Page 490 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Cake Filtration 445

ash, that is, Na 2 CO 3 in about 10 L water (1.0 lb (12) Vacaville. The Vacaville plant treats water from
alum, 1.0 lb DE and 1.0 lb soda ash in 1 gal Lake Berryessa, delivered by the Putah South
water). Effluent turbidities were reported to be Canal. The plant is operated in the summer and
lower for a given coarse grade of DE without a fall months when demands are higher and raw-
decrease in run time, which was reported not to water turbidities are low. The septum is tubular
decrease. stainless steel, 60 mesh. During operating seasons,
(8) Saratoga. Pre-coat-to-body-feed transition: pre-coat the plant operates 8–10 h to meet peak diurnal
recirculated 30 min, then valves open automatic- demands and terminal headloss is never reached.
ally for raw water, body feed, and effluent. When The plant is not operated when turbidity 15
influent turbidities are high plant has difficulty in NTU. Pre-coat is applied by circulating the DE
maintaining effluent turbidities <1NTU;operators from the suction side of the pump basin through
claim blending of DE grades (at 2:3:3) lowers the septum and back to the basin until all of the pre-
effluent turbidities. Body-feed suspension by add- coat is deposited (about 15 min). A mass of polymer
ing 11.36 kg perlite to 1250 L tank (25 lb in 330 (Nalco Ulitron 8157) is fed to the raw-water screen
gal) gives 9100 mg=L body-feed concentration. area over a 15 min period and body feed is started at
Leaf septum is 1626 mm (64 in.) round. Primary thesametimewhilewaterfromthe filters continues
filter has 44 vertically oriented leaves; the second- to be circulated. When filtered water turbidity is
ary filter has 36. Pressure differential at start-up is <0.40 NTU, the filtersgoonline (10–30 min after
about 27.6 kPa (4 psi). the start of body feed). A second mass of polymer is
(9) Blackhawk. Pre-coat-to-body-feed transition: recir- added just after the filters go on line. One hour after
culate pre-coat 15 min, then start raw-water pumps, going on line a third mass of polymer is added.
body-feed pumps, open influent valve, open filter-to- Body feed is paced with buildup of differential
waste valve until effluent turbidity is 0.6–0.8 NTU pressure; when terminal headloss is reached, all
(typically 45–60 min). Stones plug up after extended body feed will have been applied; the run is termin-
use. Well water may be blended with surface water ated by operators when peak demand is met,
to maintain influent turbidities <0.9 NTU using two typically in 8–10 h.
basins adjacent to the plant (one pond for each
source). Tube septa are each 108 mm (4.25 in) diam- 14.4 OPERATION
eter 1219 mm (48 in.) in length, giving 0.1116
2 2
m =tube (1.20 ft =tube); there are 349 tubes=tank The operation involves ordering diatomite, maintaining
2 2
giving 25.5 m (275 ft ) septum area per tank equipment and instruments, monitoring instruments, record-
(which is less than the calculated number). ing and archiving data from instruments, reporting on oper-
(10) Georgetown. Pre-coat-to-body-feed transition: recir- ation, maintaining order and cleanliness, storage and disposal
culate pre-coat 18 min, then filter is brought on line. of waste diatomite, etc.
The mixing of DE in body-feed tank is approximate.
Filter tank=septum manufacturer is Dreco, Model
14.4.1 OPERATING PROTOCOL
No. AWP6-42-275.
(11) Rawlins. Pre-coat-to-body-feed transition: water is Pre-coat and body feed are the important factors in the
recirculated for 20 min at 1200 gpm after which diatomite filtration process. Executing this involves charging
influent and effluent valves are opened. Pre-coat tanks with needed masses, metering flows, and operating
2
2
cake is at 0.37 kg=m (0.075 lb=ft ), to give 1.6 mm pumps. Much of this may be automated, leaving the operator
thickness. Pre-coat slurry is about 272.7 kg (600 lb) with the role of monitoring operations (after the software is
perlite in 1136 L (300 gal) water and recirculates programmed with values needed for operation).
about 20 min at a rate of 4542 L=min (1200 gpm).
Body feed is mixed in a 11,355 L (3,000 gal) tank 14.4.1.1 Pre-Coat Deposit
with perlite added at the rate of 0.454 kg Guidelines for pre-coat deposit have ranged at 0.50 L(DE)
2
2
perlite=37.85 L (1 lb=10 gal) water to give a slurry 1.00 kg=m (0.10–0.20 lb=ft ) (Baumann et al., 1962, p. 1114).
concentration of 12,000 mg=L and 12.6 mg=Lina Since about 1980, the tendency has been toward the higher end
raw-water flow of 10.825 mL=day (2.86 mgd) for a of the range.
slurry feed rate such that the slurry lasts 48 h. Sep-
tum are 1.981–2.234 m (6.5–7.0 ft) diameter, 14.4.1.1.1 Pre-Coat Tank Charge
depending on manufacturer. The Durco filters have The pre-coat tank is filled with a given amount of water and
39 leaves each filter and the Industrial has 36 leaves. with a given mass charge of diatomite that results in the
Water supply is from wells and a spring; each source specified pre-coat area density. Table 14.4 gives examples of
can be brought directly to the plant or discharge to a pre-coat slurry concentrations (and associated mass charges
holding pond. Wells are high in sodium and when and water volumes for several plants). The mass charge is
used are blended with the spring water. given as