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498 P. Kajitvichyanukul et al.
4.4. Biological Activated Carbon Adsorption: Biofilm
Biological activated carbon (BAC) has been attracting great attention and is being applied
to various treatment processes to remove pollutants from drinking water. BAC processes
were discovered and developed during the 1970s in drinking water treatment plants, and now
are in full-scale operation in some 60 plants or more in Europe, USA, and Japan (100, 101).
BAC was first defined by Rice and Robson (102) as a water or wastewater treatment system
in which aerobic microbial activity is deliberately promoted in a GAC system. This process
provides simultaneous adsorption of nonbiodegradable matter and oxidation of biodegradable
contaminants in a single reactor (103). GAC is widely used in drinking water after ozonation
to eliminate by-products such as aldehydes and ketones, or chlorination to eliminate chlorinated
disinfection by-products such as toxic halogenated organic compounds and unpleasant
odor. These pollutants are removed by adsorption, chemical reduction, and mechanical
filtration. This new combination process has an advantage over conventional activated carbon
treatment for the removal of organic substances. The use of BAC treatment makes the whole
facilities compact and the life of carbon longer (104). With the assistance of the bioactivity on
BAC, the reduction of TOC and other organic substances is considered to maintain a longer
service time before GAC regeneration (105). In addition, BAC is more efficient than simple
biological oxidation systems without an adsorption surface. Thermal regeneration of activated
carbon is not often required because of the influence of the biofilm. Furthermore, partial
biological regeneration of GAC owing to sorption, and later, metabolism of slowly biodegradable
carbon can be observed in practice (106, 107). This also leads to an extended life of
the carbon column.
Two noncontradictory hypotheses about the mechanism of bioregeneration were purposed.
The degradation mechanism of adsorbed substances due to exoenzymes from microorganisms
was proposed (107). A biofilm containing bacteria in the macropores and their “exoenzymes”
in the micropores was established. The biodegradation in pores takes place by means
of exocellular enzymes, which are capable of diving into micropores and interact with
adsorbed substrate and promote its hydrolytic decay (107, 108).
In another hypothesis (108, 109), the development of biological processes on the activated
carbon surface in combination with adsorption processes, desorption, and diffusion in pores
leads to partial regeneration of activated carbon. The reverse of an adsorption concentration
gradient can be created by reaching a critical rate of biodegradation in the biofilm and in the
liquid phase. The adsorbed substances are desorbed back into the biofilm, and also through it
into the liquid phase, and become accessible to the microbial degradation (109). It was
reported that biofilm develops on BAC even with very high shear stress from backwashing
(110). The negative influence of toxic substances on bacterial biofilm is also protected by
activated carbon (110). The biological regeneration (bioregeneration) of activated carbon
leads to higher efficiency of BAC than the consecutive adsorption and biological treatment.
Many researches have focused on application of BAC in drinking water treatment. Amsterdam
Water Supply (AWS) used biological activated carbon filtration (BACF) for the removal
of NOM in general and the removal of organic micro-pollutants in particular. Six years of
operation of BACF in the River-Lake Waterworks (31 Mm 3 /year) have shown that successive