water characteristics and samplingrapid sand filters ... ssf design design a slow sand filter to...

FILTRATIONWater Treatment Course

AAiT, Zerihun Alemayehu

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.



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




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 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



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.



EXAMPLE SOLUTION



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.



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.



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 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.



TYPICAL SLOW SAND FILTER

Sand filter

bed

Gravel

Schmutzecke

Supernatant

water

System of underdrains

Weir Raw water

Finished

water



TYPICAL SLOW SAND FILTER



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



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



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



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



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.



RAPID SAND FILTER



OPERATION OF A RSF

Terminal head loss.

Constant rate

filtration



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 - 58Not

available40 - 43 47 - 52

Not

available

Hardness Moh 6.5 -7.5 5.6 7 2 - 3 Low



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



FILTER HYDRAULICS



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



REYNOLDS NUMBER



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.



SOLUTION

Calculate head loss for anthracite

Calculate head loss for sand



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



SOLUTION



SOLUTION…



EXAMPLE 5

Determine the depth of the expanded sandfilter bed being designed for Example 4.



SOLUTION

water characteristics and samplingrapid sand filters ... ssf design design a slow sand filter to...

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