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Modeling with CFD

Modeling with CFD

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    CFD software provides insight to what is actually happening inside a structure, evident in this model of the San Diego Miramar water treatment plant's flow splitter box that shows velocity contours. Photos: COM

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    Shown here is construction of the San Diego Miramar water treatment plant, which provides 140 mgd to nearly 500,000 residents.

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    Above: CFD results, such as this graphic of velocity magnitude contours in the chlorine contactor at the Los Angeles West Basin Municipal Water District water recycling plant, illustrate fluid flow within a structure. Left: A CFD analysis at the West Basin Municipal Water District plant revealed that the residence time of the existing chlorine contactor at the increased flow rate was sufficient, and adding another contactor was unnecessary.

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    These two graphics illustrate the modeling of the velocity contours of the Scott Candler water filter plant's channel with (top) and without (bottom) a taper. Results showed that a rectangular channel was more effective than the tapered design, saving more than $75,000 in additional costs.

“Across the nation, cities' populations are growing and infrastructure continues to age. Combining these factors with increasingly stringent water quality regulations imposes significant challenges upon utility managers,” said Marc Serna, P.E., senior engineer for the West Basin Municipal Water District, Los Angeles. “How am I going to best accommodate the needs of my community, not only for today, but also far into the future? As utility managers, we need to implement the appropriate tools and new technologies available to us to find the best solutions for maintaining, upgrading, and expanding our facilities.”

These issues are present across the country and sound assistance is a welcome addition in evaluating water and wastewater projects. One of the latest technological innovations in the response to these challenges is computational fluid dynamics (CFD)—a new design tool that is ensuring new treatment plants and upgrades are efficient and cost effective.

CFD models are used to model hydraulic designs. The model quantitatively predicts what will happen when fluid flows within a structure. During CFD analyses, engineers reproduce real-scale geometry to model a 3-D structure. Simulations of different conditions are run, providing data and valuable visualizations of what will happen in real life when these conditions occur. These results are then applied to the design—as a source of answers, suggestions, and strategies.

CFD predicts hydraulic performance inside a structure by using the principles of fluid flow, providing the ability to troubleshoot and confirm efficient designs. In essence, it allows engineers and decision makers to visualize what occurs within a structure at a level of detail not possible to witness onsite or even through sampling.

The technology is applicable to the most detailed processes of water or wastewater treatment infrastructure—from ozone and chlorine contactors to flow-splitting structures and ultraviolet (UV) reactors. The insights achieved through CFD's cost-effective, side-by-side comparisons provide answers to specific operation-related questions, including flow distribution, mixing characteristics, residence time, areas of flow, dead zones and short-circuiting, and sedimentation.

Using CFD during a project's design achieves process efficiencies and confirms theoretical equations. By modeling the flow of a prospective design, CFD provides insight into how a facility will perform, which creates a design environment conducive to solutions based on mathematics and sound science. For water and wastewater facilities, this has far-reaching benefits, such as cost savings and optimal designs, and it also provides peace of mind and enhanced confidence in design decisions.

Fine-Tuning Designs

The San Diego Miramar water treatment plant has a big responsibility—servicing more than 500,000 customers in the northern portion of the city, which also happens to be the fastest growing region in San Diego County. Peak water demands were exceeding the plant's 140 mgd capacity, and a local water shortage was predicted by 2008. Therefore, the expansion and upgrade of Miramar became essential to the city to alleviate the region's treated water shortage.

During the design of the plant, a CFD analysis was performed on the facility's proposed ozone contactor to fine-tune the final design by identifying potential modifications that would simplify the proposed design, while maintaining or improving process performance. By using CFD, potential capital and operation and maintenance cost savings were identified. The analysis was performed on basin features affecting flow characteristics (i.e., areas of short-circuiting and dead zones), specifically the straightening vanes included to reduce dead zones around each 180-degree channel bend.

Simulations performed with and without the straightening vanes revealed that their inclusion did not improve residence time. As a result, the superfluous straightening vanes were eliminated from the design, saving an estimated $300,000 in construction cost.

Maximizing Existing Facilities

The West Basin Municipal Water District's water recycling plant uses secondary effluent from Los Angeles' Hyperion wastewater treatment plant to provide high-quality recycled water to various non-potable water users. Recently, the district initiated a series of upgrades and expansions to better serve its growing needs.

For the expansion, the ability to visualize performance was key in developing an appropriate design. Designers considered that an additional chlorine contactor would not be necessary if the existing contactors, which were designed to accommodate 5 mgd each, could accommodate the expansion flow condition of 6.9 mgd.

A CFD analysis revealed that the residence time of the existing contactor at the increased flow rate was sufficient, and adding another contactor was unnecessary. To confirm the accuracy of the results, a tracer study—a traditional pilot test that is used to determine contactor residence time—was performed and reaffirmed that the residence time met the Department of Health Services' requirements.

The West Basin Municipal Water District saved an estimated $3 million to $5 million in design and construction costs by eliminating the perceived need for an additional chlorine contactor.

“The results from our CFD analysis allowed us to make a wiser, more cost-effective decision for this part of our expansion,” said Serna. “When it comes to changes or additions in existing facilities such as these, efficiency, economy, and quality are extremely important. The innovative application of CFD on our project not only saved the district money, but it also provided a better solution all around.”

Advances In UV Reactors

Recent research has demonstrated that CFD can be used to accurately predict the UV dose in flow-through UV reactors. The result is the ability to design improved, more efficient UV reactors and to validate reactor designs for drinking water disinfection applications—providing plants with access to technology that will produce a more effective treatment process.

In addition, CFD is capable of providing the data needed to calculate a UV reactor's reduction equivalent dose (RED)— a measure of the UV dose required to inactivate a target organism (e.g., Cryptosporidium) in flow-through reactors, taking into account the irradiance field and hydrodynamics of the water being treated.

The U.S. Environmental Protection Agency's UV Disinfection Guidance Manual uses RED when establishing inactivation credit for Cryptosporidium, Giardia, and viruses—the harmful microorganisms that UV reactors aim to disinfect while treating water. Receiving an inactivation credit means that a treatment facility passes the EPA's regulations for disinfection of these pathogens. Therefore, when a facility uses CFD to model its UV reactor's efficiency, the performance of the reactor is known, providing confidence that the facility operator will obtain the needed EPA credit through a field validation test required under the regulations, thereby streamlining the approval process.

“UV disinfection research and application is a cutting-edge facet of water treatment technology,” said Christopher Schulz, International Ultraviolet Association treasurer and senior vice president of CDM, which has its headquarters in Cambridge, Mass. “The ability to use CFD during UV projects is a significant advancement. Whether analyzing a UV facility piping layout, facilitating reactor design, or predicting the outcome of reactor validation testing, CFD is quickly becoming a major contributor to this field.”

For example, CFD was recently used to compare a manufacturer's UV reactor conceptual designs to a product already on the market. The designs were tested on UV dose delivery (RED), residence time distribution, pressure drop, and hydrodynamic flow patterns. CFD results provided the information needed to determine which design to proceed with, build, and test, rather than building prototypes of all options. By answering design efficiency questions before building a model, CFD saved the manufacturer both time and money during product development. The end result is an improved reactor that will meet design and performance goals.

CFD is quickly becoming the chosen tool for achieving optimal designs. Designers predict that, in time, CFD will completely eliminate a regulatory agencies' requirement for hydraulic pilot testing—yet another future efficient benefit of its application. Replacing hydraulic pilot testing as a confirmation of design will make the compliance phase of a project more cost-effective and streamlined. As such, CFD will become ingrained as the means toward not only design facilitation, but also for regulation compliance validation.