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

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 = 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? 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)

Slow Sand Filter Schematic A. Valve for raw water inlet and regulation of filtration rate B. Valve for draining unfiltered water C. Valve for back-filling the filter bed with clean water D. Valve for draining filter bed and outlet chamber E. Valve for delivering treated water to waste F. Valve for delivering treated water to the clear-water reservoir Filter Cake B Sand F D E Gravel Underdrains C

SSF components The SSF system incorporates five parts: The Housing (Tanks) The Supernatant The Schmutzdecke The Filter Sand The Underdrain Medium

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 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 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 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 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 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 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 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 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 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 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)

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 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 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 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 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 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 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 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