The Chemical Principles and Practices in the Purification for Domestic use of Water from Rivers and Reservoirs in the UK

In a modern society, water is one of the things in life we often take for granted. When we turn on the tap, we expect water that is clean, safe and suitable for all household tasks. But in recent years, there has been a growing concern about contamination of our water supplies. No water is 100 percent pure. It contains contaminants from natural and man-made sources, such as minerals, gases, bacteria, metals and chemicals. Many of these contaminants are harmless. However, some impurities can adversely affect your health. Others damage equipment, stain laundry and fixtures and emit odours.

Methods used to improve the quality of water are referred to as treatment or conditioning. Water treatment refers to systems that reduce harmful contaminants in the water, dealing with health and safety of the water. High levels of coliform bacteria, nitrates, arsenic, lead and pesticides are examples of harmful contaminants that must be treated before water is safe to drink.

Water conditioning refers to water problems that affect water taste, colour, odour, hardness and corrositivity rather than health and safety. The presence of high levels of magnesium, calcium, iron, manganese and silt are common contaminants that require water conditioning. It is common to use both treatment and conditioning methods to improve water quality.

Fine sand, silt, clay and other particles are removed using a mechanical filter or sedimentation. Bacterial contamination is treated by chlorination; however, this may form toxic organic compounds in the water, such as trihalomethanes, so an activated carbon filter is used to remove these. Trihalomethane compounds are suspected carcinogens, so it is extremely undesirable to have these present in the water supply. The activated carbon filter will also remove taste, odour, some pesticides, radon and help filter out particles. Dissolved iron and manganese can be removed using an oxidising filter. Heavy metals such as lead, mercury, arsenic or cadmium are ...

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The use of chlorine is the oldest and most common disinfection method for water supplies. Chlorine is inexpensive and readily available, reliable, easy to use and monitor, and effective against most pathogenic bacteria, virus and cyst organisms. It also kills non-pathogenic iron, manganese and sulphur bacteria. Chlorine is also a strong oxidising agent, which causes a problem mineral such as soluble iron and manganese to change to an insoluble precipitate so it can be filtered from the water.

Ultraviolet light is a relatively new method of disinfecting private water systems. Ultraviolet radiation adds nothing to the water and does not produce any taste or odour. The UV light is produced by a mercury vapour lamp, which produces a disinfecting dose, rated in microwatt-seconds per square centimetre (MWs/cm2). Values of 20,000 MWs/cm2 kill most types of pathogenic bacteria. However, viruses are more resistant and variable and need up to 45,000 MWs/cm2.

An ultraviolet water treatment device is quite simple. The most common design consists of a stainless cylindrical chamber with a cylindrical mercury arc lamp located in it. Water enters one end of the chamber, flows through the chamber around the lamp and exits the other end. To be effective as a disinfection treatment, ultraviolet radiation must pass through every particle of water. The thinner the water film and the slower the water flow, the more effective the system will be. Also, the water cannot have any turbidity, suspended soil particles, or organic matter. As a consequence, ultraviolet light treatment is only used after the water has been thoroughly filtered.

Filtration represents a broad category of treatment systems used to remove particles, taste, odour, some organics and minerals, and some bacteria from the water. Mechanical or sedimentation filters simply retain debris as water passes through the filter unit. Mechanical filters are most effective for removing particles such as sand, silt, ferric iron, algae and some bacteria. Their effectiveness will depend on the particle size and the exit clearance of the filter. Activated carbon filtration is a common treatment to remove offensive tastes and odour, colour, chlorine and volatile organic chemicals, pesticides and trihalomethanes. Activated carbon will not remove bacteria, dissolved metals such as iron, lead, manganese and copper, or chlorides, nitrates and fluorides. Activated carbon filters, usually made up of granulated, powdered or block carbon, act like a sponge with a large surface area to absorb contaminants in the water.

Oxidising filters are used mainly to remove iron, manganese and hydrogen sulphide. Manganese greensand is a common chemically reactive medium designed to remove iron and manganese that is in solution. The greensand will also act as a filter and catch iron and manganese precipitates that have been oxidised. The unit works by providing oxygen to the iron and manganese from the greensand bed. As a result, these minerals change from their soluble to insoluble form. The precipitated minerals become trapped as rust particles within the greensand filter bed. Neutralising filters are typically used for pH modification, or treating acidic water. A neutralising filter is normally a pressure filter tank filled with limestone chips. As the water passes through the filter bed, calcium carbonate is dissolved into the water and the water pH is increased, reducing its acidity.

Reverse osmosis treatment decreases dissolved minerals in the water. It successfully treats water with high salt content, and dissolved minerals such as nitrate, sulphate, calcium, magnesium, potassium, manganese, aluminium, fluoride, silica, boron and bicarbonate. Reverse osmosis is also effective with some taste, colour and odour-producing chemicals, certain organic contaminants, and specific pesticides.

Although the reverse osmosis membrane is capable of rejecting virtually all microorganisms, it can develop pinholes or tears that allow bacteria or other microorganisms to pass into the treated water. So reverse osmosis is used only for bacteriologically safe water.

Since there are many methods of water purification, and different techniques can lead to the same good quality of water, the above mentioned methods are not always used in conjunction with each other. Many are just one step of a multi-phase commercial treatment process, or can be used individually in home water treatment systems.