The 7.5-mgd Main Street Water Reclamation Facility of Gainesville Regional Utilities (GRU) in north central Florida was built in 1930. The older, 100-hp surface aerators in its two aerobic digesters were impeded often during startup by solids concentration. However, running the aerators constantly consumed massive amounts of power and could over-aerate and kill the microorganisms.
Operators needed in-tank DO measurements to maximize aeration efficiency and maintain 28 to 33% reduction in solids. Yet, the traditional membrane-type DO sensors they used required replacement about every two months. Between sensor replacements, operators were cleaning and recalibrating sensors once a week. Concern over accuracy precluded use of DO input to the plant SCADA system to automatically control aeration.
A new solution
In May 2003, GRU began testing a Hach LDO dissolved oxygen probe in one of the
digesters. The probe applies a luminescence technique in which excited luminescent material in the probe sensor emits light as it relaxes at a rate proportional to oxygen concentration.
Performance of the probe pleasantly surprised the GRU staff: Its readings consistently compared well to a handheld DO meter, and it relieved operators of maintenance chores required by the membrane-type sensors. A month later, GRU installed a second LDO probe in the second digester.
Soon after, GRU installed similar LDO probes in the three aeration basins at the Main Street plant and two basins at its second 14.9-mgd Kanapaha plant. There, aeration basin DO readings typically measured 2 to 2.5 ppm or more. Operators found the probes were more accurate than the previously used membrane-type sensors—and that a set point of 1.2 ppm was adequate for proper aeration.
According to GRU Director of Water Reclamation Facilities James Hope, the probes have presented no operational or maintenance issues in 21 months of service. Operators have been able to reduce the amount of power needed to maintain healthy microorganism populations. They also have enough confidence in DO measurements to send them to a PID control loop to run the aeration process around DO set points. With reduced maintenance and automated control, GRU did not need to replace four operators who left the utility through attrition.
Hope reported that GRU has saved labor previously spent in weekly sensor calibration, saved the cost of bimonthly membrane replacement, reduced salaries and reduced electricity consumption thanks to automated control—all for a total annual savings exceeding $350,000.
In 2004, GRU installed a 1.5-mgd, 1,000-hp nitrification carrousel at the Kanapaha facility, accompanied by LDO probes to control oxygenation. Total electric use for the expanded plant and the Main Street plant in fiscal 2003-2004 came in more than $127,000 under budget.
Hope was quick to give credit to the LDO probes, "The bottom line tells the story. Our improved DO monitoring program is key to our effective, efficient operation."