Modeling the Integration of Adsorption with Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 181

compounds. The addition of PAC reduced not only the effluent concentrations, but
also the off-gas concentrations of volatile compounds. For example, without PAC
addition 90% of influent 1,2,4-trichlorobenzene was found in the off-gas while
the rest stayed in the liquid effluent. However, this compound also has a high
sorption potential onto activated carbon. PAC dosages ranging from25 to 200mg L 1
removed this compound by 69-94%. The nonadsorbed fraction remained mostly in
the off-gas. Similarly, in the absence of PAC, 93% of the nonvolatile lindane (Henry
constant H c 10 7 atm.m 3 /mol) stayed in the effluent while 7% of it was

biosorbed. PAC dosages at 25-200 mg L 1 reduced lindane to nondetectable levels.
In another study, an empirical model was developed to define the removal of
soluble substrates as a function of sludge age and carbon dosage in a PACT system
[44]. The model was applied to the removal of TOC and the priority pollutant
4,6-dinitro-o-cresol in the absence and presence of activated carbon. The results
clearly showed that at the same sludge age the effluent concentrations of TOC and
the priority pollutant were decreased to lower values at increased PAC dosages.
Another attempt to model the PACT process was made by O'Brien [45]. This
model takes into consideration the behavior of organic priority pollutants and
includes biodegradation, adsorption, and stripping. The following mass balance is
made for each priority pollutant:

¼ 4.3

" #

Accumulation
of compound i

Input rate
of i

Output rate
of i

Stripping rate
of i

¼

Biodegradation rate
of i

Adsorption
rate of i

(6.91)

In numerical terms, this mass balance is expressed as follows:

V dS e

dt ¼

Q : S 0 QS e K st S e V K bio S e XV K ads X PAC S e V (6.92)

where
Q: flow rate (L 3 /T),
S 0 : influent compound concentration (M s /L 3 ),
S e : effluent compound concentration (M s /L 3 ),
X: biomass concentration (M x /L 3 ),
K st : stripping rate coefficient (1/T),
K bio : biodegradation rate coefficient (L 3 /M x .T),
K ads : adsorption rate coefficient (L 3 /M c .T),
X PAC : aeration tank PAC concentration (M c /L 3 ),
V: volume of the aeration tank (L 3 ).

Under steady-state conditions the accumulation term is set to zero:

0 ¼ QS 0 QS e K st S e V K bio S e XV K ads X PAC S e V (6.93)

As seen in Eq. (6.93), the model assumed that biodegradation is first-order in
both substrate and biomass concentration. Cometabolic removal and toxic effects