Providing sufficient clean water is essential in any community, and the pumps and other machinery needed to do so require large amounts of power. Indeed, for many cities and towns across Germany, the cost of electricity to operate water infrastructure is the single largest expense in their annual budget.
Water treatment plants are typically designed and constructed by civil engineers, working closely with mechanical engineers who oversee the facilities’ mechanical and process engineering components. Other team members include environmental and process engineers, as well as biotechnology engineers, who are responsible for biological wastewater management. They ensure the health of the microorganisms that break down organic materials in sewage, and ultimately the sanitary performance of the facility.
Mar 27, 2025
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One highly relevant discipline that is typically missing in such projects, however, is computational fluid dynamics (CFD), the science of predicting how fluids and the materials of which they are made flow. According to Dr. Martin Armbruster, Managing Director of hydrograv GmbH, this is too often a missed opportunity. “Mercedes-Benz or Porsche would never put an automobile on the road without optimizing the aerodynamics extensively using simulation,“ he observes. “In our field, however, it is still exotic to optimize an engineer’s design using CFD. Because this rarely happens, we often build too large and inefficient.“
Hydrograv was founded in Dresden in 2004 as a spinoff company from the Karlsruhe Institute of Technology and the Technical University Dresden, and specializes in CFD simulation for water resource management. Using customized open source software, hydrograv advises facility operators on how to eliminate inefficiencies and improve water quality. The company also assists engineers during the planning of new facilities, using simulation to optimize efficiency during the design phase. This makes it possible to identify and rectify potential deficiencies in processes involved in mechanical or biological water purification early — long before the first cubic meter of concrete is poured.
CFD simulations of water management facilities model complex multiphase flows, an approach that is only practical using high-performance computing (HPC). Although hydrograv operates its own computer servers for smaller simulation tasks, running simulations at this scale would not be possible on its own. This is why in 2019 the company began using supercomputing resources at the High-Performance Computing Center Stuttgart (HLRS) as an integral part of its simulation pipeline.
“Considering the amount of parallel simulation that we now do, our work wouldn’t be possible if we didn’t have HLRS as a partner,” Armbruster remarked. The partnership is an impressive example of the opportunities that small companies can derive from HLRS’s high-performance computing systems, as well as how HPC can contribute to sustainability.
One key component of a water treatment facility is the aeration tank, where microorganisms break down organic pollutants in waste water. For these microorganisms to work optimally, it is important to provide evenly distributed, dissolved oxygen throughout the tank. This is usually accomplished using blowers and fine-bubble aeration systems that efficiently introduce oxygen into the tank. The principle is similar to oxygen enrichment in aquariums, but on a much larger scale and with specially designed technical systems for energy efficiency and process control.
The placement of the aeration elements (aerators) in the aeration tank is of decisive importance for an efficient distribution of oxygen. “Although the most inexpensive and easiest to maintain solution might be to place aerators in a single area, this is not optimal from a flow perspective,” Armbruster explained. A poorly chosen distribution of aerators around a tank can cause air to reach the surface of the water too quickly and escape into the atmosphere. As a result, not enough oxygen dissolves in the water. This can require pumping more air into a tank than is really needed, an inefficient approach that leads to unnecessarily high energy usage.
For complex multiphase flows, the validation of CFD models is an essential part of achieving reliable simulation results. In a comparison of simulated and measured velocities in an aeration tank, the figure shows a close match. Image: hydrograv GmbH. (click image to enlarge)
Using simulation, Hydrograv can realistically predict how oxygen will be distributed in a tank based on the distribution of aerators and different operating scenarios. A study based on engineering practice showed that optimizing aeration could increase the amount of oxygen introduced into the tank by more than 40% using the same amount of power. “When you consider that running aerators is the largest expense in a water treatment plant, which itself is often the largest energy consumer in a city, you can clearly see why optimization using CFD can be extremely important,” Armbruster said.
In addition to improving aeration, CFD can also be used to optimize the placement of pumps and the geometries of water tanks and canals. When circulation is not optimized, dead zones can develop where waste water is not evenly distributed, leading to inconsistent water treatment. Suboptimal flow conditions can also lead to deposits that impede flow and increase the risk that a tank could overflow. This, in turn, forces pumps to work harder, raising energy usage and operating costs. In addition, an unfavorable inflow can reduce the service life of a pump, leading to increased wear and mechanical stress.
Using CFD simulations, hydrograv can make targeted suggestions about structural improvements that increase circulation, such as the ideal placement of guide walls to direct water flow and promote mixing. Such optimizations help to design complex water treatment processes to be more efficient and to reduce their energy usage.
In a project with the city of Erlangen, for example, hydrograv ran CFD simulations at HLRS that enabled its water management facility to make major improvements in the secondary clarification step. The approach was so successful that it was possible to completely shut off a filtration system that was previously needed to reach water quality goals. This optimization has reduced energy usage by 500,000 kWh per year.
In another project, hydrograv supported the city of Hanau in optimizing its secondary clarification tank. Avoiding a need for the city to spend 35 million Euros to reconstruct the facility, hydrograv provided an efficient solution for adjusting inflow that could be seamlessly integrated into the existing tank. At a cost of just 1.5 million Euros, the solution not only saved the city money but also for the first time enabled it to reach demanding targets for reducing phosphorous concentration in its water. hydrograv GmbH was awarded the Saxon Environmental Prize for its outstanding contribution to environmental protection.
Since its founding, hydrograv has completed more than 1,000 simulation projects, successfully serving clients across Germany, Europe, and around the world. Its need to run many simulations now far outstrips the capabilities of its in-house computing servers. Establishing a contract with HLRS to use its supercomputer has thus become absolutely essential for its work.
Using a node on HLRS’s supercomputer, hydrograv’s simulations can often take 24 hours, with large-scale simulation projects lasting up to a week. For some projects, the company must also run multiple simulations of this size to arrive at an optimized solution. With its software installed at HLRS, hydrograv can access the system from Dresden, giving it a powerful and flexible asset that is now integrated into its simulation workflows.
This image shows a simulation project in which the introduction of oxygen in an aeration tank was increased by 43 percent through flow optimization using CFD simulations. The simulation results were confirmed by measurements carried out by W. Frey. Image: hydrograv GmbH. (click image to enlarge)
“At times we might have 3 or 4 clients in a month, each of which requires running 5 or 10 simulations. If next month we don’t have the same computing needs, however, it would be difficult to decide how many computers to put in our server room,” Armbruster observed. “This is the advantage of running these simulations at HLRS. We can access exactly the amount of large-scale computing power we need, when we need it.”
For Hydrograv, knowing that its software and data are protected in a secure environment is also extremely important. “It is very advantageous to be able to do this in a German national high-performance computing center, as we and our clients can be sure that our know-how stays in the country,” Armbruster said.
The use of CFD in the design of water treatment infrastructure is still unusual. Considering the large number of facilities across Germany and Europe, and that the cost of simulation is much lower than that of wasting energy, Armbruster hopes that the approach will be used more often in the future. Doing so could not only help communities in reducing costs, but also make sanitation and energy infrastructure more sustainable.
— Christopher Williams
Funding for Hawk was provided by Baden-Württemberg Ministry for Science, Research, and the Arts and the German Federal Ministry of Education and Research through the Gauss Centre for Supercomputing (GCS).
