Coagulation/floculation as a key unit in the decentralized surface water treatment systems in upland rural areas of Vietnam: Results from a lab scale to a pilot scale system

Abstract

Surface water is a primary water source for domestic water treatment systems in many countries around the world, especially in developing countries. The coagulation-flocculation process has been widely applied in water treatment systems due to low cost and easy-to-operate process. This study focuses on optimizing the operational conditions (dosage and pH) in the coagulation-flocculation process with different coagulants for the water supply in upland rural areas of Vietnam, from the lab scale to the pilot scale system. Two prehydrolyzed coagulants, polyaluminium chloride (PAC) and polyferric sulfate (PFS), and aluminum sulfate (alum), were used in the coagulation-flocculation testing. Response surface methodology (RSM) was applied to design the experiments, analyze the results, and optimize the response variables. The results showed that the quadratic regression models of PAC, PFS, and alum were statistically significant at the 95 % confidence level (p < 0.05), and the p values of lack of fit for these models were insignificant (p > 0.05). The optimum conditions for 48.3 % DOC and 92 % turbidity removal using PAC were a dosage of 15 mg/L and a pH of 7.98. Similarity, 47.5 % DOC and 90 % turbidity removal were observed at 43.5 mg/L of PFS and pH 7.08. However, the lower removal efficiencies (37.2 % DOC and 86.7 % turbidity) were recorded at 60 mg/L of alum and pH 7.45. PAC and PFS were found to be more effective than alum in both DOC and turbidity removal from surface water. Notably, a combination of the coagulation process with aeration and a pre-filter system including a 1 micron polypropylene filter column, activated carbon, and cation exchange at the optimal operation conditions of coagulation through the RSM approach can be applied to treat Ba river water into domestic water that meets the national standard.