Rapid and slow sand filter from spm standard textbook for university exams

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Rapid and Slow Sand Filters

From Park's Textbook of Preventive and Social Medicine

Introduction

Filtration is the second stage in water purification. 98-99% of bacteria are removed by filtration, apart from other impurities. Two types of filters are in use:

Slow sand filters (biological filters)
Rapid sand filters (mechanical filters)

I. SLOW SAND FILTER (Biological Filter)

Slow sand filters were first used in 1804 in Scotland, then in London. During the 19th century their use spread worldwide. They are generally accepted as the standard method of water purification.

Fig. 4 - Slow sand filter (Park's SPM)

Elements of a Slow Sand Filter

(1) Supernatant Water

Depth: 1 to 1.5 metres above the sand bed
Provides a constant head of water to overcome resistance of the filter bed and promote downward flow
Provides a waiting period of 3 to 12 hours for partial purification by sedimentation, oxidation and particle agglomeration
The level of supernatant water is always kept constant

(2) Sand Bed

Fig. 5 - Section of filter bed showing layers (Park's SPM)

Thickness of sand bed: ~1 metre
Sand grains: preferably rounded, effective diameter 0.2 to 0.3 mm
Sand must be clean, free from clay and organic matter
Supported by a layer of graded gravel 30-40 cm deep (prevents fine grains from entering drainage pipes)
1 cubic metre of filter sand presents ~15,000 sq. metres of surface area
Rate of filtration: 0.1 to 0.4 m³/hour/sq. metre of sand bed surface
Water percolates through sand very slowly (taking 2 hours or more)

Purification processes in the sand bed:

Mechanical straining
Sedimentation
Adsorption
Oxidation
Bacterial action

The Vital Layer (Schmutzdecke)

When a filter is newly laid, it acts only as a mechanical strainer. Very soon, the surface gets covered by a slimy layer called the "Schmutzdecke" (also called vital layer, zoogleal layer, or biological layer).

Slimy and gelatinous in nature
Consists of: threadlike algae, plankton, diatoms, and bacteria
Formation of this layer = "ripening" of the filter
Takes several days to form fully; when formed, extends 2 to 3 cm into top of sand bed
The vital layer is the "heart" of the slow sand filter
Functions: removes organic matter, holds back bacteria, oxidizes ammoniacal nitrogen into nitrates
First few days' filtrate is run to waste until vital layer is fully formed

(3) Under-Drainage System

Located at the bottom of the filter bed
Consists of porous or perforated pipes
Dual purpose: provides outlet for filtered water + supports the filter medium above

(4) Filter Control

Equipped with valves and devices to maintain a constant rate of filtration
Venturi meter measures bed resistance ("loss of head")
When resistance builds up, the regulating valve is opened
When "loss of head" exceeds 1.3 metre - uneconomical to run the filter

Filter box dimensions (top to bottom):

Component	Depth
Supernatant water	1 to 1.5 metres
Sand bed	1.2 metres
Gravel support	0.30 metres
Filter bottom	0.16 metres

Total depth of filter box: 2.5 to 4 metres; usually rectangular; built wholly or partly below ground.

Filter Cleaning

Filter may run for weeks or even months without cleaning
When bed resistance is too high and regulating valve is fully open - time to clean
Method: Scraping off the top 1-2 cm of sand layer, washing the sand, and returning it
After cleaning, the filter must re-ripen (re-form the vital layer) before returning to service
First filtrate is again run to waste

II. RAPID SAND FILTER (Mechanical Filter)

Construction

The construction of a rapid sand filter is essentially the same as the slow sand filter with these differences:

Sand grain size: effective diameter 0.4 to 0.7 mm (coarser than slow sand filter)
Gravel bed below the sand
Rate of filtration: up to 5-15 m³/hour/sq. metre - i.e., 40 to 50 times faster

Pre-treatment Required

Before the water enters a rapid sand filter, it must be pre-treated with:

Coagulation - addition of alum (aluminium sulphate)
Flocculation - formation of "floc" from suspended particles
Sedimentation - settling of floc in settling tanks

This pre-treatment is essential because rapid sand filters do NOT develop a vital layer (Schmutzdecke) the way slow sand filters do.

Filter Cleaning - Backwashing

Rapid sand filters clog up quickly and must be cleaned every 24-48 hours. Cleaning is done by backwashing - reversing the flow of water through the sand bed:

Dislodges impurities and cleans the sand bed
Washing is stopped when clear sand is visible and wash water is clear
The whole process takes about 15 minutes
In some filters, compressed air is used as part of the backwashing process

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 (40-50 times) that of a slow sand filter
Washing of the filter is easy (backwashing takes only 15 min)
More flexibility in operation

III. COMPARISON TABLE: Rapid vs. Slow Sand Filter

(Park's Textbook, Table 3)

Feature	Slow Sand Filter	Rapid Sand Filter
Also called	Biological filter	Mechanical filter
First used	1804, Scotland	Later development
Sand grain size (effective diameter)	0.2 - 0.3 mm	0.4 - 0.7 mm
Rate of filtration	0.1 - 0.4 m³/hr/m²	5 - 15 m³/hr/m² (40-50x faster)
Vital layer (Schmutzdecke)	Present - essential	Absent
Pre-treatment	Not required (storage only)	Coagulation + flocculation + sedimentation required
Bacteria removal	~98-99%	Less efficient (depends on pre-treatment)
Filter cleaning	Scraping (every weeks/months)	Backwashing (every 24-48 hours)
Cleaning time	Hours	~15 minutes
Space required	More	Less
Cost	Lower running cost	Higher (pre-treatment needed)
Suitable for	Low turbidity water	High turbidity water
Flexibility	Less	More

Key One-Liners for Exams

Slow sand filter was first used in 1804 in Scotland
Filtration removes 98-99% of bacteria
Vital layer = Schmutzdecke = heart of slow sand filter; takes several days to form ("ripening")
First filtrate of a newly cleaned/new filter is run to waste
Rapid sand filter is 40-50 times faster than slow sand filter
Rapid sand filter cleaned by backwashing (~15 min)
Slow sand filter cleaned by scraping (runs for weeks/months before cleaning needed)
Loss of head exceeding 1.3 metre - filter should be taken off service

Source: Park's Textbook of Preventive and Social Medicine, Chapter on Water Supply - pp. 812-813

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