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5.3 Coagulation and flocculation

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 1 μm in size and are termed colloids. They have poor settling characteristics and are responsible for the colour and turbidity of water. They include clays, metal oxides, proteins, micro-organisms and organic substances such as those that give the brown coloration to water from ‘peaty’
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5.2 Preliminary treatment

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 enter a preliminary settlement tank or storage reservoir. It then passes through screens again and goes to the treatment works. Screens may be classified by the size of their openings as coarse or fine, and may be in the forms of bars or continuous belts. Coarse screens are u
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5.1 Introduction

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 streams) and rivers. The safety of the water is of utmost concern – several million people die each year after consuming contaminated water. The primary aim in water treatment is the elimination of any pathogenic micro-organisms present. All the above-mentioned sources can be subject to pol
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4.7 Summary

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. Biodegradable compounds, nutrients and dissolved oxygen must be available for the metabolic activities of the algae, fungi, bacteria and protozoa which are at the lowest level of the food chain. In addition, plant and animal growth cannot occur outside narrow ranges of temperature and pH.
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4.6 Tidal rivers and estuaries

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.

A rather precise definition of an estuary is ‘a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with fresh water derived from land drainage’. This excludes large bays with little or no freshwater flow, and large brackish seas and inl
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3.5.5 Biological indicators

A great many biological species and individuals occur in normal streams. These often differ markedly in their sensitivity to environmental factors, and likewise the tolerances of various species to different types of pollution vary considerably. The major groups of organisms that have been used as indicators of environmental pollution include bacteria, fungi, protozoa, algae, higher plants, macroinvertebrates and fish. The benthic ‘bottom living’ macroinvertebrates are particularly suitab
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3.5.4 Fungi

Fungi (e.g. species such as Streptomycin which are used for manufacture of antibiotics, and yeast) are generally unicellular non-photosynthetic organisms which can tolerate acid conditions. They are capable of degrading highly complex organic compounds. They utilise much the same food sources as bacteria but they require less nitrogen since their protein content is lower. Fungi play an important role in sewage treatment.

In polluted water, particularly near to a
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3.5.3 Protozoa

Protozoa are microscopic single cell animals. They utilise solid substances and bacteria as a food source. They can only function aerobically, and in a stream which contains little organic degradable matter they can become a predominant microbial type. They play an important part in sewage treatment where they remove free-swimming bacteria and help to produce a clear effluent.

In an aquatic environment, there are three main types of protozoa:

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3.5.2 Bacteria

Bacteria are organisms of special significance to the study of clean and polluted waters because they break down organic matter. While most of them are not harmful to humans, some bacteria (e.g. Clostridium) are pathogenic. Most bacteria are retained on a filter of pore size 0.45 μm and all bacteria are trapped on a filter of 0.22 μm. They are important in sewage treatment, and in solid waste disposal. They are extremely abundant in almost all parts of the aquatic en
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3.5.1 Algae

Algae are photosynthetic organisms that are generally aquatic; they are primary producers. Many freshwater algae are of microscopic size, but when amassed can be seen as a green, brown or blue-green scum. Blue-green algae are capable of producing toxins and these have caused the death of wild animals, farm livestock and domestic pets which have consumed the contaminated water. The toxins can produce a painful rash on human skin. The extract below shows what happened off the west coast
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3.5 Biological characteristics of natural waters

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 relevance to water quality is discussed further in the sections that follow.


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3.4.2 Radiological aspects

Environmental radiation comes from a range of naturally occurring and anthropogenic sources, with the former estimated to contribute more than 98% of the radiation dose experienced by people (excluding medical exposure). Any exposure to radiation can lead to cancer, and the greater the exposure, the greater the risk.

The contribution that drinking water makes to radiation intake is very small, and is due largely to naturally occurring radionuclides (isotopes of an element which are unst
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3.4.1 Plant nutrients

Plant nutrients are necessary in varying amounts for the growth, reproduction and well-being of growing plants.

Of the major nutrients of plants, nitrogen and phosphorus are important growth-limiting factors in primary production (i.e. they are likely to run out before any other element needed by the plants). Both nitrogen and phosphorus enter watercourses from natural leaching by water of the soluble nitrates and phosphates found in soils and rocks, as well as from sewage effluent and
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3.4 Chemical characteristics of natural waters

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 as rainwater percolated through the soil and rocks. In addition, some gases will dissolve in rainwater during its passage through the air.

The substances present in water may be conveniently grouped into:

  1. those from dissolved gases such as
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3.3.3 Temperature

All aquatic organisms have a fairly well-defined temperature tolerance range and this determines their distribution. Temperature affects the saturation concentration of dissolved oxygen (as seen in Table 2). An increase in water temperature will reduce the oxygen solubility as well as increase the metabolic activity of aquatic organisms. The combination of these two effects means that oxygen demand by organisms increases just when oxygen supply is being reduced.

Coarse fish such as perc
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3.3.2 Speed of flow/turbulence

To get nutrients and dissolved oxygen to all parts of a body of water, good mixing is important. We have had one example in the thermal stratification in deep lakes in summer (Figure 7), where poor mixing and reduced sunlight lead to the bottom layers not being supplied with the necessary conditions for plant growth. The same principle applies to rivers and streams. Fast-moving turbulent streams mix and agitate the water, aiding the transfer of oxygen from the atmosphere to the river, and car
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3.3.1 Turbidity, colour and suspended solids

As water runs off the land, there are some substances which do not dissolve but are taken along as suspended solids. Then, depending on their sizes and the velocity of the river, the solid particles may settle out at a certain point or be carried on further. Quantities are affected by seasonal changes and tend to be higher in winter because of increased storm runoff due to higher rainfall and melting snow.

The quantity of suspended solids (measured in g m<
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3.3 Physical characteristics of natural waters

A river's physical characteristics include:

  • clarity/turbidity

  • colour

  • speed of flow/turbulence

  • odour

  • the presence of plants and macroscopic animal life.

The physical characteristics are determined by location, geology and climate of the catchment area. In turn they influence the chemical and biological characteristics of the watercourse.

The physical appearance m
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3.2 Dissolved oxygen

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. Oxygen dissolved in the water is also needed in the biodegradation of organic matter by aerobic (oxygen-consuming) bacteria. A measure of this oxygen demand can be obtained experimentally and is defined as the biochemical oxygen demand (BOD). The BOD i
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3.1 Water, the medium of life

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, even in its ‘natural’ state, is never pure H2O but contains a variety of soluble inorganic and organic compounds. Water can also carry large amounts of insoluble material in suspension. The amounts and types of impurities vary with location and time of year, and determine so
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