Scaling in Reverse Osmosis Systems

Scaling in reverse osmosis refers to the precipitation of sparingly soluble inorganic compounds present in the feedwater onto the membrane surface and feed spacer.

Precipitation usually occurs when the ionic product of a certain salts exceeds the solubility product (Antony et al. 2011). Therefore, scaling is directly related to the concentrations of inorganic ions in the feedwater and to the recovery (ratio of the permeate water to the feedwater) of the RO system. At higher recovery, the concentrate water in the last stage becomes more concentrated and, therefore, in many cases, exceeds the solubility limit for several types of salts, resulting in scaling (Kucera 2015). Therefore, the concentration factor is calculated as in Equation 12.9:

where, C_c = concentration in concentrate (brine); and C_f = concentration in feed water

Concentration factors depend upon the recovery of the systems as in Equation 12.10

where R = system recovery; and f = rejection when f= 100%

Scaling is considered as one of the major challenges of RO application, and it is one of the main factors that limit system recovery in RO. The consequences of scaling are permeability decline of the membrane, increase in pressure drop, increase salt passage, and shorter lifetime of membranes resulting from frequent cleanings. For a scaled membrane, higher than normal operating pressure is required to produce permeate water, which results in higher operational costs (Kucera 2015).

Scaling Compounds on the RO Membranes

Several types of scaling such as calcium carbonate, calcium sulphate, barium sulphate, calcium phosphate, calcium fluoride, strontium sulphate, and silica may occur on the membrane surface.

Calcium Carbonate Scaling

Calcium carbonate is one of the most common scales encountered in RO applications (Kucera 2015). The formation and degree of CaC0₃ scaling mainly depends on the concentrations of calcium, bicarbonate and pH in the feedwater, as well as the recovery of the RO (Tzotzi et al. 2007. Antony et al. 2011). Other factors which affect the precipitation of CaC0₃ are temperature, TDS and the presence of inorganic ions and organic substances.

In the literature, six forms of CaC0₃ scale deposits are reported to exist, depending on the experimental conditions and presence of foreign substances (impurities), such as three anhydrous forms (calcite, aragonite, and vaterite), two hydrated forms (calcium carbonate monohydrate and calcium carbonate hexahydrate) and one amorphous calcium carbonate (Brecevic and Nielsen 1989, Chakraborty et al. 1994, Elfil and Roques 2001, Coleyshaw et al. 2003).

Calcium Sulphate Scaling

Calcium sulphate is one of the common specie of the nonalkaline scales encountered on the RO membrane surface (Antony et al. 2011). The CaS0₄ precipitation can occur when the ionic product of the Ca²⁺ and S0₄²' ions exceeds solubility product (K_sp) according to the following reaction Equation 12.12:

where x can be 0, 'A, or 2, based on different forms of calcium sulphate.

The calcium sulphate scale can occur in three different forms: gypsum (CaS0₄-2H₂0), hemihydrate (CaS0₄T/2H₂0) and anhydrite (CaS0₄), where gypsum is the commonly encountered form at ambient temperatures of 20°C (Lee and Lee 2000, Antony et al. 2011).

Barium Sulphate Scaling

Barium sulphate precipitation results in very hard deposits on the membrane surface (Boerlage et al. 2000b). The solubility of the barium sulphate is very low (lxi O'⁵ rnol/L or 2.33 rng/L in pure water) (Van der Leeden 1991). Therefore, concentrate water at very low recoveries can be supersaturated with respect to barium sulphate. It is worth mentioning that precipitation is not only governed by the supersaturation, but also depends on the precipitation kinetics that involve the formation of nuclei and further crystal growth. Boerlage et al. (2002a) reported that BaS0₄ has a long stable phase prior to nucleation in the supersaturated state. Consequently substantial supersaturation is allowable.

Calcium Phosphate Scaling

Calcium phosphate scaling can occur on the membrane surface when high concentrations of calcium and orthophosphate ions are present in the feed water (Greenberg et al. 2005, Chesters 2009). Calcium phosphate can precipitate in various forms including but not limited to such as amorphous calcium phosphate, dicalcium phosphate dihydrate (CaHP0₄-2H₂0), dicalcium phosphate anhydrous (CaHP0₄), octacalcium phosphate (CasH₂ (P0₄)₆-5H₂0), tricalcium- phosphate (Ca₃ (P0₄)₂), and hydroxyapatite (Ca₅ (P0₄)₃0H) (Dorozhkin 2017). Calcium phosphate scaling is, in particular, important in waste water treatment due to the rather high phosphate concentrations.

Mechanism of Scaling in Reverse Osmosis

Scale formation is a complex process in which both crystallization and hydro- dynamic transport mechanisms are involved (Oh et al. 2009). There are two types of crystallization identified as heterogeneous (also called surface crystallization) and homogeneous (bulk crystallization). Surface crystallization refers to the formation of crystals on the surface of the RO membrane, while in the case of the bulk crystallization process, crystals form in the bulk solution and then precipitate on the membrane surface. The mechanism of crystallization is illustrated in Figure 12.6.

Prediction of Scaling

The commonly applied method to determine the scaling potential are briefly described in the following sections.

Scaling Indices

There are a number of indices available to measure the scaling tendency of the sparingly soluble salts in a water solution. The most commonly used in RO applications are:

Saturation index (SI): Saturation index is calculated by subtracting the logarithm of the thermodynamic solubility product (Log K_sp) from the logarithm of the ion activity product (Log IAP) as in Equation 12.13.

FIGURE 12.6 Scale formation mechanism in RO system. (Source: Oh et al. 2009).

when

SI = 0, the concentrate is in equilibrium SI > 0, the concentrate is supersaturated SI < 0, the concentrate is under saturated

Supersaturation ratio (S_r): Supersaturation ratio can be calculated by dividing the square root of ion activity product (IAP) by the square root of the thermodynamic solubility product (K_sp) as in Equation 12.14

when

S_r= 1, the concentrate is in equilibrium S_r > 1, the concentrate is supersaturated S_r < 1, the concentrate is under saturated

Scaling Prediction with Computer Software

A number of commercial programs are available which can be used to predict the scaling potential in RO. Most of these programs are developed by antisealant suppliers and membrane manufacturers. The programs include but not limited to:

• • Genesys Membrane Master (MM4) - Genesys International
• • Sokalant RO-Xpert-BASF
• • Hyd-RO-dose - French Creek Software
• • Argo Analyzer-Suez
• • Avista Advisor - Avista Technologies, Inc.
• • Proton-American Water Chemicals
• • WAVE-DuPont membrane projection software
• • IMSDesign- Hydranautics membrane projection software

Techniques to Control Scaling

To prevent scaling in RO applications, various chemical, physical, and mechanical approaches have been proposed, which can be summarized into three groups: optimization of operating parameters and system design; altering feed- water characteristics; and addition of scale inhibitors (Antony et al. 2011).

Addition of Scale Inhibitors

Scale inhibitor (antisealant) additions to feedwater is one of the most widely used and an effective techniques to prevent scaling in RO applications (Pervov 1991, Lee et al. 1999, Greenlee et al. 2010). A number of commercial antisealants are available that are designed for specific types of scale. The commonly used antisealants in RO applications can be categorized in three different groups based on their compositions and properties, and include: polyphosphates, phosphonates/ organophosphates, and poly acrylates/polymaletes (Antony et al. 2011, Van Enge- len and Nolles 2013). A factor contributing to the attractiveness of antisealants is the low-dose requirement to overcome scaling problems (Antony et al. 2011).

Operate at Low Recovery

Lowering RO recovery is also a scaling control technique, though it is not a desirable one as this technique leads to a high concentrate production and specific energy consumption (kWh/т³). In this method, the recovery of the RO is decreased to an extent where the sparingly soluble salts remain undersaturated. In addition, the effect of concentration polarization is also diminished with this approach.

Acid addition

Addition of acid is one of the earliest techniques to prevent the precipitation of calcium carbonate. The solubility of calcium carbonate is increased when pH of the water is lowered. Acidification is also not an attractive approach since massive amount of acid is needed to lower the feedwater pH when the concentration of bicarbonate is high.