A BRIEF OF ALUMINIUM SULFATE
ALUM
Aluminum Sulfate is Al2(SO4)3 and is commonly called alum or TAWAS
Aluminum sulfate is produced from the reaction of freshly precipitated pure aluminum hydroxide
with sulfuric acid ; the resulting solution is then evaporated and allowed to crystallize
Alum has been part of our lives ever since the time of early Egyptians, who used it in dyeing and
purification. Today, it is one of the most widely used and versatile industrial chemicals.
Most of the alum produced today is used in
- The pulp & paper industry as well as raw water and waste water treatment. It is inexpensive
and effective for a broad range of treatment problems because it can function as a coagulant.
- As a coagulant, alum removes turbidity, suspended solids and colloidal color, reduces
biochemical oxygen demand and clarifies potable, process and wastewater
Alum is available in solid form as a powder, granular, kibbled & lump form. It can also be purchased
and fed as a liquid
Alum can be effective in the pH range of 5.5 to 7.8, but seems to work best in most water
supplies in a pH range of 6.8 to 7.5.
- Below a pH range of 5.5, alkalinity in the water supply is generally insufficient. The aluminum
ions become soluble rather than insoluble and do not participate in the hydration and reactions
necessary to make the alum effective as a coagulant. In these instances the plant may
experience higher than normal filtered water turbidities, and much of the aluminum will pass
through the filters.
- When the pH level of the water is above 7.8 after the addition of the alum, the aluminum ions
again become soluble, and the efficiency of coagulation is decreased. Under these conditions,
aluminum ions again penetrate the filters, and post filtration alum coagulation can occur in the
clear well and in the distribution system in some cases.
FACTORS INFLUENCING COAGULATION
Effects of pH... The pH range in which a coagulation process occurs may be the single most important factor in propern coagulation. The vast majority of coagulation problems are related to improper pH levels. Whenever possible, coagulation should be conducted in the optimum pH zone.
Effects of salts... Since no natural waters are completely pure, each will have various levels of cations and anions such as calcium, sodium, magnesium, iron, manganese, sulfate, chloride, phosphate, and others. Some of these ions may affect the efficiency of the coagulation process.
Nature of turbidity ... The turbidity in natural surface waters is composed of a large number of sizes of particles. The sizes of particles can be changing constantly, depending on precipitation and manmade factors. When heavy rains occur, runoff into streams, rivers, and reservoirs occurs, causing turbidity levels to increase. In most cases, the particle sizes are relatively large and settle relatively quickly in both the water treatment plant and the source of supply
Water temperature °... Cold water temperatures can cause two factors which add to the difficulty of the coagulation process. As water temperatures approach freezing, almost all chemical reactions occur more slowly.
Mixing Effects ... Poor or inadequate mixing results in an uneven dispersion of the coagulant. Unfortunately, many older plants were designed with mixing facilities which generally do not accomplish mixing in the most efficient manner. As a result, it becomes necessary to use higher than necessary dosages of coagulant to achieve an optimum level of efficiency in the process. The effects of low turbidity and cold water temperatures can tend to aggravate the lack of adequate mixing facilities in some plants.
Effect of the coagulant ... The choice of the proper coagulant for the given conditions is of critical importance in maintaining an efficient coagulation scheme under widely varying conditions. The chemicals most commonly used in the coagulation process are Aluminum Sulfate, Ferric Chloride, Ferric Sulfate, and Cationic Polymers
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