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5.6 Filtration

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 through. The action of filters is complex and in some types of filter biological action also takes place. Sand filters are used in water treatment to remove the fine particles which cannot be economically removed by sedimentation. They have been effective in removing Cryptosporidium, a pr
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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. Susp
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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 inland
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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 suitable a
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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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2.7 Infiltration

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 with which water can flow through the subsurface layer) and by the existing degree of saturation of the soil. Infiltration can be impeded by outcropping impermeable rocks or by paved areas, and also by the presence of finegrained soils with a low permeability (such as clay). At certain times
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3.3 The experiential model of learning

The main proponent of this approach to learning, David Kolb, put forward a theory which he intended to be sufficiently general to account for all forms of learning (Kolb, 1984). He argued that there are four distinctive kinds of knowledge and that each is associated with a distinctive kind of learning. The four kinds of learning are:

  • concrete experiencing

  • reflective observation

  • abstract analysis

  • activ
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3.4.5 Fretting fatigue

An additional possibility was considered. It was known that there was significant movement of the bridge during passage of traffic, because users had noticed it many times when crossing. The joints would thus have been subjected to rotary motion around the pin in order to accommodate such vibrations. Could these have caused fatigue crack growth at the bearing surfaces?

Contact between a circular and a flat plate creates so-called Hertzian stresses at the contact zone: compressive at the
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3.4.2 Analysis of the eye-bar steel

Many sections were taken of the steel near the fracture to examine its microstructure, and were compared with different parts of the same eye bar as well as with other eye bars. The sections showed a steel core surrounded by a zone that could be identified as being of higher strength due to the presence of martensite.

Martensite is a strong, hard phase of steel usually formed by rapid quenching from a high temperature.

XPS, X-ray photoelectron spectroscopy, gives information about
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3.4 Analysis of eye bar 330

One particular broken part was recognised quickly as part of an eye bar. There were 146 eye bars in the original bridge, and they were safety-critical because if broken the main chains could be threatened. The eye bar was identified as being from the top joint in the hanging chain nearest the bank and next to the Ohio tower, one of the two lower bars on the outside of the bridge facing north, upriver (Author(s): The Open University

3.3.3 Reassembling the parts

As the wreckage was pulled from the river it was examined and identified, and any failures of the metal components were recognised and tagged. This was a mammoth task, given that virtually the whole bridge had fallen into the water, including all the road decks, trusses, chains and hangers, eye bars and the two towers. The parts were then reassembled and all the failed or fractured components photographed and catalogued. Over 90 per cent of the bridge components were collected together and re
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3.3.2 Planning the investigation

A plan was needed to determine the chain of events leading up to and during the collapse. That sequence would necessarily depend on which parts had broken first, and a fault tree would enable a plan of action in isolating the cause (or causes) of the disaster. Such a systematic approach is known as fault-tree analysis or FTA, and is part of the armoury of methods used by accident investigators. With large-scale and devastating accidents, all possibilities, however remote, need evaluation in t
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3.3.1 Sequence of events

It was important to establish the precise sequence of events leading up to and during the collapse. From which part had the collapse started? Why had so much of the structure been destroyed? Was there any prior warning of the failure? What part had the weather conditions at the time played?

Eyewitnesses were plentiful, and each had a different perspective of the bridge as it fell. There were some common parts to their statements. Most of the witnesses, especially survivors from vehicles
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3.3 The investigation

The investigation took three years to complete, although critical evidence emerged within weeks of the accident.

Some possibilities could be ruled out immediately. For example, there were rumours of supernatural forces at work that night, but very little solid evidence of the ‘Mothman’ emerged, either there or anywhere else. The Mothman was a demon purportedly haunting the bridge, which has supposedly appeared as a portent of similar disasters around the world. Such stories would ha
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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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2.6 Corrosion in stressed products – stress corrosion cracking (SCC)

If a stress exists in a product exposed to a corrosive environment, the rate of corrosion can then increase and be extremely localised, such as at the tip of a growing crack. Furthermore, some specific chemicals are so aggressive that corrosion will occur at relatively low stress levels, such as those created during manufacture, for example. The residual stress in a component can then be enough to trigger crack growth and failure.


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2.3 Corrosion processes

For many materials, degradation processes are simply one or a series of chemical reactions that act to erode or deteriorate the material. The deterioration of metals is a little more complex than that of non-metals because metals are electrical conductors. Local electrochemical cells freq
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2.1 Introduction

Structures are not always doomed to fail, but they do usually have a limited useful life. Exceptions include many of the monuments that have survived from the ancient world, such as the Great Pyramid in Egypt (Figure 11a), the Pont du Gard in southern France (Author(s): The Open University

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