Application of response surface methodology in the analysis of parameters influencing the removal of turbidity and nematodes in direct filtration process

Nematodes are well-known due to their resistance to disinfectants and their ability to ingest and carry pathogens to water distribution systems. Coagulation and filtration play significant roles in removing these organisms in water treatment plants. Investigating the effect of different parameters on the nematodes removal has always been an interest to researchers. However, the simultaneous evaluation of these parameters needs a comprehensive statistical analysis. In this study, the effects of ferric chloride dosage, filter media type, and filtration rate were investigated on the removal efficiency of turbidity and nematodes in direct filtration process using response surface methodology and central composite face-centered design. Based on the results, the average removal efficiency of turbidity, motile nematodes and non-motile nematodes in single-media filter were 96.14, 94.02 and 41.39%, respectively. Meanwhile, these numbers for dual-media filter were obtained as 96.61, 95.76 and 46.01%, respectively. With the increase in coagulant dosage, the removal efficiency of nematodes was improved significantly. Furthermore, the removal efficiency of non-motile nematodes was increased as the filtration rate decreased. However, an increase in the filtration rate led to an unexpected increase in the removal efficiency of motile nematodes. This distinct behavior of the nematodes led to the independence of total nematodes removal from the filtration rate. According to the results, using the direct filtration process is suggested, when the turbidity of raw water is lower than 5 NTU. However, primary disinfection must be applied to immobilize the nematodes and compensate the weakness of granular beds in the removal of motile nematodes.