In this unit operation, gas transfer is promoted between water and air.
The purposes of aeration is –
a) To add oxygen to water for imparting freshness.
b) Expulsion of CO2, Sulphide and other volatile substances causing taste and odor.
c) To precipitate impurities like Fe and Mn in various forms.
The rate of gas transfer is a function of concentration gradient i.e. difference in gas concentration between gas and liquid plane. The limiting value of concentration gradient depends on solubility of gas or saturation concentration. The saturation concentration of gas decreases with increasing temperature. The rate of aeration of gas is governed by area of interface between gas and liquid, the thickness of interface film and time of contact.
The overall gas transfer equation useful for practical purpose is
Cs – C
--------- = e – Kf x t x A/V
Cs – Co
Where,
Cs = Saturation concentration, mg/lit.
Co = initial concentration of gas, mg/lit
C = concentration of gas after time t, mg/lit
Kf = overall mass transfer coefficient, cm/hr
A/V = interfacial surface area to unit volume of water.
t = time of contact, hr
Aeration systems may involve either bringing air in contact with water orb ringing water in contact with air.
Types of aerators:-
1) Spray aerators.
2) Waterfall or multiple trays.
3) Cascade aerators.
4) Mechanical aerators.
Spray aerators: - Water is sprayed through nozzles upward into the atmosphere and broken up into droplets. The installation consists of pipe-network with fixed nozzles and with necessary outlet arrangements. It is efficient and can be expected to remove 70-90 % of CO2 and 90-99 % of H2S and adds to the appearance of the water treatment plant.
Waterfall or multiple trays: - Water is discharged through the riser pipe and distributed on a series of trays containing coarse media such as coke, stones or ceramic balls ranging from 5-15 cm in diameter to increase the efficiency. This is beneficial for iron removal. The CO2 removal efficiency varies between 65-90 % and 60-70 % for H2S.
Cascade aerators: - Water is allowed to flow downward on a series of steps and baffles. The exposure time can be increased by increasing the number of steps. The head requirement varies from 0.5 to 3 m and the space required is varies between 0.5 to 0.65 Sqm/KLD. The gas transfer efficiency is less compared to spray aerators. The removal efficiency varies between 20-45 % of CO2 and 20-35 % of H2S.
Diffused Air Aerators :- It consist of a tank with a network of perforated pipes with nozzles and plates provided at the bottom to release fine bubbles of compressed air which then rise through the water being aerated. The time of contact is increased due to lower velocity of up going fine air bubbles compared to falling water drops. The amount of air required ranges from 0.06 to 1.0 cum of air/KL of water treated. The power requirement of blower varies between 3-10 Watts/KL capacity. This would also help to mix chemicals for subsequent treatments.
Mechanical Aerators:- These are not normally used in water treatment because of availability of more economical alternatives but find application in waste water treatment.
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