Figure 2. Decrease in K and NO3 over time in (a) AN1, (b) AN2, (c) CA2.

Best fit determined by moving average.

Potential for using anaerobic settling tanks to optimize denitrification in an ecologically-engineered wastewater treatment system

Ed Anderson, Anna Brunner, and Elizabeth FabisResearch Project for Systems Ecology (ENVS 316)

IntroductionThe Living Machine at Oberlin College is an ecologically

engineered wastewater treatment system designed to remove organic matter, harmful nutrients, and pathogens. Influent moves through a series of tanks, each containing organisms and conditions that provide specific wastewater treatment functions (Fig. 1).

Influent enters the system’s anaerobic settling tanks (AN1 and 2), where organic nitrogen, present in human waste, is converted to ammonium by ammonification. Water then flows into the closed aerobic tanks (CA1 and 2), where nitrification converts the ammonium to nitrate. Later in the Living Machine, an artificial wetland is supposed to remove the nitrate through denitrification. This wetland has the necessary anaerobic environment to convert nitrate to harmless nitrogen gas, but lacks sufficient carbon for the reaction to take place.1 As a result, the effluent has higher nitrate levels than desired.

High levels of nitrate can cause algal blooms in aquatic ecosystems and “blue baby” syndrome in humans, so it is important that wastewater treatment systems adequately decrease nitrate concentrations.2 Recycling effluent through the AN tanks, an ideal environment for denitrification, may remove more nitrate from the system.3 One potential consequence is a phenomenon known as “popping,” where nitrogen gas causes the bottom sludge to float, introducing more carbon- and nitrogen-rich material into the water.

To test the ability of the AN tanks to remove high concentrations of nitrate, we added potassium nitrate (KNO3) to the AN tanks and

observed changes in concentration over time.

After taking a set of initial samples, we added 4.10 kg of technical grade KNO3, in solution, to AN1 and AN2, with 2.05 kg added to each tank. We regularly took samples from AN1, AN2, CA2, and the clarifier to assess nitrate removal. Samples from AN1 and AN2 were taken after mixing the surface of each tank with the sampling pole to reduce spatial variability. All samples were then filtered through cheese cloth to remove particulates.

Nitrate and potassium concentrations were measured on a Dionex ion chromatograph. Potassium was used as a passive tracer to observe relative nitrate concentration decrease due to dilution and mixing, as it is biologically inert. In addition, we measured ammonium with an Orion probe to assess whether nitrification (the conversion of ammonium to nitrate) was occurring as an adverse result of our project. Biological oxygen demand (NBOD and CBOD) were also calculated, using standard methods, to determine the amount of nitrogenous and carbonaceous matter present in the water.

Results Results (cont.)Ammonium, NBOD, and CBOD levels increased as an indirect

result of nitrate addition. In both AN1 and AN2, both CBOD and NBOD levels approximately doubled over the course of our experiment (Fig. 4a). This indicates that “popping” of nitrogen gas may have occurred, subsequently disrupting the bottom sludge and leading to the ammonium increase in AN1 seen in Fig. 4b.

However, the increased ammonium from the AN1 tank was relatively absent in CA2 (Fig. 4b). This indicates that ammonium, upon entering the aerobic environment of the CA tanks, was converted to nitrate through nitrification. The increase in nitrate concentration in the CA2 tank over time supports this explanation (Fig. 2c).

Our measurements show that a significant amount of denitrification occurred in the AN tanks. However, our experiment suggests recycling could increase, rather than decrease, the amount of nitrate in the effluent. The production of nitrogen gas caused by denitrification increased the ammonium in the AN tanks. This increase lead to more nitrification in the CA tanks, and thus a greater amount of nitrate in the system. Therefore, we can conclude denitrification at high concentrations in the AN tanks will ultimately increase nitrate in the Living Machine. However, this phenomenon may not occur with the smaller amount of nitrate expected from an effluent recycling loop. Further research should be done to quantify this effect at lower concentrations of added nitrate. Without further study we cannot recommend a recycling loop.

References1 Haineswood and Morse. 2003. Low Organic Carbon Limits Denitrification in

the Marsh of an Ecologically Engineered Wastewater Treatment Facility at Oberlin College. ENVS316 Research Project

2 US EPA, Toxicity and Exposure Assessment for Children’s Health, “Nitrates and Nitrites; TEACH Chemical Summary,” US EPA. <http://www.epa.gov/teach/chem_summ/Nitrates_summary.pdf>

3 Operations and Maintenance Manual: A Living Machine for Oberlin College. 2000. Living Technologies, Inc.: Burlington.

Conclusion

Figure 4a. CBOD and NBOD of water samples collected immediately prior to the addition of nitrate on 11/6/07 and ten days later. Standard error, n=3. Figure 4b. Ammonium concentration as measured by Orion probe. Best fit line is moving average.

a. b.

As Fig. 2 demonstrates, nitrate is decreasing faster than potassium, which means that more nitrate is being lost than can be accounted for by dilution alone. The discrepancy is accounted for by denitrification. We used this data to calculate the amount of nitrate removed by dilution and denitrification. Fig. 3 demonstrates this for AN1.

Figure 1. Diagram of Oberlin College’s Living Machine.

Flow Anaerobic Open Aerobic

ClarifierArtificialWetland

Effluent Holding

Tank

UV FilterPressure

Booster Tank

Restrooms

Recycling Line

ClosedAerobic

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MethodsFigure 3. Cumulative NO3 loss in AN1. Loss due to denitrification equals total loss minus loss attributed to dilution.

0

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K and NO3 Concentrations in AN2

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