Lime softening membranes

Bergman, R.A.(Nov. 1995) Florida - A Cost Comparison Update, Membrane Softening vs. Lime Softening. International Desalination and Water Reuse. 

Softening refers to removing calcium and magnesium hardness by chemicals. However, sihea, alkahnity and other constituents are also removed during lime softening. Other water softening methods include ion exchange and NF membrane separation. 

The solubihty of sihca is a function of pH and temperature. Lime softening and lime plus soda ash softening are most effective in removing sihca. Other options are process related (a) run the RO system at reduced recovery, (b) increase the feed water temperature - sihca solubihty increases with temperature, and (c) use silica inhibitors. Colloidal silica is difficult to remove by IX because it is not ionised, and can foul the resins when the levels are high. Colloidal silica flocculates easily in boundary layers resulting in severe fouhng. It can be removed by UF membranes with a MW CO of up to 100,000 Da. Since the solubihty of the sihca increases below a pH of about 6.0 and above a pH of about 9.0, the actual solubihty of sihca in the concentrate stream is further affected by the pH of reject water. 

Nanofiltration softening membranes may be an economic alternative to conventional softening. Possible advantages of membrane filtration for softening include smaller space requirements, no lime requirement, superior quality water, and less operator attendance. As for all membrane separation processes, suitability of softening membranes for a given apphcation must be made on a case-by-case analysis. 

Bergman, R.A. (1995) Membrane softening versus lime softening in Florida a cost comparison update. Desalination, 102,11-24. 

Fouled membrane used in MF of wastewater was visualized with the ESEM technique to assess the effectiveness of lime-softening pretreatment prior to filtration. ESEM revealed that the fouling layer with pretreatment process was more porous, thus enhancing the membrane filterability [50]. In another study, the suitability of SEM and ESEM were compared to visualize colloidal materials on the membrane surface [42]. In this case, ESEM visualization was preferred due to its ability to perform observation in hydrated states. However, this technology has also been used in a limited way in MBR study, due to its lower image resolution than SEM. 

O M) costs were lower than for membrane softening, but the relative difference in costs decreased with larger facihties from a factor 2 for a production of 4000 m /d to 15% for a production capacity of about 50,000 m /d. If additional treatment processes are added to hme softening to match the better membrane softening permeate quality, or if some water can be bypassed around the membranes and blended to produce water comparable to the finished water in the lime softening plant, the cost of NF is much lower than for hme softening. 

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). 

One process sometimes used for low flow rates (<50 m /h) is lime or caustic soda softening followed by cross-flow microfiltration (XMF) and RO poHshing [21]. The MF membrane (pore size <0.2 pm) is tubular with a diameter of 1.27 or 2.54 cm. Due to the large diameter of the tubes, the membranes can handle feeds with sohd levels of up to 5% at a very high membrane flux (375—500 huh). The XMF filtrate is of high quality with turbidity <0.1 NTU and SDI < 3.0. 

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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