Combination of Activated Carbon with Biological Wastewater Treatment at Full-Scale - Activated Carbon for Water and Wastewater Treatment - page 126

APHA units, respectively. In an activated sludge system the corresponding values
were as 57mg L 1 and 1020 APHA units. At the design PAC dose of 180mg L 1 ,a
full-scale PACT system achieved 31.2mg DOC/L, 6.7mg BOD 5 /L and 483 APHA
color units in the effluent with removal efficiencies of 82%, 96%, and 66%,
respectively [4]. For further details on the startup and steady-state operation at this
plant the reader may refer to literature [6].
Introducing a more biodegradable wastewater to an existing PACT plant treating
organic chemical wastewaters may be a convenient way of diluting a high-strength
wastewater and increasing the biodegradation potential. In that respect, screening
tests were performed by mixing various types of industrial effluents with the
original wastewater of Chambers Works in laboratory scale continuous-flow PACT
reactors [5]. When agricultural wastewater (40% of total BOD) was added to the
original wastewater, BOD and DOC removal efficiencies increased from 95.2% to
96.7% and from 81.6% to 86.7%, respectively. The test showed that addition of a
more biodegradable organic waste slightly increased the removal efficiency in the
PACT reactor [5].
The wastewater from an organic chemical factory was treated at full-scale in an
activated sludge system that was upgraded by PAC addition [7]. The factory was
producing several kinds of organic chemicals such as substituted phenols, ben-
zene derivatives, nitrated aromatic substances, aniline derivatives, chlorinated
hydrocarbons, solvents, plastics, and pesticides. After neutralization, settling, and
equalization of wastewater, the activated sludge units received a wastewater with
a high organic content (COD: 1700-4000mg L 1 , BOD 5 : 1100-2700mg L 1 ).
Operational difficulties were encountered since biological units suffered from
overloading. Despite equalization and pretreatment, the quality of wastewater
was very variable. Sudden changes in the composition of wastewater led to
decreases in sludge activity, and to sludge bulking and foaming. This observation
is typical for industries that utilize batch-type productions and change their pro-
cesses frequently to produce different kinds of chemicals. Two cylindrical activated
sludge units were operated with aeration volumes of about 1500m 3 each. One
of the biological units was operated as a control, without PAC addition, while the
other received different PAC doses. Under equal loading rates, in the first month,
the organic content of the effluent at the PAC dose of 70mg L 1 was 30-40% lower
than the control unit. The treatment efficiency reliably approached 85%, while that
of the control unit went down to 67%. The PAC addition decreased the sludge
volume index (SVI) from 35 to 21mL g 1 and improved sludge thickening. It also
enabled higher hydraulic loading to be applied. PAC addition also decreased
sludge foaming to a minimum. An important result was also that 20-50%
decrease was achieved in the concentration of micropollutants compared to the
control unit. The optimal PAC dose was relatively low ( o 100mg L 1 ) despite
the high organic content of the influent.
Another example showed that the PACT s system was able to reduce organic
compounds in a chemicals production wastewater to much lower levels than a con-
ventional activated sludge, although the sludge age in the latter was more than twice
that in the PACT s system (Table 5.1) [8]. The effluent concentrations in Table 5.1