Clean water is one of the most basic necessities of our lives. Our health depends on it. What transpired in the Flint water crisis in Michigan recently has shocked the nation. President Obama declared a state of emergency and there are demands that the Governor of Michigan steps down. It all started when, in order to reduce cost, the City of Flint officials decided to use Flint River water for residential consumption without adding orthophosphate, a chemical that coats the pipe interior thereby inhibiting any leaching of lead.
As a result, people who used this water have suffered from serious health issues and the matter is being handled at the Federal level with the involvement of the EPA, the lead federal agency for protecting drinking water and wastewater utility systems. Water systems, a part of our national security infrastructure, must be designed using state of the art tools in order to make them safe and efficient.
Advanced methods such as computational fluid dynamics (CFD) models are available to predict performance and root cause analysis of accidents that may happen in the water supply systems. Such models use basic sciences such as physics of pipe flow and the chemical/biological processes that happen under certain conditions in water systems. With proper understanding of these reactions, one can create predictive models that can assess the damage caused to the piping as well as quantify the amount of hazardous material in water over a period of time. Such ability is vital in alerting authorities to take preventive measures before it gets out of hand. With the advent of the internet of things (IoT), smart controls can also be added along with predictive tools to create a smart and secure supply system.
Computational fluid dynamics has established itself as the main tool in the design and troubleshooting of physical and chemical systems. In this regard, ANSYS CFD tools provide the necessary mathematical models to design, troubleshoot or improve processes and equipment used in the water industry. With the release of ANSYS 17.0, our electrochemistry model is further enhanced, which adds more physics to deal with problems such as pipe and surface corrosion. ANSYS engineers have worked extensively with major corporations and equipment manufacturers helping them improve their processes and ensuring contaminant and pathogen free water production. The pictures below highlight the use of CFD in water and waste water treatment plants.
Mixing of water and chemicals to improve quality is a basic operation which depends on the placement of mixing devices. This step is optimized using CFD. Figure 1(a), (b) and (c) show how a particle bed is agitated by an impeller at the bottom of a water tank as time progresses. Such type of simulations help in visualizing the effect of paddle speed on the relative mixing of different type of solid and liquid materials inside a reactor.
Same technique can be used with gas-liquid mixing. In the next example, air is introduced at the bottom of a mixing tank. Air bubbles through the liquid and exits at the free surface while the paddles are rotating. During this process, oxygen gets absorbed in the liquid. Figure 2(a) shows a CFD setup with sparger at the bottom and two mixing paddles. The high and low velocity areas as well as air volume fractions can be visualized as a function of air injection rate and the impeller speed. The figure below also shows volume fraction of air in the tank at different times.
Other bioreactors can also be modeled similarly. Apart from the stirred tank type shown above, airlift types are also popular due to their simple design. Figure 3 shows an example of an airlift reactor modeled using ANSYS Fluent’s eulerian multiphase model along with population balance model. This allows us to compute bubble size distribution and gas hold up in different parts of the reactor. The breakup and coalescence of bubbles is also computed as this is an important aspect in determining the correct interfacial areas for heat and mass transfer processes. Figure 3(a) shows the air-water interface. Bubble size distribution and oxygen concentration are shown in 3(b) and 3(c) respectively. This type of analysis helps in locating regions with maximum and minimum gas hold up.
ANSYS CFD can also be used to model the ultra violet (UV) disinfection process used in the water industry. The light emitted by the UV lamps can neutralize the water-borne pathogens. The radiation module computes the strength of this UV radiation field around the lamps — shown in 4(a) — while the core fluid flow solver predicts the residence time. The combination of residence time and radiation field strength then provides a means to compute the effectiveness of this kill process. The possible paths of the bacteria are shown through the water duct system in 4(b). As the pathogens pass near the lamp and go through its radiation field, they get enough radiation dosage that changes their DNA and hence neutralizes them.
In summary, ANSYS CFD tools are used by major OEM, city governments, municipal corporations and services companies to design, improve and troubleshoot water treatment processes and systems. Using advanced analysis tools such as CFD and linking them to IoT will play a big role in ensuring that our water supplies are safe, secure and available to all. Clean water and clean air is a right, not a privilege. Let's be smart in our use of these resources. Let's keep them clean for the generations to come, our children!