Inorganic fouling of pressure-driven membrane processes — A critical review

Abstract

One of the major limitations of the application of membrane processes in water and wastewater treatment is inorganic fouling. Despite the extensive studies on concentration polarization and inorganic scaling in membrane filtration, the fundamental mechanisms and processes involved in inorganic fouling are not fully understood. This paper critically reviews the mechanisms and models of concentration polarization and inorganic fouling in pressure-driven membrane processes. Effects of operating parameters and membrane properties on the formation of inorganic scale at the membrane surface are also evaluated. Future research areas that need to be pursued to alleviate inorganic fouling problems in membrane installations are discussed.

Introduction

Membrane is a selective barrier between two phases that restricts the transport of particulate, colloidal, and dissolved chemical species other than solvent or water. The selective transport is achieved based on the differences in the physical and/or chemical properties of permeating components across the membrane. In recent years, membrane processes are extensively employed in the textile, pharmaceutical, pulp and paper, semi-conductor, tanning and leather, mining, electroplating, dairy, food and beverage processing, and biotechnology industries as well as for water and wastewater treatment. In water and wastewater industries, four types of membranes (microfilter, ultrafilter, nanofilter and reverse osmosis membrane) are widely used for the removal of hardness, color, biogenics, and disinfection by-products and their precursors to produce required quality of processed water.

Membranes used in water and wastewater industry can be broadly classified into two major groups: porous membranes and non-porous membranes. Porous membranes separate particles based on sieving, straining, or size exclusion. Microfiltration, ultrafiltration and loose end nanofiltration membranes are examples of porous membranes [1]. Non-porous membranes separate molecules based on the differences in solubility or diffusivity between the solvent and the solute in the membranes [2], [3]. Tight end nanofiltration and reverse osmosis membranes are typical non-porous membranes. These membranes differ in their pore sizes, operating pressures, and applications, which are summarized in Table 1.

Traditional methods to remove pollutants from water and wastewater are coagulation, flocculation, sedimentation, sand filtration, ion exchange, electrodeposition, extraction, precipitation and biological degradation, etc. Most of which have disadvantages of operating in a successive steps of heterogeneous reactions, or distribution of substances between different phases that usually require a lengthy operating period and a large area [4], [5]. Membrane processes are of great interest because they reduce the number of unit operations, recycle process water, and recover valuable products for other applications [5]. Additional inherent advantages, such as selective separation, continuous and automatic operation, relatively low capital/operating cost, easy scale-up and low space requirement, made membrane filtration an attractive alternative compared to conventional treatment [6], [7], [8], [9].

Because of the potential of the membrane applications, many membranes have been and are being developed by different manufacturers (e.g., Nitto Denko Corp., Koch, Toray Industries, Dow/FILMTEC, GE Osmonics, etc). Most of the membranes are composed of organic polymers, such as aromatic polyamide, polysulphonates, polyvinyl alcohol, piperazineamide, polyimide, and polyacetylene [10], [11]. Table 2 shows some of the commercially available membranes. Inorganic membranes are also used for the rejection of organic molecules and ion separation in aqueous solutions. The major strength of inorganic membranes is their higher chemical, thermal, and mechanical stability compared to organic membranes [12]. Applicability of inorganic membranes is of great importance in non-aqueous filtration due to their stability in organic solvents [13]. High temperature-resistant inorganic membranes have been prepared from titania, silica–zirconia, and alumina [13].

Despite its potential in water treatment, certain limitations prohibit membrane process from large-scale and continuous operation ([14]. One of the major limitations arises from membrane fouling caused by different inorganic salts [15], which reduces permeate flux, increases feed pressure, decreases product quality and ultimately shortens membrane life [16], [17]. Consequently, membrane fouling increases the costs by increasing (1) energy consumption, (2) system down time, (3) necessary membrane area, and (4) construction, labor, time, and material costs for backwashing and cleaning processes. Successful application of membrane technology, thus, requires efficient control of membrane fouling ([18]). However, lack of clear understanding of the fouling mechanisms is a challenge in water treatment using membrane technology. In the past decade, numerous studies had been performed to investigate concentration polarization (CP), cake or gel layer formation, membrane pore blocking, and other fouling mechanisms on different membranes ([20], [21] and the reference cited therein). These studies suggested that membrane fouling due to inorganic salts is dependent on several factors, including but not limited to, membrane characteristics, module geometry, feed solution characteristics and operating conditions [22], [23], [24]. Moreover, several hypotheses have been introduced to explain the observed fouling behaviors. The objective of this paper is to provide a state-of-science review of membrane fouling caused by inorganic salts in water and wastewater treatment. This critical review addresses the strengths and deficiencies of the models and theories proposed to date, summarizes the experimental investigations, and projects future research needs on inorganic fouling.