3.7 FILTRATION

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3.7 FILTRATION
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Regulatory requirements for filtration and
disinfection

• USEPAs (www.epa.gov) Surface Water Treatment
  Rule
• (SWTR), promulgated on June 29, 1989 (Federal
  register
• CFR Parts 140 and 141, 0. 27486-27568) requires
  community
• water systems to disinfect all surface water
  and requires
• filtration for most surface water sources.
• The supplementary information published with the
  rule
• presented recommended minimum levels of
  disinfection to be
• credited to the following defined filtration
  processes
• conventional filtration, direct filtration,
  diatomaceous earth
• filtration, and slow sand filtration

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SWRT assumed Log removals and Turbidity
Requirements
From table IV-2 in supplementary information
p.27511 Special provision was made for slow
sand filters to exceed 1 ntu in some cases
providing effective disinfection was maintained
Conventional and direct filtration include
coagulation, flocculation, and in case
conventional filtration, sedimentation
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• CLASSIFICATION OF FILTERS
• A number of different types of filters are used
  in potable water treatment and they may be
  described by various classification schemes
• Based on the type of medium used
• granular bed filters - sand, coal (anthracite),
  dual
• media (sand coal), or mixed (coal,
  sand, and garnet)
• precoat filters - use very thin layer of very
  fine medium
• such as diatomaceous earth (DE)
• Based on loading rate slow sand filters, rapid
  sand filters, or
• high rate sand filters
• Based on hydraulic arrangements gravity filters
  are open to
• atmosphere pressure filters are pressure
  driven
• depth filtration if the solids are removed
  within the granular
• material (rapid granular bed filters) and cake
  filtration if the
• solids are removed on the entering face of
  granular material
• (precoat filters)

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For instance gravity slow sand filters means a
filter open to atmosphere
filter loaded with sand
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The vertical pressure filter system is applied
for all filtration applications with flow rates
of 2 MGD or less. Single or dual media beds are
available with gravel support bed or media
retaining underdrains.
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• What the EPA says about slow sand filters"When
  used with source water of the appropriate
  quality, slow sand filtration may be the most
  suitable filtration technology for small
  systems."  source Small Systems Compliance
  Technology List For The Surface Water Treatment
  Rule. USEPA, August 1997, Office of Water EPA
  815-R-97-002, page 27

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Process description
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Types of Particle Capture
Brownian Diffusion (labelled A) Sedimentation or
Inertia (labelled B) Interception (labelled C)
Hydrodynamic (labelled D) Escaped Particle
(labelled as E)
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Grain Size Characteristics

• The grain size analysis begin by placing the
  sieve screens in ascending order with the largest
  opening on top and the smallest opening on the
  bottom. Then cumulative frequency distribution is
  plotted
• E - effective size
• U - uniformity coefficient
• Xg- geometric mean
• Sg - standard deviation

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• Effective size
• E P10 0.3
• Uniformity coefficient
• U P60/P100.85/0.32.8

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Backwashing

• is a routine procedure which is usually performed
  on a weekly basis depending on what type of
  filter is on your system.
• When headloss through the filter reaches a set
  value the filter is backwashed to remove the
  accumulated solids
• Filtered water is used for backwashing, which
  consumes from 1 to 5 of the product

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Headloss in expanded media
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Requirements for Backwashing

• Head loss in expanded media
• hle(?s- ?)/ ?(1-ee)Le
• where ?s is density of medium particles
• ? is density of water
• ee is the porosity of the expanded medium
• Le id the depth of expanded media
• Le/L(1-e)/(1-ee)

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• Backwash velocity
• the velocity to expand particles will vary as a
  function of particle size
• The suitable size to calculate the backwash
  velocity is d90
• Remf(33.7)2(0.0408)Ga2-33.7
• Gad3 (?(?s- ?)g/?2
• Remf ?vmfd/ ?

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