Launch Slideshow

Continuously sequencing reactor

Down to a science

Down to a science

  • Continuously sequencing reactor

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    Continuously sequencing reactor

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    Source: Schreiber LLC

    The CSR process cycles through the aerobic, anoxic, and anaerobic phases in a single basin.

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    Photo: Clayton County Water Authority

    The Shoal Creek water reclamation plant in Clayton County, Ga., uses a continuously sequencing reactor for denitrification.

  • Continuously sequencing reactor

    http://www.pwmag.com/Images/tmp3C2%2Etmp_tcm111-1333344.jpg?width=150

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    Continuously sequencing reactor

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    The CSR process cycles through the aerobic, anoxic, and anaerobic phases in a single basin.

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Process Details

As an activated sludge process, the Schreiber CSR uses the normal three phases—aerobic, anoxic, and anaerobic—but provides all three in a single basin via process cycling, or sequencing. First, with the aeration on, the basin is aerobic, typically for several hours at a time. Then the blowers are turned off, and for the next 30 to 90 minutes, the basin is anoxic. Finally, as the blowers remain off, the basin undergoes anaerobic conditions for 15 to 45 minutes. Significantly, the actual duration of each phase depends on biological loadings.

Conditions that control when the CSR system moves from one phase to the next are called phase duration controlling parameters. For the older Northeast WRF, this is largely decided by timers, while at the Shoal Creek WRF, the controlling element is the analyzers. The CSR process is optimized when the system minimizes the time required to finish an entire sequence of aerobic, anoxic, and anaerobic process phases while completely fulfilling the biological objectives of each phase. A CSR system under normal conditions will exhibit a range of 3 to 12 cycles per day.

Control Via Analyzers

The Shoal Creek plant employs analyzers in its process control strategy. The system continuously monitors the soluble nutrient parameters in the mixed liquors of the reactor basins. Analyzers measure concentrations of ammonia, nitrates, and phosphates at all times during the three phases. Since these concentrations all vary in a predictable manner in each of the three phases, the data measured can be used to monitor and control the process.

The process prevents a phase from ending until it has been completed. The system's ability to bring each phase to completion is limited by the accuracy and repeatability of the monitoring devices. The SchreiberFlex control system optimizes each phase and initiates preemptive actions to ensure each phase reaches completion.

The control system has a series of other “watchdog” components and programmable logic controllers that protect the process from errant analyzers and other practical concerns, but the fundamental concept of the system is to use the ammonia/nitrate/phosphate analyzer data and, when necessary, dosing systems to drive each phase toward biological completion, then promptly move the process into the next phase.

This analyzer-based strategy addresses the twin concerns of efficiency and effectiveness inherent in timer-based control systems. The result is a system that more consistently and more fully completes each process phase in a minimum amount of time. This optimization yields the lower TN values exhibited in the effluent from the Shoal Creek plant. The advanced analyzer/dosing control system facilitates process optimization to a greater extent.

The performance of the CSR process at Shoal Creek has continued to improve. A three-day effluent trending chart from the plant's supervisory control and data acquisition system shows that the nitrate concentrations are typically less than 2 parts per million (ppm), and that only four times in that span of days did nitrate levels exceed 2.5 ppm; even those excursions were limited in duration. During the three-day span, phosphorus remained consistently around 1 ppm, with only one excursion approaching 2 ppm. Also, the SchreiberFlex system consistently maintained the effluent ammonia at less than 0.5 ppm, reaching 2.25 ppm for only one short period. The analyzers require some operator attention, but the extra effort appears to be well worthwhile.