Read the following text and speak on every stage of water treat- ment.
Text C. Conventional Water Treatment. Sedimentation and Flotation. Filtration
Sedimentation is the process of precipitation of sediment (matter that settles to the bottom of a liquid under the force of gravity) which is ac- complished in the sedimentation tank. A settling [sedimentation / precip- itation] tank is a tank in which suspended matter is removed either by quiescent settlement or by continuous flow and extended retention time to allow deposition. Sedimentation is used to remove settleable suspend- ed solids from waters which are high in sediment content after coagula- tion and flocculation processes. The sedimentation basin is located close to the flocculation basin so the transit between does not allow settlement or floc break up. Types of sedimentation tanks include:
· rectangular with horizontal flow;
· circular with radial flow;
· hopper-bottomed with upward flow.
The amount of floc settling out of the water depends on the retention time of the water in the basin (minimum 4 hours) and the depth of the basin (there are shallow or deep basins). As particles settle, a layer of sludge is formed at the bottom of the tank. Sludge is thick, soft, wet mud or a similar viscous mixture of liquid and solid components which is then removed and treated. The amount of sludge is usually 3-5% of the total volume of water treated. The cost of treating and disposing of sludge is a significant part of the operation cost of a water treatment plant.
An alternative technique to sedimentation is flotation. It is the use of gas bubbles for increasing the buoyancy of suspended solids and rising the particles through the water to float on the surface of the water to be collected by a skimmer. The advantage of flotation over sedimentation is more complete removal of small or light particles in a shorter time.
Filtration is the process of separating particles from a liquid by pass- ing the liquid through a medium (filter) that will not pass the particles.
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Even after coagulation and flocculation, sedimentation does not remove all suspended impurities from the water to make it crystal clear and safe. The remaining non-settling floc still causes turbidity and contains micro- organisms. Suspended solids, colloidal material (algae, silt, iron, manga- nese), bacteria, germs, and other microorganisms are filtered out by pass- ing the water through a bed (a layer) of granular material (usually fine sand, gravel, garnet, pulverized coal or related substances), or through a matrix of fibrous material supported on a perforated core*. However, soluble materials such as salts and metals in ionic form are not removed by filtration.
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There are several classifications of filters:
· according to the direction of flow through the filter bed (down- flow, upflow, biflow, radial flow, horizontal flow);
· according to the type of filter media used (sand, coal, anthracite, coal-sand, multilayered);
· according to flow rate (slow, rapid).
Most modern water treatment plants now use rapid dual-media filters following coagulation and sedimentation. A dual-media filter consists of a layer of anthracite coal (for trapping most of the large floc) above a layer of fine sand (for trapping smaller impurities). This process is called in-depth filtration. In order to enhance in-depth filtration, mixed-media filters (with a third layer of fine-grained, dense mineral called garnet at the bottom of the bed) are used in some treatment plants. Rapid filters have certain advantages over slow filters: they require much less surface area, they are easier to clean and more reliable. Backwashing [backwash
/ back-flushing] is the reverse of the direction of flow through the filter for cleaning the filter bed clogged by particles removed from the water. The development in filter technology doesn’t stand still. Membrane fil- tration is increasingly becoming popular as an advanced water and wastewater treatment process. There are various possibilities of mem- brane filtration: microfiltration; ultrafiltration; reverse osmosis; nanofil- tration.
After filtration, the water moves into a disinfection chamber.
*a matrix of fibrous material supported on a perforated core – ре- шетка (сетка) из волокнистого материала, закрепленная на перфо- рированном каркасе
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17. Read the following text and say what the purposes and charac- teristics of every stage of water treatment are and what activities each of them includes.
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