How Efficient is Your Mixed Tank?

A midwestern wet corn miller contacted us when it began experiencing treatment issues. Wet corn milling plants typically produce a broadly diversified line of products including maltodextrin products, corn syrup solids, starches, ethyl alcohol, and more. The company’s wastewater treatment plant employed two stages of biological treatment – anaerobic pretreatment and aerobic final treatment with effluent discharge to a local river.

The anaerobic pretreatment system had a number of reactors operated in parallel. ATI was hired to determine the actual hydraulic efficiency of the tanks, which describes the retention time distribution of the fluid and flow regime in the system. For anaerobic reactors, the better the mixing, the longer the actual retention time, the better the biomass-to-substrate contact, the better the performance.

The hydraulic efficiency (in actual tanks) can be influenced by flow disturbances in the inlet and outlet zones, the proximity of inlet and outlet piping, density and convection currents caused by temperature, density differences, tank size and shape, ineffective mixing or distribution, and the extent of dead spaces.

ATI provided the engineering assistance necessary for the dye study to determine the actual retention time and information needed for future planning efforts. An ATI engineer and scientist traveled to the site to set up the dye study and train plant personnel.

To conduct the study, Rhodamine WT dye was injected into the system. A total of 800 samples were collected from the anaerobic system effluent and recycle streams over 20 days by plant personnel. ATI personnel measured the dye concentrations of the samples and analyzed the results using the Fluorometer.

The study determined the actual hydraulic retention time of each anaerobic reactor and proved that the tanks were hydraulically inefficient. The next challenge is to design mechanical upgrades to the tanks to improve their hydraulic efficiency, thus improving their treatment performance.

Interestingly, we have used the dye measurement test in a variety of beneficial applications, including the following:

• Calibration of flow meters
• Discharge outfall dispersion modeling
• Thermal plume definition
• Effluent mixing definitions