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

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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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Future permit limits are a genuine concern for those in wastewater treatment—especially total nitrogen (TN) limits. Most forecasters predict TN limits will be cut from the current 5 to 10 mg/L range down to somewhere around 3 mg/L. However, TN removal is not new; development of treatment systems has been underway for more than 40 years.

The Clayton County (Ga.) Water Authority (CCWA) has an interesting TN removal history. Located in the southern part of metro Atlanta, the county has experienced tremendous growth over the past two decades. Having long been a pioneer in land application and reuse, in 1986 the CCWA became the first water utility in the state to implement biological nutrient removal (BNR). Starting with the Modified Ludzack-Ettinger (MLE) process, they progressed to the Continuously Sequencing Reactor (CSR) process used today.

The earliest BNR water reclamation facility in Clayton County used MLE. A circular version was installed at the CCWA Northeast Water Reclamation Facility (WRF) in 1986. As reported in the April 1988 issue of PUBLIC WORKS, the process yielded better-than-expected results, attaining an 84% removal rate; TN values reported in the first year averaged 4.2 mg/L.

Schreiber LLC, Trussville, Ala., supplied the circular MLE as a version of its counter-current aeration (CCA) system. The single reactor was a 171 foot diameter, 3.1 million gallon basin with a concentric circular baffle wall separating it into a center ½ million gallon denitrification zone—the anoxic “center basin” and an outer 2.6 million gallon aerated zone (the aerobic “annular ring”). The diffused aeration system consisted of ceramic diffusers mounted on an aeration bridge, which continuously rotated about the basin. The movement of the diffuser assemblies through the water by the aeration bridge provided mixing for the reactor and allowed the aeration to be completely independent of mixing.

In the late 1980s, development of the membrane diffuser offered new options. Unlike with ceramics, the diffusers now could be turned completely off without fouling, allowing creation of anoxic and anaerobic conditions within the basin. This allowed the circular CCA basin to be transformed into a CSR.

In the early 1990s, plans were underway to double the capacity of the original Northeast WRF. However, instead of just adding a copy of the existing circular MLE, engineers decided to test Schreiber's newly developed CSR design. They constructed the new train in the CSR configuration, then ran the original circular MLE and new CSR side by side. The experiment demonstrated that the new CSR configuration was superior, and the original MLE then was reconfigured to a CSR.

Further population growth in the county called for additional treatment capacity. In 2002, a dual-train CSR plant was commissioned at the new Shoal Creek WRF. Although the two plants both now operate CSRs, the results are quite different, primarily because of the new facility's enhanced automated control systems.

The Shoal Creek facility on average produces an effluent with TN concentrations of less than 2.5 mg/L—within the anticipated future permit limits in the 3 mg/L to 4 mg/L range. Although the older plant produces an excellent effluent, its typical TN runs from 4 to 6 mg/L. Data from the Shoal Creek CSR plant demonstrates that a single-sludge system can achieve very low TN levels. Historically, single-sludge systems were thought to be incapable of such results.

Unmandated TN Excellence

Ironically, there are currently no TN permit limits on wastewater treatment plants in Georgia. One might ask why CCWA plants strive to produce effluents with such low TN values when it is not mandated.

The first answer is an economic one. Attaining excellence in TN removal offers the economic benefit of recovering both alkalinity and oxygen. Secondly, such performance demonstrates an environmental-stewardship mindset, which translates into good public relations.

There is also an educational benefit. By voluntarily operating these plants to achieve a superior effluent, personnel will be better prepared to respond to TN permit limits if and when they are mandated.

Also, the county water system practices “indirect reuse.” Effluent from the two plants goes into either a waterway or wetlands, eventually discharging into the CCWA's drinking water reservoir. The Northeast WRF point discharges into a creek 7 miles upstream of the area's drinking water plant. The Shoal Creek WRF has a land-application discharge permit. It discharges into a constructed wetlands, then to the Shoal Creek drinking water reservoir. Because of this indirect reuse, the CCWA is concerned about the nitrate levels in the effluent affecting drinking water in the region.