Thorough Mixing vs. Energy Consumption

Authors:

  • Sonia M. Oton, P.E., Srinivas D. Gidugu, P.E., Paul D. Saurer, P.E., Janice R. Carroll, P.E. - Hazen and Sawyer, P.C.
  • Salil M Kharkar, P.E., Rouben Der Minassian - District of Columbia Water and Sewer Authority
  • Matthew S. Osit, P.E., Daniel A. Solimando, P.E. - New York City Department of Environmental Protection

As more and more wastewater treatment plants upgrade to meet stringent nutrient removal limits, and simultaneously work to reduce operating costs, the focus on efficiency in each unit process and in all types of equipment has increased. Mixing is a critical element of the nutrient removal process, in both the anoxic and swing zones. About 70% of total energy consumption is spent in mixing and aeration. Historically, vertical shaft mixers have been widely used in this type of application and little thought has been dedicated to reducing energy consumption in mixing applications. However, a new vertical shaft mixer, the hyperclassic mixer recently introduced into the US market, by Invent Umwelt of Germany, consumes approximately half the energy as conventional vertical shaft mixers. The hyperclassic mixer, widely used in Europe, features a hyperboloid-shaped impeller constructed of fiberglass reinforced plastic, equipped with integrated motion fins that create a radial flow directed towards the outer walls of the tank, and operates rag-less.

This paper will present the evaluation, pilot and full scale testing of this type of mixer in two rigorous US applications. These include: (a) a three year pilot test of two hyperclassic mixers in 15’ deep tanks, followed by installation of 90 units at a 150 mgd plant in New York City, (b) thorough mixing testing results of the hyperclassic and conventional vertical shaft mixers in 36’ deep basins at a 370 mgd plant in the District of Columbia. Findings from evaluation will include: flow mixing patterns and mixing test results, operations and maintenance requirements, capital and operating costs of the hyperclassic and conventional vertical shaft mixers.

Bowery Bay Wastewater Treatment Plant, New York City
Full scale tests were conducted with two vertical shaft mixers installed in Pass B of Aeration Tank No. 6. Figure 1 shows an hyperclassic mixer installed in Tank No. 6. Several criteria including the ability to maintain uniform solids distribution throughout the anoxic zone, maintenance of low DO concentrations, establishment of a well defined hydraulic profile for supporting effective denitrification, capital and operation and maintenance (O&M) costs were used by the City College of New York to evaluate the mixers. The suspended solids concentration profiles were developed across the horizontal section of the anoxic zones at elevations of 3, 7 and 12 feet below the water surface. DO and ORP were measured using the YSI 556 multi parameter probe. The hydraulic efficiency of the tank was determined by conducting a tracer study using Rhodamine WT, a fluorescent dye. The dye tests indicated that the Invent mixer behaved like three completely mixed reactors in series compared to two mixers in series for submersible mixers. The study concluded that the hyperclassic mixers were more efficient than the existing submersible mixers.

Technology site visits to the plants in Germany, Holland and Belgium supported the City College findings and confirmed the broad application and durability of these mixers in wastewater treatment facilities.

The lower horsepower and minimal preventive maintenance requirements result in significant energy savings and lower capitalized costs to the plant. Full scale operation of these mixers is expected to begin in the third quarter of 2009.

Blue Plains Wastewater Treatment Plant (DCWASA), Washington, DC:
A thorough mixing test was conducted in two Nitrification Reactors to demonstrate that the basins are thoroughly mixed. Thorough mixing is defined as having the biological solids in suspension such that the solids concentration at any given point is within +/- 10% from the basin arithmetic average concentration. Samples were collected in Reactor 8, currently operating two 20 Hp conventional vertical shaft mixers, and in Reactor 11, currently operating two 10 Hp hyperclassic mixers. At each Reactor, samples were withdrawn from ten locations, at elevations of 6, 16 and 27 feet below the water surface, for a total of 30 samples per Reactor. Figures 2 and 3 show the total suspended solids concentration profiles for each Reactor. Installation of additional 66 hyperclassic mixers is expected to begin in the summer of 2009.

While both Reactors meet the “thorough mixing” criterion, the 10 Hp mixer power costs are approximately half the 20 Hp mixer. At a unit power cost of 0.10 $/Kw-h, the 10 year present worth power cost per mixer is $46,000 and $87,000 for the hyperclassic and the conventional vertical shaft mixer, respectively.

REFERENCES:
The City College of New York Department of Civil Engineering (2005) Evaluation of Anoxic Zone Mixers at the Bowery Bay WPCP. Report presented to NYCDEP.

For a copy of the full paper, please contact the author at soton@hazenandsawyer.com

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Horizons Fall 2017 (pdf)

Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.

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