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Slow Sand Filtration
First used in the U.S. in 1872, slow sand filters are the oldest type of municipal water filtration.
Today, they remain a promising filtration method for small systems with low turbidity or algaecontaining source waters. Slow sand filtration does not require pretreatment or extensive operator
control—which can be important for a small system operator with several responsibilities.
What is slow sand filtration?
Slow sand filtration is a simple and reliable
process. They are relatively inexpensive to
build, but do require highly skilled operators.
The process percolates untreated water slowly
through a bed of porous sand, with the influent
water introduced over the surface of the filter,
and then drained from the bottom.
Properly constructed, the filter consists of a
tank, a bed of fine sand, a layer of gravel to
support the sand, a system of underdrains to
collect the filtered water, and a flow regulator
to control the filtration rate. No chemicals are
added to aid the filtration process.
Design and operation simplicity—as well as
minimal power and chemical requirements—
make the slow sand filter an appropriate technique
for removing suspended organic and inorganic
matter. These filters also may remove pathogenic
Slow sand filtration reduces bacteria, cloudiness,
and organic levels—thus reducing the need for
disinfection and, consequently, the presence of
disinfection byproducts in the finished water.
Other advantages include:
• Sludge handling problems are minimal.
• Close operator supervision is not necessary.
• Systems can make use of locally available
materials and labor.
Slow Sand Filter
Effluent flow
control structure
Raw water
Raw water
Ground level
Support gravel
Under drain
Telescoping valve
To sewer or
raw water
Filtered water
Sand filter bed
Slow Sand Filtration
Typical Treatment Performance of Conventional Slow Sand Filters
Water Quality
<1.0 NTU
1-3 log units
Enteric Viruses
2-4 log units
Giardia Cysts
2-4+log units
Cryptosporidium Oocysts
>4 log units
Dissolved Organic Carbon
Dissolved Organic Carbon
Trihalomethane Precursors
Heavy Metals
Zn, Cu, Cd, Pb
Fe, Mn
Source: Adapted from Collins, M.R. 1998.
A granular activated carbon (GAC)
sandwich filter is a modified slow sand
filter that removes organic material.
This filter uses a base sand layer that is
approximately 1 foot deep, an intermediate
GAC layer approximately 0.5 feet, and
a top sand layer approximately 1.5 feet
deep. This modified slow sand filter
effectively removes pesticides, total organic
carbon, and THM precursors.
Slow sand filters are less effective at
removing microorganisms from cold
water because as temperatures decrease,
the biological activity within the filter
bed declines.
Slow sand filters also provide excellent treatedwater quality. (See Table 1.) Slow sand filters consistently demonstrate their effectiveness in removing
suspended particles with effluent turbidities below
1.0 nephelometric turbidity units (NTU), achieving
90 to 99 + percent reductions in bacteria and
viruses, and providing virtually complete Giardia
lamblia cyst and Cryptosporidium oocyst removal.
Process Description
Slow sand filters require a very low application
or filtration rate (0.015 to 0.15 gallons per minute
per square foot of bed area, depending on the
gradation of the filter medium and the quality of
the raw water). The removal action includes a
biological process in addition to physical and
chemical ones.
Slow sand filters do have certain limitations.
They require a large land area, large quantities
of filter media, and manual labor for cleaning.
A sticky mat of biological matter, called a
“schmutzdecke,” forms on the sand surface,
where particles are trapped and organic matter is
biologically degraded. Slow sand filters rely on this
cake filtration at the surface of the filter for particulate straining. As the surface cake develops during
the filtration cycle, the cake assumes the dominant
role in filtration rather than the granular media.
Water with high turbidity levels can quickly clog
the fine sand in these filters. Water is applied
to slow sand filters without any pretreatment
when it has turbidity levels lower than 10 NTU.
organic chemicals, dissolved inorganic
substances, such as heavy metals, or trihalomethane (THM) precursors—chemical compounds that may form THMs
when mixed with chlorine. Also,
waters with very fine clays are not
easily treated using slow sand filters.
When slow sand filters are used with surface
waters that have widely varying turbidity levels,
infiltration galleries or rough filters—such as upflow gravel filters—may be used to reduce turbidity.
Waters with a very low nutrient content may
impair turbidity removal since some nutrients
must be present that promote biological
ecosystem growth within the filter bed.
Slow sand filters do not completely remove all
Pilot testing is always necessary when designing
slow sand filters. Currently, engineers are not
able to predict the performance of a slow sand
filter with a specific quality of raw water.
Operation of a small pilot filter, preferably over
several seasons of the year, will insure adequate performance of the full-scale plant.
Remember, after the designer sets the parameters—such as the plant filtration rate, bed depth,
and sand size—there is little a plant operator can
do to improve the performance of a slow sand
filter that does not produce satisfactory water.
is needed for production equalization and demand.
Slow sand filter pilot plant testing does not have
to be expensive. Pilot plant testing has been done
using manhole segments and other prefabricated
cylindrical products, such as filter vessels.
Monitoring and Operation Requirements
A slow sand filter must be cleaned when the fine
sand becomes clogged, which is measured by the
head loss. The length of time between cleanings
can range from several weeks to a year, depending
on the raw water quality. The operator cleans
the filter by scraping off the top layer of the filter
bed. A ripening period of one to two days is
required for scraped sand to produce a functioning
biological filter. The filtered water quality is poor
during this time and should not be used.
Slow sand filter pilot facilities operate over long
periods of time—up to a year—but the level of
effort can be quite low, consisting of daily checks
of head loss, flow rate, water temperature, and
turbidity and taking coliform samples.
Since the purification mechanism in a slow sand
filter is essentially a biological process, its efficiency
depends upon a balanced biological community
in the schmutzdecke. Therefore, filters should
operate at a constant rate. When operation is
stopped, the microorganisms causing bacteriological degradation of trapped impurities lose their
effectiveness. Intermittent operation disturbs the
continuity needed for efficient biological activity.
Allowing the filter to operate at a declining rate
is one way of overcoming this problem. Declining
rate filtration produces additional water, which is
generally satisfactory. Moreover, the declining-rate
mode may be applied during overnight operation,
resulting in significant labor savings.
Storing filtered water is essential at a slow sand
filter plant for two reasons. First, because of the
importance of establishing biological activity,
using chlorine ahead of the filter is inappropriate,
and the operator must provide disinfectant
contact time in a storage basin. Second, storage
In some small slow sand filters, geotextile filter
material is placed in layers over the surface. In this
cleaning method, the operator can remove a layer
of filter cloth periodically so that the upper
sand layer requires less frequent replacement.
In climates subject to below-freezing temperatures,
slow sand filters usually must be housed.
Uncovered filters operating in harsh climates
develop an ice layer that prevents cleaning. Thus,
they will operate effectively only if turbidity levels
of the influent are low enough for the filter to
operate through the winter months without cleaning.
In warm climates, a cover over the slow sand filter
may be needed to reduce algae growth within
the filter.
Before cleaning a slow sand filter, the operator
should remove floating matter, such as leaves
and algae. When one unit is shut down for
cleaning, the others are run at a slightly higher
rate to maintain the plant output.
Design Summary of a Slow Sand Filter
Design parameters
range of values
Filtration rate
Area per filter bed
0.15 m3/m2•h (0.1–0.2 m3/m2•h)
Less than 200 m2
(in small community water supplies to ease manual filter cleaning)
Number of filter beds
Minimum of two beds
Depth of filter bed
1 m (minmum of 0.7 m of sand depth)
Filter media
Effective size (ES) = 0.15–0.35 mm; uniformity coefficient (UC) = 2-3
Height of supernatant water
0.7–1 m (maximum 1.5 m)
Generally no need for further hydraulic calculations
Maximum velocity in the manifolds and in laterals = .3 m/s
Spacing between laterals = 1.5 m
Spacing of holes in laterals = 0.15 m
Size of holes in laterals =3 mm
Source: Vigneswaran, S. and C. Visvanathan. 1995
Underdrain system
Standard bricks
Precast concrete slabs
Precast concrete blocks with holes on the top
Porous concrete
Perforated pipes (laterals and manifold type)
After cleaning, the unit is refilled with water
through the underdrains. This water can be
obtained from an overhead storage tank or by
using water from an adjacent filter. When the
clearwell is designed, the temporary reduction
of plant output should be considered, ensuring
that sufficient water is available for the users.
Once the filter is cleaned, the microorganisms
usually re-establish and produce an acceptable
effluent. In cooler areas, ripening may take a
few days. Even then, if the effluent’s turbidity is
sufficiently low, the water supply can be resumed
after one day with adequate chlorination.
Slow sand filter monitoring and operation is not
complicated. Daily tasks include reading and
recording head loss, raw and filtered water turbidity, flow rates, and disinfectant residual. If
necessary, the operator should adjust the flow to
bring water production in line with demand.
In addition, with the promulgation of the
Surface Water Treatment Rule, each day the
operator needs to use the flow data and disinfectant residual data to calculate contact time
values and determine if disinfection is sufficiently rigorous. These duties may require one
to two hours unless automated.
Where can I find more information?
American Water Works Association. 1993. Back
to Basics Guide to Slow Sand Filtration. Denver:
American Water Works Association.
American Water Works Association. 1994. Slow
Sand Filtration: International Compilation on
Recent Scientific and Operational Developments.
Denver: American Water Works Association.
Clark, R. M., and D. A. Clark. 1995. Drinking
Water Quality Management. Lancaster,
Pennsylvania: Technomic Publishing Company.
Collins, M. R. 1998. “Assessing Slow Sand
Filtration and Proven Modifications.’’ In Small
Systems Water Treatment Technologies: State of
the Art Workshop. NEWWA Joint Regional
Operations Conference and Exhibition.
Marlborough, Massachusetts.
National Research Council. 1997. Safe Water
from Every Tap: Improving Water Services to
Small Communities. Washington, D.C.: National
Academy Press.
U.S. Environmental Protection Agency.1990.
Environmental Pollution Control Alternatives:
Drinking Water Treatment for Small
Communities. Washington, D.C.: Office of
Water. EPA/625/5-90/025
U.S. Environmental Protection Agency. 1998.
Small System Compliance Technology List for
the Surface Water Treatment Rule and Total
Coliform Rule. Washington, D.C.: Office of
Water. EPA/815/R/98/001.
Vigneswaran, S. and C. Visvanathan.1995.
Water Treatment Processes: Simple Options.
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