Activated Carbon Treatment - Public Water Systems - page 577

mg / L during the spring season. Higher doses of 25-30 mg / L are used during peak
events.
PAC facilities include a storage bin receiving truckloads of PAC blown into the
storage bin. A large day tank is prepared with a PAC suspension (typically 1.0-lb / gal
[0.12 kg / L] concentration) and metered into the flow. The day tank is prepared and
density measured each shift. Table 17-4 summarizes the operating criteria for the PAC
facilities at this plant.

Real-World PAC Example: Council Bluffs, Iowa
The Narrows Water Treatment Plant of the Council Bluffs Water Works in Council
Bluffs, Iowa, operates a 16.9-mgd (64-ML / d) water plant. 33 Two water sources are
available: the Missouri River or wells. Figure 17-13 shows a flow diagram of the
plant. The water is split-treated with up to 11.2 mgd (42.4 ML / d) lime softened and
the remainder alum coagulated to achieve a product water with approximately 160
mg / L hardness. The split depends on the water source (river or wells) and the hardness
in the source waters, which varies during the year.
PAC can be added before the presettling basin or with other chemicals in the rapid
mix before flocculation. The dose point is selected before the point of lime addition
to take advantage of improved adsorption on the PAC at the low pH. Multiple feed
points are used at various periods, depending on the softening needs. Generally speak-
ing, the feed point is selected to avoid interference from the high pH during lime
softening.
Table 17-5 shows the operating criteria for the PAC facilities at the Florence plant.
The main reason for PAC application has historically been for taste and odor control.
Recently, PAC has been found to be effective to control agricultural chemicals in
runoff, specifically atrazine. PAC doses are increased when the potential for agricul-
tural chemicals in the river increases, such as increases in turbidity due to increased

TABLE 17-4. Operating Criteria for Florence Water Treatment Plant,
Omaha, Nebraska
Item Value

Facilities
Maximum production 158 mgd (598 ML / d)

Average flow 45 mgd (170 ML / d)

PAC dose
Typical 6 mg / L

Maximum 30 mg / L

Annual usage 100-150 tons / year (91,000-136,000 kg / year)

PAC metering
Day tank size 1,500 gal (5,700 L)

Day tank density 1.0 lb PAC / gal (0.12 kg PAC / L)

Metering pump
capacity

9,600 gpd (36,000 L / d)

PAC storage
Bin capacity (total) 8,000 ft 3 (227 m 3 )

Typical refill interval 1 year