water characteristics and samplingrapid sand filters ... ssf design design a slow sand filter to...
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AAiT Water Treatment
By Zerihun Alemayehu
FILTRATION
Filtration involves the removal of suspended and colloidalparticles from the water by passing it through a layer orbed of a porous granular material, such as sand.
AAiT Water Treatment
By Zerihun Alemayehu
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.
AAiT Water Treatment
By Zerihun Alemayehu
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
AAiT Water Treatment
By Zerihun Alemayehu
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 Water Treatment
By Zerihun Alemayehu
RATE OF FILTRATION
Rate of filtration (loading rate) is the flow rate of waterapplied per unit area of the filter. It is the velocity of thewater 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
aA
Qv
AAiT Water Treatment
By Zerihun Alemayehu
EXAMPLE
A city is to install rapid sand filters downstream of theclarifiers. The design loading rate is selected to be 160m3/(m2 d). The design capacity of the water works is 0.35m3/s. The maximum surface per filter is limited to 50 m2.Design the number and size of filters and calculate thenormal filtration rate.
AAiT Water Treatment
By Zerihun Alemayehu
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.
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 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
By Zerihun Alemayehu
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 scrapedoff using mechanical plant and this exposes a new layerof clean sand. Water is then decanted back into thefilter and re-circulated for a few hours to allow a newSchmutzedecke to develop. The filter is then filled to fulldepth and brought back into service.
wet harrowing: lower the water level to just above theSchmutzdecke, stirring the sand and therebysuspending any solids held in that layer and thenrunning the water to waste. The filter is then filled tofull depth and brought back into service.
AAiT Water Treatment
By Zerihun Alemayehu
TYPICAL SLOW SAND FILTER
Sand filter
bed
Gravel
Schmutzecke
Supernatant
water
System of underdrains
Weir Raw water
Finished
water
AAiT Water Treatment
By Zerihun Alemayehu
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
DESIGN CRITERIA FOR SSFParameter 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
AAiT Water Treatment
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
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
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.
AAiT 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.
AAiT Water Treatment
By Zerihun Alemayehu
OPERATION OF A RSF
Terminal head loss.
Constant rate
filtration
AAiT Water Treatment
By Zerihun Alemayehu
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
AAiT Water Treatment
By Zerihun Alemayehu
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 - 58Not
available40 - 43 47 - 52
Not
available
Hardness Moh 6.5 -7.5 5.6 7 2 - 3 Low
AAiT Water Treatment
By Zerihun Alemayehu
FILTER HYDRAULICS
The loss of pressure (head loss) through a clean stratified-sandfilter with uniform porosity was described by Rose:
where hL = frictional head loss through the filter, mva = approach velocity, m/sD = depth of filter sand, mCD = drag force coefficientf = mass fraction of sand particles of diameter dd = diameter of sand grains, mϕ = shape factor and = porosity
AAiT Water Treatment
By Zerihun Alemayehu
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
AAiT Water Treatment
By Zerihun Alemayehu
EXAMPLE 3
A dual medium filter is composed of 0.3 manthracite (mean size of 2.0 mm) that is placedover a 0.6 m layer of sand (mean size of 0.7 mm)with filtration rate of 9.78 m/h. Assume the grainsphericity is = 0.75 and a porosity for both is 0.40.Estimate the head loss of the filter at 15oC.
AAiT Water Treatment
By Zerihun Alemayehu
SOLUTION
Calculate head loss for anthracite
Calculate head loss for sand
AAiT Water Treatment
By Zerihun Alemayehu
EXAMPLE 4Estimate the clean filterheadloss for a proposed newsand filter using the sand. Usethe following assumptions:loading rate is 216 m3/d.m2 ,specific gravity of sand is 2.65,the shape factor is 0.82, thebed porosity is 0.45, the watertemperature is 10oC, and thedepth 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
AAiT Water Treatment
By Zerihun Alemayehu
EXAMPLE 5
Determine the depth of the expanded sandfilter bed being designed for Example 4.