The continuously intensifying human population growth has led to the demand for clean water to be steeply increased while the gap between water demand and supply is getting wider [1
]. In addition to conventional sources, other sources of water are currently being considered for use such as groundwater, water hold by dams, brackish water and water reuse generated by wastewater treatment. Given that underground water can be efficiently pumped to the surface even in remote areas using electrical or diesel generators driving various types of pumps [3
], it can be a major auxiliary water source.
Wastewater and water treatment methods include both physical methods such as sedimentation and filtration (membranes, media filtration) and chemical methods such as coagulation, pH adjustments, addition of anti-scalants and acids [4
]. Regardless of its domestic, municipal, or industrial origin, wastewater is a serious environmental constraint that dictates effective treatment for its safe discharge in the aquatic environment. Wastewater has been regularly identified among other substances as containing hazardous chemicals including metals (e.g., As, Pb, Cr, Cd and Zn), toxic compounds such as endocrine disruptors, dyes and a strong, pungent odor due to the high content of organic matter. On the other hand underground water or also known as ground water is, though, prone to contaminants either naturally occurring or augmented by human activity to pollutants and hazardous substances that need to be removed as for the water to be constituted safe for consumption. Heavy metals, such as mercury, copper and lead can cause serious health problems in excessive amounts, including reduced growth and development, autoimmune diseases, cancer, organ damage, nervous system damage and in extreme cases, death [6
]. Another main contaminant is natural organic matter (NOM), synthetic detergents, nutrients such as phosphate and ammonia, heavy metals (HM), coliform bacteria as well as other microorganisms. NOM is a complex mixture of compounds including fulvic acid, humic acid (HA) [7
] and humin formed through decay of plant and animal material in nature and is present in numerous sources. It is composed of a range of small, low molecular weight species such as carboxylic amino acids and proteins and larger, higher molecular weight species (from 0.5–30 kDa) such as humic and fulvic acids in high concentrations [8
Currently, integrated membrane systems treatment is becoming widely popular due to their feasibility, process reliability, commercial availability, modularity, relative insensitivity in case of raw water processing and lower operating costs. Integrated membrane systems have also been proposed as the most suitable solution for decentralized wastewater treatment [9
], should this be needed due to generation of waste from rural industry (farming, livestock breeding, biogas generation through anaerobic digestion), other industrial waste producing activities such as food, beverage and dairy processing or the local population growth and activities (correctional facilities, health and wellbeing settlements, community centers) [10
]. The well thought out, designed and implemented use of membranes can decrease capital cost, reduce chemical usage, and require little maintenance [10
Membranes can offer high productivity both in terms of product recovery as well as pollutants retention and low operational cost compared to other competing technologies, since there is no water phase change and often minimal or no use of chemical additives [11
]. Among the numerous material arrangements for membranes, ceramic membranes are more and more employed in the drinking water and wastewater treatment industries when compared with organic and polymeric counterparts due to their resistance to extreme operating conditions and numerous available and sustainable cleaning protocols [12
]. This allows longer service lifetime and highly efficient filtration performance. Tubular membranes modules provide a modest surface area to volume ratio, and thus the highest cost per unit area of all cylindrical membrane geometries, but also provide potentially the greatest turbulence promotion and the best access to the membrane surface [13
Regardless of their many advantages, membranes are susceptible to fouling, an action that can be detrimental for the successful and continuous plant operations [14
]. Several strategies such as chemical cleaning protocols and backflushing with air or liquids, can be put in place so the separation and mechanical characteristics of membranes should not change in the long run. Other strategies include combining coagulation with either inorganic or organic coagulants, with membrane treatment which can potentially enhance pollutants retention and reduce membrane fouling. Precipitation of coagulated colloids at high coagulants concentration or high ionic strength in the feed reduces the feasibility of inorganic substances as coagulation aids, but organic coagulants have recently been preferred as they do not experience high precipitation, which gives them an easier to handle constitution [15
Therefore, in this narrative review the authors will attempt to explain the phenomenon of ceramic microfiltration membranes fouling occurring mainly in wastewater and water treatment. The review will be examining the pressing matter of water scarcity across the world, setting the tone of the necessity of usage of alternative source of water, introducing the concept of pressure driven membrane filtration, its types and categories, discussing ceramic microfiltration membranes and the occurring fouling phenomenon as well as current applied in practical methods of its treatment, attempting to extrapolate greater awareness in a relatively under-investigated matter within the last decade (as well as collating the main prevention and management methods, such as coagulation and cleaning.
2. Water Crisis
Climate change has not only brought great attention in greenhouse (GHG) emissions and their detrimental environmental impact but also to the necessity to maintain clean water supply for drinking purposes as well as for irrigation, agriculture, and industrial uses. For adequate living standards, countries need to maintain annual renewable water resources (ARWR) of at least 2000 m3
/capita, while a country with ARWR of 1000–2000 m3
/capita can possibly suffer occasional and localized water shortages [16
With 1000 m3
/capita being the threshold critical value [17
] countries with less than this will suffer serious water shortages that would strongly impend economic development, human health, and well-being. With an ARWR less than 500 m3
/capita, a country is likely to experience ‘absolute water scarcity’.
In the EU, changing weather conditions, droughts and water shortages have shown a substantial increase over the past fifteen years, with increasing severity clearly demanding judicious, efficient and effective water management. Within the Mediterranean basin over 50% of the population is evidently affected by water stress during the summer months, while the phenomenon is no longer confined to certain areas but will be most possibly affecting at least half Europe’s river basins by 2030 [21
]. It has been found that a low stream flow conditions can lead to an imposed wastewater reuse rates in drinking water treatment plants of up to 20% [22
]. This demonstrates why the effective treatment of wastewater is such a necessity.
Water reuse is supported by a fit-for-purpose approach, based on risk assessment, therefore achieving risk minimization through multi-barrier criteria, including water-treatment barriers and physical barriers to limit contact. Until recently, the focus on water reuse concerns has been on the well documented risks of microbiological parameters to human health [23
]. Other parameters are currently being investigated such as effluent organic matter (EfOM) control within the scope of reduction and removal in the treated water brings significant benefits, namely by decreasing color, odor, and synthetic organic compounds [25
]. Conventional methods of disinfection, such as chlorination, produce carcinogenic and hazardous by-products such as trihalomethane (THM), haloacetic acids (HAA), haloacetonitriles and haloketones which have an adverse effect on human health [27
]. EU countries, including the United Kingdom, have regulated the levels of THM in drinking water at 100 μg/L, while in the US, the US Environmental Protection Agency (USEPA) has set the levels at 80 μg/L with the HAA limit is 60 μg/L [31
]. Membrane filtration has been successfully used in water treatment for NOM removal [31
], with microfiltration (MF) being one of the most efficient membrane processes, although fouling due to NOM has been identified as a major problem as NOM particles tend to bind not only among each other or with other substances but also on the membrane surfaces [34
Water reuse is a vital tool for extending the water life cycle and in full compliance with the circular economy incentives but it has not been adapted to its full potential. Climate change has driven new global strategies, for instance those of the International Organization for Standardization (ISO/TC 282 Water Reuse) and, at an EU level, the targeting of a substantial increase in recycling and safe reuse globally by 2030 (United Nations (UN) Sustainable Development Goal on Water, SDG 6). These led to the establishment of water reuse as a top priority area (Strategic Implementation Plan of the European Innovation Partnership on Water) and the specific objective of water reuse maximization (Blueprint to safeguard Europe’s water resources). Therefore, the European Commission proposed in May 2018 new rules to stimulate and facilitate water reuse in the EU for agricultural irrigation [37
Other than the European South , climate change is also strongly affecting the already challenged in terms of water scarcity countries of the MENA (Middle East/North Africa) region (i.e., Algeria, Bahrain, Djibouti, Egypt, Iran, Iraq, Israel, Jordan, Kuwait, Lebanon, Libya, Morocco, Oman, Palestinian territories of Gaza and West Bank, Qatar, Saudi Arabia, Syria, Tunisia, United Arab Emirates, Yemen). The MENA region has 6% of the existing global population, but only 1% of the world’s freshwater resources [41
]. The countries in the region depend on seasonal rainfall, have very few rivers, some of which carry runoff from other countries, and often rely on fragile, occasionally non-renewable, aquifers. Currently MENA countries will have ARWR of less than 1000 m3
/capita with a projected decline to below 500 m3
per capita by 2025. The increasing competition for good quality water among different water-use sectors in the MENA region countries has decreased freshwater allocation to agriculture [42
]. With the increase in wastewater generation, its productive use in agriculture has increased, as farmers have no alternative sources of reliable irrigation water [43
] as the water taken away from agriculture is then diverted to non-agricultural uses. Overall, information on the productivity potential of wastewater, and its impacts on the environment, social and economic conditions of the dependent farming communities, is limited [46
3. Membrane Technology and Applications
Microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) membrane separation processes are strongly emerging technologies that can be used in several separation processes. Membrane processes are progressively being used further in several fields substituting conventional concentration, separation and purification techniques. Nowadays, membrane processes can be found in numerous industries including water treatment for domestic and industrial water supplies, waste treatment (separation of salt or other minerals, deionization), chemical (organic material separation, gas separation, recovery and recycle chemicals), pulp and paper (replacing the evaporation process, fiber and chemicals recovery), leather and textile (sensible heat recovery, pollution control and chemicals recovery), food and beverage, metallurgy (metal recovery, pollution control, air enriching for combustion), pharmaceutical, automotive, diary, food and biotechnological (separation, purification, sterilization and by-product recovery), medical (artificial organs, control release, pharmaceutical, blood fractionation, sterilization and water purification) and the petrochemical industries [47
There are numerous advantages that benefit from the use of membrane technologies, including their easy combination with other processes (hybrid processing), continuous separation, easy up-scaling, separation under mild conditions, low energy consumption and nonrequirement of additives. The growth and application of membrane technology is also pushed by the demand on industry for improved environmental solutions and cleaner technology.
Filtration is a physical process that involves the separation (removal) of particulate and colloidal matter from a liquid. Filters are categorized into three general groups [38
]: (1) depth filtration (2) surface filtration and (3) membrane filtrations. Depth filtration includes the removal of suspended materials within and on the surface of the filter bed. Sand and anthracite are usually used as filter media. In surface filtration, the suspended material is eliminated by straining through an exterior surface (e.g., filter cloth, diatomaceous earth filtration) [48
]. The range of particle sizes in membrane filtration is extended to include dissolved constituents (typically to 1.0 μm). Membranes serve as selective barriers that allow the passage of constituents and retain other constituents. MF, UF, NF and reverse osmosis (RO) are operated with a hydrostatic pressure difference as the most used membrane driving forces in water and wastewater treatment [38
]. The general characteristics of the varying membrane processes [49
] are further reported in Table 1
. It is necessary to operate a MF process with high surface velocity and low TMP. However, an optimization for the forces is needed for appropriate operation. Tangential flow across the membrane surface is preferable since it provides a continuous scouring action and hence reduces the membrane fouling layer due to feed stream debris and macromolecules.
4.1. Pressure Driven Membrane Processes
Membranes have been defined as engineered barriers that remove colloids, molecules or salt [51
] using a non-fibrous, engineered barrier, through a size exclusion mechanism. Based on pore size, shape and chemical/physical properties, membranes can separate different particles, organisms and chemical species. These systems of membrane filtration are commonly known as pressure or vacuum driven processes.
Membrane-driven processes (Figure 1
) can conveniently remove various-sized organic matter, from small solutes (through NF) to macromolecules (through UF) or suspended matter (through MF) [53
]. The permeate obtained after undergoing microfiltration and ultrafiltration can be feasibly reused in different stages, including rinsing, washing, and cleaning of industrial plants [54
Applied pressure, forces the solvent and various solute molecules through the membrane, whereas other molecules are impermeable to various extents dependent on the structure of the membrane. Pore sizes are reduced further down the filtration ladder, thus the resistance of the membranes to mass transfer increases and so, the applied pressure has to be increased to obtain the same flux. All these processes are well-established technologies developed at all levels of industry.
Physical treatments such as sedimentation, flotation, and adsorption, as well as barriers such as bar racks, screens, deep bed filters, and membranes are the methods of choice for the purification of surface water and wastewater, due to their low costs and minimal environmental impacts. The application of membrane technologies to wastewater treatment has expanded over the last few decades, with continuous reduction of their costs and extension of the application possibilities [56
]. Nominal or absolute pore sizes are often used when describing filtration capabilities of membrane materials. However, this number does not indicate the removal efficiency of the membrane. Filtering particles close in size to the pore distribution of the membrane can get quite complicated, since often particles that are smaller than most pores are removed not through sieving but through probabilistic interception in the depth of the filter media. In addition, particles may be excluded through electrostatic repulsion and adsorption to the membrane material. Over time, wear and tear takes its toll on the membrane through the deposition of particles and cake formation thus obscuring the pores of a membrane and increasing its removal efficiency [57
Schematic representation of microfiltration (MF), ultrafiltration (UF), nano filtration (NF) and reverse osmosis (RO) separation principles [58
Schematic representation of microfiltration (MF), ultrafiltration (UF), nano filtration (NF) and reverse osmosis (RO) separation principles [58
General characteristic of membrane processes [60
General characteristic of membrane processes [60
|Membrane Pores Size||MF||UF||NF||RO|
|Typical separation mechanism||Sieving||Sieving||Sieving, charge effect, adsorption, solution diffusion ||Solution–Diffusion (diffusion limitation), convection)|
|Pore size (nm)||100–10,000||2–100||0.5–2||Unknown|
|Retention|| || || || |
|• Monovalent ions||–||–||–||+|
|• Multivalent ions||–||–/+||+||+|
|• Small organic compounds||–||–||–/+||+|
|Energy consumption (kWh/m3)||0.4||3.0||5.3||10.2|
4.2. Filtration Mode
There are usually two main types of filtrations carried out in membrane separation processes: dead-end and cross-flow filtration. The dead-end filtration is normally used on small scales in laboratories whereas cross-flow filtration is the main process used on large scales in many industries including desalination [66
4.3. Dead-End Mode
Dead-end mode is a filtration method where the complete feed flow is forced perpendicular to the membrane surface, which allows the retained matter to build up on the membrane surface, due to clogging, and form a type of cake layer. The formed cake layer thickness increases with filtration time and consequently the permeate recovery rate decreases with an increased cake layer thickness. Dead-end mode is considered as the most basic form of filtration. It could be a practical technique for concentrating compounds [68
4.4. Cross-Flow Mode
In this method of filtration (Figure 2
), two forces are involved: a shear force where the feed suspension flows parallel to the surface of the membrane; and a perpendicular force on the membrane surface generated by the trans-membrane pressure. Cross-flow mode is considered a mature mode of filtration, which is regularly used as a standard technique for liquid processing and concentration of product. There are numerous advantages of this mode including low energy consumption, an increase product yield, selective and consistent separation, low maintenance and no additives, flocculants, or chemicals required [69
]. However, the perpendicular force is responsible for the formation of concentration polarization or gel layer, a build-up of retained material on the membrane surface [70
6. Membrane Manufacturing
There is a wide range of available membrane materials that are employed in industrial process and more specifically in water and wastewater treatment. They vary more widely in chemical composition than in bulk morphology. The production of membranes (Table 3
) can be by stretching, sub-atomic particle bombardment combined with etching and, in the case of ceramic membranes sintering [82
]. Nowadays, most drinking water production membranes are made of polymeric material, due to the fact that they are significantly less expensive than membranes constructed of other materials. Membrane material properties greatly affect the design and operation of a filtration system, factoring aspects such as mechanical strengths or chemical reactivity.
For example, membranes made of polymers that react with oxidants commonly used in drinking water treatment should not be used with chlorinated feed water, while a membrane with greater strength can obviously withstand greater TMP thus higher operational pressures can be applied. Likewise, a bi-directional strength membrane has the advantage of allowing cleaning operations to be performed from either the feed or the filtrate side.
Polymeric membranes have developed extensively giving rise to two types of membranes: isotropic and anisotropic. Nitrocellulose and cellulose acetate were first used in membrane manufacturing but were replaced relatively quickly by more sophisticated materials such as polyamide, polysulfone, polycarbonate and a number of advanced polymers [83
On the other hand, membranes made of inorganic materials are generally having superior chemical and thermal stability. In the past, inorganic membranes, namely ceramics, were used for a single industrial application known as the enrichment of uranium hexafluoride (235U) by Knudsen flow through porous ceramic membranes [84
]. This though has changed with inorganic membranes continuously gaining grounds with being able to be fabricated in various structures and pore sizes, especially as MF and UF membranes.
Membrane manufacturing procedures and applications [85
Membrane manufacturing procedures and applications [85
|Membrane Materials ||Manufacturing Procedure||Industrial Applications|
|Ceramic||Pressing, sintering of fine powders followed by sol-gel coating||MF, UF, aggressive (high concentration of acid and alkali chemicals for cleaning) and/or highly fouling media|
|Stretched polymers||Stretching of partially crystalline foil||MF, aggressive media, sterile filtration, medical technology|
|Track-etched polymers||Radiation followed by acid etching||MF, polycarbonate (PC) or polyethylene terephthalate (PET) materials. Analytical and medical chemistry, sterile filtration|
|Supported liquid||Formation of liquid film in inert polymer matrix||Gas separations, carrier-mediated transport|
|Integral asymmetric, microporous||Phase inversion||MF, UF, nanofiltration (NF), Gas transfer(GT) |
|Composite asymmetric, microporous||Application of thin film to integral asymmetric microporous membrane to produce TFC||NF, RO, pervaporation (PV)|
|Ion exchange||Functionalization of polymer material||Electrodialysis (ED)|
6.1. Inorganic Membranes
In industrial applications, four kinds of inorganic materials have been used. These are ceramic membranes, glass membranes, metallic membranes (including carbon) and zeolitic a subcategory of ceramic membranes. Generally, metallic membranes are achieved by the sintering of metal powders (e.g., stainless steel, tungsten or molybdenum) and they have not gained much popularity, probably due to their high the cost a complex manufacturing. Glass membranes (silicon oxide or silica, SiO2
) are primarily prepared by leaching on demised glasses and are rarely used to date [87
Ceramic membranes have been widely applied in the industry, they are commercially available in numerous sizes and arrangements, although due to their high cost compared to their polymeric counterparts, their application has been limited until recently in the field of food, beverage and pharmaceutical industry [88
However, their exceptional advantages, chemical and thermal stability as well as robust structural stability have attracted interest to their potential use in the treatment of waste streams [89
]. Ceramic filters are fabricated using alumina, zirconia or zeolite, materials that withstand extreme pH, pressure conditions and high flux rates [90
]. These characteristics facilitate effective cleaning with acidic or alkali solutions, indicating ceramic membranes as ideal candidates for processing complex effluent streams of sludge nature [80
]. They have gained widespread popularity due to their specific properties as they can withstand high temperatures which are applied for instance in membrane reactors in which they contain the catalytically active sites and function as separation barrier as well [91
]. Their uses regarding waste treatment have been expanded to the metal processing industry and surface engineering, in applications such as recycling and disposal of degreasing and rinsing baths; treatment of oil/water emulsions; recovery of heavy metals; cleaning of wastewater from grinding processes and treatment of wastewater from glass and glass-fiber production while regarding the environmental applications while environmental applications do include COD/BOD reduction; oil/water separation; recovery of pharmaceuticals and pesticides; retention of micro-organisms, heavy metals and radioactive substances; recycling of water from swimming pools and purification of the drain from sewage plants. MF is an economically viable alternative to traditional separation techniques such as centrifugation and rotary vacuum filtration.
Ceramic membrane configuration though still allows the deposition of particles in the inner side of the channels, forming a cake, which may hinder the permeate flux.
MF on the mechanism of the sieve effect. Thus, micro-structure parameters (pore size, thickness, and porosity) of ceramic membranes affect permeate flux and retention. The resistance of ceramic membranes is measured via water permeability experiments [92
6.2. Recent Developments of Membrane Materials
Since the main incentive in membrane formulation and fabrication research is fouling resistance, membranes possessing low affinity to pre-identified foulants in the feed suspension need to be developed. Natural foulants such as organic matter tends to be negatively charged; thus, membranes should possess a negative charge, in order to repel the foulant. The use of membranes for industrial process wastewater is limited by their resistance to extreme pH conditions and key organic solvents. Membranes for the development of biosensors [94
] and molecularly imprinted polymeric membranes for separation of molecules are some of the most recent developments in membrane technology [95
]. There is a wide range of chemical/physical mechanisms that can be used in membranes manufacturing, and that is one of the most attractive aspects of membrane processes. Therefore, successful applications will continue to be developed in the future. However, their industrial success will be governed by their advantages relative to other competing products and by their acceptance in the market.
8. Current Developments in Module Configurations
Ceramic membranes have been at the epicenter of recent advances and research in membrane development and further growth in the area of water and wastewater treatment [103
] with the scope being focused in results being geared towards creating a greater membrane area, without compromising mechanical strength.
Commercial manufacturers of ceramic membranes and research engineers have focused for the most part on alternative materials and designs that are less complicated [104
]. The challenge faced by suppliers is to cut the cost of ceramic membranes in order to make them further available and more economically competitive with the much less expensive polymeric membranes.
In terms of method combinations, aeration combined with submerged membrane systems is yet another recent development. Submerged membrane systems, which were originally developed for membrane bioreactors (MBRs) in the late 1980s are large-area membrane filtration modules submerged in a tank where the permeate is removed at low TMP. They were considered as an alternative to conventional filtration systems. MBRs are an example of a hybrid process and in a submerged system turbulence is provided by coarse bubble aeration. It has also been presented by [105
] that air-water two-phase flow can significantly improve membrane flux compared to single phase pumped liquid flow. Trials are still being conducted on static and moving turbulence promoters and turbulence promotion modifications to modules, including work that has been done over a ten-year period such as intermittent jets, where the feed is pumped coaxially through the membrane tube at fixed intervals through a nozzle [109
]. It was noted up to 2.5-times increase in flux for bentonite suspensions. Pulsed flow has also been studied by several researchers [110
]. In this case, pulses of flow are generated in the feed or permeate channel, creating changes in the velocity gradient. The use of inserts for turbulence promotion, especially in practical applications has gained more popularity [114
9. Properties of Ceramic Membranes
Ceramic membranes possess various advantages; they are capable of separating mixtures physically, they are ecologically friendly due to their extended shelf life, and more favorable than other separation technologies, often no additives are required and there is no limitation to process temperature. Ceramic membranes can withstand high temperature filtration (up to 500 °C) and extreme pH conditions (1–14). They can be cleaned with aggressive chemicals, organic solvents, or hot water and steam.
Ceramic membranes are chemically, mechanically, and thermally stable. They possess the ability of steam sterilization and backflushing; high abrasion resistance; high fluxes; high durability; bacteria resistance; possibility of regeneration; dry storage after cleaning.
Ceramic membranes have an asymmetric structure and consist of a coarse support, which is covered by several layers with decreasing pore size. Polymeric membranes are relatively unstable, but that problem has been greatly reduced or eliminated by using ceramic membranes [106
]. The development of ceramic membranes has induced a moderate revival in the use of static turbulence promoters in cross-flow membrane filtration [115
]. Running costs are limited by closed production cycles and continuous processes.
The high weight and considerable production costs of ceramic components are some of the disadvantages. However, costs are compensated for by a long service life. Polymeric membranes, on the other hand have limited stability (chemically, physically, and biologically), thus restricting the conditions of membrane processes applied.
12. Ceramic Membrane Fouling
The main hindrance to the widespread of pressure driven membranes is the occurrence of fouling phenomenon on the membrane surface. Membrane fouling can cause a decline in permeate flux and deteriorating of permeate quality [124
]. It is generally caused by dissolved or suspended components in the feed. Such components include dissolved inorganic and organic components, bacteria, colloids, and suspended solids. These components can interact with either the membrane surface and/or the fouling layer. Membrane fouling can also be influenced by the hydrodynamics of the filtration process. Fouling is usually classified as reversible and irreversible [125
]. Concentration polarization, gel layer formation and osmotic pressure are examples of reversible fouling [126
]. These phenomena are easier to resolve than irreversible fouling. Examples of irreversible fouling include cake layer formation, adsorption, and pore blocking (Figure 5
]. There are three types of pore blocking, i.e., complete, standard, and intermediate [127
The severity of fouling as well as the effectiveness of the cleaning method is revealed by the level of flux recovery. Concentration polarization and fouling are directly attributed to flux decline. Concentration polarization occurs when dissolved and/or colloidal materials concentrate on or very near the membrane surface while fouling is the gradual build-up of contaminants on the membrane surface [131
]. Fouling of membrane is influenced by the filtration process hydrodynamics, the interactions between the membrane and the foulants in the feed stream, and between the fouling layer and the foulants [132
]. Flux decline during MF can be very harmful on the economics of a given membrane operation and to tackle this problem, various measures are taken by organizations and companies [134
12.1. Fouling Phenomena
Understanding the fouling causing phenomena (Figure 5
) and the mechanisms that cause them is pivotal to develop effective control methods and develop longstanding effective membrane operation processes (Figure 6
). Particle adsorption and filtration-induced particle deposition are the two most important fouling phenomena in MF membranes, occurring through mechanisms such as concentration polarization and cake layer formation.
Particle adsorption on the membrane surface is usually irreversible and can occur even in the absence of filtration. In water treatment applications, the foulants are usually adhesive due to hydrophobic interactions, hydrogen bonding, van der Waals attractions, and extracellular macromolecular interactions amongst others [136
]. Judicious choice of membrane material, size and properties can limit fouling caused by particle and molecule adsorption. For example, hydrophobic membranes usually have a stronger tendency to foul, particularly with proteins and yeast. These membranes do not facilitate water flow through their pores at average operating pressures (<1 bar).
On the other hand, the particle deposition on the membrane surface is usually reversible non-adhesive fouling phenomenon [137
]. Unlike irreversible fouling the membrane surface chemistry plays a weak role in the reversible fouling [138
]. Accumulation of cell debris, organics, and other retained particles on top the membrane surface are examples of particle deposition reversible fouling.
12.2. Concentration Polarization
Concentration polarization is defined as the solute tendency to build up at the membrane-solution interface within a concentration boundary layer. The retained solutes can build up at the membrane surface and the concentration increases gradually. This concentration builds up will as a result produce a diffusive flow back to the feed bulk.
12.3. Cake Layer
Cake formation is attributed to material accumulation on the surface of the membrane, which effectively leads to cake layer. The flow of permeate drives the particles to the membrane surface to form a cake layer on the membrane except if a very high shear rate is applied to prevent the cake layer formation. Long term fouling would be resulted by the accumulation of undetachable cake layer on the membrane surface [139
]. In studies regarding river basin water clarifications [142
], it has been found that the impact of cake formation on membrane fouling has more serious consequences than the adsorption of small substances with the membrane’s pores.
12.4. Fouling and Retention of Particles due to Natural Organic Matter (NOM)
Flux decline caused by natural organic matter (NOM) fouling is major problem in membrane filtration of brackish and surface water [144
]. NOM interactions with membranes are main cause of NOM fouling [145
]. The NOM mixture has a specific chemical nature. The charge, configuration, and chemical potential of NOM during filtration are affected by many operating parameters of the process such as pH, ionic strength, ion compositions, temperature, and pressure.
pH plays an important role in the effectiveness of membrane processes to NOM removal. At pH values 6–8, NOM rejection is higher and NOM fouling is lower than pH higher than 8 [147
]. This can be attributed to the increase in NOM molecular size and charge repulsion forces at a pH above 8. At higher pH, the water flux decreases which indicate that the charge of the membrane surface and pores plays a major role in the level of membrane flux and retention. The raise in pH causes the membrane surface and pores to become more negatively charged because of anion adsorption. This reduces the pores size and hence results in flux decrease and retention increase [149
]. At low pH, NOM becomes very stable due to the fact that NOM molecules would contain approximately equal amount of carboxylate (COO-) and carboxyl (COOH-) groups which would lower the interactive forces between the fouling components and the membrane surface [150
The adsorption of humic acids (HA) on the membrane surface affects the surface charge of the membrane and makes it more negatively charged. It is widely accepted that HA aggregates with larger molecular weight has higher adsorption potential [151
]. This charge effect can be minimized by increasing the pH which causes electrostatic repulsion between HA and the membrane surface. Raising the pH can result in an increase in the hydrophilicity of HA.
The effect of the membrane pore size on the permeation of HA solutions has been previously evaluated [152
]. It was found that concentration polarization is more prevalent in membranes with larger pore size. Additionally, the effect of pH is greater for the membranes with higher permeability, while in many cases the flux was non-linearly varied with varying pressures.
The effect of NOM and humic substances on the deposition and retention of inorganic colloids by hydrophilic and hydrophobic membranes, have been previously investigated [153
]. A number of process parameters have been tested including ionic strength, pH, calcium concentration, primary colloid size, and NOM concentration. The results showed that the particles with size close to the membrane pore size caused larger flux decline. In the presence of electrolyte solution and at pH values close to that of surface water, the membranes were able to fully reject the colloid aggregates and hence the flux decline was depending on the deposition on the membrane rather than the primary colloid size. This applies mainly to surface water with high turbidity by no organic content. The addition of organics into the electrolyte solution with aggregated colloids can cause the organics to adsorb on the aggregate surface and fouling increases compared to aggregates without organics. If the organics were first mixed with colloids and then mixed with electrolyte solution, charge stabilization of the colloids can occur due to adsorption of the organics on the colloid surface. As a result, rejection falls to almost zero and fouling becomes fully dependent on primary colloid size. Rejection can be increased by destabilization of the colloids using calcium.
In membrane processes, increasing feed flowrate increases both recovery and permeate flowrate up until an optimal feed flow is attained, and then recovery starts to decrease [154
]. According to some researchers a low flux–high recovery process is more appropriate than high flux–low recovery approach for direct application on seawater [155
] as well as highly fouling surface water due to possible severe membrane fouling and plugging of fibers [156
]. The High flux–high recovery system can cause a raise in TMP. The advantages of operating with higher flux have to be balanced against associated disadvantages such as increase in chemical cost and backwashing.
Major fouling challenge comes from NOM fraction consisting small, neutral, and hydrophilic compounds [157
]. It has been observed [158
] that permeate flux decline comes mostly from hydrophobic fraction of NOM whereas the hydrophilic fraction caused much less fouling. NOM with larger molecular weight fraction contributed to the formation of the fouling layer since the size of these NOM is usually larger than the membrane pore size and hence cause surface fouling.
A number of investigations found that in MF, cake formation and pore plugging were responsible for membrane fouling as they reduce pore size and increase retention. Internal pore adsorption of calcium-organic flocs reduces the internal pore diameter and consequently increases rejection. The characteristics of membranes rejection function do not depend on initial membrane characteristics as much as the fouling state of the membranes and the nature of the foulants [160
The size and shape of macromolecular solutes and operating pressure play a significant role in membranes fouling [161
]. Colloid stability greatly affects fouling when lowering colloid stability worsens its degree and makes thicker deposits on the membrane surface. It was also found that increasing the shear rate helps to reduce concentration polarization of HA in MF membranes. The molecular diameter was found to be more useful for describing the membrane sieving mechanism.
Many studies found that humic substances cause irreversible fouling in MF membranes. For example, aggregate HA was responsible for the initial stage of fouling in a hydrophilic MF membrane. The effect of HA and fulvic acid (FA) was studied [161
] on membrane performance and found that HA was responsible for a 78% decline in flux compared to only a 15% decline with FA, and it has been hypothesized that this could be due to the HA’s aromatic and hydrophobic properties, adsorptive behavior and greater MW that led to tendency to foul.
14. Membrane Cleaning
The product flow, during long membrane performance, constantly decreases due to progressive adherence of different foulants to the membrane’s surface, the matter which raises hydraulic resistance in the membrane module and diminishes its active surface. To restore the initial flux levels, chemical regeneration procedures may be performed to remove the build-up of foulants, in the cleaning process. Cleaning may be defined as a process where material is relieved of a substance that is not an integral part of the material [207
The goal of cleaning is to acquire a physically clean structure. A considerable body of research proposed cleaning procedures for fouled membranes, as well as mild cleaning regimes and environmentally friendlier cleaning procedures, such as regimes in which purified enzymes and detergents are used in order to remove biologically derived foulants that foul polymer membranes. Attractive alternatives have been proposed to replace classical cleaning regimes such as the use of an enzymes as standalone processes or combined with biodegradable detergents [208
]. Enzymes are regarded as ideal cleaning agents due to their highly specificity for the reactions which they catalyze as well as the substrates they interact with. Generally, cleaning is carried out in different physical, chemical, and biological forms.
In the case of chemical cleaning, the first step is to find suitable materials to be used as cleaning agents [209
]. The choice of the suitable materials rests on the feed composition and the layers precipitated on the membrane surface which is performed in most cases by trial and error. The choice of the cleaning solution does not depend on the foulant type only, but also on the membrane compatibility with the cleaning solution at the cleaning temperature [210
]. The wrong choice of a cleaning agent can adversely compromise the performance of the membrane. The selected cleaning agent must be safe, chemically stable, cheap, and easily washable with water, as well as capable of dissolving most of the precipitated fouling materials on the membrane surface without damaging it [211
]. Poor permeate flux due to irreversible fouling can only be recovered by chemical cleaning or by mechanical backwashing or both. Cleaning agents (Table 3
) usually belong to the categories of bases, acids, enzymes, surfactants, and disinfectants and combinations of these categories [212
Appropriate selection of cleaning chemicals entails a thorough understanding of the foulant’s chemical properties. Chemical effects are now largely understood. The higher the foulant molecular weight and charge ratio, the greater is the fouling rate by potable water. Fouling is also increased by the presence of divalent cations [213
]. An increase in the electrostatic potential of the cleaning medium due to higher charge density, polarity, or pH, restrains the attraction forces and thereby increases cleaning efficacy [213
14.1. Cleaning Reagent Performance
Mass transfer, is the second defining cleaning mechanism, is believed to be the main barrier to effective chemical cleaning. The chemical agent is prevented from reaching the foulant unless sufficiently high concentrations are used to overcome the attraction forces [214
Several researchers [215
] have proposed six steps to summarize membrane chemical cleaning process, as follows:
Bulk reaction (hydrolysis and other) of cleaning reagent as the cleaning in place (CIP) is introduced.
Cleaning agent is transported to membrane surface.
Cleaning agent transits through foulant layers to membrane surface.
Cleaning reactions solubilise and detach foulants.
Waste cleaning agent with suspended foulants transported to interface.
Finally, transport of waste matter to the bulk solution from retentive side of membrane.
Based on the electrostatic equilibrium model (Figure 8
), forces that retain the foulant at the membrane surface are minimized during cleaning as a step towards its physical removal. Hence the selection of the cleaning agent should depend on the nature of the foulant, whether it is organic or inorganic, or acidic or basic, as well as the charge state. Table 4
shows the physic-chemical mechanisms that describe the functioning of the most commonly available membrane cleaning agents that are used to clean potable water.
14.2. Caustic Soda
Sodium hydroxide (NaOH) solutions are used when membrane chemical resistance is a problem. Usually, 1% NaOH concentration is used at the pH levels of about 11–12, or less. It interacts with the weakly acidic organic matter, usually with the carboxylic and phenolic functional groups. It also aids breaking of polysaccharides and proteins into smaller molecules of sugars and amides [217
]. NaOH also saponifies fats and solubilizes proteins. A large body of research demonstrates the efficiency of NaOH in washing away whey protein deposits from MF membranes [218
]. It can also expand NOM molecules, thus allowing higher mass transfer and movement of the cleaning agent to the membrane surface. The hydroxide is also thought to remove inorganic colloids and silicates by increasing solubility and electrostatic repulsion. More permeate could be recovered when NaOH is used at the threshold value concentration which varies for different foulants and membrane materials and degree of fouling [219
Sodium hypochlorite (NaOCl) and hydrogen peroxide (H₂O₂) are among the oxidants that are used in membrane cleaning procedures. Sodium hypochlorite is very common, yet there is no general agreement on its preferable use. Oxidants degrade NOM functional groups to ketonic, carboxyl and aldehyde groups which makes them readily hydrolysable at high pH levels. This could explain why when alkaline cleansing agents combined with oxidants, the oxidants become more effective, especially where organic foulants dominate [220
Acids are used to remove multivalent cationic particles found in hard water such as salts and metal hydroxides. Nitric acid (HNO₃) has been shown to solubilize inorganic materials that contain bases such as calcium phosphate (Ca3
). Rinsing inorganic membranes such as zirconia with HNO3
gives higher water fluxes [221
]. Mineral acids, especially hydrochloric acid (HCl) and sulfuric acid (H2
), are in common use because of their low costs. They are effective for both cleaning in place (CIP) and chemical enhanced back flush at pH of down to 1.0, acids are used more commonly for the removal of mineral scaling.