¿Por qué no es eficaz añadir solamente sulfato de aluminio para remover el fósforo y el arsénico.

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For now, it is evident that the addition of only aluminum sulfate as a coagulant for purification or depuration of water is a treatment tool that only contributes to the turbidity removal (clarification) but never to the elimination of phosphorus and arsenic dissolved in it, needing more alkaline conditions than those resulting from coagulation process.

The missing link in coagulation (Calcium Hydroxide)

The sanitary engineering research in Europe during the 1980s and 1990s, showed that it was much more convenient to accelerate the formation of aluminum hydroxide with alkali, instead to wait the slow hydrolysis reaction of aluminum sulfate in water, using Calcium Hydroxide (hydrated lime) as a source of hydroxyl ions, to produce the following chemical reaction:

Al2(SO4)3 + 3Ca(OH)2 → 3CaSO4↓ + 2Al(OH)3↓

The benefits of using hydrated lime with aluminum sulfate were significant, firstly because although a chemical product was added to the water (Calcium ion Ca+2), it was immediately converted into an insoluble substance reacting with the observer added sulfate ion forming calcium sulphate, an inorganic polar adsorbent material of high absorbent activity, able to discolor and deodorize sewage water leaving other components unchanged if its addition were made in stoichiometric amounts. Additionally, the sufficiently insoluble gel of aluminum hydroxide is obtained (Ksp = 2.99 x 10-34) that performs the function of clarification, and the calcium ion (Ca+2) is incorporated and able to make a further removal of phosphate and arsenate in form of insoluble calcium salts according to the following chemical reactions:

3Ca+2 + 2PO4−3 → Ca3(PO4)2↓

3Ca+2 + 2AsO4−3 → Ca3(AsO4)2↓

These reactions, as well as those relating to the removal of aluminum phosphates and arsenates, require the existence of these anions in its free form, completely deprotonated, which undertakes to maintain a pH in the water to be purified, above 10. This condition is satisfied in a simple manner, ensuring a measured and controlled lime excess above the stoichiometric ratio to raise the pH between 11 or 12, yielding the following optimizations:

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WWDS GT® is Waste Water Depuration Station − Gas Technology.

The processes of coagulation and clarification of water, using the combination of aluminum sulfate and calcium hydroxide, is part of the design of the Waste Water Depuration Station using Gas Technology (WWDS GT®) from the point of view of eliminating the stimulating factor of eutrophication of natural water bodies and the protection of national lakes as a water resource, and becomes important with the constant complaint of the presence of arsenic in drinking water supplied in the towns of Mixco and Chinautla in the Department of Guatemala, as a tool for purification treatment too.

Whereas today, the potable water disinfection does not use Chlorine gas (Cl2) or hypochlorite (ClO−) because their hepatocarcinogenic implications due to the generation haloforms in drinking water, it is appropriate to indicate that the Gas Technology it can also be applied as a Water Purifying Station (WPS GT®) because their processes involve clarifying and partially disinfecting with aluminum sulfate and calcium hydroxide as indicated above (which removes arsenic), complete oxidation using ozone (O3) which removes any organic and inorganic pollution including disinfection, and neutralization and final refinement with Carbon Dioxide (CO2).

Aluminum and Alzheimer’s Disease

The use of aluminum salts in water treatment for drinking or for depuration, has been subjected to restriction due to still unproven suspicion of being linked to Alzheimer's disease. It has been recommended the use of organic flocculants, but they are unable to perform the removal of phosphorus and arsenic as described above, and only work for the removal of suspended solids and clarification.

Review of Ballester and Suay about exposure to aluminum and Alzheimer's disease, has allowed to demonstrate a causal association that has not yet identified, but not fully proven as decisive, with the emergence of this disease in populations consuming drinking water with aluminum ion levels above 0.1 mg/L (100 µg/L) for long periods of time and on a specific condition for age.

International regulations on water quality, have set a limit of 0.2 mg/L as the maximum level for the concentration of aluminum in drinking water, so that in preventive plan against the given uncertainty of a real cause and effect, it has been suggested its reduction to the levels of disease risk values.

Using Aluminum Sulfate, for purposes of purification or depuration, with the complement of excess of calcium hydroxide, which ensures the complete conversion of the aluminum ion present in Aluminum Hydroxide, ensures that the final levels of this cation, will be below of any level of risk, due to its considerable aqueous insolubility.

The extremely low solubility product constant for aluminum hydroxide (Ksp = 3.0 x 10−34) indicates that "virtually" nothing of aluminum ion will be dissolved in drinking or purified waters, because its residual concentration would be approximately 0.000049 mg Al/L (49 nanograms per liter of aluminum ion, not directly measurable by present analytical techniques). These concentrations are lower than mineral leaching from basin soils rock itself, and would make the process of coagulation with aluminum sulfate and calcium hydroxide at pH above 10, a safe way to remove suspended solids, phosphorus and arsenic (as well as antimony and bismuth) for water treatment and purification, without the risk of inducing pro-Alzheimer apparent effect.

Additionally, it is important to note that the single application of Aluminum Sulfate with no Calcium Hydroxide can leave uncertainty in the final levels of aluminum in potable or treated water, because the process of spontaneous hydrolysis as discussed above is dependent of the pH of the water being treated, still incomplete at pH values below 7.

It should be added that the same applies to any other metal ion present that will precipitate and sediment at pH 10, in the form of hydroxide or hydrated oxide, also allowing the removal of inorganic toxic contaminants.

Conclusion

The water coagulation process for purification or depuration, using only Aluminum Sulfate, is intended only for clarification (removal of