A multi-parameter analyzer helps a Kentucky treatment plant step up to the plate and meet a new phosphorus limit in its permit

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In 2001, the Lexington Fayette Urban County Government West Hickman Creek Wastewater Treatment Plant in Nicholasville, Ky., was starting Phase II of its third upgrade since 1972, when the state imposed a phosphorus limit.

Total phosphorus hovered around 6 mg/l, but the permit required 1 mg/l monthly average and 2 mg/l daily maximum from May through October. Engineers from Tetra-Tech, the Lexington-based company designing the expansion, converted the anaerobic digesters to aerated sludge holding tanks and the primary clarifiers to biological phosphorus removal (BPR) fermentation tanks.

They also modified the two-staged activated sludge nitrification system and added two final clarifiers, chlorine contact, and three new 2-meter belt presses to replace the old ones.

As part of phosphorus removal, operators feed sodium aluminate precipitant at Zone 2 in the aeration tanks before the wastewater enters the final clarifiers. 'The chemical enhances the BPR process by binding to the phosphorus in the return activated sludge and waste sludge,' says Jim Worten, supervisor senior of plant operations.

To monitor phosphorus removal, engineers selected the PHOSPHAX orthophosphate analyzer from Hach Co. and the ChemScan UV-4100 analyzer from ASA Analytics, which measures nitrite, nitrate, ammonia-nitrogen, and orthophosphate at eight treatment stages in real time.

The system also allows the plant to begin denitrification in its clarifiers, returning fewer nitrates to the BPR process and thereby shortening detention times. Today, the phosphorus in the plant discharge to West Hickman Creek averages 0.5 mg/l.

Assembly required
The 270-acre facility has a peak design capacity of 64 mgd, but flows average 22 to 23 mgd. The plant treats more than 8 billion gallons annually, serving a population of 150,000 in Fayette and north Jessamine Counties.

Contractors installed the ChemScan system in four months. 'They took tanks and channels out of service, core-drilled through the walls, and installed Bettis (Emerson Process Management) electric valves,' says Worten. 'Then they synchronized the solenoids to open and close the valves simultaneously and in sequence. An extensive piping system of 1.5-inch PVC Schedule 80 pipe connects everything.'

ChemScan representatives programmed the computer and trained the staff, who found the learning curve uncomplicated. 'I sat down with the manual and went right through it,' says Worten. 'It's all keypads and touchscreens.'

The analyzer receives samples from eight treatment stages. On a 60-minute cycle, it monitors the influent channel entering the BPR tank, the midway point in the tank, and the effluent end. It draws influent and effluent samples off the first- and second-stage aeration basins, and from the line returning activated sludge to the BPR tank.

Lines flush automatically for three to four minutes to ensure fresh samples. Then the system analyzes ammonia, nitrite and nitrate levels. When analyzing phosphorus levels, the lines flush again before sampling. Monitoring of the four parameters takes eight minutes, and monitoring for only the nitrogen parameters takes five minutes.

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