Surface water softening

This section describes only the influence produced on the flotation process by two main surfactant types—foamers and collectors. Meanwhile, there is a complex of compoimds used in real processes pH adjustment, hydrophilisation of surfaces, water softeners , high-molecular compounds (flocculating as well as deflocculating agents), and others. The choice of flotation modes and bouquets of reagents is determined in every particular case depending on mineral composition of raw materials (nature of solids), water quality, aeration intensity and other factors. 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Filtration is used in addition to regular coagulation and sedimentation or precipitation softening for removal of soHds from surface water or waste water. This prepares the water for use as potable, boiler, or cooling makeup. Waste water filtration helps users meet more stringent effluent discharge permit requirements. 

Supply of MU water for a medium-pressure (450 psig) WT boiler, from a surface water source with very variable suspended solids and hardness (sugar refinery, South Africa). The process used is a. carbonate removal using hot-lime precipitation softening coupled with silica adsorption by magnesia addition b. clarification in anthracite filters and c. cation ion-exchange for the balance of hardness removal. 

Borax is used as a cleaning agent and water softener that removes ions of elements such as magnesium and calcium that cause hard water. When these hard water elements are mixed with soap, they prevent soap from sudsing and form a scum or residue that is deposited on hard surfaces. Borax can ehminate this residue ring by replacing the Mg and Ca ions with the more soluable Na and ions. Borax is the third most important boron compound. 

In this way the atmosphere assists in softening those waters, which owe the whole or a portion of their hardness to the above calcareous salt. The atmosphere, however, is a source of contamination as well as of purification, and it is on this account that the Editor always condemns the nse of surface water for dietetic purposes. Ehkenbep.G states that, exclusive of inorganic substances, he has detected three hundred and twenty species of organic forms in the dust of the... 

In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening by replacement of Ca2-with Na+ or another ion. When dehydrated they have important catalytic applications, promoted by the Brensted acid sites, and by the large area of internal surface. They are used for the cracking of petroleum and for the isomerisation of hydrocarbons, where limited pore size exerts a shape selectivity, which allows one desirable product to be formed in high yield. 

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. 

For this low hardness, surface water case, the antisealant operating cost is lower than that for the softener. Again, the affect of the softener on improved membrane performance is not known and should be considered for this surface water source. 

Sodium is present in soils and water as NaCl. In arid environments, NaCl accumulates in the surface and groundwater owing to irrigation and high evapotranspiration. Other activities such as road salting and water softening may also contribute NaCl to natural waters. For additional information on human health effects and drinking water levels, see Shelton (1989). 

Most softeners consist of molecules with both a hydrophobic and a hydrophilic part. Therefore, they can be classified as surfactants (surface active agents) and are to be found concentrated at the fibre surfaces. Most softeners have a low water solubility. Therefore softening products are usually sold as oil in water emulsions containing 20-30 % solids. The softener molecules typically contain a long alkyl group, sometimes branched, of more than 16 and up to 22 carbon atoms, but most have 18 corresponding to the stearyl residue. Exceptions to this molecular structure are the special categories of silicones, paraffins and polyethylene softeners. About one-third of the softeners used in the textile industry are silicone based. 

Ion exchange is similar to adsorption in that both involve affinity between a species i in solution and functional groups on a substrate surface ion exchange may even be considered as a particular case of adsorption, where coulombic affinity is prevalent in a completely reversible process. This reversibility is promoted by competition between two ions for the same adsorption sites. In common water-softening systems, calcium ions replace sodium ions in the ion exchange resin, according to the following reaction ... 

A two-step waste treatment process has been shown to reduce softener actives by more than 90% [3]. The rest is diluted in surface waters. 

Well water samples were collected from residential taps prior to any in-place water softening units. Water samples taken during a spring flood water period were collected from inside taps after a full open flush of 30 minutes. This assured that the sample collected was groundwater withdrawn from the carbonate aquifer some distance from the well. Stream samples were taken at the center of the channel near the stream surface. The cross sectional stream samples were taken equidistant apart at 1/3 the water depth at each lateral position. Water samples were stored less than a week at 5°C. 

Although ion-exchange is most often thought of in terms of surface as described above, it can also apply to exchange of ions from within a phase, as in the use of zeolites in water softening, or sodium for potassium exchange when K-feldspar is altered to albite at high temperatures. 

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