Combined macromolecular adsorption and coagulation for improvement of membrane separation in water treatment

Abstract

Fouling is the biggest obstacle facing the operation of RO desalination plants. Seawater contains many foulants that foul RO membranes, such as suspended particles, natural organic matter (NOM), microorganisms, and heavy metals. Different processes such as coagulation, flocculation, acid treatment, pH adjustment, addition of anti-scalant, and media filtration have been used as conventional pretreatment. Nowadays, membrane systems are utilized for pretreatment because of their feasibility, process reliability, plant availability, modularity, relative insensitivity in case of raw water, and lower operating costs. Natural organic matter and heavy metals are present in all water sources. They are of particular concern in desalination due to their toxicity and due to their effects on RO membrane fouling. Natural organic matter is a complex mixture of compounds formed from the breakdown of plant and animal material in the environment. Natural organic matter contains humic substances (HS) among other constituents. Heavy metals usually exist as free ions, but they also have a tendency of binding with HS. Consequently, heavy metals retention by ultrafiltration (UF) membranes is possible even though heavy metals have molecular sizes lower than the pore sizes of the membranes because of HS-metal complex formation. In this study, P005F UF membrane retention of humic substances, Cu and Zn, and its fouling is investigated with and without the aid of poly diallydimethylammonium chloride (PDADMAC) and copolymer of dimethyl aminoethyl acrylate (CoAA) polyelectrolyte coagulants. The conditions studied are salinity level, humic substances (HS) concentration, heavy metals concentration, and polyelectrolyte's type and concentration.