This article was downloaded by: [University of California, San Diego]On: 17 November 2014, At: 19:58Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and EnvironmentalEngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesa20

Effects of dissolved oxygen on biological nitrogenremoval in integrated fixed film activated sludge (IFAS)wastewater treatment processTongchai Sriwiriyarat a , Wiyaporn Ungkurarate b , Prayoon Fongsatitkul b & SopaChinwetkitvanich ba Department of Chemical Engineering, Faculty of Engineering , Burapha University ,Chonburi, Thailandb Department of Sanitary Engineering, Faculty of Public Health , Mahidol University ,Bangkok, ThailandPublished online: 06 Mar 2008.

To cite this article: Tongchai Sriwiriyarat , Wiyaporn Ungkurarate , Prayoon Fongsatitkul & Sopa Chinwetkitvanich (2008)Effects of dissolved oxygen on biological nitrogen removal in integrated fixed film activated sludge (IFAS) wastewatertreatment process, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and EnvironmentalEngineering, 43:5, 518-527, DOI: 10.1080/10934520701796481

To link to this article: http://dx.doi.org/10.1080/10934520701796481

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Journal of Environmental Science and Health Part A (2008) 43, 518–527Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934520701796481

Effects of dissolved oxygen on biological nitrogen removalin integrated fixed film activated sludge (IFAS) wastewatertreatment process

TONGCHAI SRIWIRIYARAT1, WIYAPORN UNGKURARATE2, PRAYOON FONGSATITKUL2,and SOPA CHINWETKITVANICH2

1Department of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi, Thailand2Department of Sanitary Engineering, Faculty of Public Health, Mahidol University, Bangkok, Thailand

The objective of this research was to determine the effects of dissolved oxygen on the biological nitrogen removal in the IntegratedFixed Film Activated Sludge (IFAS) and Modified Ludzack-Ettinger (MLE) systems. The carbonaceous and nitrogen removals wereinvestigated at the COD/Nitrogen (C/N) ratios of 4, 6, and 10, and the dissolved oxygen (DO) concentrations of 2, 4, and 6 mg/L. Theexperimental results indicate that the C/N ratios of 4, 6, and 10 and the DO concentrations of 2, 4, and 6 affected insignificantly onthe chemical oxygen demand (COD) removal, but significantly on the nitrogen removal as the consequences of different nitrificationand denitrifcation rates in both systems. The COD removal was nearly completed throughout this study because glucose was used asa primary carbon source in the wastewater and both systems were operated at high SRT relative to the minimum SRT requirementfor COD removal. The experimental conditions used in this study apparently led to nitrite accumulation in both IFAS and MLEsystems. It is suggested that there is no benefit of installing media in the IFAS system at the C/N ratio of 10 because the system wasunderloaded with the nitrogen. The lower DO concentration, the greater denitrification in the anoxic zone was achieved because nitritenitrogen was used as an electron acceptor. At the C/N ratios of 4 and 6, the IFAS system was higher in capacity for nitrification as aresult of attached biomass on the support media in the aerobic zone. The DO concentration of 6 mg/L is required to maximize thenitrification rates in the systems under these experimental conditions resulting in greater oxidized nitrogen for denitrification in theanoxic zones. The denitrification in the aerobic zone of the IFAS system is not evaluated due to unavailability of nitrite information.The optimal DO concentrations for biological nitrogen removal in the IFAS system at the C/N ratios of 4, 6, and 10 in this studywere 6, 6, and 2 mg/L, respectively.

Keywords: Nitrification, denitrification, IFAS, dissolved oxygen, BNR, C/N ratio, biofilm.

Introduction

The primary objective of the activated sludge/biofilm hy-brid system, called Integrated Fixed Film Activated Sludge(IFAS) in the United States, is to sustain nitrification inthe system at the critical conditions, especially at low tem-perature in winter. This process integrates the biofilm intothe activated sludge process by placing the support mediainto the aeration zone. The attached biomass on the sup-port media increases the solid retention time (SRT) in thesystem without causing overloading problems to the final

Address correspondence to Tongchai Sriwiriyarat, Departmentof Chemical Engineering, Faculty of Engineering, Burapha Uni-versity, Muang, Chonburi 20131, Thailand; E-mail: sriwiri@buu.ac.thReceived August 31, 2007.

clarifier or expanding the aeration basin. The observationsfrom both bench-scale and full-scale studies have demon-strated that nitrification, denitrification, and carbonaceousremoval in the aerobic zone can be enhanced in the activatedsludge system at low SRT and temperature.[1−5]

Simultaneous nitrification and denitrification in the aer-obic zone have also been observed in the IFAS process.The denitrification in the aerobic zone is accomplished inthe anoxic zone of biofilm layers. The amount of oxidizednitrogen was denitrified between 30% and 88% in the full-scale wastewater treatment process in Maryland, USA inwhich the aerobic zones were installed with 30,000 metersRinglace media.[3] The nominal hydraulic retention time(HRT) of this system was 6 hours. Sen et al.[4] also reportedthat the fractions of denitrification in the aerobic zone ofthe systems, which were maintained at the dissolved oxy-gen (DO) concentration greater than 6.0 mg O2/L and the

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

Dissolved oxygen and nitrogen removal in wastewater treatment 519

aerobic SRT of 3.1 days, were 14% and 24% with integratedRinglace and sponge media, respectively. The fractions ofdenitrification in the aerobic zone increased proportionallywith the suspended SRT maintained in the systems.

High DO is normally required to complete nitrification inthe aerobic zone, but would probably result in low denitrifi-cation in the aerobic and anoxic zones of the IFAS system.Several IFAS studies have supplied the DO about 6.0 mgO2/L in the aerobic zone to obtain the maximum capac-ity of the system for nitrification.[1−5] The objective of thisstudy was to determine the effects of DO concentration onthe IFAS wastewater treatment process, which was operatedunder different nitrogen loadings and DO concentrationsin parallel with a conventional Modified Ludzack-Ettinger(MLE) system, for biological nitrogen removal includingdenitrification in the aerobic zones.

Materials and methods

Two conventional MLE configuration pilot-scale biologi-cal nitrogen removal systems as illustrated by Figure 1 wereoperated in parallel at Mahidol University, Bangkok, Thai-

land under different nitrogen loadings and dissolved oxygen(DO) concentrations to determine the effects of DO con-centrations on the biological nitrogen removal. A 0.42 m2

of 2.54-centimeter type Bioweb media was installed into theaerobic tank of one system, referred to hereafter as IFAS,where as another MLE system was used as a control system.Each square meter of 2.54-centimeter type Bioweb is madeof fabric knitted as web-like mesh consisting of 711 hexag-onal loops of which each loop can be linearly extended toabout 14 cm/loop resulting in a total length of 99.5 m/m2.Each system consisted of one anoxic (7.2 L) reactor andone aerobic (22.2 L) reactor in series. To enhance denitrifi-cation in the anoxic zone, a nitrate recycle (NR) flow rateof 150%Q and a returned activated sludge (RAS) flow rateof 100%Q, where Q is an influent flow rate, were carriedout in each system. The total volume of each system wasapproximately 29 liters, with a small volume variation de-pending upon the quantities of media and biofilm in theIFAS system.

Both systems were operated at the hydraulic retentiontime (HRT) of 6.8 (± 0.2 SD) hours, the SRT of 5.5 (± 0.5SD) days, and the temperature of 28.4 (±0.68 SD)◦C, where

Fig. 1. The configurations of pilot-scale MLE and IFAS systems.

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

520 Sriwiriyarat et al.

Table 1. Wastewater characteristics at different C/N ratios.

Influent characteristics Phase I Phase II Phase III Unit

Total Influent COD 334 (± 28) 342 (± 15) 346 (± 9) mg COD/LTotal Influent Nitrogen (TKN) 33 (± 2) 53 (± 2) 76 (± 2) mg N/LTotal Influent Phosphorus (TP) ≈ 6 ≈ 6 ≈6 mg P/LApplied COD Loading 1.16 (± 0.09) 1.19 (± 0.05) 1.20 (± 0.04) kg COD/m3-dayApplied Nitrogen Loading 0.12 (± 0.01) 0.18 (± 0.01) 0.26 (± 0.01) kg N/m3-dayC/N Ratio 10.1 (± 1.1) 6.5 (± 0.4) 4.5 (± 0.2) mg COD/mg N

The number in parenthesis is the standard deviation (SD) of the mean value; Carbon/Nitrogen Ratio (C/N Ratio); Chemical Oxygen Demand(COD); Total Kjeldahl Nitrogen (TKN); Total Phosphorus (TP); Nitrogen (N); Phosphorus (P).

SD is a standard deviation. These systems were fed with asynthetic wastewater diluted from a liter of stock solutioncomposing of 30 g of glucose, 0.001 g of K2HPO4, 0.001 gof KH2PO4, 0.001 g of CaCl2, 14 g of NaHCO3 and 8.5 g ofMgSO4 resulting in the wastewater characteristics as listedin Table 1 at a flow rate of about 100 L/day. The influentpH was 8.3 (± 0.1 SD) as a result of supplemented alka-linity. The different amounts of urea were added to adjustthe C/N ratios to about 4, 6, and 10. The SRT was main-tained by wasting daily the appropriate amount of sludgefrom the aerobic reactor of each system. Both systems wereinitially fed with the sludge from the Sri-Phraya municipalwastewater treatment plant located in Bangkok, Thailand,and were operated for a period of time allowing sludge ac-climatization with the feeding wastewater. The data series atsteady state conditions of each experimental condition werecollected for analyses during a period of over 22 months.

The systems were monitored for mixed liquor sus-pended solid (MLSS), mixed liquor volatile suspended solid(MLVSS), chemical oxygen demand (COD), nitrate nitro-gen (NO3-N), ammonia nitrogen (NH4-N), Total KjeldahlNitrogen (TKN), pH, and dissolved oxygen (DO) concen-tration. The MLSS, MLVSS, TKN, NO3-N, NH4-N andCOD were analyzed in accordance with Standard Meth-ods for the Examination of Water and Wastewater.[6] TheDO concentration was measured by a DO meter. To deter-mine the amount of attached biomass, a syringe was usedto scrub the attached biomass from the supporting media.The syringe was specifically designed to provide a high wa-ter pressure spray. The biofilm suspending in the collectedwater were centrifuged to remove some excess water andsubsequently the suspended solids (SS) and volatile sus-pended solids (VSS) concentrations were determined ac-cording to the MLSS and MLVSS procedures. Sludge set-tling characteristics were evaluated by the SVI values. Thespecific oxygen uptake rates (SOURs) were determined di-rectly from the aeration tanks of both systems after turningoff air supply. The sludge was then slowly mixed while theDO concentrations were being monitored.

Results and discussion

The IFAS and MLE systems, which were operated in par-allel under the same experimental conditions, were com-

pared at different C/N ratios and DO concentrations toevaluate their effects on the system performances for bio-logical nitrogen removal. As shown in Figure 2, the carbonremoval efficiencies of the MLE and IFAS systems werenearly completed throughout this study because glucose,which is a readily biodegradable organic matter, was usedas a sole carbon source in the synthetic wastewater andboth systems were operated at high SRT with respect to theminimum SRT requirement for COD removal.[7]

Both systems could equally remove the COD with av-erage removal efficiencies of 98.8% (±0.64 SD) and 98.8%(±0.56 SD) for the MLE and IFAS systems, respectively.The experimental results indicate that there were insignifi-cant differences in COD removal efficiencies at the DO con-centrations and C/N ratios used in this study. In addition,there were little variations in the organic loadings betweenexperimental conditions; thereby, a graphical plot of CODmass fractions is used to represent the COD removed in theanoxic, aerobic, and final clarifiers with a total of 1.0. Thefractions in Figure 2 indicate that the organic matters (>80%) were primarily removed in the anoxic zones of bothsystems for denitrification and carbon oxidation. Further-more, the fraction of COD removed in the anoxic zone ofthe IFAS system was greater than the one in the MLE sys-tem at the same C/N ratio and DO concentration.

It is well known that the IFAS system enhances nitrifica-tion in the aerobic zone generating more oxidized nitrogen,which is recycled to the anoxic zone. It appears that thefractions of COD removed in the anoxic zones increase asthe DO concentrations and C/N ratios decrease. It is be-lieved that the DO, which is recycled from the last zone ofaerobic reactor, does not contribute to greater fraction ofCOD removed in the anoxic zone; otherwise, there wouldbe more carbon removal in the anoxic zones at the DO con-centration of 6 mg O2/L than the one of 2 mg O2/L. Asthe C/N ratios decreased, there would be more ammoniumnitrogen to be nitrified and more oxidized nitrogen to begenerated depending on TKN removal efficiencies leadingto more oxidized nitrogen recycled to the anoxic zones. Asthe DO concentrations decreased, it is postulated that ni-trite nitrogen is generated at higher degree than the nitratenitrogen resulting in more nitrite nitrogen recycled to theanoxic zone at low DO concentrations. However, the nitritenitrogen data is not available because nitrite accumulation

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

Dissolved oxygen and nitrogen removal in wastewater treatment 521

Fig. 2. The fractions of COD removed in different locations and the COD removal efficiencies of the MLE and IFAS systems atdifferent DO concentrations and C/N ratios.

is not expected in this study at the DO concentrationexceeds 2 mg O2/L, but there are further discussions inthis section to support the assumption that nitrite nitrogenis used for denitrification in the anoxic zone.

The fractions of ammonium nitrogen removed at differ-ent locations along with the removal efficiencies of bothIFAS and MLE systems are illustrated by Figure 3. TheTKN removal efficiencies between the MLE and IFAS sys-tems were significantly different at the C/N ratios of 4 and6 as the consequences of different nitrification rates at theexperimental conditions with the exception of experimentsat the C/N ratio of 10. The experimental results indicatethat both systems were capable to archive the same TKNremoval efficiencies of 100% at the C/N ratio of 10, re-gardless of DO concentrations because both systems wereunderloaded with respect to the nitrogen at the operatingtemperature and SRT. It is suggested that the DO concen-tration of 2 mg/L was sufficient for both systems at theC/N ratio of 10 to complete nitrification in the systems.Therefore, the maximum capacity of the IFAS system fornitrification cannot be evaluated and the benefits of inte-grating the fixed film media in the system for nitrificationare not achieved at the C/N ratio of 10.

In contrast, both systems provided the similar decliningtrends in the TKN removal efficiencies at the C/N ratiosof 4 and 6 when the DO concentrations decreased from6 to 2 mg O2/L. It appears that both systems were over-loaded regarding to the influent nitrogen and the oxygensupplies were limited for nitrification at low DO concentra-

tions. However, the declining rates of the IFAS system weremuch lower than the ones of the MLE system, resultingin higher nitrification in the IFAS system than the MLEsystem. The slopes of the changes in the TKN removal ef-ficiencies by the changes in DO concentrations in Figure 3were 11.6, 10.0 for the MLE system and 5.5 and 7.8 for theIFAS system at the C/N ratios of 6 and 4, respectively. Atthe C/N ratio of 4, the fixed film media integrated into theaerobic zone of the IFAS system clearly doubled the systemcapacity for nitrification as compared with the MLE systemat the DO concentrations of 2, 4, and 6 mg O2/L. Figure 3also indicates that ammonium nitrogen were nitrified in thefinal clarifiers of both systems. It is possible that the HRTis insufficient to complete the nitrification in the aerobiczones of both systems. Even though the HRT of the systemwas 6.8 hours, the HRT of aerobic zones including recycleflows for the MLE and IFAS systems were 90 minutes and85 minutes, respectively.

Table 2 lists the rates including oxygen uptake rates(OURs), specific oxygen uptake rates (SOURs), and ni-trification rates in the MLE and IFAS systems. The ni-trification rates were determined by mass balance analy-ses around the aerobic reactors. Since the COD removalin the aerobic zones were insignificant, the OURs andSOURs would response primarily to the nitrification ac-tivities in the aerobic zones of both systems. The OURsin the IFAS system increased as the C/N ratios decreasedand were as high as about 2, 3, and 4 times the rates in theMLE system at the C/N ratios of 10, 6, and 4, respectively.

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

522 Sriwiriyarat et al.

Fig. 3. The mass of TKN removed in different locations and the TKN removal efficiencies of the MLE and IFAS systems at differentDO concentrations and C/N ratios.

Conversely, the SOURs in the IFAS system were lower thanthe MLE system at the C/N ratio of 10, but were greaterat the C/N ratios of 6 and 4, respectively. The possibleexplanations are that the system was underloaded with re-spect to the nitrogen and the ammonium oxidations weremainly accomplished by the suspended biomass at the C/Nratio of 10. The SOURs in the IFAS system were greaterat the C/N ratios of 4 and 6 when the system was over-loaded with the nitrogen, so both suspended and attachedbiomass fully nitrified the ammonium nitrogen. The graph-ical plot of Figure 4 confirms that the nitrification rates of

both systems increased linearly as the SOURs increased,but the IFAS systems was higher in the nitrification ratesthan the MLE system when the SOURs was above 4.6 mgO2/g MLVSS-h. The nitrification rates in Table 2 and to-tal nitrification in Table 3 also demonstrate that the IFASsystem has greater capacity for nitrification than the MLEsystem. The OURs and SOURs in both systems decreasedas the DO concentrations decreased from 6 to 2 mg O2/Lindicate that oxygen was limited for nitrification reactions.Therefore, it is suggested that the optimal DO concentra-tions for maximum nitrification rates in both systems under

Table 2. OUR, SOUR, and nitrification rate of the MLE and IFAS systems.

OUR (mg/L-min) SOUR (msg O2/g VSS-h) Nitrification rate (mg/L-h)

C/N Ratio DO (mg/L) MLE IFAS MLE IFAS MLE IFAS

10 6 0.38 0.79 7.9 5.5 3.3 4.24 0.35 0.78 7.4 5.7 3.3 4.22 0.28 0.62 6.0 4.6 3.3 2.8

6 6 0.56 1.54 11.4 10.6 6.0 7.84 0.45 1.38 9.8 10.1 3.8 5.52 0.35 1.32 7.0 9.6 5.2 6.3

4 6 0.53 1.70 11.0 12.0 5.7 11.24 0.38 1.60 8.6 11.8 4.8 10.22 0.35 1.49 7.1 11.0 2.9 7.6

VSS is volatile suspended solids and is a combination of MLVSS and biofilm converted equivalently to VSS; Oxygen Uptake Rate (OUR); SpecificOxygen Uptake Rate (SOUR); Modified Ludzack-Ettinger (MLE); Integrated Fixed Film Activated Sludge (IFAS); Dissolved Oxygen (DO);Carbon/Nitrogen (C/N).

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

Dissolved oxygen and nitrogen removal in wastewater treatment 523

Fig. 4. Relationship between nitrification rates and SOURs of the MLE and IFAS systems.

overloading conditions is 6.0 mg O2/L for both systems atthe experimental operating conditions.

The mass balance analyses of nitrate nitrogen aroundeach reactor in both systems are plotted in Figure 5 alongwith the effluent concentrations at different C/N ratios andDO concentrations. It appears that there are little amountsof nitrate nitrogen generated from ammonium oxidationin both systems at all C/N ratios and DO concentrationswith the exception of DO concentration of 2 mg O2/L atthe C/N ratio of 4. The amounts of nitrate nitrogen gen-erated at the DO concentration of 6 mg O2/L were greaterthan at the DO concentrations of 4 and 2 as a result ofhigher nitrification, respectively. However, the quantitiesof nitrate nitrogen were not proportionally related to theamounts of ammonium nitrogen oxidized in the systems

as listed in Table 3. It is generally believed that there is noconsiderable denitrification in the aerobic zone of the MLEsystem because the DO concentration exceeds 2 mg O2/L.Therefore, it is clear that there are some great amounts ofoxidized nitrogen were still not accounted for and this ox-idized nitrogen should be the nitrite nitrogen. In addition,there were greater than 80% of organic matters removed inthe anoxic zones of both systems, but only little amount ofnitrate nitrogen used for denitrification in the anoxic zones.The quantities of oxidized nitrogen, which were generatedfrom the ammonium oxidation and were not accounted for,support the postulation of COD removal by nitrite nitrogenin the anoxic zones as discussed previously. Interestingly,the IFAS system functioned normally by generating nitratenitrogen in the aerobic zone at the C/N ratio of 4 with the

Table 3. Nitrification and denitrification at different DO concentrations and C/N ratios.

Total Mass nitrate Denitrification Unaccountednitrification generated in anoxic zone Effluent nitrate oxidized(g/day) (3) generated (g/day) (4) (g/day) (5) nitrogen (g/day) (6) nitrogen (g/day) (7)

C/N DOratio (1) (mg/L) (2) MLE IFAS MLE IFAS MLE IFAS MLE IFAS MLE IFAS

10 6 1.9 2.1 0.4 0.3 0.2 0.1 0.2 0.1 1.5 1.74 1.8 2.0 0.4 0.2 0.2 0.2 0.2 0.1 1.3 1.62 1.9 1.9 0.3 0.1 0.1 0.1 0.1 0.0 1.6 1.7

6 6 3.2 3.9 1.9 0.7 0.4 0.4 1.5 0.3 1.3 2.94 2.7 3.4 1.1 0.5 0.4 0.3 0.8 0.2 1.6 2.82 2.5 3.5 0.3 0.3 0.2 0.2 0.1 0.1 2.2 3.1

4 6 3.4 5.8 1.6 1.3 0.1 0.6 1.4 0.7 1.9 3.74 2.9 5.3 1.2 0.6 0.2 0.2 0.9 0.3 1.8 4.42 2.3 5.1 0.3 6.6 0.1 2.9 0.2 3.7 2.0 −5.2

Carbon/Nitrogen (C/N); Dissolved Oxygen (DO); Modified Ludzack-Ettinger (MLE); Integrated Fixed Film Activated Sludge (IFAS).

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

524 Sriwiriyarat et al.

Fig. 5. The fractions of nitrate nitrogen in different locations and effluent nitrate concentrations of the MLE and IFAS systems atdifferent DO concentrations and C/N ratios.

DO concentration of 2 mg O2/L as shown in a negativevalue of unaccounted oxidized nitrogen in Table 3. Addi-tionally, the nitrate nitrogen was denitrified equally in theanoxic reactor and the final clarifier as shown in Figure 5.

Table 4 lists the theoretical calculations and experimentalresults of COD utilizations for carbon oxidation and den-itrification reactions. The COD removals by the DO fromthe NR recycle (150%Q) for carbon oxidation and by the

nitrate nitrogen for denitrification in the anoxic zone aretheoretically calculated (4) and experimentally determined(5), respectively. It is assumed that there is no DO in theRAS recycle as the oxygen should be used up by nitrifica-tion and carbon oxidation in the final clarifier. The column(6) lists the amounts of COD removed by another electronacceptor, which is expected to be the nitrite nitrogen, aftersubtracting the amounts of COD used by the DO and the

Table 4. The COD utilized by oxidation-reduction and denitrification reactions in the anoxic zones of the MLE and IFAS systems atdifferent C/N ratios and DO concentrations.

Experimental Calculatedtotal COD Experimental COD utilized Calculated Effluentutilized in Calculated nitrate utilized by another nitrite SKN +

anoxic zone COD in anoxic zone electron required NO3-N(g/day) (3) utilized by (g/day) (5) acceptor (g/day) (6) (g/day) (7) (mg/L)8)

C/N DO oxygenratio (1) (mg/L) (2) MLE IFAS (g/day) (4) MLE IFAS MLE IFAS MLE IFAS MLE IFAS

10 6 27.1 28.2 2.2 0.2 0.1 20.6 23.2 1.8 2.0 2.0 2.54 26.2 34.7 1.5 0.2 0.2 20.2 30.3 1.8 2.6 2.0 1.02 32.0 31.0 0.7 0.1 0.1 28.4 29.2 2.5 2.5 1.5 0.4

6 6 27.3 34.0 2.2 0.4 0.4 17.1 24.3 1.5 2.1 16.4 3.84 30.0 30.1 1.5 0.4 0.3 21.8 23.1 1.9 2.0 17.2 4.02 32.4 33.2 0.7 0.2 0.2 27.7 29.1 2.4 2.5 15.2 6.8

4 6 33.0 31.6 2.2 0.1 0.6 28.7 18.0 2.5 1.6 38.0 13.24 31.6 31.1 1.5 0.2 0.2 25.5 25.2 2.2 2.2 41.0 15.22 33.2 32.5 0.7 0.1 2.9 30.0 −23.9 2.6 −2.1 39.6 18.2

Soluble Kjeldahl Nitrogen (SKN); Chemical Oxygen Demand (COD); Modified Ludzack-Ettinger (MLE); Integrated Fixed Film Activated Sludge(IFAS); Nitrate Nitrogen (NO3-N).

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

Dissolved oxygen and nitrogen removal in wastewater treatment 525

Table 5. Calculated denitrification in the aerobic zones of both MLE and IFAS systems.

Total Calculatednitrification or Calculated Effluent denitri. inunoxidized N Denitr. in nitrite used nitrate aerobic

generated anoxic zone in anoxic nitrogen zone Denitr.(g/day) (g/day) zone (g/day) (g/day) (g/day) in aerobic

C/N DO zone (%)Ratio (mg/L) MLE IFAS MLE IFAS MLE IFAS MLE IFAS MLE IFAS IFAS

10 6 1.9 2.1 0.2 0.1 1.8 2.0 0.2 0.1 −0.3 −0.2 —4 1.8 2.0 0.2 0.2 1.8 2.6 0.2 0.1 −0.4 −0.9 —2 1.9 1.9 0.1 0.1 2.5 2.5 0.1 0.0 −0.9 −0.8 —

6 6 3.2 3.9 0.4 0.4 1.5 2.1 1.5 0.3 −0.2 1.1 284 2.7 3.4 0.4 0.3 1.9 2.0 0.8 0.2 −0.3 1.0 282 2.5 3.5 0.2 0.2 2.4 2.5 0.1 0.1 −0.2 0.7 20

4 6 3.4 5.8 0.1 0.6 2.5 1.6 1.4 0.7 −0.6 2.9 504 2.9 5.3 0.2 0.2 2.2 2.2 0.9 0.3 −0.4 2.6 482 2.3 5.1 0.1 2.9 2.6 −2.1 0.2 3.7 −0.6 0.6 12

Modified Ludzack-Ettinger (MLE); Integrated Fixed Film Activated Sludge (IFAS); Nitrate Nitrogen (NO3-N); Carbon/Nitrogen (C/N); DissolvedOxygen (DO).

nitrate nitrogen. Column (7) shows the amount of nitritenitrogen, which will be theoretically required to remove theremaining COD in the anoxic zones.

14

O2 + H+ + e− → 12

H2O (1)

15

NO−3 + 6

5H+ + e− → 1

10N2 + 3

5H2O (2)

13

NO−2 + 4

3H+ + e− → 1

6N2 + 2

3H2O (3)

CODNO2 − N

= 1711 − 142YN

(4)

YN = Y1 − kd SRT

(5)

The calculations are based on the kinetic coefficientswhich a true yield coefficient (Y) and a decay coefficient(kd) are 0.67 g biomass COD /g substrate COD and 0.05d−1, respectively.[8] The numbers 1.42 and 1.71 are a massof COD/mass of biomass produced and an oxygen equiv-alent of nitrite nitrogen oxidation-reduction reactions in gO2/g NO2-N, respectively. The calculations are based onstoichiometric relationships as shown in Equations (1) and(3) [(0.25 × 32)/(0.33 × 14) = 1.71]. Since the ammoniumnitrogen is available for cell synthesis in the anoxic zone,the amount of nitrite nitrogen is assumed not to be usedfor cell synthesis. The calculations of column (7) are basedon the COD/NO2-N ratio for denitrification determinedby the equations (4) and (5). The amounts of nitrite nitro-gen required to remove the COD in the anoxic reactors ofcolumn (7) suggest the nitrite nitrogen were accumulatedin both systems. If the assumption is made that all nitritenitrogen are used for denitrification in the anoxic zone be-cause there were some COD removed in the aerobic zones,the optimal DO concentration for total nitrogen removal

based on SKN and nitrate nitrogen in column (8) is 6 mgO2/L at the C/N ratios of 4 and 6.

Several studies has reported that various factors leadingto partial nitrification reaction causing nitrite accumulationsuch as high pH, short HRT, high temperature over 25◦C,and low DO concentrations, and SRT.[9−12] The SHARONprocess[12] was experimentally feasible at high pH (8.1–8.4)and temperature (30◦C). The pH used in this study wasabout 8.3. The ammonium oxidizer bacteria (AOB) growfaster than the nitrite oxidizer bacteria (NOB) at the tem-perature higher than 25◦C, which is corresponding to thetemperature of 28.4◦C used in this study. Peng [9] reportedHRT of 6–9 h. can cause nitrite accumulation in the A/Osystem because the NOB is washed out from the system.The HRT of the systems used in this study was 6.8 h. Sev-eral factors used to operate the systems in these experimentscould possibly cause the nitrite accumulation in both MLEand IFAS systems.

Due to unavailability of nitrite nitrogen information, thedenitrification in the aerobic zone of the IFAS system couldnot be experimentally quantified. If the assumptions can bemade that all nitrite are used for denitrification in the sys-tems, there should be no nitrite in the effluents. The infor-mation from Tables 3 and 4 are used to calculate the den-itrification in the aerobic zones of both systems as shownin Table 5. The negative numbers in the table may repre-sent that there is no dentrification in the aerobic zones. Thenumbers are positive at the C/N ratios of 4 and 6 of theIFAS system representing the existence of dentrification inthe aerobic zone. Less denitrification in the aerobic zone isexpected if there is nitrite nitrogen in the effluent. Neverthe-less, the effects of DO on the denitrification in the aerobiczone of the IFAS systems cannot be evaluated.

The suspended biomass maintained in the MLE andIFAS systems and the attached biomass per media area and

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

526 Sriwiriyarat et al.

Table 6. Biomass concentrations and sludge settling characteristics of the MLE and IFAS systems.

MLSS (mg/L) MLVSS (mg/L) Biofilm SVI (mL/g)

C/N Ratio DO (mg/L) MLE IFAS MLE IFAS g/m2 mg/L∗ MLE IFAS

10 6 2990 2880 2830 2780 312 5891 304 2854 2800 2820 2740 2760 290 5475 293 2732 2720 2760 2690 2680 284 5363 276 275

6 6 3860 2940 3720 2860 308 5833 238 2894 2800 2760 2740 2640 292 5521 296 2932 3020 2990 2930 2850 287 5433 262 258

4 6 3520 2880 3360 2780 304 5755 276 3304 2950 2650 2830 2590 293 5540 322 3432 3130 2950 2970 2830 278 5261 275 275

Average 3088 2848 2979 2752 294 5564 282 291SD 374 110 343 95 11 217 25 28

Biofilm in gram per square meter is converted to VSS in the unit of milligram per liter; Volatile Suspended Solids (VSS); Carbon/Nitrogen (C/N);Dissolved Oxygen (DO); Mixed Liquor Suspended Solids (MLSS); Mixed Liquor Volatile Suspended Solids (MLVSS); Sludge Volume Index (SVI);Modified Ludzack-Ettinger (MLE); Integrated Fixed Film Activated Sludge (IFAS); Standard Deviation (SD).

VSS equivalent are listed in Table 6. The attached biomasscontributed significantly to the total amounts of biomassmaintained in the IFAS system providing longer SRT andresulting in higher nitrification in the IFAS system. The ra-tio of biomass between the MLE and IFAS systems wereapproximately 1:3; thereby, the maximum capacity for ni-trification system can be achieved at low C/N ratio of 4.Good sludge settling characteristics were also observed inboth systems even though the sludge volume indices (SVIs)were about 300 ml/g. It is found that there is no significantdifferent in the sludge characteristics between the MLE andIFAS systems.

Conclusion

The effects of DO concentrations on the biological nitrogenremoval in the MLE and IFAS systems under different C/Nratios were evaluated. The following conclusions can bedrawn from the experimental results.

1. The COD removal was nearly completed because glu-cose was used as a primary carbon source in the wastew-ater; therefore, there is no effect of DO concentration onthe COD removal in both systems. However, the lowerDO concentration maintained in the aerobic reactor, thegreater fractions of COD were removed by denitrifica-tion by nitrite nitrogen.

2. Under system underloading conditions with respect tonitrogen at the C/N ratio of 10, the optimal DO con-centration for nitrification is 2 mg O2/L. There is nobenefit of installing media in the IFAS system at thisC/N ratio. However, when the systems were operatedat the C/N ratios of 4 and 6 and were under systemoverloading conditions, the IFAS system was superiorto the MLE system in terms of nitrification capacity as

a result of supplemented biomass in the system. The op-timal DO concentrations for the C/N ratios of 4 and 6is 6 mg O2/L to achieve the maximum nitrification ratesgenerating more oxidized nitrogen for denitrification inthe anoxic zones resulting in lower total nitrogen in theeffluents.

3. Due to unavailability of nitrite information, the effectsof DO on denitrification in the aerobic zone of the IFASsystem cannot be experimentally concluded, but the the-oretical calculations confirms the existences of denitrifi-cation in the aerobic zone.

4. The operating conditions used in this study includingtemperature, pH, HRT, and SRT led to nitrite accumu-lation in the system. Further investigations about usingthe nitrite pathway for biological nitrogen removal atlow DO concentrations in the IFAS system are required.

Acknowledgments

The authors would like to acknowledge the research fund-ing, which was financially supported by the research anddevelopment fund of the Faculty of Engineering, BuraphaUniversity, Thailand and Post-Graduate Program in Envi-ronmental Technology (ADB-Supported Program), Mahi-dol University, Thailand. The authors also appreciate EntexTechnologies, Inc., USA for providing the Bioweb mediaused in this study.

References

[1] Liu, H. Utilization of Captor sponges to maintain nitrification anddenitrification in BNR activated sludge at low aerobic MCRTs,M.S.Thesis; Virginia Tech: Blacksburg Virginia, USA, 1996

[2] Randall, C.W.; Sen, D. Full-scale evaluation of an integrated fixed-film activated sludge (IFAS) process for enhanced nitrogen removal.Water Sci. Technol. 1996, 33(12), 155–161.

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014

Dissolved oxygen and nitrogen removal in wastewater treatment 527

[3] Sen, D.; Mitta, P.; Randall, C.W. Performance of fixed filmmedia integrated in the activated sludge reactors to en-hance nitrogen removal. Water Sci. Technol. 1994, 30(11), 13–24.

[4] Sen, D. COD removal, nitrification and denitrification ki-netics and mathematical modeling of an integrated fixedfilm activated sludge (IFAS) system, Ph.D. Dissertation;Virginia Tech: Blacksburg Virginia, USA, 1995.

[5] Sen, D.; Randall, C.W.; Liu, H. Effect of suspended solidsMCRT on COD and nitrogen removal kinetics of biofilm sup-port media integrated in activated sludge systems. Proceedingsof the Water Environment Federal, 68th Annual Conference& Exposition, Miami Beach, Florida, USA, Oct 21–25, 1996;Water Environment Federal: Alexandria, Virginia, 1995; 603–614.

[6] APHA; AWWA; WEF. Standard Methods for the Examinationof Water and Wastewater, 20th Ed.; Greenberg, A.E., Clesceri,L.E., Eaton, A.D., Eds.; American Public Health Association;Washington DC. USA, 1998.

[7] Tchobanoglous, G.; Burton, F.L.; Stensel, H.D. Suspended growthbiological treatment processes. In Wastewater Engineering: Treat-ment and Resue, 4th Ed.; Metcalf & Eddy, Inc: New York, 2003;680.

[8] WEF. Biological and chemical systems for nutrient removal, A spe-cial publication under Technical Practice Committee, Water Envi-ronment Federation; Alexandria, VA, USA, 1998, 81.

[9] Peng, Y.; Chen, T.; Tian, W. Nitrogen removal via nitrite at normaltemperature in A/O process. J. Environ. Sci. Health Pt. A 2003,38(6), 1007–1015.

[10] Peng, Y.; Zhu, G. Biological nitrogen removal with nitrificationand denitrification via nitrite pathway. Appl. Microbiol. Biotech-nol. 2006, 73, 15–26.

[11] Khin, T.; Annachhatre, A. P. Novel microbial nitrogen removal pro-cesses. Biotechnol. Adv. 2004, 22, 519–532.

[12] Hellinga, C.; Schellen, A.A.J.C.; Mulder, J.W.; Loosdrecht, M.C.M.;Heijnen, J.J. The SHARON process: an innovative method for nitro-gen removal from ammonium-rich wastewater. Water Sci. Technol.1998, 37(9), 135–142.

Dow

nloa

ded

by [

Uni

vers

ity o

f C

alif

orni

a, S

an D

iego

] at

19:

58 1

7 N

ovem

ber

2014