[[Image:Icon_biosand.png|right|80px]][[Image:roughing filter 2.jpg|thumb|right|300px200px|Horizontal roughing filter with slow-sand filtration. Sunderbans Water Filtration Unit, West Bengal. Photo: [http://www.wherevertheneed.org.in/projects/indian-projects/sunderbans-water-unit/ Wherevertheneed.org]]][[Image:roughing filter 3.jpg|thumb|right|300px200px|Horizontal roughing filter with slow-sand filtration. Water passes through the different filter beds before it is ready to drink. Photo: [http://www.wherevertheneed.org.in/projects/indian-projects/sunderbans-water-unit/ Wherevertheneed.org]]][[Image:upflow roughing filter.jpg|thumb|right|300px200px|Upflow roughing filter. <br>Photo: WHO.]][[Image:roughing filter chart.jpg|thumb|right|300px200px|Filter Classification. Chart: [http://www.greenstone.org/greenstone3/nzdl?a=d&c=hdl&d=HASH01ccfcc122c3ed3492c9f50c.6.1&sib=1&p.a=b&p.sa=&p.s=ClassifierBrowse&p.c=hdl SANDEC.]]]__NOTOC__<small-title />
Roughing filters are often used to pretreat water by removing suspended solids from the water that could rapidly clog a slow sand filter. Roughing filters can also considerably reduce the number of pathogens in the water, as well as the amount of iron and manganese. There are many types of roughing filters with different flow directions (downflow, upflow and horizontal flow filters), and with different types of filter medium (e.g. sand, gravel, coconut husk fibre). Upflow roughing filters are relatively cheap and easier to clean than downflow or horizontal flow filters.
'''Performance''': If raw water with a turbidity below 50 NTU is used as the source for a roughing sand filter, the outflow has a turbidity below 12 NTU. Approximately 84–98% of suspended solids are removed. Better results are obtained with two or three filters in series.
===Suitable conditions==='''Use''': As a pre-treatment pretreatment stage prior to slow sand filtering or other purification processes.
===Construction, operations and maintenance===
The filters should preferably be operated on a continuous basis. Operation consists of regulating the water flow and checking the turbidity of the effluent. Flow, turbidity and maintenance data are written in a logbook. If the turbidity gets too high, the filter may become clogged. In such cases, the filter should be cleaned about once a month, while leading the effluent to outlet. The inlet and outlet boxes are then cleaned, and backwashing and refilling are done twice. The monthly cleaning is performed by the caretaker and takes about half a day. No special assistance from users is required to clean the filters. Every two months, all valves should be completely opened and closed, to keep them from becoming stuck.
* Roughing filters only remove some of the solids and pathogens in the water, and additional treatment is needed.
===Costs===
'''Initial cost''': Reported construction costs are US$ 20–40 per m3 of water per day, for a structure designed to be in operation for 24 hours a day.
===Field experiences===
'''Kenya study'''<br>
The rapid sand filters of Salaga, a district centre located in West Africa, were never filled with filter media and the population of the town was therefore exposed to unsafe water. The old treatment plant had to be extended and replaced by an appropriate treatment scheme. Roughing and slow sand filters were thus tested in a pilot plant shaded by a large baobab tree. The location was chosen by the external experts suffering from the Ghanaian sun. The pilot plant was constructed by the local water authority, and the expatriates supervised the start of the field tests. However, African killer bees also chose the same large baobab tree as their habitat and were very much attracted by the white skin of the foreigners. Under remote supervision by the expatriates, the field tests were continued by the local staff which gained experience and confidence in a treatment scheme they wit/have to run in future at full scare and without foreign assistance. Thus, the African killer bees somehow contributed to developing local sustainability.
===Acknowledgements===
* Brikke, François, and Bredero, Maarten. [http://www.washdoc.info/docsearch/title/117705 Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff] or ([http://www.who.int/water_sanitation_health/hygiene/om/wsh9241562153/en/ alternative link]). World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.
* [http://www.washdoc.info/docsearch/title/119858 Surface water treatment by roughing filters - A design, Construction and Operation manual (1996)]. SANDEC.
* Onyeka Nkwonta and George Ochieng. [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Fwww.academicjournals.org/ijps/pdf/pdf2009/September/Nkwonta%2Fijps%2Fpdf%2Fpdf2009%2FSeptember%2FNkwonta%252020%2520and20and%2520Ochieng20Ochieng.pdf&ei=yPdYUJ28J5G88wS52IH4AQ&usg=AFQjCNEdF1bjeIkr9ODrFN0pTZjiMMSuXw&sig2=JMgRrWYg2GTmdvHzH5_ILg&cad=rjt Roughing filter for water pre-treatment technology in developing countries: A review]. Tshwane University of Technology, South Africa. July, 2009.