Decontamination of water by combined solar advanced oxidation processes and biotreatment - Solar Energy Sciences and Engineering Applications - page 383

sequence. Such an approach often involves at least one AOP step and one biologi-
cal treatment step (Mandal et al., 2010). Whether the AOP or the biological process
comes first in the treatment line, the overall purpose of reducing costs is nearly the
same as minimizing AOP treatment and maximizing the biological stage. The individ-
ual biological and chemical oxidation efficiencies must be calculated to find the optimal
operating conditions for the combined process. This involves detailed knowledge of
both biological and chemical processes. Therefore, several analytical parameters must
be monitored during each step of the treatment line. The usual chemical parameters
measured are total organic carbon (and/or chemical oxygen demand), the concentra-
tion of specific pollutants in the target wastewater, and heteroatoms from contaminants
completely degraded during the AOP treatment released (Cl, N, P,…) as inorganic
species (Cl − ,NO 3 ,PO 3 4 ,…) into the medium. Toxicity analyses (with organisms
like Vibrio fischeri, Daphnia magna , activated sludge, etc.) and biodegradability tests
(using activated sludge) are very important to ensure that AOP effluent conditions are
suitable for treatment by conventional biodegradation. The following sections high-
light the main parameters necessary for proper evaluation of an AOP to determine the
best way of combining it with a biotreatment.

12.3.1 Average oxidation state
One of the most widely used parameters in wastewater biodegradability assessment
is the Average Oxidation State (AOS), which can be calculated with Equation 12.3.1
(Scott and Ollis, 1995), where TOC (total organic carbon) and COD (chemical oxygen
demand) are expressed in moles of C/L and moles of O 2 /L, respectively:

TOC

COD
TOC

−

AOS = 4 × (12.3.1)

The AOS is from 4 (for the

most reduced state of carbon, CH 4 ). The AOS, which varies with treatment time and
indicates the oxidation state of the organic compounds in the wastewater, can be used
to determine how the AOP is modifying them. Figure 12.3.1 shows an example of
how the AOS commonly evolves during AOP wastewater treatment. Although the
AOS rises rapidly at first, this increase later slows down, suggesting that the chemical
nature of the reaction intermediates generated did not vary significantly after certain
stage. Furthermore, when contaminants are oxidised before mineralization, it usually
means that biodegradability is increasing, as in the transformation of chlorophenol
into phenol, and phenol into oxalic acid, for example. This is because when the AOS
stabilizes, oxidation is producing mineralization (the last step of the process), and no
further substantial change in wastewater biodegradability is expected.
So if high biodegradability is to be achieved, it must be before or right at the
moment the AOS stabilizes. From that point on, the photocatalytic treatment only
mineralizes organic carbon, even though the chemical nature of the organic compounds
does not change substantially. It may therefore be concluded that the AOS provides
indirect information about wastewater biodegradability. The example in Figure 12.3.1
shows the TOC, COD and AOS during solar photo-Fenton treatment of an industrial
waste water. As observed, the AOS rises to a maximum of around 2 and remains there

+ 4 (for the most oxidized state of carbon, CO 2 )to −