Rapido and sand filter from spm standard textbook for university exams

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I now have all the content needed for a complete answer. Here is the comprehensive notes on Rapid Sand Filter from Park's Textbook of Preventive and Social Medicine:

Rapid Sand Filter (Mechanical Filter)

Source: Park's Textbook of Preventive and Social Medicine, pp. 813-815

Introduction

In 1885, the first rapid sand filters were installed in the USA. Since then, they have gained considerable popularity especially in highly industrialized countries.

Rapid sand filters are of two types:

Gravity type - e.g., Paterson's filter
Pressure type - e.g., Candy's filter

Steps of Purification

The following steps are involved in purification of water by rapid sand filters:

Flow diagram of a rapid sand filtration plant showing River → Alum → Mixing chamber → Flocculation chamber → Sedimentation tank → Filters → Chlorine → Clear water storage → Consumption

Fig. 6 - Flow diagram of a rapid sand filtration plant (Park's SPM, p. 814)

1. Coagulation

Raw water is treated with a chemical coagulant - alum (aluminium sulphate)
Dose: 5 to 40 mg/L or more, depending on turbidity, colour, temperature, and pH of the water

2. Rapid Mixing

Treated water is subjected to violent agitation in a "mixing chamber" for a few minutes
Ensures quick and thorough dissemination of alum throughout the bulk of water

3. Flocculation

Water undergoes slow and gentle stirring in a "flocculation chamber" for about 30 minutes
Mechanical flocculator (most widely used) consists of paddles rotating at 2 to 4 rpm, driven by motors
This produces a thick, copious, white flocculant precipitate of aluminium hydroxide
Thicker the precipitate/flock diameter = greater the settling velocity

4. Sedimentation

Coagulated water is led into sedimentation tanks where it is detained for 2 to 6 hours
Flocculent precipitate along with impurities and bacteria settle down
At least 95% of the flocculant precipitate must be removed before water enters rapid sand filters
Sludge is removed periodically without disturbing operation

5. Filtration

Partly clarified water is subjected to rapid sand filtration

Filter Beds

Parameter	Details
Surface area per unit	~80 to 90 m² (~900 sq. feet)
Filtering medium	Sand
Effective size of sand particles	0.4 to 0.7 mm
Depth of sand bed	~1 metre (2½ to 3 feet)
Gravel layer below sand	30 to 40 cm (1 to 1½ feet)
Depth of water on top of sand bed	1.0 to 1.5 m (5 to 6 feet)
Rate of filtration	200 million gallons per acre per day (m.g.a.d.)

The gravel layer supports the sand bed and permits filtered water to move freely to the under-drains.

Filtration Process

The "alum-floc" not removed by sedimentation is held back on the sand bed
It forms a slimy layer comparable to the zoogleal layer in slow sand filters
It adsorbs bacteria and effects purification; oxidation of ammonia also takes place
As filtration proceeds, suspended impurities and bacteria clog the filters
When "loss of head" approaches 7 to 8 feet, filtration is stopped and backwashing begins

Backwashing

Rapid sand filters need frequent washing - daily or weekly, depending on loss of head
Washing is accomplished by reversing the flow of water through the sand bed
Backwashing dislodges impurities and cleans the sand bed
Washing is stopped when clear sand is visible and wash water is clear
Whole process takes about 15 minutes
In some rapid sand filters, compressed air is also used as part of backwashing

Advantages of Rapid Sand Filter over Slow Sand Filter

Can deal with raw water directly - no preliminary storage needed
Filter beds occupy less space
Filtration is rapid - 40 to 50 times that of a slow sand filter
Washing of the filter is easy (backwashing)
There is more flexibility in operation

Comparison of Rapid Sand Filter vs. Slow Sand Filter (TABLE 3)

Feature	Rapid Sand Filter	Slow Sand Filter
1. Space	Occupies very little space	Occupies large area
2. Rate of filtration	200 m.g.a.d.	2-3 m.g.a.d.
3. Effective size of sand	0.4-0.7 mm	0.2-0.3 mm
4. Preliminary treatment	Chemical coagulation and sedimentation	Plain sedimentation
5. Washing	By back-washing	By scraping the sand bed
6. Operation	Highly skilled	Less skilled
7. Loss of head allowed	6-8 feet (2-2.5 m)	4 feet (1.5 m)
8. Removal of turbidity	Good	Good
9. Removal of colour	Good	Fair
10. Removal of bacteria	98-99%	99.9-99.99%

Key exam point: Slow sand filter is superior to rapid sand filter in removal of bacteria (99.9-99.99% vs 98-99%). However, rapid sand filter is better for removal of colour and is much faster in rate.

Key Numbers to Remember for Exams

Parameter	Value
Alum dose	5-40 mg/L
Flocculation time	30 minutes
Sedimentation time	2-6 hours
% flock removed before filtration	≥ 95%
Sand size	0.4-0.7 mm
Sand depth	1 metre
Rate of filtration	200 m.g.a.d.
Backwash time	15 minutes
Loss of head to stop filtration	7-8 feet
Bacterial removal	98-99%

(Park's Textbook of Preventive and Social Medicine, pp. 813-815)

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