Water Treatment Plant (WTP): Water is one of the most important utility used in textile industry specially in textile wet processing industry. Without water no one can imagine scouring, bleaching, dyeing, finishing etc. There are mainly four source of water, such as rain water, surface water, subsoil water and deep well water. Mainly in Bangladesh deep well water is available most of the company used deep well water. The water is used in boiler, scouring, bleaching, dyeing, finishing and must be soft and clean.
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:
No scale formation
No corrosion
No fouling
The treated water is called ‘De-mineralized Water’ and the plant where it is treated is called Water Treatment Plant.
There are two types of water are prepared in water treatment plant:
Clarified water for washing purpose.
Soft water for dyeing purpose.
Purpose of Water Treatment Plant:
To remove hardness of water and purification of water.
To supply water to the dye house as stander quality.
To supply water to the boiler as it does not damage boiler.
Standard for Textile Dye House Water Supply:
Table-1: Standard water quality for dye house
Parameter
Permissible concentration
Color
Color less
Smell
No bad smell
Water Hardness
<5 ppm
pH value
should be 7-8 (Neutral)
Dissolve solid
< 1 ppm
Inorganic salt
< 500 ppm
Iron (Fe)
< 0.1 ppm
Manganese (Mn)
< 0.01 ppm
Copper (Cu)
<0.005 ppm
Nitrate (NO3)
< 50 ppm
Nitrate (NO2)
< 5 ppm
Solid content
< 50mg/L
Quality Requirements of Boiler Water: Boiler needs very special quality of water.
Table-2: Acceptable limits of property of water in boiler
Properties
Acceptance limits
Appearance
Clean with residue
Residue as hardness
0.050 dH
Temporary CO2
0 mg/L
Temporary Fe
< 0.02 mg/L
pH
>9
Conductivity
< 2500μs/cm
Feed water temp.
70oC to 90oC
Process Sequence of Water Treatment Plant:
Deep tube well
↓
Raw water tank
↓
Pump
↓
Filter
↓
Softener pump
↓
Brain tank (Sand, stone, carbon)
↓
Sand filter (Sand, stone)
↓
Softener filter (Resin)
↓
Brain tank
↓
Soft water tank (13ft)
↓
Boaster pump
↓
Soft water to various units
Step Discussions:
1. Water from river: Water comes from various rivers in water treatment plant.
2. Flash mixer: In this segment Al2(SO4)3, polymer etc. are mixed in water.
3. Softening plant: In this segment activated carbon are used as a softener.
4. Soft water to various units: After softening of water it is supplied in various units for various purposes.
Fig: Block diagram of Water Treatment Plant
The water flow direction is clear at the above picture. Here water flow through the first hollow cylinder and moves under the cylinder then it passes through the second to third by means of over flow. A few sludge is sediment at second cylinder and a lot of sludge is sediment at the third. From third cylinder water is passing to the filter chamber is also followed by the over flow of water. At the second cylinder water stayed for a little time as its volume is poor enough than the third that’s why few sludge is estimated over there. In filtration chamber water is kept for a long duration like 22 hours which is allowed to sediment a lot of sludge remaining in the treated water. At the outlet of filtration chamber there is a stone filtration and fabric filtration system has applied. After filtration chamber water is kept at collection tank. This water is clarified water. It is directly used for knit dyeing. And for boiler and woven dyeing water is allowed to pass through a carbon screening chamber and a softener. For water softening geolyte is used that means sodium permutit.
Requirements for Water Treatment Plant: 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:
When designing a new system choose corrosion resistant material to minimize the effect
Apply protective painting
Protect catholically using sacrifice metals
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:
Temperature
Alkalinity or Acidity
Amount of scale forming material present
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:
Dirt and silt
Sand
Corrosion products
Microbial organism
Aluminum phosphates
Process in Water Treatment Plant: The treatment process can be divided in two sections:
Pre-treatment methods
Demineralization methods
1. 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 and 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:
Mechanical methods
Chemical methods
2. Demineralization Methods: The following methods are normally used for demineralization of water:
Membrane based physical process and electro chemical process
Separation by Phase change method
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 and Maintenance cost
Storage capacity
Other Factors to be Considered:
Short or long term requirement of equipment or service
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
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