Sodium softening pretreatment

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 ... 

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. 

Table 15.3 Projected operating costs for an RO pretreatment system with and without sodium softener. 

A polishing sodium softener can operate at a higher service flow rate than a primary softener. Instead of being limited to 6-8 gpm/ft, a polishing softener can operate at 10-15 gpm/ ft. The same 500-gpm RO system that requires two 120-inch diameter softener vessels for pretreatment, would only require two 84-inch diameter vessels to post-treat the RO permeate. 

Where hardness removal is required, the simplest pretreatment method for smaller, lower pressure boiler plants (below 200-300 psig) is to use a cation-exchange softener. This removes the calcium and magnesium at source and converts the bulk of temporary hardness salts into sodium bicarbonate (NaHC03), which decomposes to form sodium carbonate (soda ash) but does not scale under normal boiler operating conditions. 

Where sodium bicarbonate is present (as when MU water pretreatment includes a water softener), given enough time and temperature it decomposes to form hydrate alkalinity, carbon dioxide, and water ... 

Maximum values of specific conductance are often not achievable without exceeding maximum T alkalinity values, especially in boilers below 900 psig (6.21 MPa) with greater than 20.0% MU water whose alkalinity is >20% of TDS naturally or after pretreatment by lime-soda or sodium cycle ion exchange softening. Actual permissible conductance values to achieve any desired steam purity must be established for each case by careful steam purity measurements. The relationship between conductance and steam purity is affected by too many variables to allow its reduction to a simple list of tabulated values. 

Scale prevention methods include operating at low conversion and chemical pretreatment. Acid injection to convert COs to CO2 is commonly used, but cellulosic membranes require operation at pH 4 to 7 to prevent hydrolysis. Sulfuric acid is commonly used at a dosing of 0.24 mg/L while hydrochloric acid is to be avoided to minimize corrosion. Acid addition will precipitate aluminum hydroxide. Water softening upstream of the RO By using lime and sodium zeolites will precipitate calcium and magnesium hydroxides and entrap some silica. Antisealant compounds such as sodium hexametaphosphate, EDTA, and polymers are also commonly added to encapsulate potential precipitants. Oxidant addition precipitates metal oxides for particle removal (converting soluble ferrous Fe ions to insoluble ferric Fe ions). 

A CEDI system can produce up to 18-megohm-cm water at 90-95% water recovery. Recovery by the CEDI system is a function of the total hardness in the feed water to the system. In general, 95% recovery can be realized at a feed water hardness of less than 0.1 ppm as calcium carbonate.16 This is typically attained if the pretreatment to the CEDI consists of either 2-pass RO or sodium-cycle softening followed by RO. Recovery that is achievable is a function of the feed... 

To prevent these failures, constant monitoring of the pretreatment system is necessary. Alarms should be installed on critical systems, such as the ORP associated with the sodium bisulfite feed. Particle monitors could be used to detect channeling or carry over through filters. Hardness analyzers with alarm should be installed on the effluent from softeners. 

The analysis of solvent and perfumes in formulations is performed by GC. For solvent analysis in surfactants sample pretreatment with organic solvent is used for the separation of inorganic impurities. Regarding surfactants and complementary compounds, applications may be mentioned such as the separation of a hardness agent (sodium laurate) in liquid laundry detergent by GC and the Hofmann degradation and analyses of cationic surfactant of the alkyltrimethyl- and dialkyldimethylammonium type in fabric softener and hair rinse. 

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