On removing little particles with big particles Filtration Theory On removing little particles with big particles BY- Prakash Sundram

Topics to be Covered Filtration How Filters Work:  Filtration Mechanisms Filter Materials Types of Filter Principles of Slow Sand Filtration Slow Sand Filters vs. Rapid Sand Filters

Filtration The resultant water after sedimentation will not be pure, and may contain some very fine suspended particles and bacteria in it. To remove or to reduce the remaining impurities still further, the water is filtered through the beds of fine granular material, such as sand, etc. The process of passing the water through the beds of such granular materials is known as Filtration.

Particle Removal Improve taste, appearance Removes odour, turbidity Removes Pathogens: bacteria, viruses, protozoa

How Filters Work: Filtration Mechanisms SEDIMENTATION INTERCEPTION BROWNIAN DIFFUSION INERTIA

Sedimentation The mechanism of sedimentation is due to force of gravity and the associate settling velocity of the particle, which causes it to cross the streamlines and reach the collector.

Interception Interception of particles is common for large particles. If a large enough particle follows the streamline, that lies very close to the media surface it will hit the media grain and be captured.

Brownian Diffusion Diffusion towards media granules occurs for very small particles, such as viruses. Particles move randomly about within the fluid, due to thermal gradients. This mechanism is only important for particles with diameters < 1 micron

Inertia Attachment by inertia occurs when larger particles move fast enough to travel off their streamlines and bump into media grains.

Contact Points

Filter Materials Sand Effective size(D10) Uniformity coefficient(D60/D10) Gravel Other material

Filter Media- Fine Sand Filter Bed Criteria for top layer of filter media  Size of particles: ~0.15-0.35 mm Particles must be uniform Free of organic matter, loam, and clay Depth greater than 0.6 m (every 0.3 meters of extra sand will help the filter media to last an extra 3-4 years) Different sized sand particles

Filter Media – Gravel Layer Construction: Separate into 3 grades Top Finest 2-8 mm Middle Size 8-16 mm Bottom Coarsest Size 16-32 mm Purpose Supports the sand  Contributes to uniform filtration Coarse gravel

Other Materials Instead of using sand, sometimes, Anthrafilt is used as filter media. Anthrafilt is made from anthracite, which is a type of coal-stone that burns without smoke or flames. It is cheaper and has been able to give a high rate of filtration.

Types of Filter Slow sand filter Rapid-sand filter Multimedia filters

The Slow Sand Filter Large scale community water treatment system Water filters through a layer of sand with gravel base Gravity is the driving force Collective pipes No chemicals added Small pore space traps and removes particles Formation of biofilm above the sand contributes to the reduction of bacteria, viruses, protozoa and colloidal particles Slow sand filter

Advantages long design life Can use local materials and labor Inexpensive and easy to construct Minimal sludge handling problems Close operator supervision is not necessary Filters enough water for a community (up to 15,000 gal/day) Reduces bacteria, cloudiness, and organic levels Minimal power and chemical requirements

How does the filter remove contaminants? The schmutzdecke consumes and adsorbs/absorbs organic contaminants Layers of sand strain out particulate contaminants due to the small pores created by fine sand particles A layer of Schmutzde

Typical Slow Sand Filter Schematic

Filter Maintenance To address head loss or slow flow rate Scrape off the schmutzdecke and top layer of sand Clean sand or add new, clean sand to the top of filter The length of time between cleaning can range from several weeks to a year depending on the raw water quality Filtered water quality will be poor for a few days after cleaning (water should not be used until the biological layer builds up again)

Biological Layer: The Schmutzdecke Due to the organic and biological matter in the water to be filtered, a bio-layer grows on top of the sand layer The bio-layer contributes to water treatment by consuming organic contaminants including bacteria and viruses A four week period of saturation before drinking is suggested so the layer fully develops  The majority of biological activity occurs in the top 20 cm of the filter Biofilm Development Layer of Schmutzdecke

What will make the filter suitable for rural communities? Simple filtration technology Low construction and operation cost   Excellent removal of pathogenic organisms Good removal of turbidity Low energy consumption Minimal sludge handling  Does not need constant operator attention

Filtration-Influencing Factors Treatment conditions Influent microorganism density Biological condition of sand bed Established biota on ripened bed More effective removal Better turbidity control Water temperature Cold water inhibits biological processes Filtration rate Increased removal with lower rate Sand size Increased removal with smaller size Influent nutrient levels Promote biological growth

Rapid Sand Filters Typically closed, pressurized units Handle high flow rates: 20 gpm/ft2 Downside: very high head loss (30-90 ft) Only really good for low solids process streams with some sort of pre-trt Backwashing can be made automatic

Rapid Sand Filter (Conventional US Treatment) Specific Gravity 1.6 2.65 Depth (cm) 30 45 Size (mm) 0.70 0.45 - 0.55 5 - 60 Anthracite Influent Sand Gravel Drain Effluent Wash water

When is Backwashing Needed The filter should be backwashed when the following conditions have been met: The head loss is so high that the filter no longer produces water at the desired rate; and/or Floc starts to break through the filter and the turbidity in the filter effluent increases; and/or A filter run reaches a given hour of operation.

Backwashing Wash water is treated water! WHY? Anthracite Only clean water should ever be on bottom of filter! Sand Influent Gravel Drain Effluent Wash water

Multimedia filters They consist of two or more layers of different granular materials, with different densities. Usually, anthracite coal, sand, and gravel are used. The different layers combined may provide more versatile collection than a single sand layer. Because of the differences in densities, the layers stay neatly separated, even after backwashing.

Sand Filters vs. Rapid Sand Filters Base material: 3 to 65 mm in size and 30 to 75 cm in depth 3 to 40 mm in size and 60 to 90 cm in depth Filter sand: E.Z between 0.2 to 0.4 and U.C between 1.2 to 3.0 E.Z between 0.30-0.55 and U.C between 1.2 to 1.8 Rate of filtration: 100-200 l/h/sq.m 3000-6000 l/h/sq.m Flexibility: No flexibility according to demand Quite flexible according to demand Post treatment: Almost pure water obtained Disinfection required Method of cleaning: Scrapping and removal of dirty skin Sand is agitated and backwashed with or without compressed air Loss of head: Initial limit 10 cm and final limit is 0.8 to 1.2 m Initial loss is 0.3m and final limit is 2.5 to 3.5m

Sand Filters and Raw water Tank

New Rapid Gravity Filters

Sludge Storage

Adapted from the following: http://www.unh.edu/erg/wttac http://www.ewb-greatlakes.org/workshop/29Sept07_1430_RobinCollins_SlowSandFilters.pdf http://feeds.eng.fiu.edu/pdf/FALL08HANDOUTS/CEE/ENV5406/6/Filtration.pdf http://openlearn.open.ac.uk/mod/resource/view.php?id=185902 http://www.oasisdesign.net/water/treatment/slowsandfilter.htm http://www.ndwc.wvu.edu/pdf/dw/publications/ontap/2009_tb/slow_sand_filtration_DWFSOM40.pdf http://www.doh.wa.gov/ehp/dw/Publications/ http://www.esemag.com/0304/maine.html -->good case st http://pubs.nrc-cnrc.gc.ca/rp/rppdf/s02-025.pdf

Image Sources http://www.marlerblog.com/E_coli_lge.jpg http://www.speaking-up.com/blog/wp-content/hepatitisa1.jpg http://www.co.durham.nc.us/departments/ceng/images/Utility_Division/Wastewater_Treatment_Plant-001.jpg http://puritech.co.za/images/Ceramic%20Filter.gif http://www.drinking-water.org/assets/181x/00000295.jpg http://ttt.astro.su.se/~magnusg/large/Boiling_water.jpg http://www.dahlias.net/dahwebpg/Soil/ImgSoil/AlumSulf25.JPG http://www.acrest.org/admin//public/img/biosand-filter03.jpg www.drinking-water.org www.sanjosepartners.org/whatwedo/mattsblog http://www.kettering.edu Precast beds : from manufacturers sucs as Milton Precast www.miltonpipes.co.uk http://megphed.gov.in/gallery/hills/hills11.jpg www.slowsandfilter.org/illustrations.html www.ci.sandy.or.us http://www.aguadechuva.com/artigosuteis/topico5/desenvolvimentodoschmutzdecke_arquivos/image002.jpg http://www.scielo.br/img/revistas/esa/v10n4/a07fig02.gif http://www.shared-source-initiative.com/biosand_filter/images/sand.jpg www.water.siemens.com/.../aquacarb_1240_R.jpg http://www.surferswithoutborders.or Source: Vigneswaran, S. and C. Visvanathan. 1995 www.zeolitesupport.com/acids_and_bases_phscale.jpg prometheus.mse.uiuc.edu/glossary/biofilms/ biosandfilter.org http://waterdiagnostics.com/shop/images/turbidity.jpg www.unep.or.jp