When the City of Lubbock, Texas master plan and feasibility studies were completed in 2012, the City and their consulting engineer, Alan Plummer Associates, Inc., had to come to terms with two facts: 1) Lubbock is growing most rapidly in the northwest quadrant of the City and 2) The City’s main waste water treatment facility was located at their Southeast plant across town. The existing interceptor from the northwest part of the City is no longer large enough to handle peak flows. It was clear that a project must be completed to reduce current and future loadings to the existing interceptor to prepare for future growth. While many design options were analyzed, it was determined a new plant that could handle 50% of the daily flow and 100% of peak flow in the northwest part of town was warranted. Yet, building a plant in this location posed new challenges previous projects in the City’s system had not encountered.
Phosphorous Limit Drives Tank–In-Tank Design Concept
The new plant treatment process was designed for a tight phosphorous limit of 0.1% mg/L to reduce algae growth in the receiving water. Given the phosphorous limit, membrane bioreactors (MBRs) were selected to be used in the treatment process. While MBRs have been proven to be a highly effective treatment tool, they require a consistent flow to work at peak efficiency. And since the City’s peak wet-weather events are greater than the daily maximum flow rate, the required peak flow storage volume is greater than volume for daily flow equalization . Therefore, the “tank in a tank” design idea was born. In conjunction with Preload’s engineering staff, the consulting engineer analyzed many variations of tank quantities, sizes, and configurations in an effort to determine the most cost-effective method to maximize the MBR performance, while providing odor control to the equalization tank. The City and Alan Plummer’s engineers ultimately chose to combine two tanks into one, with an inner tank to serve as the equalization basin and outer tank to serve as a reservoir for peak flow events.
The Inner Tank Purpose and Design
The inner tank is a 1.25-MG reservoir that is used for daily flow equalization. The tank is covered and the headspace is exhausted to an odor control system. The tank floor is outfitted with a series of 84 diffusers to create a large-bubble mixing system. Since the inflow has already been through screening and grit removal, the diffusers were added to keep fine solids from settling, while adding minimal dissolved oxygen (to protect the downstream anoxic process). Alan Plummer’s engineers designed the mixing system to be fully-automated and remotely operated. The large-bubble mixing system is capable of varying the mixing zones with the depth in the conical basin to allow the basin to be fully drained during low-flow periods.
The inner tank was specified to be an AWWA D110, Type III Wire-Wound Prestressed Concrete Tank with an inside diameter of 90’ – 0” and side water depth of 25’ – 0.” The dome is Preload’s standard 3” cast-in-place application and was installed to prevent gaseous odor release. The subgrade design called for a 10” structural sloped floor. Preload installed approximately 37 miles of prestressing wire on 27 – 26’-11 ½ ” x 10’ –3 ½ ” panels.
The Outer Tank Purpose and Design
The outer 2.75-MG reservoir is designed for a peak wet-weather event. And although the outer peak flow storage basin is not covered for odor control, five water cannons are provided for easy wash-down after the peak flow conditions have subsided.
The outer tank is also an AWWA D110, Type III Wire-Wound Prestressed Concrete Tank wall design with an inside diameter of 165’ – 0” and side water depth of 21’ – 4 ½.” Preload installed approximately 76 miles of prestressing wire on 48 – 22 5 ½” x 10 – 3 ½” panels. The panels weighed in at over 9 tons/panel requiring a large crane for erection.
A Milestone Project
While the, “tank in a tank design” has been designed and constructed in other areas of the country, this project represents a milestone for the City of Lubbock as being the first in their system and the first in Alan Plummer Associates, Inc.’s portfolio. It also represents Preload’s first, “tank in a tank” installation in the state of Texas.
Preload is proud to be part of The City of Lubbock’s Northwest Water Reclamation Plant project. Congratulations on your new plant, ground storage tanks and your vision to prepare your system and community for continued growth.
Owner: City of Lubbock, TX
Design Engineer: Alan Plummer Associates Inc. – Fort Worth, TX – www.apaienv.com
General Contractor: Eagle Contracting
Preload District Sales Manager – Tom S. Kleppe, PE – email@example.com