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3.7 FILTRATION - PowerPoint PPT Presentation

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3.7 FILTRATION. Regulatory requirements for filtration and disinfection . USEPA’s ( 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

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Regulatory requirements for filtration and disinfection

  • USEPA’s (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

Filtration Processes

Log removal*

Turbidity requirement






= or < 0.5 ntu in 95% of samples each month and never >5 ntu




= or < 0.5 ntu in 95% of samples each month and never >5 ntu

Slow sand



= or < 1 ntu in 95% of samples each month and never >5 ntu**

Diatomaceous earth



= or < 1 ntu in 95% of samples each month and never >5 ntu

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


  • 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)

For instance: gravity slow sand filters means a filter open

to atmospherefilter loaded with sand

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.

  • 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

Backwash water outlet

Process water inlet


Gravel support media or


Process water outlet

Backwash water inlet

Process description

Types of Particle Capture

  • Brownian Diffusion (labelled A)

  • Sedimentation or Inertia (labelled B)

  • Interception (labelled C)

  • Hydrodynamic (labelled D)

  • Escaped Particle (labelled as E)

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

Grain Size Characteristics

Effective size

E = P10 = 0.3

Uniformity coefficient

U = P60/P10=0.85/0.3=2.8

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


Headloss in expanded media

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


Requirements for Backwashing

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


Ga=d3 ((s- )g/2

Remf= vmfd/ 

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