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Potable water treatment
Potable water treatment

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5.4 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 water carried. The construction material depends on stresses imposed during use and other factors (size, ground conditions, etc.).

The video you are about to see shows many of the treatment processes which you have studied, together with some new ones, such as the Pulsator. Please watch the video then answer the following questions.

Video, click to watch 'Potable Water Treatment Plant'

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Self-assessment questions

SAQ 31

How are the pathogens in the reservoir of water inactivated?

Answer

By UV radiation from sunlight.

SAQ 32

What is the coagulant used to bring the dead algal cells together?

Answer

Ferric sulphate.

SAQ 33

Why does the water have to be softened?

Answer

Because hard water forms scale in pipes and home appliances. This eventually leads to blockage of pipes, and reduced efficiency. Hard water also results in scum formation, and does not lather easily.

SAQ 34

What happens to the dirty water produced from cleaning of the rapid gravity sand filters?

Answer

It goes to lagoons where the solids settle out. The supernatant is then sent back to the reservoir.

SAQ 35

How is the granular activated carbon regenerated?

Answer

By burning off the adsorbed organics.

SAQ 36

What are the various ways in which ozone is used in the treatment plan?

Answer

It is used for several purposes: to kill the algae, to breakdown pesticides, to oxidize any trace organics, to kill bacteria and viruses, and to destroy carcinogens, and eliminate odour and taste-conferring compounds.