Can a wastewater treatment plant, designed for nitrification, achieve substantial denitirfication? Yes-If the plant has been designed with higher operating sludge ages there is an opportunity to set aside a portion of the treatment train for denitrification without compromising permit parameters. If the facility has available capacity, the chances for a successful denitrification operation are even better.

Treatment Process Description

Wayne Township's Mountain View STP is a 13.5-mgd plug flow system. The plant consists of primary treatment, equalization, aeration (using surface aerators which provide a complete mix within each of four basins), lime addition, secondary settling, chlorination/dechlorination and post aeration. Currently, the daily flow ranges between 5.5 and 7.5 mgd with a yearly average of 6.8 mgd.

The aeration system has been altered to operate as a modified step aeration facility. Operating in this mode, along with additional alterations made to the secondary tanks, the facility has performed consistently well. Over the past decade, it has produced an effluent with a CBOD5 and TSS in 1-3 mg/l range and ammonia-N in the 0.1 to 0.2 mg/l range.

The facility discharges to the Singac Brook, which directly feeds the Passaic River just below the confluence of the Passaic and Pompton Rivers at Two Bridges. This entry point in the Passaic is between the two drinking water intakes of the Passaic Valley Water Company. Depending on the water companies pumping configuration, the Mountain View effluent can enter either of the two drinking water intakes.

Increasing Drought Conditions

During the last decade, the area has seen a number of droughts with increased severity. Under these extreme conditions, treatment plant effluent becomes the majority of the flow in the Passaic and Pompton Rivers. All treatment plants in this area have been designed to nitrify, but with the exception of one; none have designed denitrification capability.

Hence, most of the ammonia-N conversion to nitrate-N is placed directly on the Pompton or Passaic Rivers. During normal river flows, this nitrate-N is not a problem. However, during droughts the situation changes particularly if the river temperature decreases. Nitrate-N concentrations then begin to increase with a drastic drop in nutrient utilization by river algae.

Drought Committee Formation and Progress

During the 1999 drought, a "Drought Committee" was formed which included representatives from water companies, wastewater treatment plants, and DEP. The committee was charged with the responsibility of analyzing various aspects of the river and contributory effluents to determine whether a public health problem could exist. Various issues and methods were discussed including the removal of nitrate-N concentration from treatment plant effluents. Consultants were either hired by the State to look over dischargers processes or solicited by the wastewater discharges directly to see what could be done. It was decided that some of the plants in the area may be able to reduce nitrate-N biologically.

Mountain View STP's Participation

Pre-denitrification Operation: Prior to starting the investigation for denitrification, Wayne Township had modified the aeration system from the typical complete mix/plug-flow system to a modified step aeration operation.

The original system was operated by keeping the dissolved oxygen in the last two aeration tanks at a level between 2 and 3 mg/l. Some of the aerators would be turned to slow or off occasionally to maintain this DO level.

Additionally, Wayne's effluent permit conditions are met at the end of the second aeration tank. A third tank was kept on line to run the system in an extended mode to reduce sludge volume for incineration. Typical monthly average nitrate-N concentrations, prior to denitrification, range from 12 to 24 mg/l.

Post-denitrification Operation: After a trial period, a new operational method was established which incorporated a denitrification zone at the end of the second aeration tank to the beginning of the third. Timers were installed in selected aerators such that they could be turned on at low speed for short periods for mixing purposes only.

Most of the time, four additional aerators are kept off. The two aerators that are on timer are currently operated for fifteen minutes four times/day on slow speed, merely to prevent excessive settling of the activated sludge. Occasionally, a third aerator in the second tank may have to be placed on for short periods. To adjust DO (dissolved oxygen), a 2 to 3 mg/l DO concentration is maintained at the beginning of the second aeration tank and the end of third tank. Once the 2-3 ppm DO is reached, the rest of the second aeration tank is used do drop the DO as low as possible, hopefully to zero. Once the rest of the flow enters the third aeration tank, any remaining Dissolved Oxygen drops to zero.

In addition, this system has not produced any additional sludge in comparison to the extended step aeration mode operated prior to adding the denitrification zone.

Typical nitrate-N levels in the new configuration now range from 6 to 12 mg/l.

Another noteworthy benefit is an estimated 80 horsepower is being saved each day the plant is being operated in this manner.

Additionally, Wayne is experimenting with different oxygen values in other parts of the system in hopes of increasing nitrate-N removals. Due to reliability that has been incorporated into the modification, the Township will continue to operate in this manner and reduce as much nitrate-N as possible provided DEP permit parameters continue to be met.

If you have any questions about this article, please contact Chris Hoffman, Bureau of Engineering South, at (609) 984-6840.