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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. 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.
33:
189:, 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. 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
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212:, opened the first slow sand filtration plant, 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.
280:. 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. Typically, in the UK slow sand filters have a bed depth of 0.3 to 0.6 metres comprising 0.2 to 0.4 mm sand. The throughput is 0.25 m/h.
283:
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
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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
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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
55:
product. They are typically 1–2 m (3.3–6.6 ft) 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 for the filters, which typically have a loading rate of 200–400
275:
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
415:
Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system. Many municipal systems in the U.S. initially used slow sand filters, but as cities have grown, and because of their need to treat high-turbidity source waters, they subsequently installed
348:
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
404:
as being a superior technology for the treatment of surface water sources in small water systems. 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
130:, selling his unwanted surplus to the public. 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
71:
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
173:
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.
224:
The raw water is let in to the filter room slowly from the pipe on the right. The water will pass through the sand layers down to the bottom of this room. The
Schmutzdecke layer can be observed in this
319:
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.
374:
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
67:
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
304:
656:
802:
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The practice of water treatment soon became mainstream, and the virtues of the system were made starkly apparent after the investigations of the physician
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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
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in London in 1829. This installation provided filtered water for every resident of the area, and the network design was widely copied throughout the
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921:
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401:
300:
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Centre for
Affordable Water and Sanitation Technology, Biosand Filter Manual: Design, Construction, & Installation," July 2007.
485:
Buchan, James. (2003). Crowded with genius: the
Scottish enlightenment: Edinburgh's moment of the mind. New York: Harper Collins.
357:
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.
89:
930:, United States Environmental Protection Agency, EPA 815-R-97-002 August 1997. Slow sand filtration is listed on, p. 24.
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330:, since it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service.
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Slow sand filters differ from all other filters used to treat drinking water in that they work by using a complex
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80:. Slow sand filters now are also being tested for pathogen control of nutrient solutions in hydroponic systems.
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was formed at the same time, water filtration was adopted throughout the country, and new water intakes on the
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levels to operate efficiently. In summer conditions with high microbial activity and in conditions when the
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had not yet been developed, Snow's observations led him to discount the prevailing theory. His 1855 essay
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conclusively demonstrated the role of the water supply in spreading the cholera epidemic in
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716:"Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities."
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924:, Geneva, November 1992. Slow sand filters recommendations listed on, p. 38.
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An Act to make better
Provision respecting the Supply of Water to the Metropolis
444: – Type of wastewater treatment system with a fixed bed of rocks or similar
922:"UN High Commissioner for Refugees (UNHCR) Water Manual for Refugee Situations"
435:
139:
928:"Small System Compliance Technology List for The Surface Water Treatment Rule"
981:
955:
520:
Christman, Keith. (1998). The history of chlorine. Waterworld, 14 (8), 66–67.
154:
841:
Small System
Compliance Technology List for the Surface Water Treatment Rule
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United States
Environmental Protection Agency (EPA)(1990). Cincinnati, OH.
239:
182:
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to purify the water supply dates to 1804, when the owner of a bleachery in
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32:
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that stated that diseases were caused by noxious "bad airs". Although the
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312:
119:
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Johnson, George (March 1914). "Present Day Water
Filtration Practice".
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Slow sand filters work through the formation of a gelatinous layer (or
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48:
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in
Democratic Republic of Congo and other countries to aid the poor.
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126:, John Gibb, installed an experimental filter created by engineer
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is formed in the first 10–20 days of operation and consists of
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860:. Denver, CO: American Water Works Association. pp. 1–2.
652:
National
Drinking Water Clearinghouse (U.S.), Morgantown, WV.
381:
Slow sand filters, due to their simple design, may be created
365:
of the sand bed sits a supernatant layer of unpurified water.
438: – Water filtration technique based on slow sand filters
397:
385:. DIY-slow sand filters have been used by organisations like
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For municipal systems there usually is a certain degree of
166:
530:
Gunn, S. William A.; Masellis, Michele (23 October 2007).
382:
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Typical configuration of a housed slow sand filter system
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in the top few millimetres of the fine sand layer. The
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Water treatment came to the United States in 1872 when
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672:
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occurs more quickly and pre-treatment is recommended.
613:"History | Poughkeepsies' Water Treatment Facility"
946:. New York: P. F. Collier & Son Company.
76:, where they are used to treat water supplied to
953:
801:: CS1 maint: bot: original URL status unknown (
727:
734:. New York: John Wiley and Sons. p. 353.
533:Concepts and Practice of Humanitarian Medicine
843:(Report). EPA. August 1997. EPA 815-R-97-002.
791:. Archived from the original on 6 April 2016.
420:, due to increased demand for drinking water.
402:United States Environmental Protection Agency
917:(Report). Geneva: World Health Organization.
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307:works on the principles of slow sand filters
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708:
341:is turbid, blinding of the filters due to
153:. Snow was sceptical of the then-dominant
893:. Doctors Without Borders. Archived from
619:. Poughkeepsies' Water Treatment Facility
333:Slow sand filters require relatively low
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392:Slow sand filters are recognized by the
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360:In the base of each bed is a series of
276:matrix and soluble organic material is
163:On the Mode of Communication of Cholera
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887:"Learn More: Water (slow sand filter)"
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90:History of water supply and sanitation
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496:"Brief history during the Snow Era"
315:have a number of unique qualities:
24:
934:Reynolds, Francis J., ed. (1921).
591:10.1002/j.1551-8833.1914.tb14045.x
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151:1854 Broad Street cholera outbreak
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993:
677:. Oxford: Pergamon. p. 374.
195:Metropolitan Commission of Sewers
181:introduced the regulation of the
731:Handbook of Public Water Systems
571:American Water Works Association
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663:Tech Brief Fourteen, June 2000.
718:Document no. EPA/625/4-89/023.
673:Water Research Centre (1977).
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56:litres (0.20–0.40 m) per
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697:: CS1 maint: date and year (
675:Water Purification in the EEC
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789:"WHO – Slow sand filtration"
763:"Tearfund – Biosand filters"
469:Filtration of water supplies
378:for poor and isolated areas.
118:The first documented use of
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475:, World Health Organization
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10:
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972:Environmental soil science
943:Collier's New Encyclopedia
857:Water Filtration Practices
559:, (15 & 16 Vict. C.84)
136:Chelsea Waterworks Company
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83:
854:Logsdon, Gary S. (2011).
394:World Health Organization
27:Water purification device
728:HDR Engineering (2001).
453:
142:in the ensuing decades.
654:"Slow Sand Filtration."
305:Artificial infiltration
201:were established above
112:London epidemic of 1854
967:Appropriate technology
891:Refugee Camp Project -
376:appropriate technology
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210:Poughkeepsie, New York
159:germ theory of disease
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353:While many municipal
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912:Slow sand filtration
910:Huisman, L. (1974).
659:6 April 2016 at the
323:is drained to waste.
179:Metropolis Water Act
171:dot distribution map
169:, with the use of a
18:Slow sand filter bed
583:1914JAWWA...1a..31J
216:Method of operation
937:"Filter bed"
418:rapid sand filters
309:
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45:water purification
38:
829:on 16 March 2006.
741:978-0-471-29211-1
448:Rapid sand filter
124:Paisley, Scotland
41:Slow sand filters
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155:miasma theory
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536:. Springer.
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245:Schmutzdecke
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240:Schmutzdecke
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362:herringbone
343:bioclogging
149:during the
128:Robert Thom
69:ultraviolet
956:Categories
879:References
684:0080212255
400:, and the
369:Advantages
328:redundancy
88:See also:
60:per hour.
773:5 October
693:cite book
506:9 January
339:raw water
335:turbidity
237:layer or
147:John Snow
100:John Snow
49:raw water
901:27 March
797:cite web
747:28 March
657:Archived
599:41224153
425:See also
387:Tearfund
288:Features
278:adsorbed
261:rotifera
257:protozoa
249:bacteria
235:hypogeal
225:picture.
134:for the
104:clusters
579:Bibcode
349:filter.
273:Annelid
265:bryozoa
231:biofilm
108:cholera
84:History
65:biofilm
53:potable
864:
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681:
623:18 May
597:
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269:snails
199:Thames
193:. The
191:Europe
187:London
78:London
915:(PDF)
827:(PDF)
820:(PDF)
595:JSTOR
473:(PDF)
454:Notes
398:Oxfam
311:Slow
253:fungi
982:Sand
903:2007
862:ISBN
803:link
775:2020
749:2010
736:ISBN
699:link
679:ISBN
625:2017
538:ISBN
508:2014
271:and
177:The
167:Soho
587:doi
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