Page 251 -

SLOW SAND AND DIATOMACEOUS EARTH FILTRATION 9.27

Principal advantages of the vacuum filter system are as follows:

• For small plants, the initial cost is about one-third the cost of a stainless steel pressure
filter of the same capacity.
• It is easy to observe the precoat and cake accumulation because the unit is open.
• Precoat, filter, and sluicing cycles may be operated manually because the unit is open.
• Operation and maintenance of the units generally requires fewer skills than does pres-
sure filter operation and maintenance.
The principal disadvantages are as follows:

• The relatively low filtration rate requires more or larger units than for pressure units.
• Filter units must be protected from the weather because wind, rain, or ice disturbs the
filter cake, and sunlight promotes algae growth.

Ancillary Facilities
DE is a fine powder that has abrasive properties when handled both in dry form and in
slurries. Special precautions must be taken to protect plant staff against dust, and equip-
ment and pipelines must be protected against erosion. Because of its abrasive properties,
DE breaks down in size under turbulent operating conditions.
DE Delivery. DE can be delivered in 50-1b (22.7-kg) plastic-lined paper bags, in 900-1b
(408-kg) woven plastic bags, or in bulk truck or rail car loads. The 50-1b (22.7-kg) bags
are usually stacked about 12 to a pallet and are handled with a forklift truck. Bags are
then individually lifted by plant staff and discharged through a bag breaker equipped with
a dust collector that empties into a dry feeder hopper or into a slurry tank.
The 900-1b (408-kg) bags are delivered, two to a pallet, in an "over-and-under" stack.
Special forklift trucks and unloading frames are used for handling and unloading the bags.
Bags are transferred from the pallet with a sling to a monorail and then moved into place
over a discharge hopper. A spout, tucked into a flap in the bottom of the bag, is unrolled
and attached to a discharge spout, and the bag is then unloaded with the assistance of a
vibrator mounted on the holding frame. Discharge may be to a pneumatic conveyor for
dry bulk storage or to a wetting chamber for pumping and storage as slurry. Because bags
are completely enclosed, dust collectors are not required for unloading facilities.
Bulk delivery by truck or railcar may be made directly to storage silos with a pneu-
matic system similar to equipment used for handling bulk carbon or lime.
Slurry Storage. For small operations, DE slurry is usually stored in fiberglass-reinforced
plastic tanks and, for large volumes, in reinforced concrete basins. All storage units must
be equipped with continuously operating mixers to keep DE in suspension. Depending on
slurry concentration, it may be difficult to resuspend DE once it has been allowed to settle.
To limit attrition of DE in the turbulence of mixing, slurries should be used within 3 days.

Slurry Conveyance. Steel pipelines convey slurry either by gravity or by pumping. The
flow must be continuous, and at the end of a conveying cycle, the pipeline must be com-
pletely flushed. The line should not be flushed to the point of delivery unless flushing wa-
ter volume is measured and considered in determining the concentration of DE in the
slurry at the point of use. Another method used is to deliver flushing water to a holding