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6.4 Summary

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 mains from the service reservoirs to the consumers. The head of water necessary for flow is provided by positioning the service reservoir at the correct level or by pumping. A pumped pipeline is called a rising main. The size of mains pipelines depends on the quantity and flow rate of
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5.15 Summary

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 of the aggregated particles produces sludge and partially purified water. The purification process is completed by filtering and disinfecting the water before distribution. Some waters may require additional treatment: for example, nitrate removal, or fluoridation, or further removal of organ
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5.14.4 Solar distillation

The energy available in solar radiation can be harnessed to distil sea water. In a simple and inexpensive solar still system designed by the Technical University of Athens, for the island of Patmos, sea water is first pumped to a feed reservoir from which it flows by gravity, when required, into a large shallow basin divided into long narrow sections (Figure 36). Separating these channels are concrete strips, which provide access for maintenance. The interior surface of the entire basin is li
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5.11 Plumbo-solvency

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 established areas. The Drinking Water Directive has set a maximum admissible concentration of 10 μg 1−1 lead in water, to be achieved by the year 2013. The obvious solution to this problem is to remove all lead piping but this is a costly exercise. As an interim measure, the water l
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5.10 Fluoridation

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 the individual wishes it or not.

Many waters do, however, have a natural fluoride content (Figure 33) and it has been suggested that the presence of fluoride in a concentration of 1.0 mg l−1 is beneficial in preventing dental decay. Above this concentration there is the
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5.9.3 Nanofiltration

Nano is a prefix that means 10−9, i.e. very, very small. You may have come across nanotechnology. Nanofiltration is similar to reverse osmosis and employs membranes that are capable of physical sieving and diffusion-controlled transport. Nanofiltration systems operate at much lower pressures than reverse osmosis systems, but yield higher flow rates of permeate. The quality of the permeate is not as good as with reverse osmosis, with particles in the size range 0.0005–0.005 μm
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5.9.2 Ultrafiltration (UF)

This employs membranes with smaller pores (0.001–0.02 μm) than those for microfiltration and utilises much greater pressure (up to 3000 kPa). An atomic mass unit is 1/12 of the mass of a neutral atom of the most abundant isotope of carbon, i.e. I.66X 10−27 kg.

Commonly, the membranes are made of polysulphone, polyacrylonitrile, polyamide and cellulose acetate. Inorganic ceramic membranes are also used. Owing to its ability to remove very small particles, UF is mainly use
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5.9 Membrane filtration

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 acts as a sieve.

It is increasingly becoming popular as an advanced treatment process for water (especially for removal of Cryptosporidium) and wastewater (where water reuse takes place), and various possibilities are:


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5.8.2 Removal of trace organic compounds

After conventional treatment, water may still contain trace concentrations of synthetic organic compounds, which, if left in the water, can lead to taste and odour problems. The problem is most likely to arise where the raw water source has been badly polluted. The problem can be solved by including the process of granular activated carbon adsorption after the filtration process. Activated carbon is carbon which has been activated by heating in the absence of oxygen. This resu
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5.8.1 Nitrate removal

Nitrate in water has become a significant problem and the EU Directive sets a maximum admissible concentration of 50 g m−3 measured as NO3. This is equivalent to 11.3 g m−3 as N. High nitrate levels can cause cyanosis or methaemoglobinaemia in babies. Legislation allows the designation of nitrate-vulnerable zones and these help to prevent nitrate levels in natural waters increasing in affected areas.

Ion exchange is used in some
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5.7.2 Elimination of pathogens through solar disinfection

The lack of safe drinking water in many developing countries has prompted research into simple methods of disinfecting small quantities of water. One such investigation at the University of Beirut in the Lebanon revealed that 99.9% of total bacteria in a water sample could be destroyed by 300 minutes exposure to direct sunlight. In effect this means that if you left a sample of water in a translucent container, a lot of the bacteria in it would be killed.

Research to date has concentrat
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5.7.1 Mixed oxidant gases system

This is a relatively new system of disinfection. It involves electrolysis of high-purity NaCl brine to produce a mixture of chlorine dioxide, ozone and hypochlorite. This mixture is separated within the electrochemical cell by a membrane, or by exploiting density difference, and is then metered into the water requiring disinfection. The mixed oxidant gases are generated on demand and this is a great safety advantage, compared with having storage tanks of chlorine on site. The source for the d
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5.7 Disinfection

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 their complete removal by sedimentation and filtration, so the water must be disinfected to ensure its quality. Disinfection is the inactivation of pathogenic organisms and is not to be confused with sterilisation, which is the destruction of all organisms.

Worldwide, chlorine is the most
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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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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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2.1 Introduction

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 (Figure 1). The identifiable mechanisms of the cycle are complicated not only by the characteristics of air-water-land interfaces across which the cycle operates, but also by climatic factors which vary in both time and space. The various operations and mechanisms within the cycle are illustra
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3.4 Conclusion

The headings alongside each of the activities in this article were there to remind you of the three different types of learning to which you were introduced in Section 2: memorising, understanding and doing. The three models of the learning process that were discussed in the reading – acquisitive, constructivist and experiential – have strengths particularly for each of these three kinds of learning.

Some learning goals require that we know information accurately and can recall it w
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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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1.1 Effective course study

Research into how people study effectively suggests that it is important to pay attention not only to the content of what we are trying to learn but also to the process of our learning. Time spent on the process of how you are learning need not be a distraction from achieving your learning goals. It should support your efforts to achieve them.

However, thinking about the process of your own learning is not something which typically forms part of most formal courses of study. Most people
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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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