Process Biochemistry 44 (2009) 578–583

Short communication

Impact of carbon to nitrogen ratio on nutrient removal in a liquid–solid circulatingfluidized bed bioreactor (LSCFB)

Mohammad Islam a, Nakhla George a,*, Jesse Zhu b, Nabin Chowdhury a

a Department of Civil and Environmental Engineering, University of Western Ontario, London, Canada N6A 5B8b Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Canada N6A 5B8

A R T I C L E I N F O

Article history:

Received 6 June 2008

Received in revised form 21 January 2009

Accepted 5 February 2009

Keywords:

Total nitrogen

Simultaneous nitrification–denitrification

A B S T R A C T

The performance of a liquid–solid circulating fluidized bed bioreactor (LSCFB) with anoxic and aerobic

beds and lava rock as a biofilm carrier media was used to investigate the impact of the COD/N ratio on the

process performance, with particular focus on total nitrogen removal. Three different COD/N ratios of

10:1, 6:1 and 4:1 were tested at an empty bed contact time of 0.82 h. More than 90% of the influent

organic matter was removed throughout the study with 58% removal in the anoxic column in Phase III.

Total nitrogen removal efficiencies in Phases I–III were 91%, 82% and 71% and simultaneous nitrification–

denitrification (SND) occurred in the aerobic downer. The LSCFB demonstrated tertiary effluent quality

at COD/N ratio of 10:1 and 6:1 with soluble biochemical oxygen demand (SBOD) <10 mg l�1 and total

nitrogen (TN) <10 mg l�1.

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1. Introduction

Many industrial effluents, agricultural wastes or other humanactivities generate high strength ammonium wastewater. One ofthe key design parameters for any biological nutrient removalsystem is the influent C, N and P ratios [1]. At high C to N ratio (highCOD/TKN), the heterotrophic microorganisms tend to out-competethe nitrifying organisms [2] impacting nitrification [3], while atlow C to N ratio often the process becomes carbon limited fordenitrification [4]. Study of the treatment of shrimp aquaculturewastewater by sequence batch reactors (SBR) by Fontenot et al. [5]showed an optimum C:N ratio of 10:1 for maximum nitrogen andcarbon removal. Another investigation of SBR using syntheticwastewater showed optimal C:N ratio of 11.1 for simultaneousnitrification–denitrification [6].

Fixed film reactors, such as fluidized bed/expanded bed [7–8],biofilm reactor [9] and UASB [10], have been successfullydeveloped for the treatment of domestic and industrial waste-water, primarily due to the capability for high biomass retention[11]. The patented liquid–solid circulating fluidized bed bioreactor(LSCFB) [12] can be an alternative to the traditional activatedsludge process commonly used in biological wastewater treatmentfor reuse [13]. The premise of the LSCFB is particle recirculationbetween two columns where two distinct environments aremaintained. An anoxic environment, conducive to denitrification is

* Corresponding author. Tel.: +1 519 661 2111x85470; fax: +1 519 850 2921.

E-mail address: gnakhla@eng.uwo.ca (N. George).

1359-5113/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2009.02.003

maintained in the riser, while the downer is aerobic to promotenitrification. Furthermore, as a result of particle recirculationbetween the riser and the downer enhanced biological phosphorusremoval (EBPR) is also achieved. The primary advantages of theprocess are low hydraulic retention time and a reduced footprint,as well as elimination of primary and secondary clarifiers. Theprocess has been demonstrated to treat municipal wastewater(MWW) and achieve biological C and N efficiencies removal of 90%and 80% at C:N ratio of 10 [14]. Chowdhury et al. [13]demonstrated that at different empty bed contact times (EBCTs)ranging from 0.44 to 0.82 h, ammonia removal efficiency was 95%.

The application of LSCFB for wastewater treatment is still at itsinfancy. The aim of this study was to investigate the effect ofinfluent COD/N ratio on the process performance of LSCFB, withparticular focus on total nitrogen removal. The primary objective isto determine the optimal COD/N ratio for LSCFB treatment ofMWW, based on effluent quality and specific activity of attachedbiomass.

2. Materials and methods

In this study, a laboratory-scale LSCFB was operated at a room temperature

(22 � 2 8C) to treat synthetic wastewater (SSW). The acetate rich wastewater is

characterized by COD, TKN and TP concentrations of 244 � 21, 27 � 3.1 and

4.25 � 0.43 mg l�1 and mineral salt was prepared using stock solution (NiCl�H2O,

75 mg l�1; CoCl2�6H2O, 75 mg l�1; CuCl�2H2O, 200 mg l�1; ZnCl2, 125 mg l�1;

MnCl2�4H2O, 1250 mg l�1; FeCl3�6H2O, 750 mg l�1; (NH4)6Mo7o24�4H2O, 200 mg l�1;

H3BO3, 125 mg l�1; MgSO4�7H2O, 14 g l�1; CaCl2�H2O, 6 g l�1) which were mixed with

tap water at a volumetric ratio of 0.004:1. The COD/N ratio was varied by the addition

of ammonia as ammonium chloride, with appropriate additional alkalinity augmented

by NaHCO3.

Fig. 1. Schematic of LSCFB.

M. Islam et al. / Process Biochemistry 44 (2009) 578–583 579

2.1. Reactor description

A laboratory plexiglass LSCFB was built as shown in Fig. 1. The system consists of

two columns, the riser column (volume 0.77 l and internal diameter of 2 cm) which

was operated as anoxic reactor and the downer column (volume 0.3.3 l and internal

diameter of 7.6 cm) which was operated as aerobic reactor. A 0.65 m high, 20 cm ID

liquid–solid separator was installed at the top of riser. The downer-separator, which

also served as the final clarifier was installed at the top of the downer with 0.88 m

height and 14 cm internal diameter. The details of arrangement and recirculation

connections were given elsewhere [14] and the design and operating conditions are

given in Table 1a.

Lava rock particles with an average diameter of 0.7 mm (300–1100 mm), bulk

density 1521 kg m�3, true density of 2560 kg m�3, porosity of 33%, and surface area

Table 1aOperational conditions of the LSCFB.

Phase I

Run time (d) 37

Influent flow, Qin (l d�1) 48 � 2

Average organic loading (kg COD m�3 d�1) 2.88

Average nitrogen loading (kg N m�3 d�1) 0.33

R–R recirculation ratio (QR–R/Qin) 5.25 � 0.5

D–R recirculation ratio (QD–R/Qin) 6.0 � 0.5

D–D recirculation ratio (QD–D/Qin) 8.0 � 0.5

HRTa (h)

Anoxic 0.39

Aerobic 1.65

EBCTb (h)

Anoxic 0.23

Aerobic 0.59

Bed expansion (e) (%)

Anoxic 25

Aerobic 25

Estimated SRT (d)

Anoxic 17

Aerobic 23

Shear forcec (dyn/cm2)

Anoxic 54

Aerobic 10

Yield (Y) (g VSS g�1 COD) 0.11

Estimated particle recirculation (g d�1) 250

a HRT calculated as volume of reactor divided by flow.b EBCT calculated as the product of the compacted bed height and column cross-secc Liquid shear force calculated using equation by Patel et al. [19]

of 8.94 m2 kg�1 were used as the carrier media for biofilm in the LSCFB. No

disintegration of the media was noticed during the operation. Prior to the start up of

the LSCFB, 700 g of fresh particles were fed in the anoxic column, and 1800 g in the

aerobic column with corresponding compact bed volumes of 0.46 and 1.18 l,

respectively. The system was operated with periodic particles recirculation

between the bottom of the riser and the downer.

The LSCFB was operated at empty bed contact time of 0.82 h and a nominal

hydraulic retention time (nHRT), calculated as the product of the EBCT and (one-

compacted bed porosity of 33%), of 0.55 h at all C/N ratios. Detailed calculation of

EBCT and nHRT is reported elsewhere [15,13]. The system was fed with SWW at

the bottom of the riser column at a flow rate (Q) of 48 � 2 l d�1. Air was supplied at

the bottom of downer column at a flow of 0.315 � 0.1 m3 d�1. Both anoxic and aerobic

beds were fluidized by recirculation of fluid from the liquid–solid separators, above

the riser and downer, respectively. The recirculation flow rate in the riser was

0.54 m3 d�1 (0.252 and 0.288 m3 d�1 from the riser- and downer-separator,

respectively) and in the downer was 0.384 m3 d�1, with corresponding recirculating

superficial liquid velocities of 1.39 and 0.26 cm s�1 in the riser and downer,

respectively.

2.2. Inoculation and reactor startup

The system was inoculated with 15 l of return activated sludge from the Adelaide

Wastewater Treatment Plant, London, Ontario, enriched with ammonia and

alkalinity addition for two weeks to augment nitrifiers activity, which peaked at

0.13 g NH4+-N g VSS�1 d�1. The system was operated for two weeks without sludge

disposal, during which particles were coated with biomass with a biofilm thickness

varying from 300 to 400 mm in the riser and <100 mm in the downer, and the

attached biomass varied from 4 to 8 mg VSS g�1 lava rock.

2.3. Batch activity tests

The specific microbial activities of nitrifiers and denitrifiers were measured on

10–15 g of bioparticles collected from the riser and downer of the LSCFB. Maximum

ammonia oxidation rates per attached biomass unit (mmax/Y), expressed as mg NH4-

N g�1 particle d�1 were measured using a 0.5 l batch reactor, equipped with a

magnetic stirrer, and a diffuser for purging air to maintain dissolved oxygen

concentration of 3–4 mg l�1. The same setup was used for anoxic denitrification

studies without air (maintained air tight to avoid intrusion of oxygen and equipped

with magnetic stirrer). For nitrification, initial NH4-N concentrations ranging from

25 to 30 mg l�1 with an additional alkalinity of 250 mg l�1 as CaCO3 were used

where as for the denitrification 20–25 mg l�1 of sodium nitrate and 300–400 mg l�1

of acetic acid were added. NH4-N and NO3-N levels were monitored for 6–7 and

Phase II Phase III

45 43

48 � 2 48 � 2

2.88 2.88

0.52 0.68

5.25 � 0.5 5.25 � 0.5

6.0 � 0.5 6.0 � 0.5

8.0 � 0.5 8.0 � 0.5

0.39 0.39

1.65 1.65

0.23 0.23

0.59 0.59

25 25

25 25

18 13

28 23

54 54

10 10

0.12 0.15

tional area divided by the influent wastewater flow.

Fig. 2. Performance of LSCFB at different phases: (a) COD removal and (b) nitrogen removal.

M. Islam et al. / Process Biochemistry 44 (2009) 578–583580

3–4 h, respectively, to determine the maximum nitrification and denitrification

rates of the bioparticles.

2.4. Analytical methods

Influent samples from feed tank, anoxic effluent from riser-separator and final

effluent from downer-separator were collected in airtight bottles and refriger-

ated at 4 8C prior to analysis. Total suspended solids (TSS), volatile suspended

solids (VSS), biological oxygen demand (BOD) and total Kjeldhal nitrogen (TKN)

were analyzed according to the STANDARD METHODS [16]. Dissolved oxygen

(DO) was measured using Thermo Orion (810 A+) meter. Total chemical oxygen

Table 1bInfluent and effluent characteristics.

Parameter (mg l�1) Influent (mg l�1) Effluent (mg l�1) C

Phase I

pH 6.9–7.1 7.1–7.5

Alkalinity (mg CaCO3 l�1) 224 � 27a, 293 � 31a, 311 � 29a 102 � 20 (13)

TCOD mg l�1 244 � 21 19 � 5 (13)

SCOD mg l�1 203 � 18 12 � 4 (13)

NH4-N mg l�1 24.2 � 3.3a, 41.5 � 4.1a, 58.1 � 4.3a 0.5 � 0.1 (13)

NO3-N mg l�1 <0.2 3.2 � 0.7 (13)

TP-P mg l�1 4.2 � 0.4 3.1 � 0.4 (13)

TSS mg l�1 5 � 2 10 � 5 (13)

VSS mg l�1 1 � 1 7 � 4 (13)

TKN mg l�1 27 � 3.1a, 44.1 � 3.8a, 60.3 � 4.2a 2.4 � 0.6 (13)

TN mg l�1 27 � 3.1a, 44.1 � 3.8a, 60.3 � 4.2a 5.6 � 0.6 (13)

BOD5 mg l�1 158 � 23 12 � 4 (13)

SBOD5 mg l�1 136 � 19 5 � 2 (13)

The values in the parentheses are the number of samples.a Influent Phases I–III.

demand (TCOD), soluble chemical oxygen demand (SCOD), and total phosphorus

were measured using HACH methods and testing kits (HACH Odyssey DR/2500).

NH4, NO3, NO2 and PO4 were measured using ion chromatography (IC, Dionex

600, USA) equipped with CS16-HC and AS9-HC columns, respectively. Biomass

attachment on the support media mg VSS g�1 clean particles, was measured

according to APHA methods [16]. Approximately 8–10 g bioparticles collected in

a 50 ml vial from each of the column and sonified for 3 h at 30 8C to detach the

biomass from the particle using a Aquasonic sonicator (Model 75HT, ETL

Laboratory Testing Inc., New York). The VSS content of the detached biomass was

measured using standard methods [16] and the sonified particles were weighted

after drying at 550 8C for 1 h.

OD: N = 10:1 Effluent (mg l�1) COD: N = 6:1 Effluent (mg l�1) COD: N = 4:1

Phase II Phase III

7.2–7.6 7.1–7.6

118 � 23 (12) 137 � 31 (13)

20 � 7 (12) 24 � 6 (13)

9 � 3 (12) 9 � 4 (13)

0.83 � 0.1 (12) 7.1 � 1.8 (13)

5.3 � 0.8 (12) 6.9 � 1.9 (13)

2.4 � 0.4 (12) 2.3 � 0.6 (13)

12 � 6 (12) 18 � 7 (13)

8 � 5 (12) 13 � 5 (13)

2.8 � 0.2 (12) 10.61 � 1.3 (13)

8.1 � 1.1 (12) 17.51 � 1.8 (13)

13 � 4 (12) 19 � 7 (13)

7 � 3 (12) 8 � 5 (13)

Ta

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1.

M. Islam et al. / Process Biochemistry 44 (2009) 578–583 581

3. Results and discussion

3.1. General performance

Three different nitrogen loading rate (NLR) were used. Forclarity, the different operational phases are denoted by verticallines in Fig. 2. Measured influent and effluent COD, NH4-N, NO3-Nand TKN concentrations are shown in Fig. 2a and b. Fig. 2 showsthat the system delivered stable effluent quality throughout thestudy periods with little temporal variation. Upon increasing theNLR, the LSCFB took only 6–7 d (between days 92 and 97 in Fig. 2)to stabilize.

Table 1b, summarizing the pseudo-steady-state effluentcharacteristics of indicates that the LSCFB achieved more than90% COD removal throughout the study irrespective of the COD/Nratio. Decreasing the COD/N impacted COD utilization in the anoxiccolumn (as elaborated upon later). Approximately 37% of theinfluent was COD oxidized in the anoxic column in Phase I, whichincreased to 58% in Phase III. However, effluent TSS and VSSconcentrations increased slightly from 10 � 5 mg l�1 to 18 � 7mg l�1 TSS as the COD/N ratio decreased to 4. Effluent SBOD5 wasbelow 10 mg l�1 throughout the study period. Ammonia removalefficiencies in all phases were excellent, despite decreasing from 98%in Phase I to 85% of the influent ammonia in Phase III without anynitrite accumulation. Effluent NO3-N concentrations increased by115% upon decreasing the COD/N ratio from 10 to 4, indicating carbonlimitation in the anoxic column (discussed in Section 3.3). Totalnitrogen removal efficiencies in Phases I–III were 91%, 82% and 71%. Itshould be noted that conventional modified Ludzack–Ettinger (MLE)processes typically achieve about 80% nitrogen removal in municipalwastewater at C/N ratio of 8:1 to 10:1 range [17]. Approximately35 � 10% of the influent phosphorus was removed during the study,and was not significantly impacted by the COD/N ratio. Wastebiomass from LSCFB contained 3.3–3.5% phosphorus of dry biomassweight (as VSS) whereas typical phosphorus content in activatedsludge of 1.5–2% [17], suggesting the occurrence of enhancedbiological phosphorus removal. Approximately 1.7–2 mg l�1 ofphosphorus release was observed in the anoxic riser while 2.7–3.5 mg l�1 of phosphorus uptake was observed in the aerobic downer.

Effluent SBOD concentrations were 5 � 2, 7 � 3 mg l�1 and TSSconcentrations averaged 10 � 5, 12 � 6 mg l�1 in Phases I and II,respectively. Similarly effluent ammonia, nitrate and total nitrogenconcentrations in Phase II were 0.83 � 0.1, 5.3 � 0.8 and 8.1 �1.1 mg l�1, respectively. USEPA suggested BOD and TSS concentrationof <20 mg l�1 for industrial reuse and aquaculture and total nitrogenof<10 mg l�1 for groundwater recharge [18]. Thus, the LSCFB effluentquality meets the aforementioned reuse criteria at a COD/N ratio of6:1 without any supplementary carbon addition.

Table 2 presents the observed nitrification and denitrificationrates, along with the biomass detachment coefficients for nitrifiersand heterotrophs in the LSCFB. Biomass detachment coefficientswere based on the relative activities of nitrifiers (downer) andheterotrops (riser) in the liquid phase and on the lava rock as givenby the equations shown in Table 2 [19]. Nitrification rates (Table 2,Column 6) in the downer clearly indicate an increase of 31% inautotrophic biomass in Phase II and 41% in Phase III relative toPhase I. Scrutiny of data in Table 2, Column 8 confirmed that theamount of attached biomass on the lava rock in the aerobic downerincreased by 50% over Phase I, as the COD/N ratio decreased to 4.Bulk ammonia concentration in the downer also increased as theNLR increased. Similarly, specific nitrification activity (Table 2,Column 4) of bioparticle increased by 45% from Phase I to Phase III.Thus, ammonia utilization capacity in Phase III would increase to2.2 times Phase I. In Phases I and II, since the volumetric ammonialoading rates (Table 1a) were lower than the maximum nitrifica-tion rates (Table 2, Column 5) of 0.40 and 0.66 kg N m�3 d�1, based

M. Islam et al. / Process Biochemistry 44 (2009) 578–583582

on the measured attached biomass activity, almost all of theinfluent ammonia was removed with effluent NH4-N concentra-tions well below 1 mg l�1. However, although the maximumnitrification rate of 0.87 kg N m�3 d�1 in Phase III was higher thanthe applied load of 0.68 kg N m�3 d�1, still ammonia accumulatedin the effluent and removal efficiency deteriorated (Fig. 2b). It isinteresting to report the occurrence of simultaneous nitrificationand denitrification in the aerobic downer (discussed in Section3.3), although oxygen concentration in the downer was main-tained at 3 � 0.5 mg l�1 throughout the study period. Since enoughattached biomass existed on the lava rock to support full nitrification,it can be inferred that the limiting factor in nitrification is O2 diffusionin the biofilm which will be expanded upon later. Based on theperformance of the LSCFB in Phase III, it appears that the maximumachievable nitrification capacity is 0.6 kg N m�3d�1.

3.2. Sludge yield

Approximately 1016� 91, 1216 � 107 and 1450 � 95 mg VSS d�1

were discarded from the system in Phases I, II and III, respectively, thustranslating into remarkably low yields of 0.11, 0.12 and 0.15 (r2 = 0.99,derived from a plot of cumulative dry weight of biomass (as VSS) Vscumulative COD removal, data not shown here) g VSS g�1 COD. Table 1bshows that the effluent TSS concentrations increased from 10� 5 to18� 7 mg l�1 when the COD/N ratio decreased from 10 to 4. Thisincrease in effluent biomass concentration is due to the increase in NLRthat produced more NO3-N and hence more COD is consumedanoxically, impacting anoxic biofilm thickness and detachment rates,as discussed later.

It is apparent from Table 2, Column 9 that the detachment ratecoefficients in the anoxic column for Phases I and II were fairlyconstant at 0.057–0.059 d�1, but increased sharply in Phase III to0.077 d�1. Although 58% of the influent COD was consumed

Table 3Nutrient mass balances of the LSCFB at different COD/N ratio.

Mass in influent (g d�1) Anoxic column,

Mass consumed (g d�1)

Aero

Mass

Phase I (COD/N = 10:1)

COD 11.73 � 0.36h 4.33 � 0.46 (3.21)b 4.7

TKN 1.34 � 0.04

NH4-N 1.16 � 0.1 0.05 � 0.002 (0.04)c 1.1

NO3-N 0 � 0 0.9 � 0.07 �1.0

TP 0.20 � 0.02

PO4-P 0.19 � 0.03 �0.08 � 0.02 (0.02)d 0.1

Phase II (COD/N = 6:1)

COD 12.24 � 0.24 6.02 � 0.55 (5.41)b 3.1

TKN 2.13 � 0.01

NH4-N 1.99 � 0.03 0.07 � 0.05 (0.06)c 1.9

NO3-N 0.00 � 0 1.51 � 0.16 �1.8

TP 0.20 � 0.01

PO4-P 0.19 � 0.03 �0.10 � 0.04 (0.03)d 0.1

Phase III (COD/N = 4:1)

COD 12.05 � 0.26 6.95 � 0.46 (7.13)b 1.9

TKN 2.88 � 0.07

NH4-N 2.79 � 0.05 0.08 � 0.06 (0.09)c 2.4

NO3-N 0.00 � 0 1.92 � 0.22 �2.2

TP 0.19 � 0.01

PO4-P 0.18 � 0.03 �0.08 � 0.02 (0.04)d 0.1

a COD equivalent based on VSS, nitrogen (N) content of 8.5–9% and phosphorus conb COD consumption based on process yield, NO3-N and PO4-P consumption in th

3.39 � 0.9 + 2 � 0.08 = 3.21.c Estimated N synthesis; estimation based on process yield of 0.11, 0.12 and 0.15 g VS

waste sludge; i.e. 0.11 � 4.33 � 0.085 = 0.04.d Estimated P synthesis; estimation based on process yield of 0.11, 0.12 and 0.15 g VS

the waste sludge; i.e. 0.11 � 4.33 � 0.034 = 0.02.e COD % closure = (4.33 + 4.77 + 0.94 + 1.43)/11.73.f Nitrogen % closure = (0.9 + 0.15 + 0.26 + 0.1)/1.16.g Phosphorus % closure = (0.15 + 0.03)/0.20.h Standard deviations for 11–12 samples; average � S.D.

anoxically in Phase III, significantly greater than the 37% of Phase I,sludge yield still increased. This may be due to the greaterdetachment of biomass in the anoxic column in Phase III, asreflected by an increase in detachment rates from 0.059 to0.077 d�1 (Table 2, Column 9), effectively reducing the anoxicsludge age from 18 to 13 d (Table 1a).

3.3. Mass balances

Table 3 shows the steady-state mass balances for COD, TKN,NH4-N, NO3-N, TP, and PO4-P where a positive value indicatesremoval and a negative value indicates formation. The observedaverage N and P content of biomass were 8.5–9% and 3.3–3.5% byweight, respectively. Percent closures have been calculated usinginfluent and effluent COD, TN, TP concentrations and the masswastage from the LSCFB. The high percentage mass balanceclosures of 90–98% for COD and nitrogen, as well as 83–90% forphosphorus attest to the reliability of the data. COD consumptionin the anoxic and aerobic columns and nitrogen loss throughdenitrification have also been considered to attain the percentclosure. It is interesting to report that Table 1b shows DBOD5 to DPratio is close 100:1, implying no EBPR. However upon calculating Pincorporated in the sludge based on yield, DPsynthesis = 0.4–0.5 mg l�1; contrasted with a phosphorus consumption ofapproximately 1–2.3 mg l�1, reflecting EBPR. Calculated net Premovals (i.e. uptake–release, Table 3, Columns 3 and 4) were 1.2,1.5 and 1.9 mg l�1 closely matching the observed removals of 1.1,1.8 and 1.9 mg l�1 from Table 1b in Phases I, II and III, respectively.

3.3.1. Anoxic column

From Table 3, approximately removal of 0.90–1.92 g NO3-N d�1,release of 0.08–0.1 g P d�1 and 4.33–6.95 g COD d�1 utilization wereobserved in the anoxic column. Anoxic COD consumption in Phase I

bic column,

utilized (g d�1)

Mass liquid

effluent (g d�1)

Mass waste

sludge (g d�1)

Percent

closure (%)

7 � 0.42 0.94 � 0.09 1.43a 98e

0.26 � 0.01 0.1a 96f

3 � 0.1 (0.05) 0.03

4 � 0.07 0.15 � 0.01

0.15 � 0.02 0.03a 87g

4 � 0.04 (0.02) 0.13 � 0.05

3 � 0.83 0.96 � 0.11 1.71 94

0.38 � 0.02 0.11 95

5 � 0.02 (0.03) 0.04 � 0.02

4 � 0.02 0.25 � 0.03

0.12 � 0.01 0.04 85

7 � 0.05 (0.01) 0.11 � 0.04

3 � 1.15 0.92 � 0.10 2.07 93

0.83 � 0.05 0.13 98

5 � 0.04 (0.025) 0.34 � 0.07

8 � 0.04 0.32 � 0.05

0.11 � 0.01 0.05 83

7 � 0.05 (0.01) 0.09 � 0.04

tent 3.3–3.5% of waste sludge, respectively.

e anoxic column. Taking the ratio of DP/DS = 1:2 mg PO4-P/mg COD [21]; i.e.

S g�1 COD, COD consumption in the respective columns and nitrogen content in the

S g�1 COD, COD consumption in the respective columns and phosphorus content in

M. Islam et al. / Process Biochemistry 44 (2009) 578–583 583

was 60–70 mg COD l�1, Phase II was 118–125 mg COD l�1 and PhaseIII was 145–155 mg COD l�1 (ffi58%) of the influent flow, comparableto the 63.9% observed by Zhan et al. [20] an anoxic biofilm reactor ata C/N ratio of 11. Anoxic COD utilization increased by 61% from PhaseI to Phase III with the reduction in the C/N ratio. Estimated CODconsumption for denitrification based on process yields is 3.21–7.13 g COD d�1. The differences between estimated and observedanoxic COD balances in Phases II and III are only 3% and 10%.Approximately 0.05–0.08 g NH4-N d�1 was utilized for biomasssynthesis, closely matching the estimated NH4-N for biomasssynthesis of 0.04–0.09 g NH4-N d�1 suggesting that there is no otherNH4-N uptake mechanism such as struviating. Using Eq. (1) [17]shows that approximately 3.39–3.63 g COD were utilized todenitrify 1.0 g NO3-N, considering the observed yield biomass yield(Y) of 0.11–0.15 g VSS g�1 COD. Based on stoichiometry, 0.5–1.1 gnew cells were produced and 0.74–1.6 l nitrogen gas was producedupon consumption of NO3-N. Offline batch experimental resultsconfirmed that 3.9 � 0.3 g COD consumption was required to denitrify1.0 g NO3-N. Approximately a consumption of 2 g COD g�1 P releasewas observed. COD utilization for P synthesis in the anoxic column isapproximately 0.04 g d�1.

COD utilization during denitrification ¼ 2:86

1� 1:42� Y(1)

3.3.2. Aerobic column

COD oxidation gradually decreased in the aerobic column from4.77 g COD d�1 (41% of influent) in Phase I to 1.93 g COD d�1 (16%of influent) Phase III. Approximately 1.13–2.45 g NH4-N d�1 wereconsumed in the aerobic column, out of which 0.9–1.92 g NO3-Nwere produced. Estimated ammonia utilized for cell synthesis wasapproximately 0.03–0.05 g NH4-N d�1. The N consumed less thesum of the nitrates and estimated N consumed for synthesisindicates that about 0.04–0.14 g N (Table 3, Column 4) wereunaccounted for, thus suggesting the potential for SND despite anambient DO of 3 � 0.5 mg l�1. Approximately 0.8 mg l�1 NH4-N wasutilized during SND in Phase I that increased to 3 mg l�1 when theCOD/N ratio was decreased to 4. Phosphorus uptake in the aerobiccolumn was 0.14–0.17 g P d�1, with approximately 0.04–0.05 g P d�1

utilized for biomass synthesis in both the anoxic and aerobic columnsand the rest by PAOs. Thus, the average net P uptake due to EBPRcalculated as the difference between aerobic P uptake minus the Prelease and P used for synthesis, in Phase I, II, and III are 0.02, 0.03, and0.05 g d�1, respectively.

4. Summary and conclusions

The LSCFB was operated at three different COD/N ratios toexamine the process performance and biofilm properties andhence enhance understanding of the nutrient removal process. Thefindings of this study led to the following conclusions:

� Unlike other systems which require a COD/N ratio of 8–10, theLSCFB could optimally remove nitrogen at a COD/N ratio of 6:1.The nitrification rate at 22 � 2 8C was 0.0382 � 0.004 g NH4-N g�1 VSS d�1 with total nitrogen removal of 82% was observed atthe aforementioned ratio.

� Approximately 37–58% COD was oxidized in the anoxic column,16–41% oxidized in the aerobic column.� The maximum nitrification capacity of the LSCFB was found

0.60 Kg N m�3 d�1 and nitrification rate in Phase III was limitedby O2 diffusion in the biofilm.� Simultaneous nitrification and denitrification occurred in the

aerobic downer, and increased with decreasing COD/N ratioaffecting the removal of as 3 mg N l�1.� Reduction in the COD/N ratio from 10:1 to 4:1 increased riser

detachment coefficient in the riser from 0.057 to 0.077 d�1.

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