Civil Engineering Reference
In-Depth Information
In cases where the diatomite plant will be found to produce water at a lower total
cost than any practical alternative
•
In cases where the lower first cost of a diatomite filter installation may be the
major factor in the final choice of the plant
•
For emergency or standby service at locations experiencing large seasonal vari-
ations in water demand, when the lower first cost of the diatomite filter may prove
to be economical
•
Some of the important operating parameters of diatomite filters have been summar-
ized:
11
A precoat of 2.45 to 4.89 lb / sq ft (12 to 25 kg / m
2
) is applied to prepare the
filter.
•
A continuous feed of filter aid as body feed is necessary to prevent the cake from
clogging with the particles being filtered out.
•
Acceptable cleaning of the filter will maintain at least 95 percent of the septum
area available for flow after 100 filter run cycles.
•
Because of the precoat, DE filters do not require filter-to-waste upon start-up.
•
If the flow to the filter is disrupted, the filter cake drops off the septum. When
the filter is restarted, clean diatomite and filtered water should be used to recoat
the filter to reduce the potential for passage of pathogens.
•
It may be necessary to adjust the body feed rate in proportion to the raw-water
turbidity to prevent a short run.
•
Although filter runs can be 2 to 4 days long, decomposition of organic matter
trapped in the filter cake may necessitate shorter runs to avoid taste and odor
problems.
•
Vacuum DE filters offer the advantages of not requiring pressure vessels and being
visible during backwash; however, they have the disadvantage of an increased
potential for release of gases, which can cause shorter runs.
•
DE filters can provide very effective removal of cysts, algae, and asbestos. In
some cases, alum coating of the DE improves performance.
•
The rate of body feed and size of diatomite used are critical variables affecting
the length of the run.
•
Upflow Filters
Upflow filtration has an obvious theoretical advantage, because coarse-to-fine filtration
can be achieved with a single medium, such as sand, with almost perfect gradation of
both pore space and grain size from coarse to fine in the direction of filtration (upward).
Since the bed is backwashed in the same direction but at higher flow rates, the desired
relative positions of fine media are maintained or reestablished with each backwash.
The inherent advantage of upflow filtration has long been recognized, and, under lab-
oratory conditions, the anticipated high filtration efficiency has been verified by several
workers.
The difficulty with upflow filtration comes when the headloss above a given level
exceeds the weight of the bed above that level, at which time the bed lifts or partially
fluidizes, allowing previously removed solids to escape in the effluent. In Russia, bed
