Civil Engineering Reference
In-Depth Information
TABLE 17-5. Operating Criteria for the Council Bluffs
Water Treatment Plant
Item
Parameter
Facilities
Maximum production
16.9 mgd (64 ML / d)
Average flow
9.2 mgd (34.8 ML / d)
PAC dose
Typical
25 mg / L
Maximum
40 mg / L
PAC metering
Day tank size
35,000 gal (132,000 L)
PAC storage
Bin capacity (total)
70,000 lb (32,000 kg)
runoff in the basin. PAC dose is typically 20 mg / L and is adjusted to maintain atrazine
below 50 percent of the maximum contaminant level. Higher doses are used during
peak events.
PAC facilities include a storage bin receiving 50-lb (23-kg) bags of PAC. An auger
feeder feeds a large day tank to prepare a PAC suspension. The PAC suspension is
dosed by using an eductor arrangement to control doseage.
Real-World PAC Example: Sioux Falls, South Dakota
Seasonal taste and odor problems occur in the Sioux Falls, South Dakota, water supply
in spring when the snow melts or precipitation events occur. Water quality degradation
is a result of high organics and humic materials. Taste and odor problems and organics
are treated by adding PAC at various locations throughout the water treatment plant.
The City of Sioux Falls operates a softening / filtration plant rated for 54 mgd (200
ML / d) maximum-day capacity. Two water sources are available: the Big Sioux River
or shallow wells in the Big Sioux Aquifer or Skunk Creek Aquifer. Figure 17-14 is
a flow schematic of the water treatment plant, and Table 17-6 summarizes the oper-
ating criteria for the PAC facilities.
PAC can be added at a static mixer in the plant influent piping, in conjunction with
the lime at the solids contact units, or in the recarbonation basins prior to filtration.
The dose point in the influent before the point of lime addition could take advantage
of improved adsorption in the PAC at the low pH; however, in this case the effective-
ness of this particular PAC has not decreased substantially at the high pH experienced
in the softening basins. Multiple feed points are used at various periods, depending
on the treatment needs.
The reasons for PAC application are organics reduction and taste and odor control.
The PAC is fed routinely throughout the year and has effectively reduced complaints.
PAC dose is typically 5 mg / L and is adjusted to 20 mg / L or higher during some
episodes. Since the city has an alternative groundwater source, they curtail surface
water usage during the most severe taste and odor episodes. This step is dependent on
the availability of groundwater supplies, surface water quality, and duration of the taste
and odor episode.
A slurry tank is used for PAC storage. The slurry storage tankage consists of three
42,000-gal (160 m
3
) compartments for a total storage volume of 126,000 gal (480 m
3
).
