How we treat your drinking water

Nearly all the water we supply is first collected in reservoirs. These reservoirs either supply directly a nearby Water Treatment Works (WTW) or they release water into rivers which we then take out in the lower reaches.

The sort of treatment we provide depends on the nature of the water coming into our Water Treatment Works. Water from springs for example only requires disinfection, because it is usually already very clean having been filtered naturally. Water from reservoirs or from rivers usually requires extensive treatment and this is the task carried out by our modern Water Treatment Works.

1 Screening – the water passes through a coarse mesh to remove leaves and other debris.

2 Coagulation – small particles in the water which are to small to be removed by the screens are "coagulated" using an approved chemical to bind them into larger particles.

3 Clarification – the coagulated particles are then separated from the clarified water to form sludge which is removed for disposal.

4 Filtration – the clarified water now passes through sand and gravel which takes out any remaining particles. These filters are washed regularly. The water is now completely clean and colourless.

5 pH correction – if the water is too acid, lime is added to adjust the pH value (the measure of acidity). Water that is too acid can corrode pipes and fittings.

6 Final disinfection – the water is disinfected, usually with chlorine, to make sure it is free from bacteria and safe to drink.

7 Storage – Fully treated water then leaves the WTW and is piped to covered local storage reservoirs from which is then piped to households and businesses.

2. Discuss the following information in small groups and answer the questions:

– What do we need to get the water from our water Treatment Works to our customers?

– What quality parameters do we test each year?

– How do they cream waste water in Wales?

– What is the top priority for cleaning waste water accordingly to new European legislation?

– How do they treat your drinking water? Describe the process.

– What are the main stages of fully treated water?

3. Abstract writing. Pair work. Write 6-8 sentence abstracts of four text that you have already read. Summarize all the principal information. Present your summaries to the class and choose the summary that is the best.

Text 5.

1. Read the text below and find the answers to the following questions:

– How do they treat your waste water?

– How do you understand screening and removing grit?

– What is primary treatment?

– Describe two main types of secondary treatment.

– What is the aim final settlement?

– What is sludge treatment used for?

How we treat your waste water

Our sewerage network takes away waste water from houses and businesses for treatment at one of our Sewage Treatment Works (STW) or to a Sea Outfall where currently only partial or no treatment is provided. The investment programme for the next five years will provide sewage treatment at most of the Sea Outfalls.

1 Screening and removing grit – when it reaches the STW, the waste water is passed through screens which remove rags, paper, plastics and so on. Grit, which comes from road drainage, may also be removed in specially designed tanks or channels.

2 Primary treatment – after screening and grit removal, the sewage then flows into settling tanks where solid material sinks to the bottom. This solid material is called "sludge" and is removed from the tanks for further treatment.

3 Secondary treatment – there are two main types of secondary treatment. The first is called "biological filtration" and the second "activated sludge". Both rely on the action of natural bacteria to break down the organic matter that is left in the sewage after primary settlement. With biological filtration, the sewage is spread over a filter bed of small stones. As the sewage seeps down, bacteria, fungi and other organisms feed on the organic matter leaving a virtually clear liquid flowing from the base of the filter bed. With the "activated sludge" method, the sewage goes into a deep channel where sludge containing the bacteria which digest the organic matter is added and the whole process is then accelerated by blowing in air.

4 Final settlement – after secondary treatment, the nearly clean water goes into another settlement tank where remaining particles sink to the bottom. After final settlement the water is clean enough to go back into a river.

5 Sludge treatment – sludge is a by-product of sewage treatment. It is treated to destroy harmful organisms and to remove its unpleasant smell. Once treated, it is then a valuable fertiliser which can be used on farmland.