ENHANCED BIOLOGICAL NUTRIENTS REMOVAL USING THE COMBINED FIXED-FILM REACTOR WITH BYPASS
FLOW
H.U.NAM,J.H.LEE,C.W.KIM M and T.J.PARK*M
Department of Environmental Engineering,Pusan National University,Pusan,609-735,South Korea
(First received 1March 1999;accepted in revised form 1July 1999)
Abstract ÐThe possibility of e ective internal carbon source usage for removing nitrogen and phosphorus simultaneously in a ®xed-®lm reactor was studied using the operation strategy with bypass ¯ow.Tests were made to con®rm whether nitrogen and phosphorus from municipal wastewater were eliminated e ectively in the ®xed-®lm reactor with bypass ¯ow by increasing the bypass ¯ow ratio from 0to 0.4.The ®xed-®lm reactor used in this experiment was a combined A 2/O process and bio®lm process.The bypass ¯ow was applied in this experiment unit and the part of the in¯uent was directly fed to an anoxic reactor for e ective denitri®cation.The bypass ¯ow ratio applied in this unit was 0,0.3and 0.4based on the in¯uent ¯ow rate.The removal e ciencies of COD,NH +4±N and T±P were observed to be higher than
87.2%,75.2%and 52.8%in all runs,respectively.Further,the optimal operational conditions for phosphorus removal were estimated when the bypass ¯ow ratio was 0.4with the internal recycle ratio of 0.5and the external recycle ratio of 0.5on the basis of the in¯uent ¯ow rate.The removal e ciencies in the bypass ¯ow ratio of 0.4were 88.0%for NH +4-N and 68.0%for T±P.Large di erences in the removal of phosphorus resulted from varying the bypass ¯ow ratio.With the bypass ¯ow ratio of 0,0.3and 0.4,the removal e ciencies for T±P were 52.8%,61.6%and 68.0%,respectively.It is suggested that the bypass ¯ow in the ®xed-®lm reactor can achieve complete denitri®cation and can be helpful for improving phosphorus removal.#2000Elsevier Science Ltd.All rights reserved
Key words Ðcombined ®xed-®lm reactor,bypass ¯ow,denitri®cation,phosphorus uptake,A 2/O process,internal recycle,external recycle,anoxic condition
INTRODUCTION
The potential impact of discharged nutrients on the oxygen resources of receiving waters can best be il-lustrated by looking at the amounts of organic mat-ter that can be generated by the nutrients compared to the amount of organic matter in untreated sew-age.The COD of raw sewage in Korea is typically about 200±250mg/l,whereas the phosphorus con-tent is around 4±6mg/l,depending on wheth
er or not a phosphate detergent ban is in place,and the nitrogen content is 20±40mg/l (Choi,1996).If 1kg of phosphorus was completely assimilated by algae and used to manufacture new biomass from photo-synthesis and inorganic elements,a biomass of 111kg with a COD of 138kg would be produced,assuming that algae composition can be represented by C 106H 263O 110N 16P.Thus,the discharge of 5mg/l phosphorus could potentially result in COD pro-duction equivalent to 690mg/l,or more than
double the COD of the organic matter in the untreated sewage (Randall et al .,1992).
It is probable that either nitrogen or phosphorus will be the limiting nutrient controlling eutrophica-tion because of the relatively large quantities required for biomass growth compared to other nutrients such as sulfur,potassium,calcium,and magnesium.Conventional wisdom in recent years has been that phosphorus is typically the limiting nutrient in freshwater environments,whereas nitro-gen is typically limiting in estuaries and marine waters (Sedlak,1989).
The Bio®lm process has many characteristics and advantages (Park et al .,1995,1996):(1)the ®lms used by the system e ciently remove nitrogen due to the use of bacteria such as nitrifying bacteria that have both a slow growth rate and a long gener-ation time;(2)wide spectrum pollutant removal can be a
chieved due to the existence of more species of organisms in the ®lm compared with the activated sludge process;(3)the treatment capacity per unit volume of the process is remarkably larger than activated sludge process because of the larger bio-mass amount per unit volume;(4)compared with
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the activated sludge process,less surplus sludge is produced.More sludge produced is consumed by organisms of higher tropic levels existing in the ®lm and there is less surplus sludge produced;(5)the process is energy e cient and convenient to oper-ate/maintain;and (6)the process has a stable oper-ation e ciency.The process can sustain and adapt ¯uctuations of hydraulic and organic loading,since it possesses a larger amount of biomass and a longer food chain compared with the activated sludge process.On the other hand,the bio®lm pro-cess has some shortcomings:(1)a large amount of initial capital is necessary due to the large amount of carriers and their support;and (2)the tiny par-ticles of broken anaerobic ®lm layers,which do not settle well,sometimes lead to higher turbidity in the e uent (Lee et al .,1996;Su and Ouyang,1996).The objectives of this paper are to develop a new ®xed-®lm re
actor with bypass ¯ow for removing nutrients from sewage,to assure the fundamental data for upgrading by operating a laboratory scale study,and to e ectively use an internal carbon source by bypass ¯ow to cut down on the expendi-ture of the external carbon source for removing nutrients.Therefore we think that an economical process could result from this study.Consequently,we hope that this new process can resolve the con-¯icts associated with operation of a conventional nutrient removal process.
MATERIALS AND METHODS
Experimental conditions and setup
One unit of a laboratory scale reactor capable of per-forming continuous experiments for nutrient removal was used,including anaerobic/anoxic/aerobic reactors in series.
Figure 1shows the schematic diagram of the process which is a combined A 2/O process and bio®lm process.The process has two recycle ¯ows:one is an internal re-cycle ¯ow from the aerobic reactor to the anoxic reactor for denitri®cation,the other is an external recycle ¯ow from the clari®er to the anaerobic reactor for phosphorus release.The internal recycle ratio and the external recycle ratio were both 0.5,based on the in¯uent ¯ow rate.Also,another special ¯ow,the bypass ¯ow,was applied in the ®
xed-®lm reactor,and part of the in¯uent was directly fed to the anoxic reactor for e ective denitri®cation.The e ec-tive volumes of the anaerobic,anoxic and aerobic reactors were 10l,6l and 18l,respectively;the total e ective volume of all reactors was 34l.Small volume of the anoxic reactor should be helpful in improving speci®c denitri®cation rate in the anoxic reactor (Randall et al .,1992).The operating conditions of the lab-scale exper-iments are shown in Table 1.All of the reactors were ®lled with net-type SARAN media having a porosity of 96.3%,a packing ratio of 40/30/20%in anaerobic/anoxic/aerobic reactors based on the volume of each reactor and a speci®c surface area of 400m 2/m 3.The media packing ratio and characteristics used in this study are shown in Table 2.
An agitator was installed in each of the anaerobic and anoxic reactors.Air was supplied through two ®ne-bubble bar-type di users at the bottom of the aerobic reactors by a blower with a capacity of 150l/min.The air ¯ow rate in the aerobic reactor was maintained at a constant rate of 16l/min (208C,1atm)over the total operation period.The temperature in the anaerobic reactor was kept at 37228C by a temperature controller.The COD concentration of the synthetic wastewater was 250mg/l,NH +4±N was 20mg/l and T±P was 8mg/l.A sodium bicarbonate bu er was maintained at 200mg CaCO 3/l to prevent a pH drop,which is caused mostly by nitri®cation and limited alka-linity in synthetic municipal wastewater.Acclimation and operation
For this study,seed sludge was obtained from the exist-ing sewage treatment plant in Pusan,Korea and accli-mated to the synthetic municipal wastewater of 0.1kg COD/m 3/day for about 15days.During the start-up period,the air ¯ow rate was controlled so as to make the bio®lm formed on the media surface be easily detachable.Once acclimated,the bypass ¯ow ratio was changed to 0(Run 1),0.3(Run 2)and 0.4(Run 3),based on the in¯u-ent ¯ow rate in order to evaluate the performance of the ®xed-®lm reactor with bypass ¯ow on nitrogen and phos-phorus removal.In a steady-state condition,reactors were operated for more than three weeks to collect data.Analysis of samples
In¯uent samples were collected twice a week and e u-ent samples every 3days.Samples for the determination of soluble components were immediately ®ltered using 0.45m m ®lter paper and cooled in order to prevent further reaction after sampling.All the samples except NO Àx ±N,which was measured by HPLC (Waters,USA),were
per-
Fig.1.Schematic diagram of CFFR (Combined Fixed
Film Reactor).
Table 1.Operating conditions of lab-scale experiments
Run number
HRT (h)Internal recycle ratio (%)
External recycle ratio (%)
Bypass ¯ow ratio (Q )
Anaerobic
Anoxic Aerobic Total 1  1.50.9  2.8  5.2505002  1.50.9  2.8  5.250500.33
1.5
0.9
2.8
5.2
50
50
0.4
Enhanced biological nutrients removal 1571
formed according to Standard Methods (19th).The methods for sampling analysis are given in Table 3.
RESULTS AND DISCUSSIONS
Removal of organic compounds
The COD concentrations in the e uents of Run 1,2and 3are shown in Fig.2.This ®gure presents the re
sults obtained from the three di erent oper-ation conditions (Run 1,Run 2and Run 3)in which the di erent bypass ¯ow ratios from 0to 0.4were applied using only the one waste strength of 250mg COD/l.In this study,the notations A,B,C,D and E indicate the in¯uent,anaerobic e uent,anoxic e uent,aerobic e uent and ®nal e uent,respectively.All three cases indicate that the e uent COD concentrations were almost constant although the bypass ¯ow ratio increased.The amount of reduced COD in the anaerobic reactor was highest for Run 1without bypass ¯ow.The COD concen-tration of the anaerobic e uent was lowest for Run 3with a 0.4bypass ¯ow ratio,because the high bypass ¯ow ratio caused the in¯uent fed into an-aerobic reactor to be small.It was found out that
the COD removal e ciencies of 88.8%,87.2%and 89.6%in Run 1,2and 3,respectively,were su-perior to the 79.4±83.0%obtained from the extended aeration submerged bio®lm process at 0.05±0.50kg COD/m 3/day (Wang et al .,1991).The dilution by external recycle caused changes of COD concentration and fermentation of anaerobic bac-teria in the anaerobic reactor caused COD removal.Nitrogen removal:nitri®cation and denitri®cation Figure 3shows the relationship between 2NH +4±N concentration and C/N ratio in the aerobic reac-tor of Run 1,2and 3,respectively.NH +4±N con-centrations of in¯uent in the aerobic reactor were 11.23±12.30mg/l,10.98±11.60mg/l and 10.51±10.77mg/l for Run 1,2and 3,respectively.NH +4±N concentrations of in¯uent in the aerobic react
or decreased as the bypass ¯ow ratio increased from 0to 0.4,but the di erence was only 0.46±1.79mg/l.It seems that the variation of in¯uent in the aerobic reactor at Run 1was larger than that at Run 2and
Table 2.Media packing ratio and characteristics
Item
Anaerobic zone Anoxic zone Aerobic zone Media type
SARAN 1000D SARAN 1000D SARAN 1000D Media size (mm)
20Â100Â29020Â100Â29020Â190Â350Number of packing media 1EA
3EA
6EA
Speci®c surface area (m 2/m 3)400400400Speci®c weight (kg/m 2)37.5637.5637.56Media surface area (m 2)
7.1750.905  3.893Media packing ratio (V/V,%)
40
30
20
Fig.  2.COD pro®les through each stage at di erent
bypass ¯ow ratios in the CFFR process.
Table 3.Sample analysis methods
Parameter Method
DO DO meter,model 58(YSI Inc,USA)
pH pH meter,HM-14P (TOA Electronics,Japan)
COD Cr Open re¯ux methods (Standard Method 19th edition)NH +4±N Nesslerization method (Standard Method 19th edition)NO Àx ±N HPLC (Waters,USA)
T±P
Stannous chloride method (Standard Method 19th edition)Alkalinity
Titration method (Standard Method 19th
edition)
Fig.3.Relationship between NH +4±N concentration and
C/N ratio in the aerobic reactor with bypass ¯ow.
H.U.Nam et al.
1572
Run 3.It was also discovered that the bypass ¯ow e ectively stabilizes NH +4±N concentration in the e uent from the anoxic reactor.In the aerobic reactor,the average amounts of removed ammonia were 7.01mg/l,7.74mg/l and 8.20mg/l in Run 1,2and 3,respectively.NH +4±N removal was greatest in Run 3,since the higher bypass ratio caused the lower C/N ratio.If the C/N ratio is below 5,nitri-®ers like nitrosomonas and nitrobacter will take the opportunity to become more active than the hetero-trophic bacteria concerning carbon source removal in aerobic condition,so the C/N ratio in the aerobic reactor determines the dominance of two species (Tchobanoglous and Burton,1991).
Figure 4illustrates the relationship between the concentration of nitri®ed ammonia and the con-sumed alkalinity in the aerobic reactor of the CFFR process.Approximately 7.14mg of alkalinity (as CaCO 3)ar
e consumed per mg of NH +4±N oxi-dized,assuming full nitri®cation in aerobic con-dition (Randall et al .,1992).The concentrations of nitri®ed NH +4±N in the aerobic reactor were 6.80±7.57mg/l,7.40±8.11mg/l and 8.08±8.39mg/l during Run 1,2and 3,respectively.Due to nitri®cation nitri®ed NH +4±N concentrations increased as the bypass ¯ow ratio increased from 0to 0.4,but the variations of nitri®ed NH +4±N reduced as the bypass ¯ow ratio increased.The solid line indicates the alkalinity consumption due to nitri®cation in the aerobic reactor.Also,consumed alkalinity in the aerobic reactor increased as the bypass ¯ow ratio increased whereas the variations of consumed alkalinity reduced.The proportional coe cient (6.91)of the relationship between the nitri®ed ammonia and the consumed alkalinity in this study was smaller than the theoretical proportional coe -cient (7.14).This di erence indicates that a part of the NH +4±N is consumed in cell synthesis.Note that these results are comparable with the results from the simultaneous nitri®cation and denitri®ca-tion reactor with HRT of 15±17h (Moriyama et al .,1990).
Figure 5shows the concentrations of NH +4±N,
organic±N and NO À
x ±N of the aerobic e uent in each run.The sum of organic±N,NH +4±N and
NO À
x ±N could be regarded as T±N.NH +4±N removal of each run was mainly achieved in the aerobic reactor and was caused by nitri®cation of autotrophic bacteria and assimilation of carbon-aceous bacteria.On the other hand,the denitri®ca-tion of each run was mainly achieved in the anoxic reactor and fraction of NO Àx ±N removed was caused by denitri®cation of heterotrophic bacteria.In Fig.5,as the bypass ¯ow ratio was increased from 0to 0.4,the NO Àx ±N concentrations of e u-ent decreased from 0.40mg/l to 0.01mg/l and the T±N removal e ciencies was gradually increased from 66%to 74%.According to these results,it was pointed out that Run 3with bypass ¯ow ratio of 0.4was the more e ective for T±N removal since complete NO Àx ±N removal could be achieved in the anoxic reactor in Run 2and Run 3with bypass ¯ow and without NO Àx ±N accumulation in the fol-lowing aerobic reactor.It was also found out that the e ect of C/NO Àx ±N ratio on denitri®cation in the ®xed-®lm reactor system was signi®cant.C/NO Àx ±N ratio was developed by considering that the amount of COD used could be accounted for the cell synthesis and the amount of COD oxidation by NO Àx ±N reduction was due to cell energy pro-duction (Sedlak,1989).It indicates that the lack of organic source in an anoxic condition could cause incomplete NO Àx ±N removal at short HRT (0.9h)in the anoxic reactor.
The concentrations of in¯uent NO Àx ±N,e uent NO À
x ±N and ORP in the anoxic reactor are shown in Fig.6.The concentrations of in¯uent NO Àx ±N in the anoxic reactor were    6.47±6.93mg/l,7.64±7.71mg/l and 8.01±8.08mg/l during Run 1,2and 3,respectively.The concentrations of e uent NO Àx ±N in the anoxic reactors were 0.17±0.39mg/l,0±0.12mg/l and 0.±0.02mg/l during Run 1,2and 3,respectively and the variations of e uent NO Àx ±N were reduced as the bypass ¯ow ratio increased from 0to 0.4.It is suggested that the bypass ¯ow is helpful in the e ective removal of NO Àx ±N in
the
Fig.4.Relationship between nitri®ed ammonia and con-sumed alkalinity in the aerobic
reactor.
Fig.5.Concentrations of NH +4±N,organic±N and NO À
x ±
N of the aerobic e uent in each run.
Enhanced biological nutrients removal 1573
anoxic reactor because it is capable of supplying
enough carbon to eliminate NO Àx ±N.The NO À
x ±N removal e ciencies were 94.2%,97.3%and 98.5%in the anoxic reactor in this study.As the denitri®-cation in the anoxic reactor was progressing,the ORP values under À300mV were obtained.The ORP variation tended to be similar to those during the removal of NO Àx ±N in the anoxic reactor.The variations in the ORP values reduced as the bypass ¯ow ratios increased from 0to 0.4.The ORP values in this study were signi®cantly lower than those of other studies (Charpentier et al .,1989;Peddie et al .,1990;Wareham et al .,1993).Plisson-Saune et al .(1996)reported that sul®des hav
e a great impact on ORP values so that a 0.07mg S±sul®des/l concen-tration increase,in the absence of oxygen,leads to a 100mV fall in the ORP value.In the present study,SO À4concentration in the anoxic reactor was 0.60,0.56and 0.58mg SO À4/l in each run,respect-ively.It was supposed that the residual SO À4caused the ORP value to be low.
Phosphorus removal:P release and P uptake Figure 7indicates the changes of T±P concen-tration in the each stage of Run 1,2and 3.The varied amounts of T±P concentration in the anaero-bic reactor were À0.51±0.33mg/l.In the anaerobic reactor the variations of the T±P concentrations were caused by the e ect of diluting by the external recycle and phosphorus release.Some phosphorus was released by phosphorus accumulating bacteria like (Kerrn-Jespersen et al .,1994).Nicholls and Osborn (1978)suggested that the anaerobic stage was necessary to allow selectively take up acetates into the cells using stored polyphosphates as the energy source and releasing phosphates in the liquid phase.
More phosphorus was released in Run 1without bypass ¯ow than in Run 2and Run 3.Since part of the in¯uent was bypassed to the anoxic reactor,phosphorus releases in Run 2and Run 3was smal-
ler than in Run 1.In the anoxic reactor,phos-phorus uptake by phosphorus accumulating bac-teria occu
rred.Run 2and Run 3with bypass ¯ow took up more phosphorus than that of Run 1,because more organic compounds were fed into the anoxic reactor by bypass ¯ow during Run 2and Run 3.
reactor4Phosphorus uptake in the anoxic reactor could be called the ``®rst phosphorus uptake''because its mechanism was di erent than the phosphorus uptake in the aerobic reactor.Kerrn-Jespersen and Henze (1993)reported that phosphorus accumulat-ing bacteria can be divided into two groups:one is capable of utilizing only oxygen as an electron acceptor and the other is capable of utilizing both oxygen and nitrates as electron acceptors.The trend of phosphorus uptake in the aerobic reactor of the ®xed-®lm reactor with bypass ¯ow was mostly simi-lar to that in the anoxic reactor.In the clari®er,some phosphorus was released but it was so small as to be negligible.When carbon dioxide was bubbled through the phosphorus accumulating bac-teria,or when acid was added a substantial release of phosphate took place and this release was called the ``secondary release''(Barnard,1984).
The rate of COD consumption and the rate of T±P transformation in each operation of the ®xed-®lm reactor are shown in Fig.8.In the anaerobic reactor of Run 1,the rate of COD consumption and phosphorus release in the anaerobic reactor were the largest,whereas the COD consumption rate and the phosphorus uptake rate in the anoxic reactor were smallest among the three di erent op-eration conditions.On the other hand,the vari-ations of COD consumption rate in the aerobic reactor were cont
rary to that in the anoxic reactor and the phosphorus uptake rate in the aerobic reac-tor decreased as the bypass ¯ow ratio increased.The amounts of the total phosphorus uptake were larger in Run 2and Run 3with bypass ¯ow than in Run 1without bypass ¯ow,but the amounts of the total COD consumption were mostly similar in
the
Fig.7.Changes of T±P concentration in the ®xed-®lm
reactor with bypass
¯ow.
Fig.  6.Relationship between denitri®cation and ORP
value in the anoxic reactor with bypass ¯ow.
H.U.Nam et al.
1574

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