معالجة مياه الشرب(الفلاتر الرملية لترشيح المياه)

Water Filtration

Rapid Sand Filtration

In rapid sand filtration much higher application velocitie s are used.

Filter Filtration occurs through the depth of the filter.

filter application rates are often expressed as volumetric flow rate per area=gal/min-
ft2
A comparison of rapid and slow sand filtration is

Filtration Type Application Rate

m/hr gal/ft2 –day

Slow Sand 0.04 to 0.4 340 to 3400

Rapid Sand 0.4 to 3.1 3400 to 26,000

Hydraulic Loading Rate

Let’s compute the hydraulic loading rate on our

filters in lab:

Flowrate: 1,000 ml/min

Area of filter: 3.5 inch diameter filter

Loading rate= flow rate/area=3.954gpm/ft2

A hydraulic loading rate of 3.954 gpm/ft could be

classified as
:
A high-end direct filtration (1–6 gpm/ft
)
A mid-range rapid filter (range of 2–10 gpm/ft)

with 5 gpm/ft normally the maximum design rate

To convert the hydraulic loading rate to the U.S

standard of gpd/ft , convert minutes to days:-

flow rate=3.954x60min/hrx24hr/day=5.694gpd/ft2

The water above the filter provides the hydraulic
pressure (head) for the process:-.

The filter medium is above larger gravel , rock , or other media for support.

Below the rock is usually an under drain support of some type.

The water flows through the filter and support media exiting from a pipe below

Most modern filters employ two separate filter media in layers

The lower layer is composed is composed of, fine dense media, often sands

The upper layer is composed of a less dense, coarse media ,often anthracite coal

The coarse upper layer removes larger particles before they reach the fine layer, allowing the filter to operate for a longer period before clogging

As the filter begins to clog from accumulated solids, less water will pass through it.

At some point cleaning is required.

Usual filter operation before cleaning is from a few hours to 2 days

Cleaning is accomplished by reversing the flow of water to the filter, or backwashing

The backwash velocity is sufficient to fluidize the that is, to suspend the bed with the
reverse flow

After backwashing, the filter is again placed in
Operation

Description of a Typical Gravity Filter System
During filtration, the water enters above the filter
media through an inlet flume
.
After passing downward through the granular media and collected in the supporting gravel bed, it is under drain system

During backwashing, wash water passing upward through the filter carries out the impurities that
accumulated in the media.

The flow is directed upward, hydraulically expanding the filter media.

The water is collected in the wash–water troughs that discharge to the outlet flume.

The filters are placed on both sides of a pipe gallery that contains inlet and outlet piping and wash–water drains and wash–water inlet lines.

A clear well for storage of filtered water is located under a portion of the filter bed area.

Filter Media
filter media should possess the following qualities:

Coarse enough to retain large quantities of floc
Sufficiently fine particles to prevent passage of
suspended solids

Deep enough to allow relatively long filter runs

Graded to permit backwash cleaning
A filter medium is defined by effective size and
uniformity coefficient

Effective size is the 10–percentile diameter; that is, 10% by weight of the filter
material is less than this diameter, D10

Uniformity coefficient is the ratio of the 60– percentile size to the 10–percentile size (D60 /D10)
Conventional sand medium has an effective size of 0.45-0.55 mm, a uniformity coefficient less than 1.65
A sand filter bed with a relatively uniform grain size can provide effective filtration throughout its depth
Multimedia Filters
Dual–media filter beds
usually employ anthracite and sand However, other materials have been used other materials have been used, such as activated carbon and sand

Multimedia filter beds generally use anthracite, sand, and garnet
However, other materials have been used, such activated carbon, sand, and garnet as
The main advantages of multimedia filters are compared to single–medium filters
.
Longer filtration runs

Higher filtration rates

.
The ability to filter a water with higher turbidity
The advantages of the multimedia filters are due to
. The media particle size
. The different specific gravities of the media
. The media gradation
Traditional Filtration
A typical scheme for water filtration consists of:-
flocculation with a chemical coagulant and sedimentation prior to filtration .
Under the force of gravity water passes downward through the media that collect the floc and particles.
When the media become filled or solids break through a filter bed is cleaned by backwashing.
Filtration rates following flocculation and sedimentation are in the range of 2–10 gpm/ft with 5 gpm/ft
normally the maximum design rate
Direct Filtration
The process of direct filtration does not include
sedimentation prior to filtration
The impurities removed from the water are collected and stored in the filter .
Contact flocculation of the chemically coagulated
particles in the water takes place in the granular media.
Successful advances in direct filtration are
attributed to:
Development of coarse–to–fine multimedia filters Improved backwashing systems, and
Availability of better polymer coagulants
Filtration rates in direct filtration are usually
1-6gpm/ft

Reffered by
Colonel.dr
BAHAA BADR EL-DIN

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