What technology do you need to turn your wastewater treatment plant into an energy supplier?

The Marselisborg wastewater treatment plant uses a holistic approach to optimize its system. In addition to energy-efficient components, it is above all the consideration and optimization of the actual processes that provides a high potential for energy savings. In the past, the so-called 60-30-10 rule applied: 10% of the total savings potential was accounted for by the most efficient engine, 30% by its speed control, and 60% by process optimization and adaptation. In this Tech Insight, learn more about pump and fan speed control.

In Marselisborg, the operators, together with external experts, have precisely analyzed the sludge treatment and all clarification stages in the plant, equipped them with sensors that provide accurate data from the processes at any time and feed it into a control center. The master computer then evaluates these and returns the control signals resulting from the data to the equipment in the field, which uses them to set the optimum speeds and power data for blowers, fans and pumps, resulting in the significant savings. But why is that?

Speed control of pumps and fans with quadratic load torque

Pumps and fans are excellent for achieving savings. Especially for fans and centrifugal pumps, i.e. fluid flow machines with a quadratic load characteristic, the energy consumption decreases in the 3rd power with the speed.

Therefore, it makes sense to equip these devices with a quadratic characteristic curve with powerful frequency converters and thus optimally adapt their speed to the respective current power requirement. This is because in most cases the pumps and fans in water/wastewater technology are designed for the "worst case", i.e. for the maximum power required. However, this only occurs extremely rarely, and the rest of the time they naturally only run at partial load. And this is where Danfoss VLT® AQUA Drive frequency converters come into play. But beware: not all pumps and fans are suitable for speed control.

In the past, a centrifugal pump was usually controlled by means of butterfly valves, swirl valves or three-way valves. As a result of the throttling, the operating point of the machine shifted along the pump characteristic curve. There is only a minimal reduction in the energy required compared to the nominal operating point of the pump.

Graphic: Relationship pressure/flow rate

In the case of pump control via the speed, the operating point shifts along the system characteristic curve. The required energy is reduced to the third power compared to throttle control! For example, a centrifugal pump requires only one-eighth the power at half the speed. This behavior applies analogously to all turbomachines, fans, pumps, blowers with a quadratic characteristic curve.

In the characteristic curve diagram, in addition to the pump and system characteristic curve, some efficiency limits are also shown. Both throttle control and speed control move the operating point out of the efficiency optimum.

At about 32 Hz, the additional losses of the pump begin to exceed the savings. In the system shown, the energy-optimal frequency is 38 Hz. If the pump were not speed-controlled, the energy balance would be even worse.

Systems in water technology usually have to be designed for peak load. This inevitably results in a high proportion of part-load operation. Manufacturers of turbomachinery are now taking this fact into account. In some cases, they design their aggregates in such a way that the optimum efficiency is around 70 percent of the flow rate. Therefore, when retrofitting existing plants or designing new ones, users should pay attention to where the optimum efficiency lies when selecting the turbomachinery to be used. With the part load profile of your application, it is easy to determine whether such a selection makes sense for your plant.

Energy consumption of a selected pump at speed control.

Speed control of pumps and fans with square load torque

All process data converge in the central control system in Marselisborg. The large amount of data is then used to determine the necessary speeds for optimal control of the pumps, fans and blowers, so that the microbes work optimally, the throughput times are precisely adapted to the degree of contamination and the throughput and energy consumption are optimized. For this purpose, many data paths run through the plant.

VLT® AQUA Drive frequency inverters can be integrated into such systems in a variety of ways. Thus, they can be controlled via analog lines by means of current or voltage in a certain range. For this purpose, a 0/4..20 mA or 0..10 mA signal is available at the correspondingly configured input terminals. V and thus regulates the speed. However, this is rarely found in large plants today.

Fieldbuses – traditional or Ethernet-based – are used more frequently. Here, a lively exchange takes place via speed specifications from the master computer. Nowadays, frequency inverters also increasingly supply parameters such as motor current, voltage, speed and other system data. Then they also take over the task of network nodes, where users can also connect additional sensors via unused control interfaces in order to save further network nodes only for sensors. The intelligent VLT® inverters can also evaluate the measured values directly in the inverter, thus reducing the load on the network by reducing data transmission. At the same time, this pre-evaluation in AQUA Drive also conserves the computing power of the master computer, which is thus available for other tasks at the control level. For example, to optimize the fermentation of the sludge into biogas, thus further increasing the efficiency of the overall system.

After all, every kilowatt hour not used reduces resource consumption for energy and lowers CO2-emissions of the entire plant.

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