Improving Nutrient Removal While Reducing Energy Use at Three Swiss WWTPs Using Advanced Control

Abstract

Aeration consumes about 60% of the total energy use of
a wastewater treatment plant (WWTP) and therefore is a major
contributor to its carbon footprint. Introducing advanced process
control can help plants to reduce their carbon footprint and at the
same time improve effluent quality through making available unused
capacity for denitrification, if the ammonia concentration is below
a certain set-point. Monitoring and control concepts are cost-saving
alternatives to the extension of reactor volume. However, they also
involve the risk of violation of the effluent limits due to measuring errors,
unsuitable control concepts or inadequate implementation of the
monitoring and control system. Dynamic simulation is a suitable tool
to analyze the plant and to design tailored measuring and control
systems. During this work, extensive data collection, modeling and fullscale
implementation of aeration control algorithms were carried out at
three conventional activated sludge plants with fixed pre-denitrification
and nitrification reactor zones. Full-scale energy savings in the range of
16–20% could be achieved together with an increase of total nitrogen
removal of 40%.

Full text

Improving Nutrient Removal While Reducing
Energy Use at Three Swiss WWTPs Using
Advanced Control
Leiv Rieger
1,2
*, Imre Taka´cs
3
, Hansruedi Siegrist
1
ABSTRACT: Aeration consumes about 60% of the total energy use of
a wastewater treatment plant (WWTP) and therefore is a major
contributor to its carbon footprint. Introducing advanced process
control can help plants to reduce their carbon footprint and at the
same time improve effluent quality through making available unused
capacity for denitrification, if the ammonia concentration is below
a certain set-point. Monitoring and control concepts are cost-saving
alternatives to the extension of reactor volume. However, they also
involve the risk of violation of the effluent limits due to measuring errors,
unsuitable control concepts or inadequate implementation of the
monitoring and control system. Dynamic simulation is a suitable tool
to analyze the plant and to design tailored measuring and control
systems. During this work, extensive data collection, modeling and full-
scale implementation of aeration control algorithms were carried out at
three conventional activated sludge plants with fixed pre-denitrification
and nitrification reactor zones. Full-scale energy savings in the range of
16–20% could be achieved together with an increase of total nitrogen
removal of 40%. Water Environ. Res.,84, 170 (2012).
KEYWORDS: Biological nutrient removal, aeration control, activated
sludge model, energy savings, denitrification, control system design,
ammonia control.
doi:10.2175/106143011X13233670703684
Introduction
Aeration contributes about 60% to the total energy consump-
tion of WWTPs (Ingildsen, 2002). Reducing aeration by
innovative control to the minimum level required to guarantee
a desired effluent quality helps saving costs and significantly
reduces the carbon footprint of the plant. Costs savings however
must not increase the risk of effluent quality violations in terms
of total nitrogen and phosphorus content nor must it increase
the carbon footprint of the plant through elevated greenhouse
gas emissions.
The use of online sensors for automated process control
has been the topic of various studies since the early seventies
(compare Ingildsen, 2002; Olsson and Newell, 1999); first with
a focus on DO control and later also on biological nutrient
removal (Ingildsen, 2002; Olsson et al., 2005; Yuan et al., 2002).
Although several publications claimed that the reliability of
sensors improved significantly (e.g., Jeppsson et al., 2002), data
quality and signal availability can still be seen as the limiting
factor for the use of process control in practice. Worldwide, the
number of full-scale implementations of advanced process
control in wastewater treatment is still scarce (Ingildsen, 2002;
Jeppson et al., 2002; Palmer et al., 2007; Weijers, 2000).
During the present work, extensive measuring campaigns,
simulation studies and full-scale implementations of aeration-
control concepts were carried out. All three WWTPs evaluated
are conventional activated sludge plants with fixed volumes for
pre-denitrification and nitrification and cover a range of plant
sizes from 35,000 population equivalents (p.e.) (13,000 m
3
/d),
130,000 (40,000 m
3
/d) and 600,000 p.e. (205,000 m
3
/d). This
should guarantee the applicability of the study to other plants.
The primary goal of this project was to demonstrate that
increasing nitrogen removal in parallel with a reduction of the
energy consumption is viable using control technology. The
excess capacity recovered in the optimized nitrification process,
particularly during summer time, is used to introduce anoxic or
even anaerobic phases and therefore allows increased biological
nitrogen and phosphorus removal. Thanks to the increased
biological phosphorus removal, chemical doses can be reduced
and the sludge production decreases. The objectives were to test
different control strategies at a variety of full-scale plants to find
the best control concepts for specific conditions.
Procedure
A general procedure for design and implementation of
process control strategies (Figure 1) was followed in all three
case studies. The objectives should be defined together with the
responsible plant manager and the operator-in-chief. Data
quality evaluation is essential for a successful design and should
begin directly after the start-up of the optimization study and
should accompany all measuring campaigns. The dynamic
model provides the platform to design, evaluate, adapt and
finally select the optimal control concept(s), which will then be
implemented and tested. Finally, it is proposed to evaluate the
success on the basis of a cost-benefit calculation.
Definition of Objectives. The goals of the studies were to
increase the nitrogen removal in parallel with a reduction of
energy consumption. The constraints were to keep the effluent
1
Eawag, Swiss Federal Institute of Aquatic Science and Technology,
Environmental Engineering, Ueberlandstr. 133, PO Box 611, CH-8600
Duebendorf, Switzerland; e-mail: hansruedi.siegrist@eawag.ch.
2
*EnviroSim Associates Ltd., McMaster Innovation Park, 175 Longwood
Rd S, Suite 114A, Hamilton, Ontario, L8P 0A1, Canada; e-mail:
rieger@envirosim.com. Corresponding author.
3
EnviroSim Europe, 15 Impasse Faure
´, 33000 Bordeaux, France; e-mail:
imre@dynamita.com.
170 Water Environment Research, Volume 84, Number 2

concentrations below the specified regulation limits. The Swiss
ammonia limit of 2 mg N/L in the 24-hour composite sample
was the main boundary for the aeration control concepts tested.
Priority was on nitrogen removal, but if enough free capacity
exists in the plant, it should be used for enhanced biological
phosphorus removal (Bio-P) to reduce the dosage of P-
precipitants. The reduction in sludge production leads to lower
sludge disposal costs.
Special emphasis was placed on the implementation of the
measuring and control system. This required understandable
and adaptable control concepts, suitable sensors with low
maintenance requirements and tools for monitoring the data
quality of the measuring devices.
Data Quality Evaluation and Control. Good data quality is
essential for reliable modeling results as well as for effective
control systems. A concept for data collection, quality evalua-
tion, and reconciliation in optimization studies (Rieger et al.,
2010; see Figure 2) was used in this work: After analyzing
existing data and locating gaps in routine measurements, a few
additional measurements were taken to calculate mass balances
on the basis of redundant information. The combination of
different mass balances allowed systematic measuring errors to
be detected, isolated, and identified (Thomann, 2008). Addi-
tional experiments were carried out to obtain values for the
precision of the equipment and analytical methods and to
evaluate the response times of the measuring devices (Rieger
et al., 2003) and the actuators (Rieger et al., 2006). Intensive
measuring campaigns were planned to include redundant
information for quality checks (e.g., mass and flow balances).
The implementation and success monitoring data sets were
evaluated for data quality too.
Simulation-supported Controller Design. The goals of
process control in wastewater treatment can be summarized as
follows:
NTo meet effluent discharge requirements.
NTo achieve good disturbance rejection.
NTo optimize operation to minimize the operating cost.
In this work the main goal was to reduce operating costs
(including effluent taxes) with the constraint of meeting the
effluent limit requirements. The behavior of the control system
during peak loading was tested and special feed-forward
controllers designed to increase the control authority against
influent variability.
The specific control goal is to keep the ammonia concentra-
tion at the end of the aerated zone near a defined set-point. This
is done by increasing or decreasing the amount of air which is
blown into the reactor. Due to the Monod dependence, the
modeled nitrification rate reaches ca. 80% of the maximum at
DO concentrations of 2 mg/L (depending on the half saturation
constant, which in this study was chosen as 0.5 mg/L).
Consequently, the ammonia control loop has been put on top
of the conventional DO control loop. This guarantees DO
concentrations below a limit of 2–3 mg O
2
/L, even if the
ammonia is above the set-point. Below the ammonia set-point,
the oxygen input will be decreased and capacity for an enhanced
denitrification arises in parallel with a reduction in energy
consumption.
Whether the oxygen input can be switched off completely or
only reduced to a minimum depends on the kind of aeration
system used. If ceramic aerators are used, a minimum airflow
Figure 1—Procedure to design and implement process
control strategies.
Figure 2—Concept of data collection, evaluation, and reconciliation for simulation studies (Rieger et al., 2010).
Rieger et al.
February 2012 171

rate has to be maintained, whereas membrane aerators can
normally deal with the air supply being completely switched off.
To guarantee a sufficient degree of mixing, additional mixers
have to be installed or an overlying control scheme has to re-
start aeration after a certain period of time. The length of the
unaerated interval depends on the sludge settling behavior and
should be selected based on an experimental analysis.
If the ammonia sensor is installed at the end of the aerobic
zone (feed-back concept), the signal may be too late for a control
action to manage peak loading. At plants with a highly dynamic
influent combined with plug flow behavior and small reactor
volumes, this can lead to violation of the effluent limits. This
effect is increased if sensors with a high response time (.30
min.) are used (Alex et al., 2003). One solution is to combine
feed-back and feed-forward concepts. An additional ammonia
sensor upstream of the aeration zone (e.g., in the primary
effluent or in one of the unaerated reactors) provides the
required advance signal to prepare the plant for a peak load.
The evaluated aeration control concepts enhance the existing
DO control loop (Figure 3) by an ammonia control loop. The
following basic concepts have been tested:
NOn-off control of the blowers based on the ammonia
concentration (Figure 4)
NDO set-point adjustment based on the ammonia concen-
tration (Figure 5)
NCombination of feed-back and feed-forward control using
a maximum condition (Figure 6).
Feed-forward Controller. On the basis of an additional
ammonia sensor in the effluent of the primary clarifier and
a flow measurement, the feed-forward controller calculates the
time when full aeration is necessary. A simplified model is used
which compares the ammonia load in the effluent of the primary
clarifier with an estimated nitrification capacity (eq 1).
nitstate~LNH4,Inf
rnit:VolBR
ðÞ ð1Þ
where:
L
NH4,Inf
5Measured NH
4
-N influent load [g/d]
r
nit
5Nitrification rate [g/(m
3
*d)]
Vol
BR
5Volume aerated bio-reactors [m
3
]
The nitrification rate is estimated according to eq 2 for the
aeration strategy with the lowest air demand (this could mean
intermittent aeration or not all tanks aerated).
rnit~XANO :mmax,T:taer
YANO ð2Þ
where:
X
ANO
5Concentration of nitrifiers (from steady state
and low aeration strategy, see Eq. 3) [g/m
3
]
m
max,T
5Actual max. growth rate at given temperature
5m
max
* exp(0.105*(T-20uC)) [1/d]
m
max,20
51.0 [1/d] (Max. growth rate at 20uC)
Y
ANO
50.24 [g COD/g N] (Autotrophic yield)
t
aer
50.15 [-] (reduction factor for intermittent
aeration and low DO conc.)
T5Actual temperature [uC] (in this study 15uCis
used)
The concentration of nitrifiers depends on the mean influent
load of ammonia (eq 3):
Figure 3—DO control loop.
Figure 4—DO control loop +ammonia on-off control.
Figure 5—DO +ammonia control loops.
Figure 6—DO +ammonia feed-forward/feed-back control.
Rieger et al.
172 Water Environment Research, Volume 84, Number 2

XANO~LNH 4,removed :YANO:SRT
VolBR:1zbANO,T:SRTðÞ
ð3Þ
where:
L
NH4,removed
5Mean NH
4
-N influent load 2
NH
4
feed-back controller set-point [g/d]
SRT 5Sludge retention time [d]
b
ANO,T
5Max. decay rate at given temperature
5b
ANO,20
* exp(0.105*(T-20)) [1/d]
b
ANO,20
50.2 [1/d] (Max. decay rate at 20uC)
If the load is higher than the nitrification capacity, increased
aeration is triggered after a delay time. The delay is necessary
to take the hydraulic retention time in the anoxic zone into
account. The combination of feed-forward and feed-back is
implemented by a selector using a maximum condition, that is
the controller with the higher air demand is selected.
Simulation studies. The plant models were calibrated and
validated on the basis of intensive measuring campaigns at
WWTP Morgental and WWTP Thunersee. In addition, the
hydraulic behavior of the plants was evaluated by tracer
experiments using sodium bromide. For WWTP Werdhoelzli,
a representative data set was created by analyzing the available
routine data and 11 diurnal variations. The simulation studies
were carried out to evaluate the potential of the plants and to
design, test and finally select the best controller for each specific
plant.
The evaluation of the simulation results at WWTP Thunersee
was based on the existing effluent load tax of the canton of
Berne, whereas a slightly modified effluent load tax is used for
the studies at WWTP Morgental and WWTP Werdhoelzli,
where no official tax exists.
Effluent quality tax in the canton of Berne (KGSchG, 1996):
NNH
4
-N 54 CHF/kg N (<4$/kg N)
NNO
3
-N 51 CHF/kg N (<1$/kg N)
NPO
4
-P 530 CHF/kg P (<30$/kg P)
Effluent quality tax (used at WWTP Morgental and WWTP
Werdhoelzli):
NNH
4
-N 51 $/kg N (for concentrations ,2 mg N/L)
NNH
4
-N 54 $/kg N (for concentrations .2 mg N/L)
NNO
3
-N 51 $/kg N
NPO
4
-P 530 $/kg P
The energy costs were calculated on the basis of 0.15 $/ kWh.
The costs of the phosphorus precipitants were set to 3 $/kg P
precipitated and the costs for the precipitation sludge disposal to
7 $/kg P.
Full-scale Implementation. On the basis of the simulation
studies, the selected control concepts were adapted to several
constraints of the plants and implemented on a full scale at
WWTP Morgental and WWTP Thunersee. Due to financial
restrictions, no additional mixing devices were installed in the
normally aerated zones, and consequently the sludge settled
during phases without aeration. To prevent problems with
permanently settled sludge, the settling behavior was tested and
as a result the maximum duration of non-aerated phases were
set to 20–30 min in both full-scale implementations followed by
a short re-aeration phase.
Control of Success. Measuring campaigns were carried out
at WWTPs Morgental and Thunersee to validate the simulation
results. The cost-benefit calculations included annual costs for
additional investments and operation of the control systems
under evaluation and compared them against savings in terms of
energy and chemicals consumption, sludge disposal and effluent
quality (based on the effluent tax).
Case Studies
Two full case studies were carried out for WWTP Morgental
and WWTP Thunersee. For WWTP Werdhoelzli, two simula-
tion studies on ammonia-based control and the implementation
of the AI-process (alternating influent, intermittent aeration)
scheme were performed during this work.
WWTP Werdhoelzli. WWTP Werdhoelzli treats the
wastewater from the city of Zurich and several nearby villages.
With a current capacity of 550,000 p.e. (205,000 m
3
/d), WWTP
Werdhoelzli is the largest treatment plant in Switzerland and
must remove substantial quantities of nitrogen because the
receiving river Limmat flows into the Rhine with special
demands on nitrogen limits. The plant was initially designed
for nitrification only. In 1997 two anoxic compartments
comprising 28% of the total volume were implemented in each
of the twelve aeration tanks (Figure 7). This allowed 55–60% of
the nitrogen to be removed (Siegrist et al., 2000).
Definition of Objectives. WWTP Werdhoelzli expressed
interest to further improve nitrogen removal and reduce
aeration energy. This work on ammonia-based control is part
of a major plant upgrade planning, which also includes separate
treatment of digester supernatant. Therefore, scenarios for
separate treatment and controlled dosing of the supernatant
were included in this study.
Data Quality Evaluation and Control. Since only a relative
comparison is to be made, only basic data quality control was
carried out. The typical weekly variation used is based on the
analysis of the routine measurements of the plant laboratory of
the years 2000 and 2001 and eleven measured diurnal
variations.
Simulation-supported Controller Design. A simulation study
for WWTP Werdhoelzli was carried out to compare the costs of
the aeration energy, the phosphate precipitation and an effluent
quality tax for different aeration control concepts. An important
Figure 7—Process scheme WWTP Werdhoelzli (one of twelve lanes).
Rieger et al.
February 2012 173

cost factor is the reduction of the phosphate precipitants and
the sludge production due to enhanced biological phosphate
removal (Bio-P). The ASM3 with the EAWAG Bio-P module
(Rieger et al., 2001) was selected as the biological process model.
Only a basic calibration of the biological model was carried out.
The energy consumption and amount of air used were calibrated
based on a six-month data set with daily averages.
Besides the current DO control system operating at 15uC, four
main versions were investigated (Table 1). Version 1 includes
a feed-back control of the DO set-point based on ammonium
sensors at the end of the aeration tanks, whereas V1a comprises
additional oxygen feed-back control of the valve serving the last
two aeration zones. Version 2 combines the feed-back control of
version 1a with a feed-forward control based on an ammonium
sensor in the primary effluent. Version 3 comprises of
a controlled dosage of digester supernatant based on the NH
4
load in the primary effluent and version 3a is a separate
treatment of the supernatant.
The feed-forward controller was simulated according to
equations 1 to 3. Table 2 shows the step-wise increase in
aeration intensity for the feed-forward controller for WWTP
Werdhoelzli. Note that only a ratio of 1.4 (NH
4
load/nitrification
capacity) triggers an action by the feed-forward controller. This
is due to the fact that the disturbances should mainly be dealt
with by the feed-back controller. The feed-forward controller
should provide a fast reaction to sudden ammonia peaks in the
influent.
The simulated results show that there is an enormous
potential for measuring and control technology to reduce
operating costs and simultaneously improve effluent quality.
The best control version without separate supernatant treat-
ment (V3) reduces the energy cost by 25% and chemical
addition by more than 50% compared to the current version. If
an effluent load tax is also taken into account, a total cost
reduction of about 30% in relation to the actual situation can be
attained. Looking at the benefits, the savings in energy of about
$425,000/yr greatly exceed the additional annual costs of about
$50,000 for the measuring and control system and its operation.
Net savings increase to about $1,400,000/yr if the effluent tax
($225,000/yr) and the reduction of precipitants and sludge
disposal ($800,000/yr) due to the biological P-removal are
included (Figure 8).
The version (V3a) with separate digester supernatant
treatment (Anammox) increases net energy savings by another
$80,000/yr (difference of aeration energy for the oxidation of
300 t NH
4
/yr supernatant separately instead of in the activated
sludge treatment). Total net savings then increase to about
$2,400,000/yr (effluent tax: $550,000/yr; precipitants and
sludge reduction $1,300,000/yr due to biological P-removal)
from which about $400,000/yr have to be deducted for
separate supernatant treatment (Siegrist et al., 2004). The
total nitrogen removal efficiency improves to better than 80%
(Figure 9).
The examined feed-forward control did not improve the
effluent conditions because all control scenarios fall below the
effluent limit of 2 mg NH
4
/L. However, feed-forward control
decreases the peaks in energy consumption caused by ammonia-
based feedback. This is an advantage because the energy price
takes into account the maximum electrical power uptake.
Uncertainties in the simulation results are mostly due to the
input data with typical diurnal and weekly variations and general
model limitations. In particular, the model prediction for
denitrification and enhanced biological P-removal during low
oxygen concentration is of limited accuracy. Other limitations
are the response times of the sensors and the actuators, which
were not taken into account in this study. Low oxygen
concentrations could also increase the nitrite concentration
and thus inhibit the biological P-uptake. Low DO concentrations
may also favor the growth of filamentous organisms and con-
sequently can lead to bulking and or foaming. The potentially
negative effects of low oxygen concentrations were therefore
investigated also.
Implementation. Based on a second simulation study,
WWTP Werdhoelzli decided to implement the A/I process
scheme including NH
4
-based control. The plant upgrade has
been started but not finalized yet.
WWTP Morgental. The WWTP Morgental (Figure 10) is
a single-stage activated sludge plant built in 1975 for the removal
of organic matter of 83,000 p.e. (31,000 m
3
/d). Since 1994, the
Table 1—Modeled control concepts at 156C with parameter settings (WWTP Werdhoelzli).
No Control concept Parameter
V0 Actual control concept DO set-point 52 mg/L, return sludge 525,000 m
3
/d (1 lane) Valve for
last two aeration registers minimum 70% open
V1 DO set-point control based on NH
4,out
at the end of the
aeration tank (NH
4
feed-back control)
NH
4,eff
.1.8 mg/L 5. DO set-point 52mg/L
NH
4,eff
,1.6 mg/L 5. min. airflow 50.7 m
3
/(h * aerator), valve of last
aeration register minimum 70% open
V1a V1 +control of valve for last aeration register based on DO
at the end of aerated zone
According to V1 +possible reduction of valve opening to minimum 10% to
reach minimal airflow
V2 DO set-point control based on combination of NH
4
feed-back
and feed-forward control
According to V1a +feed-forward control based on an NH
4
sensor in the
primary effluent (Eq. 1 – 3)
V3 Feed-forward control of digester supernatant (DS) dosage According to V1a +control of DS dosage based on the ammonium load in
the primary effluent
V3a Separate treatment of digester supernatant According to V1a +separate treatment of digester supernatant
Table 2—Settings for feed-forward controller WWTP Werdhoelzli.
Control loop nit
State
DO set-point
Aer1+2.22
,1.8 0
Aer3 .1.4 2
,1.2 0
Rieger et al.
174 Water Environment Research, Volume 84, Number 2

plant has had to nitrify the wastewater of 42,000 p.e. (15,500 m
3
/d).
During the upgrade, two reactors were assigned for pre-
denitrification. Approximately 35,000 p.e. (13,000 m
3
/d) are
currently connected to the plant. In 1999 and 2000 additional
digester supernatant from a nearby WWTP was treated. The
final effluent is discharged into Lake Constance.
WWTP Morgental is not well suited to aeration control. The
plant consists of six lanes, and each lane in the biological stage is
divided into two sub-lanes. The aeration system consists of four
identical root-type blowers whose output cannot be reduced
below a minimum of 70%. This leads to step changes in the
aeration capacity during the speed-up or shutdown of blowers.
Moreover, the blower capacity cannot be throttled to the
required minimum during night hours or other low loading
periods. The air distribution is designed as a one-collector
system. Since no stabilizing pressure control is possible (due to
the step changes), the aeration of the single lanes is quite
different and variable. The calculation of the aeration capacity is
based on the average DO concentration of all the lanes. The
valves of the single lanes are used for DO control in the lanes.
The intention was to use the valves only for fine-tuning the air
distribution, but in reality the control loops do not have enough
control authority to reach the set-point, thus resulting in
a varying oxygen input in the different lanes.
Moreover, the wastewater distribution is not equalized. Lanes
one to three are supplied with less particulate matter than lanes
four to six. The outermost left- and right-hand lanes are also
supplied with more particulate compounds.
Figure 8—Modeled ammonia and nitrate load (left) and annual costs (right) for the investigated control concepts (WWTP Werdhoelzli).
Figure 9—Total net savings compared to V0 including energy costs, effluent load tax and P-precipitation (WWTP Werdhoelzli).
Rieger et al.
February 2012 175

The waste activated sludge is withdrawn from the last aerated
reactor and pumped back to the primary clarifier to be finally
removed with the primary sludge. Due to several shortcomings
of the primary clarifier, this leads to a significant re-seeding with
activated sludge and consequently to an accumulation of inert
particulate matter in the system.
The plant is equipped with membrane aerators so that the
aeration can be switched off completely. The DO control loop
uses a DO probe in the middle of the first aerated reactor. Only
one of the two sub-lanes is equipped with a DO sensor.
Definition of Objectives. The main goal at WWTP Morgental
was to reduce the energy consumption, since no effluent quality
tax and no other limits for total nitrogen exist. Enhanced
biological phosphorus removal was also outside the scope of the
study.
Data Quality Evaluation and Control. An extensive data
quality control was performed at WWTP Morgental. It included
the evaluation of all flow and concentration measurements. The
complete operating program for the various measurements in
the laboratory was analyzed, including of testing the micropip-
ettes, scales etc. Finally, the mass rates were evaluated by
combined mass balances (Thomann, 2008). Numerous in- and
ex-situ devices were installed on the plant to monitor its
processes and provide an input for the control loops. A new
monitoring concept was developed on the basis of this
experience (Thomann et al., 2002; Rieger et al., 2004a). In
November 2001, six different ex-situ analyzers and in-situ
sensors for ammonia were tested according to ISO 15839 (2003)
and a field testing protocol (Rieger et al., 2005).
Simulation-supported Controller Design. It could be shown
in the simulation study that the WWTP Morgental has great
potential for increasing the nitrogen removal and reducing the
energy consumption through the use of suitable measuring and
control concepts.
On the basis of intensive measuring campaigns of 10 and
11 days respectively, the plant model was calibrated and
validated using ASM3 as the biokinetic model (Gujer et al.,
1999) with a parameter set for Swiss municipal wastewater
(Koch et al., 2000). The simulated control concepts take the
technical and process constraints of the plant into account.
Table 3 shows the investigated control concepts.
All the concepts were tested with the actual blower limitation
due to the step changes and with an optimized blower unit
where a continuous change in capacity was possible (Figure 16).
The comparison resulted in a difference of 1 to 2% reduction in
energy consumption if the blower unit was optimized. The
absolute cost reduction is between 350 and $1,000/yr. For the
base case, a higher reduction of $1,800/yr can be attained due to
insufficient control authority for throttling the aeration capacity
to a minimum except with intermittent aeration.
The evaluation shows that the on-off concept combined with
feed-forward control on 3 mg DO/L (concept 4) gives the best
results with respect to effluent concentrations and energy
consumption. The feed-back part guarantees aeration according
to the requirements and the feed-forward part provides an early
signal for increased aeration during peak loadings. Use of a DO
set-point of 3 mg/L during peak loadings guarantees that enough
oxygen is available in both aerobic reactors (Figures 11, 12, and 13).
The reduction potential of the energy consumption reaches
$26,000/yr for the selected concept, which is 70% of the actual
energy requirement. If an effluent quality tax is applied in
addition, the increased nitrogen removal yields a reduction
potential of $65,000/yr (Figure 14) or an increase in the nitrogen
removal by 48% compared with the actual capacity (Figure 15).
Figure 10—WWTP Morgental with measuring locations.
Rieger et al.
176 Water Environment Research, Volume 84, Number 2

For a cost-benefit calculation, the annual costs were estimated
at $12,000/yr including investment and operational costs for
a lifetime of 10 years with an interest rate of 5%. In relation to
the energy reduction, this results in a net benefit of $14,000/yr. If
an effluent quality tax is taken into account, the net benefit
would increase to $53,000/yr.
Implementation. The control concepts were applied in stages
due to mainly technical constraints. After the first measuring
campaigns and an extensive data analysis and evaluation, the
starting situation could be characterized as follows:
NThe aeration capacity could not be reduced to the required
minimum at low loading situations (the blower unit is
significantly oversized after changing to fine-bubble aeration).
NDosing of digester supernatant in one pulse resulted in
effluent peaks of up to 10 mg/L (or more than 70 mg/L in
the influent to the biological stage).
NUnequal distribution of wastewater (flow and particulate
matter).
NUnequal distribution of air.
NDO sensors only in one of two sub-lanes.
First Optimization Step. The first step was to equalize the
dosing of digester supernatant and to introduce intermittent
aeration on the basis of the average DO concentration in order
to prevent DO saturation during the night hours.
During the period of intermittent aeration and due to the
unequal air and wastewater distribution certain lanes do not get
enough oxygen and others run into DO saturation. In the
aeration phase, the blower control loop tries to reach the DO
set-point of 2 mg/L. However, the actual value for the controller
is the average DO concentration of all lanes with the assumption
of equal DO concentrations in all lanes. Since the DO controller
of the single lanes has insufficient control authority, this leads to
Table 3—Investigated control concepts in the simulation study for WWTP Morgental.
No. Control concept Parameter
0 Actual control concept DO set-point 52 mg/L
1 DO set-point adjustment due to the ammonia concentration If NH
4,BB
,1.5 mg/L, than DO set-point 50.5 mg/L
If NH
4,BB
.1.8 mg/L, than DO set-point 52.0 mg/L
2 On-off of the blowers due to the ammonia concentration If NH
4,BB
5 Min. ,1.5 mg/L 5. Blower 30 min off
If NH
4,BB
within time interval .1.8mg/L 5. Blower on
3 Concept no. 2 plus feed-forward control Time delay FF control 525 min
Relation NH
4
influent load/nitrification capacity (Eq. 1 and 2)
X
AUT
580gCSB/m
3
, Volume 5458 m
3
,t
int
5Part of aerated time at
intermittent aeration 50.15
Relation set-point aeration on 51.8
Relation set-point aeration off 51.6
Combination feed-back and feed-forward by maximum condition
4 Concept no. 3, but with increased DO set-point for
feed-forward control
DO set-point at feed-forward 53 mg/L
Figure 11—DO concentrations in the second aerated reactor for different feed-forward control concepts (WWTP Morgental).
Rieger et al.
February 2012 177

a DO deficiency in some lanes and consequently to increased
ammonia concentrations in the effluent from the entire plant.
A first intensive measuring campaign was carried out to
obtain data for calibrating the dynamic simulation model. A
tracer experiment was used to characterize the hydraulic
behavior of the plant.
The ammonia control concepts were also tested in one pilot
lane during this phase. The plant model was validated with
a second intensive measuring campaign (with a DO set-point
adjustment due to the ammonia concentration). Accordingly, it
was decided to implement the ammonia control at the whole
plant. A simulation study was carried out to predict the reduction
potential of the energy consumption and the nitrogen discharge.
In the middle of this phase, the butterfly valves in lanes 1 and 2
were replaced by iris valves in order to test the effect of more
suitable valves on the control authority of the DO controller. The
result was sufficient control authority if the DO concentration was
higher than the set-point because the influence of the other lanes
predominates at DO concentrations below the set-point.
Second Optimization Step. To reduce the investment costs,
only three ion-sensitive ammonia sensors were installed in the
lanes with the higher loads. During this phase, all the ammonia
feed-back control concepts modeled in the simulation study
were tested. The DO set-point adjustment was applied using an
intermittent aeration concept. Due to financial restrictions, the
feed-forward concepts were not applied to the plant. The
Figure 12—Ammonia concentration in the second aerated reactor for different feed-forward control concepts (WWTP Morgental).
Figure 13—Ammonia effluent concentration for the modeled control concepts (WWTP Morgental).
Rieger et al.
178 Water Environment Research, Volume 84, Number 2

average of the three ammonia sensors was used as the input to
the control loops.
The result was that the average ammonia concentration could
be kept near the set-point of 1.5 mg N/L, but the concentrations
in the lanes varied strongly. Moreover, the DO concentrations in
the lanes varied so greatly that one lane had DO concentrations
close to zero during the intermittent aeration (Figure 17). The
DO concentration also varied between the two sub-lanes due to
the lack of a second DO probe and additional actuators to
control both lanes.
Figure 14—Reduction potential of the modeled control concepts (WWTP Morgental).
Figure 15—Comparison of the ammonia and nitrate effluent load for the modeled control concepts (WWTP Morgental).
Rieger et al.
February 2012 179

The conclusions of this phase were:
NDue to the averaging, the control loop was unable to
equalize the varying ammonia concentrations
NThe air distribution during the intermittent phase was
insufficient
NThe air distribution between the two sub-lanes was
insufficient
Third Optimization Step. In this step, the following mea-
sures were taken to overcome the problems identified:
NAutomatic equalization between the lanes equipped with
ammonia sensors through a decrease of the DO set-point in
the lanes with a higher nitrification rate.
NIncrease of the sludge retention time in the lanes without
ammonia sensors.
NReplacement of the remaining butterfly valves by wedge-
gate valves (lanes 3 to 6).
After fine-tuning of the valve control parameters, the result
was acceptable, although the air distribution during intermittent
operation is still critical.
Due to ongoing problems with the ammonia sensors, the
benefit resulting from the measures could not be evaluated over
a longer period within the schedule of this work. The last
measuring campaign carried out by EAWAG followed the
second optimization step. Within this period and when the
sensors worked, the reduction of the energy consumption was
less than the potential savings calculated in the simulation study,
but in the range of 20%. At the same time, the nitrogen removal
increased by approximately 40%.
Control of Success. It could be shown that ammonia control
leads to a significant reduction in energy consumption and in an
increase of nitrogen removal even with unfavorable plant
designs. The cost-benefit analysis for WWTP Morgental (based
on the simulation study) results in a ratio of 0.46 between the
annual costs (amortization of investment and operational costs
Figure 16—Reduction potential for energy consumption for the modeled control concepts (WWTP Morgental).
Figure 17—DO concentrations in the different lanes during the second optimization step (WWTP Morgental).
Rieger et al.
180 Water Environment Research, Volume 84, Number 2

for service, maintenance and chemicals) and the potential energy
reduction.
WWTP Thunersee. WWTP Thunersee is an activated
sludge plant mainly treating municipal wastewater of 130,000
p.e. (40,000 m
3
/d) (actual and design capacity) with an industrial
influent of approximately 30,000 p.e. After the upgrading of the
biological stage in 1998, the plant operates according to the A
2
O
process scheme for enhanced biological phosphorus removal
(Figure 18) and consists of two completely independent lanes
with separate sludge systems. Each lane is divided into two sub-
lanes in the biological stage and is connected to four secondary
clarifiers. The return activated sludge is mixed per lane before it
is pumped back to the first reactors of the two sub-lanes
(Figure 19). An annual effluent quality limit for total phosphorus
of 0.5 mg P/L is mandated by the authorities.
The plant was already able to control the aeration in each
reactor in several ways on the basis of DO and ammonia. Two
blower units per lane are installed and each of the blower units
serves one pair of parallel reactors in the two sub-lanes
(Figure 20). One blower unit is controlled by a DO con-
trol loop based on the average DO concentration of the two
sub-lanes. The air distribution is also controlled by the DO
concentration. The valve of the sub-lane with the higher DO
concentration is throttled. The air supply to the first reactor can
be reduced by intermittent aeration. The second reactor can be
controlled by means of a DO set-point adjustment in five steps
based on the NH
4
concentration.
Definition of Objectives. The goal of the study at WWTP
Thunersee was to reduce the operational costs. Because of the
effluent quality tax levied by the canton of Bern (see above), the
main optimization goal is to increase the biological phosphorus
removal and the nitrogen removal. The energy consumption also
represents a significant share of the operational costs and its
reduction is consequently the second goal of the study.
Data Quality Evaluation and Control.AtWWTPThunersee
as well, great efforts have been made to check the data quality.
Unlike the case of WWTP Morgental, however, EAWAG only
supplied the methods and acted as a contact in the event of
questions. This approach started a whole campaign of plant
analysis by the plant staff. The acceptance of their own
measurements increased greatly after the campaign, although an
erroneously calibrated photometer and imprecise test kits had
Figure 18—A2O process scheme and measurement locations of WWTP Thunersee.
Figure 19—Biological stage of WWTP Thunersee.
Rieger et al.
February 2012 181

been detected. A photometer was consequently acquired from
another manufacturer and used for the subsequent measurements.
The response times of all important on-line devices were
measured with special experiments. All on-line sensors were
monitored using a software tool specifically developed for this
purpose (Rieger et al., 2004).
Simulation-supported Controller Design. The plant model
based on the ASM3 with the EAWAG Bio-P module (Rieger
et al., 2001) was calibrated for a data set of 10 days in November
2000 and validated based on online and lab data from November
2002 to June 2003. In March 2003 a tracer test using sodium
bromide revealed insufficient mixing and back flows between the
non-aerated reactors. Changes to the separation walls, closing of
a short circuit, and re-direction of the internal recycle to reactor
5 were done in the test lane before the full-scale tests were
started. For the controller design study the 2000 data set was
used as model input but as the base of comparison the internal
recycle was redirected to reactor 5 and complete mixing in all
tanks was assumed. The control concepts were investigated in
Table 4.
After evaluation of the results, concept 6 (a 4-step controller
combined with a feed-forward part, see Figure 21) was selected
as the best choice with respect to effluent-quality tax, energy
consumption, dosing of precipitants and sludge disposal. The
controller reduces the air supply into the two reactors in steps
on the basis of an ammonia measurement at the end of the last
aerated tank. The feed-forward controller assures greater safety
with respect to peak loading and allows the feed-back controller
to be run at the optimum without additional risk factors due to
the delayed signal.
The annual costs arising from the need for additional
measuring devices and control approaches were estimated for
the cost-benefit calculation. They consist of investment and
operational costs for a lifetime of 10 years with an interest rate of
5%. The reduction potential for effluent-quality tax, precipitants
and precipitation sludge disposal is 40%, or $260,000/yr in
absolute numbers (Figure 22). The energy reduction potential
was calculated as 30% or $120,000/yr. The reduction in energy
consumption alone would be much higher than the annual costs
of the control system of $16,000/yr. The total resulting net
revenue was calculated to be $360,000/yr.
Regarding the environmental benefit, the nitrogen removal
could be increased by 135 t N/yr or 60% compared to a full
aeration in both reactors (Figure 23).
The reduction potential for phosphorus is also high due to
better denitrification (Figure 24). Precipitation was not directly
modeled in the simulation study, but in the analysis all
Figure 20—Allocation of the blower units to the reactors in one
lane (WWTP Thunersee).
Table 4—Investigated control concepts in the simulation study for WWTP Thunersee.
No. Control concept Parameter
0 Prior situation DO set-point in both reactors 52 mg/L
(IR in fourth reactor) Internal recycle in 4
th
reactor
1Base case for study DO set-point in both reactors 52 mg/L
(IR in fifth reactor) Internal recycle in 5
th
reactor
2 Reactor aer1+aer2: DO set-point adjustment based on NH
4
aer1: NH
4,BB
,1.6 mg/L 5. DO set-point 50.5mg/L
NH
4,BB
.1.8 mg/L 5. DO set-point 52mg/L
aer2: NH
4,BB
,1.3 mg/L 5. DO set-point 50.5mg/L
NH
4,BB
.1.5 mg/L 5. DO set-point 52mg/L
3 aer1: On-off control aer1: NH
4,BB
,1.6 mg/L 5. intermit. aeration: 30 min on/30 min off
NH
4,BB
.1.8 mg/L 5. DO set-point 52mg/L
aer2: Base case aer2: DO set-point 52 mg/L
4 aer1: On-off control aer1:NH
4,BB
,1.6 mg/L 5. intermit. aeration: 30 min on/30 min off
NH
4,BB
.1.8 mg/L 5. DO set-point 52mg/L
aer2: DO set-point adjustment based on NH
4
aer2:NH
4,BB
,1.3 mg/L 5. DO set-point 50.5mg/L
NH
4,BB
.1.5 mg/L 5. DO set-point 52mg/L
5 4-step controller see Figure 21
6 4-step controller combined with feed-forward control Concept 5 +feed-forward control based on NH
4
-N
PC
(Eq. 1 – 3)
Time delay feed-forward control 515 min
Relation NH
4
influent load/nitrification capacity
Relation set-point aeration on aer1 52: DO set-point 2 mg/L
Relation set-point aeration off aer1 51.8: acc. to 4-step controller
Relation set-point aeration on aer2 51.4: DO set-point 52 mg/L
Relation set-point aeration off aer2 51.2: acc. to 4-step controller
Rieger et al.
182 Water Environment Research, Volume 84, Number 2

phosphorus above 0.5 mg/L was calculated as precipitated. An
increase in the phosphorus eliminated by Bio-P leads to a lower
consumption of precipitants and thus to less chemical sludge to
be disposed of. When the 4-step controller was used, the
biological phosphorus removal increased by 50% or 12 t P/yr.
One part of this reduction is caused by enhancement of the
anaerobic zone achieved by returning the internal recycle into
the 5th instead of the 4th reactor.
Implementation. The findings of the simulation study pro-
vided the basis for discussing implementation of additional
control concepts with the plant manager and the operator-in-
chief. It was decided to test three different control concepts
against the current situation (Table 5 and Figure 25). The base
case includes an increase of the anaerobic zone to four instead of
three reactors. Based on the results from the tracer study, the
partition walls between the non-aerated reactors were modified
in order to achieve better mixing and prevent back flows. In the
test phase, the first of the two lanes was used for the experiments
while the second lane was operated with full aeration and served
as a reference.
The comparison of the experimental and reference lanes
shows a reduction in energy consumption of 10% for control
concept no. 1, 11% for no. 2 and 16.5% for no. 3 (Figure 26)
compared with the base case (DO set-point of 2 mg DO/L in
both reactors).
The reason for the limited reduction is the insufficient control
authority of the implemented control concepts (Figure 27) to
reduce the oxygen concentrations to very low levels. The mean
value of ammonia in the last aerated reactor during the test of the
third control concept was 1 mg N/L, whereas a concentration of
2 mg N/L should be maintained by the ammonia controller.
Nevertheless, the total nitrogen elimination was increased by 40%
with the third control concept (Figure 28). The results were
calculated by comparing the 24-h composite samples of the
experimental and reference lanes. A reduction potential for
phosphorus precipitation cannot be given because a basic pre-
cipitation took place in both lanes. The precipitant dosage is
controlled manually by the operators on the basis of the on-line
measurements for phosphate and the 24-h composite samples.
Control of Success. The reduction in energy consumption
and total nitrogen discharge achieved is less than the results
obtained in the simulation study for the best scenario, but still
represents a significant improvement with 16% (energy) and 40%
(total nitrogen). The better results obtained in the simulation are
Figure 21—Scheme of the 4-step controller (WWTP Thunersee).
Figure 22—Reduction potential of the modeled control concepts (WWTP Thunersee).
Rieger et al.
February 2012 183

due to a higher reduction potential of the air supply and
consequently a better control authority for reaching the
ammonia set-point.
The cost-benefit analysis for WWTP Thunersee results in
a ratio of 0.1 between the annual costs (amortization of
investment and operational costs for service and maintenance)
and the reduction in energy consumption and effluent-quality
tax for nitrogen.
Practical Aspects of Aeration Reduction in WWTPs
The control of aeration in an activated sludge system by
ammonia can lead to a significant reduction of the total energy
Figure 23—Comparison of the ammonia and nitrate effluent load for the modeled control concepts (WWTP Thunersee).
Figure 24—Phosphorus effluent load for the modeled control concepts without chemical P-precipitation (WWTP Thunersee).
Rieger et al.
184 Water Environment Research, Volume 84, Number 2

consumption in conjunction with decreased effluent concentra-
tions of total nitrogen. However, this is only one part of the cost
calculation. The need for reliable and accurate sensor values
as an input for the control concepts requires more monitoring
and service operations to be performed. Moreover, there are
concerns that low oxygen concentrations can lead to serious
problems in plant operation. During several measuring cam-
paigns at WWTP Thunersee, the practical and cost aspects of
a significant reduction of aeration were evaluated.
Hypothesis. Two hypotheses for the effects of low oxygen
concentrations were formulated and examined:
NLow oxygen concentrations lead to increased growth of
filamentous bacteria resulting in the production of foam
and scum.
NLow oxygen concentrations lead to higher concentrations of
nitrite in the effluent.
In addition, a long-term evaluation should evaluate practical
problems occurring as a result of greatly reduced oxygen
concentrations.
Methods. The first hypothesis was evaluated using fluores-
cence in-situ hybridization (FISH) to measure the abundance of
M. parvicella and nocardioform actinomycetes. A rapid quanti-
fication method was applied which was suitable for practical use
in wastewater treatment systems (Hug et al., 2005). In addition,
the foam coverage of the reactors was monitored and the foaming
potential evaluated using a batch test method developed by Ho
and Jenkins (1991). The accumulation of nitrite was monitored
with a submerged spectrometer sensor (Rieger et al., 2004b).
Table 5—Implemented control concept and duration of the experiments in WWTP Thunersee (schemes in Figure 25).
No. Control concept Duration of experiment
1 aer1: Intermittent aeration Nov. 2002 – Feb. 2003
aer2: DO set-point 52 mg/L
2 aer1: Intermittent aeration (2 states) Feb. 2003 – March 2003
aer2: DO set-point 52 mg/L
0 Base case (DO set-point 2 mg/L in both reactors) March 2003 – April 2003 and July 2003
3 aer1: Aeration based on blower capacity of aer2 June 2003
aer2: DO set-point adjustment based on NH
4, BB
Figure 25—Schemes of the tested control concepts for WWTP Thunersee. Maximum blower capacity is 300%, 100% means that one of
three blowers operates at maximum output.
Rieger et al.
February 2012 185

First Results and Discussion. Hypothesis 1. Microthrix
parvicella always occurred at very high concentrations although
it decreased during one time period. This trend was identical in
both independent lanes (no sludge mixed) and is consistent with
the commonly observed seasonal variations of this microorgan-
ism. Hence, the reduction does not seem to be caused by the
changed conditions due to the control strategies implemented.
With regards to nocardioform actinomycetes, none of the typical
branched filaments were found, but rather non-filamentous
types that did not vary over time. The variations of the foam
coverage and foaming potential could be linked to the type and
amount of the P-precipitants (FeClSO
4
, AlCl
3
) but not to the
implemented control strategies.
Hypothesis 2. The nitrite measurements (Figure 29) showed
increased concentrations if the aeration is reduced only in the
first aerated reactor. When a reduction was implemented in both
reactors, the concentrations increased at first, but decreased to
normal values of less than 0.1 mg/L after one week. The
differences between laboratory and sensor values can be related
to ongoing reactions in the 24 h-composite sample. Although
the concentrations in the reference lane also increased, the
nitrite concentrations in the pilot lane were higher.
Long-term Evaluations. Experience from these and other
sensor implementations and evaluations showed that the
maintenance and monitoring outlay using advanced aeration
control plays a major part in the cost calculations. Depending on
the measuring device, this outlay is between 0.5 and 3 h/week and
device on average. This means that at most plants the staffing
requirements will account for the main part of the annual costs.
At WWTP Thunersee, a significant part of the cost reduction
due to lower average aeration was lost again due to short peaks
of high energy consumption, since the energy price structure for
this plant takes into account the maximum electrical power
consumption in a quarter of an hour.
After three years of applying the control strategies at WWTP
Morgental, corrosion problems of the concrete were detected at
the end of the second aerated reactor. This was due to a too
excessive manual reduction of the airflow in the last sector of
Figure 26—Measured reduction in energy consumption (WWTP Thunersee).
Figure 27—Ammonia concentration during the test of the third control concept (WWTP Thunersee).
Rieger et al.
186 Water Environment Research, Volume 84, Number 2

this reactor leading to permanently settled and therefore
anaerobic sludge. No further problems were reported after
resetting the valves.
Conclusions
Ammonia controlled aeration in an activated sludge system
leads to a significant reduction of the total energy consumption
in conjunction with decreased effluent concentrations of total
nitrogen. Table 6 shows the reductions in energy consumption
and the increase in total nitrogen removal for the three plants
under study.
Despite significant benefits, the implementation of advanced
aeration control is still a complex task. It should take into
account not only the measuring and control system but also
practical aspects such as the manpower required to monitor and
maintain the sensors. A well-adapted control strategy must place
special emphasis on the technical equipment and the effect of
peak consumption on the energy price and not only on the
possible cost reduction due to lower average aeration. Possible
cost drawbacks due to filamentous growth and foam formation
should be considered, although no direct influence of the control
strategies on the abundance of M. parvicella or nocardioform
actinomycetes was observed.
The application of measuring and control systems requires
greater knowledge and effort on the part of the operators. In the
first place, the online sensors have to be maintained and
monitored. The lack of a systematic monitoring concept will
jeopardize optimum operation.
The measuring and control equipment should be regarded as
a single system. In practice, problems often arise because the
time delays resulting from the sensors, aeration system or
oxygen input are not taken into account during the planning
phase. Dynamic simulation is a suitable tool for explicitly
including these delays. Another problem is the control authority
needed to reach the given set-points. In the study for WWTP
Thunersee, it could be shown that the reduction potential in the
air supply is insufficient to keep the ammonia concentration
near the set-point.
The crucial point for practical applications of the control
concepts is their economic viability. Most plants in Switzerland
Figure 28—Measured reduction in nitrogen discharge (WWTP Thunersee).
Figure 29—NO
2
-N from the on-line sensor and the lab results of 24h-composite samples for lanes 1 (pilot) and 2 (reference) during the
test phases (WWTP Thunersee).
Rieger et al.
February 2012 187

can break even only by means of energy and precipitant
reduction. An effluent quality tax exists in only two cantons.
Regarding the sustainable application of measuring and control
concepts, it should be decided based on detailed analysis if the
benefit exceeds the real costs. The best solution will be the
simplest concept that still yields a significant benefit in
comparison with the annual costs.
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Sci. Technol., 48 (11–12), 345–354.
Table 6—Overview of simulated and full-scale energy savings and improvements of total nitrogen removal.
WWTP Morgental 35,000 PE WWTP Thunersee 130,000 PE WWTP Werdhoelzli 600,000 PE
Simulation Full-scale Simulation Full-scale Simulation
Energy 230% 220% 230% 216.5% 225%
TN removal +48% +40% +60% +40% +32%
Rieger et al.
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