Improving Simultaneous Nitrogen Removal Performance through Magnetite Addition Jose Jimenez1*, Steven Woodard2, Matthew Vareika3, Denny Parker1 1
Brown and Caldwell, 850 Trafalgar Court, Suite 300, Maitland, FL 32751, USA Haley & Aldrich, USA 3 Siemens, USA *To whom correspondence should be addressed. Email:
[email protected] 2
ABSTRACT Simultaneous nitrification7denitrification (SND) has significant potential advantages over conventional nitrogen removal systems including simpler process design, lower carbon, and energy requirements. However, these cannot be fully realized until the factors controlling SND are better understood. The low DO environment required for SND is conventionally considered more susceptible to sludge bulking, which have limited the applicability of this process due to their negative effects on the overall secondary clarification capacity of the plants. Therefore, this study examined the effect of magnetite addition as a ballast7aid to control and improve settling characteristics of mixed liquor from SND systems. Batch test results and full7scale operating data were analyzed to quantify the effect of magnetite on the settling characteristics of the mixed liquor under SND conditions. The results from both, the batch experiments and the full7scale plant, showed significant settling rate improvements with the use of magnetite. Additionally, the batch test results indicated an unexpected improvement in the SND performance with the addition of magnetite. Activity testing for nitrification showed that the batch reactor with magnetite provided almost 70 percent higher rates than the reactor without the magnetite. The improved nitrogen removal through SND experienced in the bench7scale reactor and full7scale systems might indicate growth of biofilm within the magnetite7ballasted floc, enhancing the performance of the SND system. Alternatively or in addition to this proposed mechanism, the electrically conductive magnetite particles could be serving to increase the rates of both nitrification and denitrification via a proposed mechanism of enhanced interspecies electron transfer (IET). Full7scale operating data from a treatment plant operating under SND with magnetite addition was evaluated for this study and showed the ability to meet low effluent nitrogen concentrations without external carbon addition, while maintaining very high settling rates. KEYWORDS Simultaneous nitrogen removal, nitrification, denitrification, settling, magnetite INTRODUCTION Simultaneous nitrification7denitrification (SND) refers to the process where nitrification and denitrification occur concurrently in the same reactor under identical operating conditions. SND is a well7known phenomenon in biological nutrient removal (BNR) activated sludge systems and it largely depends on the bioreactor configuration, dissolved oxygen concentration, organic
carbon availability, and floc size (Pochana and Keller, 1999; Daigger and Littleton, 2000; Littleton et al., 2003; Ju et al., 2007; and Jimenez et al., 2010). SND is a point of interest for designers and operators of wastewater treatment plants as it may have significant potential advantages over conventional nitrogen removal systems. However, while potentially offering significant benefits, SND faces several challenges in design, control and operation. In fact, Jimenez et al. (2010) indicated one of the main factors limiting the applicability of SND processes is the severe low DO bulking conditions in many SND facilities which have limited the applicability of these processes due to their negative effects on the overall secondary clarification capacity of the plants. In a survey presented by them, it was shown that most of the SND plants surveyed operated at average sludge volume index (SVI) values in excess of 150 mL/g with 90th percentile SVI values often surpassing 250 mL/g. The low DO environment required for SND processes are conventionally considered more susceptible to sludge bulking, primarily because of the excessive growth of filamentous bacteria (Grady et al. 1999, Jenkins et al. 2003, Martins et al. 2004). In order to resolve the low DO bulking issue in many SND plants, many studies have focused on understanding the mechanisms involved in low DO bulking and have tried to develop operating control strategies to mitigate such issues (Jue et al., 2007), with mixed results. Other studies have focused on investigating physical means for improving the settling properties of activated sludge flocs, with promising results (Catlow and Woodard, 2009). OBJECTIVES This study examines the effect of magnetite addition as a ballast7aid to control and to improve settling characteristics of mixed liquor from SND systems, as well as additional possible benefits of the magnetite7ballast material on the performance and design considerations of SND systems. MATERIAL AND METHODS BioMag™ Process The BioMag process is a ballasted flocculation7aided wastewater treatment system that uses magnetite to increase the specific gravity of the biological flocs in the activated sludge process. This process was developed and patented by Cambridge Water Technology (Woodard et al., 2010) and is currently owned by Siemens. Magnetite (Fe3O4) is an inert iron ore, with a specific gravity of approximately 5.2 and with a strong affinity to activated sludge mixed liquor solids. Figure 1 presents a general schematic of the BioMag process. The process works by impregnating the biological flocs with magnetite as seen in Figure 2. In this process, the mixed liquor is mixed with magnetite in a continuously mixed tank before entering the activated sludge reactor. After clarification, the return activated sludge (RAS) is conveyed from the clarification system back to the biological reactors. Waste activated sludge is sent to a magnetite / WAS separation system for removal and recovery of the magnetite prior to sludge processing.
Figure 1. BioMag™ Schematic (provided by Siemens)
Figure 2. Biological floc impregnated with magnetite (from Catlow and Woodard, 2009) The magnetite removed from the WAS is recovered and sent to a mixing tank to be reused. The magnetite separation and recovery process starts with a shear mill that applies high shear forces to break up the floc into smaller particles, releasing the magnetite for recovery After the shear mill, the WAS flows to a rotating magnetic drum to separate the magnetite from the sludge. Once separated, the WAS is sent to solids processing facilities, whereas the recovered magnetite is sent to the magnetite mix tank before being re7introduced into the reactor. Make7up magnetite is added to maintain a design magnetite7to7MLSS weight ratio ranging from 0.5 to 2.0, depending on specific design conditions.
Increasing the specific gravity of the floc increases settling and thickening rates, and allows operators to appreciably increase the mixed liquor concentration in the biological reactors, while still maintaining adequate settling and thickening in the secondary clarifiers. This allows activated sludge systems to increase their capacities within the existing infrastructure by maximizing the loading rates to the clarification process. The process also facilitates, in some instances, nitrogen and phosphorus removal by allowing plants to increase the solids residence time (SRT) and free up existing aeration tankage for use as anoxic and/or anaerobic zone(s). Settling Tests Mixed liquor samples with and without magnetite addition were collected from a plant operating under SND mode to quantify the benefits of magnetite in terms of sludge settling characteristics. For the purpose of this study, sludge volume index (SVI) and settling rate measurements were conducted following the methodology referenced in the WERF/CRTC Protocol for secondary clarifier testing (Wahlberg, 2001). In addition to testing various biological suspended solids concentrations, a range of magnetite ballast ratios with and without polymer were analyzed. Laboratory6Scale Experiments The laboratory7scale system consisted on both batch tests and continuous tests experiments starting with mixed liquor samples from a treatment plant operating under SND mode to investigate the effect of the magnetite on the nitrogen removal process. Two 207L parallel bench7 scale reactors (Reactor A contained no magnetite whereas Reactor B contained magnetite at a ratio of 0.75 mg of magnetite per mg of volatile suspended solids) were operated continuously under SND mode for an extended period of time to examine the effect of magnetite on SND. The reactors were aerated continuously to maintain a bulk DO concentration of 0.3 to 0.5 mg/L and were operated at a target SRT of 8.5 days, MLVSS between 2,500 and 3,000 mg/L and a water temperature of about 20oC. The reactors were operated at a 2.57hour cycle, three cycles per day. Each cycle consisted on rapid filling time followed by a 27hour reacting cycle and then a 307 minute settling and decanting period. Before the start of the test, mixed liquor samples were drawn from the batch reactor for TSS and VSS determination. The pH in the batch was monitored continuously and maintained in the 6.977.2 range. At the start of the batch test, 2 gallons of high7concentration synthetic wastewater was added to the mixed liquor. The target initial conditions for the batch test experiments were as follows: soluble COD of 150 mg/L, ammonia of 20 mg N/L, and orthophosphate of 5 mg P/L. After addition of the influent, mixed liquor and effluent samples were collected after 2.5 hours. Effluent samples were filtered immediately and analyzed for soluble COD, ammonia, nitrate, and nitrite. Mixed liquor samples were analyzed for TSS and VSS. Nitrification Rate Measurements Specific nitrification rates were measured using 47L bench scale reactors with provisions for DO and pH probe access. Mixing was provided by a magnetic stir bar to ensure proper mixing. During the test, the DO was maintained between 2.5 and 4 mg/L using an air7stone and a fish pump. pH was maintained at 7.5 at the beginning of each experiment by adding sodium bicarbonate. DO, pH and temperature values were recorded directly throughout the experiment.
For AOB and NOB experiments, 47L samples were collected from the SND reactors and aerated for 30 minutes, and then spiked with 20730 mg/L ammonium chloride and sampled continuously for 1 hour at 207minute intervals. All collected samples were analyzed for NH37N, NO27N, and NO37N. NH37N was measured using HACH colorimetric method TNT plus 832. NO37N was analyzed using HACH colorimetric method TNT plus 836. NO27 was analyzed using the standard colorimetric method HACH Nitriver 3 reagent and TNT plus 840. All samples were filtered immediately and analyzed within 2 hours. PCR6based Anammox6specific amplification and gel electrophoresis PCR7based analysis were conducted by Dr. Ramesh Goel at the University of Utah and made available for this study by Siemens. Using the DNA extracted from the samples as template, specific region of Anammox DNA was amplified employing different primer sets: AMX368F and AMX820R (for Kuenenia/ Brocadia), AMX820F and BS820R (for Scalindua), Pla46F and AMX1480R (for all anammox belonging to Planctomycetes). PCR reaction volume of 25 HL included 12.5 HL 2X Mastermix, 0.1mg/mL BSA, 1.0 HL of DNA template and 1.0HL of each primer. The final volume of 25 HL was reached by adding nuclease free water. The PCR products were then analyzed on 1% agarose gel and against 1 kb DNA ladder. RESULTS AND DISCUSSION Batch Experiments Mixed liquor samples were collected three times per week from the reactors and poured in a settlelometer to measure the SVI. After the SVI analysis, the mixed liquor samples were returned to the batch reactors. Table 1 summarizes the SVI results from both reactors during the testing period. These results show that the reactor operating with magnetite provided significantly lower SVI values than the reactor operating without the magnetite. It should be noted that mixed liquor samples from both reactors were collected and analyzed for microscopic identification. The results indicated that both reactors contained excessive low7DO filamentous bacteria; however, this did not affect the settling conditions in Reactor B. During the testing period, new mixed liquor had to be brought from the wastewater treatment plant to re7seed Reactor A due to extremely high SVI that where affecting the overall ability to perform the experiments. Table 1. Sludge Volume Index (SVI) Comparison for Reactor A and Reactor B SVI (mL/g) Average Minimum Maximum
Reactor A 195 150 300
Reactor B 75 50 88
Figure 3 presents the effluent nitrogen results from both reactors during the testing period. Based on these results, the following was noted: • • • • •
• •
•
Both reactors achieved low effluent ammonia concentrations at low DO conditions. It appears that complete nitrification was achieved in both reactors (conversion of ammonia to nitrate). Reactor B provided overall lower ammonia levels than Reactor A, especially after 60 days of operation. Nitrite levels in both reactors were low throughout the testing period. No nitrite accumulation was observed in the reactors. Nitrogen mass balances around both reactors indicated that SND was occurring since there was a fraction of un7accounted nitrogen which was converted to nitrogen gas. Nitrate levels in Reactor A averaged approximately 7 mg/L whereas Reactor B averaged approximately 3.5 mg/L NO37N. The denitrification performance of Reactor B, after 60 days of operation increased significantly compared to Reactor A. For the last 60 days of operation, Reactors A and B averaged 6 mg/L NO37N and 2.5 mg/L NO37N, respectively under similar operating conditions. For the last 60 days of operation, Reactor A and Reactor B averaged approximately 75 and 90 percent removal efficiencies for total inorganic nitrogen (TIN). Overall, Reactor B provided a higher degree of SND than Reactor A under the same operating conditions. These results might indicate that the magnetite, given its long retention time in the system, could be acting as more than just an inert ballasting agent, but could also develop a biofilm to enhance the performance of the SND system; similar to what develops in some IFAS systems. Another possible explanation for the enhanced SND performance is a phenomenon recently observed by Kato et al. (2012). Their results suggested that magnetite can work as an electron shuttle in microbial ecosystems, where microbes use electrically conductive magnetite particles as conduits of electrons. This resulted in efficient interspecies electron transfer (IET) and cooperative catabolism. Microbial cultures enhanced with magnetite nanoparticles demonstrated IET rates more than 107fold higher than control cultures, appreciably speeding up the oxidation of acetate and the associated reduction of nitrate. However the significance of this supplemental mechanism for support of SND needs further investigation, as the magnetite particles in cited work are much smaller and with much higher surface area, providing for easier pathways for particle and bacterial connections, than would occur with the larger magnetite particles with lower surface area that are used for ballasting in the BioMag™ process.
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Figure 3. Ammonia, nitrite and nitrate concentrations from batch reactors A and B
Activity Test Results for AOB and NOB and Molecular Fingerprinting Specific nitrification rates were measured from mixed liquor for both reactors after 90 days of operation. Figure 4 and Figure 5 summarizes the results of the activity tests for both reactors for AOB and NOB. Table 2 presents the specific nitrification rates for AOB and NOB for Reactor A and Reactor B. From the results presented in Figures 4 and 5 and the results summarized in Table 2 one can conclude that the mixed liquor biomass in Reactor B experienced nitrification rates that are almost 70 percent higher than the reactor without the magnetite, suggesting that the ballasting material is providing further benefits to Reactor B, in addition to settling improvements. The activity test results indicate that both AOB and NOB are active in both reactors; hence, complete nitrification was being achieved during the low DO, SND conditions. Similarly to the batch experiment results, the superior performance of Reactor B might indicate that the magnetite could be acting as a support for biofilm formation within the structure of the floc, increasing the activity of the biomass. Alternatively or in addition to this proposed mechanism, the electrically conductive magnetite particles could be serving to increase the rates of both nitrification and denitrification via the proposed mechanism of enhanced IET cited above. The enhanced bench scale nitrification/denitrification rates for magnetite7impregnated mixed liquor are consistent with observations of pilot and full scale BioMag systems. Unusually high total nitrogen removal efficiencies have been observed for BioMag in sequencing batch reactor, Modified Ludzack7Ettinger (MLE) and conventional activated sludge systems, all without supplemental carbon addition (Catlow and Woodard, 2009; Moody et al., 2011; Lubenow et al., 2011; Bishop et al., 2012). 30
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Reactor A 3.51 3.22
Reactor B 5.95 5.75
Molecular fingerprinting results provided by Siemens and performed by Dr. Ramesh Goel at the University of Utah for anammox bacteria showed that mixed liquor samples from a BioMag facility tested positive for anammox bacteria including for the specific strains Kuenenia, Brocadia and Scalindua. The positive results for Kuenenia, Brocadia and Scalindua are encouraging because these specific anammox bacteria have been shown to be participating in anammox reactions (Park et al., 2010; Kotay et al., 2012). However, based on the anammox activity tests, it was quite obvious that BioMag’s mixed liquor sample did not show anammox activity. Sturbridge Wastewater Treatment Plant As part of this study, long term operational data from a BioMag facility was evaluated to understand the nitrogen removal potential of the BioMag process. The Sturbridge Wastewater Treatment Plant was selected, based on its relatively long operational history using the BioMag™ process. The Town of Sturbridge Massachusetts WWTP consists on a 1.57MGD activated sludge plant using an MLE process. The treatment facility was upgraded in 2010 with the BioMag™ technology to increase its wet weather capacity, while providing nutrient removal capabilities (Catlow and Woodard, 2012). The authors presented details associated with the BioMag improvements at Sturbridge. Figure 6 presents an aerial view of the biological reactors and
secondary clarifiers at the Sturbridge WWTP. Table 3 presents a summary list of average operational parameters for 2012.
Figure 6. Aerial view of biological reactors and secondary clarifiers at the Sturbridge WWTP (provided by Siemens Industry) Table 3. Summary of Operating Data for the Sturbridge WWTP, MA Parameter Influent flow (MGD) Influent BOD5 (mg/L) Influent TSS (mg/L) Influent TKN (mg/L) Influent NH37N (mg/L) Influent TP (mg/L) Influent C:N ratio Total SRT (days) Aerobic SRT (days) MLVSS (mg/L) Dissolved Oxygen (mg/L)
Operating Value 0.50 260 190 45 30 5.5 5.7 15 12 4,500 < 1.0
As part of this study, settling column tests were conducted at Sturbridge at various biological suspended solids concentrations, a range of magnetite ballast ratios, with and without polymer in order to understand the benefits of the magnetite in terms of settling characteristics for this SND facility. The variable solids interface in the settling column was measured at Sturbridge and plotted versus time in Figure 7. Table 3 summarizes the settling test results. In this table, the unitless magnetite ratios, expressed as the ratio of magnetite to biological suspended solids, ranged from 0.5 to 1.3. In the samples where polymer was added, the dose varied from 1.5 to 2.5 mg/L. As shown in Figure7 and Table 3, the range of sludge7specific parameters determined from the field testing resulted in significantly higher settling velocities as compared to control
reactor (no magnetite). Predicted settling velocities at Sturbridge, MA ranged from 5 to 42 times the control reactor at a solids concentration of 2,500 mg/L, and from 11 to 67 times the control reactor at 9,500 mg/L. These results validate the observation obtained through the independent bench7scale experiments presented previously on the positive impact of the magnetite on the settling rates of the SND reactors.
Figure 7. Settling velocities at various conditions at Sturbridge, MA Table 3. Summary of Sludge6Specific Settling Parameters Magnetite Ratio No Magnetite (Control) 0.5 – 0.75 0.5 – 0.75 0.8 – 1.3 0.8 – 1.3
Polymer 77 No Yes No Yes
V0 (ft/hr) 7.1 25.3 217.8 75.0 255.1
k (L/g) 0.37 0.255 0.392 0.2485 0.305
Final effluent nitrogen data for 2012 are presented in Figure 8; which depicts lower than expected nitrogen concentrations, especially for nitrate. Although the Sturbridge WWTP was designed to meet a total nitrogen concentration of less than 10 mg/L, the facility has surpassed the expected performance meeting TIN levels of less than 3 mg/L, with no external carbon addition and operated under SND mode.
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Figure 8. Historical secondary effluent nitrogen species for the Sturbridge WWTP
CONCLUSIONS The results of this study indicate that the magnetite ballast has a significant impact on both the settling characteristics of and nitrogen removal efficiency of SND systems. The impact of magnetite ballast with and without polymer can increase settling velocities by as much as 36 and 10 times, respectively, when compared to mixed liquor from SND reactors. The improved nitrogen removal through SND experienced in the bench7scale reactor and full7scale BioMag systems might indicate growth of biofilm within the magnetite ballast floc, enhancing the performance of the SND system. In addition to this proposed mechanism, the electrically conductive magnetite particles could be serving to increase the rates of both nitrification and denitrification via a proposed mechanism of enhanced IET. ACKNOWLEDGMENT Data from the Sturbridge Wastewater Treatment Plant was provided by Mr. Shane Moody. The team wants to thank Dr. Charles Bott at the Hampton Road Sanitation District and Mr. Pusker Regmi from Old Dominion University for performing nitrification and anammox activity testing and for providing insights in the possible nitrogen removal mechanisms for the BioMag process. Finally, the team wants to acknowledge Dr. Ramesh Goel at the University of Utah for conducting PCR7based analysis.
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