Sedimentation or filtration

With sedimentation one must remember that substantially the only driving force is the difference in specific gravity between the phases. The apparent difference can be increased by, for instance, applying a centrifugal force or by increasing the mass of individual particles through flocculation, but the basic difference is a given factor which cannot be altered. Sedimentation is. 

Almost always drier than settling Almost always clear Design control possible 

Filtration, on the other hand, gives an almost unlimited amount of design control since it relies on a man-made septum, so chosen that it will retain those particles which must be separated. Filtration systems, however, are far less suitable for continuous production, sometimes not even for automatic production, and tend to be more expensive per volume treated than sedimentation systems. 

From this it follows that one must first of aU look at sedimentation. In this respect it is important to note that even if sedimentation does not achieve exactly the separation required it may still be a valuable first step in a process and may be followed by filtration. A typical example of this is the well-known clarification of water in water works. Here sedimentation with or without flocculation is first carried out, and the still somewhat hazy overflow is then passed through polishing filters. To achieve the end result purely by sedimentation or purely by filtration would be prohibitively expensive. The combined procedure, however, is both practical and reliable. 

Since filtration and separation are still to a very large extent an art rather than a science it is important that right from the beginning one obtains a feel for the material. We suggest therefore that before taking any actual 

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Tricalcium Phosphate. Commercial tricalcium phosphate (TCP) is actually an amorphous basic calcium phosphate close to hydroxyapatite in composition. Because of its extremely low solubiUty in water, TCP is precipitated almost quantitatively from dilute phosphate solutions with a slurry of hydrated lime. TCP is separated by dmm-, spray-, or flash-drying the TCP slurry, with or without intermediate sedimentation or filtration steps. It is used as an industrial-grade flow conditioner and parting agent. 

The choice between a sedimentation or filtration centrifuge for a particular application will depend on the nature of the feed and the product requirements. 

This section describes the treatment techniques currently used or available to remove or recover wastewater pollutants normally generated by aluminum forming facilities. In general, these pollutants are removed by oil removal (skimming, emulsion breaking, and flotation), chemical precipitation and sedimentation, or filtration.6- 3... 

Table 1 shows treatment costs for the technology (based on a processing rate of 20 gpm) in comparison to other groundwater treatment technologies (i.e., chemical reduction and precipitation, chemical precipitation with sedimentation or filtration, activated carbon adsorption, ion exchange, reverse osmosis, and electrodialysis) (D168869, Table 13). 

Chemical precipitation with sedimentation or filtration Relies on proven technology limited installation for chromium removal 192,000 64,000 Effectiveness limited low removal efficiencies are reported in literature. 

Waste water, containing a variety of colloids, is often treated by polymeric flocculants such as polyacrylamides to precipitate the colloidal material and make it better filterable. Quite often these flocculants are high-molecular-weight weakly charged polyelectrolytes. Bacterial suspensions resulting from biological waste water treatment must also be flocculated before they can be properly separated from the purified effluent by sedimentation or filtration. 

The special feature of Burger s process is the observation that after complete consumption of the bromofurfural, the formation of difurfural continues indefinitely. Thus, after a start with bromofurfural, a continuous addition of furfural results in a continuous precipitation of solid difurfural (melting point 264.5 C), which can be removed continuously by sedimentation or filtration. By contrast, if the process is started without the bromofurfural, no difurfural is formed. 

Swellable polymers Crosslinked polymers are 3-dimensional networks and can easily be separated by. sedimentation or filtration. Slightly crosshnked polymers such as, e.g., polystyrene crosslinked with 0.5—3% 1,4-divinylben-zene have to be used in solvents in which they swell, otherwise mass transport may be completely stopped. 

An alternative method of treating wastewaters is to add powdered carbon to a tank of solution using mechanical stirrers or air spargers to keep the particles suspended. With the fine particles, the adsorption is much more rapid than with granular carbon, but large equipment is needed to remove the spent carbon by sedimentation or filtration. The treatment with powdered carbon can be done... 

Chemical agents cause coagulation — agglomeration of dispersed particles into bigger units, when pollutants are incorporated into the formed floccules of ferric or aluminium hydroxide. The floccules are separated from water by mechanical methods, for example, sedimentation or filtration. 

The technique for the removal of iron and manganese during water treatment employs the oxidation of bivalent well-soluble forms to multivalent low-soluble hydrated oxides, which can be removed from water either by sedimentation or filtration. Oxidizing agents in this case are atmospheric oxygen, chlorine, potassium permanganate, ozone and chlorine dioxide. 

Raw soy sauce may be adjusted to standard salt and nitrogen concentrations. It is then pasteurized at 70—80°C to inactivate enzymes and microorganisms, enhance the unique product aroma, darken color, and induce formation of floes, which facilitates clarification. After heating, the soy sauce is clarified by either sedimentation or filtration. Kaolin, diatomite, or alum may be added before filtration to enhance clarification. The clear supernatant is packed immediately into cans or bottles. In some cases, preservatives, such as sodium benzoate and paraoxy-benzoate, may be used. 

The effect of flocculant is to increase the solids content of the thickened slurry or filter cake besides increasing the rate of sedimentation or filtration. Full advantage cannot always be taken of the flocculant in the case of rotary vacuum filtration because the dense, heavily flocculated pulp sometimes has a slippery texture which does not adhere well to the drum. It may then be necessary to use rather small quantities of flocculant to optimize the process as a whole. 

OH are the charge-determining ions, and, hence, the colloidal stability is sensitive to the pH. By adjusting the pH, low C values for most of the materials may be reached so that aggregation readily occurs and the materials can be removed by sedimentation or filtration. In fact, for aggregation to occur, it would be most favorable if some components have a positive value for and others a negative. The aggregation of colloidal particles is discussed in more detail in Chapter 16. 

The solubility of heavy metals is highly pH dependent (Fig. 6.3). Lead shows a solubility minimum at ca. pH 10 with respect to the hydroxide and pH 8-10 with respect to carbonate species. Adjusting the effluent pH can therefore cause the precipitation of the metals which are subsequently removed by sedimentation or filtration. Often a flocculating or coagulating agent is added to improve the sedimentation characteristics of the precipitate. The pH chosen for the precipitation will depend upon the range of metals to be removed and must be optimized for each individual effluent. It is sometimes necessary to employ a two-stage pH treatment to remove all metals. 

If one ignores impingement, it would be true to say that all mechanical solid-liquid separation systems are based on one of two principles sedimentation or filtration (see Figure 17.1). 

The sulfur produced is easily separated by sedimentation or filtration fi-om the Fe-EDTA-solution. Regeneration of the aqueous Fe-EDTA-solution is done by oxygenation according to Eq. (4.8) ... 

Precipitation An aqueous chemical reaction that forms an insoluble compound which is commonly removed by sedimentation or filtration. 

Adsorption of As by AC can be improved by impregnation of AC emplo)dng suitable chemicals (e.g., sulfur contain textile dyes for As(III) and Fe(III) salts for removal of As(V) from aqueous solutions (Ansari, 2007). Recently, an interesting study was carried out by Ahna et al. (2013) on the As removal from water using activated carbon (AC). The authors tested different activated carbons modified with iron hydroxide for their ability to adsorb As from water. They concluded that iron modified activated carbons are efficient adsorbents for As at concentrations lower than 300 [xg. According to Mohan and Pittman (2007) Activated carbon can remove 60% As(V) and As(III) but this removal percentage of As is not sufficient to reach drinking water quality. However, sedimentation or filtration processes are required in case of powdered activated carbons, which in turn adds extra cost to the technology. 

Some suspended solids remain in the water after primary treatment. These are removed by filtration, sedimentation or air flotation. Flocculation agents may be added to consolidate the solids, making them easier to remove by sedimentation or filtration. Activated sludge is used to digest water-soluble organic compounds, either in aerated or anaerobic lagoons. Steam-stripping is used to remove sulfides and/or ammonia, and solvent extraction is used to remove phenols. 

The characteristic of a micro-homogeneous mixture is that its constituents cannot be separated by mechanical methods, such as sedimentation or filtration. In the nomenclature of physical chemistry, a true solution is monophasic, all its constituents being present as part of the same physical state, or phase. 

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