purification of water on large scale public health dentistry

The supernatant water (1 to 1.5 metres), sand bed (1.2 metres), gravel support (0.3 metres) and filter bottom (0.16 metres) together form the filter box. It is an open rectangular box, 2.5 to 4 metres deep and is built wholly or partly below ground. The walls may be made of stone, brick or cement.

Filter cleaning:

When the bed resistance increases to such an extent that the regulating valve has to be kept fully open, it is time to clean the filter bed. The supernatant water is then drained off and the sand bed is cleaned by scraping off the top portion of the sand layer to a depth of 1-2 cm. After 20 or 30 scrapings, the thickness of the sand bed will have reduced and a new bed is constructed.

RAPID SAND OR MECHANICAL FILTERS

Before being subjected to rapid sand filtration, the water has to undergo coagulation, rapid mixing, flocculation and sedimentation.

The steps involved are,

1. Coagulation: the raw water is first treated with a chemical coagulant such as alum.

2. Rapid mixing: the treated water is then subjected to violent agitation in a "mixing chamber" for a few minutes. This allows a quick and through dissemination of alum throughout the bulk of the water.

3. Flocculation: this phase involves a slow and gentle stirring of the treated water in a "flocculation chamber" for about 30 minutes. This results in the formation of a thick, copious, white flocculent precipitate of aluminum hydroxide.

4. Sedimentation: the coagulated water is then led into sedimentation tanks

where it is kept for 2-6 hours, when the flocculent precipitate together with impurities and bacteria settle down in the tank.

5. Filtration: the partly purified water is then subjected to rapid sand filtration.

Filter Tank

Filter beds:

Wash Troughs

Filter Sand

Graded Gravel

Perforated Pipes

Filter Floor

Each unit of the filter bed has a surface of 80 to 90 m². Sand is the filtering medium. The effective size of the sand particles is between 0.4 -0.7 mm. The depth of the sand bed is usually about 1 metre. Below the sand bed is a layer of graded gravel, 30 to 40 cm deep. The gravel supports the sand bed and permits the filtered water to move freely towards the under drains. The depth of the water on the top of the sand bed is 1.0 to 1.5 m. The rate of filtration is 5-15 m³/m²/hour.

Filtration removes the remaining alum floc not removed by sedimentation. As filtration proceeds, the suspended impurities and bacteria clog the filters resulting in their reduced efficiency called "loss of head".

Filter cleaning: Back washing

Rapid sand filters need frequent washing aily or weekly, depending upon the loss of Dead. Washing is accomplished by reversing the flow of water through the sand bed, which is called "back washing". It helps in dislodging the impurities and cleaning up
the sand bed. The washing is stopped when the wash water is sufficiently clean. The whole process of washing takes about 15 minutes.

ADVANTAGES

Slow sand filter

Rapid sand filter

It is simp

1. AMIT KUMAR SINGH BDS FINAL YEAR GROUP-B Purification of water On large scale

2. TABLE OF CONTENT INTRODUCTION Purification of water on a large scale Storage Filtration Slow sand or BIOLOGICAL filters Vital layer FILTER BOX FILTER CLEANING RAPID SAND OR MECHANICAL Filter beds Filter cleaning back washing

3. PURIFICATION OF WATER Water purification is a crucial process for ensuring the safety and quality of our water supply. It involves removing impurities, contaminants, and harmful microorganisms from water sources to make it suitable for consumption and other uses. Water purification can be done on a large scale, such as in municipal water treatment plants, or on a small scale, such as in homes or for individual use. INTRODUCTION

4. PURIFICATION OF WATER ON A LARGE SCALE Storage The first stage of large-scale water purification involves storage, where the raw water is held in reservoirs or tanks. During storage, several natural processes occur that contribute to water purification. Filtration Filtration is the second stage, where the water is passed through filters to remove suspended particles and other impurities. There are two main types of filters used in large-scale water purification: slow sand filters and rapid sand filters.

5. STORAGE Physical Action During storage, gravity causes about 90% of suspended impurities to settle to the bottom within 24 hours. This makes the water clearer, allowing light to penetrate. Chemical Action Aerobic bacteria use dissolved oxygen to oxidize organic matter in the water, reducing free ammonia and increasing nitrate concentration . Biological Action Pathogenic organisms gradually die out during storage. However, prolonged storage can lead to the growth of algae , which can impart bad smell and color to the water. The bacterial count drops significantly in the first 5-7 days .

6. PHYSICAL ACTION CHEMICAL ACTION BIOLOGICAL ACTION

7. FILTRATION Slow Sand Filters Slow sand filters are biological filters that use layers of sand and gravel to remove impurities. A vital layer of microorganisms forms on the surface of the sand, contributing to the reduction of disease-causing organisms. Rapid Sand Filters Rapid sand filters are mechanical filters that use a bed of sand to remove suspended particles. Before rapid sand filtration, water undergoes coagulation, rapid mixing,and sedimentation to remove larger particles.

8. SLOW SAND FILTERS Supernatant Water The supernatant water layer above the sand bed provides a constant head of water to overcome filter bed resistance and promotes downward water flow. It also allows time for sedimentation, oxidation, and particle agglomeration. It is 1-1.5 meter above sand bed Sand Bed The sand bed, about 1 meter thick , consists of graded sand particles with an effective diameter between 0.2 and 0.3 mm . Water percolates slowly through the sand, undergoing mechanical straining, sedimentation, adsorption, oxidation, and bacterial action . sand bed is supported by graded gravel which is 30-40cm deep rate of filtration 0.1 – 0.4 meter cube per hour per square meter

9. Filter Control Valves Filter control valves maintain a constant filtration rate . A venturi meter measures bed resistance , and the regulating valve is adjusted to maintain a steady flow. Under Drainage System The under drainage system at the bottom of the filter bed consists of perforated pipes that provide an outlet for filtered water and support the filter medium.

10. VITAL LAYER A slimy growth, known as the "Schmutzdecke," vital layer, zoogleal layer, or biological layer, forms on the surface of the sand bed after a few days of operation. This layer consists of algae, plankton, diatoms, and bacteria. The vital layer is essential for removing organic matter, holding back bacteria, oxidizing ammoniacal nitrogen into nitrates, and purifying the water. It extends 2 to 3 cm into the top portion of the sand bed.

11. The formation of the vital layer is known as "ripening " of the filter. The supernatant water, sand bed, gravel support, and filter bottom together form the filter box, which is an open rectangular box built wholly or partly below ground.

12. FILTER CLEANING When the bed resistance increases, the filter needs cleaning. The supernatant water is drained, and the sand bed is cleaned by scraping off the top portion of the sand layer to a depth of 1-2 cm. After multiple scrapings 20-30 times , a new sand bed is constructed. Rate of filtration is 5-15 meter cube per hour per square meter .

13. RAPID SAND FILTERS Coagulation Raw water is treated with a coagulant, such as alum, to cause small particles to clump together . Rapid Mixing The treated water is subjected to violent agitation in a mixing chamber to distribute the coagulant evenly. For few minutes Flocculation The water is slowly stirred in a flocculation chamber to form a thick, flocculent precipitate of aluminum hydroxide .This process runs for 30 minutes Sedimentation The coagulated water is then led into sedimentation tanks, where the precipitate settles to the bottom . Filtration The partly purified water is then subjected to rapid sand filtration to remove any remaining alum floc.

14. FILTER TANK Sand is the filtering medium, with an effective size of 0.4-0.7 mm. Filter Beds Each unit of the filter bed has a surface area of 80 to 90 m². Filter Sand Graded Gravel A layer of graded gravel supports the sand bed and allows filtered water to flow freely to the under drains. Perforated Pipes Perforated pipes collect the filtered water and direct it to the outlet. Filter Floor The filter floor supports the entire filter bed and prevents sand from escaping.

15. FILTER CLEANING : BACK WASHING Rapid sand filters require frequent washing to remove accumulated impurities. Backwashing involves reversing the flow of water through the sand bed, dislodging impurities and cleaning the sand. The washing is stopped when the wash water is clean, and the entire process takes about 15 minutes .

16. CONCLUSION Water has a profound influence on human health. At a very basic level, a minimum amount of water is required for consumption on a daily basis for survival and therefore access to some form of water is essential for life. However, water has much broader influences on health and wellbeing and issues such as the quantity and quality of the water supplied are important in determining the health of individuals and whole communities.

17. REFERENCES SOBEN PETER 7 TH EDITION IMAGE REFERENCES https://www.civilconcept.com/different-between-slow-sand-filterssf-and-rapid-sand-filterrsf/ https://fitness365days.com/blog/tag/sun-charged-water/ https://www.technologynetworks.com/immunology/ebooks/large-insights-into-microorganisms-366302

18. Thank you