Success of Onlin Leads to Closing for Wastewater 2 Process Control of Consent Order Treatment Plant

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In 2003, Plant City, a small Florida city of 30,000 residents, faced a very big problem. The Florida Department of Environmental Protection (FDF.P) determined that the city's wastewater treatment plant was not consistently operating within its permit limits, and placed it under a consent order.

Plant City is a highly industrialized area. 'Our industrial loading is very diverse and includes metal treating, fiberboard construction material, food processing, and dairy operations, which definitely affects the operation of our plant' said Patrick Murphy, the city's chief plant operator. 'During the week we typically get five-day carbonaceous biochemical oxygen demand, orCBOD5,ashigh as 500 mg/I. coming into the plant, but on the weekends when the manufacturers are closed, it can decrease to only 100 mg/L. To operate within our permit limits we must be able to quickly respond to these changes, and thai can only happen by providing experienced operators with accurate information in time for them to adjust the process.'

Plant City management decided to invest in a new plant to bring its operations into compliance so the consent order could be lifted. I( was apparent, however, that new tanks and aeration basins alone wouldn't sufficiently improve operations; it would also require better process control.

Plant City faced a challenge common to many small wastewater treatment plants, as well: turnover of its experienced operators. 'Small plants serve as the industry's training ground,' said Murphy. 'It's been our experience that soon after our operators become experienced and licensed, they are likely to leave for higher paying positions at other facilities. Part of our challenge is then to bring in untrained people who may have been flipping burgers last week and train them to operate the plant.'

Two state-of-the-art online nutrient analyzers, manufactured by ASA Analytics, were installed in the new plant to take the guesswork out of managing the process. 'Our analyzers are the heart of the plant and our staff can operate them with minimal training,' Murphy explained. 'The online analyzers are used to monitor orthophosphate, ammonia, nitrite, and nitrates, and they literally run the plant by telling the programmable logic controller, or PLCs, when and how fast to run the blowers and how to adjust the valves to keep the process in compliance for total nitrogen.'

The analyzers automatically draw samples from the aeration basins and test for orthophosphate, which can vary significantly, depending on the industrial flows. This provides the operators with the information needed to add the optimal amount of ferric sulfate. 'Under typical flow conditions, we're supposed to pump two gallons of ferric sulfate per mg/L of phosphorus, hut because we don't know when a heavy load of phosphorous is coming in, timely information allows us to adjust our flow rate for actual flow and load conditions,' said Murphy.

The ChemScan Model 4100 uses automated ultraviolet-visible spectrometry to measure absorbance levels across 256 wavelengths of ultraviolet and visible light.
A single-process analyzer can automatically draw samples and monitor up to eight sample points, detecting ammonia, nitrite, nitrate, and orthophosphate as individual parameters from multiple process sample points or basins.

Plant City's master lift station features six 100 horsepower (hp) pumps that pump the flow into three anoxic tanks for conditioning and further grit removal. It then moves on to the aeration basins, which were designed to operate as simultaneous nitrification and den-itrification, or with distinct aerobic and anoxic zones. Under the current configuration, the flow first enters the aeration basin into an anoxic zone; it then proceeds around the tank where the four oxygen valves are usually open, sustaining the aerobic zone that breaks down the ammonia, converting it to nitrite and nitrate. The mixed liquor again flows through the anoxic zone where dcnitrification occurs.

The analyzers draw samples from two grinder pumps located in each of the three aeration basins. They then test for nitrate, nitrite, and ammonia and instruct the PLCs to adjust the blowers to optimize the nitrification and dcnitrification processes.

'I wasn't working at the plant when it was placed under the consent order, but T could see how the previous operating conditions could have led to the problems,' said Murphy. 'They were using dissolved oxygen probes and grab samples, but couldn't do them frequently enough or adequately control the aeration system to maintain a compliant process.'

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