Slow Sand Filters
Overview
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As a means of water purification, slow sand filters constitute a
simple, efficient design and may be constructed using local
resources
A SSF entails a porous bed of graded sand fortified by an
underlying layer of gravel. Raw water enters the filter bed and
undergoes purification.
Principal application: treatment of low turbidity water (<20 NTU)
for distribution as potable water (maintenance intervals shorten
with water > 20 NTU and SSF may not function at all when water
turbidity > 200 NTU)
Mechanisms for removing impurities: sedimentation, adsorption,
straining, chemical and microbiological processes
SSFs extricate solids, precipitates, turbidity, and bacterial particles
from a water supply
Construction: Explanation
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A SSF constitutes a porous bed of graded sand which is fortified
by an underlying layer of gravel
Raw water enters the filter medium from a raw water tank
Note: a pre-filter or other form of pre-treatment is required if the
raw water has a high turbidity (>20 NTU) or if it is necessary to
reduce the oxygen demand of the raw water (see card #13)
passes through the filter and undergoes purification
exits into the treated water tank
Construction: Design
If the tank is
being fed by
pumps:
situated at a
higher
elevation than
the filter to
allow for a
generous
operating
range
Inlet to the
treated water
tank should be
slightly more
elevated than
top of filter
sand; prevents
filter from
being
accidentally
drained if
treated water
tank is
emptied
Filtration Rate
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Filtration Rate = flow (m3/h) / filter surface area (m2)
Expressed as m3/m2/h or m/h
Maximum recommended rate: 0.3 m/h
Ideal rate: 0.2 m/h
One Filter Bed or Two?
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Because the biological film requires 2-7 days to re-develop after
each cleaning, recommended use of 2 SSF filters
Used interchangeably
Second SSF put in operation approximately 7 days before routine
cleaning of the first SSF (maintain constant supply of potable
water)
Exceptional case: Oxfam filter; requires only one filter (see card
#21)
SSF components
The SSF system incorporates five parts:
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The Housing (Tanks)
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The Supernatant
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The Schmutzdecke
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The Filter Sand
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The Underdrain Medium
Housing (Tanks): Explanation
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For most SSF designs, there must be three tanks: one to harbor the raw
water, another one to house the filter, and a final one to hold the treated
water
Filter size and number depends upon filtration rate and water demand
Raw water tank size depends upon operating range, turbidity and settling
rate
Treated water tank size depends upon contact time required for
chlorination and whether storage is to be supplied on site or at some other
time and place
Note: if a pre-filter is necessary (turbidity >20 NTU), another tank is
required to house it
Appropriate kinds of tanks: 1) those with non-reactive surfaces
(plastic tanks)
2) fiberglass lined galvanized tanks
3) poly or concrete tanks with a
capacity of 200 to 100,000 liters
Housing: Operation and Maintenance
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If flow is stopped for more than one day, a tank must be drained
entirely (in this case, anaerobic conditions in the filter sand will
have been engaged, infusing the water with a lasting bad taste)
Because recontamination of treated water can occur, periodic
chlorination of treated water tanks is necessary
Algae in the filter tank is not necessarily harmful
The Supernatant
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The supernatant is the water situated above the filter sand
Purpose of the supernatant: 1) provides hydraulic head to water
treatment
2) engenders a detention time of a
couple of hours for purification
3) supplies a temperature buffer that
both regulates the filter and
maintains the Schmutzdecke
Most designs require a minimum depth of 0.5-1.5m
The Schmutzdecke: Explanation
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The Schmutzdecke is a biologically active layer situated on top of
the filter sand (top 0.5 – 2 cm of filter bed) that initiates the
process of breaking down pathogens into inorganic, innocuous
molecules
Also works as a fine, mechanical filter
Constitutes a stratum of decomposing organic matter, iron,
manganese and silica
Ripens within 2-3 weeks after filter implementation
Note: the Schmutzdecke is only one example of a biologically
active zone. Further biological activity occurs below the
Schmutzdecke (at a depth up to ~ 0.4-0.5m)
The Schmutzdecke: Maintenance
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Disperse the energy of water entering the filter: the
Schmutzdecke should not be disturbed (in intermittent sand
filters, a diffuser plate is used to achieve this end)
Keep the sand bed wet: addressed in SSF design, achieved by
situating the outlet of filter at a higher level than the sand
Provide a supply of oxygen: used in the incapacitation and
consumption of pathogens; an adequate oxygen level is > 3 mg/l;
in order to supply this level of oxygen, the raw water might have
to be aerated to increase its oxygen content or pre-treated to
decrease oxygen demand
Keep the temperature at a moderate level
Take into consideration ripening time: 2-3 weeks
The Filter Sand: Explanation
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Situated directly underneath the Schmutzdecke
Contains a biological zone that continues the purification initiated
by the Schmutzdecke (lowest depth of this region: ~ 0.4-0.5 m)
May be collected from local riverbanks
Minimum depth 0.6 meters (sufficient for short term use of SSF);
nevertheless, SSFs often include an extra 0.3-0.5 m depth of sand
to allow for successive scrapings
Effective size (D10): 0.15-0.35mm
Uniformity coefficient (D60/D10): <3
Maximum size: 3mm
for guidance; in practice almost
Minimum size: 0.1mm
any sand will work
The sand must be replaced approximately every 3-4 years
Filter Sand: Pre-Cleaning
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Pre-cleaning of sand required to
remove fine and organic material
If source of sand is a river, this
cleaning can be done at a spot
along the river where there is
plentiful water
Another means: the drum
arrangement- 1) fill drum with
sand
2) add water
3) permit water to
flow into bottom of
drum
4) wash silt away
in the overflow
5) continue
flushing until water
becomes clear
The Underdrain Medium
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Constitutes a series of gravel layers encompassing the underdrain
pipes
Note: finest gravel situated directly beneath the sand; coarsest
gravel envelops underdrain pipes
Alternative to layers of gravel: interpose a layer of synthetic fabric
between filter sand and gravel encompassing underdrains
Gravel should be of a uniform size for easy passage of water and
large enough not to pass through slots in drains
All gravel underlaid by 50 mm layer of sand to protect tank lining
Purpose: 1) supplies an unbarred channel for treated water
exiting out of filter bed
2) fortifies bed of filter sand
3) prevents sand from migrating to the slots
The Underdrain Medium: Diagram
While,
specifically, this
diagram refers to
the Oxfam
package, its
general design is
characteristic of
most SSFs
Operation and Maintenance: Filter Bed
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Scraping the Schmutzdecke: 1) drain the supernatant (usually by
continuing filtration with no influent)
to 20 cm below sand surface
2) scrape off 1 inch of Schmutzdecke
and underlying sand from top of
filter bed
3) refill the filter from bottom of bed
using filtered water to preclude air
entrapment
Common filter run time (time between cleanings): 2-20 weeks,
variation depends upon raw water turbidity (which depends upon
the seasons) and filtration rate
Operation and Maintenance: Personnel
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Personnel requirements for construction of sand filter package:
unskilled personnel can construct an SSF package in a matter of
3-4 weeks; also, this group can initiate sand preparation even
before the tanks are designed and ordered
Personnel requirements for operation: one day operator and one
night operator per SSF package are necessary
Training: operators must be trained to maintain a constant flow
rate, head, and temperature
Advantages and Disadvantages of SSFs
Advantages
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Considerably reduces
pathogenic bacteria, viruses
and cysts, to engender
potable water w/o further
purification
No machinery necessary
Disadvantages
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Can only effectively purify low
turbidity water (w/o other
means of purification, i.e.
pre-filter)
Specific Types of SSFs: Oxfam package
Design
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encompasses one revolutionary development-a geotextile filter fabric:
A) situated on top of the sand filter
B) retains much of the matter strained from the
untreated water during purification and therefore
amount of sand removed during cleaning
C) use reduces cleaning interval and time taken for filters to recover their
biological treatment ability: allows the package to be operated with just
one filter as opposed to 2 filters in parallel
This arrangement is only adequate if there is a regular supply of chlorine
to disinfect product water for period immediately after filter cleaning when
filter is ‘recovering’
Entails the general SSF design: the Schmutzdecke as well as the other SSF
components situated underneath the fabric
Note: Oxfam package includes all components of SSF except treated water
tanks, which must be ordered separately
Specific Types of SSFs: Oxfam Package 2
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Cleaning the Filter
1) the water is lowered to approximately 200mm below the sand
surface
2) the fabric is rolled up and taken out of the tank
3) after having been washed, the fabric is reinstated on the sand
bed
Note: because the fabric in its entirety may be difficult to move,
cutting up the fabric into smaller pieces is recommended
Depending upon the conditions, it may also be necessary to
remove a 10 mm layer of sand from the top of the filter bed
Specific Types of SSFs: Oxfam Package 3
Specific Types of SSFs: Dug and Lined Filter
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A roughing filter design that may be applied to the construction of slow
sand filter
uses as a filter chamber an excavated basin covered with a plastic sheet
Specific Types of SSFs: Small-Scale Drum Filters
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Construction
Uses a 200 liter drum as a filter bed
Includes a pre-filter when turbidity of entering water > 30 NTU
Specific Types of SSFs: Small-Scale Drum Filter 2
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General Information
Filtration rate must be < 0.2 m/h
Necessary to maintain constant flow of entering water (tank
storing raw water must be higher than filter to induce constant
rate through gravity)
Size of each sand particle: 0.3-1.6 mm
Filter should never run dry unless it is out of use for more than
one day; in this case, the filter must be drained
Convenient for use in small service centers
Specific Types of SSFs: Ceramic Candle Filters
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Operated through three means: gravity, pumping, and siphonic action
1) Gravity Operated Filters: A) can produce 1 liter/hour/filter element
B) appropriate for domestic needs
2) Filters Using Pumps: adequate to serve the needs of larger-scale
ventures
3) Filters Using Siphonic Action: derives efficacy from its simplicity
always order spare filter elements in addition to ordinary filter as filter needs
replacing after long and extensive use
Spare filter elements can also be used to filter water with a plastic bucket for the
upper container and a jerry can for the filtered water
Specific Types of SSFs: Ceramic Candle Filters 2
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Maintenance
When the candle becomes lined with filtered-out particles, a nylon
brush is used to remove the amassed slime
Each cleaning also removes some of the ceramic candle, resulting
in candle’s eventual loss in diameter
candle needs to be
replaced
Indication of this condition: if circular gauge, included in each kit,
can be slipped over candle, the candle needs to be replaced.
Otherwise, it does not
Specific Types of Slow Sand Filters: Intermittent SSF
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Design: Explanation
Constitutes a cement container filled with layers of sand and
gravel
Plastic pipe exits bottom of filter and returns up the outside;
forms a sort of trap, reminiscent of trap under household sinks
Untreated water poured into container through a diffuser plate
(controls rate of flow)
Water passes through a biologically active region 2 inches below
water surface (equivalent of Schmutzdecke) and the strata of
gravel and sand
Clean water exits container through pipe; travels to clean water
receptacle
Specific Types of SSFs: Intermittent SSF
References
“Biosand Filter: Biological Action”: http://biosandfilter.org/biosandfilter/index.php/item/320
“How the Water Is Cleaned”: http://www.purewaterfortheworld.org/the_filter.asp
“MSF-UAE: Refugee Camp Project-Learn More: Water (slow sand filter)”: http://www.msf
uae.ae/en/refugeecamp/le arnmore/water/slow_sand_filter.htm
RedR-Engineers for Disaster Relief, Jan Davis and Robert Lambert, “Engineering in Emergencies: A Practical Guide For Relief
Workers”, ITDG Publishing, 1995
“Slow Sand Filter Bed”: http://en.wikipedia.org/wiki/Slow_sand_filter_bed
“Slow Sand Filter: Slow Sand Filtration”: http://www.agr.gc.pfra/water/slowsnd_e.htm
“Slow Sand Filtration”: http://www.oasisdesign.net/water/treatment/slowsandfilter.htm
“Slow Sand Filtration”: http://ewr.cee.vt.edu/environmental/teach/wtprimer/slowsand/slowsand.html
WHO/SEARO-Technical Note No.5 “Emergency Treatment of Drinking Water-Point of Use”: http://doultonusa.com/doulton_
water_filters/Emergency-treatment-of-drinking-water.htm