Duffany, Fountain, JulianOptimizing Nitrogen Removal in a Sequencing Batch ReactorCEE 453
Findings Report:
Optimizing Nitrogen Removal in a Sequencing Batch Reactor
PI’s:
Matthew Duffany
Matthew Fountain
Timothy Julian
5/11/04
Version 1.0 5/11/04Page 1 of 25
Duffany, Fountain, JulianOptimizing Nitrogen Removal in a Sequencing Batch ReactorCEE 453
Table of Contents:
Version 1.0 5/11/04Page 1 of 25
Duffany, Fountain, JulianOptimizing Nitrogen Removal in a Sequencing Batch ReactorCEE 453
Topic Page
Abstract...... 3
Introduction...... 3
Objectives...... 4
Methods...... 4
Nitrate Probe Analysis...... 6
Nitrogen Removal Discussion...... 13
Appendix 1:Plant Setup...... 17
Appendix 2: Reactor Setups...... 18
Appendix 3: Waste Characteristics...... 19
Appendix 4: Academic / Industrial Designs...... 20
Appendix 5: Nitrate Probe Calibration...... 22
Appendix 6: Aeration Level...... 23
Works Cited...... 25
Version 1.0 5/11/04Page 1 of 25
Duffany, Fountain, JulianOptimizing Nitrogen Removal in a Sequencing Batch ReactorCEE 453
Abstract:
The initial goal of this project was to establish a system for optimized nitrogen and BOD removal from a wastewater stream in an activated sludge batch reactor. The plan for optimization included the implementation of online nitrate measurement combined with Process Controller V2 LabView software to manage reactor state changes. However, initial testing of the nitrate probe (an Analytical Sensors Inc gel filled LogIT ion selective electrode) suggested that the device was unsuitable for the current project intentions. At this point experiments were run to confirm probe difficulties and research was begun to establish industry and academic criteria for nitrification and denitrification in process. This report presents a summary of findings into the difficulties encountered with the online probe (ranging from voltage fluctuations under varying conditions to ionic interference in different conditions) and condenses the state of the art on nitrogen (biological and chemical aspects, sensitivities, kinetics and processes) removal as it pertains to lab tested small scale batch reactors.
Introduction:
Much ongoing research is focused on the analyses of varied waste water treatment strategies. One of the increasingly important criteria is nitrogen concentrations in waste effluent. The major sources of nitrogen in municipal wastewater are urea and fecal matter. Nitrogen is important to treat as ammonia nitrogen is a serious fish poison even in small concentrations[1], nitrate nitrogen can lead to eutrophication of coastal (nitrogen limited) ecosystems even in low effluent concentrations[2], and nitrite creates human health concerns by causing blue baby syndrome in children (nitrite in concentrations of 1 mg/L). The nitrogen can be present as any of these species due to the bacterial pathways:
To this extent, drinking water is regulated by the EPA at 1 mg/L for nitrite-nitrogen and 10 mg/L for nitrate-nitrogen.
However, while the nitrification cycle is simply described, its biological kinetics forms a slightly more complicated process. To begin with, a typical nitrogen removal in bio-solids is the volatilization of ammonia, this method is difficult to achieve in wastewater treatment as very small amounts of gaseous ammonia will be lost at pH values less than 8.5 as hydrogen ions are a component in the equilibrium[3]:
Therefore, the ammonia must instead be oxidized and taken through the reactions shown above to nitrate so that denitrification can later be used to remove the nitrogen to a harmless form of diatomic nitrogen gas.
Nitrification is a relatively specialized process that will only be accomplished through the Nitrosomonas and Nitrobacter species of bacteria. These bacteria are obligate aerobes (their populations will only grow under aerobic conditions) but they can survive long periods of anaerobic conditions. They are both autotrophs that oxidize ammomia to provide energy for cell synthesis, but Nitrosomonaskinetics is the limiting rate in the process, leading to solutions typically low in nitrite compared to ammonia or nitrate concentrations[4].
Once the nitrogen has all been converted to nitrate the next step is to induce the action of denitrifying bacteria. These bacteria come from a range of diverse species including Pseudomonas, Micrococus, Archromobacter, Thiobacillus, and Bacillus. These species are mostly facultative (able to use either oxygen or nitrate as the terminal electron acceptor in respiration), and thus oxygen concentrations must be kept low as oxygen provides greater energy and is thus preferred by the organisms (oxygen respiration of glucose yields 686 Kcal / mol while nitrate yields only 570 Kcal / mol).[5]
More specific analyses of these processes follow below.
Objectives:
The initial goal for this project consisted of establishing an online control of an activated sludge batch reactor to optimize both nitrogen and BOD removal. To this aim funding was requested and received for an online nitrate probe. However, the nitrate probe proved unsuitable due to extreme fluctuations for the online control of the plant. At this point the project was shifted to an analysis of effects on the nitrate probe and what factors may be causing these variations, and whether or not they were correctable. The experimental studies on the probe also led us to delve into the literature surrounding the difficulties with both nitrification and denitrification, and the eventual report became a combination of analysis on the probe and a study on applying industrial and academic techniques of nitrogen removal in a sequencing batch reactor in the hopes that future groups would be able to advance our work into a fully functional model.
Methods:
An activated sludge sequencing batch reactor was set up to simulate the activity in an activated sludge single tank treatment plant. While treating waste as a batch process is unlikely in an industrial or municipal setting where extremely large volumes would be neccessary, the approximation served adequately to experiment with nitrogen removal as it related to oxygen and timing. If specific methods were to be devised, a model of a continuous flow reactor would need to be constructed and tested as well. The details of the plant design can be found in Appendix 1: Plant Setup, and the precise volumes, timings, and alternative inputs of the multitude of experiments run in this project can be found in Appendix 2: Reactor Setups. The characteristics of the waste flows can be found in tables provided in Appendix 3: Waste Characteristics. In a general sense the plant was constructed, tested for structural integrity and leaking, and then run continuously (other than a 2 day downtime due to computer virus infection of the lab) for a six week period. During this period the different reactor setups were tested to determine different information about the nitrate probe and BOD levels. The reactor was run according to Process Controller v2 software developed by Monroe Weber-Shirk, as a piece of LabView code which controlled different states of the reactor as shown in the appendix through a series of solenoid valves controlling waste input and air input. Oxygen was introduced to the reactor through a ceramic diffuser to maximize dissolution into the waste solution. The reactor was also brought into effectiveness by the introduction of 4 L of activated sludge from the Ithaca Waste Water Treatment Facility. It was also important to refrigerate the waste so that no degradation would occur in the waste bottle which would falsely serve as a “pretreatment.” Furthermore, it was necessary to load the waste into the tank before the water so that the water could be used to flush the pipes that were outside the refrigerator and prevent a bioaccumulation from growing on the waste in the piping and possibly blocking future flow. The different states run in the reactor can be summarized as the following:
- Add Waste: input of 140 mL of 20x concentration stock waste from the refrigerator; controlled through flowrate of the peristaltic pump.
- Add Water: input water (with stock 2 and 3) into the reactor until 4 L of total volume is recorded by a pressure sensor (calibrated to account for altered density of water due to waste and sludge).
- Aerate: air pumped into the ceramic diffuser at air rate controlled by solenoid valves while the reactor is stirred by a stirbar on a stir plate set at 8. Operated on a timer system.
- Anaerobic: no aeration occurs but stirring continues at setting number 8. Operated on a timer system.
- Settling: ceasing of aeration and stirring, sludge allowed to settle to the bottom of the tank. Operated on a timer system.
- Discharge: with no stirring or aeration the water is removed from the top of the settled sludge though a port at 1.2 L height in the tank, along with wasted sludge that allows mass balance with growth to retain constant sludge volume. Controlled by pressure sensor recording minimum volume.
- Repetition of steps above
Nitrate Probe Analysis:
The nitrate probe was calibrated as shown in Appendix 5. The nitrate probe was first soaked in water for 15 minutes, then a diluted nitrate solution for 2 hours. At this point the probe was washed clean and placed into distilled water with different concentrations of nitrate in order to measure the corresponding voltages. The probe values show a distinctively curved calibration line, implying a similar voltage to interpreted value ratio as a pH meter. This result seems sensible as both meters use voltage from the diffusion of a solution through a membrane.
One of the major problems with the nitrate probe is that the calibration was accomplished in distilled water with only nitrate ions present. In the activated sludge reactor there are many other chemicals present. The probe will suffer from interference in solutions with high ionic strength, and this was one issue that had to be tested in the reactor.
The initial testing of the nitrate probe consisted of measuring the voltage output from the probe while it was immersed in the tank during our standard reactor cycle of adding waste and water, aerating, letting sit anaerobically, settling, and discharging effluent. By applying the calibration curve to the voltage received from the probe while it was immersed in the reactor, the nitrate concentration can, hypothetically, be determined. This concentration is inversely related to the voltage received. This relationship is shown in Figure 1, where nitrate voltage is measured during one complete cycle and is converted into nitrate concentration. The two graphs are mirror images of each other due to the inverse relationship between voltage and nitrate concentration. In addition, variations in the voltage of approximately 0.02 volts correspond to variations in nitrate concentration of roughly 0.18 ppm/100. This demonstrates the magnification effect that occurs when converting from voltage to concentration of nitrate.
Figure 1: This figure shows the voltage read by the nitrate probe and corresponding nitrate concentration over the period of one wastewater treatment cycle. This demonstrates the indirect relationship between voltage output by the probe and nitrate concentration in the reactor.
Once the relationship between the voltage and the nitrate concentration was determined, the nitrate concentration could be constantly measured and monitored in the reactor. This was a vital component of our proposed research as our initial goal was to use the nitrate probe as a real-time feedback monitor of conditions in the reactor (online reactor control). However, the measurements provided by the nitrate probe were not always accurate. The magnitude that the nitrate concentration wavered, as measured by the probe, inhibited its use as a real-time feedback monitor of conditions in the tank. While the reactor was run for multiple cycles under the typical Wuhrman Configuration conditions of aerobic, anaerobic, settle, discharge / fill, the nitrate was measured. The results were then compared to expected results and it was discovered that the nitrate probe was displaying wavering concentrations of nitrate that did not follow the expected conditions. This is demonstrated in Figure 2, which shows the nitrate concentrations in two complete cycles, along with idealized expected nitrate concentrations and dissolved oxygen in the tank.
Figure 2: This figure shows the expected nitrate concentration along with the actual nitrate and dissolved oxygen concentrations as demonstrated by the nitrate and oxygen probes. While the oxygen probe is demonstrating expected results, the nitrate probe wavers significantly throughout the cycle with only small correlation to the expected nitrate concentration trends. This demonstrates that the nitrate probe is not yielding expected results.
The project direction (of the laboratory module) was now devoted to discerning the cause for the wavering nitrate readings so that the probe could be used to in conjunction with the rest of the reactor to optimize waste treatment.
To understand whether the nitrate probe was recognizing nitrate concentration or it was recognizing concentration of ion interference, a burst of nitrate (0.971 g NaNO3) was added to the tank while the wastewater treatment was in the anaerobic state. The concentration of nitrate was meanwhile measured using the probe. A nitrate concentration spike was demonstrated, as expected, by the probe as shown in Figure 3while the reactor was being aerated. A burst of dextrose (10 g) was then added to the tank two hours after the nitrate was added. The purpose of this was to provide the activated sludge with a source of chemical oxygen demand that would increase the rate of denitrification. The expected result of this would be a sharp decline in the concentration of nitrate.
Figure 3: This figure shows the nitrate concentration in the reactor after a spike of 0.971 g of sodium nitrate (NaNO3) and 10 g dextrose were added to the tank two hours apart. The concentration of nitrate spikes, and then slowly declines while in the anaerobic state. As soon as the dextrose is added, the nitrate concentration drops dramatically and remains low even when the reactor switches to the aerobic stage.
The results demonstrated that the nitrate probe was recognizing changes in nitrate concentration. However, the nitrate concentration that was being measured by the probe must clearly not have been the only contributor to the measurement because, as shown above, the nitrate probe was not giving expected results in the full cycle controlled situation. Because this experiment was run in only in aerobic / anaerobic conditions, it was postulated that electrical interference caused by the complete cycle run through may have caused the wavering nitrate readings.
At this point, further experiments were run. To test this electrical hypothesis, the nitrate probe was removed from the tank and placed in a separate beaker with a steady diluted nitrate concentration while the wastewater reactor was run. If the nitrate concentration changed even slightly over the period of one cycle, this would demonstrate that state changes were causing electrical interference. Because the nitrate concentration remained steady over the period of one cycle, electrical interference was ruled out as the cause for error. However, as stated, the nitrate concentration in the beaker was composed entirely of distilled water and sodium nitrate, with no other ionic interference present.
The next possible source of error examined was the ionicor electrical interference created from the dissolved oxygen probe which was measuring dissolved oxygen only a few centimeters from the nitrate probe, as shown in the lab set-up in Appendix 1. The dissolved oxygen probe was removed from the tank in order to determine any difference in the nitrate probe readings, but there was no change in the nitrate readings. The dissolved oxygen concentration displayed by the DO probe equilibrated, but the nitrate probe reading did not change. This demonstrates that the proximity to the working dissolved oxygen probe did not influence the nitrate probe.
Because the nitrate probe was carefully calibrated using a solution of diluted nitrate, we determined that the activated sludge (or other compounds) in the wastewater tank wasa possible source of interference. In order to test this hypothesis, we ran fourseparate sets of cycles while measuring nitrate, slowly removing different aspects of the wastewater treatment plant. If the removal of one of the components in the reactor resulted in expected unwavering results, we could rule that it was the cause of interference. The first cycle tested was the normal running mode of the tank, in which every aspect of the reactor was included: water, waste, and activated sludge. The second cycle tested removed the activated sludge and ran the reactor with only water and waste. The third cycle tested consisted of running only water with a nitrate spike added at the beginning. The results are shown as follows, where Figures 4, 5and6demonstrate the results from water – water and nitrate spike -- water and waste – and -- water, waste and activated sludgerespectively.
Figure 4:This figure demonstrates the results from running through a cycle with water after a spike of nitrate was added. Because there is no activated sludge in the tank to treat the nitrate, the concentration should remain relatively stable. Though this is true for the aerobic state, the measured nitrate spikes in the anaerobic state and remains high.
The nitrate spike shown at the beginning of Figure 4 corresponds with the results demonstrated in Figure 3 that the nitrate probe recognizes an increase in nitrate level. However, the nitrate level is supposed to slowly increase in the aerobic stage, and then decrease in the anaerobic stage as expected in Figure 2. Clearly, the multiple spikes that occur in the anaerobic stage do not conform to this projection. After the multiple spikes occur, the nitrate concentration remains high until the reactor discharges the waste and is refilled. As the only components present in the tank are water and the initial spike of nitrate, the switch to anaerobic conditions should have no effect on the nitrate levels whatsoever. This, however, was not demonstrated by Figure 4.