Objective 02: Filters

Question 1

Filter

Answer 1

A bed of porous material through which water is passed, either by gravity or under pressure, in order to clarify it of suspended materials.

Question 2

What is the purpose of filter with regard to Water Pretreatment?

Answer 2

The porous material will trap suspended matter from the water, clarifying the water.

Question 3

What is the usual filter medium?

Answer 3

1. Sand

2. Anthracite

Question 4

Why is sand avoided as a filtering medium when the filtered water will become boiler feedwater?

Answer 4

The water may absorb silica which could deposit as scale in the boiler.

Question 5

What happens to a filter when it has been in service for a period of time?

Answer 5

The filter bed becomes “plugged” with the suspended matter from the water.

Question 6

How does one fix a plugged filter bed?

Answer 6

Backwashing to remove the matter.

Question 7

Backwashing

Answer 7

The direction of flow though the filter is reversed, thus freeing the trapped material from the bed and washing it to waste.

Question 8

Describe the components of a Simple Gravity Filter using sand as filter media?

Answer 8

1. The filter has a rectangular, reinforced concrete shell
2. The shell is made of wood or steel and may be circular in shape
3. The bed of sand is usually 40-80 cm deep
4. The gravel support bed is 30-60 cm deep and prevents loss of sand

Question 9

How does a Simple Gravity Filter work?

Answer 9

The water enters at the top and flows downward through the filter bed to the outlet at the bottom.

Question 10

How does a typical Gravity Filter with automatic backwash work?

Answer 10

1. The inlet water normally passes downward through the filter bed and up through the outlet pipe
2. As impurities accumulate in the filter bed, the water has a difficult time getting through the bed and the level in the backwash pipe rises until it eventually flows over the loop at the top
3. The backwash pipe begins to siphon the water from the space above the bed, causing the backwash storage water to flow from the storage space through a pipe to the underside of the bed. It then passes upward through the bed to waste, thus backwashing the bed.
4. When the storage water level drops below the end of the siphon breaker pipe, the backwashing will stop and normal flow through the filter bed will resume, first filling the backwash storage space and then passing through the outlet to service
5. The filling of the backwash storage space provides a rinsing period before the filter returns to service.

Question 11

Describe the components of a typical Pressure Filter

Answer 11

1. Steel shell
2. A bed of anthracite
3. Strainer Plate Support Bed
4. Pressure Gauges at the inlet and bottom outlet

Question 12

How does a typical Pressure Filter work?

Answer 12

Impure water is pumped under pressure from the top, through the filter bed and then through stainless steel strainers located below the bed and into the bottom outlet.

Question 13

How does one know when backwashing is required in a Pressure Filter?

Answer 13

When the pressure difference between the inlet and the outlet reaches a certain value, usually from 35 to 55 kPa.

Question 14

What specially occurs in Pressure Filters during backwashing?

Answer 14

A rotary surface washer may be used to loosen material from the surface of the bed. The water leaves the nozzles of the rotary washer at high velocity, which causes the washer arm to revolve.

Question 15

How do filters frequently fit into the grand scheme of pretreatment?

Answer 15

1. Impurities are coagulated and settled in a settling tank

2. Water passed through filters for final clarication

Question 16

How can Filters be made more effective in the absence of Settling Tanks?

Answer 16

Feeding a coagulant to the filter