Jartest testing in water treatment

1. Purpose

- Determine the optimal pH value of the flocculation process by alum, PAC.

- Determine the optimal dosage of alum, PAC of the flocculation process.

An illustration of a Jartest experimental model

2. Theoretical basis

Flocculation treatment involves adding a chemical called flocculation to the water that can be enough to cause very small particles to turn into large particles to settle. Usually the flocculation flocculation process occurs through the following 2 stages:

- Stage 1: The flocculation itself generates hydrolysis, the process of forming colloidal solution and condensation.

- Stage 2: Neutralize adsorption and filter impurities in the water:

The result of the above processes is the formation of large settling particles.

To perform the flocculation process, suitable flocculants are added to water such as: Al₂ alum (SO₄)₃, FeSO₄ type iron alum or FeCl₃ type or PAC alum. These alum are introduced into the water in the form of a dissolved solution.

When the alum is added to water, they dissociate into Al3 + ions, then these ions are hydrolyzed to Al(OH)₃: 

Al³⁺ + 3H₂O → Al(OH)₃ + 3H⁺

In the above hydrolysis reaction, in addition to Al(OH)₃, which is the decisive factor in the effect of the resulting coagulation, also releases H⁺ ions. These H⁺ ions will be reduced by the natural alkalinity of the water (assessed by HCO^3-). In case the natural alkalinity of the water is low, not enough to neutralize H + ions, it is necessary to alkaline the water. The most commonly used alkaline chemical is lime (CaO). In other cases, soda (Na₂CO₃) or soda (NaOH) can be used.

The following are the factors that affect the flocculation process when using aluminum alum.

2.1 pH value of water

When natural water has added Al₂(SO₄)₃, its pH value is reduced because Al₂(SO₄)₃ is a type of salt consisting of strong acid, weak base. Its hydrolysis can increase the acidity of the water. For flocculation has an effect, mainly the pH value of the water after alum is added. So the pH values ​​below are all pH values ​​of water after adding alum.

The pH value of water has a great influence on the flocculation process in many ways.

(1) The effect of pH on the solubility of aluminum hydroxide.

It is a typical bisexual hydroxide. The pH value of water that is too high or too low is enough to dissolve it, causing the excess aluminum content in the water to increase.

When the pH value drops as low as 5.5 or less, Al(OH)₃ has a clear effect as an alkaline, causing the Al3 + content in water to increase greatly, as follows:

Al(OH)₃+ Al3 + + 3 H2O ®3H +

When the pH value rises to 7.5 or more, Al(OH)₃ acts as an acid, causing the AlO^2- radical in water to appear as the following reaction (3-6):

AlO^2- + 2H₂O (3-6) ® Al (OH) 3 + OH-

When the pH value reaches 9 or more, the solubility of Al(OH)₃ rapidly increases, eventually becoming a solution of aluminum salts.

When SO₄^2- is present in water, in the pH range = 5.5 ~ 7 in the precipitate has alkaline sulfate salts, very little is dissolved. In this range, when the pH value changes high, the alkaline sulfate salt is in the Al₂(OH)₄SO₄ form, when the pH is low in the Al₂(OH)₄SO₄ form.

In summary, in the pH range of 5.5 to 7.5 the residual aluminum in the water is very small.

The charge of colloidal particles in aqueous solution is related to the composition of ions in water, especially with the concentration of H + ions. So the pH value for the electric properties of collo

(2) The effect of pH on the colloidal aluminum hydroxide particle charge.

idal particles has a great influence. When 5

(3) Effect of pH on organic substances in water

Organic matter in water such as plants rotting, when the pH is low, the colloidal solution of humic acid has a negative charge. This time it is easy to use the reducing flocculation. When the pH is high, it becomes a soluble humic acid salt. Therefore, the reduction efficiency is relatively poor. Use this type of reduced aluminum salt, most suitable at pH = 6 ~ 6.5.

(4) The effect of pH on the flocculation rate of the colloidal solution

The smaller the value of the colloidal potential x of the colloidal solution, the weaker the repulsion between particles, so its flocculation rate is zero, that is, the isoelectric point is reached, the faster the flocculation speed. When its flocculation potential is maximum (x) Colloidal solution is formed from an amphoteric compound, its potential and isoelectric point are mainly determined by the pH value of water. Aluminum hydroxide and humic substances, clay forming colloidal solution in natural water are both hermaphrodite, so pH is the main factor affecting the flocculation rate.

From a number of reasons above, for a specific type of water, there is no method to calculate the optimal pH value, but only determined by experiment. The water quality is different, the optimal pH value is different, that is, the same water source, the seasons, the optimal pH value can also change.

When using aluminum salts as flocculants, the optimal pH value is generally within the range of 6.5 ~ 7.5. The general rule is that when the amount of flocculation is relatively small, the natural colloidal solution in water is mainly based on its own flocculation process, so a relatively low pH is appropriate. because then the positive charge of the aluminum hydroxide colloidal solution is relatively large. As such it is very beneficial to neutralize its electricity. When x is negative of the self-adhesive solutio