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Photograph of influent at St Cloud WWTP, Florida, taken during a Hydro International operational performance evaluation test

Real-world performance evaluation - St Cloud, Florida

Situation

As part of a new wastewater treatment plant (WWTP), the City of St Cloud, Florida’s Southside Wastewater Treatment Facility installed an 18’ diameter in-situ Grit King® advanced hydrodynamic separator. As part of the contract, Hydro International was required to verify equipment performance. This took place under the guidance of the plant’s management team, including plant administration, the contractor Wharton Smith, and the engineering firm Jones Edmunds.

The Southside Wastewater Treatment Facility is designed to cope with an average flow of 6 million gallons per day (MGD), a current peak of 18 MGD, and a future peak of 27 MGD. From experience at other plants in the area, the engineer knew that St. Cloud would require high-performance advanced grit removal equipment to prevent damage from sugar sand, which is prevalent across Florida’s treatment plants. 

Real-world evaluation

Wastewater treatment plants do not operate under laboratory conditions, so treatment plant operators should ask for reliable evidence that the real-world performance of their grit removal system matches up to the performance claims and laboratory test results provided prior to installation.

Meaningful evaluation of equipment performance once it is installed in the field requires an understanding of the characteristics of the grit that is entering a plant. As the St. Cloud Southside Treatment Facility was entirely new, this required field testing using seed sand, and carefully controlled flows. This provided an added benefit in that it allowed the plant to precisely quantify the amount of grit entering the Grit King unit and compare it to the amount of grit that the equipment was capturing.

Field testing equipment at existing plants with established wastewater flow can be difficult to accurately characterize. Obtaining a mass balance can be problematic due to fluctuating flows and the resultant changes in grit loading rates, as well as flow stratification from temperature differences between the influent entering a plant and standing water inside the plant. This new system, in a new plant, provided the ideal opportunity to accurately determine the equipment’s performance with a great deal of precision.

Testing methodology

Five separate tests were conducted at flows ranging from 2 MGD to 9 MGD to verify performance within flow ranges that the plant was likely to encounter in typical operations once online. Seed sand was introduced into the influent channel 30 feet upstream of the Grit King® unit. The seed sand concentrations ranged from 350 mg/l at 9 MGD to 1,500 mg/l at 2 MGD. The amount of seed sand introduced was controlled by an eductor to match the flow regimes being tested. The seed sand also had sizes similar to what the plant was likely to encounter in their operational influent.

In Test 1, the effluent pump had higher velocities than the influent pump (resulting in lower removal rates). For Test 2, the pumps were switched (resulting in higher removal rates). Tests 3-5 used the same pump for both influent and effluent to ensure that the exact same criteria were used to provide comparable data points.

The collected samples were sent to an independent lab in Tampa, FL to verify the sizes of the collected grit.

Measured overall removal rates ranged from 93% to 99% of all the influent grit that was introduced into the Grit King unit.

Results

St Cloud WWTP Grit King perfomance test results

This Grit King unit was guaranteed to capture 90% of all grit of 106 microns and larger in the specifications, and with removal rates ranging from 92.8% to 99.9% all performance tests demonstrated performance which was significantly better than the levels required in the specifications.

This is an example of a unit outperforming its pre-installation performance claims, but unfortunately installed units do not always meet the claims made of them, and real-world performance may not reflect performance achieved in the necessarily artificial conditions of the laboratory. When it comes to a grit removal system, ultimately all that matters is how much grit is passing through it when it is operational - operators should bear this firmly in mind and should not hesitate to test installed systems and verify the performance of their equipment.

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