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Slow Sand Filters. Overview. As a means of water purification, slow sand filters constitute a simple, efficient design and may be constructed using local resources

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Overview
Overview

  • 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
Construction: Explanation

  • 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
Construction: Design

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

If the tank is being fed by pumps: situated at a higher elevation than the filter to allow for a generous operating range


Filtration rate
Filtration Rate

  • 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
One Filter Bed or Two?

  • 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
SSF components

The SSF system incorporates five parts:

  • The Housing (Tanks)

  • The Supernatant

  • The Schmutzdecke

  • The Filter Sand

  • The Underdrain Medium


Housing tanks explanation
Housing (Tanks): Explanation

  • 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
Housing: Operation and Maintenance

  • 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
The Supernatant

  • 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
The Schmutzdecke: Explanation

  • 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
The Schmutzdecke: Maintenance

  • 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
The Filter Sand: Explanation

  • 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
Filter Sand: Pre-Cleaning

  • 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
The Underdrain Medium

  • 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
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
Operation and Maintenance: Filter Bed

  • 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
Operation and Maintenance: Personnel

  • 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

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

No machinery necessary

Disadvantages

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

Advantages and Disadvantages of SSFs


Specific types of ssfs oxfam package
Specific Types of SSFs: Oxfam package

Design

  • 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
Specific Types of SSFs: Oxfam Package 2

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 dug and lined filter
Specific Types of SSFs: Dug and Lined Filter

  • 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
Specific Types of SSFs: Small-Scale Drum Filters

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
Specific Types of SSFs: Small-Scale Drum Filter 2

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
Specific Types of SSFs: Ceramic Candle Filters

  • 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
Specific Types of SSFs: Ceramic Candle Filters 2

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
Specific Types of Slow Sand Filters: Intermittent SSF

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



References
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


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