Integration of Activated Carbon Adsorption and Biological Processes in Wastewater Treatment - Activated Carbon for Water and Wastewater Treatment - page 48

Y yield coefficient (M x /M s ),

k max ¼ maximum specific substrate utilization rate (

¼

m max /Y), (M s /M x . T).

In the above notation, the subscripts s and x represent substrate and biomass,
respectively.
Below the S min concentration no biomass can be kept in the system [3]. The S min
concept is relevant in water and wastewater treatment and in soil and groundwater
bioremediation.
However, bacteria can still consume a pollutant if there is another substrate at a
concentration high enough to support the growth of existing population. In such
cases, the compound at the higher concentration is denoted as the primary sub-
strate whereas the pollutant at the lower concentration becomes the secondary
substrate. In contrast to cometabolism, the secondary substrate serves as a source
of carbon and energy.
Cometabolic removal of substrate If a substrate (pollutant) is entirely resistant
to biodegradation under the specific condition, there is still a possibility of removal
by cometabolism. Cometabolism is the biological transformation of a nongrowth
(cometabolic) substrate by bacteria through enzymes which can only be induced in
the presence of a growth substrate (primary substrate) providing energy for cell
growth and maintenance. Since the cometabolic transformation of a substrate
yields no carbon and energy benefits to the cells and cannot induce production of
enzymes, a growth substrate must be supplied to the microorganism, at least
periodically, for the growth of new cells that produce the necessary enzymes.
Cometabolic degradation of various chlorinated organics by phenol, toluene
or propane oxidizers is widely reported (Figure 3.4). For example, phenol serves
as an ideal growth substrate in cometabolic transformation of chlorinated
phenols. Phenol is used by phenol oxidizers to induce the necessary enzymes
for the degradation of chlorinated phenols that have a structural analogy with
phenol. As exemplified in Figure 3.5, in the presence of phenol (bisolute case),
2-chlorophenol (2-CP), which is normally nonbiodegradable under aerobic condi-
tions (single solute case), acts as a cometabolic substrate and is efficiently
dechlorinated [4].
Also, easily biodegradable compounds such as glucose and dextrose can serve
as primary or growth substrates in cometabolism. Aerobic cometabolism of
chlorinated compounds is also possible using the inorganic ammonia as primary
substrate [5]. Cometabolism is also commonly encountered under anaerobic
conditions.
Cometabolic degradation of (micro)pollutants is often encountered in the
treatment of domestic, municipal, and industrial wastewaters, sanitary and
hazardous landfill leachates, in bioremediation of polluted soils and groundwaters,
and in drinking water treatment.

3.2.1.1.3 Biological Removal of Micropollutants Due to the relatively low
concentrations of organic micropollutants in wastewater treatment systems, bio-
degradation is often modeled using a pseudo first order degradation expression [6].