Biological Nutrient Removal Meets Strict N.C. Limits

The North Cary Wastewater Treatment Plant, one of two plants in Cary, N.C., recently redesigned its treatment process to meet stringent new state limits on nitrogen and phosphorus discharges using a totally biological nutrient removal process.

The process has consistently met the new limits of no more than 5.5 mg/l total nitrogen and no more than 2.0 mg/l total phosphorus. During a 30-day performance test period in September 1997, the plant average was 0.7 mg/l phosphorus and 2.9 mg/l total nitrogen without the use of supplemental chemicals.

The process, called BioDenipho, is monitored and controlled with a programmable logic controller (PLC), and uses dissolved oxygen (DO) measurements to indicate process status as the mixed liquor goes through the nitrification-denitrification phases necessary to complete nutrient removal. It replaced the plants original countercurrent system, which was designed for BOD and ammonia removal (nitrification).

Cary officials selected the treatment process (developed by Krüger Inc., also based in Cary, N.C.) because it provided high biological nutrient removal capabilities with low energy consumption.

The new North Cary plant went on line on May 20, 1997. Operators used the summer months to optimize the process and by August the plant achieved total compliance, averaging less than 4.0 mg/l total nitrogen and less than 1.0 mg/l phosphorus.

The nutrient removal process begins when the wastewater enters one of two separate trains, each consisting of the following components: n a four-stage anaerobic selector positioned ahead of the oxidation ditches; n two oxidation ditches designed for a series flow pattern and alternating process conditions;
n a three-stage secondary anoxic zone; and n a reaeration zone.

Raw wastewater enters the four stage anaerobic selector and remains in each cell approximately 30 minutes for a total anaerobic retention time of two hours. Return activated sludge (RAS) is added to the influent in the second stage, and low velocity submerged mixers maintain solids suspension and minimize turbulence. Anaerobic conditioning of the mixed liquor promotes the growth of microorganisms necessary for phosphorus removal at a later stage of the process.

After leaving the anaerobic selector, the mixed liquor flows into phased isolation ditches for biological nitrogen and phosphorus removal. Conditions in the ditches alternate between oxic and anoxic in order to achieve nitrification and denitrification without internal recycle pumping. Oxic conditions are maintained by rotor surface aerators. In the presence of dissolved oxygen (DO), the microorganisms convert ammonia to nitrate. In the anoxic phase, the organisms strip oxygen molecules from the nitrates, producing nitrogen gas which escapes to the atmosphere.

The phosphorus removal process begins when RAS is mixed with raw influent in the anaerobic selector. Microorganisms in the sludge release stored phosphorus under anaerobic conditions and store soluble BOD inside the cell. When these organisms enter the oxic phase in the isolation ditch, the stored BOD is converted to CO2, water, and new cell mass.

In the oxic condition, the organisms restore their intracellular phosphorus content by removing phosphorus from the mixed liquor. Because the cells are also reproducing, the phosphorus uptake is greater than the amount solubilized in the anaerobic selector. Phosphorus is then removed from the system in the waste sludge.

Aeration and Mixing

In order to facilitate nitrogen and phosphorus removal, each 18-foot-deep oxidation ditch is equipped with four 9.0 meter 60 HP Maxi-Rotors, two 9.0 HP POPL-I submersible mixers, one 5.0 meter motorized weir, one DO probe, and one liquid level monitor. The PLC controls the operation of the rotors in response to alternating process phases and the DO level detected by the probes.

Aeration and Mixing

While the submersible mixers operate continuously to prevent settling of the mixed liquor suspended solids, the rotors switch on and off as DO levels approach pre-determined low and high points.

Aeration and Mixing

The submergence level of the rotors is critical to energy efficient operations. If they are positioned too deep in the basin, energy consumption increases. If the position is too shallow, aeration is inadequate. The PLC maintains the proper rotor submergence level by raising and lowering the motorized effluent weirs. The PLC also controls the hydraulic flow pattern through the two interconnected oxidation ditches, again by raising or lowering the weirs.

Aeration and Mixing

After the effluent leaves the ditches, the total nitrogen concentration is further reduced in a RAS enhanced secondary anoxic zone equipped with low speed mixers. Following this anoxic treatment, the mixed liquor passes through a reaeration zone before entering the secondary clarifiers, then undergoes UV disinfection prior to discharge into Crabtree Creek.

Low Maintenance with PLC

Along with consistent NPDES permit compliance, ease of operation is one of the best features of the BioDenipho system. The plant’s PLC and SCADA systems were configured by Krüger on site after the plant was in operation to ensure that the computer functions were fully coordinated with plant processes.

Low Maintenance with PLC

All North Cary WWTP personnel were involved with the programming of the PLC-based control system and received comprehensive training in the BioDenipho process. Despite the significant increase in plant size, the process has remained manageable because operators no longer have to manually adjust DO levels. Continuous online control of the DO levels allows the plant to maintain optimal DO concentrations. Rotor operation is held to the minimum run time necessary to achieve the desired oxic conditions, saving energy and maintenance costs.

SCADA: Operations at a Glance

The SCADA system at the North Cary WWTP provides animated displays representing plant operations, making it possible for operators to check on process flows and equipment performance in a matter of seconds. With a standard desktop computer, the viewer can look at an overview of the entire plant or zoom in on a particular component or process.

SCADA: Operations at a Glance

The system stores historical data, allowing personnel to track plant performance and optimize operations. The data can be converted to a specific format for reporting purposes. The SCADA alarm/pager feature can be programmed to notify operators if a process or component needs attention. This is useful during periods of low staffing or unmanned operations.

Designed for Growth

The North Cary WWTP currently processes 6.0 mgd, but the plant is designed and permitted for 10.0 mgd. When construction work restricted operations at the South Cary WWTP, the North plant assumed an extra 1.0 mgd to reduce the South plant’s load. The town of Morrisville, four miles from Cary, had been subject to a building moratorium due to inadequate wastewater treatment facilities until assisted by the North Cary WWTP. By treating an additional 350,000 gallons per day, North Cary makes it possible for Morrisville to support new construction and attract new residents.