Energy-Saving Mixing System Pays Big Dividends

July 2, 2015
This article discusses how significant energy savings were achieved at the Madison Metropolitan Sewerage District in Madison, Wis., by replacing older, larger submersible mixers in the biological selector basins with lower energy submersible mixers. This was done as part of an overall plant goal to reduce energy consumption.
Aerial view of Nine Springs WWTP (Madison, Wis.).

By replacing older, larger submersible mixers in biological selector basins with lower energy submersible mixers, the Madison Metropolitan Sewerage District was able to achieve significant energy savings. This was done as part of an overall plant goal to reduce energy consumption.

Madison Metropolitan Sewerage District (MMSD; Madison, Wis.) manages the regional wastewater service and treatment for the Madison area of Wisconsin. MMSD provides regional wastewater collection and treatment services for 43 municipal customers, including cities, villages, utility districts, and sanitary districts. With a total service area of about 180 square miles, the District serves a population of around 330,000 and handles an average daily wastewater volume of 42 million gallons per day (MGD). The City of Madison is MMSD's largest customer, contributing about 70 percent of the total wastewater flow.

Each municipality served by MMSD owns and operates a local sanitary sewer collection system that feeds into the District's regional system. The MMSD-owned regional collection system includes 17 pumping stations, 92 miles of gravity interceptor sewer and 29 miles of force main. All wastewater collected within the MMSD system is conveyed to and treated at its Nine Springs Wastewater Treatment Plant.

Preliminary treatment at the Nine Springs plant is accomplished by fine screening followed by vortex grit chambers. The District uses three fine screens (6-mm openings) called band screens. Unscreened wastewater flows into the center of the "band" and then passes perpendicular to the direction of inlet flow through the 6-mm openings, yielding screened wastewater downstream of the screen, with screened materials held on the inside of the band. The band screen rotates to bring the screened materials to the top of the band where reuse water is used to flush the screenings off the inside of the screen and into a trough for conveyance to compaction.

Grit removal follows the screens. Three grit chambers remove grit by vortex action, with settled grit falling into a lower sump for pumping to grit classification. De-gritted raw wastewater then leaves the grit chamber and flows by gravity to the primary clarifiers.

The fine screening and grit chambers remove mainly inorganic materials that are disposed of in a landfill. Following the grit chambers, primary settling tanks are used to remove materials that settle and float from the wastewater.

The activated sludge process that takes place in the aeration tanks and final clarifiers then provides biological secondary treatment. Nine Springs is an enhanced biological phosphorus removal (EBPR) conventional activated sludge facility and uses Xylem's Sanitaire fine bubble ceramic disc diffusers to provide aeration. These fine bubble diffusers are 9 inches in diameter and provide high-density, full-floor coverage in any tank configuration.

Alan Grooms stands with one 2.5-HP mixer and one 4.0-HP mixer used in selected mixing zones.

Biosolids that are separated from the wastewater during primary and secondary treatment are thickened, digested, thickened again, and then applied to local cropland as a soil conditioner and fertilizer. These biosolids are marketed by MMSD under the name "Metrogro". Further, the treated effluent water from the Nine Springs plant is pumped to the District's discharge points on Badfish Creek and Badger Mill Creek. The wastewater treatment facility also disinfects all effluent seasonally (mid-April to mid-October) using ultraviolet (UV) disinfection.

Significant Energy Demands

An undertaking of this magnitude demands a large amount of energy. At the Nine Springs Wastewater Treatment Plant, significant energy savings were achieved by replacing submersible mixers in biological selector basins with lower energy submersible mixers. In addition, all of the mixers were about 16 years old and nearing the end of their useful lives.

The appearance of problematic scum was solved with a trial approach by increasing the mixer power to 4 HP.

Evaluation of the mixing system in the biological selector basins ultimately revealed the potential for significant savings as a result of recent process improvements that removed heavier solids prior to biological treatment. By removing these heavier solids with the aforementioned grit removal and fine screening, less mixing energy was required to suspend the remaining solids in the selector basins.

Proposals from manufacturers were solicited for reduced mixer sizes that would provide good mixing under the new conditions. A unique aspect of these proposals was the requirement to provide a trial mixer in order to evaluate actual plant performance as well as allow the electrical and mechanical maintenance crews to more closely evaluate the equipment under "field conditions."

Bulk of other mixer zones where the 2.5-HP models are sufficient.

Field trials supported Xylem's claims that adequate mixing could be achieved with lower energy input. Based on the results of the trial as well as the observations of the staff involved, it was recommended that Nine Springs select the Xylem proposal moving forward.

As a result, in 2013, MMSD decided to move forward with Phase 1 of the project using Xylem's new Flygt Brand 4630 compact, low-energy replacement mixers in 26 of the biological selector basins. Phase 2 of the project will be installed in 2015 and will involve replacing the remaining 10 selector zone mixers with model 4640 mixers in areas that require additional mixing energy.

Flygt compact mixers easily blend highly contaminated fluids, high-density or high-viscosity liquids, and liquids with fibrous material. Features of these space-saving mixers include compact, robust, direct-drive motors; large-volume oil housings; zero leakage; non-clogging, fiber-handling hydraulics; and higher available efficiencies with an optional jet ring. Guide bar systems for any tank used together with Flygt's lifting equipment enable easy access for inspection and service.

The field trial also revealed some important performance aspects that would otherwise have been undiscovered until after purchase if a more traditional solicitation had been done (i.e., no in-basin trial). This allowed modification of the project prior to final purchase to better meet the demands of the system.

From the trial, MMSD basically learned that a known small quantity of surface scum that formed on the selector basins with the old mixers during the winter turned rapidly into a major issue with the lower energy mixers. Given that it was discovered during evaluation, staff was able to modify the purchase to replace a smaller number of mixers at that time, pending further evaluation for a solid solution later. Flygt then offered a mixer that was slightly more powerful than the existing mixers, so staff was able to evaluate how the new alternative handled the scum issue. The mixer performed acceptably, and staff moved ahead and ordered the last block of replacement mixers.

Once the Phase 2 installation is fully operational, the new mixing system is projected to use about 40 percent of the energy as before. After only three months, the performance of Phase 1 was meeting projections, and the estimated payback on the project will be under three years, not including a Wisconsin Focus on Energy (FOE) grant that was approved for the project.

Grant Specifics Outlined

An FOE (Focus on Energy) grant is offered to incentivize the adoption of energy-saving measures and projects. There are prescriptive and custom grants. Prescriptive grants basically have an approved list of improvements and equipment that, if implemented, equal a grant of "X" (i.e., each old light fixture replaced will result in a grant of $X), subject to terms and conditions. Prescriptive grants leave no latitude for deviation. Failure to conform will normally result in losing the grant, even if the measures adopted actually make more sense or save more money.

If a situation arises that is not already covered in a prescriptive grant, then a custom grant is in order. A custom grant -- which is what MMSD used -- basically proposes a project, states what the "before" and "after" energy use will be, and what the project will cost. FOE then takes this worksheet and the provided figures and determines if the project is eligible for a grant, and if so, how much it will be. The size of the grant is determined by a formula that accounts for cost of energy, total cost to implement, total savings, paybacks, and more. The recipient of a custom grant receives the money at the end of the project, once FOE verifies that the proposed project has actually been installed and commissioned.

In the case of MMSD's initial grant, the District was originally offered $73,628.64, but it was reduced to $32,498 due to modification of the project as well as final costs coming in much lower than expected (the grant has to be applied for before bidding). Phase 2, which comprises the final 10 mixers, has been offered a grant of $13,739. Since the mixer price was known, this figure is likely to be close to the final number. The only variable will be the cost to modify the motor control centers.

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