Chemical Engineering Spot: River Water Treatment Process

RIVER WATER TREATMENT:

Raw water from river is sent to settling basin. Settling basin is divided into two channels & provided for removal of large particle size of suspended solid. Removed muddy water is discharged to the river. Raw water is transferred to water treatment unit.

CLARIFICATION:

Clarification is a system normally used to remove suspended solids in raw water. Raw water from raw water reservoir is pumped to clarifier.

Clarification parameter:

· P^Hhas to be maintained near 7-8.5

· Free chlorine in water has to be maintained 1 ppm, where chlorine is used to ensure that bacteria are removed from water.

In clarification, chemicals such as Alum, Chlorine, Soda ash, Coagulant aid are mixed in reaction zone. In reaction zone, an agitator is used to mix chemicals with water and increase the size of suspended particles by Coagulant. Another is used to sweep the sludge at the bottom of the clarification section to blow out the sludge.

Functions of chemicals that are used in Clarification:

Ø Alum [Al₂(SO₄)₃]: Alum is the common name for aluminum sulfate Al₂ (SO₄)₃, which is often used as a coagulant in water treatment. When alum is used for coagulation the following reaction takes place-

Al₂(SO₄)₃+ H₂O = Al(OH)₃ + H₂SO₄

Here Al(OH)₃carrying positive charges which comes in contact with colloidal particles with negative charges and forms feather like substances called flock.

Ø Chlorine: Chlorination is the application of chlorine to water to accomplish some definite purpose. Chlorination is also called disinfection. In water treatment processes, chlorination can be used as a pretreatment process or as part of the primary treatment of water. Chlorine is used to kill the Bacteria, micro organism. Chlorine is added at such quantity to maintain the concentration of chlorine at 1 ppm.If its concentration is less than 1 ppm that means there are still remaining some bacteria or micro organism. 1 ppm concentration ensures that there are no more bacteria present. In this processing the flocks become bigger in size and ultimately settle down allowing clear water to flow upward. This clean water is then separated and known as clarified water and the flocks deposited at the bottom is known as sludge.

Ø Coagulant Aid: Coagulant aid is a poly electrolyte. Poly acryl amide (PAA) is used to remove very fine particles.

Ø Sulfuric Acid (H₂SO₄): Sulfuric acid is used if pH is greater then 8.5.

Ø Sodium Hydroxide (NAOH): Sodium hydroxide is used if pH is less 7.

Ø Soda Ash (Na₂CO₃): If alum is dosed in excess quantity for any reason depression of P^H of treated water is observed for the formation of H₂SO₄. In this case Na₂CO₃ is added to maintain P^H close to neutral.

Now the clarified water has a turbidity of 1 to 3 with P^Hrange of 7.5 to 8.5

Reservoir: From the clarification section water is sent to a reservoir by a pump from where water is sent to another reservoir through sand filter. This water is called treated water.

TREATED WATER TREATMENT

Treated water is divided into 3 flows-

a. Hydrant water: Hydrant water is used for emergency fire extinguishing purpose. One pump is usually used with a standby diesel pump in case of power failure. A constant pressure of 8kg/cm²is maintained in pipeline.

b. Cooling water: Treated water is stored in cooling water storage pit from where it is then delivered to cooling tower as make-up water.

Ø Roll of cooling water in process industries:

In process industries many coolers such as Heat exchangers are used. Water is mostly used as cooling medium in these industries. After receiving heat from hot zone temperature of water rises. The reuse of this water with higher temperature is less effective. So it retains the effectiveness of cooling water, it requires to release heat in the environment. The increased trend of water consumption has led the technologist to conserve water or reuse water.

c. Pure water: Treated water is stored in pure water storage pit from where it’s used for many purposes.

PURE WATER TREATMENT

Pure water is divided into 3 flows.

a. Drinking water: Water is supplied directly to office and resident for drinking and washing purposes.

b. Service water: Water is supplied to intercoolers of air compressors of the factory.

c. DM water: DM water is produced from pure water by passing the water through Cation bed, Degasifier, Anion bed, De-aerator, Mixed bed polisher.

Demineralization of water:

The unit consists of the following steps-

Cation bed (Cation exchange resin)
Degasifier (removes CO₂)
Anion bed (Anion exchange resin)
De-aerator (removes the dissolved oxygen)
Mixed bed polisher (for further polishing)

Filtered water is first fed to the cation exchanger consisting of cation exchange resin which removes the dissolved cation from water. This water is then passed through degasifier (decarbonator) where unstable carbonic acid dissociates into water & carbondioxde. After degasification, anions are removed from water by passing through anion exchanger. The simplified overall reaction may be shown as follows:

In Cation bed: R-h + M-X = R-M + H-X

Mⁿ⁺is usually Ca ⁺⁺, Mg⁺⁺, Na⁺, AI⁺⁺⁺etc.

In Decarbonator :

H₂CO₃D CO₂ h + H₂O

In Anion bed : R-OH + H-X = R-X + H₂O

Xn- is usually CO^3--, CI^-, SO₄^--, silicate etc.

Where, R-H = Cation resin R- OH = Anion resin

M = Cation X = Anions

The cation & anion exchange resins:

Ø Cation exchange resin:

Ø Anion exchange resin:

Dematerialized water is called pure water and sent to the pure water storage tank. Its quality is normally like as:

Conductivity: 10 Ω/cm

Silica: 0.8 ppm.

P^H: 7- 8.5

De-aerator:

It is a unit where pressure is maintained about 10 mm (Hg) as to remove the dissolved oxygen from polished water.

Polished water:

Dematerialized water along with condensate are passed through mixed bed ion-exchangers (MBP) for further polishing (deionization). After which water has a silica content less then .01 ppm, conductivity less then 1.0µΩ/cm. The water is stored in polished water storage tank from which it is supplied to high pressure boiler through a de-aerator. The Catoin & Anion exchange resins used in MBP are as follows:

Ø Anion exchange resin:

Ø Cation exchange resin:

Regeneration of anion & cation beds:

Continuous operation of ion-exchangers reduces exchangeability of the resins. Exchange inability can be measured by conductivity (more than 10µΩ/cm) and by silica content (analyzed by laboratory: 0.8 ppm). Chemicals used for regeneration are.

Cation bed:98% concentrated Sulfuric acid (H₂SO₄) is used for the regeneration of Cation Bed.

R-M + H₂CO₄ D H-R + M/ SO₄

Anion Bed:Sodium Hydroxide (NaOH) is used for the regeneration of Aation Bed.

R-X + NaOH D R-OH + Na/X

Reactions occur in different unit of water treatment plant:

1. Cation Exchange tower:

NaSiO₃ + R-SO₃H g R-SO₃- Ng + Na₂SiO₃

This process produces acid such as HCI, H₂SO₄ etc. Acidity of which indicates pH value around 2.8 ~ 3.5.

2. Decarbonization tower (Degasification):

HCO₃g CO₂ h + H₂O

This process reaction makes the anion resin lighter which lowers running cost.

3. Mixed bed polisher:

Ø Cation Exchange resin :

NaSiO₃ + R-SO₃H g R-SO₃Na + H₂SiO₃

Ø Anion Exchange resin:

H₂SO₃ + R-N-OH g R = N- SiO₃+ H₂O

The above mentioned reactions take place successfully which produce the ultra-high purity of water showing pH value around 7.0 and the electric conductivity is as low as 0.5µΩ/cm.