Membrane scaling calcium carbonate

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material. 

Adsorption of color onto an RO membrane is favored when the compounds are hydrophobic or positively charged. As with other organics, a high pH (>9) helps to minimize fouling with color, but causes other concerns, including calcium carbonate scaling. 

Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP < Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as ... 

Besides calcium carbonate, there are three other calcium-based compounds that will scale RO membranes. These compounds are calcium sulfate, phosphate, and fluoride. Although there are no specified feed water guidelines for these compounds, they are worth investigating. 

Calcium phosphate has become a common problem with the increase in treatment of municipal waste-water for reuse. Surface waters can also contain phosphate. Calcium phosphate compounds can contain hydroxyl, chloride, fluoride, aluminum, and/ or iron. Several calcium phosphate compounds have low solubility, as shown in Table 7.2. Solubility for calcium carbonate and barium sulfate are also shown by comparison. The potential for scaling RO membranes with the calcium phosphate compounds listed in Table 7.2 is high and will occur when the ion product exceeds the solubility constant. This can occur at orthophosphate concentrations as low as 0.5 ppm. Sodium softening or antisealants together with low pH help to control phosphate-based scaling. 

Figure 11.4 depicts a cross-section of a membrane with a layer of calcium carbonate scale on the surface. The concentration of calcium at the membrane surface, Z, is higher than that in the bulk feed, X, since the concentration at the surface has reached saturation. The membrane passes salts based on what concentration is actually next to the membrane. In this case, the membrane is exposed to a saturated concentration, not the lower bulk solution concentration. Even though the percent passage of calcium through the membranes stays constant, the scaled membrane will yield higher permeate concentration of calcium. This is because the concentration of calcium that the membrane is exposed to at the membrane surface is higher than the bulk solution concentration of calcium, [Z],[X]. 

Low-pH cleaners are typically used to address calcium carbonate scale and iron oxide deposition. These cleaners are usually formulated using only acid, such as acetic, hydrochloric, or sulfamic. Figure 13.5 shows the effects of temperature and pH on the removal of calcium carbonate from a membrane.7 As the figure shows, lower pH, and higher temperatures are more effective at restoring permeate flow than higher pH and lower temperatures. 

Fortunately, calcium carbonate scale does not always precipitate immediately when the solubility product is exceeded. A considerable degree of supersaturation can be tolerated for a while. In fact, the method of contact stabilization (II) for preventing scale in sea water stills is based on this fact. If supersaturation conditions near the membrane are not allowed to prevail long enough, scale will not form there. Thus it becomes very important to study the process of... 

Bicarbonate (HC03 ) results from the chemical decomposition of dissolved CO2 with the hydroxide ion in water. A portion of the bicarbonate present in the feed water can be converted to carbonate as a result of pH changes owing to concentration of salts on the concentrate side of the membrane. Such cases require the addition of acid or a crystal growth inhibitor to avoid calcium carbonate scaling. 

Scaling of membranes by pH-sensitive electrolytes occurs at anion-exchange membranes because OH ions transfer through the membranes when water splitting occurs. The OH ions increase the pH within the membrane and at the interface on the concentrating side so that pH-sensitive substances, such as calcium carbonate, precipitate. 

NACE Test Methods for Determining Water Quality for Subsurface Injection Using Membrane Filters (TM0173) describes methods for evaluating water quality of injection water, NACE Test Methods for Laboratory Screening Tests to Determine the Ability of Scale Inhibitors to Prevent the Precipitation of Calcium Sulfate and Calcium Carbonate from Solution (for Oil and Gas Production Systems) (TM0374) is used to rank order scale inhibitors at 158°F (70°C). 

Conflicting results on tlie effect of flow rate or fluid velocity on scale deposition have been reported by Andritsos and Karabelas [21]. They showed that the flow rate either increased or decreased the mass of scale deposited over a certain period of time. Clearly, this is due to the nature of the solid-fluid interface layer and the shear sti ess induced to the scale by the flow. These two incompatible forces are further complicated by other parameters such as the presence of additives, which may exert a significant effect on the nature of the interface layer as well as on the characteristics of the scale deposited [26]. Fluid velocity also affects the orientation of the growing scale or crystals [2, 27, 28]. The tendency is that the scale to orient to the direction of the fluid flow [2]. A rather recent study on gypsum and calcium carbonate scaling of membrane desalination by He and co-workers [29] shows that a faster flow rate increases more deposition of gypsum scale. 

For RO applications, a positive LSI or SDSI indicates that the influent water has a tendency to form calcium carbonate scale. In these cases, pre-treatment in the form of softening (either with lime or ion exchange), or via the use of antisealants and/or acid is required. Note that most membrane manufactures recommend an LSI of +1.8 or lower in the concentrate with antisealant feed to control scaling. 

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