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14.8 CHAPTER FOURTEEN

• A batch meter and flush connection, with gauges for monitoring supply pressure
• A single rotary volumetric feeder (because carbon is not fed constantly at this facility,
only a single feeder has been provided)
• An eductor, equipped with a rotameter for measuring water supply
• A solenoid valve on the water supply line that keeps the water flowing for a period of
time after the feeder is turned off to flush the discharge line
• Sample connections
• Cleanouts

For the PAC system illustrated in Figure 14.1, the maximum plant flow rate is 90 mgd
(340 ML per day). At maximum flow and an average dosage of 1.5 mg/L, the tank pro-
vides 26 days of storage. At a maximum feed rate of 10 mg/L, only 4 days of storage is
available.
Plants using PAC on a regular basis should consider at least two slurry tanks so that
a shipment of carbon can be placed into one tank before the other is empty. However,
most applications can be satisfied by a single storage tank. A carbon slurry storage tank
can be circular or square and made of steel or concrete. Tank storage capacity should be
approximately 20% greater than the maximum carbon load delivered by railroad car or
truck. A protective lining can be provided to inhibit corrosion, but unlined concrete tanks
are usually sufficient. Mechanical mixers should be provided in the storage tank to keep
the PAC slurry in suspension. All metal surfaces should be type 316 stainless steel.
Slurry can be pumped from the main storage tank to a day tank that holds the volume
of slurry to be fed over a period of several hours, or slurry can be fed from the main stor-
age tank directly to the chemical feeder. The day tank, if provided, should be plastic or
steel with a corrosion-resistant lining. The tank must be equipped with a mixer to keep
the slurry in suspension. Slurry should then flow by gravity to the volumetric feeder.
An eductor usually moves the slurry from the feeder to the application point. Piping
should slope downgrade to the application point, with provisions for flushing any carbon
that may settle out and clog the pipe. Avoid any vertical drops of pipe down off a header
unless the drops are used continuously. If they are not used continuously, PAC will set-
tle in the bottom of them and plug the pipe. Because of the abrasive nature of carbon
slurry, piping should be corrosion- and erosion-resistant, such as rubber, plastic, or stain-
less steel. Pump impellers and mixing blades in the slurry tank and day tank should also
be of stainless steel or fiberglass. To avoid feed pipe clogging, a minimum flow velocity
of 5 ft/s (1.5 m/s) should be maintained in the pipelines. Care must be taken to provide
long-radius elbows.
Wet activated carbon removes oxygen from the air, and as a result, slurry tanks or
other enclosed spaces containing carbon may have seriously reduced oxygen levels. Per-
sonnel who must enter these spaces should use an oxygen meter to check the atmosphere
and also have attached safety belts and another worker present to pull them from danger
if necessary.

Common PAC System Operating Problems
The most common operating problem with PAC is chemical handling. Most dry feed sys-
tems currently in use can be labor-intensive. Dust is a major problem if a dry feed sys-
tem is used, but the amount of dust allowed to become airborne can be minimized by
good design practice.
A problem that is not uncommon is PAC passing through the filters and entering the
distribution system, provoking complaints from consumers. Black water is usually caused