be maintained for about 100 min and 180 min at filtration velocities of 80 m/h and 60 m/h, respectively. The coagulant dosage of 1 mg/L represents very economical efficient conditions of filtration. 2) Although particles are efficiently removed by fiber filter (mostly particles larger than 5 microns), the head loss is small compared with conventional sand filtration. 3) The fiber filter with in-line injection of polymeric coagulant is able to replace the conventional step of flocculation–coagulation in drinking water production of surface water. Its main advantages are the small dosage of coagulant and the very short residence time (about 1 min). 4) Despite more frequent backwashing than conventional rapid sand filter, the productivity is high and the production of backwash water is small (about 3%). These conclusions are valid for treatment of surface water where the turbidity is less than 10 NTU. The behavior of the filter during peaks of turbidity or periods with higher turbidity will be tested in the future.