IWA Poster Template

1
The Effect of Low-Concentration Ozone on the
Structure of Organic Matter and Their Removal by
a BAC Filter for Tertiary Wastewater Treatment
Introduction
The most important effect of ozone, among its
multi-functions, is to convert non-biodegradable
organic substances into biodegradable organic
substances, but not limited to direct
decomposition of organics. Therefore, ozonation
is usually followed by a biofilter using sand,
anthracite, granular activated carbon (GAC) or
other materials as filter media, to achieve a
high organic removal and/or to improve the
bio-stability of the treated water. It was found
that complete decomposition of organic matters
required higher ozone dose, but significant
conversion of the structure of non-biodegradable
organics into biodegradable ones could be
achieved at a low concentration. It is thus
suggestible that effective organic removal could
be made possible by combining low-concentration
ozonation process with biofiltration. In this
paper, this idea was practised in a pilot study
of tertiary treatment of domestic wastewater for
water reclamation using a low-concentration
ozonation (ozone dose as 1 2 mg/l) and
biofiltration process.
Results Discussion
Effect of ozonation on the structure of organic
matter
Profiles of biomass, NH3-N and NOX-N along the
BAC filter
As shown in Figure 1, it is identified that in
addition to certain kinds of aromatic acids,
cyclic hydrocarbons are dominant in the raw
water, secondary effluent from the wastewater
treatment plant. After ozonation, there is an
apparent decrease in the amount of cyclic
hydrocarbons while many chain hydrocarbons,
aliphatic acids and some newly formed organic
matter with oxygenated functional groups become
dominant.
According to the distribution curve of the
quantity of biomass along the filter depth
(Figure 3), it can be estimated that more than
80 of the biomass is accumulated in the upper
half of the filter. The BAC filter can also
achieve a removal of inorganic nitrogen to
certain extent (about 40). In Figure 3, the
profiles of ammonia (NH3-N), nitrate (NO3-N), and
nitrite (NO2-N) along the filter depth are
compared. It is interesting that in the upper
half of the filter down to the depth about 90 cm,
there is a decrease of NH3-N concentration
accompanied by an increase of NO3-N along the
filter depth, while in the lower half of the
filter, NH3-N almost keeps unchanged but NO3-N
decreases dramatically along the filter depth.
Regarding NO2-N, only a slight increase is found
along the filter depth especially in the lower
part. The ozonated water entering the BAC filter
is full of dissolved oxygen (usually higher than
9.0 mg/l), which results in a good aerobic
environment at the top part of the filter.
Sufficient dissolved oxygen and substrates supply
provides a favourable condition for the process
of nitrification where NH3-N is oxidized to NO2-N
and/or NO3-N under the action of nitrifying
bacteria. Down to the depth of 90 cm, the
concentration of dissolved oxygen drops to about
5.0 mg/l and this value almost keeps unchanged in
the lower half of the filter. Under the condition
of laminar flow through the filter, the
efficiency of oxygen transfer is often very low
from the flowing water to the biofilm on the
surface of the filter media. Therefore, an anoxic
environment may be thus resulted in the filter
layer especially inside the biofilm. This is a
condition suitable for the process of
denitrification where NO3-N is consumed to
provide oxygen for the growth of the denitrifying
bacteria and finally forms gaseous nitrogen (N2).
Figure 1 GC-MS chromatograms of organics before
and after ozonation
BDOC/DOC Ratio at Various Stages of the Treatment
Process
BDOC analysis results have provided evidence for
explaining the role of ozonation in improving the
biodegradability of organic matter. As shown in
Figure 2, although noticeable decrease has not
been found in the total concentration of DOC of
the water after ozonation, the BDOC/DOC ratio has
increased from 0.04 to 0.27, which indicates a
23 increase in the biodegradable portion of the
dissolved organic matter. Figure 2 also shows the
changes in the BDOC/DOC ratio after the ozonated
water has entered the BAC filter. At a filter
depth of 30 cm, the ratio drops to 0.17, and then
to 0.07 and 0.03 at 60 cm and 90 cm,
respectively. The BDOC/DOC ratio is as low as
0.01 in the finished water, indicating an almost
complete removal of the biodegradable portion of
the dissolved organic matter. The whole process
can achieve a total removal of DOC as about 50.
Figure 3 Profiles of biomass, NH3-N and
NOX-N along the BAC filter Layer
The process of low-concentration-ozonation and
biological activated carbon (BAC) filtration that
applied for tertiary treatment of domestic
wastewater for water reclamation can achieve a
very high removal of SS, colour, offensive odour,
and dissolved organic matter, resulting in high
quality treated water.
Figure 2 Comparison of BDOC/DOC ratio at
different stage
Conclusions
A low concentration ozonation can achieve an
effective conversion of organic functional groups
from cyclic hydrocarbons and aromatics into chain
hydrocarbons and aliphatic acids. This in due
increases the BDOC/DOC ratio showing a
significant improvement of the biodegradability.
Therefore a remarkable organic removal is
achievable by the subsequent BAC filter. The top
section of the BAC filter plays the main role of
organic removal and nitrification, while the
lower section of the filter performs the function
of denitrification. High quality treated water is
obtained by the low-concentration ozonation and
bio-filtration process.
Acknowledgement This study is supported by the
National Natural Science Foundation of China
(Grant No. 50138020)