TM 5-813-3/AFM 88-10, Vol 3
be undesirable to recycle wash water. For such a sit-
division structure upstream from flow division weirs is
uation, the wash water can be discharged to the recov-
arbitrarily established at the 100 footmark.
ery basin and then drained slowly to the plant waste
(4) Raw water transmission pipe is assumed to be
disposal system.
a flow division structure to first-stage rapid mix with a
44.5 foot, 16-inch pipe having a C value of 100.
m. Chemical application. Chemicals required for
plant operation and the purpose of each are shown in
(5) Plant units in service:
--Half of first-stage flow division structure.
table B-1. Table B-2 summarizes major features of
First-stage rapid mixer and first-stage floccu-
chemical application and related factors.
later-clarifier.
--Half of second-stage flow division structure.
Second-stage rapid mixer and second-stage floc-
culator-clarifier.
All (4) filters, with filter level control equip-
ment, etc., operating normally.
(6) Filters are to be washed when head loss ex-
ceeds approximately 8 feet.
k. Wash water. Water supply for filter backwash
can be supplied by a special pump, sized to provide the
required flow. If used, backwash pumps should be pro-
n. Chemical storage space. Chemical storage space
vided in duplicate. An alternative is an elevated wash
requirements must be analyzed in terms of required
water storage tank providing gravity flow. The capac-
application rates, shipping schedules and quantities.
ity of this tank should be at least 1.5 times maximum
In general, a 30-day supply of a given chemical, based
on estimated average feed rate, is the minimum stor-
wash water requirement for a single filter. For this ex-
age volume that should be provided. If chemicals are
ample, it is assumed that two filters will be washed in
purchased in bulk, the minimum storage volume avail-
succession, each for 15 minutes, at maximum rate. A
able should be 150 percent of the volume of one bulk
water tank having a capacity of three times the wash
shipment, or about 30 days of storage at average feet
requirement for a single filter is recommended. Its
rate, whichever is greater. For example, if the chem-
capacity will be (3)(15)(14)(14)(15) or 132,000 gallons.
ical purchase contract is for liquid alum, depending on
The wash water storage tank is refilled by pumping fil-
local conditions, the manufacturer may elect to ship as
tered water from the clearwell. Duplicate, automat-
follows: rail tank cars, 7,000 to 10,000 gallons; tank
ically-controlled, refill pumps should be provided. A
trucks, 3,600,4,000 or 5,000 gallons. For rail delivery,
single pump should be capable of refilling the wash
minimum storage should be 1.5 x 10,000 or 15,000
water tank in approximately 4 hours. A wash water
gallons. For tank-truck delivery, minimum storage
rate-of-flow controller should be provided on the main
should be 1.5 x 5,000 or 7,500 gallons. If the esti-
wash water line serving the filters. Rate of wash water
mated average alum feed rate is 30 mg/1 and the plant
flow and totalizing instrumentation with readout
is operated at design rate, 2.2 mgd, daily require-
visible during the washing process should also be pro-
ments, in terms of dry alum, are (30)(8.34)(2.2) or 550
vided.
l. Wash water recovery. Filter wash water can be re-
pounds per day. Liquid alum, as furnished by the
manufacturer, normally contains 5.4 pounds of dry
covered and recirculated through the plant. Solids con-
alum per gallon of solution. The daily alum solution re-
tained in the wash water are removed in the plant sedi-
mentation basins. Wash water recovery requires the
quirement will, therefore, be about 102 gallons. A stor-
age volume of 15,000 gallons provides about 150 days
construction of a basin into which the wash water is
discharged. The basin bottom should slope steeply
of storage at average feed rate and design flow rate; a
toward the suction pipe of the recycling pump. The
storage volume of 7,500 gallons, about 75 days. In this
example, standard shipping volumes determine stor-
capacity of the basin should be approximately equal to
age capacity. If the alum supply is to be purchased and
the value of two, maximum rate, 15-minute filter
stored in 100 pound bags, minimum bag storage space
washes, or about 90,000 gallons. For an assumed
equivalent to (30)(550) or 16,500 lbs. of alum should be
schedule of two filter washes every 12 hours, the re-
cycle pump should have a capacity of about 125 gpm so
provided. Loosely-packed, dry alum has a bulk density
that the recovery tank will be emptied in about 12
of about 50 pounds per cubic foot. The minimum bag
hours. The recycle pumps should be provided in dupli-
storage volume should, therefore, be about 330 cubic
feet arranged so that bags can be handled and stored
cate. The recovery tank should be equipped with an
overflow and a drain connection, both discharging to
on pallets. Suppliers should be consulted in advance of
the plant waste disposal system. Under unusual cir-
design regarding shipping quantities, schedules and
cumstances, associated with raw water quality, it may
costs. It maybe possible to reduce overall shipping and
B-5
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