WWTP Stuttgart – Mühlhausen – changeover to air distribution control
system (ADC)
U. Zettl1, P. Baumann1, B. Diehm2, Th. Hauck2
Weber-Ingenieure GmbH, Pforzheim, Germany
Stadtentwässerung Stuttgart SES, Stuttgart, Germany
(1
Weber-Ingenieure GmbH, Bauschlotter Straße 62, 75177 Pforzheim, Germany
2
Landeshauptstadt Stuttgart, Tiefbauamt, Eigenbetrieb Stadtentwässerung Abt. Klärwerke und Kanalbetrieb, Aldinger Straße 212, 70378 Stuttgart, Germany)
1
2
Activated sludge; aeration; dissolved oxygen control; air distribution control; energy efficiency
Reasons for action and Objective
The energy concept of Stuttgart, Baden-Württemberg, Germany, plans to reduce energy consumption from today’s level of approx. 800 GWh/a by approx. 100 GWh/a by
the year 2020. This corresponds to approx. 12.5% reduction in energy consumption.
WWTPs are the largest electric power consumers of municipalities, and the aeration is the largest single electric power consumer of WWTPs. Therefore, the reduction
of the energy consumption in the four WWTPs is an essential part of the energy concept.
The Stadtentwässerung Stuttgart (SES, Stuttgart’s urban drainage service) designs, builds and operates 4 WWTPs, approx. 1,750 km of sewer pipelines and many
more buildings for sewage disposal. The WWTP Stuttgart-Mühlhausen, which has 1,200,000 total number of inhabitants and population equivalents (PT), is one of the
10 largest WWTPs in Germany.
The waste water is treated biologically in 2 groups of tanks. The southern tank group consists of 9 aeration tanks (totaling 69,000 m³) and 5 secondary clarifiers. Whilst
the northern tank group consists of 10 aeration tanks (totaling 84,000 m³) and 7 secondary clarifiers (see Figure 1.1).
The WWTP is updated and maintained regularly to comply with the legislative framework. From 2006-2009, 3 aeration tanks in the southern tank group were rebuilt and
the aeration controlling system was replaced by an air distribution control system (ADC) which was chosen by SES. Today all of the aeration tank groups operate with
the ADC (each tank group with 2 independent control loops).
This paper describes the main principles of ADC, the essential technical details of the control loops and actuators as well as the approach taken to commission the
controller. Since 2011, the ADC has been functioning successfully. This paper will show how the energy consumption declined due to the new aeration control system.
The principle of air distribution control (ADC)
The ADC has been used for more than 20 years in almost every WWTP that Weber-Ingenieure has designed as an engineering company for the control of aeration
intensity. After checking the reference plants of the engineering company, SES decided to utilize the ADC for their aeration control system. ADC is being admitted to the
technical regulation guidelines within Germany under the regulation reference DWA-A 268 (2015). Figure 1.2 illustrates the principle control structure of ADC.
The air flow rate of the compressors (actuator) is controlled by the “average value of dissolved oxygen (DO)”. The difference between the average measurement value
of DO (current) and the set-point is used to change the air flow (control signal). This is a closed loop control for the air flow/compressor. The minimum and maximum
air flow is fixed, due to the demands of the aerators, and can be adapted to the current aerated tank volume. The DO set-point can be determined by an on-line
measurement of the ammonia concentration.
The available air flow from the compressors is distributed to the aeration zones proportional to the single set-points of DO (multi-loop control). The single DO set-points
are deduced from the average measured DO and can be considered in relation to the situation in the other zones by weighing the set-points. The pressure losses can
be reduced by adapting the minimal valve position to the air flow rate (in consideration of the characteristic curve of the valve). One of the valves must always be
completely open.
Overall there is no need to throttle the complete air flow, only the pressure losses of the air distribution have to be taken. The energy efficiency of the ADC is better than
that of most open valve principle (MOV) because of the complete opening of one valve (100 % instead of 90 % for the MOV). The calculation of the average value makes
the air distribution control less susceptible to problems in the event of failing DO measurements. In addition, it is not sensitive to pressure variations. The pressure is
measured only for the purposes of checking and monitoring.
This control system is over-determined, and, theoretically there should be problems with the DO controller. With n+1 actuating variables, a system with n controlled
process variables is controlled; many stationary solutions are admissible. The pressure can increase with the actuator air flow/compressor to any level and, simultaneously, the actuator valves can throttle the air flow to meet the DO set-points, and, subsequently, the average DO set-point. This theory was investigated by Alex et al.
in a simulation study (2015), whereby different control structures were compared (constant pressure, variable pressure control/ most open valve principle (MOV) and
ADC), and values for the tuning of the PI controller loops were approved. With the specification of one completely opened valve, there is only one admissible solution
with a little offset of the DO in the zone containing the completely opened valve. The control loops act robust, and the tuning of the controller is manageable. With a low
gain of the air flow/compressor control loop, the level of pressure-peaks is comparable to constant pressure control and MOV.
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Essential technical details of the control loops and actuators
The 3 aeration tanks of the southern tank group with a total volume of approx. 36,000 m³ were rebuilt in 2006/07 and equipped with aerators, air pipes, valves and
a vertical mixer in the swing zones in 2008/09. Three turbo compressors from 7,000 to 15,000 Nm³/h (HV Turbo Typ KA 10 SV which were constructed in 1990) supply
the south tank group with air. The 3 lines have been operated with the new equipment since 2010, and the ADC was implemented in March 2011.
The 3 aeration tanks are designed for pre-anoxic zone denitrification process each with 3 swing zones. The first swing zone can be operated as anaerobic/anoxic, the
second zone as anoxic zone (aerators can be upgraded if a swing zone becomes necessary) and the last swing zone as anoxic/aerobic (Figure 1.3). The rear areas of
the 3 lines are continuously aerated for nitrification. Alongside the flow path, fewer aerators were installed.
The air flow is distributed among the swing zones and nitrification zones of the 3 lines with 6 control valves. Each line is equipped with 2 oxygen sensors, one at the end
of the anoxic/aerobic swing zone and one at approximately two-thirds of the flow path in the nitrification zone. The process design is depicted in Figure 1.4.
The measurement of the air flow is conducive to check the specific air flow of the installed aerators (MIN-/MAX-limitations). The pressure in the air pipes is measured at
the turbo compressors and at the end of the pipes in each line. The difference between the pressure measurements represent the loss at the valves and should always
be at a minimum. Simultaneously, the (pressure loss) state of the aerators can be deduced from the system pressure.
Optionally, the anoxic/aerobic swing zones can be aerated automatically in the winter operation mode based on the measured ammonium concentration in the outlet
of the nitrification zone.
Experiences with commissioning the ADC and with the tuning procedure
Because the prior DO controller was inoperable, the aeration system had to be operated manually before the implementation of the ADC in 2012. With the implementation of the ADC, the oscillations of the DO and the pressure as well as the frequency of changes in the valve positions were damped. The control quality improved
significantly.
To make the controller transparent, all the relevant and available measurement data was switched to the process control system (PLC). Different graphics were formed
for monitoring the control loops. The data transmission had to be checked (e.g. allocation of the signals, units) and the scales of the data in the graphics had to be
defined meaningfully. All this is a fundamental requirement for the following tuning procedure. To parametrizes the controller, the initial values had to be fixed regarding
the hardware (e.g. turbo compressors, control valves, tank volumes), load conditions (e.g. variation of the load, distribution of the waste water to the lines) and the
software (e.g. the constants of the controllers and the rates for control action). This required a close collaboration between operators, planners and programmers.
The wastewater flows through a distribution channel into the 3 lines. The wastewater and, therefore, the load are unequally distributed. The ADC had to be complemented by the function “indicator line”. The line with the current lowest average DO is continuously determined by the control system as “indicator line”, and the valve
is opened to the maximum. The other control valves are geared to this in order to keep the pressure loss in the whole system to a minimum through the positions of
the control valves.
Results / reaching the objective
2010 the southern tank group required an average of approx. 23,500 kWh per day. After the commissioning and improvement of the ADC, the consumption was approx.
18,000 kWh (Figure 1.5). The northern tank group required a daily average of approx. 35,000 kWh and, after the changeover to ADC, it required approx. 25,000 kWh per
day. With the changeover to the ADC, approx. 26% of the energy could be saved. This conforms to the results of full-scale studies in Sweden (Ingildsen & Olsson, 2001).
In 2013 a specific energy consumption of 20.6 kWh per PT was measured in the southern tank group and 20.9 kWh per PT in the northern tank group. After replacing
the aerators in the 7 aeration tanks of the northern tank group in 2014, the energy consumption has decreased even more.
With improving the energy consumption, the DO set-point was lowered. Especially in the northern tank group the ammonia concentration values rose. Thereupon the
set –points of DO were increased again. Saving energy must not lead to higher ammonia concentrations or to sludge bulking.
Summary and conclusions
Because of its superior energy efficiency in consequence of one completely opened valve, the SES chose the ADC for the control of the aeration intensity. The costs of
the technical configuration are the same as of other control systems like the MOV.
A close collaboration between operators, planners and programmers is essential to commission the control system and for the tuning procedure. Whilst the operator
has to keep busy with the control system and the treatment process as well. Either applies to all control systems for the aeration.
Nomenclature
ADC
DO
ICA
MOV
PI
PLC
air distribution control
dissolved oxygen
instrumentation, control and automation
most open valve
proportional-integral
process control system
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PT
SES
total number of inhabitants and population equivalents
Stadtentwässerung Stuttgart (Stuttgarts urban drainage service)
References
Kohler, J.. (2009) Automatisierungslösungen für Belüftungssysteme, MSR-Technik in abwassertechnischen Anlagen, TAE Kontakt & Studium Band 664, ISBN 978-38169-2922-2, Germany
Jens, A., Morck, T. and Zettl, U.. (2015) Modelltechnische Überprüfung energieeffizienter Luftverteilregelungen bei Druckbelüftungen, 10. Mess- und Regelungstechnik
in abwassertechnischen Anlagen (MSR) in Kassel, Germany.
Braun, D., Sturzenegger, L.; Gresch, M. and Gujer, W. (2012): Robuste und leistungsfähige Regelungskonzepte für Kläranlagen. KA Korrespondenz Abwasser, Abfall
2012 (59), Nr. 8, Germany.
Ingildsen, P. and Olsson, G. (2001): Get out more of your wastewater treatment plant – complexity made simple. Danfoss Analytical, 2001. ISBN Nr. 87-87411-01-6
DWA-A 268 (2015): Automatisierung von einstufigen Belebungsanlagen, DWA-Regelwerk A 268 (Entwurf), März 2015
Figure 1.1 Aerial survey of WWTP Stuttgart-Mühlhausen
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Control loop for air flow
Control loop for air distribution
Set-point DO (mg/l)
DO controller
DO controller
Average DO (mg/l)
Control/regulation
of compressor
Average value
calculation
Air
Control
valve
DO sensor
Air compressor
Aeration tank
Air
Control
valve
DO sensor
Aeration tank
Possible further tanks
Figure 1.2 Principle of the ADC
Figure 1.3 Equipment of a swing zone
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Figure 1.4 Process design of one line
Figure 1.5 Average daily energy consumption in the southern tank group
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