Generally, filter beds are made
from granular materials.
- Anthracite coal
- Other granular
materials such as garnet sand and granular activated carbon
particles – silts and clays
- Colloids – very
small, finely divided solids that remain dispersed in a liquid due to their
small size and electrical charge
– bacteria, plankton, algae, cysts, oocysts, and viruses
characteristics of the water
- Nature of
- Types and degree
- Filter type and
- Adsorption – The
collection of a gas, liquid or dissolved substance on the surface and interface
zone of another material.
- Absorption – The
taking in or soaking up of one substance into the body of another by molecular
or chemical action (e.g., tree roots absorb dissolved nutrients in the soil).
- Straining – The
removal of particulates by trapping in the open spaces between the grains of
TYPES OF FILTERS
Gravity Filters – Water moves
downward through filter media by gravity.
to 150 gpd/sqft (0.03 to 0.10 gpm/sqft).
- Majority of
material removed in top several inches of media.
- Cleaned by
removing and replacing top media layer.
application due to area requirements and manual cleaning.
- Filter media –
24” to 30” deep silica sand bed.
Rapid Rate Gravity Filtration
is dependent on the media used.
media = 2 gpm/sqft
or Multi media = 4 gpm/sqft
rates permitted if justified based on raw water quality, degree of pretreatment,
media type, and other factors.
Filter media can
be single, dual or multi.
media bed is 24” to 30” deep, and made up of:
silica sand, or
crushed anthracite coal.
media bed is 24” to 30” deep containing both sand and anthracite:
sand, 9” to 12” deep, and
anthracite 15” to 18” deep.
or mixed media bed is 24” to 30” deep containing silica sand, anthracite coal,
and a layer of dense, small grained garnet sand:
Sand - 3” layer,
Sand - 9” to 12” layer, and
- 15” to 18” layer.
Filter control systems regulate
flow rates by maintaining adequate head above the media surface. This prevents
sudden flow increases, or surges, which could dislodge trapped solids.
Each filter is
equipped with a rate-of-flow control valve.
- The valve
maintains a constant rate of water flow through the filter.
- As filter clogs
the valve slowly opens to maintain the flow rate.
controller maintains a constant level of water above the media.
- Filtration rate
declines as filter clogs.
- A loss of head
gauge on a filter us used to measure the drop in pressure through a filter bed.
Filter Media Materials
The filter media
consists of a bed of silica sand and anthracite coal. Garnet sand is sometimes included
in a Mixed Media bed.
- The anthracite
coal can be replaced with Granular Activated Carbon (GAC) for organic, taste,
and odor control applications.
particle size and lower density of anthracite provides for longer filter run
and higher filtration rates.
- The filter bed
depth depends on treatment application.
probe checks should be performed to determine depth to the gravel.
- Filter media
aids such as copper sulfate and polymers.
used as a backwash aid are injected into the backwash waste in very low doses
to reduce reopening of the filters.
Silica gravel is
commonly used to support the media bed.
- An alternative
media support uses an IMS® Cap.
is a porous material which replaces the common silica gravel bed system.
used where overall filter depth is a concern, as a ½ inch thick Cap replaces 12
or more inches of silica gravel.
characteristics (permeable bed)
- Inert and easy
- Hard and durable
- Free of
- Insoluble in
- Comply with AWWA
B-100 Standard for Filtering Media
of sieve opening which permits 10 percent of media particles (by weight) to
operational problems are improved by proper selection of media effective size:
required for turbidity breakthrough.
required for filter to reach limiting or terminal headloss.
Referred to as
the measure of uniformity of filter media.
is a ratio based on sieve size.
is equal to the size of the sieve opening that will just pass 60 percent (by
weight) of a representative sample of the media, divided by the size of the
sieve opening that will just pass 10 percent (by weight) of the same sample.
UC = D60 ÷ D10
with lower UC is more uniform in size than media with higher UC. Generally, the
lower the UC, the slower the headloss buildup
Ratio of the
unit weight of the media to that of water
- Resistance to
erosion or wearing away of the particle
Filter Underdrains collect
filtered water during normal operation and distribute backwash water during backwash
System (Header/Lateral system)