Combined Effect of Slow Sand Filter and Roughing Filter While Adding a Coagulant Chemical for Improving Turbidity Removal

The prominent drinking water treatment processes practised to eliminate turbidity from surface waters are coagulation, sedimentation and rapid sand filtration. However, the possibility of removing turbidity using the combination of Roughing Filter (RF) as pre-treatment and Slow Sand Filter (SSF) was not extensively studied in the case of high turbidity occurrence in the surface water. This study aimed to evaluate the performance of the combination of RF (as the pre-treatment) and SSF in terms of turbidity removal in surface water when pre-adding PolyAluminium Chloride (PACl) as a coagulant chemical. One per cent of PACl is prepared and dosed at the rate of 20 mg/L with raw water into the water intake chamber before it reaches the RF. The raw and treated water samples were collected at six-hour intervals. The raw water samples of which turbidity level is more than 80 NTU were considered and analysed from 63 nos of trials. The turbidity level of raw water and treated water was observed in the range of 80.4 - 185 NTU and 0.3 - 5.5 NTU respectively. The turbidity removal from RF alone and SSF alone was 87.4% and 86.4% respectively. However, the average turbidity removal in the combined technique was 99.2% and the turbidity of treated water was observed to be well below the SLS 614; 2013 acceptable limit of 2 NTU in 97% of the treated samples. This methodology shall enable the proper function of the treatment plant, during the turbidity level of less than 185 NTU in raw water, by complying with SLS 614-2013.