Microfiltration of colloids and natural organic matter

Surface waters contain colloids and natural organic matter, largely compose d of humic substances. In this work the effect of natural organic matter (N OM) and humic substances (IHSS stream humic and fulvic acid reference mater ial) on the deposition and rejection of inorganic colloids (hematite) by GV WP and GVHP microfiltration (MF) membranes was studied. Parameters of inter est were solution pH, ionic strength, calcium concentration, primary colloi d size (75, 250 and 500 nm), organic-type and concentration, as well as mem brane-type and hydrophobicity, aggregate structure and colloid stability. The method of preparation of the equilibrated suspensions, and thus their a ggregation state, had a large influence on the rejection of colloids and th eir aggregates, as well as the association of particles with the membrane m aterial and flux decline. The systems studied were grouped into (a) organics in the absence of inorga nic colloids, (b) stable primary particles, (c) primary particles at pH ext remes with organics, (d) particles pre-aggregated in electrolyte solution p rior to adsorption of organics (SPO), and (e) particles stabilised with org anics (OPS). Extreme pH conditions and pre-adsorption of organics onto the particle surf ace created very stable systems (colloids retained their primary particle s ize) and the deposition of the colloids at the membrane surface was reduced significantly. This led to a penetration of the particles into the pores a nd adsorption on the pore walls leading to full rejection in the absence of organics. Flux decline was, in this case, dependent on colloid size, with the size closest to the membrane pore size causing the greatest flux declin e. In the presence of organics, membrane-colloid charge interaction and ads orption were reduced and rejection decreased to near-zero for these stable colloid systems. For aggregates the presence of organics led to a greater flux decline. Reje ction of colloids was complete, now determined by the large size of the agg regates formed, which also indicates great mechanical stability of these ag gregates. Calcium played a key role in the flux decline of all systems. In the absenc e of hematite, calcium contributed to organic aggregation and increased flu x decline. Calcium led to an increased flux for hematite aggregates (SPO) p resumably due to the formation of looser aggregates and a decreased flux fo r stable colloid-organic systems (OPS) due to destabilisation of these syst ems. (C) 2000 Elsevier Science B.V. All rights reserved.