Membrane Fouling and Scaling in Reverse Osmosis
Table of Contents:
The main “Achilles heel” for the efficient and smooth operation of the membrane-based desalination systems is membrane fouling (Flemming et al. 1997). Fouling occurs in all membrane processes such as reverse osmosis, as well as nano-, ultra-, and microfiltration. Membrane fouling results from complex physical and chemical interactions between the fouling constituents in water and between these constituents and membrane surfaces, leading to the attachment, accumulation, and/or adsorption of these constituents onto the membrane surface. As a result, water transport is hindered because of the formation of a fouling layer on the membrane surface, which eventually causes a decline in the quantity and quality of the produced water (Guo et al. 2012). The problems associated with membrane fouling are decreased membrane permeability, increased operating pressure, and increased frequency of chemical cleaning and membrane damage (Matin et al. 2011).
EFFECT OF FOULING AND SCALING
Fouling and scaling may manifest in three ways, which are detailed next.
Increasing Differential Pressure in Cross - Wound Fibers, or of Spacer Spiral Wound Elements
Particles tend to deposit and bacteria tend to grow on fibers and spacers, which results in clogging of fiber bundles and spacers. Two main mechanisms involved in fouling the membrane surface itself are identified namely; pore blocking and cake/gel formation resulting in a lower permeability of the membranes. Clogging of the spacers and bundles results in higher differential pressure (head loss). The consequences of this are; i) lower net driving pressure,
FIGURE 12.1 Damage of membrane elements resulting from clogging of bundles or spacers: a) Telescoping; b) Squeezed element; and c) Channelling (Source: Schippers et al. 2019).
which requires a higher pressure to maintain the capacity; ii) damage to elements because of telescoping in spiral-wound; channelling in spiral-wound; squeezing spiral-wound; and breaking fibers as shown in Figure 12.1.
The fouling and scaling could also result from local clogging, which causes uneven flow distribution, resulting in locally high conversion and high concentration polarization; deposition of particles; local precipitation of sparingly soluble compounds; growth and attachment of bacteria.
Increase Hydraulic Resistance of the Membrane
The increase in hydraulic resistance of the membrane mainly results from deposition and/or adsorption of material and/or bacterial growth (biomass) on the membrane surface that results in higher required pressure to maintain capacity, or lower capacity when pressure is not increased.
Decrease in Rejection Owing to Concentration Polarization in the Foul Layer
Because of fouling and scaling, concentration, polarization increases when cross- flow velocity close to the membrane decreases, mainly because of uneven flow distribution and foul layers. Consequently, the dissolved salts and organic compounds, colloidal matter, and suspended matter accumulate at the surface of the membrane. The sparingly soluble compounds might precipitate and the rejection of the salts may decrease because of higher concentration at the membrane surface.
To overcome membrane fouling, membrane manufacturers recommend cleaning in place (CIP). The general criteria to apply CIP are when:
• Normalized differential pressure (feed pressure - concentrate pressure) has increased by 15%.
In the market, there is a wide range of chemicals that are used for CIP; while using these chemicals, compatibility with the membrane must be secured.
Fouling can be classified into four types: particulate fouling, organic fouling, biological fouling, and inorganic fouling (scaling)
Particulate/colloidal fouling is caused by the accumulation of particles (> 1pm) and colloids (0.001-1 pm) on the membrane surface. These particles and colloids can be inorganic (e.g., clay minerals, colloidal silica, aluminum, iron, and manganese oxides) or organic (e.g., large polysaccharide molecules, fulvic compounds, extracellular polymer substances (EPS), transparent exopolymer particles (TEP), and proteins). Fouling potential of these foulant matters depends on several factors: feed water compositions (e.g., foulant type and concentration, pH, ionic strength), membrane properties (e.g., roughness, charge, hydrophobicity, surface functional group) and hydrodynamic conditions (e.g., flux, cross- flow velocity, recovery, temperature, module, and spacer design) (Tang et al. 2011).
In general, there are four mechanisms of particulate fouling, as shown in Figure 12.2.
FIGURE 12.2 Mechanisms of particulate fouling: a) complete blocking; b) standard blocking; c) intermediate blocking; d) cake formation/filtration.
Initially, particles/colloids begin to deposit on the membrane surface, blocking the membrane pores. This initial phase is known as pore blocking, which may entail plugging of pores (complete blocking), constriction of pores because of deposition of particles around pores entry (standard blocking), or a combination of the previous two (intermediate blocking). The next stage of particulate fouling includes the development of a cake layer on the membrane surface as additional particles continue to deposit on the initial layer. Once the cake starts to form, the cake layer takes over the role of the membrane performance and controls the transport and removal process (Pearce 2007). Since RO membranes are considered non porous, unlike MF and UF membranes, the dominant particulate fouling mechanism in RO is the cake formation (Zhu and Elimelech 1997).
Henry et al. (2012) described the mechanisms of particulate fouling as a result of a combination of four elementary phenomena (Figure 12.3), depending on the particle - particle, particle - fluid and particle - surface interactions.
Various methods were developed by researchers to assess the particulate fouling potential. However, silt density index (SD1) and modified fouling index (MFI045) are the most common methods applied for this purpose; both indices are standard testing methods in the ASTM.
Silt Density Index (SDI)
Silt density index (SDI) is commonly applied as a parameter for the fouling potential of feed water for reverse osmosis and nanofiltration plants. Silt density index is determined by measuring the rate of plugging of a 0.45 pm membrane filter at 210 kPa (30 Psi) according to the standard protocol of ASTM (ASTM 2014). SDI measures the decline in filtration rate expressed in percentage flux decline per minute. The following steps need to be taken while measuring the SDI.
FIGURE 12.3 Mechanism of particulate fouling described by Henry et al. (2012).
i. Time /] required to filter the first 500 mL is determined.
ii. Fifteen minutes (//) after the start of the first volume filtration time l2 needed to filter, the second 500 mL is determined.
iii. SD1 is calculated using equation 12.1
Shorter time (tf) has to be considered, such as 10, 5, or 3 minutes, if the plugging ratio (%P) exceeds 75%.
Despite the main advantage of the SDI method represented in its simple performance, the method has some deficiencies: i) it does not take into account the fouling mechanisms; ii) it has no linear correlation with particulate matter concentration; and iii) it has no temperature correction.
Modified Fouling Index (MFI0.45)
The modified fouling index (MFI0.45) was developed by Schippers and Verdouw (1980) to overcome the deficiencies of SDI. MFI is based on the cake filtration mechanism that occurs during a distinct period of the test. At constant pressure, cake filtration can be described by Equation 12.2.
V - filtrate volume (L or nr3) t - filtration time (s)
A - membrane area (nr)
AP - applied pressure (Pa) i] = water viscosity (Pa.s)
Rm - clean membrane resistance (nT1)
/ = fouling index (nT2)
The MFI0.45 is calculated from the slope of t/V versus V plot, and is based on the correction standard testing conditions proposed by Schippers and Verdouw (1980), as in Equation 12.3.
where r/w is the water viscosity at 20°C; AP„ represents the standard pressure (200 kPa); and A„ is the standard membrane area (13.8 x 10 4 nr).
Modified Fouling Index (MFI - UF)
MFI (0.45) and SDI fail to predict the rate of fouling in RO membranes as they make use of membranes with pore size of 0.45pm. while the particles that are smaller than 0.45 pm are more likely to be responsible for fouling in RO membranes (Schippers et al. 1981). To overcome this, the MFI - UF method has been developed using ultrafiltration membranes to capture the small colloids. MFI - UF was performed initially at constant pressure as the standard MFI045. It was verified that MFI - UF demonstrates a linear correlation with colloidal matter. However, it was found that the cake formed in the MFI - UF test performed at constant pressure is highly compressible, and consequently leads to overestimated measurements. In addition, the test period is relativity long, possibly several hours in duration (Boerlage et al. 1997, 1998, 2000a, 2002b, 2003a, 2003b). Accordingly, Boerlage et al. (2004) further developed the MFI - UF to operate at constant flux instead of constant pressure.
MFI-UF at Constant Flux
MFI-UF is measured at constant flux through UF membranes with pore size reduced to 10 kDa according to the protocol developed by (Boerlage et al. 2004) and modified by (Salinas-Rodriguez et al. 2015). At constant flux (J), cake filtration taking place during MFI - UF measurement can be expressed by the general Equation 12.4.
Fouling index (I) can be calculated from the slope of cake filtration region in the plot of P versus t (shown in Figure 12.4) using Equation 12.5. The MFI - UF value can be then calculated by correcting I for the standard testing conditions as in Equation 12.6.
It turned out that the MFI - UF constant flux depends, more or less proportional, on flux (Salinas-Rodriguez, et al. 2012). The MFI - UF can currently be measured accurately at the lowest flux of 50 L/nr.h; however, the flux in brackish RO is in the range of 20 to 30 L/m2.h. and in seawater RO systems in the range of 10-20 L/m2.h. Therefore, the MFI-UF needs to be measured at higher flux rates and then extrapolated to simulate the corresponding flux applied in the RO system to be assessed. Particulate fouling rate can be then predicted in terms of the time required for an increase in net driving pressure (NDP), using the prediction model shown by Equation 12.7.
where Q is the particle deposition factor, incorporated in the model to simulate the portion of particles, present in the water passing the membrane, depositing on the RO membrane during the cross-flow filtration. Q can be calculated based on the operating recovery (R) and the relation between the MFI of concentrate water and the MFI of feedwater, as shown in Equation 12.8. Q value may vary between 0 and 1, indicating 0% to 100% particle deposition.
Transparent Exopolymer Particles (TEPs)
TEPs are transparent, seasonally abundant organic substances (e.g., algal blooms) in marine and freshwater environments (Passow 2002). Common bloom-forming algae produce algal organic matter (AOM) that are comprised of high-molecular weight biopolymers (polysaccharides and proteins), and often include the sticky transparent exopolymer particles (TEPs) (Villacorte et al. 2013). TEPs can be present in different shapes (e.g., strings, disks, sheets, or fibers) and in different sizes, ranging from a few nanometers in diameter up to hundreds of micrometers in length (Passow 2002, Villacorte et al. 2015). They are sticky and comprise mainly hydrophilic, negatively-charged, acidic polysaccharides. In reverse osmosis membranes, organic fouling often occurs when sticky microbial-derived biopolymers (e.g., transparent exopolymer particles) are abundant in the RO feedwater. The accumulation of such materials may result in a substantial decrease of normalized flux in RO membranes. Strong correlations between TEP produced by several algal spices and MFI- UF lOOkDa has been demonstrated showing high particulate fouling potential. (Dhakal et al. 2019; Villacorte et al. 2014). Furthermore, it may further initiate or enhance biological fouling as they can serve as a conditioning layer,
which is an ideal attachment sites for bacteria - a platform where bacteria can effectively utilize biodegradable nutrients (С, P, N) from the feedwater while excreting more extracellular substances, resulting in rapid buildup of biofilm. Moreover, the accumulated sticky substances may enhance the deposition of other colloids/particles from the feedwater to the membrane/spacers and may further aggravate fouling problems. TEP can be measured according to the protocol described by Villacorte et al. (2015).