Filtration

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Description of Filtration

•It is the process in which water passes through a filter bed made up of fine sand over a layer of supporting gravel.

•The major purpose of filtration is to remove suspended material from water.

• This suspended material can include floc from the coagulation / flocculation or coagulation, flocculation, and sedimentation processes; microorganisms.

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This is important in the slow sand filter where an accumulation of a dirty skin (Schmutzdeck layer) on the top of the filter is important to obtain a satisfactory effluent.

3- Biological metabolism

2-Sedimentation & adsorptionAs water passes through the filter bed, the suspended particles contact and adsorb (stick) onto the surface of the individual filter grains or onto previously deposited material.

Mechanisms of Filtration Process

1- Mechanical straining

Types of FiltersFilters can be classified by the filtration rate, type of filter media, or type of operation

I. Gravity Filters

II. Pressure Filters

A- Slow - Sand B- Rabid - Sand C- High – Rate: 1- Dual- Media 2- Multi - Media

A- Sand or Multi –MediaB- Diatomaceous Earth

Slow – sand Filters

•Removes pathogens

•Removes many pollutants

•Low maintenance

•Slow flow rates 0.05 gpm/sq ft

MechanismMechanism• Very little mechanical removal• “schmutzdeck” where most

treatment occurs:– A community of

microorganisms– Bed surface to 0.15 M below

• Organisms that have been identified:– Algae, bacteria, diatoms and

zooplankton• Mechanisms for removal are not

fully understood.

Rapid – sand filters

• Filter sand is specifically manufactured for filtration as are all media used for this purpose.

• A rapid-sand filter usually uses a 24-to 30-in. (0.6 – to 0.75 M) bed of fairly uniform sand grains, 0.4 to 0.6 mm in diameter. This sand is much coarser than that used for slow-sand filters.

• This accounts for many of the advantages of rapid-sand filters. One major advantage is that the coarser sand has larger void spaces, which do not fill and seal off quickly.

• Three to five layers of graded gravel are placed between the sand and under drain system. The total gravel bed is usually 6 to 18 in. (0.15 to 0.45 M) thick, depending on the under drain system.

• This gravel bed prevents sand from entering the under drains and helps distribute the backwash water evenly.

The Under DrainThe Under Drain

The under drains serve two functions:

evenly distribute the backwash water so that

the sand and gravel beds will not be disturbed.

uniformly collect the filtered water. The water

then flows into the piping system for further

treatment and then to the clear well.

Wheeler bottom

Types of under drain systems commonly usedTypes of under drain systems commonly used

Leopold bottom

Porous plates or strainer nozzles

Pipe-lateral

Comparison of slow-sand, Rapid-Sand

High-rate filters High-rate filters high-rate filters can operate high-rate filters can operate

at rates three to four times at rates three to four times those of rapid-sand filters. those of rapid-sand filters.

These filters use a These filters use a combination of filter media, combination of filter media, not just sand; all other not just sand; all other components are the same components are the same as for rapid-sand filters. as for rapid-sand filters.

Dual-media filters use sand Dual-media filters use sand and anthracite coal. and anthracite coal.

Multi-media (mixed-media) Multi-media (mixed-media) filters use three or more filters use three or more media and - usually sand, media and - usually sand, anthracite coal, and garnet.anthracite coal, and garnet.

Dual- and mixed- media filters have a Dual- and mixed- media filters have a

coarse-to-fine gradation of filter media.coarse-to-fine gradation of filter media. The layers of media approximately The layers of media approximately

maintain their respective positions in the maintain their respective positions in the

filter bed even after backwashing.filter bed even after backwashing. Some mixing makes the filter more Some mixing makes the filter more

effective by providing more contact area effective by providing more contact area

for the suspended particles. for the suspended particles. The coarse layer on top removes the The coarse layer on top removes the

larger suspended particles. The finer larger suspended particles. The finer

particles pass through this layer and are particles pass through this layer and are

removed by finer media below. So, most removed by finer media below. So, most

of the filter bed is used to remove of the filter bed is used to remove

suspended particles. suspended particles. This allows for longer filter runs and This allows for longer filter runs and

higher filtration rates because head loss higher filtration rates because head loss

does not build up as quickly as with a does not build up as quickly as with a

rapid-sand filter. rapid-sand filter.

Operation of gravity filters Operation of

gravity filters

Filtering

During filtering, water is applied to the filter to maintain a constant depth of 1 to 1.5 m over the media. Initially, the media is clean and head loss is very low. The filtration rate is kept at the desired level by using a control.

A control is important in preventing harmful surges, which can disturb the media and force floc through the filter.

As filtration progress, suspended matter builds up within the filter bed.

At some time, usually after 15 to 36 hours of operation, the filter must be cleaned by backwashing.

Backwashing

For a filter to operate efficiently, it must be cleaned thoroughly

before the next filter run begins. In addition, properly backwashed

filters require far less maintenance. Treated water is always used for

backwashing so that the bed will not be contaminated.

During filtration, the voids between the media grains fill with the filtered material (floc)..

o The grains also become coated with the floc and become very sticky

o To clean the filter bed, the media grains must be agitated violently and rubbed against each other to dislodge the sticky coating.

o The backwash rate must be high enough to completely suspend the filter media in the water.

o The backwash causes the filter bed to expand. o The media, auxiliary scour (surface wash)

equipment is recommended to provide the extra agitation needed. It is a must be with high-rate filters, since the filtered material penetrates much deeper into the bed.

A filter needs backwashing as:

• Head loss,

• filter-effluent turbidity,

• and length of the filter run

Head loss is so high that the filter no longer produces water at the

desired rate. This is know as TERMINAL HEAD LOSS and is usually

about 2.4 m.

Floc starts to break through the filter bed causing the filter effluent

turbidity to increase.

A filter run reaches 36 hours.

The decision to backwash should not be based on only one of these three factors. if a filter is not washed until terminal head loss is reached, a serious increase in filtered-water turbidity could occur well before the filter is backwashed. Likewise, depending on effluent turbidity alone can cause head loss to reach terminal conditions well before BREAKTHROUGH has occurred. This can cause the pressure in the filter bed to drop below atmospheric pressure, which causes operational problems. The turbidity should never be allowed to increase to 1 NTU before backwashing This, in turn, can lead to tastes and odors in the treated water as well as difficult-to-remove slime growth within the filter.

Usually, the amount of backwash water used should not Usually, the amount of backwash water used should not

exceed 4% of the amount of water treated for rapid- exceed 4% of the amount of water treated for rapid-

sand filters, and 6 % for dual- and multi- media. Filter sand filters, and 6 % for dual- and multi- media. Filter

runs should range between 15 and 24 hours and should runs should range between 15 and 24 hours and should

never exceed 36 hours.never exceed 36 hours. A typical backwash sequence begins by draining the A typical backwash sequence begins by draining the

water in a water in a

filter down to a level about 150 mm above the media.filter down to a level about 150 mm above the media. The surface washers are turned on and allowed to The surface washers are turned on and allowed to

operate for 1 to 2 min. This allows the high-velocity operate for 1 to 2 min. This allows the high-velocity

water jets to break up any surface layers of filtered water jets to break up any surface layers of filtered

material. material. The backwash valve is opened part-way to allow the bed The backwash valve is opened part-way to allow the bed

to expand to just above the level of the washers. This to expand to just above the level of the washers. This

provides violent scrubbing of the top portion of the provides violent scrubbing of the top portion of the

media. media. Intense scrubbing is particularly important with rapid-Intense scrubbing is particularly important with rapid-

sand filters because the top 200 mm of media does sand filters because the top 200 mm of media does

most of the suspended-solids removal. After a few most of the suspended-solids removal. After a few

minutes, the backwash valve is fully opened to allow a minutes, the backwash valve is fully opened to allow a

filter bed expansion of 20 to 30 percent. filter bed expansion of 20 to 30 percent.

With multi- media filters, the bed must be With multi- media filters, the bed must be

expanded so that the surface washers can scrub expanded so that the surface washers can scrub

the area (interface) between the coal and sand the area (interface) between the coal and sand

layers where most of the filtered material has layers where most of the filtered material has

penetrated. penetrated. The clarity of the wash water as it passes into the The clarity of the wash water as it passes into the

wash-water troughs can be used as an indicator of wash-water troughs can be used as an indicator of

when to stop washing. when to stop washing. The surface washers are usually turned off about 1 The surface washers are usually turned off about 1

min before the backwash flow is stopped. This min before the backwash flow is stopped. This

allows the bed to restratify into layers, which is allows the bed to restratify into layers, which is

particularly important for multi-media filters. particularly important for multi-media filters. If surface wash equipment is not available, a two-If surface wash equipment is not available, a two-

stage wash stage wash

should be used. The initial wash velocity should be should be used. The initial wash velocity should be

just enough to slightly expand the top portion of just enough to slightly expand the top portion of

the bed. Although not as effective as surface the bed. Although not as effective as surface

washing, this method will provide the scrubbing washing, this method will provide the scrubbing

action needed to clean the surface media grains. action needed to clean the surface media grains.

After the surface has been cleaned, the full After the surface has been cleaned, the full

backwash rate is applied. backwash rate is applied.

Turning on the backwash too quickly can severely Turning on the backwash too quickly can severely

damage the under drain system, gravel bed; and damage the under drain system, gravel bed; and

media. The time from starting the backwash flow to media. The time from starting the backwash flow to

reaching desired. Backwash flow rate should be from reaching desired. Backwash flow rate should be from

30 to 45 sec. To prevent accidents, the backwash 30 to 45 sec. To prevent accidents, the backwash

valve controls should be set to open slowly.valve controls should be set to open slowly.

Filter Backwash ProcedureFilter Backwash ProcedureStep 1 – Close Influent ValveStep 2 – Close Effluent ValveStep 3 – Surface Washers are turned on.Step 4 – Backwash Valve is opened partiallyStep 5 – Backwash Valve opened to expand bed 20 to 50%Step 6 – Backwash period = to 5 to 15 minutesStep 7 – Surface wash turned off 1 minute before end of BWStep 8 – Wash Water Valve closedStep 9 – Filter Bed is rested for 30 to 60 minutesStep 10 – Filter Effluent Valve is openedStep 11 – Filter to Waste is openedStep 12 – Filter to Waste ends when Eff. Turbidity is OKStep 13 – Filter to Waste Valve is closedStep 14 – Filter Effluent Valve is opened

Disposal of backwash water

• To avoid water pollution, backwash water

must not be returned directly to streams or

lakes.

• This water is usually routed to a lagoon or

basin for settling.

• After settling, the water is usually recycled to

the treatment plant influent.

• The settled solids are combined with the

sludge from the sedimentation basins and

treated.

Filtering to waste

Once backwashing is completed, the water

applied to the filter should be filtered to waste

until the turbidity of the effluent drops to

acceptable level. This period may last from 2 to

20 min.

In addition, the initial high filtration rates

continue to wash fine material through the filter

until the media grains become sticky and more

effectively adsorb the suspended material.

Operating problems with gravity filters

Operating problems with gravity filters

Mudball Formation Mudball Formation :

inadequate backwash and surface wash

Filter Bed Shrinkage Filter Bed Shrinkage :

Bed compresses and shrinks

Gravel Displacement Gravel Displacement

Backwash valve is opened too quickly

Excessive Media Loss Excessive Media Loss

Caused by excessive backwashing rate

Air Binding Air Binding

negative head operation and low water levels.

Thank you