Integration of Activated Carbon Adsorption and Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 56

biomass is not provided. This results then in the inhibition of the removal of bulk
organic matter. For example, 2-nitrophenol may inhibit the removal of other
organics denoted by sum parameters such as COD, BOD 5 , or TOC while its own
removal is nil or very limited under aerobic conditions. Thus, it is regarded as
nonbiodegradable and inhibitory.
Fortunately, most of the inhibitory or toxic organic pollutants are amenable to
adsorption onto activated carbon (Figure 3.6b). Their adsorption frombulkwater onto
activated carbon leads to the protection of biomass, which is retained in the biological
reactor in suspended or attached form. Moreover, microbial processes can take place
on the surface of activated carbon that convert a specific toxic organic pollutant into an
innocuous one. This beneficial effect of activated carbon is even more enhanced in
BAC systems where microorganisms are attached to the GAC surface (Section 3.3).
Various methods can be used to test the inhibitory behavior of a substance in the
absence and presence of activated carbon. Monitoring of the Specific OxygenUptake
Rate (SOUR) is a common procedure in the assessment of the inhibitory response of
a wastewater due to presence of organic and/or inorganic substances [17]. Phenol is
well known for its inhibitory effects on biological processes, and has therefore often
been employed as a model compound in testing the effect of activated carbon on
biological processes. For example, PAC addition to activated sludge enabled con-
tinuous operation with inlet concentrations of phenol up to 1000 mg L 1 [18].
Heavy metals constitute themajor group of inorganics that have adverse effects on
biological processes. For example, copper, zinc, nickel, lead, cadmium, and chro-
mium can react with microbial enzymes to retard or completely inhibit metabolism.
On the other hand, as shown in Figure 3.6, heavy metals can also be adsorbed on
activated carbon. The affinity of activated carbon for toxic heavy metals may even
eliminate the need for metal precipitation prior to biological treatment [19].
3.2.2.3
Concentration of Substrates on the Surface of Activated Carbon
Most of the organic pollutants in wastewater treatment are found nowadays at very
reduced concentrations. Under these conditions, most substrates can be used as
secondary substrates only since microorganisms are unable to utilize them as the
main energy and carbon sources. In that respect, the presence of activated carbon in a
biological system is advantageous because it provides a tremendous surface on which
substrates become concentrated upon adsorption (Figure 3.6c). Accordingly, the use
of activated carbon is particularly advantageous for dilute wastewater streams.
Additionally, depending on the type of activated carbon and environment, activated
carbonhas the ability to concentrate other substrates on its surface such as oxygen and
nutrients. This concentration of various substrates further enhances biodegradation.
3.2.2.4
Retention of Volatile Organic Compounds (VOCs) on the Surface of Activated
Carbon
Depending on the extent of adsorbability, the retention of a VOC on the carbon
surface prevents its emission into the air. Additionally, if a VOC is amenable to
biodegradation, the retention on the carbon surface increases the probability of
biodegradation by suspended or attached biomass (Figure 3.6d).