Wt chapter-6-2-filter

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FILTRATION
Water Treatment Course
AAiT, ZerihunAlemayehu
AAiT Water Treatment
By Zerihun Alemayehu
FILTRATION
Filtration involves the removal of suspended and colloidal
particles from the water by passing it through a layer or
bed of a porous granular material, such as sand.

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CLASSIFICATION OF FILTERS
 Based on the filter media
 Sand filters, e.g. natural silica sand
 Anthracite filters, e.g. crushed anthracitic coal
 Diatomaceous earth filters, e.g. diatomaceous earth
 Metal fabric filters (microstrainers), e.g. stainless
steel fabric filter.
 Based on the depth of filter media
 Deep granular filters, e.g. sand, dual‐media and
multi‐media (combination of two or more media),
granular activated carbon
 Precoat filters, e.g. diatomaceous earth, and
powdered activated carbon, filters

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 Based on the rate of filtration, sand filters can be
further classified as
 Gravity filters
 Slow sand filters
 rapid sand filters
 high‐rate sand filters
 Pressure filters

RATE OF FILTRATION
Rate of filtration (loading rate) is the flow rate of water
applied per unit area of the filter. It is the velocity of the
water approaching the face of the filter:

where va = face velocity, m/d = loading rate, m3/d.m2
Q = flow rate onto filter surface, m3/d
As = surface are of filter, m2

s
a
A
Q
v 

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EXAMPLE
A city is to install rapid sand filters downstream of the
clarifiers. The design loading rate is selected to be 160
m3/(m2 d). The design capacity of the water works is 0.35
m3/s. The maximum surface per filter is limited to 50 m2.
Design the number and size of filters and calculate the
normal filtration rate.
EXAMPLE SOLUTION

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MECHANISM OF FILTRATION
 The theory of filtration basically involves, transport
mechanisms, and attachment mechanisms.
 The transport mechanism brings small particles from
the bulk solution to the surface of the media.
a) gravitational settling,
b) diffusion,
c) interception and
d) hydrodynamics.
MECHANISM OF FILTRATION
 They are affected by physical characteristics such as
size of the filter medium, filtration rate, fluid
temperature, size and density of suspended solids.
 As the particles reach the surface of the filter media, an
attachment mechanism is required to retain it. This
occurs due to
 (i) electrostatic interactions
 (ii) chemical bridging or specific adsorption.

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SLOW SAND FILTERS
 In SSF water is allowed at a slow rate through a bed of
sand, so that coarse suspended solids are retained on or
near the surface of the bed.
 Loading rate of 2.9 to 7.6 m3/d.m2
 The raw water turbidity has to be < 50 NTU.
 The filtering action is a combination of straining,
adsorption, and biological flocculation.

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SLOW SAND FILTERS
 Gelatinous slimes of bacterial growth called ‘schmutzdecke’
form on the surface and in the upper sand layer, consists of
bacteria, fungi, protozoa, rotifera and a range of aquatic
insect larvae.
 The underlying sand provides the support medium for this
biological treatment layer.
 Slow sand filters slowly lose their performance as the
Schmutzdecke grows and thereby reduces the rate of flow
through the filter. requires refurbishing
CLEANING SLOW SAND FILTERS
 Scrapping: the top few mm of sand is carefully scraped
off using mechanical plant and this exposes a new layer
of clean sand. Water is then decanted back into the
filter and re‐circulated for a few hours to allow a new
Schmutzedecke to develop. The filter is then filled to full
depth and brought back into service.
 wet harrowing: lower the water level to just above the
Schmutzdecke, stirring the sand and thereby
suspending any solids held in that layer and then
running the water to waste. The filter is then filled to
full depth and brought back into service.

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TYPICAL SLOW SAND FILTER
Sand filter
bed
Grave
l
Schmutzecke
Supernatant
water
System of underdrains
WeirRaw water
Finished
water
TYPICAL SLOW SAND FILTER

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TYPICAL SSF CONSTRUCTION DETAILS
ADVANTAGES AND DISADVANTAGES
 Advantages
 Simple to construct and supervise
 Suitable where sand is readily available
 Effective in bacterial removal
 Preferable for uniform quality of treated water
 Disadvantages
 Large area is required
 Unsuitable for treating highly turbid waters
 Less flexibility in operation due to seasonal variations in raw
water quality

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DESIGN CRITERIA FOR SSF
Parameter Recommended level (UK experience)
Design life
Period of operation
Filtration rate
Filter bed area
Height of filter bed
Initial
Minimum
Effective size
Uniformity coefficient
Height of underdrains + gravel layer
Height of supernatant water
10-15 year
24 h/day
0.1 – 0.2 m/h
5-200 m2/filter (minimum of two filters)
0.8-0.9 m
0.5-0.6 m
0.15-0.3 mm
< 3
0.3-0.5 m
1 m
EXAMPLE. SSF DESIGN
Design a slow sand filter to treat a flow of 800 m3/day.
 Solution:
 assuming a filtration rate of 0.15 m/h,
 Required tank area = (800/24) x (1/0.15) = 222 m2
 Use a tank 23 m long x 10 m wide.
 From Table 6.1, the height of the tank require is:
 System underdrain + gravel ≈ 0.5 m
 Filter bed ≈ 0.9 m
 Supernatant water ≈ 1 m
 Therefore, total tank height = 2.4 m and tank dimension
becomes 23 m long x 10 m wide x 2.4 m high

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RAPID SAND FILTERS
 The most common type of filter for treating municipal
water supplies.
 During filtration, the water flows downward through
the bed under the force of gravity.
 When the filter is washed, clean water is forced upward,
expanding the filter bed slightly and carrying away the
accumulated impurities. This process is called
backwashing.
ADVANTAGES AND DISADVANTAGES
 Advantages
 Turbid water may be treated
 Land required is less compared to slow sand filter
 Operation is continuous.
 Disadvantages
 Requires skilled personnel for operation and maintenance
 Less effective in bacteria removal
 Operational troubles

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TYPICAL GRADATION OF RSF
after backwashing, the larger
sand grains settle to the bottom
first, leaving the smaller sand
grains at the filter surface.
Allows in-depth filtration:
provides more storage space for
the solids, offer less resistance to
flow, and allows longer filter runs.
TYPES OF RSF
 RSF based on filter material, three types:
 Single‐media filters: these have one type of media,
usually sand or crushed anthracite coal
 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.

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RAPID SAND FILTER
OPERATION OF A RSF
Terminal head loss.
Constant rate
filtration

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GRAIN SIZE CHARACTERISTICS
 Sieve analysis  a plot on semi‐log paper of the
cumulative frequency distribution,
 Geometric mean (Xg) and
 Geometric standard deviation (Sg)
 Effective size, E, or 10 percentile, P10,
 E = P10 = (Xg/Sg)‐1.282
 Uniformity coefficient, U, or ratio of the 60 percentile to
the 10 percentile, P60/P10.
 U = P60/P10 = (Sg)1.535

RSF FILTER MEDIA TYPICAL PROPERTIES
PROPERTY UNIT GARNET LMENITE SAND ANTHRACITE GAC
Effective Size,
ES
mm 0.2 - 0.4 0.2 - 0.4 0.4 - 0.8 0.8 - 2.0 0.8 - 2.0
Uniformity
Coefficient, UC
UC 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 2.4
Density, ρρ g/mL 3.6 - 4.2 4.5 - 5.0 2.65 1.4 - 1.8 1.3 - 1.7
Porosity, ε % 45 - 58
Not
available
40 - 43 47 - 52
Not
available
Hardness Moh 6.5 -7.5 5.6 7 2 - 3 Low

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FILTER HYDRAULICS
The loss of pressure (head loss) through a clean stratified‐sand
filter with uniform porosity was described by Rose:
where hL = frictional head loss through the filter, m
           va = approach velocity, m/s
           D = depth of filter sand, m
          CD = drag force coefficient
             f = mass fraction of sand particles of diameter d
            d = diameter of sand grains, m
           ϕ = shape factor and = porosity
FILTER HYDRAULICS

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FILTER HYDRAULICS…
The hydraulic head loss that occurs during backwashing is
calculated to determine the placement of the backwash troughs
above the filter bed.

where De = depth of the expanded bed, m
 = porosity of the bed and s= porosity of the expanded bed
f = mass fraction of sand with expanded porosity
Laminar Turbulent
SETTLING
VELOCITY

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REYNOLDS NUMBER

EXAMPLE 3
A dual medium filter is composed of 0.3 m
anthracite (mean size of 2.0 mm) that is placed
over a 0.6 m layer of sand (mean size of 0.7 mm)
with filtration rate of 9.78 m/h. Assume the grain
sphericity is = 0.75 and a porosity for both is 0.40.
Estimate the head loss of the filter at 15oC.

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SOLUTION
 Calculate head loss for anthracite

 Calculate head loss for sand

EXAMPLE 4
Estimate the clean filter
headloss for a proposed new
sand filter using the sand. Use
the following assumptions:
loading rate is 216 m3/d.m2 ,
specific gravity of sand is 2.65,
the shape factor is 0.82, the
bed porosity is 0.45, the water
temperature is 10oC, and the
depth of sand is 0.5 m.
Sieve No % retain d(mm)
8-12 7.3 2
12-16 17.1 1.42
16-20 14.6 1
20-30 20.4 0.714
30-40 17.6 0.0505
40-50 11.9 0.0357
50-70 5.9 0.0252
70-100 3.1 0.0178
100-140 0.7 0.0126

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SOLUTION
SOLUTION…

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SOLUTION…
EXAMPLE 5
Determine the depth of the expanded sand
filter bed being designed for Example 4.

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SOLUTION

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Wt chapter-6-2-filter

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