Mechanistic study of ultrafiltration membrane fouling in the separation of molecular-size characterized pulp and paper mill effluents

Abstract

The use of ultrafiltration (UF) to treat pulp mill effluents is limited by fouling of membrane surfaces and pores. In prior studies with pulp mill effluents, no efforts were made to relate the molecular-size distribution of the solute molecules to the membrane fouling mechanisms. This dissertation presents a novel protocol for obtaining certain essential size distribution parameters, such as, the average molecular weight (M(w)), average molecular number (M(n)) and heterogeneity index (HI) to describe complex industrial wastewaters including the extraction (E)-stage effluent. This novel molecular sizing protocol was verified using challenge solutions containing solutes of known molecular weight. In these tests, the measured M(w)'s were within ±5% of the expected M(w)'s. A comprehensive model to describe the UF membrane productivity during the treatment of E-stage effluent was developed. This model accounts for variations in membrane, feed water and operational variables. The feed water variables that were incorporated into the model include molecular size distribution, viscosity and concentration. Also, included in the model are the operational variables such as trans-membrane pressure and cross-flow velocity. This model predicted the fluxes for the E- and oxygenated E-stage (E₀) effluents within an error of 9%. The UF membrane fouling by E-stage effluent was quantified employing fouling potential factors. An increase in the molecular weight cut-off (MWCO) of the membranes resulted in increased irreversible fouling possibly by increased pore plugging. Lower irreversible fouling was observed for pulp mill effluents with high M(w)'s. The role of surfactants in reducing the membrane fouling was also discussed. The ratio of M(w) of feed wastewater to the MWCO of the membrane, denoted by λ, effectively represents the ratio of the average diameter of the solute molecule to the nominal diameter of the pore. Membrane rejection and fouling potentials were related to λ. The measured apparent diffusion (D) and mass transfer (k) coefficients for the E- and E₀-stage effluents across the membranes were found to confirm to D ∼ M(w)⁻⁽⁰·³³ ᵗᵒ ⁰·⁴⁸⁾ and k ∼ D⁰·⁶⁶.