coagulant material

Coagulant

The separation of fine and colloidal particles takes a long time, so chemical coagulants are used to enlarge the particles and reduce the settling time. When a coagulant is added to water, some of it is used to adjust the electrical potential of the particles, and some of it is combined with the alkalinity of the water. For example, when alum is added to water, aluminum hydroxide is produced, which, while settling, also removes suspended particles from the environment.

Al2(SO4)3 + 6H2O→2Al(OH)3↓ + 3H2SO4

The acid produced in the above reaction reacts with the alkalinity of the water and produces carbonic gas. This gas interferes with the coagulation process and causes them to float, and as a result, pH control is necessary to prevent the production of CO2 and to perform the coagulation process better. The coagulants used can be divided into two categories:

• Cations such as iron and aluminum ions that are absorbed on the surface of the particles.
• Water-soluble organic substances that cause the production of cations in the solution, most of which are polymeric materials.

Characteristics of coagulants

Coagulants must have the following characteristics:

• High coagulation power
• No toxicity in the water.
• Cost-effectiveness.

One of the advantages of mineral coagulants is that they help absorb phosphates effectively. Phosphates are resistant to biological treatment and biological interactions.

Most mineral coagulants are compounds of iron and aluminum and, due to their acidic properties, reduce the pH of the water. The pH of water affects the surface charge of particles and the sedimentation of flocs. It is necessary to adjust the pH of water with the help of substances such as lime water or soda.

The use of metal salts is based on the neutralization of the colloidal electrical charge. Due to the production of a large volume of sludge due to the use of mineral coagulants, coagulant coagulants are used with regard to economic issues and operational efficiency. Due to the importance of the subject, a number of mineral coagulants are described below.

Some examples of mineral coagulants

A) Aluminum sulfate or white alum (Al2(SO4)3.18H2O)

This substance is also known as alum and is used more than other substances. This substance has optimal performance in the pH range of 5.5 to 7.5. Aluminum sulfate produces aluminum hydroxide with alkaline substances in water, which is the central nucleus of particle aggregation and causes the production of larger particles. If the alkalinity of the water is low, lime or sodium carbonate is usually used. This operation must be done before adding alum to the water. The reactions that alum gives with lime or sodium carbonate or alkaline substances in water are as follows:

Reaction with lime:

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

Reaction with sodium carbonate:

Al2(SO4)3 + 3Na2CO3 + 3H2O → 2Al(OH)3 + 3Na2SO4 + 3CO2

Reaction with calcium bicarbonate:

Al2(SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 +6CO2

The use of alum increases the hardness of water and produces carbonic gas, which is a corrosive agent. In addition, if sodium and potassium salts are high in water, colloidal substances are formed as fine sediments.

According to studies conducted in recent years, aluminum can be effective in causing Alzheimer's disease, which causes forgetfulness in people. For this reason, some articles have declared the use of aluminum compounds in water purification to be dangerous in recent years. However, this issue has not been mentioned in drinking water standards.

For the effectiveness of alum and pH adjustment, the added amounts of alum and alkaline materials must be appropriate. Typically, for each milligram of alum, 0.5 milligrams of sodium carbonate, 0.7 milligrams of lime, and 0.54 milligrams of calcium carbonate are appropriate for each liter of water.

B) Sodium aluminate (Na2Al2O4)

This material is aluminum oxide stabilized by caustic soda, and its industrial composition contains several percent of additional hydroxide. The use of sodium aluminate does not require additional alkalinity. Its use in combination with aluminum sulfate has also been reported. In addition to coagulating particles, this substance is also effective in lightening water.

The reaction of sodium aluminate in water is as follows:

NaAlO2 + Ca(HCO3)2 + H2O → Al(OH)3↓ + CaCO3↓ + NaHCO3

C) Iron coagulants

The characteristic of these compounds is to create larger masses than those produced with aluminum salts. Iron compounds work better at low temperatures and lime is also necessary in this process. Iron coagulants include iron compounds and ferric compounds.

- Ferrous sulfate or green alum (FeSO4.7H2O):Lime is required in the use of this substance, like the previous coagulants, because the alkalinity of most waters is not sufficient to react with ferrosulfate and produce ferric hydroxide masses, and the appropriate pH for the process is in the range of 8.5 to 11. Ferrous sulfate initially produces ferrohydroxide with lime and then is oxidized by oxygen dissolved in water and converted to ferric hydroxide. Therefore, the presence of some oxygen in the water is necessary for this reaction, which is about 0.03 mg of oxygen for every mg of ferrosulfate per liter of water.

- Ferric sulfate (Fe2(SO4)3.7H2O):This compound is corrosive.

Ferric compounds have the following advantages over other coagulants:

• Performance over a wide pH range (from 4 to 11)
• Removal of taste, odor and color
• Faster settling performance compared to alum due to the heavier mass produced from ferric hydroxide
• Removal of manganese at pH=9
• Removal of hydrogen sulfide

Ferric sulfate can be used without lime and is more economical than other materials.

The reactions of ferric sulfate with alkaline water are as follows:

Fe2(SO4)3 + 3Ca(HCO3)2 → 2Fe(OH)3↓ + 3CaSO4↓ + 6CO2

Fe2(SO4)3 + 3Ca(OH)2 → 2Fe(OH)3↓ + 3CaSO4↓

Ferric hydrate produced in the above reactions is the main nucleus for the formation of colloidal flocs. Ferric chloride sulfate mixture, which is obtained from the oxidation of ferrous sulfate by chlorine, is a type of ferric coagulant that works in the pH range of 4 to 11.

6FeSO4 + 3Cl2 +9Ca(HCO3)2 → 6Fe(OH)3 + 6CaSO4 + 3CaCl2 +18CO2

Ferric chloride (FeCl3.6H2O):This substance is corrosive and difficult to store due to its deterioration from air and moisture.

The reactions that this substance gives with calcium bicarbonate and calcium hydroxide are as follows:

2FeCl3 + 3Ca(HCO3)2 → 2Fe(OH)3 + 3CaCl2 + 6CO2

2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2

Despite the limitations of this material, its use is preferred over alum in most cases due to its advantages. These advantages include:

• Ability to create larger and heavier flocs
• Efficiency in cold water
• Performance in the appropriate pH range (the pH range of drinking water is between 5 and 7.5)
• Reasonable price
• Less amount of material required compared to other coagulants due to high coagulation

Given that aluminum in water poses a risk of pathogenicity, the use of ferric chloride is preferred. However, ferric chloride delays sedimentation in waters with high carbon dioxide content and produces a colored iron solution complex in waters with high organic matter content, both problems being solved by adding chlorine before or at the same time as ferric chloride.

d) Magnesium oxide (MgO)

Activated magnesium oxide is highly effective in removing water-soluble silica. This material works at pH around 9.5 and above and is used in lime-soda lightening units. The amount of coagulant added to the water depends on the temperature and pH of the environment. The larger the particles, the lower the amount of this material used. Magnesium oxide is usually used for water with high color and turbidity, but if the magnesium ion in the water is sufficient, then only lime needs to be added to the water to form a jelly-like precipitate of Mg(OH)2. This precipitate absorbs suspended particles as it sinks and removes them from the environment.