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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.
22:
178:, 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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201:, 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.
269:. 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.
272:
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
353:
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
311:
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
44:
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
264:
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
404:
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
337:
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
393:
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
119:, 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
60:
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
162:
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.
213:
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
308:
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.
363:
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
56:
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
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645:
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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
127:
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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910:
875:
805:
777:
751:
390:
289:
629:
Centre for
Affordable Water and Sanitation Technology, Biosand Filter Manual: Design, Construction, & Installation," July 2007.
474:
Buchan, James. (2003). Crowded with genius: the
Scottish enlightenment: Edinburgh's moment of the mind. New York: Harper Collins.
346:
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.
78:
919:, United States Environmental Protection Agency, EPA 815-R-97-002 August 1997. Slow sand filtration is listed on, p. 24.
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319:, 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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69:. 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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705:"Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities."
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8:
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913:, 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
433: – Type of wastewater treatment system with a fixed bed of rocks or similar
911:"UN High Commissioner for Refugees (UNHCR) Water Manual for Refugee Situations"
424:
128:
917:"Small System Compliance Technology List for The Surface Water Treatment Rule"
970:
944:
509:
Christman, Keith. (1998). The history of chlorine. Waterworld, 14 (8), 66–67.
143:
830:
Small System
Compliance Technology List for the Surface Water Treatment Rule
703:
United States
Environmental Protection Agency (EPA)(1990). Cincinnati, OH.
228:
171:
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to purify the water supply dates to 1804, when the owner of a bleachery in
46:
21:
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that stated that diseases were caused by noxious "bad airs". Although the
331:
301:
108:
587:
209:
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Johnson, George (March 1914). "Present Day Water
Filtration Practice".
281:
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Slow sand filters work through the formation of a gelatinous layer (or
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83:
37:
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in
Democratic Republic of Congo and other countries to aid the poor.
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115:, 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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179:
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849:. Denver, CO: American Water Works Association. pp. 1–2.
641:
National
Drinking Water Clearinghouse (U.S.), Morgantown, WV.
370:
Slow sand filters, due to their simple design, may be created
354:
of the sand bed sits a supernatant layer of unpurified water.
427: – Water filtration technique based on slow sand filters
386:
374:. DIY-slow sand filters have been used by organisations like
241:
315:
For municipal systems there usually is a certain degree of
155:
519:
Gunn, S. William A.; Masellis, Michele (23 October 2007).
371:
285:
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
62:
661:
334:
occurs more quickly and pre-treatment is recommended.
602:"History | Poughkeepsies' Water Treatment Facility"
935:. New York: P. F. Collier & Son Company.
65:, where they are used to treat water supplied to
942:
790:: CS1 maint: bot: original URL status unknown (
716:
723:. New York: John Wiley and Sons. p. 353.
522:Concepts and Practice of Humanitarian Medicine
832:(Report). EPA. August 1997. EPA 815-R-97-002.
780:. Archived from the original on 6 April 2016.
409:, due to increased demand for drinking water.
391:United States Environmental Protection Agency
906:(Report). Geneva: World Health Organization.
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296:works on the principles of slow sand filters
699:
697:
330:is turbid, blinding of the filters due to
142:. Snow was sceptical of the then-dominant
882:. Doctors Without Borders. Archived from
608:. Poughkeepsies' Water Treatment Facility
322:Slow sand filters require relatively low
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381:Slow sand filters are recognized by the
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349:In the base of each bed is a series of
265:matrix and soluble organic material is
152:On the Mode of Communication of Cholera
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876:"Learn More: Water (slow sand filter)"
204:
79:History of water supply and sanitation
632:
620:
710:
485:"Brief history during the Snow Era"
304:have a number of unique qualities:
13:
923:Reynolds, Francis J., ed. (1921).
580:10.1002/j.1551-8833.1914.tb14045.x
449:
140:1854 Broad Street cholera outbreak
14:
982:
666:. Oxford: Pergamon. p. 374.
184:Metropolitan Commission of Sewers
170:introduced the regulation of the
720:Handbook of Public Water Systems
560:American Water Works Association
398:
836:
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798:
770:
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655:
652:Tech Brief Fourteen, June 2000.
707:Document no. EPA/625/4-89/023.
662:Water Research Centre (1977).
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512:
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45:litres (0.20–0.40 m) per
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686:: CS1 maint: date and year (
664:Water Purification in the EEC
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778:"WHO – Slow sand filtration"
752:"Tearfund – Biosand filters"
458:Filtration of water supplies
367:for poor and isolated areas.
107:The first documented use of
7:
464:, World Health Organization
413:
276:
10:
987:
961:Environmental soil science
932:Collier's New Encyclopedia
846:Water Filtration Practices
548:, (15 & 16 Vict. C.84)
125:Chelsea Waterworks Company
76:
72:
843:Logsdon, Gary S. (2011).
383:World Health Organization
16:Water purification device
717:HDR Engineering (2001).
442:
131:in the ensuing decades.
643:"Slow Sand Filtration."
294:Artificial infiltration
190:were established above
101:London epidemic of 1854
956:Appropriate technology
880:Refugee Camp Project -
365:appropriate technology
297:
286:
215:
199:Poughkeepsie, New York
148:germ theory of disease
104:
26:
342:While many municipal
292:
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212:
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901:Slow sand filtration
899:Huisman, L. (1974).
648:6 April 2016 at the
312:is drained to waste.
168:Metropolis Water Act
160:dot distribution map
158:, with the use of a
572:1914JAWWA...1a..31J
205:Method of operation
926:"Filter bed"
407:rapid sand filters
298:
287:
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34:water purification
27:
818:on 16 March 2006.
730:978-0-471-29211-1
437:Rapid sand filter
113:Paisley, Scotland
30:Slow sand filters
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174:companies in
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144:miasma theory
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121:James Simpson
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99:cases in the
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489:the original
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234:Schmutzdecke
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229:Schmutzdecke
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394:treatment."
351:herringbone
332:bioclogging
138:during the
117:Robert Thom
58:ultraviolet
945:Categories
868:References
673:0080212255
389:, and the
358:Advantages
317:redundancy
77:See also:
49:per hour.
762:5 October
682:cite book
495:9 January
328:raw water
324:turbidity
226:layer or
136:John Snow
89:John Snow
38:raw water
890:27 March
786:cite web
736:28 March
646:Archived
588:41224153
414:See also
376:Tearfund
277:Features
267:adsorbed
250:rotifera
246:protozoa
238:bacteria
224:hypogeal
214:picture.
123:for the
93:clusters
568:Bibcode
338:filter.
262:Annelid
254:bryozoa
220:biofilm
97:cholera
73:History
54:biofilm
42:potable
853:
727:
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612:18 May
586:
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258:snails
188:Thames
182:. The
180:Europe
176:London
67:London
904:(PDF)
816:(PDF)
809:(PDF)
584:JSTOR
462:(PDF)
443:Notes
387:Oxfam
300:Slow
242:fungi
971:Sand
892:2007
851:ISBN
792:link
764:2020
738:2010
725:ISBN
688:link
668:ISBN
614:2017
527:ISBN
497:2014
260:and
166:The
156:Soho
576:doi
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Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
