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West Hickman Creek Wastewater Treatment Plant

Posted on: Tuesday, 20 September 2005, 03:00 CDT

West Hickman Creek Wastewater Treatment Plant

Location: Nicholasville, Ky.

Startup date: 1972

Service population: 200,000

Number of employees: 26

Average flow: 24 mgd (91,000 m^sup 3^/d)

Design flow: 33.8 mgd (128,000 m^sup 3^/d)

Peak flow: greater than 75 mgd (284,000 m^sup 3^/d)

Annual operating cost: $3 million

The West Hickman Creek Wastewater Treatment Plant (Nicholasville, Ky.) has been using single-stage nitrification and biological nutrient removal for more than 4 years to meet a phosphorus limit of 1 mg/L. The plant's discharge usually has less than 0.8 mg/L in total phosphorus with ammonia-nitrogen averaging less than 0.2 mg/ L.

The West Hickman Creek Wastewater Treatment Plant (Nicholasville, Ky.) uses biological phosphorus removal to maintain an average effluent concentration of 0.8 mg/L in total phosphorus. The plant experimented with chemical addition to increase phosphorus removal efficiency, but the chemicals interfered with nitrogen removal in the plant's single-stage nitrification process.

Upon entering the plant, raw influent is screened, has grit removed, and then flows to a high-flow diversion box, which routes design flows to the biological phosphorus removal (BPR) and single- stage nitrification steps, and any flow in excess of the design amount goes directly to the latter stage of nitrification. The flow diversion box protects the BPR zone and first zone of aeration from high flows and subsequent washout. The box includes adjustable leaping weirs and, thus, is a passive system requiring no operator attention. The overflow point can be adjusted and usually is set at the dry weather peak flow level.

The basic principle of a BPR system is simple: An anaerobic zone is placed ahead of an aerobic zone in a single activated sludge system with the influent fed directly to the anaerobic zone. BPR can be enhanced by adding supplemental phosphorus-precipitating chemicals, such as alum, ferric chloride, and ferrous sulfate, to the activated sludge without inhibiting the BPR mechanism, if the dosages are appropriately controlled.

However, when operators at West Hickman Creek attempted to add an alum-based precipitant, the chemical bound to the phosphorus in return activated sludge and prevented nitrogen release and uptake in the biological zones. Thus, the feed was discontinued. The only way to remove the chemical and bound phosphorus from the system was through excess wasting.

The nitrification system discharges to the circular final clarifiers, which lead to disinfection by gaseous chlorine and dechlorination by gaseous sulfur dioxide. Both gas systems have containment scrubbers to prevent leaks. Finally, the flow is re- aerated before it is released to West Hickman Creek.

In 2000, the plant was upgraded from 22.3 mgd (84,400 m^sup 3^/ d) to 33.8 mgd (128,000 m^sup 3^/d) at a cost of $9.4 million. The expansion included converting the anaerobic digesters to aerobic sludge holding tanks, converting the primary clarifiers to BPR (fermentation) tanks, and converting to single-stage nitrification. Two final clarifiers were added, along with three belt filter presses. (Waste activated sludge is aerated, thickened, dewatered via belt filter press, and hauled to a landfill.)

The West Hickman Creek Wastewater Treatment Plant (Nicholasville, Ky.) uses a real-time process monitoring system to automatically measure nitrites, nitrates, ammonia-nitrogen, and orthophosphate at eight different stages of treatment. While not all four parameters are measured at each location, this setup enables real-time monitoring of phosphorus release and uptake and nitrification.

Also during the 2000 expansion, the plant installed a real-time process monitoring system that automatically measures nitrites, nitrates, ammonia-nitrogen, and orthophosphate at eight different stages of treatment. While not all four parameters are measured at each location, this setup enables real-time monitoring of phosphorus release and uptake and nitrification. It also allows the plant to denitrify a little in its clarifiers and thus return fewer nitrates to the BPR process, resulting in shorter BPR detention times.

Copyright Water Environment Federation Sep 2005

Source: Water Environment & Technology

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