The previous article in this series gave its readers a brief overview of different methods which are commonly used to treat water. In this article, we will be taking a closer look at specialized methods which are used to remove colour, odour and taste from the water. Based on the source from where the water is obtained or because of the natural pathways through which the water flows, water might get polluted and get unwanted colour, odour or taste. Many times, natural and man-made water sources get contaminated due to pollution from the industry or other commercial activities.
To be more specific, most of the times, the colour, odour and taste can be due to presence of dissolved gases such as hydrogen sulphide, organic matter, microbial content and contamination due to industrial waste containing excessive chlorine and phenol.
Here are some of the methods which are used to teat water in such situations:
Aeration is a process of mixing air thoroughly in the water. This facilitates the ingression of oxygen in the water. Aeration leads to water purification in multiple ways, including removal of carbon dioxide, iron, manganese, hydrogen sulphide etc. Aeration also helps to kill bacteria to a certain extent. Large scale aeration of water can be carried out by one of the following methods:
Air Diffusion: In this method, air is diffused through the water with the help of placing perforated pipes at the bottom of the settling tanks. Generally, the diffusion is carried out rigorously and the water is churned out at a high velocity.
Trickling Beds: This method makes use of trickling trays. Trickling trays are trays with perforated bottoms. The water from these trays trickles down to another layer of trays below and during this process, air gets mixed in it.
Spray Nozzles: In this method, nozzles are used to spray water to a height of 2 to 2.5 meters under a pressure of 0.7 to 1.15 kg/sq.cm. The spraying under pressure breaks down the water into millions of fine droplets which increases the surface area of the water and thus the rate of aeration. This is one of the most efficient methods of aeration.
Cascades: This method makes use of sequential concrete steps or weir like structures. When water is passed over these cascades, atmospheric air get mixed with it thoroughly.
One important point to note is, water should be aerated moderately. Over aeration tends to increase the corrosive nature of the water and such water needs to be de-aerated.
Treatment with activated carbon
Activated carbon acts as a great solution for removing tastes and odour from water because of its high adsorption capabilities. It attracts and entraps gases and fine organic matter which impart unwanted tastes and odour to water. Availability of activated carbon based filters and their cartridges makes it very simple to use them at small as well as large scales. Use of activates carbon also brings down the need of chlorination.
Treatment with Copper Sulphate
Copper Sulphate is mixed with water in form of a solution. It can be used either at a water source like lake or a tank or can be applied to the water at the entrance of distribution line. Normally, 0.5 to 0.65 mg of Copper Sulphate is used with a liter of treated water. Copper Sulphate puts a check on the growth of algae in the water and hence, commonly used for prevention of algae growth in large sized water reservoirs.
Excess use of Copper Sulphate can adversely affect the aquatic ecosystem by killing fishes and other creatures in the water.
Use of Oxidising Agents:
Oxidising agents like chlorine, potassium permanganate, ozone can directly oxidise the organic matter in the water which accounts for majority of tastes and odours. While these oxidising agents are easy to use, they should be used not in excess. Gases like chlorine dioxide and ozone are very effective oxidising agents but are costly which makes their use unfeasible. If the water already contains excess of chlorine, it might be required to de-chlorinize.
The methods listed above include almost all of the commonly used methods to remove odour, colour and taste. The actual selection of a method from the above list is done based on analysing the water, sources of contamination and the quantities in which different types of contaminants are present.
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