Sodium softeners

Iron and manganese can be removed from RO feed water using sodium softening or iron filters (see Chapters 8.1.6 and 8.1.5, respectively). In some cases, it may be desirable to operate with soluble iron and/or manganese through the RO system, if the entire system can be kept air tight to prevent oxidation of the metals into suspended solids. 

Barium and strontium can be reduced in RO feed water using sodium softening (see Chapter 8.1.6). Antisealant can be used to control or inhibit scaling without reducing the concentration of either species (see Chapter 8.2.3). 

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. 

Calcium fluoride scale can form when the concentration of fluoride is as low as 0.1 ppm if the concentration of calcium is high. Scaling will occur when the ion product exceeds the solubility constant. Antisealants or sodium softening can be used to control calcium fluoride scale. 

As described previously, calcium can be removed or reduced in RO feed water using sodium softening or lime softening (see Chapters 8.1.6 and 8.3, respectively). 

Sodium softeners are used to treated RO influent water to remove soluble hardness (calcium, magnesium, barium, and strontium) that can form scale on RO membranes. Once known as sodium zeolite softeners, zeolites have been replaced with synthetic plastic resin beads. For sodium softeners, these resin beads are strongly acidic cation (SAC) polystyrene resin in the sodium form. The active group is benzene sulfonic acid, in the sodium, not free acid, form. Figure 8.12 shows styrene-divinylbenzene gel cation resin. Equation 8.4 shows the softening reaction for calcium exchange ... 

Figure 8.13 Sodium softening reaction for calcium exchange.

Theoretically, hardness in the effluent from a sodium softener can be less than 1.0 ppm as CaC03. In practice, effluent quality from a sodium softener is dependent on the influent water quality. Higher influent hardness leads to higher effluent hardness. The total dissolved solids (TDS) concentration in the effluent from the softeners should be equivalent to that of the influent water since no TDS is removed through the softener instead sodium replaces hardness in the treated stream. The effluent will have a much higher concentration of sodium and lower concentration of hardness than the influent has. 

For optimal operating of the sodium softener, the feed water to the system should adhere to these guidelines ... 

Cold lime and lime-soda softening is conducted in a solids-contact clarifier (see Chapter 8.1.1.). Cold lime softeners are typically followed by filtration and sodium softening to remove the balance of hardness from the water. 

Sodium softener The softener removes hardness and any residual soluble iron from the RO feed water. 

Sodium softening is used to remove soluble hardness from water, including calcium, magnesium, barium, and strontium. As discussed in Chapter 8.1.6, sodium softeners are commonly used to pre-treat RO feed water to reduce the potential for scaling the membrane with hardness scales. In the next two sections, the placement of the sodium softener, either before or after the RO system, as well as the use of sodium softeners versus antisealants are discussed. 

Traditionally, sodium softeners have been used as pretreatment to RO. Sodium softeners remove hardness and metals, such as iron and manganese, that scale, foul, or degrade RO membranes. Anecdotally, softeners are also used to help reduce suspended solids and SDI from surface or other highly fouling feed waters prior to RO. The sodium softener, in essence, acts as another barrier in front of the membrane. 

The disadvantage with using sodium softening as RO pretreatment is that the softener must treat not only the permeate volume but also the volume of water that will become the reject. In other words, the softener must be large enough to treat the entire feed volume to the RO. This brings up two issues ... 

These two issues have prompted several users to move or consider moving the sodium softener from in front of the RO to after the RO, to polish the RO permeate. Post treatment of RO permeate is often necessary because the RO does not reject 100% of the hardness in the feed water. And, since the feed water is not softened, the concentration of hardness in the RO effluent will be higher than if the feed water were softened prior to the RO. Depending on the application of the permeate, polishing with a softener to remove hardness may be required. 

The sodium softener is only treating the RO permeate, typically about 75% of the feed flow rate. 

A polishing sodium softener can operate at a higher service flow rate than a primary softener. Instead of being limited to 6 - 8 gpm /ft2, a polishing softener... 

One 84-inch diameter vessel will generate about 1,700 gallons of 10% brine waste. About 44,200 gallons of chloride-free water will be required to dilute the chloride to meet discharge limits, or only about 60% of that required when the sodium softener was located in front of the RO. 

The choice of sodium softening or antisealant is specific to each application. It is very difficult to make a blanket statement that one technology is better than the other, since both are effective at minimizing the potential for scaling RO membranes. Some designers prefer using sodium softeners because of the additional barrier they provide in front of the RO. Others prefer antisealant to eliminate the need to handle brine waste (see Chapter 15.1.1). 

For practical purposes, it generally makes sense to use sodium softeners for RO feed flow rates of less than about 20 gpm rather than conventional, flow-proportional control difficulties in feeding based on flow control are exacerbated at low flow rates because of limited turn down on proportioning chemical feed pumps at low flow rates. However, when Nalco Company s 3D TRASAR technology is used, antisealant can be fed to RO systems operating feed flow rates as little as 3 gpm.2... 

Table 15.1 Operating costs considerations for sodium softeners and antisealant feed systems.

Operating Cost Issue Sodium Softeners Antisealant Feed System... 

Table 15.1 lists operating cost issues for sodium softeners and antisealant feed systems. The three largest expenditures listed in Table 15.1 are the salt and resin amortization for the sodium softener options and the antisealant itself for that option. To provide examples of these costs in greater detail, consider the following cases. 

The sodium softener selected to treat this water is a duplex, 150-gpm system with 54-inch diameter by 72-inch side sheet vessels. Each vessel contains 50ft3 of resin. Since the duplex system operates with one vessel on line and one vessel in stand-by, the total system regenerates 2.4 times per day. At a salt dosage of 15 pounds per cubic foot, the system uses about 1,800 pounds (820 kg) of salt per day or 27.4 tons per month, assuming a 100% operating factor. At a salt cost of 80 per ton, the total month cost for salt is about 2,200.3... 

The softener is a 48-inch diameter unit with 35ft3 of resin. The softener will regenerate once per day with 151b/ft3 of salt. At a salt cost of 80 per ton, the sodium softener would require about 638 per month to operate. 

Concern With Sodium Softener Without Sodium Softener ... 

For RO pretreatment, NF is typically used to pre-soften and reduce color from RO feed water (when appropriate NF membranes are employed). Nanofiltration replaces sodium softening (for hardness removal) and augments clarification (for color removal). 

In many cases, additional pretreatment is required prior to the HERO process. Additional pretreatment can include cold lime softening, sodium softening, and ultrafiltration or microfiltration. These pretreatment unit operations are required when the total hardness is very high and/or the concentration of suspended solids is high. 

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