Nutrient Removal ProjectSimultaneous Nitrification and Denitrification in

a Sequencing Batch Reactor

Will LambertRobert NwaokoroStephen Russo

CEE 45312/8/04

Abstract

Nitrogen removal is important part of wastewater treatment, especially in coastal regions where nitrogen loading of oceans and estuaries can cause algal blooms. The most common method of removal uses microbes. Unfortunately, there are several steps to achieve denitrification with an organic nitrogen feed. Many bacteria convert organic nitrogen to ammonia, but only certain anaerobes, namely Nitrobacter and Nitrosomonas, are responsible for the conversion of ammonia to nitrite and nitrate, or nitrification. Furthermore, it is only from the nitrite or nitrate form that anaerobic denitrifying bacteria can produce nitrogen gas. Therefore, traditional nitrogen removal in waste treatment plants occurs in first and aerobic stage and then an anaerobic stage. This can either occur in separate tanks or in one tank that is sequenced to go through each state. Recently, further research has been done on simultaneous nitrification and denitrification. It seems that at very low levels of dissolved oxygen, anaerobic zones can be produced, allowing both processes to occur concurrently.

The purpose of this research is to explore the possibilities of simultaneous nitrification and denitrification in a sequencing batch reactor at a laboratory scale. We used aeration cycles of 8, 18, and 24 hours and a dissolved oxygen level of 0.5 mg/L. Removal efficiencies were on the range of 10-20%, and increased with longer aeration times. Due to unexpected experimental difficulties, we were not able to take as many samples as we would have liked, so there are many options for further research.

Introduction

The objective of our research is to evaluate the efficiency of simultaneous nitrification and denitrification (SND) in a sequencing batch reactor (SBR). Nitrogen removal is an important part of wastewater treatment. Nitrogen is a limiting nutrient for algal growth in oceans and estuaries. Furthermore, organic nitrogen in wastewater is converted to nitrates, which can cause Methemoglobinemia (“blue-baby” syndrome) if it is present in drinking water. Biological nitrogen removal has been highly favored by wastewater treatment plants due to the increasingly stringent effluent discharge requirements on nitrogen. Biological nitrogen removal involves ammonification (converting organic nitrogen to ammonia), nitrification (biological oxidation of ammonium nitrogen to the nitrite), and denitrification (reduction of nitrate to free nitrogen gas via a nitrite intermediate). Traditional nitrogen removal in wastewater treatment plants occurs in separate aerobic and anaerobic stages. SND systems tend to be a cheaper means of achieving these requirements as it uses less bio-reactor volume, consumes less time, produces less sludge and has lower energy costs than current sequential nitrification and denitrification systems.

A potential problem with the SND setup is the anoxic requirement of the denitrification process which we intend to synchronize with the aerobic nitrification process. An aerobic condition (a measurable bulk liquid dissolved oxygen (DO)) inhibits the nitrifying bacteria Nitrosomonas and Nitrobacter, while an anaerobic situation (bulk liquid DO 0.5 – 2 mg/L) inhibits denitrifying bacteria such as Thiobacillus denitrificans and Pseudomonas denitrificans. Denitrification occurs in a series of steps

where nitrate is converted to nitrite and ultimately to gaseous nitrogen. A general schematic of the nitrogen cycle as it occurs in activated sludge treatment is given below:

Figure 1: Schematic of the nitrogen cycle in wastewater treatment plants.

A compromise situation at which both nitrification and denitrification can occur simultaneously requires the bioreactor to operate at the minimum DO concentration necessary to support nitrification ( 0.5 mg/L) (Munch, et al 1996).

The major challenge is creating the optimal conditions necessary for an efficient SND process: appropriate hydraulic/solids retention times, optimal aeration rates, and suitable feed introduction strategy. These conditions must be met while minimizing odor generation and the accumulation of free ammonia gas. Another potential difficulty is the selection of filamentous bacteria at low DO that interfere with settling and can cause sludge bulking in a reactor. However, these same bacterial flocs (sludge bulking) may provide sites of anoxic zones where denitrification can take place.

Ideally, dissolved oxygen control should be based on levels of nitrate and nitrite in the system, but access to reliable nitrogen probes is limited. Therefore, airflow rates will be controlled by the DO level in the system using the proportional integral derivative (PID) control. Nitrate and nitrite levels will be measured using the Hach Cadmium Test with the aid of a UV Spectrophotometer. Ammonia levels in the system are monitored using the micro-phenate test, also with the aid of a UV Spectrophotometer. Assuming that all organic nitrogen is converted to ammonia, these tests allow us to measure total nitrogen in an effluent sample. The nitrogen input of the waste is known, so nitrogen removal is easily calculated. Reported total nitrogen removal efficiencies vary between 73% and 91% (Sverdlikov, et al 1999). The Sverdlikov Group also found that complete nitrification, 90% denitrification, and 91% total nitrogen removal efficiency is attainable at high hydraulic and volumetric loading rates (1999).

Denitrification (anaerobic)

Nitrification (aerobic)Nitrosobacters Denitrification (anaerobic)

Nitrification (aerobic)Nitrosomonas

Ammonification

Assimilation

Ammonia

Nitrite

N2 (gas)

Organic Nitrogen

Nitrate

Fixation

Plant Setup and CharacteristicsTo implement Simultaneous Nitrification-Denitrification, we used an automated

Sequencing Batch Reactor. The reactor system was built using these main components:

4 L reactor vessel magnetic stirrer peristaltic pump pipe-thread solenoid control valves tubing, bulk head fittings, clamps and clamp holders. dissolved oxygen probe and pressure sensors diffuser stone data acquisition and process controller

The reactor vessel was mounted on the magnetic stirrer base, set to a stir speed of 5. The synthetic waste solution stored in the shelf refrigerator was input to the reactor using the peristaltic pump controlled by the process controller. The solenoid control valves were an integral part of the plumbing system by controlling:

the flow of compressed air the drainage of effluents the regulation of waste and tap water influent

The dissolved oxygen probe was fastened by the clamps and situated such that contact with aeration bubbles via the diffuser stone was limited. The pressure sensor provided the process controller with reactor fluid volume. The process control software allowed us to automate the SBR using desired set points.

Plant Operation

Figure 2: Schematic of the reactor system used. Air delivery system shown in detail.

To operate the plant, the above components were configured as illustrated in Figure 2 above. A test run of the system identified and led to the repair of any leaks in the plumbing connections. The plant was then configured to execute five states involved in wastewater treatment cycle in a SBR:

Fill with waste Fill with water Aerate Settle Drain

The above setup was designed to automatically cycle through the five states in sequence, repeating at the completion of each cycle. The process controller automatically defaults to an off-status if any error occurs in any of the steps. However, we were unable to implement a configuration for a system shutdown in case of a sensor read failure.

Fill with Waste

A 100x stock synthetic waste was diluted to a 20x feed waste and both were kept in the shelf refrigerator. Table 1 below shows the recipe for the desired solution in our plants.

S1

Air Supply

N2

DO probe

Stir bar

200 kPa Pressure sensor

Needle ValveSolenoid Valve

Accumulator7 kPa Pressure sensor (optional)

S2

1.5 mm ID x 5 cm restriction

Table 1: 100x stock synthetic waste contents and their concentrations.

Compound Chemical Formula Concentration (mg/L)

Starch 84.40Casein 125.00Sodium acetate C2H3O2Na3H20 31.90Capric acid C10H20O2 11.60Ammonium chloride NH4Cl 75.33Potassium phosphate K2HPO4 6.90Sodium hydroxide NaOH 175.00Glycerol C3H8O3 12.00

Refridgerated storage prevented microbial degradation of the synthetic waste until its use in the reactor. The synthetic waste contained 40.9 mg/L of nitrogen and a chemical oxygen demand (COD) of 325 mg/L. The waste flow was configured to enter the reactor via a Y-piping connection that also fed the tap water. During the Fill with Waste step, the peristaltic pump was configured by the process controller to:

flow waste feed for 18.3 seconds deliver 140 mL of waste

These configurations were based on the measured peristaltic pump flow rate of 459 mL/min.

Fill with Water

The tap water delivered consisted of “modified” tap water and was made by adding the contents in Table 1 below to tap water.

Table 2: "Modified" tap water contents and their concentrations.

Compound Chemical Formula Concentration (mg/L)

Magnesium sulfate MgSO47H2O 69.60Sodium molybdate NaMoO42H2O 0.15Manganese sulfate MnSO4H2O 0.13Cupric sulfate CuSO44H2O 0.08Zinc suflate ZnSO47H2O 0.48Calcium chloride CaCl22H2O 22.50Iron chloride FeCl36H2O 18.33Cobalt chloride CoCl26H2O 0.42

This modified tap water was fed from a central supply that was passed through the same piping as the synthetic waste. This periodic flushing minimized the bacterial build-up on the waste delivery line. The Fill with Water state was configured by the process controller to deliver the “modified” tap water until two conditions were met:

1. 350 seconds elapsed, which corresponded to the flow rate in the Fill with Waste state.

2. the reactors volume reach 4 L, averaged over 5 seconds.

The second condition acted as a fail-safe if the peristaltic pump failed to deliver the appropriate amount of fluid. The pressure sensor attached at the bottom of the reactor provided the necessary data to monitor the volume.

Aerate

The aeration state was configured to flow air into the reactor and stir the reactor’s contents. The aeration time was varied (8, 18, and 24 hours) in order to identify its impact on the extent of the SND process. The aeration rate was controlled by implementing a Proportional, Integral and Derivative (PID) control system. The target DO level was set to 0.5 mg/L by the PID control. PID sets the value of the control parameter (airflow) based on the sum, the integral and the derivate of the error. Equation 1 below is the general PID function:

, equation 1

where, Kc is controller gain (tuning parameter), TI is the integral time (tuning parameter), TD is the derivative time (tuning parameter), is the difference between measured value and set point (measured oxygen concentration minus desired oxygen concentration), / t is the error rate of change (note that this is the same as the dissolved oxygen concentration rate of change), is the area under the curve of the error as a function of time, and u(t) is the airflow rate that the controller sets. The difference between the process variables and the user-defined set-point is the error (which is reduced to zero to find an output, in this case the airflow rate.

Equation 1 is simplified when brought to the user-interface of LabView, as shown below in Equation 2 :

P I D

. equation 2

For a response to be shown in airflow rate, the values of P, I, and D must be changed accordingly from their default zero values. LabView then calculates the respective values of the PID parameters found in Equation 1.

To identify appropriate values for the values of P, I, and D, trial and error method was utilized to observe the changes in the DO of the reactor. When the DO approached 0.5 mg/L, effective control was established.

The aeration state relied heavily on the performance of the diffuser stone and the DO probe. The DO probe was recalibrated and its membrane was changed on a weekly basis, which ensured its accurate acquisition ability. The diffuser stone was changed once it became clogged and ceased to deliver air.

Settle

The settle state lasted for 1 hour after aeration in order to settle the sludge and biomass sufficiently, to prevent evacuation from the reactor. The allocated settling time was observed to be more than ample for settling the bio-solids in the reactor, but was left unchanged.

Drain

The drain state was the final state in the operational cycle for the SBR, and discharged water from the reactor. The state ran until a residual volume of 1.2 L in the reactor remained, which ensured a continual microbial population in the reactor.

Plant Activation

The plant was activated by the introduction of 4 L of waste-mixed liquor from the City of Ithaca wastewater treatment plant.

Experimental Methods

Testing Methods

The extent of SND occurring in the reactor was assessed by measuring the ammonia and total nitrate concentrations, as ammonia-nitrogen and nitrate-nitrogen. These tests were used as indicators of reactor performance, as two different microbial populations were attempted to be maintained in the reactor. This method was used instead of the Total Suspended Solids (TSS) method because the dominating bacterial population was not known. Satisfactory levels of nitrification and denitrification were hoped to be established via these methods.

Essential Parameters

The essential parameters in the tests were the concentration of nitrogen as ammonia and the concentration of nitrogen as nitrite/nitrate. The total nitrogen in the synthetic waste is known, so assuming all organic nitrogen is converted to ammonia, this information allows us to determine nitrogen removal.

Ammonia Standards via the Micro-Phenate Method

The micro-phenate method was used to determine the total ammonia concentration in the reactor as ammonia-nitrogen. Standards for the micro phenate method were prepared with the following species:

Phenol solution: prepared by introducing 1.11 mL of liquefied phenol (>89%) into a 1.5 mL cuvet with a micro-pipette. Dilution to 10 mL by the addition of 95% v/v ethyl alcohol under the fume hood.

Sodium nitroprusside: prepared by dissolving 50 mg sodium nitroprusside in 10 mL E-pure (deionized) water. Storage in a plastic bottle which was protected from light.

Alkaline citrate: prepared by dissolving 200 g trisodium citrate and 10 g sodium hydroxide in deionized water diluted to 1 L.

Sodium hypochlorite: Clorox®. Oxidizing Solution: prepared for each test by mixing 1 mL of the alkaline citrate

solution with 0.25 mL of Clorox in a 4.5 mL plastic cuvete.

Ammonia standards were prepared by dissolving 1.389 mg of ammonium chloride in 1 mL of water, diluted to 1 L, making a solution containing 1 mg/L of nitrogen. The stock solution was diluted with deionized water to concentrations of 0, 0.2, 0.4, and 0.6 mg N-NH4/L.

1 mL of each of the ammonium standards was pipetted to a separate 1.5-mL cuvete in a fume hood, with the following additions:

40 mL phenol solution 40 mL sodium nitroprusside solution 100 mL oxidizing solution

The cuvete samples were covered and left to stand for a minimum of 1 hour inside a bench drawer. Analysis then followed at an absorbance of 640 nm using a UV-spectrophotometer. The measured absorbances were used to obtain a calibration curve.

Nitrate-nitrogen Standards

The nitrate concentration in the reactor was determined as total nitrate-nitrogen, using the cadmium gravimetric method with Nitriver® and Nitraver® reagent packets

produced by the Hach Corporation. Nitrate-nitrogen standards were prepared by creating a solution of sodium nitrate in distilled water at a concentration of 100 mg/L. This stock solution was diluted into concentrations of 0, 2, 6 and 10 mg/L of nitrate-nitrogen. 0.5 mL of each of the standards was pipetted into a cuvete and a Nitraver® packet was added under the fume hood. The cuvet was then covered, shaken for 3 minutes, and allowed to stand for 30 seconds. Nitriver® packets were added and the new solutions were shaken for 30 seconds and allowed to stand for 10 minutes. The absorbance of these standards was also measured using the UV-spectrophotometer at a wavelength of 542 nm, which generated a calibration curve.

Sampling

Samples were taken from the reactor at the end of the aeration state, filtered using a Millipore filter and stored in the shelf refrigerator. For the ammonia samples, no smell of hydrogen sulfide was detected, so no further additional treatment was required. These samples were diluted by 10x and 100x and were treated using the micro-phenate procedure described above. The dilution was performed because we were not aware of the residual ammonia-nitrogen concentration in reactor at the end of the respective aeration phases. It was discovered from the standards preparation that a highly concentrated solution would give inaccurate absorbance readings (the absorbance readings would exceed the accuracy limits of the spectrophotometer). The 10x dilution fell in the range of the standard calibration curve and its use was continued.

The nitrate-nitrogen samples were acquired from the reactor in a similar way outlined above. These samples were treated using the nitrate-nitrogen procedure previously described. The UV-spectrophotometer was used to determine the nitrate-nitrogen concentration in the samples.

Results and DiscussionsPID Control

PID control was set to maintain a DO level of 0.5 mg/L in the reactor during the aeration state. To assess the performance of the control mechanism two concurrent DO data sets, during the aeration state, were analyzed against time. The data was collected on November 30, 2004 and December 1, 2004 from 4:42:54 PM to 9:42:54PM and from 10:55:20 PM to 3:55:20 AM. For each data set, the average DO and its standard deviation was:

4:42:54 PM to 9:42:54PM, values taken from 4:51:54PM on, when DO dropped below target value of 0.5 mg/L

o Average DO: 0.505 mg/Lo Standard Deviation: 0.015

10:55:20 PM to 3:55:20 AM, values taken from 11:04:20PM on, when DO dropped below target value of 0.5 mg/L

o Average DO: 0.538 mg/Lo Standard Deviation: 0.042

The graphs of DO response versus time are shown below in Figure 3 and 4. PID control accurately maintained an approximate DO of 0.5 mg/L in the reactor throughout the aeration state, as the sample figures below clearly show. We were originally planning on changing the values for P and I, trying to maximize control, but the program worked much better than expected and there was no reason to change the values.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

4:42:54 PM 5:54:54 PM 7:06:54 PM 8:18:54 PM 9:30:54 PMTime

DO

(mg/

L)

DO (mg/L)

Figure 3: Dissolved Oxygen (mg/L) vs. Time during aeration state on 11/30/04 from 4:42:54 PM to 9:42:54 PM.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

10:55:20 PM 12:07:20 AM 1:19:20 AM 2:31:20 AM 3:43:20 AMTime

DO

(mg/

L)

DO (mg/L)

Figure 4: Dissolved Oxygen (mg/L) vs. Time during aeration state on 11/30/04-12/1/04 from 10:55:20 PM to 3:55:20 AM.

UV Spectrometry Analysis: Standards

The results obtained from the UV spectrometry test performed in the Hach cadmium test and the micro-phenate test are provided below:

Figure 5: UV spectrometry curves for nitrate and nitrite standards.

-0.5

0

0.5

1

1.5

2

2.5

0 200 400 600 800 1000

Wavelength (λ)

Abso

rban

ce

0 mg/L

2 mg/L

6 mg/L

10 mg/L

y = 0.1611x - 0.0451R2 = 0.9954

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 2 4 6 8 10 12

Concentration (mg N-NO3/L)

Abso

rban

ce a

t 542

λ

Figure 6: UV spectrometry curves for ammonia standards

We chose to analyze the nitrate and nitrite standards at 542 λ and the ammonia standards were analyzed at 640 λ because these were the wavelengths with the most observable curves. It should be noted that the cadmium test for nitrate and nitrite had to be run several times on standards before an acceptable linear relationship was determined for the standards. We found that it was important to make sure the timing of each step in the cadmium test was the same from sample to sample in order to achieve accurate results. Acceptable results for the ammonia standards were achieved on the first try. The results of the standards analyzed at of the tests at their appropriate wavelengths are provided below:

Figure 7: The linear region of the nitrogen concentration as nitrate and nitrite as determined by tests on prepared standards.

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 200 400 600 800 1000

Wavength (λ)

Abso

rban

ce

0 mg/L

0.2 mg/L

0.4 mg/L

0.6 mg/L

y = 0.436x - 0.0158R2 = 0.9737

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Concentration (mg N-NH3/L)

Abso

rban

ce a

t 640

λ

Figure 8: The linear region of the nitrogen concentration as ammonia as determined by tests on prepared standards.

UV Spectrometry Analysis: Samples

These linear calibration curves were used to estimate the concentrations of ammonia, nitrate and nitrite in the samples extracted from the reactor.

The plant was run with at bulk DO target of 0.5 mg/L with aeration times of 8, 18 and 24 hours. Samples taken from the reactor at the end of these aeration times were analyzed and the results obtained are presented and discussed below.

Table 3: Nitrate-nitrogen concentrations at different aeration times for a DO of 0.5 mg/L.

Aeration Time (hr)

Absorbance at 542 λ

Concentration (mg N-NO3/L)

24 0.63 4.2118 0.54 3.648 1.84 11.72

Table 4: Ammonia-nitrogen concentrations at different aeration times for a DO of 0.5 mg/L.

Aeration Time (hr)

Absorbance at 640λ

Diluted Concentration (mg N-NH3/L)

Actual Concentration (mg N-NH3/L)

24 1.22 3.16 31.5718 1.22 2.83 28.34

8 1.05 2.44 24.43

Table 5: Nitrogen Removal efficiency.

Aeration Total N N Removal % RemovalCycle (hr) Remaining (mg/L) (mg/L)  

24 32.33 8.57 20.96%18 35.19 5.71 13.96%8 36.15 4.75 11.61%

At a DO target of 0.5 mg/L, the first aeration cycle was run for 24 hours. At the end of this aeration cycle, the sample taken from the reactor was analyzed for total nitrate and ammonia concentrations. The results obtained indicated a total ammonia concentration of 28.3 mg N-NH3/L and a total nitrate concentration of 4.22 mg N-NO3/L. We know that the initial nitrogen concentration in the tank is 40.9 mg/L, indicating that a total of 8.38 mg/L of Nitrogen was removed by the bacterial process. This corresponds to 20.5% removal of nitrogen.

From the 24 hour data, it can be noted that most of the nitrogen remaining is in the form of ammonia. This could indicate to insufficient DO level being maintained in the reactor, a plausible explanation considering the predominant dark color observed in the bio-solids in the reactor during the settling stages. This dark color often serves as an indicator of a predominantly anoxic condition in the reactor. Another plausible explanation could be that we allowed the SND process to extend for far longer than necessary such that the aerobic bacterial population was suffocated in the largely anoxic zones in the reactor and simply ceased to perform effectively anymore. A flaw in our process, in this regard, was our inability to test for viable biomass concentrations in the reactor.

Next, we reduced the aeration period to 18 hours, from which we observed a nitrogen removal rate of 13.8%. At the 18-hour aeration cycle, we observed an increase in the residual ammonia-nitrogen concentration to 31.6 mg/L and a decrease in nitrate-nitrogen to 3.64 mg/L.

Finally, we achieved 11.7% removal in the 8-hour aeration cycle. However, a much higher percentage of residual nitrogen was in the form of nitrate and nitrite for the 8-hour cycle. The concentration of ammonia for this test was the lowest of the three.

The trends in these results are interesting and possibly a bit suspicious. In all cases, the ammonia-nitrogen was in the highest concentration, indicating that anoxic conditions predominated in our tank. However, the nitrogen removal efficiency in the 24-hour and 18-hour tests were high compared to the levels of nitrates in the tank, indicating that denitrification was efficient once nitrogen reached the nitrite or nitrate form. The

concentration of nitrates for the 8-hour cycle was very high. This could be an indicator that most of the nitrification is occurring early, but that denitrification does not occur until later. However, the relatively low concentrations of ammonia-nitrogen for this cycle are hard to explain. Ammonia-nitrogen should increase initially, but we expect to see decreasing concentrations of ammonia as nitrogen removal increases. The opposite was observed in our experiment.

A possible solution to this problem would have been to run an aeration cycle for 24-hours and to take samples during that same cycle at more frequent intervals. By doing this we would eliminate any variance in the waste feed or bacterial population from one sample to the next, and would be able more accurately show how the nitrogen concentrations in each form change with time. We ran out of Hach cadmium testing materials and were unable to perform this test, but it would be an interesting topic for further investigation.

ConclusionWe were unable to collect enough data to come to any encompassing conclusions

concerning simultaneous nitrification and denitrification. We were able to achieve some nitrogen removal, so it seems like the process is possible on the laboratory scale. It also seems like long aeration times are required to get enough oxygen into the system to allow for nitrification. We were able to measure total nitrogen in our system without a working nitrogen probe, making the project somewhat successful. We were also able to reach a very well controlled dissolved oxygen level using the process control software and the PID program. Finally, we found that there are many options still available to explore on this topic, but we encourage other research groups to consider our difficulties in planning their experiments.

Difficulties and SuggestionsThe difficulties encountered in running the plant were many, but the major ones

included the tendency of the reactor to produce a lot of effervescent during the initial aeration stages immediately following the tap water-feed step. This led to our reactor overflowing regularly creating a messy situation. We also had overflow problems when there were power supply issues in the lab. We had our “fill with water” step dependent on the pressure sensor at the bottom of the tank, and found that whenever there was a power supply problem, these failed, but the peristaltic pump did not, and our tank overflowed.

Probably the most time consuming part of our experiment was getting the Hach cadmium test for nitrate and nitrite to work correctly. We eventually used almost all of our materials for the nitrite test just in getting the standard test to work correctly. We would suggest to anyone using this test to be careful in the timing. It is not crucial that

the timing is exactly as instructed in the testing kit, but it is important to be consistent from one sample to another. It is also important to make sure that there is no contamination from once sample to the next because it is done with very small volumes. The ammonia test, however, was much easier. We would suggest this method for future experiments, especially those which require collection of a large quantity of data.

If we were to do this project again, we would run the plant at aeration times that were sequentially increased by 2 hours from a 2-hour initial point. Furthermore, we would take the samples from the same cycle. For example, we would run a 24-hour aeration cycle and take samples every 2 hours during that time. This would help reduce the variability in the reactor conditions and provide more meaningful data. It may be hard to have a 24-hour time period when an investigator is available every 2-hours, but it would be worth it when compared to the numerous problems encountered trying to run several cycles. Other further research could include trying to obtain simultaneous nitrification and denitrification at different levels that ranging from 0.5 to 2.5 mg/L. We chose 0.5 mg/L based on our research, but it would be interesting to see how good of a choice this was.

References

David P. Whichard (2001), Nitrogen Removal From Dairy Manure Wastewater Using Sequencing Batch Reactors, M.Sc Thesis, Virginia Polytechnic Institute and State University. URL at http://http://scholar.lib.vt.edu/theses/available/etd-8082001103610/unrestricted/David_Whichard_ETD.pdf .

Sverdlikov A.I., Scherbina G.P., Sverdlikov A.A., Nakhla G.F, and Lugowski A, Simultaneous Nitrification-Denitrification and Clarification in Pseudoliquified Activated Sludge Reactors, URL at http://alumni.iatp.org.ua/publications/ files/2004071911200536.doc

Ekama, G. and Wentzel, M.C. (1999) Denitrification Kinetics in Biological N&P Removal Activated Sludge Systems Treating Municipal Wastewaters, Water Sci. Tech., 39(6), 69.

Daigger, G.T. and Littleton, H.X. (2000), Characterization of Simultaneous Nutrient Removal in Staged, Closed-Loop Bioreactor, Water Environment Research (72): 330-339.

Fuerhacker, M., Bauer, H., et al. (2000), Approach for a novel control strategy for simultaneous nitrification/denitrification in activated sludge reactors, Water Research (34):2499-2506.

Munch, E.V., Lant, P., and Keller, J. (1996), Simultaneous Nitrification and

Denitrification in Bench-Scale Sequencing Batch Reactors, Water Research, (30):277-284.

Pochana, K., and Keller, J. (1999), Study of Factors Affecting Simultaneous Nitrification and Denitrification (SND) Water Science and Technology (39): 61-68.