The task of the association for sewage treatment of Alme-Riehe, based in Sibbesse in Lower Saxony, is to feed the wastewater accumulated by its members to the sewage treatment plant in Östrum, clean it and then discharge it without harm. In addition to the city of Bad Salzdetfurth, the communities of Lamspringe and Sibbesse, as well as a company from the food industry, are members of the association.
An outdated control solution with failure-prone switchgear made it necessary to modernize the wastewater lifting system of the sewage treatment plant in 2019 and 2020. Among other things, the starting currents had to be reduced to allow the system to be operated optimally in emergency operation as well.
Modification during ongoing operations
Harzer Antriebstechnik was commissioned to convert the control. The ABB partner based in Bad Lauterberg in the Harz region is a specialist in controllable drive solutions in the field of industrial automation, including related services. “The modification has to be carried out during ongoing operations, because at least two pumps should always be available,” says Managing Director Marcus Warlich – a challenge that was solved well and “just in time” in partnership with the customer.
The wastewater lifting system originally consisted of four centrifugal pumps, each with an output of
30 kW, an ABB ACS550 variable speed drive (VSD) and four star/delta controls. The level control in normal operation was carried out via continuous level detection. In emergency mode, the level of the dirty water was regulated using five float switches. If the value fell below the specified limit, the motors were stopped; if exceeded, they were started. The system was controlled via the control technology of the sewage treatment plant.
Previously, the selection of the pump in the ACS550 VSD was made using a selector switch. The other pumps, depending on the fill level, were switched on via the control system in the star/delta start-up as required. In the past, if the pump preselection in the variable speed drive was unfavorable, the VSD and the star/delta control were operated in parallel on the same pump drive. The operator then switched the pump to the VSD so that its running time was twice that of the other three pumps. Another problem was that in emergency operation via the emergency power supply, operation via star/delta was no longer possible due to the high starting current. This meant that optimal plant operation was no longer ensured.
New control of the pumps
“We had to adapt the control of the pumps to reduce the starting currents for operation with the emergency power generator. In addition, we wanted to save energy and increase the system’s availability. Everything also had to be operable and controllable from the control room. The engineering team from Harzer Antriebstechnik also dismantled the components that were no longer required,” reports Marcus Warlich.
The experts at Harzer Antriebstechnik changed the star/delta combination for ABB’s PSTX softstarters and installed a ACS580 drive. The VSD controls a wide range of applications with variable torque or constant torque, including reliable pumps with a quadratic load torque. In the energy optimization mode, the maximum torque per ampere is ensured, and energy consumption is reduced. With integrated energy calculators for consumed and saved kWh, CO₂ reduction and energy cost savings, the sewage plant operator can now fine-tune the processes for optimal energy use.
An independent control for emergency operation without an additional PLC could be implemented by means of adaptive programming with the Drive AP tool, which is integrated in the VSD, as well as the graphic PC tool from ABB, which is part of the Drive Composer commissioning software.
Normal and emergency operation optimized
Today in normal operation, two pumps are operated continuously on one VSD each, and two pumps on one soft starter each. Control and pump change are conducted directly from the control system.
If the control system fails, two pumps are operated continuously in emergency mode on one VSD each, and two pumps on one soft starter each. Control and pump change take place via the adaptive program in the drive. The VSDs reliably regulate the level via the internal PID controller.
If the fill level rises above a limit value stored in the VSD, another pump with the ABB PSTX softstarter is switched on via the adaptive program. If the level falls below a certain limit value again, the mains pump is switched off first. With the help of the adaptive program, the status of each pump is monitored to ensure another pump can be controlled in the event of a fault.
With adaptive programming, an automatic pump change of the mains pump, with a soft starter, was implemented in the drive control/regulation. This ensures that the pumps are used evenly.
Important advantages
The new automation solution has several advantages for the operator, because the distribution of the operating hours among the various pumps means they are evenly utilized. Since only two pumps are allowed to run at a time, there is also redundancy in the regulating pumps and rigid network pumps. Last but not least, the use of VSDs and soft starters reduces the starting currents, because the system can be operated via an emergency power generator in an emergency situation. Should the control technology fail, the operation of the sewage treatment system is still safe.
Additional advantages result from energy saving through two regulated pumps, which guarantee an adjustment to the required flowrate. The maintenance of individual pumps in regular operation is possible without restriction, as they can be switched off for this. During the system upgrades, the emergency power generator and the transfer switch were also replaced.