Optimization of the coagulation–flocculation process for the removal of natural organic matter fractions present in drinking water sources Oswaldo Cerón Alfaro, Alejandra Martín Domínguez, Fotis Rigas, Myriam Solís-López and Rosa-María Ramírez-Zamora ABSTRACT We used an experimental design to determine the best coagulation–flocculation mechanism and the optimal operating conditions for the maximum removal of the natural organic matter fractions (hydrophobic acid (HPOA) and hydrophilic neutral (HPIN)), which are the main factors responsible for irreversible membrane fouling and the generation of disinfection by-products (DBPs). Charge neutralization and sweep mechanisms (SM) were studied using the jar test experiment, and synthetic waters prepared with different hydrophobic/hydrophilic (HPO/HPI) weight ratios by adding model compounds to represent the dissolved organic matter (DOM) fractions. Significant influence factors were identified for both coagulation mechanisms. The SM was the best one for DOM removal independent of the HPO/HPI weight ratio. The SM removed HPOA and HPIN fractions with efficiencies of 87.5–90.5% and 73.6–89.8%, respectively. The dissolved organic carbon (DOC) values of all met the recommendation proposed by the United States Environmental Protection Agency (2 mg total organic carbon (TOC)/L or 1.8 mg DOC/L) for DBPs (<100 μg/L). Furthermore, all effluents met the DOC and silt density index recommended values by membrane suppliers (<3 mg DOC/L and <5%/min, respectively) to minimize fouling potential and to extend the membrane life. Oswaldo Cerón Alfaro Myriam Solís-López Rosa-María Ramírez-Zamora (corresponding author) Instituto de Ingeniería, Coordinación de Ingeniería Ambiental, Universidad Nacional Autónoma de México, Cd. Universitaria, 04510 Ciudad de México, México E-mail: rramirezz@iingen.unam.mx Alejandra Martín Domínguez Subcoordinación de potabilización, Instituto Mexicano de Tecnología del Agua, 62550 Jiutepec Morelos, México Fotis Rigas School of Chemical Engineering, National Technical University of Athens, 15700 Athens, Greece Key words | coagulation mechanisms, disinfection by-products precursors, enhanced coagulation, hydrophobic/hydrophilic ratio, irreversible fouling membranes INTRODUCTION Natural organic matter (NOM) in soil, peat, and water is composed of a mixture of plant and animal products in var- ious stages of decomposition, together with substances synthesized biologically and chemically (Cornel et al. ; Chow et al. ). NOM consists of suspended organic matter and dissolved organic matter (DOM). DOM is divided into two main groups: humic substances (HS) (hydrophobic (HPO) fraction) and non-HS (hydrophilic (HPI) fraction) (Thurman ; Sillampää ). The amounts and proportions of DOM differ consider- ably in surface water due to its origin and seasonal variability (Bruchet et al. ; Matilainen et al. ). However, DOM from different origins has shown similar subunits such as fulvic acid (FA), hydrocarbons/tannins, aromatic amines, carboxylic acids, polysaccharides, pep- tides/amino acids identified as hydrophobic acid (HPOA), hydrophobic neutral (HPON), hydrophobic base (HPOB), hydrophilic acid (HPIA), hydrophilic neu- tral (HPIN), and hydrophilic base (HPIB) fractions, respectively (Choudhry ; Imai et al. ; Guo et al. ). DOM in raw natural water is undesirable because it is responsible for color and odor in effluents of drinking water treatment plants, complexation with heavy metals, 153 © IWA Publishing 2016 Water Quality Research Journal of Canada | 51.2 | 2016 doi: 10.2166/wqrjc.2016.041