Faster SDNRs, Carbon Storing Bugs, and Dissolved Oxygen While Piloting Supplemental Carbon
Authors:
- Katya Bilyk, Theresa Bruton, Joseph Rohrbacher, Ron Latimer, Paul Pitt - Hazen and Sawyer
Supplemental carbon is generally added to the secondary anoxic zones of biological nutrient removal tanks or denitrification filters, where little or no wastewater carbon is available for denitrification. Methanol is the most common carbon donor for denitrification at wastewater treatment plants; however, safety concerns, supply/cost volatility, and the long-term sustainability of methanol usage have led many utilities to consider alternative sources including sugar water and glycerin.
The results of supplemental carbon donor evaluations at five Mid-Atlantic Wastewater Treatment Plants (WWTPs) were previously evaluated (Rohrbacher et al., 2009). Two of these WWTPs have continued piloting glycerin products along with another similar facility. The purpose of this paper will be to share the universally applicable lessons learned to enhance treatment plant performance for nutrient removal, as well as present several unexpected observations, including enhanced specific denitrification rates (SDNRs) after glycerin acclimation, carbon storage and enhanced biological phosphorus removal (BPR).
Project Approach
Full-scale evaluations with supplemental carbon alternatives to methanol have been conducted to evaluate whether these products are effective in meeting low effluent total nitrogen (TN) limits. Bench-scale experiments were also performed to quantify the required carbon to nitrate/nitrite (NOX)-nitrogen ratio and the SDNR in a controlled environment. In this experiment, COD in the form of the carbon source and nitrate were added to a reactor; and ammonia, COD, nitrate, nitrite, dissolved oxygen (DO), and orthophosphate were monitored over time.
For the full-scale test, the supplemental carbon source was generally pumped to the application point, which was typically an anoxic zone downstream of aeration. A sampling plan that would capture the nitrate removed in the anoxic zone was devised. In the activated sludge basins this typically consisted of quantifying nitrate, nitrite, and DO into and out of the anoxic zone.
Results and Discussion
The following lessons learned and observations will be elaborated upon in the paper.
Lesson 1: DO control directly upstream of the carbon addition point is critical for optimizing the efficiency and minimizing operating costs of the full-scale plant. Figure 1 illustrates that lower DO concentrations entering the anoxic zone result in lower required COD:N ratios for denitrification. This means less supplemental carbon is used overall, and more of the anoxic zone is utilized for denitrification.
Lesson 2: A comparison of temperature-adjusted SDNRs from anoxic batch tests for a control and a glycerin-fed experimental reactor revealed significantly faster SDNRs after an extended period of glycerin addition in the full-scale process, as shown in Figure 2. This suggests that although glycerin does not require a specialist population, denitrification rates may increase after prolonged glycerin addition. In addition, the SDNR in the control, which was not fed carbon, also increased. One would expect the SDNR in the control to be representative of endogenous metabolism and fairly constant. Therefore, it is also possible that some carbon was stored in the full-scale reactor (used as the seed for the batch tests), and this carbon was released in the acclimated control reactor during the batch test.
Lesson 3: Enhanced BPR was observed as a result of carbon addition in the full-scale plant and bench-scale reactors. The mechanism of this enhanced BPR is thought to be COD storage by ordinary heterotrophic organisms in the process of carbon assisted denitrification, as shown in Figure 3. The stored COD is subsequently fermented to produce VFAs in the anaerobic cells, which would enhance BPR. Evidence of enhanced BPR and how to correct the denitrification COD:N value for BPR release and COD storage will be discussed in the paper.
Lesson 4: Incomplete nitrification in the aerobic zone prior to a post-anoxic zone can mask the effectiveness of supplemental carbon addition. This is because the reaeration zone is used for nitrification polishing (1-2 mg/L ammonia), which increases the nitrate in the BNR basin effluent. Therefore, detailed nutrient profiles are recommended for future pilots. Similarly, compliance with low nutrient limits necessitates full nitrification in the primary aerobic zone and careful operation to limit ammonia release in anoxic zones.
References
Rohrbacher, J., Bilyk, K., Bruton, T., Pitt, P., Latimer, R. Evaluation of Alternative Supplemental Carbon Sources at Four BNR Facilities. 2009 Water Environment Federation Technical Education Conference (WEFTEC), Orlando, FL
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