Slow Sand FiltersSlow Sand Filters

OverviewOverview

As a means of water purification, slow sand filters constitute a As a means of water purification, slow sand filters constitute a simple, efficient design and may be constructed using local simple, efficient design and may be constructed using local resourcesresources

A SSF entails a porous bed of graded sand fortified by an A SSF entails a porous bed of graded sand fortified by an underlying layer of gravel. Raw water enters the filter bed and underlying layer of gravel. Raw water enters the filter bed and undergoes purification.undergoes purification.

Principal application: treatment of low turbidity water (<20 NTU) Principal application: treatment of low turbidity water (<20 NTU) for distribution as potable water (maintenance intervals shorten for distribution as potable water (maintenance intervals shorten with water > 20 NTU and SSF may not function at all when water with water > 20 NTU and SSF may not function at all when water turbidity > 200 NTU)turbidity > 200 NTU)

Mechanisms for removing impurities: sedimentation, adsorption, Mechanisms for removing impurities: sedimentation, adsorption, straining, chemical and straining, chemical and microbiological processesmicrobiological processes

SSFs extricate solids, precipitates, turbidity, and bacterial particles SSFs extricate solids, precipitates, turbidity, and bacterial particles from a water supplyfrom a water supply

Construction: ExplanationConstruction: Explanation

A SSF constitutes a porous bed of graded sand which is fortified by A SSF constitutes a porous bed of graded sand which is fortified by an underlying layer of gravelan underlying layer of gravel

Raw water enters the filter medium from a raw water tankRaw water enters the filter medium from a raw water tank Note: a pre-filter or other form of pre-treatment is required if the 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 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)reduce the oxygen demand of the raw water (see card #13)

passes through the filter and undergoes purificationpasses through the filter and undergoes purification exits into the treated water tankexits into the treated water tank

Construction: DesignConstruction: 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 RateFiltration Rate

Filtration Rate = flow (mFiltration Rate = flow (m33/h) / filter surface area (m/h) / filter surface area (m22)) Expressed as mExpressed as m33/m/m22/h or m/h/h or m/h Maximum recommended rate: 0.3 m/hMaximum recommended rate: 0.3 m/h Ideal rate: 0.2 m/hIdeal rate: 0.2 m/h

One Filter Bed or Two?One Filter Bed or Two?

Because the biological film requires 2-7 days to re-develop after Because the biological film requires 2-7 days to re-develop after each cleaning, recommended use of 2 SSF filterseach cleaning, recommended use of 2 SSF filters

Used interchangeablyUsed interchangeably Second SSF put in operation approximately 7 days before routine Second SSF put in operation approximately 7 days before routine

cleaning of the first SSF (maintain constant supply of potable cleaning of the first SSF (maintain constant supply of potable water)water)

Exceptional case: Oxfam filter; requires only one filter (see card Exceptional case: Oxfam filter; requires only one filter (see card #21)#21)

SSF componentsSSF components

The SSF system incorporates five parts:The SSF system incorporates five parts: The Housing (Tanks)The Housing (Tanks) The SupernatantThe Supernatant The SchmutzdeckeThe Schmutzdecke The Filter SandThe Filter Sand The Underdrain MediumThe Underdrain Medium

Housing (Tanks): ExplanationHousing (Tanks): Explanation For most SSF designs, there must be three tanks: one to harbor the raw 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, another one to house the filter, and a final one to hold the treated waterwater

Filter size and number depends upon filtration rate and water demandFilter size and number depends upon filtration rate and water demand Raw water tank size depends upon operating range, turbidity and settling Raw water tank size depends upon operating range, turbidity and settling

raterate Treated water tank size depends upon contact time required for Treated water tank size depends upon contact time required for

chlorination and whether storage is to be supplied on site or at some other chlorination and whether storage is to be supplied on site or at some other time and placetime and place

Note: if a pre-filter is necessary (turbidity >20 NTU), another tank is Note: if a pre-filter is necessary (turbidity >20 NTU), another tank is required to house itrequired to house it

Appropriate kinds of tanks: 1) those with non-reactive surfacesAppropriate kinds of tanks: 1) those with non-reactive surfaces (plastic tanks)(plastic tanks) 2) fiberglass lined galvanized tanks2) fiberglass lined galvanized tanks 3) poly or concrete tanks with a3) poly or concrete tanks with a capacity of 200 to 100,000 literscapacity of 200 to 100,000 liters

Housing: Operation and MaintenanceHousing: Operation and Maintenance

If flow is stopped for more than one day, a tank must be drained 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 entirely (in this case, anaerobic conditions in the filter sand will have been engaged, infusing the water with a lasting bad taste)have been engaged, infusing the water with a lasting bad taste)

Because recontamination of treated water can occur, periodic Because recontamination of treated water can occur, periodic chlorination of treated water tanks is necessarychlorination of treated water tanks is necessary

Algae in the filter tank is not necessarily harmfulAlgae in the filter tank is not necessarily harmful

The SupernatantThe Supernatant

The supernatant is the water situated above the filter sandThe supernatant is the water situated above the filter sand Purpose of the supernatant: 1) provides hydraulic head to waterPurpose of the supernatant: 1) provides hydraulic head to water treatmenttreatment 2) engenders a detention time of a2) engenders a detention time of a couple of hours for purificationcouple of hours for purification 3) supplies a temperature buffer that3) supplies a temperature buffer that both regulates the filter andboth regulates the filter and maintains the Schmutzdeckemaintains the Schmutzdecke Most designs require a minimum depth of 0.5-1.5mMost designs require a minimum depth of 0.5-1.5m

The Schmutzdecke: ExplanationThe Schmutzdecke: Explanation

The Schmutzdecke is a biologically active layer situated on top of 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 the filter sand (top 0.5 – 2 cm of filter bed) that initiates the process of breaking down pathogens into inorganic, innocuous process of breaking down pathogens into inorganic, innocuous moleculesmolecules

Also works as a fine, mechanical filterAlso works as a fine, mechanical filter Constitutes a stratum of decomposing organic matter, iron, Constitutes a stratum of decomposing organic matter, iron,

manganese and silica manganese and silica Ripens within 2-3 weeks after filter implementationRipens within 2-3 weeks after filter implementation Note: the Schmutzdecke is only one example of a biologically Note: the Schmutzdecke is only one example of a biologically

active zone. Further biological activity occurs below the active zone. Further biological activity occurs below the Schmutzdecke (at a depth up to ~ 0.4-0.5m)Schmutzdecke (at a depth up to ~ 0.4-0.5m)

The Schmutzdecke: MaintenanceThe Schmutzdecke: Maintenance

Disperse the energy of water entering the filter: the Schmutzdecke Disperse the energy of water entering the filter: the Schmutzdecke should not be disturbed (in intermittent sand filters, a diffuser should not be disturbed (in intermittent sand filters, a diffuser plate is used to achieve this end)plate is used to achieve this end)

Keep the sand bed wet: addressed in SSF design, achieved by Keep the sand bed wet: addressed in SSF design, achieved by situating the outlet of filter at a higher level than the sandsituating the outlet of filter at a higher level than the sand

Provide a supply of oxygen: used in the incapacitation and Provide a supply of oxygen: used in the incapacitation and consumption of pathogens; an adequate oxygen level is > 3 mg/l; consumption of pathogens; an adequate oxygen level is > 3 mg/l; in order to supply this level of oxygen, the raw water might have 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 to be aerated to increase its oxygen content or pre-treated to decrease oxygen demanddecrease oxygen demand

Keep the temperature at a moderate levelKeep the temperature at a moderate level Take into consideration ripening time: 2-3 weeksTake into consideration ripening time: 2-3 weeks

The Filter Sand: ExplanationThe Filter Sand: Explanation

Situated directly underneath the SchmutzdeckeSituated directly underneath the Schmutzdecke Contains a biological zone that continues the purification initiated Contains a biological zone that continues the purification initiated

by the Schmutzdecke (lowest depth of this region: ~ 0.4-0.5 m)by the Schmutzdecke (lowest depth of this region: ~ 0.4-0.5 m) May be collected from local riverbanksMay be collected from local riverbanks Minimum depth 0.6 meters (sufficient for short term use of SSF); 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 nevertheless, SSFs often include an extra 0.3-0.5 m depth of sand to allow for successive scrapingsto allow for successive scrapings

Effective size (DEffective size (D1010): 0.15-0.35mm): 0.15-0.35mm Uniformity coefficient (DUniformity coefficient (D6060/D/D1010): <3): <3 Maximum size: 3mm Maximum size: 3mm for guidance; in practice almostfor guidance; in practice almost Minimum size: 0.1mm Minimum size: 0.1mm any sand will workany sand will work The sand must be replaced approximately every 3-4 yearsThe sand must be replaced approximately every 3-4 years

Filter Sand: Pre-CleaningFilter Sand: Pre-Cleaning Pre-cleaning of sand required to Pre-cleaning of sand required to

remove fine and organic materialremove fine and organic material If source of sand is a river, this If source of sand is a river, this

cleaning can be done at a spot cleaning can be done at a spot along the river where there is along the river where there is plentiful waterplentiful water

Another means: the drum Another means: the drum arrangement- 1) fill drum witharrangement- 1) fill drum with

sandsand 2) add water2) add water 3) permit water to3) permit water to flow into bottom offlow into bottom of drumdrum 4) wash silt away4) wash silt away in the overflowin the overflow 5) continue5) continue flushing until waterflushing until water becomes clearbecomes clear

The Underdrain MediumThe Underdrain Medium

Constitutes a series of gravel layers encompassing the underdrain Constitutes a series of gravel layers encompassing the underdrain pipespipes

Note: finest gravel situated directly beneath the sand; coarsest Note: finest gravel situated directly beneath the sand; coarsest gravel envelops underdrain pipesgravel envelops underdrain pipes

Alternative to layers of gravel: interpose a layer of synthetic fabric Alternative to layers of gravel: interpose a layer of synthetic fabric between filter sand and gravel encompassing underdrainsbetween filter sand and gravel encompassing underdrains

Gravel should be of a uniform size for easy passage of water and Gravel should be of a uniform size for easy passage of water and large enough not to pass through slots in drains large enough not to pass through slots in drains

All gravel underlaid by 50 mm layer of sand to protect tank liningAll gravel underlaid by 50 mm layer of sand to protect tank lining Purpose: 1) supplies an unbarred channel for treated waterPurpose: 1) supplies an unbarred channel for treated water exiting out of filter bedexiting out of filter bed 2) fortifies bed of filter sand2) fortifies bed of filter sand

3) prevents sand from migrating to the slots3) prevents sand from migrating to the slots

The Underdrain Medium: DiagramThe Underdrain Medium: Diagram

While, specifically, this diagram refers to the Oxfam package, its general design is characteristic of most SSFs

Operation and Maintenance: Filter BedOperation and Maintenance: Filter Bed

Scraping the Schmutzdecke: 1) drain the supernatant (usually byScraping the Schmutzdecke: 1) drain the supernatant (usually by continuing filtration with no influent)continuing filtration with no influent) to 20 cm below sand surfaceto 20 cm below sand surface 2) scrape off 1 inch of Schmutzdecke2) scrape off 1 inch of Schmutzdecke and underlying sand from top ofand underlying sand from top of filter bedfilter bed 3) refill the filter from bottom of bed3) refill the filter from bottom of bed using filtered water to preclude airusing filtered water to preclude air entrapmententrapment Common filter run time (time between cleanings): 2-20 weeks, Common filter run time (time between cleanings): 2-20 weeks,

variation depends upon raw water turbidity (which depends upon variation depends upon raw water turbidity (which depends upon the seasons) and filtration ratethe seasons) and filtration rate

Operation and Maintenance: PersonnelOperation and Maintenance: Personnel

Personnel requirements for construction of sand filter package: Personnel requirements for construction of sand filter package: unskilled personnel can construct an SSF package in a matter of 3-unskilled personnel can construct an SSF package in a matter of 3-4 weeks; also, this group can initiate sand preparation even before 4 weeks; also, this group can initiate sand preparation even before the tanks are designed and orderedthe tanks are designed and ordered

Personnel requirements for operation: one day operator and one Personnel requirements for operation: one day operator and one night operator per SSF package are necessarynight operator per SSF package are necessary

Training: operators must be trained to maintain a constant flow Training: operators must be trained to maintain a constant flow rate, head, and temperature rate, head, and temperature

Advantages and Disadvantages of SSFsAdvantages and Disadvantages of SSFs

AdvantagesAdvantages

Considerably reduces Considerably reduces pathogenic bacteria, viruses pathogenic bacteria, viruses and cysts, to engender and cysts, to engender potable water w/o further potable water w/o further purificationpurification

No machinery necessaryNo machinery necessary

DisadvantagesDisadvantages

Can only effectively purify low Can only effectively purify low turbidity water (w/o other turbidity water (w/o other means of purification, i.e. pre-means of purification, i.e. pre-filter)filter)

Specific Types of SSFs: Oxfam packageSpecific Types of SSFs: Oxfam package

DesignDesign encompasses one revolutionary development-a geotextile filter fabric:encompasses one revolutionary development-a geotextile filter fabric: A) situated on top of the sand filterA) situated on top of the sand filter B) retains much of the matter strained from theB) retains much of the matter strained from the untreated water during purification and thereforeuntreated water during purification and therefore amount of sand removed during cleaning amount of sand removed during cleaning C) use reduces cleaning interval and time taken for filters to recover their 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 biological treatment ability: allows the package to be operated with just one filter as opposed to 2 filters in parallel filter as opposed to 2 filters in parallel

This arrangement is only adequate if there is a regular supply of chlorine to 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 disinfect product water for period immediately after filter cleaning when filter is ‘recovering’‘recovering’

Entails the general SSF design: the Schmutzdecke as well as the other SSF Entails the general SSF design: the Schmutzdecke as well as the other SSF components situated underneath the fabriccomponents situated underneath the fabric

Note: Oxfam package includes all components of SSF except treated water Note: Oxfam package includes all components of SSF except treated water tanks, which must be ordered separately tanks, which must be ordered separately

Specific Types of SSFs: Oxfam Package 2Specific Types of SSFs: Oxfam Package 2

Cleaning the FilterCleaning the Filter 1) the water is lowered to approximately 200mm below the sand 1) the water is lowered to approximately 200mm below the sand

surfacesurface 2) the fabric is rolled up and taken out of the tank2) the fabric is rolled up and taken out of the tank 3) after having been washed, the fabric is reinstated on the sand 3) after having been washed, the fabric is reinstated on the sand

bedbed Note: because the fabric in its entirety may be difficult to move, Note: because the fabric in its entirety may be difficult to move,

cutting up the fabric into smaller pieces is recommendedcutting up the fabric into smaller pieces is recommended Depending upon the conditions, it may also be necessary to Depending upon the conditions, it may also be necessary to

remove a 10 mm layer of sand from the top of the filter bedremove a 10 mm layer of sand from the top of the filter bed

Specific Types of SSFs: Oxfam Package 3Specific Types of SSFs: Oxfam Package 3

Specific Types of SSFs: Dug and Lined FilterSpecific Types of SSFs: Dug and Lined Filter

A roughing filter design that may be applied to the construction of slow A roughing filter design that may be applied to the construction of slow sand filtersand filter

uses as a filter chamber an excavated basin covered with a plastic sheetuses as a filter chamber an excavated basin covered with a plastic sheet

Specific Types of SSFs: Small-Scale Drum FiltersSpecific Types of SSFs: Small-Scale Drum Filters

ConstructionConstruction Uses a 200 liter drum as a filter bedUses a 200 liter drum as a filter bed Includes a pre-filter when turbidity of entering water > 30 NTUIncludes a pre-filter when turbidity of entering water > 30 NTU

Specific Types of SSFs: Small-Scale Drum Filter 2Specific Types of SSFs: Small-Scale Drum Filter 2

General InformationGeneral Information Filtration rate must be < 0.2 m/hFiltration rate must be < 0.2 m/h Necessary to maintain constant flow of entering water (tank Necessary to maintain constant flow of entering water (tank

storing raw water must be higher than filter to induce constant storing raw water must be higher than filter to induce constant rate through gravity)rate through gravity)

Size of each sand particle: 0.3-1.6 mmSize of each sand particle: 0.3-1.6 mm Filter should never run dry unless it is out of use for more than one Filter should never run dry unless it is out of use for more than one

day; in this case, the filter must be drainedday; in this case, the filter must be drained Convenient for use in small service centersConvenient for use in small service centers

Specific Types of SSFs: Ceramic Candle FiltersSpecific Types of SSFs: Ceramic Candle Filters

Operated through three means: gravity, pumping, and siphonic actionOperated through three means: gravity, pumping, and siphonic action 1) Gravity Operated Filters: A) can produce 1 liter/hour/filter element1) Gravity Operated Filters: A) can produce 1 liter/hour/filter element B) appropriate for domestic needsB) appropriate for domestic needs 2) Filters Using Pumps: adequate to serve the needs of larger-scale2) Filters Using Pumps: adequate to serve the needs of larger-scale venturesventures 3) Filters Using Siphonic Action: derives efficacy from its simplicity3) Filters Using Siphonic Action: derives efficacy from its simplicity always order spare filter elements in addition to ordinary filter as filter needs always order spare filter elements in addition to ordinary filter as filter needs

replacing after long and extensive usereplacing after long and extensive use Spare filter elements can also be used to filter water with a plastic bucket for the 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 waterupper container and a jerry can for the filtered water

Specific Types of SSFs: Ceramic Candle Filters 2Specific Types of SSFs: Ceramic Candle Filters 2

MaintenanceMaintenance When the candle becomes lined with filtered-out particles, a nylon When the candle becomes lined with filtered-out particles, a nylon

brush is used to remove the amassed slimebrush is used to remove the amassed slime Each cleaning also removes some of the ceramic candle, resulting Each cleaning also removes some of the ceramic candle, resulting

in candle’s eventual loss in diameter candle needs to be in candle’s eventual loss in diameter candle needs to be replacedreplaced

Indication of this condition: if circular gauge, included in each kit, Indication of this condition: if circular gauge, included in each kit, can be slipped over candle, the candle needs to be replaced. can be slipped over candle, the candle needs to be replaced. Otherwise, it does not Otherwise, it does not

Specific Types of Slow Sand Filters: Intermittent SSFSpecific Types of Slow Sand Filters: Intermittent SSF

Design: ExplanationDesign: Explanation Constitutes a cement container filled with layers of sand and Constitutes a cement container filled with layers of sand and

gravelgravel Plastic pipe exits bottom of filter and returns up the outside; forms Plastic pipe exits bottom of filter and returns up the outside; forms

a sort of trap, reminiscent of trap under household sinksa sort of trap, reminiscent of trap under household sinks Untreated water poured into container through a diffuser plate Untreated water poured into container through a diffuser plate

(controls rate of flow)(controls rate of flow) Water passes through a biologically active region 2 inches below Water passes through a biologically active region 2 inches below

water surface (equivalent of Schmutzdecke) and the strata of water surface (equivalent of Schmutzdecke) and the strata of gravel and sandgravel and sand

Clean water exits container through pipe; travels to clean water Clean water exits container through pipe; travels to clean water receptaclereceptacle

Specific Types of SSFs: Intermittent SSF Specific Types of SSFs: Intermittent SSF

ReferencesReferences““Biosand Filter: Biological Action”: http://biosandfilter.org/biosandfilter/index.php/item/320Biosand Filter: Biological Action”: http://biosandfilter.org/biosandfilter/index.php/item/320

““How the Water Is Cleaned”: http://www.purewaterfortheworld.org/the_filter.aspHow the Water Is Cleaned”: http://www.purewaterfortheworld.org/the_filter.asp

““MSF-UAE: Refugee Camp Project-Learn More: Water (slow sand filter)”: http://www.msfMSF-UAE: Refugee Camp Project-Learn More: Water (slow sand filter)”: http://www.msf

uae.ae/en/refugeecamp/le arnmore/water/slow_sand_filter.htmuae.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 RedR-Engineers for Disaster Relief, Jan Davis and Robert Lambert, “Engineering in Emergencies: A Practical Guide For Relief

Workers”, ITDG Publishing, 1995Workers”, ITDG Publishing, 1995

““Slow Sand Filter Bed”: http://en.wikipedia.org/wiki/Slow_sand_filter_bedSlow 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.htmSlow Sand Filter: Slow Sand Filtration”: http://www.agr.gc.pfra/water/slowsnd_e.htm

““Slow Sand Filtration”: http://www.oasisdesign.net/water/treatment/slowsandfilter.htmSlow Sand Filtration”: http://www.oasisdesign.net/water/treatment/slowsandfilter.htm

““Slow Sand Filtration”: http://ewr.cee.vt.edu/environmental/teach/wtprimer/slowsand/slowsand.htmlSlow 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_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 water_filters/Emergency-treatment-of-drinking-water.htm