Science of Water : Concepts and Applications

Water Treatment Calculations 365
WATER FILTRATION CALCULATIONS
Water fi ltration is a physical process of separating suspended and colloidal particles from waste by
passing the water through a granular material. The process of fi ltration involves straining, settling,
and adsorption. As fl oc passes into the fi lter, the spaces between the fi lter grains become clogged,
reducing this opening and increasing removal. Some material is removed merely because it settles
on a media grain. One of the most important processes is adsorption of the fl oc onto the surface of
individual fi lter grains.
In addition to removing silt and sediment, fl ock, algae, insect larvae, and any other large elements,
fi ltration also contributes to the removal of bacteria and protozoans such as Giardia lamblia
and Cryptosporidium. Some fi ltration processes are also used for iron and manganese removal.
The Surface Water Treatment Rule (SWTR) specifi es four fi ltration technologies, although
SWTR also allows the use of alternate fi ltration technologies, e.g., cartridge fi lters. These include
slow sand fi ltration/rapid sand fi ltration, pressure fi ltration, diatomaceous earth fi ltration, and direct
fi ltration. Of these, all but rapid sand fi ltration are commonly employed in small water systems that
use fi ltration. Each type of fi ltration system has advantages and disadvantages. Regardless of the
type of fi lter, however, fi ltration involves the processes of straining (where particles are captured in
the small spaces between fi lter media grains), sedimentation (where the particles land on top of the
grains and stay there), and adsorption (where a chemical attraction occurs between the particles and
the surface of the media grains).
FLOW RATE THROUGH A FILTER, gpm
Flow rate in gpm through a fi lter can be determined by simply converting the gpd fl ow rate, as indicated
on the fl ow meter. The fl ow rate (gpm) can be calculated by taking the meter fl ow rate (gpd)
and dividing by 1440 min/day as shown in Equation 10.67.
Example 10.67
flow rate, gpd
Flow rate, gpm
1440 min day
Problem:
The fl ow rate through a fi lter is 4.25 MGD. What is this fl ow rate expressed as gpm?
Solution:
4.25 MGD
Flow rate, gpm
1440 minday 4,250,000 gpd
1440 minday 2951 gpm
Example 10.68
Problem:
During a 70-hour fi lter run, a total of 22.4 million gal of water are fi ltered. What is the average fl ow rate
through the fi lter in gpm during this fi lter run?
(10.67)