Introduction to Water Treatment

Water treatment is required for surface waters and some ground waters for drinking purposes. Water treatments involves the removal of pollutants generated from different sources and to produce water that is pure and suitable for human consumption without causing any long term or short-term adverse health effects. Many aquifers and isolated surface waters are high in water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes or fire control. However, clean water sources are the exception in many parts of the world, particularly regions where the population is dense or where there is heavy agricultural use. In these places, the water supply must receive varying degrees of treatment before distribution.

The available raw water must be treated and purified before they can be supplied to the public for their domestic, industrial or any other uses. The extent of treatment required to be given to the particular water depends upon the characteristics and quality of the available water and also upon the quality requirements for the intended use. Raw water may contain suspended, colloidal and dissolved impurities. The purpose of water treatments is to remove all those impurities which are objectionable either from taste and odour perspective or from public health perspective.

Impurities enter water as it moves through the atmosphere, across the earth’s surface and between soil particles in the ground. These background levels of impurities are often supplemented by human activities. Chemicals from industrial discharges and pathogenic organisms of human origin, if allowed to enter the water distribution system, may cause health problems. Excessive silt and other solids may make water aesthetically unpleasant and unsightly. Heavy metal pollution, including lead, zinc and copper may be caused by corrosion of the very pipes that carry water from its source to the consumer.

The method and degree of water treatment are important considerations for environmental engineers. Generally, the characteristics of raw water determine the treatment method. Most public water systems are relied on for drinking water as well as for industrial consumption and firefighting, so that human consumption, the highest use of the water, defines the degree of treatment. The flow chart of the process occurring in conventional water treatment plant is shown below in Fig.1.

Fig.1 Process of Water Treatment

A typical water treatment plant is diagrammed in Fig.2. It is designed to remove odour, colour, and turbidity as well as bacteria and other contaminants. Raw water entering a treatment plant usually has significant turbidity caused by colloidal clay and silt particles. These particles carry an electrostatic charge that keeps them in continual motion and prevents them from colliding and sticking together. Chemicals like alum (aluminium sulphate) are added to the water both to neutralize the particles electrically and to aid in making them “sticky” so that they can coalesce and form large particles called flocs. This process is called coagulation.

Fig.2 Diagram of a Typical Water Treatment Facility

For surface water, following are the treatment processes that are generally adopted.

1) Screening

This is adopted to remove all the floating matter from surface waters. It is generally provided at the intake point

2) Aeration

This is adopted to remove objectionable tastes and colour and also to remove the dissolved gases such as carbon-dioxide, hydrogen sulphide etc. The iron and manganese present in water also oxidized to some extent. This process is optional and is not adopted in cases where water does not contain objectionable taste and odour.

3) Sedimentation with or without Coagulants

The purpose of sedimentation is to remove the suspended impurities. With the help of plain sedimentation, silt, sand etc. can be removed. However, with the help of sedimentation with coagulants, very fine suspended particles and some bacteria can be removed.

4) Filtration

The process of filtration forms the most important stage in the purification of water. Filtration removes very fine suspended impurities and colloidal impurities that may have escaped the sedimentation tanks. In addition to this, the micro-organisms present in the water are largely removed.

5) Disinfection

It is carried out to eliminate or reduce to a safe minimum limit, the remaining microorganisms and to prevent the contamination of water during its transit from the treatment plant to the place of its consumption

6) Miscellaneous Processes

These include water softening, desalination, removal of iron, manganese and other harmful constituents.

Objectives of Water Treatment

• To remove the dissolved gases and colour of water.
• To remove the unpleasant and objectionable tastes and odours from the water.
• To kill all the pathogenic organisms which are harmful to the human health.
• To make water fit for domestic use such as cooking, washing and various industrial purposes as dyeing, steam generation etc.
• To eradicate the contaminants that are contained in water as found in nature.
• To control the impurities from scale formation.
• Pure water quality is required to minimize the corrosion, radiation levels and fouling of heat transfer surfaces in reactor facility systems.
• To prove safe potable water to the public.
• To reduce the physical, chemical and biological contaminants in water.
• To eliminate the tuberculation and corrosive properties of water which affects the conduits and pipes.

Location of Treatment Plant

The water treatment plants should be located as near to the towns as possible. If the source of water supply is tube well the treatment plant should be located in the central part of the town, so that purified water may reach the public as early as possible. If the city is very large to which water cannot be supplied from one tube well, the city should be divided into zones and a separate tube well with necessary treatment plants should be provided for each zone. If the source of water is river or reservoir, the treatment plant should be located as near the town as possible preferably in the central place. The following points should be kept in mind while giving the layout of any treatment plant.

• All the plants should be located in order of sequence, so that water from one process should directly go into next process.
• If possible, all the plants should be located at such elevations that water should flow from one plant to next under its force of gravity only.
• All the treatment units should be arranged in such a way that minimum area is required, it will also insure economy in its cost.
• Sufficient area should be occupied for future extension in the beginning.
• Staff quarters and office should also be provided near the treatment plants, so that operators can watch the plants easily.
• The site of treatment plant should be very neat and give very good aesthetic appearance.