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II. Reading Comprehension.



1. First skim the text. You’ll probably come across a number of words you don’t know. Don’t stop to look up these words, but try to understand the main points of the text.

Water Treatment and Methods of Water Treatment

 

The presence of impurities of natural and man-made origin in raw water makes this water unacceptable and dangerous for drinking. All known impurities can be divided into the following groups: a) suspended solids, b) heavy metal ions, c) dissolved organic substances, d) microorganisms, and e) phytoplankton / zooplankton.

Water treatment is application of physical and chemical processes to a water supply to remove tastes, odors, dirt and debris, to reduce “hard” elements and salts, and to destroy harmful organisms.

 

Methods of Water Treatment

 

Screening. The first step in purifying surface water is to remove large debris like sticks, leaves, trash and other large particles which may interfere with subsequent purification steps. The smaller the holes in the screen, the smaller the debris must be to pass through. Groundwater does not need screening before other purification steps.

Storage.Water from rivers may also be stored in bankside reservoirs for periods between a few days and many months to allow natural biological purification to take place. The filtered water is then treated to remove or inactivate remaining potentially harmful microscopic organisms including viruses, protozoa and bacteria. This removal step comprises part of a multistep process of disinfection which is completed by chemical and/or ultraviolet light treatment which damages and makes non-infectious any remaining viable harmful microbes. For waters that are particularly difficult to treat such as from catchments with intensive agriculture, extra physical, chemical and biological treatment steps may be necessary.

Coagulation and flocculation.Together coagulation and flocculation is a traditional purification method which works by using active chemicals called coagulants that effectively "glue" small suspended particles together so that they settle out of the water or stick to sand or other granules in a granular media filter. In a relatively new and economically attractive development polymer film with chemically formed microscopic pores called micro or ultrafiltration membranes can be used in place of granular media to filter water effectively without coagulants.

Coagulation normally works by eliminating the natural electrical charge of the suspended particles so they attract and stick to each other. The joining of the particles so that they will form larger settleable particles is called flocculation. The larger formed particles are called floe.

Sedimentation.Water exiting the flocculation basin enters the sedimentation basin, also called a clarifier. It is a large tank with slow flow, allowing floc to settle to the bottom.

The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. In order to keep the water in the basin longer, while treating the same amount of water, the basin volume must be increased. A deep basin will allow more floc to settle out than a shallow basin..

As particles settle to the bottom of the basin a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated.