PRE TREATMENT OF WATER

If the raw water has good quality than it can be directly goes in secondary treatment processes of flocculation/ coagulation and sedimentation. Prior to the secondary treatment some steps are followed. They are as follows:

• Screening
• Storage
• Chemical pre-treatment

SCREENING

Coarse screens with inclined bars of 25 mm in diameter and 100 mm spacing avoid the large floating materials from entering the treatment plant. Raking is done with the inclination of bars. The velocities set for screens is 0.5 m/s that may be automatically or manually raked down. If there is no facilitation of storage than fine screens are fitted after the coarse screens. If here is storage than fine screens are placed at the outlet of storage tanks. Fine screens have the openings with 6 mm diameter or square. Exclusive forms of screens are generally used and they are normally automatically cleaned. 

The screens can be of circular drum type or travelling belt type as in vertical escalator. Screens poses the head loss that accounts for hydraulic calculations. Another type of screening is micro screening with mesh opening of 20 micrometer to 40 micrometer. Such screens are used to treat the uncontaminated waters and moderately colored waters. 

STORAGE

Storage is required for municipal water supply systems to meet variable water demand, fire protection, and for emergency needs. The reservoirs used are of three types:

• Surface Reservoir
• Standpipes
• Elevated Tanks

Surface Reservoirs

Surface reservoirs are located at the location where sufficient water pressure is provided. They are normally covered to prevent the contamination. 

Standpipes

These are tall cylindrical tanks. Its upper portion is used for storage and its lower portion supports the structure. The standpipes with the height of 15 m are not economical but above this height the storage tanks become the choice. Water demands of residential areas changes over day. 

Elevated Tanks

In recent years elevated tanks have becoming less famous due to their high cost and due to availability of invariable speed pumps and controls that make it possible to adjust pumping rates with varying demand. The location, type of storage and storage size must be determined. This depend on population and purpose of storage.