Comprehensive Guide to Pressure Sand Filters: Effective Water Treatment Filtration Techniques

Filtration is a technique in which solids or impurities are separated from the liquid by passing the liquid through filter media. The filter media has pores. These pores will allow water to pass while retaining the solid particles.

The liquid that passes through filter media is called filtrate. The materials that are retained are called residues.

Uses of Filtration:

Filtration is used in the Chemical processing, food and beverages, air, oil, and Water Treatment plants.

We would like to specifically talk about filtration related to water treatment plants as we had already discussed Clarification of water. We will specifically talk about Pressure Sand Filters.

Pressure Sand Filter

A pressure sand filter is a type of filter used in the water treatment industry. It is used to remove suspended particles, impurities, and flocs that are carried over and not removed in sedimentation will be removed here. Water will be passed through these filters having three layers which are below:

• Anthracite
• Sand
• Gravel

• Anthracite, the largest particle, is the lightest (least dense) and settles on top. It provides large pore spaces that trap larger particles while allowing smaller particles to travel through to the layers below
• Sand is intermediate in size and density. It settles as the middle layer. The sand layer filters out particles of intermediate size while allowing smaller particles to flow through to the garnet layer
• Gravel, the larger particle than sand, is the heaviest (densest). It settles out as the filter vessel’s bottom layer to support media layers.

These layers will allow them only to pass water while retaining the impurities and suspended particles.

Operation

The service water is supplied from the clarified water transfer pump to the top of the sand filters and filtered water is obtained from the side of a sand filter and transferred to the raw water storage tank.

Backwashing of PSF

As more and more water passes through these bed layers impurities and suspended particles will get trapped on these layers and accumulation will happen on these layers. That will reduce the differential pressure.

Major Equipment for Backwashing

• PSF Backwash pumps
• Air blower for air scouring
• Backwash inlet, outlet, vent & drain valves

Conditions for backwashing of PSF

• After 1440 minutes or 24hr or 1 day
• When dp reaches 1 bar

PSF Backwashing Steps

• Drain down 10 min
• Air scouring 05 min (by Air bowers for scouring)
• Backwash 10 min (by PSF Back Washing pumps)
• Settling 60 min
• Rinse 04 minutes

1) Drain Down

Before the air scour system can be used on the media, the water level must be lowered to a few inches above the top of the media. The valve configuration listed below is used during the drain-down cycle of the sand filter system and is shown below. When the water reaches the required level in the pressure sand filter, the air scour cycle will start. This step will be done after Drain Down. The level is set by routing the rinse outlet pipe up to the level id the top of the media bed for turning down.

2)  Air Scouring

The air scouring cleans the media more thoroughly than a backwash cycle alone. The vessel in the pressure sand filter will be drained until the level in the tank is just above the media bed. when the required level is achieved then air scouring will be done. After the air scours, the filter must be backwashed with water and backwashed to remove the loosened impurities. After the air scour is complete, the vessel is refilled with water via the backwash inlet valve.

3)  Backwash

The backwash cycle is used to remove impurities that have been collected in the media bed. In Backwashing the system will operate opposite to normal operation. With sufficient flow, impurities are loosed from the media bed a carried out of the vessel through the inlet distributor and service inlet. The media bed must be expanded by 30% for the backwash to be effective. After the backwash cycle is complete, the vessel is rinsed and can then return to normal service.

4) Settling

For classifying the media in the Multi-Media Filter, It shall be settled after backwashing. None of the on-off valves are opened at this step.

(5) Rinse

Rinse The rinse cycle is used to remove any residual backwash water in the media bed. During Rining mode, the water will be sent to drain. After the rinse cycle, the vessel can be returned to normal service.

(6) BLOWDOWN

The blow-down cycle is used to remove any residual material in the media bed. The blow-down mode is the same as the rinse mode, except the time is short and the preferred process for the service.

Advantages of pressure sand filter

• It is effective in removing suspended particles and impurities
• PSF has a straightforward and robust design
• If proper maintenance is done it will have a longer lifespan

Disadvantages of pressure sand filter

• Cannot handle dissolved solids or pathogens like bacteria, fungi
• Require backwashing that results in loss of water
• Not suitable for very fine particles