IWA Poster Template

1
Treatability Evaluation of Domestic Wastewater
for a Rational Selection of Treatment Processes
for Water Reuse
Introduction
Rational selection of wastewater treatment
processes is important to meet the requirement on
treated water quality for various reuse purposes.
In this paper, the authors conducted a
fundamental study to classify the main impurities
in the raw domestic wastewater in Xian City,
China according to their physical and chemical
properties. On this basis, treatability
evaluation was conducted regarding the impurities
of each category through three kinds of typical
wastewater treatment processes, i.e. enhanced
primary treatment, secondary treatment and
advanced treatment.
Results Discussion
Classification of Impurities in Domestic
Wastewater
Treatability Evaluation of Impurities in
Domestic Wastewater
As shown in Figure 1, suspended solids (SS),
chemical oxygen demand (COD), biological oxygen
demand (BOD), total nitrogen (TN) and total
phosphorous (TP) were chosen as parameters
representing the main impurities encountered in
the raw domestic wastewater. Regarding each kind
of impurity, a 0.45mm filtration method was
applied to classify it into dissolved and
suspended matters and a thermal ignition method
was applied to classify it into organic and
inorganic matters. A matrix was thus formed to
show the composition of each kind of impurity. It
is noticeable that most of the BOD and COD are of
suspended state in the raw wastewater, as well as
half of TP and 20 of TN. Regarding SS, COD, TP
and TN, the inorganic parts take about 65, 30,
60 and 70 respectively.
Figure 3 shows the treatability of dissolved and
suspended matters of each category by three kinds
of treatment processes, where enhanced primary
treatment was done by jar test using
poly-aluminium chloride (PAC) as coagulant,
secondary treatment was done at an wastewater
treatment plant using an oxidation ditch process
and advanced treatment was done by granular
activated carbon adsorption with pre-ozonation
after the secondary treatment. Regarding the
suspended components, all the processes are
effective in the removal of almost all the
components except for suspended nitrogen where
enhanced primary treatment shows much lower
ability of nitrogen removal than the other two
processes. However, it is noticeable that
enhanced primary treatment is equally effective
in SS, COD(s), BOD(s), P(s) and NH3-N(s) removal
as the advanced treatment both of them can
almost completely remove these suspended
impurities. Regarding the dissolved matter,
enhanced primary treatment is less effective than
the secondary and advanced treatment for most of
the components. However, complete removal of the
dissolved phosphorous is only achieved by the
enhanced primary treatment. By GC-MS analysis,
dissolved organic matter were fractionated into
three groups according to their functional bonds.
As is shown in Figure 3, the enhanced primary
treatment seems to be effective to remove
aliphatic acids but less effective to remove
chain hydrocarbons and cyclic hydrocarbons.
Figure 1 Classification of Impurities in
Domestic Wastewater
HPLC Chromatograms for Molecular Weight Evaluation
Figure 2 is an evaluation of the molecular
distribution of dissolved organic matter before
and after the treatment by different processes
using high performance liquid chromatography
(HPLC) with UV detection at 254nm. The secondary
treatment, i.e. biological degradation, cannot
reduce the peaks of large organic molecules very
much. In contrary to this, by enhanced primary
treatment and advanced treatment most of the
organic matter with molecular weight larger than
1000 can be effectively removed. Several new
peaks appear in the smaller molecular range after
secondary treatment and advanced treatment. This
is believed to be resulted from biological
degradation and chemical oxidation by ozone where
larger molecules may be broken into smaller ones.
Figure 2 HPLC Chromatograms for Molecular
Weight Evaluation
Figure 3 Treatability of Various Suspended and
Dissolved Matters by Different
Treatment Processes
Conclusions
Impurities in the domestic wastewater are
classified according to their size and chemical
composition. More than 60 of the organic
substances and an amount of TP and TN in the
domestic wastewater are with a size larger than
0.45 mm showing the property of suspended matter.
Enhanced primary treatment can achieve almost
100 removal of the suspended impurities, part of
the soluble COD/BOD, and a complete removal of
the soluble phosphorous. Secondary treatment is
effective in removing most of the soluble
impurities. However, if very high quality
effluent is required, additional advanced
treatment should be applied.
Acknowledgement This study is supported by the
National Natural Science Foundation of China
(Grant No. 50138020)