Abstract |
Wastewater from olive oil mills has significant polluting properties, especially due to
the high concentration of aromatic compounds (mainly phenolics). The current project aims
to study the biodegradation of the total phenolic compounds in olive mills wastewater
(OMW) by algae, taking into account their bioenergetic strategy to biodegradate phenols
(Papazi and Kotzabasis,2007, 2008). In the meanwhile we tried to test the possibility to use
the biodegradation of the total phenolic compounds in OMW by algae as electrons source
for transferring electrons through the photosynthetic electron flow to hydrogenase yielding
large amounts of H2 according to the mechanism recently has been described by Papazi et al.
(2012).
To exclude the randomness of the results, we used individual samples of OMW from
14 different mills around Greece. The results have shown that the pH adjustment is the key
not only for the biodegradation of the total phenolic compounds but also for the production
of molecular hydrogen. When the acidic pH (pH 4-5) was configured at pH> 6,5, significant
levels of molecular hydrogen production and high biodegradation of total phenolic
compounds in OMW (in closed systems), were observed. This study has clearly
demonstrated that the two processes (production of H2 and biodegradation of toxic phenolic
compounds) are competitive, since the biodegradation of the total phenolic compound
requires the presence of oxygen [because of the essential need of (mono) oxygenases in the
biochemical pathways which are involved in the biodegradation of phenols], and the
production of molecular hydrogen needs anoxic conditions to start.
Samples of OMW (mixture of the 14 mills around Greece), which were incubated in
closed and open systems, have clearly shown that the biodegradation of the total phenolic
compounds in open systems is complete (100% degradation of the total phenolic
compound), in a period of 7 days of incubation, with and without additional algae
(Scenedesmus obliquus). The fact, there was 100% biodegradation in algae free treatments,
revealed the existence of specific micro flora (algae, cyanobacteria, bacteria, ...) in the
OMW which remains inactive, mainly due to the acidic pH of OMW. The pH adjustment of
OMW to pH: 6,5 or higher, activates the metabolism of these organisms, inducing the
biodegradation of toxic phenolic compounds. In closed systems the activation of metabolism
leads to the establishment of anoxic conditions and induces H2 production. Although the
addition of algae contributes to this process, it seems it is not necessary!
It seems that the micro flora in OMW is able under certain conditions to produce molecular
hydrogen and detoxificate the total of the phenolic compounds in OMW. All we have to do is
create the proper conditions (pH regulation, closed or open system, etc.), and the OMW’s
micro flora itself can fully biodegrade the total of the phenolic compounds in OMW and at
the same time can produce high amounts of H2. The results of the present study give for first
time the possibility to use a complete detoxification method of OMW at the place of olive
oil production (in each mill), with simultaneous production of high amount of hydrogen
which could used as an energy source. In addition to the environmental compatibility and
the sustainability of the process, the cost / benefit ratio appears to be very appealing and
ensures a wide use in the Mediterranean region and especially here in Crete in the near
future.
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