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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 env
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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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3.4.1 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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3.3.3 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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3.2 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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2.11 Storage

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 is the storage. So within that space there is a body of water whose mass is not directly controlled by instantaneous values of inflow and outflow. For example, in river flow the movement of the whole body of water in the channel is generally downstream, yet a given reach contains a volume whose
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2.9 Percolation

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 by the action of gravity through the unsaturated layers of soil or rock until it arrives at the water table. Here the percolated water joins the body of groundwater which seeps slowly to the sea.

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2.4 Condensation

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 with water vapour which condenses around small particles in the air. These particles may occur naturally, such as soil particles or salt particles residual to evaporation of sea spray, or they may be produced artificially during combustion. A measure of the necessary cooling to produce conde
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4 Introduction to systems practice

This course teaches some aspects of systems thinking and practice. But what does it mean to be a systems practitioner, and is it different to being a manager? This section attempts to answer those questions.

First, I believe a good systems practitioner will be more competent at handling complex situations, more capable of managing their working and domestic lives, and more able to learn not only how to learn but also how to act more effectively by using systemic concepts and techniques.
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2.5 Back to your definition

Now you have worked through this section, reflect on your own version of what learning is, as you drafted it for your learning file at the start of this section. Did you give more emphasis to the outcomes of learning, or to the process? Or did you find a way of balancing the two? Try to revise your own wording to your own satisfaction.

Learning is …

Do remember that definitions of learning often reflect the kinds of learning that were most important to us at the time, or were in
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1.7 Conclusions

Could both of these students have got more from their involvement with the course if they had taken time to reflect on their goals and their strengths and weaknesses, especially at the beginning of study? Alan, whose reaction to the course was positive, for example, could have learned more about how the course succeeded if he had reflected rather more in the beginning about his initial scepticism and his preference for communicating verbally rather than in writing. What was the reason for his
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1.6 Building on strengths

Activity 1

A self-review exercise for your learning file

The aim of this activity is to encourage you to take a problem-solving approach to your own learning, and to be proactive. The first part asks you to refle
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions) and is used under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.

Course image: drpavloff in Flickr ma
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3.4.6 Residual stress

One factor that can cause serious problems in any material is the presence of residual tensile stress. The problem often arises as a direct result of manufacturing, when hot material is shaped and then allowed to cool to ambient temperatures. For large castings like those needed to make the eye bars, such residual stress would be modified by the subsequent heat treatment to strengthen the steel, but had to be studied as part of the research effort into the catastrophic failure of the bridge.<
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3.4.3 Simulated environmental tests

The investigators wanted to know about the fatigue properties of the component, to find a feasible explanation of why it took 39 years for the eye bar to break. They needed information on the several stress corrosion mechanisms that were possible in the material, including hydrogen embrittlement, the effects of sulphur compounds such as H2S (hydrogen sulphide) and the effects of moisture and salt. Notched eye-bar material was loaded to failure in various environments.

In fact
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3.2 The disaster

The 39-year-old Silver Bridge collapsed suddenly at about 5 p.m. on 15 December 1967 when the roadway was filled with rush-hour traffic – 37 vehicles were trapped on the roadway.

The first signs of collapse were later recounted by the survivors. Many occupants of the cars on the bridge had felt it ‘quivering’ before it fell. Most witnesses had then heard ‘cracking’ or ‘popping’ noises, some saying that it sounded like a ‘shotgun blast’. After this, the bridge started d
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2.6.1 Stress corrosion cracking in stainless-steel structures

On 9 May 1985 the roof of a swimming pool at Uster near Zurich collapsed, killing 12 and injuring several others.

The concrete roof had been held up by a set of stainless-steel tie bars, which were found after the accident to have cracked transversely (Figure 21). Chlorine is added at qu
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Learning outcomes

After studying this course, you should be able to:

  • understand the common techniques underlying free verse and traditional forms of poetry

  • identify personal experiences that can be used when writing poems

  • understand the basic terminology and practical elements of poetry.


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2.9 How successful is Grice's theory of the meaning of utterances?

I turn now to difficulties for Grice's account of the meaning of utterances, beginning with a concern over his methodology. By focusing on examples, real or imagined, Grice attempts to draw out our intuitions and so lead us, as he has been led, to Grice 3. But perhaps our intuitions are wildly inaccurate, or wildly irrelevant. We need to check that Grice's notion of meaning, mined out of his and our intuitions, delivers what we were after when we turned to him for a theory of the meaning of u
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The Arts Past and Present: Mosaics
How can we read an image to tell us more about its ancient maker? In this album a mosaic artist, Catherine Parkinson, visits the splendidly-preserved ancient Roman mosaics at Brading Villa on the Isle of Wight. With the help of two archaeologists she discovers that the iconography reveals important clues about the villa inhabitants' world view, taste, and aspirations. Their leisure pursuits, the value placed on learning, and their views on men and women are just some of the themes revealed in
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Except for third party materials and otherwise stated (see http://www.open.ac.uk/conditions terms and conditions), this content is made available under a http://creativecommons.org/licenses/by-nc-sa/2

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