Decontamination of water by combined solar advanced oxidation processes and biotreatment - Solar Energy Sciences and Engineering Applications - page 389

additional analytical methods may be necessary for the identification of unknown
intermediate degradation products (chromatography coupled with mass spectrome-
try). Considerable effort and sophisticated analytical equipment may be necessary to
explain (Gómez-Ramos et al., 2011) why, for example, acute toxicity rises during treat-
ment by pinpointing a single intermediate product much more toxic than the original
pollutant, or when the purpose is to degrade certain contaminants to below a limit
(usually g/L) in complicated water containing other organics, and therefore COD,

TOC or HPLC/UV cannot be used.

µ

12.4 COMBINING SOLAR ADVANCED OXIDATION PROCESSES
AND BIOTREATMENT: CASE STUDIES

12.4.1 Case study A:An unsuccessful AOP/biological process
In the first example the targets are two biorecalcitrant substances used as synthesis
intermediates in the pharmaceutical industry, 2-(2, 4-dichlorophenyl)-2-(1H-imidazol-
1-dylmethyl)-1,3-dioxolan-4-ylmethanol (CAS 84682-23-5) (DIM) and 2-(2,4-
dichlorophenyl)-2-(1H-1,2,4-triazol-1-ylmethyl)-1,3-dioxolan-4-ylmethanol (CAS
67914-85-6) (DTIM). These two non-biodegradable compounds mixed in a distilled
water matrix were degraded by solar Photo-Fenton. Each contaminant was dissolved at
a concentration of 200 mg/L (COD of 700 mg/L) because they are usually found at this
concentration in industrial wastewater. Figure 12.4.1 shows wastewater degradation
by solar photo-Fenton, toxicity results and Zahn-Wellens biodegradability analyses.
It may be observed that the target compounds were susceptible to complete
degradation and mineralization by photo-Fenton. Both substances were completely
eliminated after 25 minutes of illumination (DIM and DTIM

0) with 27.5 mM of
hydrogen peroxide. The AOS was also calculated from TOC and COD results, but did
not increase until around 80% of TOC had been mineralized. Toxicity analyses showed
that inhibition remained the same until practically the end of the treatment. Further-
more, biodegradability monitored by Zahn-Wellens showed that DIM and DTIM
biodegradability were only slightly enhanced when photo-Fenton pretreatment was
extended until TOC was below 98 mg/L (at this point, biodegradability after 11 days
was 60%). From these results, it can be concluded that the best treatment option for
wastewater containing DIM and DTIM is to apply solar photo-Fenton (or other AOP)
only until almost complete mineralization. In this case a combined AOP/biological
process strategy is not feasible.

=

12.4.2 Case study B:A successful AOP/biological process
Industrial wastewater with a low organic load (TOC under 500 mg/L) can usually be
treated in a combined AOP/biological process. This is the case in the following example,
the successful treatment of saline industrial wastewater containing around 600 mg/L
of a non-biodegradable compound (R-methylphenylglycine, MPG, also from the
pharmaceutical industry) and 400 to 600 mg/L dissolved organic carbon (TOC). Tests
performed in solar photo-Fenton pilot plants [Gernjak et al., 2006] were used to design
a large-scale hybrid solar photocatalytic-biological plant with a 4 m 3 daily treatment