Integration of Activated Carbon Adsorption and Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 69

Biological reactions, namely biodegradation and/or biotransformation of organic
compounds, are assumed to take place mainly in the aeration basin. The biological
sludge continuously flows from the aeration basin into a secondary clarifier where
it is settled. A portion of the settled sludge is returned to the aeration basin to
maintain the proper F/M ratio. Generally, a surplus of activated sludge is pro-
duced. Therefore, some of the activated sludge is wasted from the aeration tank or
from the sludge return line. Waste activated sludge is introduced to the sludge
handling systems for further treatment and disposal.

3.4.1.3
Characteristics of the PACT Process
The PACT process is simply a modification of the activated sludge process. In this
aerobic system the influent enters an aeration tank where activated carbon in the
form of PAC is added to the mixed liquor. PAC is dosed in dry form or as a slurry.
Typically, PAC is added directly to the aeration tank or the sludge return line. The
dosage rate of carbon depends on the type and concentration of pollutants.
Following aeration, the mixed liquor enters the secondary settling tank, where the
slurry composed of biomass and carbon particles is allowed to settle. A small dose
of cationic polyelectrolyte may be added ahead of the secondary settling tank to
enhance solids-liquid separation and to ensure maximum overflow clarity. After
settling, filtration is optional and is usually recommended if a suspended solids
level of less than 20 mg L 1 is to be consistently obtained in the effluent [44].
Following treatment, a portion of the sludge is daily wasted from the system. Since
a portion of the PAC is also wasted with this sludge, continuous addition of virgin
or regenerated PAC is required to maintain the desired carbon concentration
in the aeration tank. Waste solids can be disposed of as slurry, dewatered to a
compact stable cake, or pumped to a Wet Air Regeneration (WAR) unit for
regeneration of carbon (Section 3.7) (Figure 3.9).

Virgin
Carbon
Storage

Polyelectrolyte
Storage
Settling
Tank

Filtration
(Optional)

Waste

Contact-Aeration
Tank

Effluent

Carbon Recycle

Thickener

Overflow

PACT ® System Process
(Single Stage, Aerobic)

To Regeneration
or Disposal

Figure 3.9 PACT s general process diagram [45] (permission received).