How to Remove Colour, Odour and Taste from Water? | Water Engineering

In this article we will discuss about how to remove colour, odour and taste from water.

Aeration:

Aeration is a process in which water is brought in intimate contact with atmospheric air to promote exchange of gases between water and atmospheric air.

The aeration of water serves the following purposes:

(i) In the course of aeration, oxygen from atmospheric air is absorbed by water thereby oxygen deficiency of water is eliminated and also freshness is imparted to water.

(ii) Carbon dioxide, hydrogen sulphide and other volatile substances imparting taste and odour to water are easily expelled by aeration.

(iii) Iron and manganese present in water are oxidized to certain extent by aeration.

The following four types of aerators are generally adopted for aeration in the treatment of water:

(a) Diffused Air Aerators:

A diffused air aerator consists of a tank or basin in which at the bottom perforated pipes are provided. The water to be aerated is filled in the tank and compressed air is blown through the perforated pipes. The air bubbles emerging from the perforations rise up from the bottom of the tank. While rising up the air bubbles come in close contact with water contained in the tank, and aeration of water is thus achieved. The tanks are generally 3 to 4.5 m deep and 3 to 9 m wide.

For effective mixing of air and water ratios of width to depth should not exceed 2:1. The tanks work on the principle of continuous flow with detention period varying from 10 to 30 minutes. The amount of air required ranges from 0.06 to 1 m³ of air per 1000 litres of water treated. The diffusers are located on one side of the tank.

(b) Cascade Aerators:

In cascade aerators water is allowed to flow downwards after spreading over inclined thin sheets and the turbulence is created by allowing water to pass through a series of steps or baffles. The number of steps is usually 4 to 6. Water is allowed to fall through a height of about 1 to 3 m. In this case removal of gas varies from 20 to 45 percent of carbon dioxide and about 35 percent of hydrogen sulphide.

(c) Spray Aerators:

In spray aerators water is sprayed through nozzles upwards into atmosphere and broken up into either a mist of droplets. The installation consists of fixed nozzles on a pipe grid and trays for collecting the sprayed water. Nozzles usually have diameters varying from 10 to 40 mm spaced at intervals of 0.5 to 1 m or more.

The pressure required at the nozzle head is usually 7 m of water but it may vary from 2 to 9 m of water. The discharge rating per nozzle varies from 18 to 36 m³/hour. Spray aerators are usually quite efficient with respect to gas transfer and can remove 70 to 90 percent of carbon dioxide and 90 to 99 percent of hydrogen sulphide.

(d) Water-Fall or Multiple Tray Aerators:

A water-fall or multiple tray aerators consists of a number of trays with perforated bottoms, arranged vertically in series. Water flowing through a riser pipe is discharged through perforated pipes into the top most trays and after flowing down through each of the lower trays, it is collected in a basin provided at the base. During the downward flow through trays water comes in contact with atmospheric air and thus aeration takes place.

Usually trays about 4 to 9 in number with spacing of 300 to 750 mm are provided. In most aerators coarse media such as coke, stone or ceramic balls ranging from 50 to 150 mm in diameter are placed in the trays to increase the efficiency of aeration.

With the media, good turbulence is created and large water surface is exposed to atmosphere resulting in an increase in the efficiency of aeration. The provision of media is also beneficial for removal of iron from water. The removal efficiencies of these aerators usually vary from 65 to 90 percent of carbon dioxide and 60 to 70 percent of hydrogen sulphide.

Treatment by Activated Carbon:

Activated carbon is used to remove colour, taste and odour from water. Activated carbon is produced by heating a carbonaceous material such as coke, charcoal, paper mill waste, saw dust, lignite, etc., in a closed vessel at a high temperature. It is then activated or oxidised by passing air, steam, carbon dioxide, chlorine or flue gases. The activation of the carbonaceous materials removes the hydrocarbons which might interfere with the adsorption of organic matter.

The activated carbon is available in various trade names such as Darco, Nuchar and Minchar. It is available in granular as well as powder form. The grains are of 6 mm size and below. Its weight is 4 kN per m³ and it is highly porous in structure. Activated carbon removes organic contaminants from water by the process of adsorption. In adsorption high surface area is the prime consideration. Granular activated carbons typically have surface areas of 500- 1400 m²/gm.

Methods of Application:

Activated carbon can be applied for treatment of water in two ways:

(a) As filter media,

(b) As fine powder feed.

As a filter media, a bed of granular activated carbon is placed instead of sand bed in the rapid sand filters. The activated carbon filter bed is supported on a graded gravel bed. Filtered water is passed through the bed of activated carbon. This will remove taste and dour from water and will also absorb organic compounds. With continuous use, the activated carbon loses its adsorptive power. It is then required to be rejuvenated.

The rejuvenation of activated carbon bed is carried out by passing live steam into the activated carbon bed through perforated pipes which are placed in the gravel bed of the filter. The process of rejuvenation may have to be carried out at intervals varying from 1 month to 1 year depending upon the quality of water treated.

In powder form activated carbon may be applied to water at various stages of its treatment as indicated below:

(i) It may be added to raw water before any treatment is carried out.

(ii) It may be added along with coagulant in mixing tank.

(iii) A portion of activated carbon may be added in mixing tank and the remaining portion may be added to water just before it enters filters. This is known as split method

(iv) It may be added just at the point where water enters filters. It may be applied at constant rate or at varying rate. In the later case the rate of application is high when filter is washed and it becomes lower and lower as filter gets clogged.

The usual dose of activated carbon varies from 5 to 20 p.p.m. However, the optimum dose of activated carbon may be determined on the basis of test results.

Activated carbon treatment has the following advantages:

(i) It helps the process of coagulation, if adopted before filtration of water.

(ii) It reduces the chlorine demand of treated water.

(iii) It removes tastes, odours and colours caused by the presence of excess chlorine, hydrogen sulphide, phenol, iron, manganese, etc.

(iv) It removes organic matter present in water.

(v) It is effective in preventing or retarding the decomposition of sludge in settling basins.

(vi) Its overdose is harmless.

Use of Copper Sulphate:

Copper sulphate CuSO₄ is used to serve the following two purposes:

(i) Removal of colour, odour and taste from water.

(ii) Control the growth of algae, bacteria and some types of aquatic weeds.

Copper sulphate is available in powder form or crystal form. It is easily soluble in water. It can be applied either in the distribution pipes or in open reservoirs. In the former case, a solution of copper sulphate of required strength is prepared and it is added just at the entry of water in the distribution system.

In the later case copper sulphate may be applied in one of the following ways:

(i) The crystals of copper sulphate are ground and converted into powder and this powder is sprinkled on the water surface in reservoir from a moving boat or an airplane.

(ii) The crystals of copper sulphate are placed in porous bags and these hags are attached behind a moving boat.

(iii) Solution of copper sulphate of required strength is prepared and it is sprayed on the water surface in reservoirs from a moving boat.

The usual dose of copper sulphate is about 0.30 to 0.65 p.p.m. Such amount of copper sulphate does not make water unfit for drinking purposes or for industrial purposes, but it may prove detrimental to certain types of fish.