1/3/2012 FILTRATION
Water Treatment Course
AAiT, Z erihun 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. AAiT
Water Treatment
By Zerihun Alemayehu
1 1/3/2012 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. 
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CLASSIFICATION
OF FILTERS
 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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2 1/3/2012 CLASSIFICATION

OF FILTERS
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 AAiT
RATE
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By Zerihun Alemayehu
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: Q
va 
As
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 AAiT
Water Treatment
By Zerihun Alemayehu
3 1/3/2012 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. AAiT
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EXAMPLE SOLUTION
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4 1/3/2012 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)
b)
c)
d)
gravitational settling, diffusion, interception and hydrodynamics. AAiT
Water Treatment
By Zerihun Alemayehu
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. AAiT
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5 1/3/2012 AAiT
Water Treatment
By Zerihun Alemayehu
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. AAiT
Water Treatment
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6 1/3/2012 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 AAiT
Water Treatment
By Zerihun Alemayehu
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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By Zerihun Alemayehu
7 1/3/2012 TYPICAL
SLOW SAND FILTER
Raw water
Supernatant
water
Weir
Schmutzecke
Sand filter
bed
Grave
l System of underdrains
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TYPICAL
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Finished
water
SLOW SAND FILTER
Water Treatment
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8 1/3/2012 TYPICAL SSF CONSTRUCTION DETAILS
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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 AAiT
Water Treatment
By Zerihun Alemayehu
9 1/3/2012 DESIGN
CRITERIA FOR
SSF
Parameter Recommended level (UK experience) 10-15 year Design life 24 h/day Period of operation 0.1 – 0.2 m/h Filtration rate 5-200 m2/filter (minimum of two filters) Filter bed area Height of filter bed Initial 0.8-0.9 m Minimum 0.5-0.6 m Effective size 0.15-0.3 mm Uniformity coefficient < 3 Height of underdrains + gravel layer 0.3-0.5 m Height of supernatant water 1 m AAiT
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By Zerihun Alemayehu
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 AAiT
Water Treatment
By Zerihun Alemayehu
10 1/3/2012 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. AAiT
Water Treatment
By Zerihun Alemayehu
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 AAiT
Water Treatment
By Zerihun Alemayehu
11 1/3/2012 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.
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Water Treatment
By Zerihun Alemayehu
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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Water Treatment
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12 1/3/2012 RAPID
AAiT
SAND FILTER
Water Treatment
By Zerihun Alemayehu
OPERATION
OF A RSF
Terminal head loss.
Constant rate
filtration
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Water Treatment
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13 1/3/2012 GRAIN


Geometric mean (Xg) and Geometric standard deviation (Sg) Effective size, E, or 10 percentile, P10, 

CHARACTERISTICS
Sieve analysis  a plot on semi‐log paper of the cumulative frequency distribution, 

SIZE
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 AAiT
Water Treatment
By Zerihun Alemayehu
RSF FILTER MEDIA TYPICAL PROPERTIES
PROPERTY UNIT Effective Size,
ES mm 0.2 - 0.4 Uniformity
Coefficient, UC UC Density, ρρ Porosity, ε Hardness AAiT
GARNET 1.3 - 1.7 g/mL 3.6 - 4.2 % 45 - 58 Moh 6.5 -7.5 LMENITE SAND ANTHRACITE GAC 0.2 - 0.4 0.4 - 0.8 0.8 - 2.0 0.8 - 2.0 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 2.4 4.5 - 5.0 2.65 1.4 - 1.8 1.3 - 1.7 Not
available 40 - 43 47 - 52 Not
available 5.6 7 2 - 3 Low Water Treatment
By Zerihun Alemayehu
14 1/3/2012 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
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FILTER HYDRAULICS
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15 1/3/2012 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
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Turbulent
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SETTLING
VELOCITY
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16 1/3/2012 REYNOLDS
NUMBER
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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. AAiT
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17 1/3/2012 SOLUTION
Calculate head loss for anthracite  Calculate head loss for sand 
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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. AAiT
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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18 1/3/2012 SOLUTION
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SOLUTION…
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19 1/3/2012 SOLUTION…
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By Zerihun Alemayehu
EXAMPLE 5
Determine the depth of the expanded sand filter bed being designed for Example 4. AAiT
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By Zerihun Alemayehu
20 1/3/2012 SOLUTION
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Any
Questions?
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21