CHAPTER 3: Theory
3.1 REVERSE OSMOSIS AND NANOFILTRATION
Separation mechanisms
Reverse osmosis and nanofiltration make use of slightly different mechanisms that allow water to pass through the membrane while solutes are rejected when pressure is applied to the system. Reverse osmosis membranes do not have distinct pores (Greenlee et al., 2009) and therefore have much slower permeation than NF membranes (Van der Bruggen et al., 2003). Solute-liquid separation is governed by solution diffusion whereby water molecules are absorbed into the membrane surface and then they permeate through the membrane by diffusion through the nonporous surface layer (Mazid, 1984).
Nanofiltration makes use of two mechanisms for solute rejection: steric hindrance (sieving) and the Donnan potential (charge effects) (Bergman, 2007). The sieving separates compounds based on their size in relation to the membrane's pore size, which is characterized by a molecular weight cut-off or the molecular weight of a solute that will be 90% retained by the membrane. For NF membranes, this is 250-2000 Daltons (Greenlee et al., 2009). The Donnan potential is an electrical potential phenomenon created as negative ions are repelled by the surface of the membrane (Greenlee et al., 2009). The electric potential between the membrane and the bulk solution allows ions to be rejected even though they are smaller than the NF membrane pore sizes (Van der Bruggen et al., 2003). Nanofiltration typically has a high rejection of divalent ions but a poor rejection of monovalent ions as the rejection of divalent ions is determined by Donnan exclusion, whereas because monovalent ions have weak electrostatic repulsion their rejection is controlled by steric hindrance (Suhalim et al., 2022).
Fundamentals of RO and NF operation
This section describes the equations used to determine the efficiency of RO and NF systems. Water recovery is typically defined as the volume percentage of the influent recovered as permeate (Kucera, 2015). However, as the aim of this study was to concentrate urine rather than recover water it shall be referred to as ‘water removal’. The concentration or volume reduction factor is the ratio between the initial and final (concentrate) volume.
Permeate flux is described by Equation 3-1 and is usually measured as the permeate flow rate per membrane area. The water transport coefficient is unique for each membrane and it can vary with pH and temperature (Kucera, 2015). Water permeation will only occur when the feed pressure exceeds the solution's osmotic pressure. The driving force for flux is the difference between the applied pressure and solution osmotic pressure. For thermodynamically ideal solutions, the osmotic pressure is described by Equation 3-2, (Greenlee et al., 2009). As the solution is concentrated, the osmotic pressure increases, resulting in a decrease in flux. Whilst diffusion of solutes through the membrane is possible, the mass
Rejection is used to describe the percentage of an influent solute that is retained by the membrane (Equation 3-4). Solution rejection increases with increasing applied pressure (Bergman, 2007). Solution flux increases with applied pressure; however, solute flux remains relatively constant. This results in an increased calculated solute rejection because progressively more solution permeates relative to the salt (Kucera, 2015). Concentration polarization is a phenomenon that can result in decreased rejection. It is defined as the occurrence of increased solute concentration at the membrane surface relative to the bulk solution (Bergman, 2007).
Flux:
J* = K*(∆P − ∆π) =+,! (Equation 3-1) Where:
Jw = Permeate flux (L m-2 hr-1)
Kw = water transport coefficient (L m-2 bar-1)
ΔP = the pressure difference across the membrane (bar)
Δπ = the osmotic pressure difference between the feed and permeate (bar)
Osmotic pressure:
π = CRT (Equation 3-2)
Where:
C = the total ion concentration (mols L-1) R = the ideal gas constant (L bar mol-1 K-1) T = the solution temperature (K-1)
Dissolved solute flux:
J- = K-(C.− C/) (Equation 3-3) Where:
Ks = solute mass transfer coefficient (m hr-1)
Cm = the solute concentration at the membrane surface (g L-1) Cp = the solute concentration in the permeate (g L-1)
Rejection:
R0 = 51 −C0,/
C0,2
7 8 × 100% (Equation 3-4) Where:
Cx,p= the concentration in the permeate (g L-1) Cx,f= the concentration in the feed (g L-1)
Fouling and scaling
RO and NF membranes can often show a decline in productivity over time. This can be attributed to either fouling or scaling because of the deposition of solids on the membrane surface. During the concentration process, inorganics precipitate out of the solution and deposit on the membrane forming a ‘scale’ (Kumar et al., 2006). Multiple mechanisms cause fouling including; pore clogging, adsorption of feed components, chemical interaction between the membrane and solutes in the feed, bacterial growth, and gel formation (Goosen et al., 2005). Potential fouling and scaling components include suspended solids (organic & inorganic), dissolved organic matter, dissolved solids (precipitation of sparingly soluble salts as concentration increases), and biological organisms (Amiri and Samiei, 2007).
Upen et al. (2000) advise that bacterial adhesion is one of the most serious forms of fouling and can be very difficult to remove. The potential for fouling can be reduced in several ways such as pre-treatment, changing operational variables, and cleaning.
Pre-treatment: Kumar et al. (2006) observed that particulate matter greater than 1 µm caused the most fouling on high pressure SWRO membranes. However, they further noted that pre-treatment using 0.1 µm microfiltration had the most significant impact on fouling reduction. For stable filtration performance, Franks et al. (2009) advise a suspended solids concentration of less than 0.5 ppm.
Operational variables: Increasing the feed velocity has also been shown to reduce the potential of scaling/ solute build-up by increasing turbulence at the membrane surface (Amiri and Samiei, 2007).
Increased turbulence and flux can also be achieved by increasing feed spacer thickness (Sablani et al., 2002). Franks et al. (2009) found that with increasing feed spacer thickness, cleaning-in-place (CIP) requirements were reduced. An antiscalant can be used to increase the concentration at which scaling components precipitate (Bergman, 2007).
Cleaning: In the case that it is too late for fouling prevention, it is possible to remove any fouling or scaling with a chemical CIP, which works by dissolving, dislodging, or breaking down the fouling/scaling component (Bergman, 2007).