|
|
| Line 1: |
Line 1: |
| − | {{stub}}
| + | Mi mi mim im afioa asfpasfp asf!!!! |
| − | | |
| − | ==Short Description==
| |
| − | *'''Problem:''' Filtering water
| |
| − | *'''Idea:''' Using the filtering propreties of sand
| |
| − | *'''Difficulty:''' Understanding the principle of the schmutzdecke. Using the filter properly (first use, continous filtering, regular replacement of the sand)
| |
| − | *'''Price Range:''' Cheap
| |
| − | *'''Material Needeed:''' A big container (best are non reactive surfaces: plastic, concrete tanks... from 200 liters up), different sand sizes, reservoir and pipes for the dirty water and for the clean water, a tap
| |
| − | *'''Geographic Area:''' Global
| |
| − | *'''Competencies:''' Simple plumber skills
| |
| − | *'''How Many people?''' From one person on
| |
| − | *'''How Long does it take?''' 1 to 4 days to buid the filter, depending on its complexity, 10 days before water should be drunk.
| |
| − | | |
| − | ==Description==
| |
| − | '''Slow Gravity Sand Filters'''
| |
| − | | |
| − | Safe drinking water can be reached with this type of sand filters.
| |
| − | Thanks to mechanical and biological action in the sand layer, slow gravity sand filters remove bacteria as well as small particles from water, making it safe to drink. This is a basic introduction to the subject and should help maintain a constant supply of clean, running water using simple technology.
| |
| − | The sand filter described below is designed for domestic use only.
| |
| − | | |
| − | '''Water source'''
| |
| − | | |
| − | It is important to have a relatively clean source of water, that means free from chemicals and heavy metalls, because a Sand Filter won't provide a sufficient filtering to turn chemically polluted water into drinkable water. It is important to test a sample of water in a laboratory before you start. Even spring water or very clear river water should be tested for chemical contaminants.
| |
| − | If your water source has a high level of contamination,try to locate a new one. If this isn't possible other methods of filtration should be used.
| |
| − | If the water is very brown, that is has a high [[turbidity]] level, it should be treated before it gets to the sand filter. For exemple stay in settling tank or pass through a [[horizontal roughing filter]] before it goes in the proper sand filter.
| |
| − | | |
| − | '''How the filter works'''
| |
| − | | |
| − | The water passes through the sand from top to bottom. Larger suspended particles will settle in the top layers of sand. Smaller particles of organic sediment left in the sand filter are eaten by microscopic organisms including bacteria and protozoans which 'stick' in the layers of slime that form around the sand particles.
| |
| − | The clean water which passes through the filter is safe to drink. If the grain size is around 0.1mm in diameter, a sand filter can remove all fecal coliforms (bacteria that originate from feces) and virtually all viruses.
| |
| − | | |
| − | '''Slow Sand Filters'''
| |
| − | | |
| − | The drawing shows a typical sand filter. The dirty water should flow into the upper part of the tank without disturbing the schmutzdecke (a layer of slime that form on the top of the sand after a few days and contains beneficial bacterias). There should be a sufficient depth of water above the sand so that enough pressure pushes the dirty water through the schmutzdecke, the filter bed and into the support gravel. This depth should ideally be about 2-3 meters (7-10 feet) even if 1.5 meters (4 feet) should also allow the filter to work properly. This depth will depend on the sand properties and the porosity of the schmutzdecke.
| |
| − | The maximum water level may be controled by using a float and a control valve or by maintaining the water lever near the overflow line.
| |
| − | Important: the filtration should be continuous, the sand should always be immerged and the schmutzdecke protected from direct sun and strong temperature differences.
| |
| − | | |
| − | The depth of sand (the filter bed) has a strong influence on the effectiveness of the filtration and should be at least (minimum) 75 cm / 1 meter (30" - 40").
| |
| − | | |
| − | Sand propreties are defined with the grain diameter and the properties ES ( Effective Size) and UC( Uniformity Coefficient). For the Slow Sand Filter, sand needs to be very fine (0,1 to 0,35mm diameter) ES typically is 12 to 40 mm (0.5" to about 1.6")and UC should be less than about 2.5.
| |
| − | | |
| − | Typical water processing rates in slow sand filters are about 2.5 m3/[m2 of filter cross-section area - day] = about 0.1 m/hour (0.33 ft/hour).
| |
| − | The filtration rate may be determined by a flow meter on the outlet tap. Unless the raw water is particularly well treated to about 20 turbidity units or less, this figure should be maintained, unless a very reliable post-ssf disinfection is in place.
| |
| − | High turbidity (lots of particles in the water) of the raw water will block the sand filter quicker, shortening the time between cleanings and lowering the water quality. It might also shorten the filter life extremely from several months to some days. The raw water should be pre-treated, for exemple with a Horizontal Roughing Filter that helps reduce the turbidity from an average of about 200 units, with occasional short-term peaks to around 1000, down to about 20. Turbidity reduction can also occur thanks holding ponds or sedimentation tanks with a floating water intake.
| |
| − | The schmutzdecke is a layer where bacteriological and physical processes occure, the main one being that most of the suspended matter is being pressed in a thin dense layer in which the pores may be less than a micron. The thickness of this layer increases with time untill the flow rates become very small,at this point, it is usually about 25 mm (1"). That is the time where a layer of the filter has to be taken away.
| |
| − | | |
| − | Bacteria and viruses are traped in the schmutzdecke. Bacterial and biological activity maximizes in the schmutzdecke but will continue up to 20 cm deep in the sand of the filter bed. A certain minimum level of dissolved oxygen should be present to support the aerobic actions that occur in the bed. After the initial installation of the Sand Filter, '''the formation of the schmutzdecke and bacterial/biological activity will take some days or weeks depending on the ambient temperature. During this period the processed water is unsafe for human consumption and should not be drunk, or must be filtered in another filter, or disinfected with other means'''.
| |
| − | Water quality tests should be done at a regular intervals until the required standard is reached.
| |
| − | | |
| − | '''Cleaning the filter'''
| |
| − | | |
| − | When clean water flow becomes very small, the sand filter must be cleaned.
| |
| − | Remove very carefully about 25 mm (1") of the top layer, which includes most of the existing schmutzdecke,with broad flat-bottomed shovels after dropping the water level to slightly below the latter. In strong hot sunlight this should be done as quickly as possible to avoid drying out and damage to the biological matter in the top layer, which will be the base of the new schmutzdecke.
| |
| − | When the filter is restarted, the processed water must again not be consummed until the required level of quality is again reached. This takes several days.
| |
| − | | |
| − | When the depth of the bed of sand will have been reduced by the cleaning processes to about 0.75 meters (30"), the original depth must restored by adding new sand. because the new sand will have no biological activity, the process is accelerated by removing and discarding the schmutzdecke, and then the top 20 centimeters of the existing bed is removed and kept to be re-use on top of the new sand. this help to restore quickly the biological activity and new schmutzdecke.
| |
| − | The sand from the existing bed should not be allowed to dry out and should be set in place again as quickly as possible.
| |
| − | | |
| − | ==Difficulties==
| |
| − | The Slow Sand Filter needs large amounts of land, because of the low filtering rates ( 10 per cent of the filtering rates allowed by rapid filtration).
| |
| − | Because of the slow rates, the clean water has to be stored in proper containers to adapt the peak demand.
| |
| − | Great care in the operation and maintenance, in particular in relation to the schmutzdecke which requires some time to form. The processed water cannot be considered safe until it has and the water that is passed through the system while the schmutzdecke is forming must be cycled back or discarded.
| |
| − | While first building the filter, the sand should be either dropped in the tank filled up water or be filled with water from the bottom to avoid the formation of air pockets in the sand bed.
| |
| − | | |
| − | ==Success Story==
| |
| − | | |
| − | | |
| − | ==Plans, Illustrations, Posters==
| |
| − | | |
| − | | |
| − | ==Contacts==
| |
| − | | |
| − | | |
| − | ==Links==
| |
| − | http://www.waterinfo.com/ssf.htm
| |
| − | http://www.photometer.com/en/abc/abc_095.htm Procedure for measuring turbidity
| |
| − | • The Davnor BioSand Water Filter
| |
| − | • Manual of Design for Slow Sand Filtration
| |
| − | • Slow Sand Filtration, Huisman L. and Wood, W.E., WHO 1974
| |
| − | | |
| − | ==Bibliography==
| |
| − | The Davnor BioSand Water Filter
| |
| − | Manual of Design for Slow Sand Filtration
| |
| − | Slow Sand Filtration, Huisman L. and Wood, W.E., WHO 1974
| |
| − | | |
| − | ==Related articles==
| |
| − | [[turbidity]],[[horizontal roughing filter]],[[How to Pasteurize Water with a Plastic Bottle?]]
| |
| − | | |
| − | =='''Categories:'''==
| |
| − | [[category:example]]
| |
| − | [[Category:Global Technology]]
| |
| − | [[Category:Easy]]
| |
| − | [[Category:Less than 50 US$]]
| |
| − | [[Category:Up to 5 Persons]]
| |
| − | [[Category:Water]]
| |
