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	Time: 20 hours Level: Introductory

	Introduction Introduction Resource Without it we are dead! Water is essential, but what processes must it go through to become fit for human consumption? This unit will guide you through the continuous cycling of water between land, open...

	Some facts about water Some facts about water Resource We can all relate to water. We know we need it to survive – indeed all the great civilizations of the world (the Egyptian, Greek, Mesopotamian, etc.) were centred around river valleys where there was a...

	1 The hydrological cycle 1.1 Introduction Resource The hydrological cycle, the continuous cycling of water between land, open water surfaces and the sea, either directly or indirectly, is an extremely complex process which has been known for a long time... 1.2 Evaporation Resource At an interface with the atmosphere, water changes its state from a liquid to a vapour in response to an increase in temperature caused by an external heat source. This temperature change is normally the... 1.3 Transpiration Resource If there were no vegetation, the rate of evaporation from land surfaces after rain would diminish rapidly to a very low value. Plants increase this rate by transpiration. In this process, water is transferred... 1.4 Condensation Resource As air rises it expands, owing to the decrease in pressure with height, and as it expands, in theory it cools at an average rate of 1°C for every 100 m of altitude. As the air cools, it becomes saturated... 1.5 Air circulation Resource At this stage, air circulation enters and plays a dual role. Firstly, winds transmit moisture horizontally from one location to another. In this way, moisture derived from oceanic evaporation can be transported... 1.6 Precipitation Resource Precipitation is defined as the depth of rainfall, or the water equivalent of snow, sleet and hail falling during a given measurement period. It may be in the form of rain, snow,... 1.7 Infiltration Resource Entry of precipitation through the soil surface and on downwards, by gravity, is known as infiltration. The rate at which this process can take place is governed by the permeability (a measure of the ease... 1.8 Surface run-off Resource In some inland drainage areas, all water is removed by evaporation and infiltration. However, precipitation not penetrating the land surface usually runs off the surface along defined channels which have... 1.9 Percolation Resource Movement of infiltrated water downwards through the zone of aeration (Figure 5) is known as percolation. The infiltrated water which does not remain held by capillary forces in the surface soils moves... 1.10 Aquifers Resource Groundwater is water that, after infiltrating and percolating through surface soils, flows into an aquifer, an underground water-bearing layer of porous rock. About one-third... 1.11 Storage Resource In a given fixed space at any phase of the hydrological cycle, there is an inflow and an outflow of water, the rates of which vary with time. The total cumulative difference between inflow and outflow... 1.12 Summary Resource The hydrological cycle is a complex process involving evaporation, transpiration, condensation, air circulation, precipitation, infiltration, surface run-off and percolation. Aquifers are an important...

	2 The natural aquatic environment 2.1 Water, the medium of life Resource The list of necessities for the provision of life includes various nutrients and water: water is one of the basic resources needed for the process of photosynthesis. Since it is an excellent solvent, water,... 2.2 Dissolved oxygen Resource Organic and inorganic nutrients are the basic food supply essential for maintaining the plants and animals in natural watercourses. Equally essential to aquatic life is a supply of oxygen, needed for respiration.... 2.3 Physical characteristics of natural waters Resource A river's physical characteristics include: 2.4 Chemical characteristics of natural waters Resource Since water is such a good solvent, it is not surprising to find many different chemical substances present in it. Water, on reaching a river, will contain inorganic and organic compounds which were dissolved... 2.5 Biological characteristics of natural waters Resource In addition to the easily visible plants and animals which live in or on a river, there are many small and often microscopic species which play a vital role in maintaining the health of a river. Their... 2.6 Tidal rivers and estuaries Resource Most of the major cities and harbours in the world are located on estuaries. The estuarine ecosystem is a unique intermediate between the sea, the land and fresh water. 2.7 Summary Resource Water in its ‘natural’ state supports a complex, yet fragile, ecosystem. The ability of natural watercourses to sustain aquatic life depends on a variety of physical, chemical and biological conditions....

	3 Water treatment 3.1 Introduction Resource Water for public supply can be obtained from underground sources by wells sunk into aquifers, or from surface sources such as purpose-built reservoirs or lakes (collecting rainwater run-off or water from... 3.2 Preliminary treatment Resource The abstracted water is first screened to remove suspended and floating debris, such as leaves or branches, which could interfere with the operation of machinery in the treatment works. The water may then... 3.3 Coagulation and flocculation Resource Coagulation is always considered along with flocculation and is used to remove particles which cannot be removed by sedimentation or filtration alone. These particles are usually less than... 3.4 Sedimentation Resource When water has little or no movement, suspended solids sink to the bottom under the force of gravity and form a sediment. You will recall that we discussed a similar process in estuaries, with solids separating... 3.5 Flotation Resource An alternative technique to that of sedimentation is flotation. This uses gas bubbles to increase the buoyancy of suspended solids. The gas bubbles attach to the particles and make their effective density... 3.6 Filtration Resource In filtration, the partially treated water is passed through a medium such as sand or anthracite, which acts as a ‘strainer’, retaining the fine organic and inorganic material and allowing clean water... 3.7 Disinfection Resource Before water can be passed into the public supply, it is necessary to remove all potentially pathogenic micro-organisms. Since these micro-organisms are extremely small, it is not possible to guarantee... 3.8 Additional treatment Resource As a result of strict standards set by the EU Directive on the Quality of Drinking Water, it is now often necessary for drinking water to have further treatment to remove components such as nitrates and... 3.9 Membrane filtration Resource Membrane filtration is a process whereby particles smaller than about 10−2 mm (which can pass through sand filters) are removed using synthetic polymeric membranes and a high pressure. The membrane effectively... 3.10 Fluoridation Resource The addition of fluoride to water has caused much controversy and public debate. The problem seems to be that some see it as the addition of a poison, and others see it as the use of mass medication whether... 3.11 Plumbo-solvency Resource Many water supplies in the UK are naturally acidic, and when this type of water is supplied through lead pipes the lead dissolves into the water. Lead pipes are dominant in many older... 3.12 Sludge treatment and disposal Resource The sludge collected in any sedimentation tank in the water treatment process has to be disposed of. In some instances the wet sludge is transported to the nearest sewage works where it is discharged into... 3.13 Groundwater treatment Resource The treatment of groundwater frequently does not extend beyond disinfection. However, groundwater may contain dissolved substances such as carbon dioxide and iron for which additional treatment (aeration,... 3.14 Desalination Resource In many parts of the world, surface water or non-saline groundwater stocks are not adequate to satisfy the water demand. While one may immediately think of the Middle East as being one such area, it is... 3.15 Summary Resource The basic water treatment process for surface waters consists of preliminary screening and storage, followed by coagulation and flocculation to allow the aggregation of colloidal particles. Sedimentation...

	4 Water supply 4.1 Introduction Resource This section is concerned with the plant used in carrying water from the treatment works to houses, farms, blocks of flats and other buildings of a community. The major components of this distribution... 4.2 Service reservoirs Resource Transmission mains convey treated water from the water treatment works to the service reservoir throughout all, or most of, the day. However, as with the demand for electricity or gas, the demand for water... 4.3 Distribution systems Resource The water from service reservoirs is distributed by a network of pipes of various sizes, laid beneath the streets, pavements and verges of our towns and cities. Any part of a distribution system can be... 4.4 Summary Resource After passing through a treatment works, water flows via transmission mains to service reservoirs which act to even out variations in consumption during a 24-hour demand period. It is then taken by distribution...

	References and Acknowledgements References Resource Acknowledgements Resource

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