Coagulant aids

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

The chemical formula for limestone is CaCOj and upon burning forms calcium oxide (CaO), which is known as burnt lime. Calcium oxide, when mixed with water, forms calcium hydroxide (Ca(OH)2). Calcium hydroxide is used to treat water as a coagulation aid along with aluminum sulfate. 

Polyacrylamide Nonionic flocculant, usually supplied as 100% active powders but also available as 30% or so emulsions. MW ranges typically from 2.5 M to 20 M. Test for suitability using jar-tester apparatus at, say, 1 to 5 ppm. It can be used as a coagulant aid for alum, and PWG are available. 

Coagulant aids may be used to improve the filtration of floes, but these are generally only of value if sufficient residence time is available for coagulation to occur before the water enters the filter. 

The elements of the process are as follows. Acid and alkaline wastestreams are collected in a neutralization tank. Here sufficient lime is added to raise the pH to 6.0. At this point, no zinc hydroxide will precipitate but a portion of the iron, calcium sulfate, and other impurities will form a light precipitate. With a coagulant aid, the mixture is sent to a clarifier where a clear overflow containing the dissolved zinc is obtained. 

Design and operational variables Chemicals Coagulation aids, media size, sludge recirculation Energy input, mass transport Residence time filter length and media diameter... 

An additional method exists for the effective coagulation of low turbidity waters that is, the addition of more particles in the form of a coagulant aid. Bentonite (a finely divided clay) and some forms of activated silica serve this function. 

Electrostatic stability plays a dominant role in many separation processes, such as filtration of industrial wastewaters. Coagulation aids (known as coagulants) are routinely used to improve the effectiveness of separation processes in such cases. Polymer-induced stability is often the method of choice, particularly in the case of concentrated dispersions for example, many pharmaceutical preparations, paints, inks, and liquid toners depend on surfactants or polymer additives for ensuring stable preparations. We see in Section 13.2 that in the case of concentrated dispersions both thermodynamic and kinetic issues often become very important. 

Many chemicals are added to water to remove contaminants such as organic matter, suspended or dissolved solids, and microbial pathogens. Among those added are alum, iron salts, polymeric coagulant aids, chlorine, and other oxidizing agents, all of which may leave residues or byproducts in the finished water. Chlorine gas often contains... 

Coagulants and coagulant aids iron and aluminum sulfates, activated silicates, alginates, synthetic polyelectrolytes and their impurities, silica, and aluminum electrolyte monomers. 

Reckhow DA, Singer PC, Trussell RC. Ozone as a coagulant aid. Proceedings of AWWA Seminar Ozonation Recent Advances and Research Needs, No. 20005, Denver, 1986. American Water Works Association 1986 17-46. 

Activated silica and clays have also been used as coagulant aids. Activated silica is sodium silicate that has been treated with sulfuric acid, aluminum sulfate, carbon dioxide, or chlorine. When the activated silica is applied, a stable negative sol is produced. This sol unites with the positively charged primary-metal coagulant to produce tougher, denser, and faster settling floes. 

In practice, irrespective of what coagulant or coagulant aid is used, the optimum dose and pFI are determined by a jar test. This consists of four to six beakers (such as 1000 ml in volume) filled with the raw water into which varying amounts of dose are administered. Each beaker is provided with a variable-speed stirrer capable of operating from 0 to 100 rpm. 

The chief nonmineral application of froth flotation is to the removal or oil or grease or fibrous materials from waste waters of refineries or food processing plants. Oil droplets, for instance, attach themselves to air bubbles which rise to the surface and are skimmed off Coagulant aids and frothers often are desirable. In one kind of system, the water is saturated with air under pressure and then is pumped into a chamber maintained under a partial vacuum. Bubbles form uniformly throughout the mass and carry out the impurities. The unit illustrated in Figure 20.12 operates at 9 in. mercury vacuum and removes both skimmed and settled... 

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