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• Slow Sand Filter For Sale
• Slow Sand Filter How It Works Water

Sand filters are used as a step in the water treatment process of water purification.

They do make a better pump/filter combo with a sand filter that is not real expensive. I would comb craigslist, ebay, and local classifieds and look for a good used pump and filter. Bottom line is the pool will not stay clean and clear without a working pump and filter.

There are three main types; rapid (gravity) sand filters, upward flow sand filters and slow sand filters. All three methods are used extensively in the water industry throughout the world. The first two require the use of flocculant chemicals to work effectively while slow sand filters can produce very high quality water with pathogens removal from 90% to >99% (depending on the strains), taste and odour without the need for chemical aids.^[1] Sand filters can, apart from being used in water treatment plants, be used for water purification in singular households as they use materials which are available for most people.^[2]

• 2Sand bed filtration concept
  • 2.2Operating regimes

History[edit]

The history of separation techniques reaches far back, as filter materials were already in use during ancient periods. Rushes and genista plants were used to fill sieving vessels that separated solid and liquid materials. The Egyptians also used porous clay vessels to filter drinking water, wine and other liquids.^[3]

Slow Sand Filter For Sale

Sand bed filtration concept[edit]

A sand bed filter is a kind of depth filter. Broadly, there are two types of filter for separating particulate solids from fluids:

• Surface filters, where particulates are captured on a permeable surface
• Depth filters, where particulates are captured within a porous body of material.^[4]

In addition, there are passive and active devices for causing solid-liquid separation such as settling tanks, self-cleaning screen filters, hydrocyclones and centrifuges.^[4]

There are several kinds of depth filter, some employing fibrous material and others employing granular materials. Sand bed filters are an example of a granular loose media depth filter. They are usually used to separate small amounts (<10 parts per million or <10 g per cubic metre) of fine solids (<100 micrometres) from aqueous solutions.^{[5]:302–303} In addition, they are usually used to purify the fluid rather than capture the solids as a valuable material. Therefore they find most of their uses in liquid effluent (wastewater) treatment.

Particulate solids capture mechanisms[edit]

Sand bed filters work by providing the particulate solids with many opportunities to be captured on the surface of a sand grain. As fluid flows through the porous sand along a tortuous route, the particulates come close to sand grains. They can be captured by one of several mechanisms:

• Direct collision
• Van der Waals or London force attraction
• Surface charge attraction
• Diffusion^[4]

In addition, particulate solids can be prevented from being captured by surface charge repulsion if the surface charge of the sand is of the same sign (positive or negative) as that of the particulate solid. Furthermore, it is possible to dislodge captured particulates although they may be re-captured at a greater depth within the bed. Finally, a sand grain that is already contaminated with particulate solids may become more attractive or repel addition particulate solids. This can occur if by adhering to the sand grain the particulate loses surface charge and becomes attractive to additional particulates or the opposite and surface charge is retained repelling further particulates from the sand grain.

In some applications it is necessary to pre-treat the effluent flowing into a sand bed to ensure that the particulate solids can be captured. This can be achieved by one of several methods:

• Adjusting the surface charge on the particles and the sand by changing the pH
• Coagulation – adding small, highly charged cations (aluminium 3+ or calcium 2+ are usually used)
• Flocculation – adding small amounts of charge polymer chains which either form a bridge between the particulate solids (making them bigger) or between the particulate solids and the sand.

Operating regimes[edit]

They can be operated either with upward flowing fluids or downward flowing fluids the latter being much more usual. For downward flowing devices the fluid can flow under pressure or by gravity alone. Pressure sand bed filters tend to be used in industrial applications and often referred to as rapid sand bed filters. Gravity fed units are used in water purification especially drinking water and these filters have found wide use in developing countries (slow sand filters).

Overall, there are several categories of sand bed filter:

• rapid (gravity) sand filters
• rapid (pressure) sand bed filters
• upflow sand filters
• slow sand filters

The sketch illustrates the general structure of a rapid pressure sand filter. The filter sand takes up most space of the chamber. It sits either on a nozzle floor or on top of a drainage system which allows the filtered water to exit. The pre-treated raw water enters the filter chamber on the top, flows through the filter medium and the effluent drains through the drainage system in the lower part. Large process plants have also a system implemented to evenly distribute the raw water to the filter. In addition, a distribution system controlling the air flow is usually included. It allows a constant air and water distribution and prevents too high water flows in specific areas. A typical grain distribution exits due to the frequent backwashing. Grains with smaller diameter are dominant in the upper part of the sand layer while coarse grain dominates in the lower parts.

Two processes influencing the functionality of a filter are ripening and regeneration.
At the beginning of a new filter run, the filter efficiency increases simultaneously with the number of captured particles in the medium. This process is called filter ripening. During filter ripening the effluent might not meet quality criteria and must be reinjected at previous steps in the plant.^[6] Regeneration methods allow the reuse of the filter medium. Accumulated solids from the filter bed are removed.^[6] During backwashing, water (and air) is pumped backwards through the filter system. Backwash water may partially be reinjected in front of the filter process and generated sewage needs to be discarded. The backwashing time is determined by either the turbidity value behind the filter, which must not exceed a set threshold, or by the head loss across the filter medium, which must also not exceed a certain value.

Rapid pressure sand bed filter design[edit]

Slow Sand Filter How It Works Water

Smaller sand grains provide more surface area and therefore a higher decontamination of the inlet water, but it also requires more pumping energy to drive the fluid through the bed. A compromise is that most rapid pressure sand bed filters use grains in the range 0.6 to 1.2 mm although for specialist applications other sizes may be specified. Larger feed particles (>100 micrometres) will tend to block the pores of the bed and turn it into a surface filter that blinds rapidly. Larger sand grains can be used to overcome this problem, but if significant amounts of large solids are in the feed they need to be removed upstream of the sand bed filter by a process such as settling.^{[5]:302–303}

The depth of the sand bed is recommended to be around 0.6–1.8 m (2–6 ft) regardless of the application. This is linked to the maximum throughput discussed below.^{[5]:302–303}

Guidance on the design of rapid sand bed filters suggests that they should be operated with a maximum flow rate of 9 m³/m²/hr (220 US gal/ft²/hr).^[7] Using the required throughput and the maximum flow rate, the required area of the bed can be calculated.

The final key design point is to be sure that the fluid is properly distributed across the bed and that there are no preferred fluid paths where the sand may be washed away and the filter be compromised.

Rapid pressure sand bed filters are typically operated with a feed pressure of 2 to 5 bar(a) (28 to 70 psi(a)). The pressure drop across a clean sand bed is usually very low. It builds as particulate solids are captured on the bed. Particulate solids are not captured uniformly with depth, more are captured higher up with bed with the concentration gradient decaying exponentially.^{[5]:302–303}

This filter type will capture particles down to very small sizes, and does not have a true cut off size below which particles will always pass. Extend volume greyed out windows 10 2017. The shape of the filter particle size-efficiency curve is a U-shape with high rates of particle capture for the smallest and largest particles with a dip in between for mid-sized particles.^[7]

The build-up of particulate solids causes an increase in the pressure lost across the bed for a given flow rate. For a gravity fed bed when the pressure available is constant, the flow rate will fall. When the pressure loss or flow is unacceptable and the filter is not working effectively any longer, the bed is backwashed to remove the accumulated particles. For a pressurized rapid sand bed filter this occurs when the pressure drop is around 0.5 bar. The backwash fluid is pumped backwards through the bed until it is fluidized and has expanded by up to about 30% (the sand grains start to mix and as they rub together they drive off the particulate solids). The smaller particulate solids are washed away with the backwash fluid and captured usually in a settling tank. The fluid flow required to fluidize the bed is typically 3 to 10 m³/m²/hr but not run for long (a few minutes).^{[5]:224–235} Small amounts of sand can be lost in the backwashing process and the bed may need to be topped up periodically.

Slow sand filter design[edit]

As the title indicates, the speed of filtration is changed in the slow sand filter, however, the biggest difference between slow and rapid sand filter, is that the top layer of sand is biologically active, as microbial communities are introduced to the system. The recommended and usual depth of the filter is 0.9 to 1.5 meters. Microbial layer is formed within 10–20 days from the start of the operation. During the process of filtration, raw water can percolate through the porous sand medium, stopping and trapping organic material, bacteria, viruses and cysts such as Giardia and Cryptosporidium. The regeneration procedure for slow sand filters is called scraping and is used to mechanically remove the dried out particles on the filter. However, this process can also be done under water, depending on the individual system. Another limiting factor for the water being treated is turbidity, which is for slow sand filters defined to be 10 NTU (Nephelometric Turbidity Units). Slow sand filters are a good option for limited budget operations as the filtration is not using any chemicals and requires little or no mechanical assistance. However, because of a continuous growing population in communities, slow sand filters are being replaced for rapid sand filters, mostly due to the running period length.

Characteristics of rapid and slow sand filters^[6][edit]

Mixed bed filters[edit]

Filters can be constructed with different layers, called mixed bed filters. Sand is a common filter material, but anthracite, granular activated carbon (GAC), garnet and ilmenite are also common filter materials. Anthracite is a harder material and has less volatile compared to other coals. Ilmenite and garnet are heavy compared to sand. Garnet consists several minerals, causing a shifting red colour. Ilmenite is an oxide of iron and titanium. GAC can be used in the process of adsorption and filtration at the same time. These materials can be used both alone, or combined with other media. Different combinations give different filter classification. Monomedia is a one layered filter, commonly consisting of sand and is today replaced by newer technology. Deep-bed monomedia is also a one layered filter which consist of either anthracite or GAC. The deep-bed monomedia filter is used when there is a consistent water quality and this gives a longer run time. Dual media (two layered) often contain a sand layer in the bottom with an anthracite or GAC layer on top. Trimedia or mixed media is a filter with three layers. Trimedia often have garnet or ilmenite in the bottom layer, sand in the middle and anthracite at the top.

Uses in water treatment[edit]

All of these methods are used extensively in the water industry throughout the world. The first three in the list above require the use of flocculant chemicals to work effectively. Slow sand filters produce high-quality water without the use of chemical aids.

Passing flocculated water through a rapid gravity sand filter strains out the floc and the particles trapped within it, reducing numbers of bacteria and removing most of the solids. The medium of the filter is sand of varying grades. Where taste and odor may be a problem (organoleptic impacts), the sand filter may include a layer of activated carbon to remove such taste and odor.

Sand filters become clogged with floc or bioclogged after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed of as a waste-water stream. In some countries, the sludge may be used as a soil conditioner. Inadequate filter maintenance has been the cause of occasional drinking water contamination.

Sand filters are occasionally used in the sewage treatment as a final polishing stage. In these filters the sand traps residual suspended material and bacteria and provides a physical matrix for bacterial decomposition of nitrogenous material, including ammonia and nitrates, into nitrogen gas.

Sand filters are one of the most useful treatment processes as the filtering process (especially with slow sand filtration) combines within itself many of the purification functions.^[8]

Challenges in the application process[edit]

In the process of water treatment, one should be aware of certain factors that might cause serious problems if not treated properly. Aforementioned processes such as filter ripening and backwashing influence not only the water quality but also the time needed for the full treatment. Backwashing reduces also the volume of the effluent. If a certain amount of water has to be delivered to e.g. a community, this water loss needs to be considered. In addition, backwashing waste needs to be treated or properly discarded. From the chemical perspective, varying raw water qualities and changes in the temperature effect, already at the entrance to the plant, the efficiency of the treatment process.

Considerable uncertainty is involved regarding models used to construct sand filters. This is due to mathematical assumptions that have to be made such as all grains being spherical. The spherical shape affects the interpretation of the size since the diameter is different for spherical and non-spherical grains. The packing of the grains within the bed is also dependent on the shape of the grains. This then affects the porosity and hydraulic flow.^[6]

See also[edit]

References[edit]

1. ^'Slow Sand Filtration'(PDF). National Drinking Water Clearinghouse.
2. ^'Household Sand Filters for Arsenic Removal'(PDF). EAWAG.
3. ^Anlauf, Harald (2003). 'Mechanische Fest/Flüssig-Trennungim Wandel der Zeit'. Chemie Ingenieur Technik. 75 (10): 1460–1463. doi:10.1002/cite.200303283.
4. ^ ^abcA. Rushton, A. S. Ward, R. G. Holdich (1996). Introduction to Solid-Liquid Filtration and Separation Technology. Wiley VCH. ISBN978-3-527-28613-3
5. ^ ^abcdeCoulson, J. M.; Richardson, J. F.; Backhurst, J. R., Harker, J. H. (1991). Chemical Engineering. Vol.2, 4th ed. ISBN0-7506-2942-8.
6. ^ ^abcdCrittenden, John C.; Trussell, R. Rhodes; Hand, David W.; Howe, Kerry J.; Tchobanoglous, George (2012). MWH's water treatment: principles and design (3rd ed.). Hoboken, N.J.: John Wiley & Sons. ISBN9780470405390.
7. ^ ^abK. J. Ives (1990). 'Deep Bed Filtration'. Chap. 11 of Solid-Liquid Separation, 3rd ed., L. Svarovsky (ed). Butterworths. ISBN0-408-03765-2
8. ^Huisman, L.; Wood, W. E. (1974). Slow sand filtration. Geneva: World Health Organization. ISBN978-9241540377.

Slow sand filters are used in water purification for treating raw water to produce a potable product. They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water. The length and breadth of the tanks are determined by the flow rate desired by the filters, which typically have a loading rate of 200 to 400 litres per hour per square metre (or 0.2 to 0.4 cubic metres per square metre per hour).

Slow sand filters differ from all other filters used to treat drinking water in that they work by using a complex biological film that grows naturally on the surface of the sand. The sand itself does not perform any filtration function but simply acts as a substrate, unlike its counterparts for UV and pressurized treatments. Although they are often preferred technology in many developing countries because of their low energy requirements and robust performance, they are also used to treat water in some developed countries, such as the UK, where they are used to treat water supplied to London. Slow sand filters now are also being tested for pathogen control of nutrient solutions in hydroponic systems.

History[edit]

The first documented use of sand filters to purify the water supply dates to 1804, when the owner of a bleachery in Paisley, Scotland, John Gibb, installed an experimental filter, selling his unwanted surplus to the public.^[1][2] This method was refined in the following two decades by engineers working for private water companies, and it culminated in the first treated public water supply in the world, installed by engineer James Simpson for the Chelsea Waterworks Company in London in 1829.^[3][4] This installation provided filtered water for every resident of the area, and the network design was widely copied throughout the United Kingdom in the ensuing decades.

The practice of water treatment soon became mainstream, and the virtues of the system were made starkly apparent after the investigations of the physician John Snow during the 1854 Broad Street cholera outbreak. Snow was sceptical of the then-dominant miasma theory that stated that diseases were caused by noxious 'bad airs'. Although the germ theory of disease had not yet been developed, Snow's observations led him to discount the prevailing theory. His 1855 essay On the Mode of Communication of Cholera conclusively demonstrated the role of the water supply in spreading the cholera epidemic in Soho,^[5] with the use of a dot distribution map and statistical proof to illustrate the connection between the quality of the water source and cholera cases. His data convinced the local council to disable the water pump, which promptly ended the outbreak.

The Metropolis Water Act introduced the regulation of the water supply companies in London, including minimum standards of water quality for the first time. The Act 'made provision for securing the supply to the Metropolis of pure and wholesome water', and required that all water be 'effectually filtered' from 31 December 1855.^[6] This was followed up with legislation for the mandatory inspection of water quality, including comprehensive chemical analyses, in 1858. This legislation set a worldwide precedent for similar state public health interventions across Europe. The Metropolitan Commission of Sewers was formed at the same time, water filtration was adopted throughout the country, and new water intakes on the Thames were established above Teddington Lock.

Water treatment came to the United States in 1872 when Poughkeepsie, NY opened the first slow sand filtration plant,^[7] dramatically reducing instances of cholera and typhoid fever which had been seriously impacting the local community. Poughkeepsie's design criteria were used throughout the country as a model for other municipalities. Poughkeepsie's original treatment facility operated continuously for 87 years before being replaced in 1959.^[8]

Method of operation[edit]

Slow sand filters work through the formation of a gelatinous layer (or biofilm) called the hypogeal layer or Schmutzdecke in the top few millimetres of the fine sand layer. The Schmutzdecke is formed in the first 10–20 days of operation^[9] and consists of bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. As an epigeal biofilm ages, more algae tend to develop and larger aquatic organisms may be present including some bryozoa, snails and Annelid worms. The surface biofilm is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support medium for this biological treatment layer. As water passes through the hypogeal layer, particles of foreign matter are trapped in the mucilaginous matrix and soluble organic material is adsorbed. The contaminants are metabolised by the bacteria, fungi and protozoa. The water produced from an exemplary slow sand filter is of excellent quality with 90-99% bacterial cell count reduction.^[10]

Slow sand filters slowly lose their performance as the biofilm thickens and thereby reduces the rate of flow through the filter. Eventually, it is necessary to refurbish the filter. Two methods are commonly used to do this. In the first, the top few millimetres of fine sand is scraped off to expose a new layer of clean sand. Water is then decanted back into the filter and re-circulated for a few hours to allow a new biofilm to develop. The filter is then filled to full volume and brought back into service.^[10] The second method, sometimes called wet harrowing, involves lowering the water level to just above the hypogeal layer, stirring the sand; thus precipitating any solids held in that layer and allowing the remaining water to wash through the sand. The filter column is then filled to full capacity and brought back into service. Wet harrowing can allow the filter to be brought back into service more quickly.^[9]

Features[edit]

Slow sand filters have a number of unique qualities:

1. Unlike other filtration methods, slow sand filters use biological processes to clean the water, and are non-pressurized systems. Slow sand filters do not require chemicals or electricity to operate.
2. Cleaning is traditionally done by use of a mechanical scraper, which is usually driven into the filter bed once the bed has been dried out. However, some slow sand filter operators use a method called 'wet harrowing', where the sand is scraped while still under water, and the water used for cleaning is drained to waste.
3. For municipal systems there usually is a certain degree of redundancy, since it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service.
4. Slow sand filters require relatively low turbidity levels to operate efficiently. In summer conditions with high microbial activity and in conditions when the raw water is turbid, blinding of the filters due to bioclogging occurs more quickly and pre-treatment is recommended.
5. Unlike other water filtration technologies that produce water on demand, slow sand filters produce water at a slow, constant flow rate and are usually used in conjunction with a storage tank for peak usage. This slow rate is necessary for healthy development of the biological processes in the filter.^{[11]:38–41[12]}

While many municipal water treatment works will have 12 or more beds in use at any one time, smaller communities or households may only have one or two filter beds.

In the base of each bed is a series of herringbone drains that are covered with a layer of pebbles which in turn is covered with coarse gravel. Further layers of sand are placed on top followed by a thick layer of fine sand. The whole depth of filter material may be more than 1 metre in depth, the majority of which will be fine sand material. On top of the sand bed sits a supernatant layer of unpurified water.

Advantages[edit]

• As they require little or no mechanical power, chemicals or replaceable parts, and they require minimal operator training and only periodic maintenance, they are often an appropriate technology for poor and isolated areas.
• Slow sand filters, due to their simple design, may be created DIY. DIY-slow sand filters have been used in Afghanistan and other countries to aid the poor.^[13]
• Slow sand filters are recognized by the World Health Organization,^[14]Oxfam,^[15] and the United States Environmental Protection Agency^[16] as being superior technology for the treatment of surface water sources. According to the World Health Organization, 'Under suitable circumstances, slow sand filtration may be not only the cheapest and simplest but also the most efficient method of water treatment.'

Disadvantages[edit]

• Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system.^[11] Many municipal systems in the U.S. initially used slow sand filters, but as cities have grown they subsequently installed rapid sand filters, due to increased demand for drinking water.

See also[edit]

Notes[edit]

1. ^Filtration of water supplies(PDF), World Health Organization
2. ^Buchan, James. (2003). Crowded with genius: the Scottish enlightenment: Edinburgh's moment of the mind. New York: Harper Collins.
3. ^'BRIEF HISTORY DURING THE SNOW ERA'.
4. ^Christman, Keith. (1998). The history of chlorine. Waterworld, 14 (8), 66-67.
5. ^'Concepts and Practice of Humanitarian Medicine'.
6. ^An Act to make better Provision respecting the Supply of Water to the Metropolis, (15 & 16 Vict. C.84)
7. ^Johnson, George (March 1914). 'PRESENT DAY WATER FILTRATION PRACTICE'. American Water Works Association. 1 (1): 31. JSTOR41224153.
8. ^'History Poughkeepsies' Water Treatment Facility'. pokwater.com. Poughkeepsies' Water Treatment Facility. Retrieved 18 May 2017.
9. ^ ^abCentre for Affordable Water and Sanitation Technology, Biosand Filter Manual: Design, Construction, & Installation,' July 2007.
10. ^ ^abNational Drinking Water Clearinghouse (U.S.), Morgantown, WV. 'Slow Sand Filtration.' Tech Brief Fourteen, June 2000.
11. ^ ^abUnited States Environmental Protection Agency (EPA)(1990). Cincinnati, OH. 'Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities.' Document no. EPA/625/4-89/023.
12. ^HDR Engineering (2001). Handbook of Public Water Systems. New York: John Wiley and Sons. p. 353. ISBN978-0-471-29211-1. Retrieved 28 March 2010.
13. ^DIY slow sand filter
14. ^'WHO - Slow sand filtration'. Archived from the original on 6 April 2016.CS1 maint: BOT: original-url status unknown (link)
15. ^'UNHCR eCentre'(PDF). Archived from the original(PDF) on 16 March 2006.
16. ^[1]

References[edit]

• 'Learn More: Water (slow sand filter)'. Refugee Camp Project -. Doctors Without Borders. Archived from the original on 28 July 2007. Retrieved 27 March 2007.
• 'Slow Sand Filtration', World Health Organization, 1974 ISBN92-4-154037-0
• 'UN High Commissioner for Refugees (UNHCR) Water Manual for Refugee Situations', Geneva, November 1992. Slow sand filters recommendations listed on, p. 38.
• 'Small System Compliance Technology List for The Surface Water Treatment Rule', United States Environmental Protection Agency, EPA 815-R-97-002 August 1997. Slow sand filtration is listed on, p. 24.
• Reynolds, Francis J., ed. (1921). 'Filter bed' . Collier's New Encyclopedia. New York: P.F. Collier & Son Company.