SCREENING AND AERATION (METHOD OF WATER TREATMENT)

1.Screening

• It is the process of removal the heavier suspended solids from the water like plants, stones, animals, tree or other waste etc.
• It also help to protect the heavy and costly machinery like pumps or pipes from any potential damage or choking from heavy suspended solids. 

Screening can be broadly classified into two types of screens:

• Coarse screens
• Fine Screens

a.Coarse screens (Bar screen/ Trash racks)

• The screen are in the form of bars of diameter 10-20 mm and spacing of 20-100 mm between them.
• These screens are always placed at inclination of 3-6 vertical to 1 horizontal as it helps in effective cleaning of screening by racking mechanism.
• When water passes through screens, loos of head of water take place, the magnitude of which is given by: 

b.Fine Screens

• Fine screens in the form of wire mesh of size 10mm.
• Under normal treatment, Fine screens are generally avoided as it get frequently choked.
• It also requires cleaning at short intervals, thus increases operational cost.
• The GOI manual suggest the use of coarse screen instead of fine screens and remove the finer particles in other treatment stages.

2. Aeration

• It is the process in which water is brought in contact of oxygen.
• Remove dissolved gases like H2S and CO2.
• Carry out the oxidation of undesirable substances like organic matter and oils
• Removes volatile liquid like phenols, humic acids from water.
• Remove the dissolved minerals like Fe, Mg etc.
• Remove colour, odour and taste.
• Increase acidity and turbidity of water.
• Generally adopted for treating of water which is devoided of oxygen such as ground water or water at bottom of lake.

Methods of Aeration

a.Spray Nozzle Method

• Most efficient method
• Pressurized water is used to increase surface area of water.
• Used when quantity of water is less.

b.Cascade Aerator

• Efficiency is less (about 40%).
• Generally used in treatment plants.

c.Spray Tower Method

• Best method to remove iron, manganese and CO2.
• require more area for functioning.

d.Diffused air Method

• It has highest operational cost.
• Used when area is limited.