Partially buried & underground tanks - Case study
Topics: potable, stratification/waterage, short-circuiting, chlorine
Overview: The underground tank is a concrete rectangular structure (226' long, 166 wide, 21 ft high) built in 1968. Divided in the center, each side has a 2.5 MG storage capacity. The partially buried conical bowl tank (115' diameter, 20' high) with a geodome roof built in 1908 and refurbished in 1975, also with a 2.5 MG storage capacity. Both tanks have common inlet / outlet structures, and both tanks use free chlorine as the disinfectant.
Conditions / Objectives: Both tanks experienced significant thermal stratification from inadequate circulation resulting in short-circuiting, inconsistent water age and water quality.
Solution: June 2011, installed two (2) GridBee® GS-12 mixers in the underground tank (one on each side of the partition), and one (1) GridBee® GS-12 mixer in the partially buried tank. Three strings of digital temperature probes were also installed in each tank to monitor vertical thermal profiles. Each string measured water temperatures at 6 inches,5 feet, and 10 feet below the surface, as well as 6 inches above the tank bottom (see illustration on page 2).
Results: The vertical profile temperature data are presented graphically below. The data clearly indicate that the GS-12 mixers performed very well in both tanks. Prior to the startup, water in the underground tank had 5°C stratification from bottom to the surface, while the partially buried and had 3°C stratification. Within 24 hours of starting up the GS-12s, water column temperatures converged to within 0.5°C. In both tanks, fill cycles were detectable by observing how bottom temperatures dipped lower than surface waters due to cooler influent water. For several hours following each fill, the GS-12 mixer converged the temperatures, rapidly mixing in the new colder water that entered during the fill cycle. As part of the study, an intentional off/on cycle of the GS-12 mixer in underground tank showed that temperatures began to stratify soon after the mixer was turned off, but again quickly converged when the mixer was turned back on. Furthermore, measurements from the three separate test locations within each tank indicate that water temperatures were uniform both vertically and horizontally, from one end of the tank to the other.
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