Civil Engineering Reference
In-Depth Information
Gravity filters operate with relatively low hydraulic headlosses through the unit,
while the pressure filters are more practical where higher headlosses are not detrimen-
tal. Pressure filters allow pumping through the unit at relatively high pressures, and
are commonly used in small plants where they tend to be more economical. In larger
plants, gravity filters are more prevalent, as pressure systems are limited by available
vessel sizes. A significant concern with pressure filters is the inability to observe the
filter during routine operation, particularly during cleaning of the filter.
Rapid Rate Gravity Filtration
Rapid rate gravity filtration is the most widely used
technology for removing turbidity and microbial contaminants from pretreated surface
water and groundwater. Flow is normally downward, with the pretreated water passing
through a granular bed. Solids accumulate within the voids and on the top surface of
the filter bed. Plugging of the filter bed gradually occurs, resulting in either increased
headloss or a reduction in flow through the filter. After a period of operation, the filter
bed is cleaned using an upward flow of water referred to as
backwashing
. Treatment
prior to the filters, consisting of chemical precipitation and sedimentation, is often
used to reduce the quantity of filter influent particulates and ensure a more efficient
removal of particulates by the filter.
Slow-Sand Filtration
Slow-sand filters are similar to single-media, rapid-rate filters
in some respects, yet they differ in a number of important characteristics. In addition
to slower flow rates (by a factor of 50 to 100 versus rapid-rate filtration for example),
slow-sand filters also:
Function using biological mechanisms instead of physical-chemical mechanisms
•
Have smaller pores between sand particles
•
Do not require backwashing
•
Have longer run times between cleaning.
•
Require a ripening period at the beginning of each run
•
Although applicable to medium- to large-size plants, they are most commonly used
for small communities because of the operational simplicity and large space require-
ments. Because of the revived interest in this filtration method, it is in some instances
being classified as new technology. Slow-sand filters are effective for removal of
Giar-
dia
and
Cryptosporidium,
providing over 3-log removal.
1
Rapid Rate Pressure Filtration
This filter type is similar to the gravity rapid rate
filters, except that the flow enters and exits the filter under pressure. The filter bed is
normally installed in a cylindrical steel pressure vessel, with connecting pressurized
piping. The pressure vessel can be a vertical or horizontal cylinder. The filtration
process is essentially the same as the gravity system, except for the elimination of
concerns associated with negative pressures in the filter bed. Pressure filters are nor-
mally used in small water systems and industrial applications for iron and manganese
removal from groundwater. Their use for filtration of surface waters or lime-softened
water is not normally permitted.
Low-Head Continuous Backwash Filters
A variation of the traditional gravity
filter is the low-head continuous backwash filter. The filter usually consists of a shallow
bed of sand about 1 ft (0.3 m) deep, with an effective size of 0.4 to 0.5 mm. The bed
