Lecture10 filtration
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Transcript of Lecture10 filtration
FILTRATION
INTRODUCTION
• Settled water turbidity is in the range 1 to 10 NTU with a typical value being 2NTU.
• EPA requires that the water turbidity 0.3 NTU.• Filtration process is required.• The most common filtration process is granular filtration
where the suspended or colloidal impurities are separated from water by passage through a porous medium.
• The medium is usually a bed of sand or other media such as coal, activated carbon, etc…
• Most common type of filter is: Rapid Sand Filter• Water moves through sand (layer of activated carbon oranthracite coal)
• Top layer removes organic compounds which contributeto taste and odor.
• Sand pores act as a screen for large suspended particles
FILTRATION
• Grain effective size is the sieve opening in mm that allow 10% of sand weight to pass or that retains 90% of sand weight.
• Degree of conformity indicates the degree of sand grains variation. It is the ratio of sieve opening that 60% of sand weight pass and the effective size.
• Ex: If 60% of sand weight pass through sieve diameter 0.7mm and the effective size is 0.35mm; then the degree of conformity =0.7/0.35=2
FILTRATION
• Rate of Filtration (ROF) = 120‐180 m3/m2/day• Area of one filter = 50 m2
• Sand layer thickness = 60‐70 cm• Gravel layer thickness = 15‐30cm • Water Depth = 120‐150cm• Number of filters = Total area / 50• Length to width ratio varies between 1 and 1.4• Add 1 or 2 standby filters for washing and operation conditions.• Preparation period = 5‐15min• Operation period = 12‐24 hrs• Washing water rate = 300‐500 lt/m2/min• Washing time = 5‐15min • Wash water tank for 2 filters• Total Nb. Of filters = Design + 1 or 2 Standby
FILTRATION – Design Criteria
• It is a vertical or horizontal steel cylinder can withstand pressure difference 2‐5 atmosphere.
• It includes filter material such as sand.• Same principle of rapid gravity filters but the filter media is
enclosed in a steel vessel and water is under pressure.• Mostly used for flow rates projects (5‐150m3/hr), in swimming
pools, drug and food industries, tertiary wastewater treatment plants.
• May be in form of Glass fiber or Poly Ethylene (PE)• Diameter= 50‐260cm; Height = 100‐750cm
Pressure Filters
• Activated Carbon Filters are used to remove organic & inorganic material.
Example:For Q=120000m3/d design the rapid sand filters and calculate the percentage of washing water.
Solution:Q=120000m3/d Rate of filtration=120-180 take 150m3/m2/d Area= 120000/150=800m2
No of filters= 800/50=16 add 2 filters for washing = 16+2=18 filterWashing Filter dimensions = 6.25*8m
Washing water= Nb of filters x Area of filter x Washing time x Washing water rate=16filters*50m2*10*0.5m3/m2/min = 4000m3
Percentage of washing water= 4000/120000=0.0333=3.33%