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 100 C0(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.