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FILTRATION. It is a process for separating suspended & colloidal impurities from water by passage through a porous medium. Filtration with or without pre-treatment effectively removes Turbidity (silt & clay) Color Micro-organisms

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## FILTRATION

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**FILTRATION**• It is a process for separating suspended & colloidal impurities from water by passage through a porous medium. • Filtration with or without pre-treatment effectively removes • Turbidity (silt & clay) • Color • Micro-organisms • Precipitated hardness from chemically softened water . • Precipitated iron & Mn from aerated waters.**Classification of filters**• I) According to direction of flow 1.up flow 2.down flow 3.radial flow 4.horizontal flow II) Types of filter media 1.granular media filters single media dual media multi media 2.fabric & mat filters**III) Driving force**(gravity filters/pr.filters) • IV) Mtd . Of flow rate control (constant rate /declining or variable rate filters) • V) Filtration rate (SSF / RSF) • SSF & RSF are down flow /granular-mediam gravity filters. • RSF operate @constant rate of filtration.**Example 14.1**Design a slow sand filter for a town with population of 20,000. Per capita water supply rate is 90 Lpcd. The expected maximum raw water turbidity is 25 NTU. Design criteria : No under drain should be provided within 600mm of the sidewall.**Solution:**Total daily water demand=20,000X90=1,80,0000 litres Add 10% extra for operational and other losses Total water demand including losses=1,980,000 litres or 1.90MLD Since the raw water turbidity is less than 30NTU the water can be directly fed to the filters. Assume average hours of operation per day as 16 hours**Rapid sand filter design**Problem :design the rapid sand filter to treat 10 million liters of raw water per day allowing 0.5% of filtered water for back washing. Half hour per day is used for back washing . Assume necessary data. Solution :**Filter hydraulics**• Actual filtration process by which water is cleaned. • Back washing operation by which filter is cleaned. • Flow through packed bed can be analyzed by classic filtration theory. • Carmen modified Darcy - Weisbach eqns for head loss in a pipe to reflect conditions in a bed of porous of uniforms size. • Carmen –Kozeny eqn .is**f' isthe friction factor related to coefft. of drag around**the particles. For laminar flow, Clean water @ 20°C is passed through a bed of uniform sand at a filtration velocity of 5m/h. The sand grains are 0.4mm dia. with a shape factor of 0.85 & a specific gravity of 2.65. the depth of bed is 0.67 & porosity is 0.4. Determine the head loss through the bed.**Design criteria under- drainage system consisting of central**manifold and laterals**Backwashing**Backwashing preceded by air-wash**Design of Rapid sand filter**step-1: Estimation of design flow (MLD/Lph):**Step-2: Determination of surface area of filter:**Step-3: Dimension of each filter unit:**Step-4: Depth of filter unit:**Step-5: Filter sand:**Design a SSF for a town with population of 20,000.Percapita**water supply rate is 90 lpcd. The expected raw water turbidity is 25 NTU. step-1: Estimation of design flow:**Step-2: Determination of SA of filter:**Step-3: Dimension of each unit**Step-4: Depth of filter unit:**Step-5: Filter sand:**SERVICE RESERVOIRFind**out capacity of SR for the following situations viz.,Power is available throughout 24 hrs.A)16h of pumping during 4am to 12noon & 1am to 9pm. B)8h of pumping during 4am to 8pm & 2pm to 6pm. Data given are:

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