Challenge

The Wallace R. Knox Wastewater Treatment Facility, in Texas, experienced solids wash-out during storm flow conditions. As the waste strength increased, too few microorganisms were left in the system to assimilate the waste. This resulted in violations of the NPDES permit. The upgraded system needed to be energy efficient, even while experiencing extremely variable loading conditions.

Solution

An Envirex®  Vertical Loop Reactor (VLR®) system upgrade was installed. This system differs from a conventional oxidation ditch because the looped flow is made over and under the horizontal baffle with the top and bottom portions of the tank being constantly flipped.

• The variability of oxygen input and multiple tank operation combine to provide optimum flexibility.
• The channel flow, divided by the horizontal baffle, does not allow any air bubbles to escape to the surface until the end of the tank.
• Air bubbles are in contact with the liquid several times longer than in a conventional aeration tank.
• Typical VLR tanks are arranged in series, usually with three or more rectangular tanks-all of equal size.

Results

The city’s VLR system has proved to be even better on aeration draw than expected, while providing excellent effluent limits. The storm flow problems were solved by diverting the raw wastewater (diluted by the rain water) into the second, third or fourth tank, depending on how dilute the waste had become.

• Since installation, the treatment plant has consistently met the effluent permit limit.
• The plant treats low load months of 4,000 lbs of BOD/day to months above 9,500 lbs/day.
• BOD effluent average is 3.2 mg/l.
• TSS effluent average is 5.2 mg/l.
• MLSS are typically in the 4,000 mg/l to 5,000 mg/l range.
• Sludge age averages in the 15 to 20 day range.
• D.O. concentrations are at design levels with the first two tanks operating in an aerated anoxic condition (<0.5 mg/l) and the last reactor operating in the range of 2 to 3 mg/l.