The Fine Line Between Thorough Mixing and Energy Consumption

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 and submersible mixers have been widely used in this type of application. Limited research has been done to reduce energy consumption in mixing applications. However, a new style of vertical shaft mixer recently introduced into the US market, the Hyperclassic mixer (which shall hereinafter be referred to as the hyperboloid mixer) as manufactured by Invent-Umwelt, can provide thorough mixing at lower energy requirements. The hyperboloid 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 without building up rags on the impeller.This paper will present the evaluation, pilot and full scale testing of this type of mixer in two rigorous US applications. These include: (a) thorough mixing testing results of the hyperboloid and conventional vertical shaft mixers in 36 feet deep basins at a 370 mgd plant in the District of Columbia, (b) a three year pilot test of two hyperboloid mixers in 15 feet deep tanks, followed by installation of 90 units at a 150 mgd plant in New York City. Findings from these evaluations will include: flow mixing patterns and mixing test results, and capital and operating costs.DCWASA Blue Plains Advanced Wastewater Treatment Plant, Washington DCA 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 Stage 4 of Reactor 8, which is currently operating with two 20 Hp conventional vertical shaft mixers (which shall hereinafter be referred to as the hydrofoil mixer), manufactured by Philadelphia Mixing Solutions, and in Stage 4 of Reactor 11, which is currently operating with two 10 Hp hyperboloid mixers manufactured by Invent-Umwelt. While both reactors meet the thorough mixing criterion, the hyperboloid mixer has demonstrated thorough mixing at a lower “power/unit volume” mixing ratio than that of the conventional hydrofoil mixer.Bowery Bay Water Pollution Control Plant, New York CityFull scale tests were conducted with two hyperboloid mixers installed in Pass B of Aeration Tank No. 6. Several criteria including the ability to maintain uniform solids distribution throughout the anoxic zone, maintenance of low dissolved oxygen (DO) concentrations, establishment of a well defined hydraulic profile for supporting effective denitrification, and capital and operation and maintenance (O&M) costs were used to evaluate the mixers. Dye tests conducted to determine the hydraulic profile indicated that the hyperboloid mixer created a flow regime that approximated like three completely mixed reactors in series. The study concluded that the hyperboloid mixers were more efficient than the existing submersible mixers. Full scale operation of these mixers is expected to begin in 2009.