Integration of Activated Carbon Adsorption and Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 54

others, like phenol, which have a relatively high biodegradability and adsorbability,
the synergism created by simultaneous treatment is obvious [15].
The main interactions taking place in a carbon environment with biological
activity are explained below. Figure 3.6 illustrates some of these interactions.

3.2.2.1
Retention of Slowly Biodegradable and Nonbiodegradable
Organics on the Surface of Activated Carbon
The presence of activated carbon in a biological system provides a vast surface area
for adsorption of organics. Hydrophobic compounds with log K ow

2 would be
preferably adsorbed onto carbon. Adsorption is particularly beneficial for biode-
gradation of slowly biodegradable and apparently nonbiodegradable organics. A
number of compounds found in domestic, municipal, industrial wastewaters,
and in landfill leachates fall into these categories. Adsorption of such compounds
onto carbon would decrease the probable inhibitory effects in the bulk solution. In
addition to the initial organics, activated carbon can also effectively adsorb the
intermediate and/or end products of biodegradation.
The presence of bulk organic matter in a wastewater would decrease the
adsorption capacity of activated carbon for a specific pollutant. Furthermore, it
should be recognized that a pollutant never exists alone in a wastewater. There-
fore, competition may occur between different types of specific pollutants for
adsorption sites on activated carbon.
Upon adsorption onto activated carbon, the retention time of an organic sub-
stance in a biological system is considerably increased. For example, in the PACT
process (Section 3.4), dissolved organics adsorbed on activated carbon are retained
in the system for a time equal to the SRT of about 10-50 days, while in conven-
tional biological systems they will be kept for a period equal to the hydraulic
retention time (HRT), typically 6-36 h [16]. This arises because PAC acts in
the same way as the biomass in the system; in other words, the 'age' of PAC in the
system is equal to that of biological sludge. In comparison to PACT, the retention
of organics may even be longer in BAC reactors since the GAC is kept for a very
long period of time in the reactor. The long retention on carbon surface enables
the acclimation of attached and suspended microorganisms to these organics,
eventually leading to their efficient biodegradation (Figure 3.6a).

W

3.2.2.2
Retention of Toxic and Inhibitory Substances on the Surface of Activated Carbon
Some organic and inorganic compounds may exert inhibitory or toxic effects on
biological processes. Such adverse effects are usually encountered during indus-
trial wastewater treatment, during leachate treatment, or in POTWs receiving a
significant input of these wastewaters.
Generally, a pollutant may exert either substrate inhibition (inhibition of its own
removal) or competitive and noncompetitive inhibitory effects on other sub-
stances. The inhibitory characteristics of a pollutant in a biological system are not
easily determined by a single parameter, but tests have to be conducted to predict