Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
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PART B. TECHNOLOGY PROFILES
2.5 Filtration Systems
Filtration systems are primarily used to purify water for domestic
consumption. Several types of filtration systems have been used
extensively in developing countries throughout the world, particularly in
Latin America and the Caribbean. These include residential filters, slow
and rapid sand filters, and dual media filters. Vertical flow pre-filters
with gravel media tested in Guatemala and up-flow solids contact filters
used in Brazil have potential for future use.
The design and application of different types of filters depend on the
volume, flow rate, and quality of the inflowing water; the desired degree
of water purification; and the use of the filtered water. The
availabilities of filtering materials and skilled personnel are also
factors to be considered in the selection of an appropriate filtration
Normally, the quality of the product water can be improved by mechanical
straining through a porous material, such as sand or gravel. Depending on
the size of the pores and the nature of the filter material, straining, or
filtering, may remove a significant portion of the undesirable contents of
the feedwater: suspended and colloidal matter, bacteria and other
microorganisms, and, sometimes, certain chemicals. The filter material may
be any porous, chemically stable material, but sand (silica and garnet) is
used most often. Sand is cheap, inert, durable, and widely available. It
has been extensively tested and has been found to give excellent results.
(Other materials have been used, some of which are described below;
others, such as the reverse osmosis technologies described previously, are
also a specialized form of filtration.)
Residential filters are a common form of filtration. They can be either
homemade or purchased commercially. The homemade filters usually consist
of a sand- or gravel-filled pipe or tub, while the commercial systems
usually have a stainless steel frame, with appropriate connections that
make installation and operation relatively simple. Many commercial filters
contain filtration media other than sand or gravel.
The basic form of residential filter, used in rural areas with no public
water supply, is the tub filter. The tub filter consists of two tubs made
of mud or clay, pottery or plastic, and joined together. The upper tub
contains the filter medium (sand, gravel, coal, stone, etc.), into which
the water to be treated is poured. It moves through the filter medium,
through holes in the base of the upper tub, to the lower tub, where it is
stored until used. A faucet is usually installed in the lower tub for
convenient access. Homemade filters, such as the tub filter, are usually
constructed of locally available materials. For example, in El Salvador,
they are constructed of a concrete pipe, approximately 0.5 m in diameter
and 1 m in length, fitted with a perforated pipe, which is placed at the
bottom of the filter in a 10 cm layer of gravel and connected to a pipe
with a 3/4-inch internal diameter from which the filtered water is
extracted. The gravel is overlain by 60 cm of sand. Both the gravel and
the sand are cleaned and dried in the sun, before use. In Mexico,
residential filters are constructed of porous volcanic rock assembled in a
wooden frame and protected by a screen. In the Dominican Republic,
residential filters are installed at the point of discharge of storage
cisterns, or at the point where water enters the houses. The frame of
these filters is usually made of stainless steel, with layers of sand,
quartzitic gravel, anthracite, and activated carbon as the filtration
A slow sand filter consists of a watertight box, fitted with an
underdrain, which supports the filtering material and distributes the flow
evenly through the filter. Many different media have been used for the
underdrain system. Bricks, stone, and even bamboo have been used for this
purpose; bamboo, however, requires frequent replacement because it is
organic and subject to decomposition. The effective size of the sand used
in slow sand filters is about 0.2 mm, and may range between 0.15 mm and
0.35 mm, with a coefficient of uniformity of between 1.5 and 3.0. In a
mature bed, a layer of algae, plankton, and bacteria forms on the surface
of the sand. The walls of the filter can be made of concrete or stone.
Sloping walls, dug into the earth and supported or protected by chicken
wire reinforcement and a sand or sand-bitumen coating, could be a
cost-effective alternative to concrete. Some Latin American countries,
such as Ecuador and El Salvador, use concrete reinforced with a minimal
amount of iron (ferrocement). Inlets and outlets should be provided with
controllers to keep the raw water level and the filtration rate constant.
Lateral pipes range from 2 to 8 in, while the bottom drains are normally
between 10 and 30 in. Bottom drains consist of a system of manifold and
lateral pipes. Figure 26 is a diagram of a typical slow sand filter.
The successful performance of a slow sand filter depends mainly on the
retention of inorganic suspended matter by the straining action of the
sand. Filtration rates usually employed in developing countries range
between 2.5 and 6.0 m3/m2/day. Higher rates may be
used after a series of tests demonstrates that the effluents are of good
quality. The system should be designed for flexibility, and should consist
of a number of separate units to enable maintenance to be performed
without interruption of the water service. The suggested number of units
for a given population size ranges from two units for a population of 2
000 up to six units for a population of 200 000.
Rapid sand filters differ from slow sand filters in the size of the
media employed. Media in rapid sand filters may range in size from 0.35 to
1.0 mm, with a coefficient of uniformity of 1.2 to 1.7. A typical size
might be 0.5 mm, with an effective size of 1.3 to 1.7 mm. This range of
media size has demonstrated the ability to handle turbidities in the range
of 5 to 10 NTU at rates of up to 4.88 m3/m2/h.
Filtration rates for rapid filters may be as high as 100 to 300 m3/m2/day,
or about 50 times the rate of a slow sand filter. The number of filters
used for a specific plant ranges from 3 filters for a plant capacity of 50
l/s to 10 filters for a plant capacity of 1 500 l/s.
A typical rapid sand filter consists of an open watertight basin
containing a layer of sand 60 to 80 cm thick, supported on a layer of
gravel. The gravel, in turn, is supported by an underdrain system. In
contrast to a slow sand filter, the sand is graded in a rapid rate filter
configuration. The sand is regraded each time the filter is backwashed,
with the finest sand at the top of the bed. The underdrain system, in
addition to performing the same functions served in the slow rate filter,
serves to distribute the backwash water uniformly to the bed. The
underdrain system may be made of perforated pipes, a pipe and strainer,
vitrified tile blocks with orifices, porous plates, etc. A clear well is
usually located beneath the filters (or in a separate structure), to
provide consistent output quantity. The minimum number of filter units in
a system is two. The surface area of a unit is normally less than 150 m2.
The ratio of length to width is 1.25 to 1.35.
Dual-media filtration uses two layers, a top one of anthracite and a
bottom one of sand, to remove the residual biological floc contained in
settled, secondary-treated wastewater effluents and residual
chemical-biological floc after alum, iron, or lime precipitation in
potable water treatment plants. It is also used for tertiary or
independent physical-chemical waste treatment in the United States and
other countries. Gravity filters operate by using either the available
head from the previous treatment unit or the head developed by pumping the
feedwater to a flow cell above the filter cells. A filter unit consists of
an open watertight basin; filter media; structures to support the media;
distribution and collection devices for influent, effluent, and backwash
water flows; supplemental cleaning devices; and the necessary controls to
sequence water flows, levels, and backwashing.
Figure 26: Slow Sand Filtration System.
Source: Edward J. Martin, Handbook for Appropriate Water
and Wastewater Technology for Latin America and the Caribbean, Washington,
D.C., PAHO and IDB, 1988.
Solids Contact Filter
These units eliminate the need for separate flocculators
and settling tanks, since they perform liquid-solid separation,
filtration, and sludge removal in a single unit process. Coagulation and
flocculation are performed in a granular medium (such as a layer of gravel
under a sand bed). The use of flocculent aids improves filtration results.
This process should be restricted to raw waters of low turbidity (up to 50
JTU) and no more than 150 mg/l of suspended solids. It is widely used,
especially in Brazil. These filters are designed for rates of filtration
between 120 and 150 m3/m2/day.
Extent of Use
Both homemade and commercially purchased residential
filters are commonly used in developing countries where the quality of
water for domestic use is poor. El Salvador, Dominican Republic, and
Mexico have promoted the use of these types of filters. In general, most
Latin American countries use residential filters for water purification,
particularly in rural areas.
Slow and rapid sand filters have been used in the rural
community of La Pinera, El Salvador. In Ecuador, slow sand filters are
used extensively for both surface and groundwaters. Filtration systems
using vertical reactors with gravel beds have been tested as a means of
pre-filtration in a water treatment plant in the municipalities of Cabañas
and Zacapa, Guatemala. Rapid sand filters are more complex to operate than
their slow sand filter counterparts, but they are widely used, especially
in areas with high turbidity and where land requirements may be an
important design consideration. Conventional rapid sand filtration plants
are widely available and widely used in Latin America and other developing
countries throughout the world.
Dual or multimedia filters are limited to developing
countries that can inexpensively acquire anthracite. The higher skill
level and energy requirements for the operation of these high rate systems
may limit their application.
Upflow solids contact filters, because of their simplicity
and low cost, could be an effective technology in many developing
countries. Brazil has successfully used this type of filtration system.
Operation and Maintenance
The filter media of homemade residential filters must be
periodically changed to maintain the filter's effectiveness. Most of the
residential filters acquired commercially can be purchased with a
maintenance contract, which will prolong their operational life.
A number of factors affect the operation and maintenance of
slow sand filters. The initial resistance (loss of head) of a clean filter
bed is about 6 cm. During filtration, impurities are deposited in and on
the surface layer of the sand bed, and the loss of head increases. At a
predetermined limit (the head loss is usually not allowed to exceed the
depth of water over the sand, or about 1 m to 1.5 m), the filter is taken
out of service and cleaned. The period between cleaning is typically 20 to
60 days. The filter can be cleaned by either scraping off the surface
layer of sand and replacing it with washed sand stored after previous
cleanings (periodic re-sanding of the bed), or washing the sand in place
with a washer that travels over the sand bed. If sand is readily
available, the former method is favored; workers with wide, flat shovels
do the scraping, removing 1 to 2 cm of the topmost material. The amount of
time this takes depends on the area of the filter bed, but it can usually
be completed in one or two days. After washing, the sand is stored and
replaced on the bed when, after successive cleanings, the thickness of the
sand bed has been reduced to about 50 to 80 cm. A sand and gravel filter
needs to be replaced every two years or so. When using the method of
washing in place, about 0.2% to 0.6% of the water filtered is required for
washing purposes. The bacteriological layer, which is the most important
layer in the filtration process, needs to be reactivated in the new
filter. Reactivation usually lasts two months.
Rapid sand filtration plants are complicated to operate,
requiring operator training in order for the plant to produce a product
water of consistent quality and quantity. The filters require frequent
backwashing to maintain satisfactory operating heads in the system (filter
runs may vary from only a few hours to as many as 24 to 72 hours,
depending on the suspended solids in the influent). Backwashing rates are
typically 0.6 m3/min or higher, for a period of several
minutes. In addition, the initial production following backwashing is
channeled to waste for several minutes. Thus, the water backwashing uses
can be as much as 10% to 15% of the total plant output. On the other hand,
rapid sand filtration plants (including chemical treatment) can
effectively treat higher solids loadings and produce higher outputs than
slow sand filters. The land area requirements are significantly lower.
Dual-media filters, like rapid sand filters, are cleaned by
hydraulic backwashing (upflow) with potable water. Thorough cleaning of
the bed makes it advisable in the case of single medium filters, and
mandatory in the case of dual- or mixed-media filters, to use auxiliary
scour or so-called surface wash devices before or during the backwash
cycle. In dual-media and mixed-media beds, such additional effort is
needed to remove accumulated floc, which is stored throughout the bed
depth to within a few inches of the bottom of the fine media. Backwashing
is generally carried out every 24 to 72 hours. The optimum rate of
washwater application is a direct function of water temperature, as
expansion of the bed varies inversely with the viscosity of the washwater.
For example, a backwash rate of 18 gpm/ft2 at 20oC
equates to 15.7 gpm/ft2 at 5oC , and to 20 gpm/ft2
at 35oC. The time required for backwashing varies from 3 to 15
minutes. After the washing process, water should be discharged to waste
until the turbidity drops to an acceptable value. Few data are available
on the operation and maintenance of the vertical reactor pre-filters
tested in Guatemala, which remain in the experimental stage.
Other operational considerations relating to the use of
filtration technologies include the use of flocculent aids. Coagulants
such as alum, ferrous sulfate, and lime may be added to aid in the
flocculation and sedimentation of particulates. The coagulant dosage is
generally determined from jar tests, and the chemicals are almost always
added with rapid mix systems. In the case of water treatment plants,
flocculation is usually performed ahead of the settling process to improve
the effectiveness of this process.
Maintenance considerations include the resolution of a
number of problems which can interfere with the consistent operation of
sand filters. These problems often are due to poor design or operation of
the filtration systems. The problems most often encountered and their
possible solutions are as follows:
- Surface clogging and cracking: This problem, caused by an overload of
solids at the thin filter layer in sand filters, can be alleviated by
using dual or multiple media, which allows deeper penetration of solids
into the bed, and, generally, longer run times.
- Gravel displacement or mounding: This problem can be alleviated by
placing a 76 mm layer of coarse garnet between the gravel supporting the
media and the fine bed material.
- "Mudball" formation: This problem can be reduced by
increasing the backwash flow rate (e.g., up to 20 gpm/ft2),
and by providing for auxiliary water or air scouring of the washed
- Sand leakage: This problem may be alleviated by adding the garnet
- Accumulation of air bubbles in the bed: This problem, which causes a
significantly increased resistance to flow through the filter, can be
minimized by maintaining adequate water depths in the clear well and
filters; frequent backwashing may help.
Level of Involvement
In many developing countries, filtration methods are introduced and
promoted by both governmental agencies and NGOs, with the full
participation of the community. This is the case in El Salvador, where the
Centro Salvadoreño de Tecnología Apropiada (CESTA) builds
and installs residential filters for rural communities at a minimum cost.
In Dominican Republic, the private sector, particularly the companies
which manufacture filtration systems, promotes the technology. In Ecuador,
NGOs like Plan Internacional and CARE actively participate in the
implementation of these technologies in order to reduce the use of
contaminated water. In Brazil, the government and the private sector are
actively involved in the development and implementation of filtration
Homemade residential filters were constructed in El Salvador at a cost
of $23. Operation and maintenance costs are about $6/year. The cost of
residential filters manufactured and commercially distributed in Dominican
Republic varies with the flow capacity of the filter. It ranges from $382
for 1 gpm to $588 for 6 gpm; this price includes installation and
maintenance. Commercially manufactured tub filters are sold in Dominican
Republic hardware stores at a price ranging from $26 to $45. The cost of
quarry filters used in Mexico was $50, with little or no operation and
maintenance cost. Figure 27 shows the construction cost of an upflow
solids contact filter a function of the filtration area.
The unit filtering cost of slow and rapid sand filters in Ecuador ranges
between $0.13/m3 and $0.20/m3. Slow sand filters
were constructed in Ecuador at a cost of $132.30 with an estimated
operation and maintenance cost of 25% of the construction cost. Table 13
shows estimated per capita costs of construction and of operation and
maintenance for slow and rapid sand filters.
Effectiveness of the Technology
Homemade residential filters can adequately reduce the level of
contaminants in water, but, because quality control tests are usually not
performed on the product water, there is a risk of some contamination
remaining after filtration. For example, quarry filters used in Mexico
reduce bacteriological contaminants by up to 90%. However, quarry filters
must be covered and protected with a screen, and a faucet at the outlet is
recommended. This filter needs to be cleaned every 3 to 4 months,
depending on the quality of the water treated.
Commercially available residential filters are usually more effective at
producing a good quality product water since quality control is performed
during the manufacturing process and a level of efficiency is initially
guaranteed. Quality product water can be further ensured through the
regular inspections performed by technicians from the supplier in the case
of systems sold with a service contract.
Slow sand filters are very effective in removing solids and turbidity
when the raw water has low turbidity and color (turbidity up to 50 NTU and
color up to 30 Pt units). Taste and odor are also improved. However, if
the raw water quality is poor, filtration is often less effective. In such
situations, roughing filters, or pre-filters, are often used before the
feedwater enters the slow sand filters. The slow sand filters are very
effective in removing bacteria; in general, their effectiveness in
removing bacteriological contaminants ranges between 80% and 99%,
depending on the initial level of contaminants and the number and design
of the filtration units. In many regions of Ecuador, the effectiveness is
close to 100%. In El Salvador, they are estimated to remove 84% and 99% of
total and fecal coliform bacteria, respectively. Reductions in the levels
of iron, manganese, and nitrate concentrations and turbidity are also
observed. Chemicals are typically not used. The flow rates for slow sand
filters are many times slower than for rapid sand and roughing filters,
and the operating filter bed is not stratified.
Multimedia filters are usually more effective, since the filtration
media combine the filtration properties of several materials. In the
system of vertical flow pre-filters used in Guatemala, turbidity reduction
ranged from 23% to 45%, and color reduction between 34% and 56%.
Filtration technologies are suitable for use throughout the
region. Homemade residential filters are better suited to rural areas
where the equipment, skills, and infrastructure necessary to provide piped
domestic water supplies are lacking. The other, more complex filtration
systems are best used at water treatment plants and are generally located
in urban areas.
- Filtration systems have a low construction cost, especially
when built using manual labor. These systems are simple to design,
install, operate, and maintain, which makes them ideal for use in areas
where skilled personnel are few.
- No chemicals are required, although flocculent aids are sometimes
used in conjunction with large-scale filtration systems; supplies of
sand can usually be found locally.
- Large quantities of washwater are not required.
- Use of filtration to pretreat water and wastewaters results in fewer
sludge disposal problems because fewer contaminants are left to be
removed during the treatment process.
- Residential filters provide adequate treatment of water for
average-sized households, particularly in rural areas.
- Filters are environmentally friendly.
- In some areas, there is a lack of locally available filtration media.
- There may be a lack of skilled personnel to operate the more
sophisticated filtration systems, particularly in rural areas.
- Use of filtration alone is recommended only for source waters with
low levels of contamination.
- Pretreatment may be required for many applications.
- Provision must be made for washing and storing used sand from sand
filters, either permanently or temporarily, and for moving sand from the
filters to the wash site and from the wash site to the storage site and
back, as needed.
- If sand from slow sand filters is to be washed, a separate backwash
facility and washwater supply may be required; treated water must often
be used for washing, which could reduce the available supply of treated
water, especially in water-poor areas.
- Precise operational control of the rate of head loss is required to
prevent air bubbles from entering and binding the system; this type of
interference is a potential problem in all types of filters.
- There may be a lack of quality control of the product water in rural
- To obtain good results from slow sand filters, the raw feedwater must
not generally have a suspended solids content of less than 50 mg/l.
Filtration is a well-accepted technology when applied in the
treatment of industrial and public water supplies. It has limited
acceptance in other applications, and at the household level in rural
Further Development of the Technology
Additional research is needed to develop more efficient
filtration media that can remove both bacteriological and chemical
contaminants. Education is needed, particularly in the rural areas, to
encourage the use of homemade filtration systems and disinfection of
household water supplies.
Figure 27: Construction Cost of Upflow Solids Contact
Source: Edward J. Martin. Handbook for
Appropriate Water and Wastewater Technology for Latin America and the
Caribbean, Washington, D.C., PAHO and IDB, 1988.
TABLE 13. Per Capita Costs of Construction, and of Operation and
Maintenance for Slow Sand Filters and Rapid Sand Filters ($).
||Cost Range :
|| Slow Sand Filter
|| Rapid Sand Filter
| 500 - 2 499
|| Construction Operation and Maintenance
|| 17.08 - 27.00 1.80 - 6.75
|| 12.84 - 15.12
2.43 - 5.40
| 2 500 - 14 999
|| Construction Operation and Maintenance
||12.19 - 19.28
0.81 - 3.04
| 10.08 - 11.88 1.22 - 2.70
| 15 000 - 49 999
||Construction Operation and Maintenance
||8.55 - 13.5
0.45 - 1.69
| 5.73 - 6.75
5.72 - 2.36
| 50 000 - 100 000
||Construction Operation and Maintenance
|| 5.33 - 8.44
0.27 - 1.01
| 3.04 - 3.58
0.91 - 2.03
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Mexico. Tel. (52-73)19-3544, 19-3567 and 19-4000 ext. 355. Fax
Carlos Cisneros E. and Osvaldo Encalada,
Instituto de Investigaciones de Ciencias Técnicas (IICT), Plan
Internacional/Care International, Cuenca, Ecuador. Tel. (593-7)840-073.
Yolanda López, Centro Salvadoreño de
Tecnología Apropriada (CESTA), Dirección 17 calle Oriente No
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