After the water goes down the drain — whether via home, commercial, or industrial uses — many of us forget about it. However, that water has a long way to go before it is put back into the environment. It is collected and sent to wastewater treatment plants where it is cleaned via chemical and mechanical processes before being discharged. Simulation is put to work in this phase of the urban water cycle as well.

Wastewater treatment (WWT) is closely regulated by stringent government standards and is essential for sustainable processes. Indeed, industry leaders regard sustainability as a social responsibility, preventing pollution and reusing water as one of our most precious resources. The goal in such environmental initiatives is to extract waste materials as efficiently as possible by maximizing throughput rates using the least amount of energy.

Simulation has been used to simulate many water clarification processes that enable designers to accommodate more stringent effluent quality limits and increase throughput. For example, engineers used Ansys CFD solutions to help optimize water treatment by simulating a new dissolved air flotation (DAF) model, in which injected air bubbles are used to transport solids in water to the surface where they can be removed. Additionally, Ansys simulation is used to visualize and predict how sediment settles in the bottom of tanks to maximize their efficiency.

Ansys fluid dynamics simulation has also been used to help design and operate WWT systems in chemical processing plants. This represents a huge step forward in an industry that has traditionally relied on replicating past designs in new plants. One example focuses on the critical step in WWT operations in which screened and filtered liquid waste are treated in a large aeration basin — essentially a large tank with inlets for receiving raw materials and an outlet for draining off treated effluent. Naturally acclimated bacteria within the basin break down the organic compounds into a safe material. At the bottom of one of the basins, lengths of pipe with small sparging nozzles blow out air bubbles that transfer oxygen into the sludge to accelerate decomposition of the waste. The decomposition process in another basin is aided by the stirring action of floating aerators, whose impellers push liquid waste together with oxygen bubbles downward toward the basin floor to discourage settling and promote a more even distribution of bacterial action throughout the mixture.

Several variables — including gas–liquid flow, turbulence, dissolved oxygen, and biochemical reaction kinetics — were combined into a large-domain model consisting of 10 million computational cells representing the entire sludge basin. Subsequent comparison of the basin’s dissolved oxygen measurements with fluid dynamics calculations showed good correlation, thus validating the accuracy of the approach. This validation gave engineers confidence in using the model in a wide range of simulations that provided insight into design changes for optimizing the performance of the sludge basin. Using the CFD model, engineers could change a few key parameters to quickly study various alternative configurations in arriving at the most efficient design for meeting performance criteria. many of the simulations used Fluent to create isosurfaces that define boundaries separating regions of various oxygen levels or liquid velocities, for example. Such displays are helpful in understanding the characteristics of a complex system undergoing multiple behaviors.

While fresh water is only a very small percentage of Earth’s overall water supply, it can still be too much of a good thing. Ansys simulations are used to represent complex fluid-structure-soil interactions to aid environmental risk assessments to prevent flooding, as well as improve water management by optimizing stormwater systems, spillways, culverts, and dams.