Environmental Engineering Reference
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improved wastewater biodegradability. Biodegradability of the mixture during pho-
totreatment was evaluated in samples taken over 28 days by the Zahn-Wellens (Z-W)
test. S1 was non-biodegradable (around 50% biodegradability in 28 days), whereas
the rest of the samples were biodegradable according to the Z-W test. S2, S3 and S4
were biodegradable after a long period of 8 days or more. However, after S5 (345 mg/L
TOC), all the samples became biodegradable in a short time (5 days or less). S6 had the
highest biodegradation percentages. Therefore, the most suitable point for combining
photo-Fenton with the biological treatment is somewhere between S5 and S6.
12.3.4 Factors to be considered in designing a combined system
Design of a combined chemical and biological wastewater treatment consider how
the characteristics of each individual treatment can improve the destruction of a
persistent contaminant. The chemical oxidant to be used (photo-Fenton or Fenton
reagent, O 3 /H 2 O 2 ,O 3 /UV, H 2 O 2 /UV, TiO 2 /UV, etc.) must be decided based on tests
to determine which has the highest rate in the key parameter selected (TOC, COD,
biodegradability, toxicity or a combination thereof) with the lowest chemical consump-
tion. The rest of the characteristics to be taken into consideration are widely known:
the ability of the chemical oxidation process, its potential for forming toxic interme-
diates or not, change in pollutant behaviour, choice of biological agent, comparison
of different cultures, comparison of acclimated and non-acclimated cultures, use of
monospecific cultures, anaerobic cultures, etc.
Measurement of the combined process efficiency depends on the purpose of the
treatment, but usually requires independent optimization of each chemical and bio-
logical step. For example, the extent of mineralization of the organic compounds may
be a measure of efficiency if highly pure water is used or the effluent has a specific
dissolved organic carbon limit. The main purpose of other treatments may be total
elimination of toxicity or of a specific pollutant. Determining the target is an essen-
tial step in combination studies since it helps define process efficiency and provides
a basis for comparing operating conditions and optimizing the process. Nevertheless,
if the influent concentration is expected to change, correct scaling and design of the
photoreactor may be complicated, because the correlation of the required treatment
time and substrate concentration cannot be estimated directly, but must be determined
experimentally.
Therefore, several analytical parameters must be monitored during each step of
the treatment train, as commented before. Chemical parameters, biological assays
for toxicity (with various organisms like Vibrio fischeri , Daphnia magna , but rec-
ommended with activated sludge, etc.) and biodegradability (always using activated
sludge) are very important to ensure the optimal conditions for complete effluent treat-
ment. In the biological system itself, and in addition to daily control analyses such as
total suspended and volatile solids, total organic carbon or chemical oxygen demand,
pH and dissolved oxygen in the system, etc., it is also essential to measure anions
and cations in the biological medium, since nutrients are vital to the microorganisms
that make up the activated sludge populations and monitoring of the nitrogen species
provides much information on nitrification and denitrification that take place during
biotreatment. This whole series of analytical parameters is necessary for engineering the
design of the combined strategy. For further understanding of the underlying processes,
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