Hydraulic modeling: The often overlooked factor in treatment plant design


Courtesy of Innovative Hydraulics

Treatment plant hydraulics is often overlooked as one of the most important aspects of treatment plant design, especially in plant upgrade scenarios where retrofitting new equipment or processes into the existing treatment train will create additional hydraulic restrictions.  Many existing plants already have a minimal amount of head available for passing peak flows through the system, which can make plant upgrades even more of a challenge.

The Bethel Park Municipal Authority located outside of Pittsburgh, Pennsylvania faced this exact problem when recently upgrading their trickling filter wastewater treatment plant.  The existing trickling filter system involved a confusing scheme of recycling flows between the one trickling filter pump station and both trickling filters.  Rather than the filters operating in series (as originally designed), they often operated in parallel due to the poorly designed pump station wet well.

In order for the trickling filters to function as intended in a true series operation, a separate pump station had to be designed that would allow flow to be pumped directly to each filter from its own pumping system.  Also included in the design were unique flow distribution boxes that managed the forward flows to the filters, return flows from the filters, and provided for the flexibility of either pump station being able to pump to either filter should one pump station be off-line.  While the concept behind the flow distribution scenarios was fairly simple, the hydraulics associated with making the entire design work was not.  Each distribution box has multiple chambers that direct the flow to its intended location within the process.  Varying floor inverts and weir elevations had to be established that would ensure that the appropriate amount of forward flow and return flows were getting to each filter.  Additionally, since the available head throughout the treatment plant was very limited, the additional head loss created by the upgraded system had to be minimal.

With a wide range of flows possible at the treatment plant (3 MGD – 18 MGD), hydraulic modeling was determined to be the best solution for analyzing all of the possible hydraulic scenarios and determine the design that would be best suited for the hydraulic limitations.  In order to model the hydraulics for the Bethel Park plant, Visual Hydraulics was determined to have the most flexibility and would allow designers to easily change hydraulic features, return flows, and model the hydraulics over the range of flows typically experienced at the plant.  In this case, the software was used to model the flow distribution boxes in terms of box size, invert elevations, and weir elevations.  Through modeling it was also determined that the best way to control the return flow to each of the trickling filters was to place gates on the distribution box effluent lines that would open and close based on the amount of flow being conveyed to the filters.  Under lower flows, the gates would remain mostly closed, allowing more flow to be recycled.  As the flows in the plant would rise (typically as the result of rainfall events), the gates would open to allow more flow to be conveyed as forward flow through the trickling filters to the secondary clarifiers.  This permits the plant to avoid unnecessarily recycling flows that are mostly rainwater, which can hurt the biological process. A diagram of the hydraulic profile created with Visual Hydraulics for the Piney Fork treatment plant is shown below:

Detailed hydraulic modeling was required to determine how much head loss should be generated by the distribution box effluent gates to produce the desired amount of flow being returned to the filters.  This had to be balanced with the amount of flow to be passed onto the next process in the system.  Since the return flows are conveyed over a weir before being pumped back to the appropriate trickling filter, the weir elevations had to be set based on the gate hydraulics as well.  Only hydraulic modeling and the flexibility that a computer generated model can provide could make this complex analysis as pain free as possible.

The Piney Fork plant upgrade project was completed in later summer of 2007.  The plant has received numerous peak flow events that have met or exceeded the plant capacity of 18 MGD since the plant upgrade was constructed.  The hydraulics of the new trickling filter system have performed as predicted, and the trickling filters are producing significantly lower BOD and NH3 concentrations than they have since they were originally placed into operation over 50 years ago.

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