Water Treatment Plant (WTP) | Process Sequence of Water Treatment Plant

Water Treatment Plant:

The water available from different water sources cannot be used directly in boilers as such. The objective of water treatment plant is to produce the boiler feed water so that there shall be:

1. No scale formation
2. No corrosion
3. No fouling

The treated water is called ‘De-mineralized Water’ and the plant where it is treated is called Water Treatment Plant.

WTP process

Process Sequence of Water Treatment Plant:
Water from river
↓
Flash mixer
↓
Clarifier
↓
Filter pack
↓
Clarified water tank
↓
Softening plant
↓
Soft water to various units

Step Discussions:
Water from river:
Water comes from various rivers in water treatment plant.

Flash mixer:
In this segment Al2 (SO4)3, polymer etc are mixed in water.

Softening plant:
In this segment activated carbon are used as a softener.

Soft water to various units:
After softening of water it is supplied in various units for various purposes.

There are two types of water are prepared in water treatment plant:

1. Clarified water for washing purpose.
2. Soft water for dyeing purpose.

Chemicals are Used in WTP:

1. Aluminum Sulphate
2. Polyelectrolyte
3. Sodium Chloride

Amount of Chemical Used:

1. Polymer- .1% solution
2. Al2 (SO4)3- 10% solution
3. Lime- 10% solution

Experiments Performed in WTP:

1. Total hardness test
2. Jar test
3. pH meter test
4. Titration for chlorine content test

Requirements of WTP:
The importance of maintaining supply of high purity water for Boiler purposes in Thermal power stations is well recognized. The presence of undesirable contaminants in water exceeding the few PPB level may lead to serious consequences.

The requirements of pure water have been met by recent developments of improved ion exchange resins and regeneration techniques.

The need for removal of dissolved solids from water arises under several situations. These range from effluent water treatment, water purification for portability, treatment as boiler feed and other industrial applications.

Why Water Treatment is Required?
If proper treatment is not done for water then Corrosion, Scaling, Microbiological contaminants and fouling will occur in the system.

a). Corrosion:
It is an electrochemical process by which a metal returns to its natural state. For e.g. Mild Steel is commonly used metal in cooling water systems and is very susceptible to corrosion. It will return to its iron oxide.

Corrosion can be prevented by or minimized by one or more of the following method:

1. When designing a new system choose corrosion resistant material to minimize the effect
2. Apply protective painting
3. Protect catholically using sacrifice metals
4. Add protective film forming chemical inhibitors.

b). Scaling:
Scale is a dense coating of predominantly inorganic material formed from the precipitation of water soluble constituents

Some common scales are:

• Calcium carbonate
• Calcium phosphate
• Magnesium salts
• Silica

Four principal factors determine whether or not water is scale formed:

1. Temperature
2. Alkalinity or Acidity
3. Amount of scale forming material present
4. Influence of other dissolved materials

Scaling can be controlled by following methods:

• Limit the concentration of scale forming minerals by controlling
• Cycle of concentration
• Feed acid to keep the Calcium carbonate dissolved
• Treat with chemicals designed to prevent scale.

c). Fouling:
It is the accumulation of solid material other than scale in a way that hampers the operation of plant equipment.

Common fouling elements are:

1. Dirt and silt
2. Sand
3. Corrosion products
4. Microbial organism
5. Aluminum phosphates

Process in Water Treatment Plant:
The treatment process can be divided in two sections:

1. Pre treatment methods
2. Demineralization methods

Pre-treatment Methods:
Pre-treatment plant removes suspended solids like clay, salt, plants, micro-organisms, etc. form raw water to give clarified water. Suspended solids can be separable or nonseparable.

Separable solids are heavier & large and can easily be removed by an aerator. Non-separable solids have finer size and taken long to settle down. Hence they are required to be flocculated.

In this, water is first dozed with lime and alum. This forces finer particles to coagulated increasing their weight and size. Non-separable solids can now be separated in clariflocculator. The clarified water is then stored in clarified water storage tanks.

Following are the different pre-treatment methods used in water treatment plant:

1. Mechanical methods
2. Chemical methods

Demineralization Methods:
The following methods are normally used for demineralization of water:

1. Membrane based physical process and electro chemical process
2. Separation by Phase change method
3. Ion exchange method

Factors for Selection of Treatment Methods: 

The following specific site factors are to be considered in choosing the treatment method:

• Feed water source and quality available
• Feed water cost and quantity available
• Final makeup water quantity and quality required
• Final make up water maximum and average flow required
• Chemical storage handling and usage cost
• Waste disposal cost and waste treatment equipment available
• Existing water and waste treatment equipment available
• Operation & Maintenance cost
• Storage capacity

Other Factors to be Considered:

• Short or long term requirement of equipment or service
• Acceptable limitations on membrane liability
• Operating labour availability
• Capital available
• Environmental policy