Decontamination of water by combined solar advanced oxidation processes and biotreatment - Solar Energy Sciences and Engineering Applications - page 386

under the curve shows the readily biodegradable fraction of COD in a wastewater
sample containing non-biodegradable contaminants partially treated by solar photo-
Fenton. In this particular case, COD was 197 mg/L and the COD rb found in the
respirometric analysis was 37.4 mg/L. Thus COD rb /COD is 0.29, so the sample is con-
sidered biodegradable and completely biocompatible for discharge into a conventional
municipal WWTP.

12.3.3 Zahn-Wellens test
Study of the inherent biodegradability of a chemical compound enables its poten-
tial for biodegradation under optimal aerobic conditions, such as in a conventional
WWTP, to be determined. In these assays, the chemicals are exposed to microorgan-
isms, sometimes previously adapted to the substance for a long period of time to
increase compound degradation. The Zahn-Wellens method (Z-W) is standardized by
a European Union protocol (Directive 88/302/EEC), and is recommended much more
for biodegradability testing than BOD for several reasons. First, the procedure is simi-
lar to a real activated sludge biological reactor, and the biomass can even adapt to the
compounds in the sample, since the Z-W test lasts 28 days, and second, biodegrada-
tion efficiency can also be evaluated by TOC and HPLC-UV to ensure the reliability
of the results. The main drawbacks of this biological assay are that it is not applicable
to volatile or semi-volatile compounds, or to compounds with water solubility under
50 mg of carbon per litre (application range 50 to 400 mg/L organic carbon).
The Z-W procedure consists of placing activated sludge (preferably from the
WWTP which is going to receive the industrial wastewater) and nutrients in contact
with the target compound as the only carbon source in the medium. This mixture is
kept under proper aeration (and agitation) and in the dark or under diffuse light for
28 days (ambient temperature around 20-25 ◦ C). The percentage of biodegradability
can be determined at any given time by monitoring total organic carbon and using
Equation 12.3.2:

1 TOC t

− TOC b

D t = − × 100 (12.3.2)

TOC a − TOC ba

where D t is the percentage of biodegradability after time t, C a is the TOC (mg/L) in
the sample measured three hours after the beginning of the experiment (to take the
effect of adsorption of the compound on the biomass into account), C t is the TOC
measured at time t (usually measured daily), C b is the blank TOC (containing only the
same amount of activated sludge as the samples, distilled water and nutrients in order
to evaluate the TOC produced only by the biomass metabolism) measured at time t
and C ba is the blank TOC measured three hours after the beginning of the experiment.
The biodegradability threshold is 70%. The biodegradability of a reference compound
such as diethylene glycol should be evaluated as a control to ensure the method is
working properly and the correct activity of the activated sludge. 70% elimination of
the TOC in this substance in less than 14 days demonstrates proper activity of the
activated sludge.
As a practical example of the use of biodegradability analyses, Figure 12.3.3
shows photo-Fenton degradation of an industrial wastewater. As photo-Fenton is an