CIVL 1101 Introduction to Filtration 1/13 Water Treatment Water Treatment Basis water treatment Coagulation consists of four processes: This process helps removes Coagulation/Flocculation g particles suspended in water. Sedimentation Chemicals are added to water to form tiny sticky Filtration particles called "floc" which Disinfection attract the particles. Water Treatment Water Treatment Flocculation Sedimentation Flocculation refers to The heavy particles (floc) water treatment processes settle to the bottom and the that combine or coagulate clear water moves to small particles into larger filtration. particles, which settle out of the water as sediment. Water Treatment Water Treatment Filtration Disinfection The water passes through A small amount of chlorine is filters, some made of layers added or some other of sand, gravel, and charcoal disinfection method is used that help remove even to kill any bacteria or smaller particles. microorganisms that may be in the water.
CIVL 1101 Introduction to Filtration 2/13 Water Treatment Water Treatment 1. Coagulation 2. Flocculation 3. Sedimentation 4. Filtration 5. Disinfection 6. Fluoridation 7. Stabilization 8. Collect and test water samples Water Treatment Water Treatment 1. Coagulation - Aluminum or iron salts plus chemicals called polymers are mixed with the water to make 5. Disinfection - Chlorine is added to reduce risks from the particles in the water stick together. remaining bacteria and other disease-causing 2. Flocculation - The coagulated particles are slowly organisms and to maintain water quality through the mixed so th t th can collide and form l i d that they llid df larger distribution pipe system system. particles, known as "floc." 6. Fluoridation - Fluoride is added to provide dental 3. Sedimentation - Water flows through a large tank benefits. which allows the "floc" to settle to the bottom of 7. Stabilization - Small amounts of lime (calcium the tank and be removed. hydroxide) or sodium hydroxide are added to make 4. Filtration - Water is passed through filters made of the water less corrosive to pipes and plumbing. sand and anthracite coal to filter out remaining 8. Collect and test water samples particles. Water Filtration Water Filtration Filters may be classified according to the Filtration is used to separate nonsettleable solids types of media used as follows: from water and wastewater by passing it through a porous medium Single–media filters: These have one type of yp The most common system is filtration through a media, usually sand or crushed anthracite coal. layered bed of granular media, usually a coarse anthracite coal underlain by a finer sand. Dual–media filters: These have two types of media, usually crushed anthracite coal and sand. Multi–media filters: These have three types of media, usually crushed anthracite coal, sand, and garnet.
CIVL 1101 Introduction to Filtration 3/13 Water Filtration Water Filtration In water treatment all three types are used; however, Filtration was actually developed prior to the discovery the dual– and multimedia filters are becoming of the germ theory by Louis Pasteur in France. increasingly popular. Particle removal is accomplished only when the Louis Pasteur (1822 – 1895) was a particles make physical contact with the surface of French chemist and microbiologist. the filter medium. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. Water Filtration Water Filtration In the 1700s the first water filters for domestic In 1854 it was discovered that a cholera epidemic application were applied. These were made of wool, spread through water. sponge and charcoal. The outbreak seemed less severe in areas where sand In 1804 the first actual municipal water treatment filters were installed. plant designed by Robert Thom, was built in Paisley, Scotland. British scientist John Snow found that the direct cause of the outbreak was water pump contamination The water treatment was based on slow sand by sewage water. filtration, and horse and cart distributed the water. He applied chlorine to purify the water, and this paved Some three years later, the first water pipes were the way for water disinfection. installed. Water Filtration Water Filtration John Snow (1813 – 1858) was an English physician and a leader in the adoption of anaesthesia and medical hyg n . hygiene. He is considered to be one of the fathers of epidemiology, because of his work in tracing the source of a cholera outbreak in Soho, England, in 1854.
CIVL 1101 Introduction to Filtration 4/13 Water Filtration Water Filtration Floc Particles Larger particles may be removed by straining Interception Straining Particles may also be removed by Flocculation sedimentation Others may be intercepted by and adhere to the surface of the medium due to inertia Filtration efficiency is greatly increased by destabilization or coagulation of the particles prior to filtration Sedimentation Filter Media Water Filtration Water Filtration Gravity Granular–Media Filtration Gravity Granular–Media Filtration Gravity filtration through beds of granular Because of the reduction in pore area, the media is the most common method removing velocity of water through the remaining voids colloidal impurities in water processing ll id l i i i i i increases, shearing off pieces of capture fl i h i ff i f floc and carrying impurities deeper into the filter Initially, surface straining and interstitial bed removal results in accumulation of deposits in the upper portion of the filter media The effective zone of removal passes deeper and deeper into the filter Water Filtration Water Filtration Gravity Granular–Media Filtration Turbidity Eventually, clean bed depth is no longer Turbidity is a measurement of the clarity of available and breakthrough occurs, carrying water run solids out in the underflow and causing lid i h d fl d i termination of the filter run Clouded water is caused by suspended particles scattering or absorbing the light Turbidity is an indirect measurement of the amount of suspended matter in the water
CIVL 1101 Introduction to Filtration 5/13 Water Filtration Water Filtration Turbidity Slow Sand Filtration However, since solids of different sizes, shapes, The early filtration units developed in Great and surfaces reflect light differently, turbidity Britain used a process in which the hydraulic and suspended solids d not correlate well. d d d lid do l ll loading l di rate i relatively l is l i l low. Turbidity is normally gauged with an instrument Typical slow sand filtration velocities are only that measures the amount of light scattered at about 0.4m/hr. an angle of 90° from a source beam. At these low rates, the filtered contaminants do The units of turbidity are usually in not penetrate to an appreciable depth within the Nephelometric Turbidity Units (NTU). filtration medium. Water Filtration Water Filtration Slow Sand Filtration Slow Sand Filtration The filter builds up a layer of filtered contaminants on the surface, which becomes the active fil i medium i filtering di Slow sand filters are cleaned by taking them off line and draining them. The organic or contaminant layer is then scraped off. The filter can then be restarted. After water quality reaches an acceptable level, the filter can then be put back on line. Water Filtration Water Filtration Rapid Sand Filtration Rapid Sand Filtration In rapid sand filtration much higher application velocities In the United States, filter application rates are often are used expressed as volumetric flowrate per area, or Filtration occurs through the depth of the filter gal/m n ft wh ch s gal/min–ft2, which is actually a velocity with atypical veloc ty w th atyp cal units. A comparison of rapid and slow sand filtration is shown in the table below Filtration Type Application Rate Filtration Type Application Rate m/hr gal/ft2–day m/hr gal/ft2–day Slow Sand 0.04 to 0.4 340 to 3400 Slow Sand 0.04 to 0.4 340 to 3400 Rapid Sand 0.4 to 3.1 3400 to 26,000 Rapid Sand 0.4 to 3.1 3400 to 26,000
CIVL 1101 Introduction to Filtration 6/13 Water Filtration Water Filtration Hydraulic Loading Rate Hydraulic Loading Rate Let’s compute the hydraulic loading rate on our A hydraulic loading rate of 3.954 gpm/ft2 could be filters in lab: classified as: Flowrate: 1,000 ml/min 1. A high-end direct filtration (1–6 gpm/ft2) Area of filter: 3.5 inch diameter filter 2. A mid-range rapid filter (range of 2–10 Flowrate gpm/ft2 with 5 gpm/ft2 normally the maximum Loading Rate design rate) Area 1, 000 ml min 1 gallon 144in 2 gpm 3.954 2 (3.5in ) 4 3,785 ml ft 2 ft 2 Water Filtration Water Filtration Hydraulic Loading Rate Hydraulic Loading Rate To convert the hydraulic loading rate to the U.S. A hydraulic loading rate of 5,694 gpd/ft2 could be standard of gpd/ft2, convert minutes to days qualifies as a rapid sand filter Flowrate Fl t Loading Rate Area Filtration Type Application Rate gpm 3.954 60 min 24 hr m/hr gal/ft2–day ft 2 hr day Slow Sand 0.04 to 0.4 340 to 3400 gpd Rapid Sand 0.4 to 3.1 3400 to 26,000 5,694 ft 2 Water Filtration Water Filtration Hydraulic Loading Rate Hydraulic Loading Rate Let’s compute the hydraulic loading rate for Let’s compute the hydraulic loading rate for flowrates in class: flowrates in class: Flowrate: 1,250 and 1,500 ml/min Flowrate: 1,250 and 1,500 ml/min Area of filter: 3.5 inch diameter filter Area of filter: 3.5 inch diameter filter Flowrate Loading Rate Flowrate of 1,250 ml/min 4.943 gpm/ft2 Area Flowrate ml min 1 gallon 144in 2 Flowrate of 1,500 ml/min 5.931 gpm/ft2 3,785 ml ft 2 2 (3.5in ) 4
CIVL 1101 Introduction to Filtration 7/13 Water Filtration Water Filtration Hydraulic Loading Rate Rapid Sand Filtration Let’s compute the hydraulic loading rate for The water above the filter provides the hydraulic flowrates in class: pressure (head) for the process. Flowrate: 1,250 and 1,500 ml/min The filter medium is above a larger gravel, rock, or other h fl d b l l k h media for support. Area of filter: 3.5 inch diameter filter Below the rock is usually an underdrain support of some type. Flowrate of 1,250 ml/min 7,117 gpd/ft2 The water flows through the filter and support media, Flowrate of 1,500 ml/min 8,541 gpd/ft2 exiting from a pipe below. Water Filtration Water Filtration Rapid Sand Filtration Rapid Sand Filtration Water Filtration Water Filtration Rapid Sand Filtration Rapid Sand Filtration Most modern filters employ two separate filter media in As the filter begins to clog from accumulated solids, less layers: water will pass through it. At some point cleaning is The lower layer is composed of a dense fine media, often sand dense, media requ red. required. The upper layer is composed of a less dense, coarse media, often anthracite coal Usual filter operation before cleaning is from a few hours to 2 days. The coarse upper layer removes larger particles before Cleaning is accomplished by reversing the flow of water they reach the fine layer, allowing the filter to operate to the filter, or backwashing. for a longer period before clogging.
CIVL 1101 Introduction to Filtration 8/13 Water Filtration Water Filtration Rapid Sand Filtration Rapid Sand Filtration Water supply Backflush water out The backwash velocity is sufficient to fluidize the bed – that is, to suspend the bed with the reverse flow. Backflush Backflush After backwashing, the filter is again placed in supply supply operation Fluidized filter Filter media media Filtered water Underdrain support Underdrain support Operation during filtration Operation during cleaning Water Filtration Water Filtration http://www.fbleopold.com/flash/media.swf Water Filtration Water Filtration Backwash Velocity Backwash Velocity The backwash velocity may be estimated using the Once the backwash velocity has been estimated, the depth following equation of the expanded filter bed may be computed L(1 ) v v s e4.5 Le 0.22 where v is the backwash velocity (ft/s) 1 vv s vs is the settling velocity of the filter media (ft/s) where L is depth of the filter media (ft) e is the porosity of the expanded filter Le is depth of the expanded filter media (ft) is the porosity of the filter media
CIVL 1101 Introduction to Filtration 9/13 Water Filtration Water Filtration Backwash Velocity Example Backwash Velocity Example Determine the required backwash velocity to expand the The backwash velocity may be estimated using the sand filters in lab to a porosity of 0.70. following equation Also, determine the depth of the expanded filter bed. v v s e4.5 Assume the following data about our lab filters: 1. Depth of sand bed 0.5 ft 0.27 ft s 0.70 4.5 2. Sand with a particle diameter of 0.5 mm or 0.02 inches with a settling 0.054 ft s velocity of 0.27 ft/s 3. Sand porosity is 0.35 Water Filtration Water Filtration Backwash Velocity Example Backwash Velocity Example Determine the hydraulic loading rate of the backwash Once the backwash velocity has been estimated, the depth of the expanded filter bed may be computed V l it 0 054ft s 0 054ft 3 Velocity 0.054 0.054 L( ) (1 ft 2s Le 0.22 0.054ft 3 7.48 gallons 86, 400s 1 vv ft 2s ft 3 day s 0.5ft (1 0.35) 34, 900 gpd The backwash loading rate 0.22 1.26 ft ft 2 is about 7 times larger than 0.054 ft 1 s the filter loading rate 0.27 ft s Water Filtration Water Filtration Backwash Velocity Group Problem Traditional Filtration Determine the required backwash velocity to expand the A typical scheme for water filtration consists of sand filters in lab to a porosity of 0.75. flocculation with a chemical coagulant and sedimentation prior to filtration filtration. Also, determine the depth of the expanded filter bed. Alum or other Assume the following data about our lab filters: coagulant Polymer coagulant 1. Depth of sand bed 0.5 ft Influent Effluent Flocculation Sedimentation Filtration 2. Sand with a particle diameter of 0.5 mm or 0.02 inches with a t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2 settling velocity of 0.27 ft/s Rapid mixing 3. Sand porosity is 0.30 t = 30 minutes
CIVL 1101 Introduction to Filtration 10/13 Water Filtration Water Filtration Traditional Filtration Traditional Filtration Under the force of gravity water passes downward When the media become filled or solids break through, through the media that collect the floc and particles. a filter bed is cleaned by backwashing. Alum or other Alum or other coagulant Polymer coagulant coagulant Polymer coagulant Influent Effluent Influent Effluent Flocculation Sedimentation Filtration Flocculation Sedimentation Filtration t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2 t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2 Rapid mixing Rapid mixing t = 30 minutes t = 30 minutes Water Filtration Water Filtration Traditional Filtration Direct Filtration Filtration rates following flocculation and sedimentation The process of direct filtration does not include are in the range of 2–10 gpm/ft2 with 5 gpm/ft2 sedimentation prior to filtration. normally the maximum desi n rate design rate. Alum or other Alum or other coagulant Polymer coagulant coagulant Polymer coagulant Influent Effluent Influent Effluent Optional mixing Filtration Flocculation Sedimentation Filtration T > 30 minutes R = 1 – 10 gpm/ft2 t = 15-30 minutes t = 1-4 hours t = 1-10 gpm/ft2 Rapid mixing Rapid mixing t = 30 minutes t = 30 minutes Water Filtration Water Filtration Direct Filtration Direct Filtration The impurities removed from the water are collected Contact flocculation of the chemically coagulated and stored in the filter. particles in the water takes place in the granular media. Alum or other Alum or other coagulant Polymer coagulant coagulant Polymer coagulant Influent Effluent Influent Effluent Optional mixing Filtration Optional mixing Filtration T > 30 minutes R = 1 – 10 gpm/ft2 T > 30 minutes R = 1 – 10 gpm/ft2 Rapid mixing Rapid mixing t = 30 minutes t = 30 minutes
CIVL 1101 Introduction to Filtration 11/13 Water Filtration Water Filtration Direct Filtration Description of a Typical Gravity Filter System Successful advances in direct filtration are attributed to: During filtration, the water enters above the filter media through an inlet flume. Development of coarse–to–fine multimedia filters Improved backwashing systems, and After passing downward through the granular media and Availability of better polymer coagulants the supporting gravel bed, it is collected in the underdrain system Filtration rates in direct filtration are usually 1–6 gpm/ft2 Water Filtration Water Filtration Operating Table Filter Bed Description of a Typical Gravity Filter System Concrete Floor Wall Floor During backwashing, wash water passing upward through the filter carries out the impurities that accumulated in Hydraulic Lines for Values the media Drain Influent Line Waste The flow is directed upward, hydraulically expanding the Effluent Line Wash Line to Clearwell filter media The water is collected in the wash–water troughs that Wash Trough Concrete Wall discharge to the outlet flume Filter Sand Graded Gravel Perforated Laterals Manifold Water Filtration Water Filtration Description of a Typical Gravity Filter System The filters are placed on both sides of a pipe gallery that contains inlet and outlet piping, wash–water inlet lines, and wash–water drains. A clear well for storage of filtered water is located under a portion of the filter bed area
CIVL 1101 Introduction to Filtration 12/13 Water Filtration Water Filtration Filter Media – Ideal Filter Bed Depth Increasing Grain Size Pore Size Water Filtration Water Filtration Filter Media – Single Medium Filter Filter Media – Dual-Medium Filter Increasing Grain Size Bed Depth Bed Depth Increasing Grain Size Increasing Grain Size Pore Size Pore Size Water Filtration Water Filtration Filter Media Filter Media Broadly speaking, filter media should possess the These attributes are not compatible. For example: following qualities: 1. Fine sand retains floc and tends to shorten the filter run 1. Coarse enough to retain large quantities of floc, 2. Sufficiently fine particles to prevent passage of 2. For a course sand the opposite would be true suspended solids, 3. Deep enough to allow relatively long filter runs, and 4. Graded to permit backwash cleaning.
CIVL 1101 Introduction to Filtration 13/13 Water Filtration Water Filtration Filter Media Filter Media A filter medium is defined by effective size and Conventional sand medium has an effective size of 0.45– uniformity coefficient. 0.55 mm, a uniformity coefficient less than 1.65 Effective size is the 10–percentile diameter; that is, 10% by weight of the filter material is less than this diameter, A sand filter bed with a relatively uniform grain size can D10 provide effective filtration throughout its depth Uniformity coefficient is the ratio of the 60–percentile size to the 10–percentile size (D60 /D10) Water Filtration Water Filtration Multimedia Filters Multimedia Filters Dual–media filter beds usually employ anthracite and sand The main advantages of multimedia filters compared to single–medium filters are: However, However other materials have been used, such as used activated carbon and sand 1. Longer filtration runs, Multimedia filter beds generally use anthracite, sand, and 2. Higher filtration rates, and garnet. 3. The ability to filter a water with higher turbidity However, other materials have been used, such as activated carbon, sand, and garnet. Water Filtration Water Filtration Multimedia Filters The advantages of the multimedia filters are due to: Any Questions? 1. The 1 Th media particle size, di ti l i 2. The different specific gravities of the media, and 3. The media gradation.