The authors investigated the possibility to reduce aeration time in one of the cycles of sequencing batch activated sludge reactors. It is known that there are microorganisms in activated sludge which can store organic materials temporarily in such forms as polyhydroxyalkanoate (PHA). It was expected that removal of organic materials in the cycle with reduced aeration was supplemented by the microbial activities to store organic materials temporarily. The authors operated sequencing batch reactors with 6 cycles/day with synthetic wastewater, and reduced aeration in one of the cycles. Short-term experiments were conducted to see the effects of aeration reduction for one time, and long-term experiments were conducted to see the effect of long term implementation of operation with aeration reduction. In both experiments, removal of DOC was greater than 92%, and no significant adverse effect was observed. The more aeration was reduced, the more PHA was carried over to the following cycles. It was estimated that about 17% to 50 % of PHA was carried over to the cycles following the cycles in which aeration was reduced. The operation with one-cycle reduced aeration was successfully implemented in the experiments. There is a big room to explore wastewater treatment technologies in the direction to flexibly control energy consumption.
The effect of reduction of aeration period on organic pollutants removal in sequencing batch activated sludge reactors
1. 1
THE EFFECT OF REDUCTION OF AERATION
PERIOD ON ORGANIC POLLUTANTS REMOVAL
IN SEQUENCING BATCH ACTIVATED SLUDGE
REACTORS
Wei SHI*
, Hiroyasu SATOH and Takashi MINO
Graduate School of Frontier Sciences, The University of Tokyo
(5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan)
* E-mail: shiwei@mw.k.u-tokyo.ac.jp
The authors investigated the possibility to reduce aeration time in one of the cycles of sequencing batch
activated sludge reactors. It is known that there are microorganisms in activated sludge which can store
organic materials temporarily in such forms as polyhydroxyalkanoate (PHA). It was expected that
removal of organic materials in the cycle with reduced aeration was supplemented by the microbial
activities to store organic materials temporarily. The authors operated sequencing batch reactors with 6
cycles/day with synthetic wastewater, and reduced aeration in one of the cycles. Short-term experiments
were conducted to see the effects of aeration reduction for one time, and long-term experiments were
conducted to see the effect of long term implementation of operation with aeration reduction. In both
experiments, removal of DOC was greater than 92%, and no significant adverse effect was observed. The
more aeration was reduced, the more PHA was carried over to the following cycles. It was estimated that
about 17% to 50 % of PHA was carried over to the cycles following the cycles in which aeration was
reduced. The operation with one-cycle reduced aeration was successfully implemented in the
experiments. There is a big room to explore wastewater treatment technologies in the direction to flexibly
control energy consumption.
Key Words : reduced aeration, power consumption, temporary carbon storage capability
1. INTRODUCTION
The amount of energy used in water and
wastewater treatment corresponds to roughly 1% of
the total electricity consumption1)
. Wastewater
treatment represents 0.1 to 0.3% of total energy
consumption in the United States2)
. In Japan, about
0.3% of total electrical power is consumed for
wastewater treatment. With the worldwide resource
limitation and increase of energy price, the
operation and maintenance cost in wastewater
treatment plants (WWTPs) tend to increase as well.
Therefore, energy and cost reduction of wastewater
treatment should be of serious interest. Aeration in
biological treatment accounts for 50 ~ 80% of the
total electrical energy consumption in WWTPs2,3,4)
.
Due to this situation, there is also much potential to
reduce energy and cost for electrical power 2,5,6)
.
While reduction of total energy consumption is
of importance, it is also valuable to reduce peak
energy usage in certain situations. The Great East
Japan Earthquake on March 11, 2011, and the
accident of the Fukushima Daiichi Nuclear Power
Plant after the quake caused serious shortage of
energy supply capacity in Japan. In disasters such as
these, not only reducing total power consumption
but also minimizing power consumption during
peak time are important concerns.
Apart from emergency situations caused by
disasters, even under normal situations, electrical
power consumption shift may be beneficial. For
example, in Japan, electricity prices are cheaper at
nighttime, because demand is less at night.
Therefore ability to shift aeration from day to night,
would be beneficial in reducing costs.
Furthermore, in a society which depends more on
natural power such as wind and solar, the electrical
supply will be more unstable. Development of
土木学会論文集G（環境），Vol.69，No.7，III_427-III_434，2013.
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2. 2
technologies which can flexibly consume energy to
compensate for the instability of the energy supply
should be of importance.
On the other hand, it is known that some of
microorganisms in activated sludge have the ability
to remove organic matter anoxically; the organic
matter removed anoxically is accumulated in
microbial cells as temporary carbon storage
materials such as polyhydroxyalkanoates (PHA)7,8,9)
.
There are several studies on PHA in activated
sludge: some of them focused on the metabolism in
enhanced biological phosphorus removal
processes10)
, and others were conducted in a
direction to produce PHA as a biodegradable plastic
material. Recently, a new research direction has
been proposed by Oshiki et al.11)
and Huda et al.12)
.
They proposed the ‘Final AeRation of Excess
sludge With Excess Loading’ (FAREWEL) process,
in which organic matter in wastewater is fixed to
activated sludge as PHA and then further converted
to methane gas.
Here, the authors propose another approach to
use PHA to improve energy efficiency in
wastewater treatment: aeration during daytime is
reduced and removal of organic pollutants is
achieved by the capability of microorganisms to
accumulate temporary carbon storage materials such
as polyhydroxyalkanoate (PHA). The temporary
carbon storage materials accumulated in daytime are
oxidized later.
In the present study, the authors conducted
preliminary experiments to see the feasibility of
operation with one-cycle reduced aeration.
Sequencing batch activated sludge reactors were
operated with 6 cycles a day using synthetic
wastewater. In the short-term experiments, aeration
was reduced in one of the 6 cycles, and the
behaviour of effluent quality and temporary carbon
storage material PHA were monitored in the cycle in
which aeration was reduced and the following cycle.
In the long-term experiments, reactors were
operated with one-cycle reduced aeration per day for
about a month to evaluate the effect of long term
implementation.
2. MATERIALS AND METHODS
(1) Activated sludge reactor
Two sequencing-batch activated sludge reactors
(SBR-A and SBR-B) were operated in parallel.
Each SBR had a working volume of 10L and was
fed with synthetic wastewater containing:
CH3COONa·3H2O 135.6 mg/L, CH3CH2COONa
64.4 mg/L, peptone 120 mg/L, yeast extract 24
mg/L, KCl 50.4 mg/L, CaCl2·2H2O 15.8 mg/L,
MgSO4·7H2O 132 mg/L, K2HPO4 43.2 mg/L and
trace element solution containing B, Fe, Mn, Mo,
Co and Cu. The influent dissolved organic carbon
(DOC) concentration was around 120 mgC/L and
chemical oxygen demand (CODCr) was around
300mg/L. Each cycle was 4hr in total with
following phases under normal operation: inflow
and anaerobic reaction for 60min, aerobic reaction
for 120min, settling for 53min, and effluent
discharge for 7min. In each cyle, 240mL of mixed
liquor was discharged to maintain the sludge
retention time (SRT) to be around 7 days. In each
cycle, 5 L of influent was fed and 5 L of treated
water was discharged to maintain the hydraulic
retention time (HRT) at 8 hr.
Dissolved oxygen (DO) concentrations were
monitored and controlled with DO meters (DO-21P,
DKK-TOA, Japan) in combination with a program
developed on LabView (National Instruments,
USA). The DO concentrations were kept between
2.3mg/L and 2.5mg/L during aerobic reaction
phases. For the monitoring of pH, pH meters
(WM-22EP, DKK-TOA, Japan) were used. The
reactors were installed in an air-conditioned room
with an ambient temperature at around 26˚C.
The experiments conducted with the SBRs are
tablated in Table 1. The reactors were operated for
170 days in parallel. Runs A-1 and B-1 were seeded
with the same activated sludge from a laboratory
reactor which had been acclimatized in the
laboratory of the authors for longer than half a year.
At the end of Runs A-1 and B-1, the settleability
was deteriorated, and the sludge was discarded.
Runs A-2 and B-2 were started with stored excess
sludge from Runs A-1 and B-1 which had been
collected before the deterioration of settleability.
Activated sludge in the two reactors were mixed,
divided into two, and then used as the seeed for
Runs A-2 and B-2. Similarly, Runs A-3 and B-3
were started with the mixture of activated sludge
collected from Runs A-2 and B-2 as excess sludge.
Short-term experiments were conducted in Runs
A-1 and B-1 to examine the effect of aeration
reduction in one of the six cycles for one day only.
The extents of aeration reduction were by 50% (day
12), 70% (day 20), 90% (day 25), or 100% (day 30),
and the same aeration reduction conditions were
applied to both SBRs in parallel. In the cycle with
reduced aeration, the anaerobic period was extended
to maintain the cycle time at 4 hours. Operation of
the reactors under normal conditions and when
aeration was reduced are illustrated in Fig. 1.
Aeration was reduced in aerobic phage. It is called
50% reduced aeration if the aeration period is
shortened from 2hr to 1hr. Similarly, 70%, 90%,
100% reduced aeration means the aeration period
was shortened to 36min, 12min and 0min,
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3. 3
respectively. These perecentage values ‘50%, 70%,
90%, 100%’ are defined as ‘reduced aeration levels’
(RA Levels). In the cycles with aeration reduction,
anaerobic reaction time was extended to maintain
the duration of the cycle at 4 hours.
Table 1 Operational condition
Period
Run Numbers
Outline
SBR A SBR B
day
1-47
Run
A-1
Run
B-1
[Short-term experiments]
day 12: 50% aeration reduction with
Run A and B
day 20: 70% aeration reduction with
Run A and B
day 25: 90% aeration reduction with
Run A and B
day 30: 100% aeration reduction with
Run A and B
day
48-109
Run
A-2
Run
B-2
[Long-term experiment]
Run A-2 : control
Run B-2 :
day 48-70 normal operation
day 71-96 50% aeration reduction
in one of the 6 cycles per day
day 97-109 normal operation
day
110-170
Run
A-3
Run
B-3
[Long-term experiment]
Run A-3 :
day 110-140 normal operation
day 141-170 50% aeration reduction
in one of the 6 cycles per day
Run B-3 :
day 110-140 normal operation
day 141-170 70% aeration reduction
in one of the 6 cycles per day
Samples were taken at different timings in the
cycles with reduced aeration (RA cycles) and in the
cycles following the RA cycles (PostRA cycles).
The first, second, and n’th cycles following the RA
cycle are referred to as PostRA1 cycle, PostRA2
cycle, and PostRAn cycle, respectively.
The long-term experiments were conducted in
Runs A-2, B-2, A-3 and B-3. Run A-2 was operated
under normal condition throughout, while in Run
B-2, normal aeration was conducted from day 48 to
day 70 and then operation with one-cycle reduced
aeration was introduced from day 71 to day 96, in
which aeration time in one of the 6 daily cycles was
reduced by 50%. After day 97, both Runs A-2 and
B-2 were operated under normal conditions. Runs
A-3 and B-3 were operated under nomal conditions
until day 140 (30 days after start of Runs A-3 and
B-3), and then operation with one-cycle reduced
aeration was introduced. Aeration time in one of the
6 cycles of a day was reduced by 50% and 70% in
Runs A-3 and B-3, respectively. Samples were
taken at the end of the aerobic phases in the cycles
with reduced aeration (RA cycles) and the cycles
following them (PostRA cycles).
Fig.1 Experimental Conductions
(2) Analytical methods
The dissolved organic carbon (DOC)
concentration in the supernatant was measured with
a TOC Analyzer ( TOC-Vcsn, Shimadzu, Japan).
The concentrations of mixed liquor suspended solids
(MLSS) were measured according to the Standard
Methods13)
. Analysis of PHA was done by gas
chromatography after methanolytic
decomposition14)
.
3. RESULTS
(1) Reactor performance
The concentrations of MLSS in Reactors A and B
during the whole experimental period were between
1,000mg/L and 3,000mg/L, as shown in Fig. 2.
Settling properties were not very good especially in
Reactor B, as can be seen from the SV30 values in
Fig. 2. In Runs A-2, B-2, SV30 values were initially
at the same level when operation with one-cycle
reduced aeration was introduced to Run B-2 on day
71. Then, while SV30 of Run A-2 was improved,
that of Run B-2 stayed at the same level. In Run
A-3 and Run B-3, SV30 increased after the
introduction of operation with one-cycle reduced
aeration, but the extent of the increase was higher in
Run B-3 than in Run A-3. In Run B-3, biomass was
washed out after day 160, and the experiment had to
be stopped in the end.
(2) Short-term experiments
Figure 3 shows the DOC concentrations during the
short-term experiments. Both in Reactors A and B,
DOC concentrations reached less than 8mgC/L at
the end of the aerobic phase regardless of reduction
of aeration. In the cycles following the RA cycles,
DOC concentrations at the end of aerobic phase
4. 0 1 2 2.4 2.8 3 4
hour
RA cycle PostRA1 PostRA2 PostRA3 PostRA4 PostRA5 cycle
Reduced Aeration Normal Aeration
Normal Aeration:
Feeding
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5. 4
Fig.3 DOC Concentration in short-term experiments
were less than 5 mgC/L, which were as low as those
in normal conditions.
Figure 4 shows the DOC and PHA
concentrations in RA cycles and PostRA cycles in
the short-term experiments. DOC concentrations
decreased rapidly from the beginning both in reactor
A and reactor B. As the influent DOC concentration
was 120 mgC /L, the initial DOC concentration was
60mgC/L. Removal of DOC remained higher than
92% in all the monitored cycles. When aeration
reduction was 50% and 70%, DOC removal was not
affected, and even when aeration reduction was 90%
or 100%, adverse effects on DOC removal were not
very remarkable. In the cycle following the RA
cycle, DOC removal remained at the usual level
(97%±1%).
As a result of the short-term experiments, effects
of aeration reduction up to 100% were examined
with synthetic wastewater. Omission of aeration did
not badly affect the effluent DOC in the short-term
7. Run A-1
Run B-1
:
day 12
50%RA
day 20
70%RA
day 25
90%RA
day 30
100%RA
Fig.2 MLSS and SV30 in reactor A and B during the
whole period
Fig.4 DOC and PHA Concentration in short-term experiments
9. 50%
70%
90%
100%
RA level:
b) Run B-1:
DOC
a) Run A-1:
DOC
c) Run A-1:
PHA
d) Run B-1:
PHA
Anaerobic Aerobic Settling
PostRA1 cycle
Anaerobic Aerobic Settling
Time:
Condition:
hr
Here, A,B is reactor A or reactor B; 50%, 70%, 90%,
100% is RA level. For example, A-50% means 50%
reduced aeration experiment in reactor A.
Calculated initial DOC concentration
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10. 5
experiments.
In the anaerobic phase of RA cycles, rapid
accumulation of PHA was observed. The
consumptions of accumulated PHA were small in
the aerobic phase of the RA cycle, and
concentrations of PHA were further increased
during the anaerobic phase of the PostRA1 cycle.
The accumulated PHA was consumed quickly in the
aerobic phase of the PostRA1 cycle.
More PHA was accumulated in the anaerobic
phases of the PostRA cycles, which indicates that
there was still room for microorganisims to
accumulate PHA in the RA cycles. At the end of the
PostRA cycles, PHA concentrations returned
quickly to the normal level, which indicates
activated sludge consumed most of stored carbon
and renewed their ability to accumulate PHA in the
next reduced aeration cycle.
The level of PHA remaining at the end of RA
cycles and the PostRA cycles was compared in
Figure 5. Higher levels of aeration reduction
resulted in more accumulation of PHA. In the case
of 50% reduction, it returned to the original level.
The more the aeration was reduced, the more PHA
remained unutilized and carried over to the next
cycle(Fig. 5,a). In Fig. 5(b), the amount of PHA
accumulated in sludge at the end of the RA cycles is
shown. PHA content in sludge was less than 5 %
even when RA level was higher than 90%.
Fig.5 PHA Concentrations at the end of RA cycles and PostRA
cycles with aeration levels
(3) Long-term experiments
Figure 6 shows the DOC and PHA concentrations
at the end of the RA cycles and the PostRA cycles
in the long-term experiments.
As are shown in Fig. 6 (a, c, e, g), the
concentrations of DOC were almost less than 6
mg/L during a one-month-long operation with
one-cycle reduced aeration every day. As the
influent DOC concentration in the experiment was
120mgC/L, the DOC removal was more than 95%
in all monitored cycles.
Figure 6 (b, d, f, h) shows the PHA
concentrations during the whole long-term
experiments. During the operation with one-cycle
reduced aeration, the PHA concentrations at the end
of the RA cycles tended to be significantly higher
than those at the end of the PostRA cycles. This
means that carbon temporarily stored in microbial
cells in the RA cycles was carried over and then
oxidized in the PostRA cycles.
4. DISCUSSION
In the present study, effects of the introduction of
operation with reduced aeration for one cycle only
and for a long period of time were investigated. The
short-term experiments were to simulate the effects
of aeration reduction under accidental shortage of
energy supply for a short period of time. The
long-term experiments were to simulate shortage of
energy supply for a longer period of time and also to
take the advantage of cheaper electricity price
during night. As far as DOC removal is concerned,
operation with one-cycle reduced aeration in the
present study was successful. The carbon which
could not be oxidized in the cycle with aeration
reduction was carried over to the PostRA cycles and
then oxidized, as can be seen in Figs. 4 and 6.
Based on the results shown in Fig. 6 (d, f and h),
the concentrations of PHA at the end of the RA
cycles were reduced during the PostRA cycles by 10
to 30 mgC/L. As the concentration of the influent
synthetic wastewater was 120mgC/L and was
diluted by half as the return sludge ratio was 50% in
the present study, it is thought that about 17% to
50% of carbon was carried over from RA cycle to
PostRA1 cycle.
In Fig. 5(a), it was shown that higher level of
aeration reduction resulted in more accumulation of
PHA. This means that PHA play an imoprtant role
in carbon removal in the operation with aeration
reduction. Yet, PHA accumulation was not
significant in the experiments with 50% aeration
reduction. This may be because most of temporarily
acumulated carbon was oxidized within the
!
15. 6
shortened aeration time, or may simply mean that
temporary storage in other forms than PHA was
more important under lower RA levels.
Additionally, Fig. 5(b) indicates PHA content in
activated sludge increased as RA level was
increased, and that increase was less at RA levels
higher than 90%.
In the present study, the effect of reduction of
aerobic period of anaerobic/aerobic SBRs treating
synthetic wastewater on the treatment performance
of DOC was investigated. The efficiency of the
aeration reduction should be evaluated from the
view of the reduction of electrical power
consumption.
It should be noted that 50% reduction of aerobic
time, for example, does not mean 50% reduction of
Fig.6 DOC and PHA Concentration in long-term experiments
16. a) Run A-2:
DOC
b) Run A-2:
PHA
DOC was monitored
from day 60 and PHA
was measured from day
70 when operation with
reduced aeration was
introduced in Run 2
(from day 48 to day 109)
17. Normal condition Reduced Aeration condition
c) Run B-2:
DOC
d) Run B-2:
PHA
e) Run A-3:
DOC
f) Run A-3:
PHA
g) Run B-3:
DOC
h) Run B-3:
PHA
:
:
DOC was monitored
from day 60 in Run 2.
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18. 7
aeration energy. Indeed, the amount of carbon
carried over to PostRA1 cycle was less than 50% of
carbon from influent, and this mean more than 50%
of carbon was oxidized during the RA cycles. It
would be necessary to more precisely evaluate the
efficiency of energy reduction in the aeration
reduced cycles. In addition, it would also be
necessary to evaluate the extra amount of energy
which is needed to oxidize carbon carried over from
the RA cycles to PostRA cycles.
However, the authors would like to discuss about
the effect of aeration time on energy consumption
under an assumption that energy consumption is
proportional to aeration time. For example, in the
case of 50% aeration reduction in one of the 6
cycles of a day is assumed to result in reduction of
50% of aeration energy in the RA cycle, and 8.3%
within a whole day. Under the above assumption,
reduction of energy consumption during peak period
was evaluated on a wastewater treatment plant
which has four SBRs each operated with 4-hr cycles
consisting of 1-hour feeding and anaerobic mixing,
2-hour aeration, and 1-hour settling and discharge.
In this simulation as shown in Fig. 7, energy for
aeration is thought to be reduced by 50% for 4 hrs in
day time. Reduction of electrical consumption by
50% would be a great contribution to reduce the
electrical power demand during peak time of urban
activities.
On the other hand, the authors observed adverse
effect of aeration reduction on the settleability of the
sludge. That is, during Runs A-2 and B-2, while
settleability of Run A-2 (without aeration delay)
was improved, that of Run B-2 (reduced aeration of
50%) maintained high SV30. During Runs A-3
(reduced aeration of 50%) and B-3 (70%),
settleability became worse only in Run B-3.
The authors found filamentous microorganisms
in the sludge when SV30 became higher. The
reduction of settleability was caused by the
overgrowth of filamentous microorganisms. The
effects of aeration reduction on filamentous
microorganisms and bulking requires further study.
There are other concerns. One of them is the
removal efficiency of nutrients. Aeration reduction
can be first focused on the application of such
treatment plants which do not have to remove
nutrients. Treatment plants located along open
ocean satisfy such condition. Another concern is if
results aquired with synthetic wastewater can be
applied to real wastewater. To respond to this
concern, similar experiments should be done with
real wastewater. Further, this study was conducted
with sequencing batch reactors. The way to apply
the aeration reduction to continuous reactors should
be studied.
While there still are many concerns, attempts to
develop wastewater treatment technology in which
energy consumption can flexibly be adjusted would
be of great use, due to limitation of energy supply,
and potential future disasters. If public works such
as wastewater treatment can reduce energy demand
by such technologies as aeration reduction, extra
electrical energy can be directed to other sectors
which require constant supply of energy.
5. CONCLUSION
The efficiency of operation with reduced aeration
was evaluated with short-term experiments and
long-term experiments.
In the short-term experiments, DOC removal was
maintained above 92% in all monitored cycles and
was not badly affected by reduced aeration. PHA
concentration variations in RA cycle and PostRA1
cycle showed a periodic storage-consumption cycle
during anaerobic and aerobic phases. The
concentrations of PHA in sludge increased as a
result of aeration reduction and reduced in the
following cycles. The more aeration was reduced,
the more PHA was carried over to the following
cycles.
In the long-term experiments, DOC removal was
higher than 95% in a one-month run with either
50% or 70% reduced aeration. Based on the
Fig. 7 Time-shifting of power consumption by 50% reduced
aeration in a 4-tank-plant
21. 8
differences of PHA concentrations between RA
cycles and PostRA cycles, it was estimated that
about 17% to 50 % of carbon stored in microbial
cells in the RA cycle was carried over to PostRA1
cycle.
In this study, the authors successfully
demonstrated the concept of operation with
one-cycle reduced aeration. There are good
opportunities to explore wastewater treatment
technologies reated to flexible control of energy
consumption.
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(Received May 24, 2013)
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