Slow Sand Filter Design Calculation
Sand filter used for water treatmentSand filters are used as a step in the water treatment process of.There are three main types;, upward flow sand filters. All three methods are used extensively in the throughout the world. The first two require the use of to work effectively while slow sand filters can produce very high quality water with removal from 90% to 99% (depending on the strains), taste and odour without the need for chemical aids. 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. Sand filters on a tomato farm inA sand bed filter is a kind of. Broadly, there are two types of filter for separating 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.In addition, there are passive and active devices for causing solid-liquid separation such as, self-cleaning screen filters, and.There are several kinds of depth filter, some employing material and others employing. Sand bed filters are an example of a granular loose media depth filter. They are usually used to separate small amounts (.
Rapid pressure filter 1=raw water, 2=filtered water, 3=tank, 4=inlet flushing water, 5=outlet flushing water, 6=retraction line, 7=scavenging air, 8=injector, 9=supporting layer, 10=filter sand, 11=flushing funnel, 12=ventilationSmaller 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.
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1.4 Hydraulic Conductivities for Slow Sand Filters 19 1.5 Summary of Designs of Slow Sand Filters in Europe as Reported by Hazen, With Units Adjusted to Fit Those Used in This Manual 21 1.6 Design Criteria for Slow Sand Filters for Rural Water Supplies 26 2.1 Present and Projected Flow Data for Village of 100 Mile House, BC 35. Slow sand filter installations. From experience gained in building slow sand filters, some general guidelines for filter design have been established as follows: Population to be served. Slow sand filtration is more cost effective than rapid sand filtration for populations up to 30,000 and possibly considerably higher. Raw water quality. Jul 01, 2012 Slow-sand water filter: Design, implementation, accessibility and sustainability in developing countries Peter A. Clark, 1 Catalina Arango Pinedo, 2 Matthew Fadus, 1 and Stephen Capuzzi 1 1 Institute of Catholic Bioethics, Saint Joseph’s University, Philadelphia, PA, U.S.A.
Gravity Sand Filter Design
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.: 302–303The 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.: 302–303Guidance on the design of rapid sand bed filters suggests that they should be operated with a maximum flow rate of 9 m 3/m 2/hr (220 US gal/ft 2/hr). 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.: 302–303This 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. 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.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 m 3/m 2/hr but not run for long (a few minutes).: 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 As the title indicates, the speed of filtration is changed in the, 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. 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, 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. National Drinking Water Clearinghouse. (PDF). EAWAG. Anlauf, Harald (2003).
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'Deep Bed Filtration'. 11 of Solid-Liquid Separation, 3rd ed., L. Svarovsky (ed). Butterworths. Huisman, L.; Wood, W.
Slow sand filtration. Geneva: World Health Organization.
Slow sand filters are used in for treating to produce a 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, where they are used to treat water supplied to. Slow sand filters now are also being tested for pathogen control of nutrient solutions in hydroponic systems. Original map by showing the of cases in the.The first documented use of to purify the water supply dates to 1804, when the owner of a bleachery in, John Gibb, installed an experimental filter, 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 for the in London in 1829. This installation provided filtered water for every resident of the area, and the network design was widely copied throughout the 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 during the. Snow was sceptical of the then-dominant that stated that diseases were caused by noxious 'bad airs'. Although the 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, with the use of a 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 introduced the regulation of the companies in, 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. The was formed at the same time, water filtration was adopted throughout the country, and new water intakes on the were established above.Water treatment came to the United States in 1872 when 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. Method of operation. 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 picture.Slow sand filters work through the formation of a gelatinous layer (or ) called the layer or in the top few millimetres of the fine sand layer. The Schmutzdecke is formed in the first 10–20 days of operation and consists of, 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, and 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. 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.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. 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. Features. Works on the principles of slow sand filtersSlow have a number of unique qualities:. 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. 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. For municipal systems there usually is a certain degree of, since it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service.
Slow sand filters require relatively low levels to operate efficiently. In summer conditions with high microbial activity and in conditions when the is turbid, blinding of the filters due to occurs more quickly and pre-treatment is recommended. 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.: 38–41While many municipal 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 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.
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 for poor and isolated areas. Slow sand filters, due to their simple design, may be created. DIY-slow sand filters have been used in Afghanistan and other countries to aid the poor. Slow sand filters are recognized by the, and the 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. 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 they subsequently installed, due to increased demand for drinking water.See also.Notes.
(PDF), World Health Organization. Buchan, James. Crowded with genius: the Scottish enlightenment: Edinburgh's moment of the mind. New York: Harper Collins.
Christman, Keith. The history of chlorine. Waterworld, 14 (8), 66-67. An Act to make better Provision respecting the Supply of Water to the Metropolis, (15 & 16 Vict. C.84). Johnson, George (March 1914). 'PRESENT DAY WATER FILTRATION PRACTICE'.
American Water Works Association. 1 (1): 31. Poughkeepsies' Water Treatment Facility.
Retrieved 18 May 2017. ^ Centre for Affordable Water and Sanitation Technology, Biosand Filter Manual: Design, Construction, & Installation,' July 2007. ^ National Drinking Water Clearinghouse (U.S.), Morgantown, WV. Tech Brief Fourteen, June 2000.
^ United States Environmental Protection Agency (EPA)(1990). Cincinnati, OH.
EPA/625/4-89/023. HDR Engineering (2001). New York: John Wiley and Sons. Retrieved 28 March 2010. Archived from the original on 6 April 2016. CS1 maint: BOT: original-url status unknown.
(PDF). Archived from (PDF) on 16 March 2006.References.