Disinfection / disinfectants / to disinfect
is the the process designed to kill most microorganisms in wa- ter, including essentially all disease-causing bacteria. destroy harmful bacteria and deactivate viruses.
Aeration / aerator / air / to aerate
mixes air with water either by spraying the water into the air or by forcing small bubbles through the water and is used primari- ly to reduce unpleasant odours and tastes.
Softening / softener / to soften
is the process of removing calcium and magnesium from the water either by chemical precipitation or by ion exchange.
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Make a summary of the text according to the following plan.
1. The title of the text is “…” .
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2. 
The text is devoted to … .
3. Such problems as… are touched upon in the text.
4. The text consists of … parts.
5. The first part deals with … .
6. The second (third, forth, etc.) part describes … .
7. The main idea of the text is to show … (to underline … / to prove … / to inform the reader about ...).
8. In my opinion, the text is useful / informative / interesting. It is worth reading.
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Skim over the text. Answer the following questions.
Text B. Conventional Water Treatment. Pretreatment. Coagulation and Flocculation
The conventional water treatment processes of greatest importance are coagulation and flocculation, sedimentation and flotation, filtra- tion, disinfection, as well as some additional treatment methods (sof- tening, aeration, carbon adsorption, distillation, deferrization, desali- nation, fluoridation, reverse osmosis, etc.).
Preliminary treatment (pretreatment) is any physical, chemical or mechanical process used before water undergoes the main treatment pro- cess. During pretreatment:
· coarse and fine screens or microstrainers may be used to remove rocks, sticks, leaves and other debris (screening);
· presedimentation settles out sand, grit and gravel from raw wa- ter. Sedimentation occurs naturally in reservoirs and is accomplished in treatment plants by settling basins (also called sedimentation basins or settling [settlement / sedimentation] tanks). Plain sedimentation will not remove extremely fine or colloidal material within a reasonable time, and the process is used principally as a preliminary to other treatment methods;
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· adding of chemicals may be added to control the growth of al- gae.
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Pict. 10. Conventional Water Treatment Process
Coagulation is a separation or precipitation from a dispersed state of suspensoid particles. Coagulation removes small particles made up of microbes, silt, and other suspended material in the water. By adding chemicals called coagulants (coagulating agents) to the water, fine non- settling particles and colloidal material form larger, heavier masses of solids by coagulation. These masses, called floc, are large enough to set- tle in basins and to be caught on the surface of filters. A precipitate forms and causes a clumping of the bacteria and other foreign particles which then settle out during the several hours of sedimentation. In this way about 85% of the bacteria and suspended particles, as well as some
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of the mineral elements (such as certain forms of iron) can be removed. The 3 main types of coagulants are inorganic electrolytes (alum, lime, ferric chloride, ferrous sulfate), organic polymers, and synthetic poly- electrolytes. Their application may have serious disadvantages because of possible negative effect on water consumers’ health. Considerable attention is focused on the development of new coagulants and floccu- lants, preferably from natural and renewable sources, which are safe for human health and biodegradable.
Coagulation is usually accomplished in 2 stages: rapid mixing and slow mixing.
· Rapid mixing serves to disperse the coagulants evenly through- out the water and to ensure a complete chemical reaction.
· Slow mixing (also called flocculation) is longer gentle agitation for promoting particle collisions and enhancing the growth of flocs. A flocculant (flocculating agent) is a reagent added to a dispersion of solids in water to bring together the fine particles to form flocs.
After flocculation the water flows into the sedimentation tanks where sedimentation or flotation is accomplished.
1. What are the most important conventional water treatment processes?
2. What is pretreatment (preliminary treatment)?
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3. What is used to remove debris during pretreatment? How is this pro- cess called?
4. What is the purpose of presedimentation?
5. Where is sedimentation accomplished in water treatment plants?
6. Why is presedimentation used as a preliminary to other treatment methods?
7. What may be added to control the growth of algae during pretreat- ment?
8. What is coagulation? What is the purpose of this method?
9. What are coagulants (coagulating agents)? What are the three main types of coagulants?
11. How are heavier masses of solids formed by coagulation called?
10. Why may the application of coagulants and flocculants have serious disadvantages?
11. What are two stages of coagulation? How is slow mixing called?
12. For what purpose is a flocculant (flocculating agent) added?
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13. Where are sedimentation and flotation accomplished?
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