Aluminum sulfate, alum, is most commonly used industrially as a coagulant in water treatment plants. Being water soluble it’s used in treating both wastewater and potable water. Used as a flocculant in order to get the water as clear and particulate free as possible.
Often, raw water, has particles that are too small to be removed by filter. The alum causes these impurities to clump together to either settle at the bottom or be easily filtered out. Turbidity and color are also resolved because the alum promotes coagulation of fine particles. When applied properly, and given enough time to work, alum
The alum promotes coagulation of fine particles which helps resolve problems of color as well a turbidity. If the process is given enough time to work and is applied properly, it not only corrects problems in the water but actually results in the removal of most of the aluminum.
Aluminum sulfate is used as a mordant in the dyeing process. It is used on paper and fabric as a dye fixative. Aluminum sufate, creates the sludge-like aluminum hydroxide when mixed with water. Along with aiding textile dye absorption, the alum, also makes the fabric’s dye water insoluble. Hence, they won’t bleed when washed.
Aluminum sulfate is used in gardening to lower soil pH for acid loving plants including: azaleas, grapes, hydrangeas, strawberries, and blueberries.
Aluminum sulfate is used in pools as a flocculant that quickly clumps together all the fine particles in your cloudy water, including green algae, so that they sink to the bottom of your swimming pool. Afterwards, simply use the vacuum to clean the waste.
In the water treatment technology the sodium aluminate is used as a coagulation and flocculation reagent, at the same time being a coagulation additive in combination with aluminium sulfate. The active element is the AlO2 anion. The use of aluminate is self‐evident when one takes into consideration the hydrolysis reactions of the aluminium salts, as follows:
a) The hydrolysis of aluminium sulfate, the most widely used coagulation reagent, takes place according to the formula:
Al2(SO4)3 + 3 H2O = 2 Al(OH)3 (gel‐cleaning agent) + 3 H2SO4
The resulting sulphuric acid should be partially neutralized by the bicarbonates present in the water. However, at high dosages or at low HCO3 ionic concentrations, water pH drops significantly creating the need for the addition of an alkaline substance (usually lime, sodium carbonate or sodium hydroxide) for pH balance.
b) The hydrolysis of sodium aluminate, which is an alkaline aluminium based reagent, takes place according to the formula:
Na3AlO3 + 3 H2O = Al(OH)3 +3 NaOH
The resulting sodium hydroxide reacts with the resulting sulphuric acid from point a); therefore a neutralization reaction occurs:
H2SO4 + 2 NaOH = 2 Na2SO4 + H2O
It follows that by using the two reagents together one can control the pH and keep an optimum coagulation pH throughout the coagulation phase. The optimum pH has to be determined by laboratory experiments, since each water has its own characteristics (turbidity, temperature, pH, color, bicarbonates content, etc.). The pH is the most important technological factor influencing the coagulation/flocculation process; it is the determining factor in establishing the efficiency of the process. Each coagulant has an optimum hydrolysis and coagulation pH level. As mentioned before, using the two reagents together allows one to precisely and easily set and keep the pH.
Sodium aluminate is dosed either diluted or undiluted. The correct dosage of useful substance per m3 of treated water is the sum of the two reagents; the ratio between the two has to be set after laboratory analysis on the water to be treated.
The advantages of using sodium aluminate in combination with aluminium sulfate, as opposed to using aluminium sulfate with lime, include:
– The possibility of maintaining an optimum and stable pH‐level throughout the water treatment process.
-The marked improvement of the coagulation process, especially at temperatures below 1000C (when the sulfate is less effective).
-The reduction of filter clogging.
– The reduction of the total coagulant dosage.
-The lowering of organic content of the treated water.
– Reducing remanent Aluminium content to a minimum.
-Completely replacing the lime, resulting in cost reduction of both manpower and maintenance.