Desalination 250 (2010) 592–597
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Desalination
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Optimization of biological nutrient removal in a pilot plant UCT-MBR treatingmunicipal wastewater during start-up☆
H. Monclús a,⁎, J. Sipma a, G. Ferrero a, J. Comas a, I. Rodriguez-Roda a,b
a Laboratory of Chemical and Environmental Engineering, University of Girona, Science and Technologic Park, Ed. Jaume Casademont, c/Pic de Peguera 15, E17003, Girona, Spainb ICRA (Catalan Institute for Water Research), Scientific and Technological Park of the University of Girona, H2O Building, Emili Grahit 101, 17003 Girona, Spain
☆ Presented at the Conference on Membranes in DProduction, 20-24 October 2008, Toulouse, France.⁎ Corresponding author. Tel.: +34 972183244.
E-mail address: [email protected] (H. Monclús)
0011-9164/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.desal.2009.09.030
a b s t r a c t
a r t i c l e i n f oAvailable online 9 October 2009
Keywords:Biological nutrient removal (BNR)Enhanced biological phosphorousremoval (EBPR)Membrane bioreactor (MBR)Polyphosphate-accumulatingorganisms (PAOs)
This study shows that an MBR pilot plant with UCT configuration is able to obtain high nutrient removalefficiency already during start-up. The biological nutrient removal (BNR) efficiencies significantly increasedtowards the end of the experimental run, achieving a COD removal efficiency exceeding 94% and N removalefficiency in the range of 89 to 93%. P removal efficiencies in the range of 80 to 92% have been obtained.During the experimental period (4 months) the evolution of the activity of polyphosphate-accumulatingorganisms, obtained from Prelease and Puptake rates, showed a small increase in the activity of polyphosphate-accumulating organisms (PAOs) and denitrifying polyphosphate-accumulating organisms (DPAOs). Thespecific phosphate accumulation at the end of the experimental run amounted to 8.0 mg P g−1VSS h−1 and3.29 mg P g−1VSS h−1, for the PAOs and DPAOs respectively. Moreover, the DPAOs activity increased fasterthan PAOs activity, i.e. from 0.36 to 0.41 of phosphate uptake rate (PUR) ratio.
rinking and Industrial Water
.
ll rights reserved.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Membrane bioreactor (MBR) technology usually results in highquality effluents with low concentration of total suspended solids(TSS), with solids removal efficiencies better than tertiary treatmenteffluent characteristics. MBR technology was initially designed toachieve high organic matter removal (i.e. COD removal), but it hasbeen demonstrated that it easily obtains efficient nitrogen removal.Indeed high COD and nitrogen removal efficiencies have been demon-strated operatingMBRs with high sludge retention times andwith theaddition of an anoxic zone [1]. However, biological phosphorusremoval is limited due to the low net biomass growth, typical forsystems with long sludge retention times (SRT), and thus limitedphosphate incorporation in new cell material.
Enhanced biological phosphorous removal (EBPR) employs theextended polyphosphate storage capacity of specialized microorgan-isms. These polyphosphate-accumulating organisms require anaerobicconditions in order to assimilate organic matter such as volatile fattyacids (VFAs)with the release of phosphorus from stored polyphosphate[2]. Phosphate is taken up, under aerobic conditions, by PAOs [3] and ashas been demonstrated, under anoxic conditions in the presence ofnitrate by denitrifying-polyphosphate-accumulating organisms(DPAOs) [4–6]. The DPAOs, in fact, denitrify nitrite and nitrate (NOx
−)using the organic matter previously stored under anaerobic condi-tions [6]. Therefore, DPAOs optimise denitrification, which can be
limited by the low quantity of organic matter present in the anoxicreactor. Moreover, phosphate uptake occurs either under anoxic oraerobic conditions increasing phosphorus removal efficiencies.
Phosphate removal is the result of phosphate incorporation in newcell material and ultimately the removal of phosphate containingbiomass. Despite the fact that processes operating at higher SRTusually are characterized by a reduced biological phosphorus removal[7], phosphate removal inMBRmay be feasible at high SRT. The reasonfor this is that the development of poly-P accumulating organisms(PAOs) is favoured in MBRs due to their competitive advantages overnon-poly-P accumulating microorganisms to survive during starva-tion periods, characteristic of an MBR due to the low F/M ratios [8].Bacteria that contain poly-P maintain longer high activity as a conse-quence of the accumulated energy source. Under carbon-limitingconditions, an MBR was capable of generating an effluent with totalphosphorus (PT) levels lower than a conventional-EBPR, because ofthe complete retention of suspended solids [9].
This paper describes the results obtained during the start-up of apilot plant MBR treating rawmunicipal wastewater, with the focus onthe biological nutrient removal.
2. Materials and methods
2.1. MBR pilot plant
The pilot plant (Fig. 1) comprises of a pre-screening system, bio-reactor (total volume 2.26 m3) with UCT configuration, i.e. anaerobic(14% of the total volume), anoxic (14%) and aerobic (23%) reactorcompartments, that are ultimately followed by a compartment (49%)
Fig. 1. Scheme of the MBR pilot plant showing the different compartments, flow directions and main instruments and equipment.
Table 1Influent wastewater characteristics.
Parameter Units Mean (SD) Max Min
COD mg COD·L−1 459 (191) 912 161BOD5 mg BOD5·L−1 293 (79.1) 430 238TKN mg TKN-N·L−1 50.6 (20.5) 123.5 24.1NH4
+ mg NH4+-N·L−1 29.1 (10.2) 59.9 13.0
NOx− mg NOx
−-N·L−1 0.3 (0.3) 1.6 0.0PO4
3− mg PO43−-P·L−1 3.63 (1.3) 7.4 1.3
C/N/P ratio 100/11/0.8
593H. Monclús et al. / Desalination 250 (2010) 592–597
with submerged hollow fibre membranes with a nominal pore size of0.1 μm.The totalmembranearea is 12.5 m2. Total suspended solids (TSS)sensors (Solitax; Hach Lange, Düsseldorf, Germany) are installed in theanaerobic andmembrane compartments. The anoxic reactor is equippedwith an ORP sensor (Alldos, Reinach, Switzerland). The anaerobic andanoxic reactor compartments are equipped with a mixer. In the aerobicand membrane reactor compartments two DO-temperature sensors(Crison,Alella, Barcelona, Spain) are installed. Furthermore, in the aerobicbioreactor compartment and in the membrane compartment a pHsensor (ProMinent, Heidelberg, Germany) and an ammonium sensor(Hach Lange, Düsseldorf, Germany) are installed respectively.
The rawwastewater is collected directly from the sewer that entersthe WWTP, after passing a first screen. The wastewater is pumpedto the pilot plant using a centrifuge pump (Grundfos, Bjerringbro,Denmark), crossing a 1 mm pore size filter and stored under mixingconditions in a 500 L buffer tank. From this buffer tank thewastewateris pumped to the anaerobic reactor compartment with a positiveadvance pump (Seepex, Bottrop, Germany) passing a second filter witha pore size of 0.6 mm to prevent large solids from entering thebioreactor and damaging themembranes. The permeate is obtained byapplying a vacuum pressure drop over the membranes using a secondpositive advance pump, which is controlled by pressure transducersthat measure the transmembrane pressure (TMP). Permeate is sepa-rated in two different tanks, backwash and permeate tank, to ensureconstantly sufficient water for backwashing the membranes. Ulti-mately, the treated effluent collected in the permeate tank isdischarged from the pilot plant to the WWTP sewer. No biomass waswasted from the MBR during these start-up periods.
In the aerobic reactor a PID control maintains the DO at 1.5 mg L−1
using two membrane air diffusers. The bioreactor is operated withthree different recirculation flows proportional to the inflow.
The pilot plant is provided with a programmable logic controller(PLC) and supervisory control and data acquisition (SCADA) systemthat acquires digital and analogical data and controls all the automaticcontrol loops of the plant: aeration, permeate and backwashing fluxes,hydraulic retention time (HRT), SRT and mixed liquor suspendedsolids (MLSS) concentration and recycles. Besides a decision support
system has been applied to control remotely and supervise theintegrated operation of MBR for wastewater treatment and reuse,integrating both the physical membrane separation process and thebiological wastewater treatment process [10].
2.2. Urban wastewater characteristics
The pilot plant is located at the Castell d'AroWastewater TreatmentPlant (WWTP), Catalonia, North-East of Spain. The pilot plant treatsmunicipal wastewater with a ratio of nutrients (C:N:P) of 100:11:0.8.The characteristics of the raw wastewater fed to the pilot plant MBRare shown in Table 1.
2.3. Analytical procedures
2.3.1. PAOs/DPAOs assaysActivated sludge samples were incubated in 2 different batch tests
in order to observe the PAOs and DPAOs phosphorus uptake andrelease potential. The sludgewas inoculated andmaintained anaerobicin the presence of sodium acetate during 3.5 h [4]. Subsequently one ofthe incubations was exposed to aerobic conditions and the other wasexposed to anoxic conditions by the addition of nitrate to a finalconcentration of 20 mg. NO3
−-N·L−1. Phosphate uptake rate (PUR)was estimated from the slope of the line describing the linear decrease
594 H. Monclús et al. / Desalination 250 (2010) 592–597
in phosphate concentration. The ratio of anoxic PUR to aerobic PUR(anoxic/aerobic PUR ratio) was used as an index reflecting the fractionof DPAOs [4].
2.3.2. FISH analysisFluorescent in-situ hybridization (FISH) analysis was performed as
specified by Amann [11] using a Cy3-labelled PAOMIX (consisting ofPAO462, PAO651 and PAO846 probes), which was used for Accumu-libacter, a known PAO, and FLUOS-labelled GAOMIX (consisting ofGAO431 and GAO989 probes). To target all bacteria, the Cy5 labelledEUBMIX (consisting of EUB338I, EUB338II and EUB338III) was used.The preparations were examined using an epifluorescence Zeisssmicroscope. The area containing specific labelled cells (Cy3 and FLUOSfor PAOMIX and GAOMIX, respectively) was quantified as a percent-age of the area of the entire bacterial population (EUBMIX).
2.3.3. Analytical methodsTotal suspended solids (TSS; 2540B), volatile suspended solids
(VSS; 2540C), total chemical oxygen demand (COD; 5220B) and totalKjeldahl nitrogen (N-TKN; 4500-Norg.B) were analyzed according toStandard Methods [12]. Ammonium (NH4
+-N; 4500-NH3 B–C) concen-tration of the supernatant was determined by distilling (Büchi B324) asample into a solution of boric acid. The ammonia in the distillate wasdetermined with a trimetric method (Tritino 719S Metrohm) using astandard H2SO4 and a pH meter. Nitrites (NO2
−-N) nitrates (NO3−-N)
and phosphates (PO43−-P) were analyzed using ionic chromatography
(Metrohm 761-Compact; 4110B).
3. Results and discussion
3.1. BNR during the start-up of the pilot plant MBR
The operational conditions for complete biological nutrient removalwere determined via simulation through an optimisation spreadsheetbased on theASM2dmodel [13]. During the start-up period, the influentflow rate and subsequently the permeate flow rate were increasedgradually from 6 L m−2 h−1 (LMH) to 10 LMH, while the DO set pointfor the aerobic compartment, external recycle, internal (anaerobic andanoxic) recirculations and sludge waste flow rates were fixed to thevalue provided by the optimizer. The external recirculation ratio, i.e.from the membrane compartment to anoxic compartment was set to1.36 times the inflow (IF). The anoxic recirculation ratio, i.e. from theaerobic to anoxic reactor compartment was set to 0.92 times the IF andthe anaerobic recirculation ratio, i.e. from the anoxic to anaerobicreactor compartment was set to 1.3 times the IF.
Table 2 summarises the operational conditions during the experi-mental study divided in two different periods. A malfunctioning of theanti-foaming dosing pump, causing too high concentrations ofantifoam in the reactor at day 46 caused a considerable TMP increaseand the consequent reduction of permeability to values as low as20 L m−2 h−1 bar−1. In order to recover the membrane permeabilitya chemical cleaning was applied (day 61) and the process was
Table 2Operational conditions of the MBR pilot plant during the experimental period.
Parameter Units Period I(0 to day 61)
Period II(68 to 110)
Operational days Days 61 42HRT Hours 22.6 15.06LMH L m−2 h−1 From 6 to 10 12Anaerobic recirculation % of the inflow 100 to 129 129Anoxic recirculation % of the inflow 80 to 92 92External recycle % of the inflow 110 to 136 136DO aerobic set point mg O2·L−1 1.5 1.5
restarted inoculating new sludge (day 68). This event is the reason forthe separation of the total experimental time in period I and II, whichthus both represent initial start-up.
Fig. 2 shows the BNR evolution during the entire experimental run.COD removal efficiencies were very high during the two periods(always around 95%). In general, nitrification was complete through-out the experimental runs achieving 0 mg NH4
+-N·L−1 in the effluent.However, in period I, nitrates were reduced from 10 mg NOx
−-N·L−1
(at day 20) to 3 mg NOx−-N·L (at day 60), which permitted the
increase of P removal achieving an effluent with low phosphate con-centration (0.3 mg PO4
3−-P) at day 54. The high concentration ofnitrates in the anoxic reactor could be recirculated to the anaerobicreactor, which negatively affects the release of phosphate and sub-sequently the phosphate uptake in the aerobic or anoxic reactorcompartment. Nitrification was also complete during period II (0 mgNH4
+-N·L−1), but the high nitrate concentration in the reactor (8 mgNOx
−-N·L−1) at the beginning also provoked a limitation of the Premoval. Nevertheless, at the end of period II, the P concentrationdecreased significantly, achieving an effluent concentration lowerthan 0.9 mg PO4
3−-P.In summary, total N removal efficiencies were a bit higher at the
end of period I (95% at day 60) than in period II (90% at day 110). Inthe case of P removal the EBPRwas higher in period I (92.1% at day 60)than in the second (80.5% at day 110). The lower value of P removalefficiency is related to the elevated nitrate concentration in the anoxicreactor, limiting the optimal conditions for Prelease in the anaerobicreactor. To obtain high removal percentages of P, anaerobic conditionsare required for the uptake and storage of easily biodegradableorganicmatter and also Prelease, to finally accumulate phosphate underanoxic or aerobic conditions [2,3].
Fig. 3 illustratesMLSS concentration, permeability andpermeateflux(LMH) in the membrane compartment during the experimental study.During the start-up, in period I, the MLSS concentration increased from1100 mg MLSS·L−1 to 6230 mg MLSS·L−1 at day 58. The permeabilitystarted at low values (40 L m−2 h−1 bar−1) to 18 L m−2 h−1 bar−1,and on-line chemical cleaning (maintenance cleaning) was appliedseveral times (day 11 and 40) to recover the permeability, but theireffectwas limited. After the chemical off-line cleaning (day61), the newrun also started at low MLSS concentration (1050 mg MLSS·L−1), andthe concentration increased to 7400 mg LMSS·L−1 at day 110. Thepermeability was recovered to high values (105 L m−2 h−1 bar−1) andthe permeate flow was also increased to 10 LMH. However, an on-linecleaningwas necessary at day 90 because the permeability decreased tovalues under 80 L m−2 h−1 bar−1.
Analysis of the inoculum sludge using fluorescent in-situ hybridi-sation (FISH) revealed that the PAO populations, at day 70, after thesecond inoculation was significantly lower than glycogen-accumulat-ing organisms (GAO). Estimations revealed a PAOs population ofaround 22% and a GAO population of around 33%. This indicates that inthe inoculum sludge a certain competition may be expected for thestorage of easily degradable organic matter.
3.2. EBPR test to measure PAOs and DPAOs activity
Phosphorous removal through bacterial assimilation, with an esti-mated biomass yield of 100 mg L−1 d−1 between days 70 and 100,taking into account the average P content of bacteria of 2% (Metcalfand Eddy, 2003), results in a P removal capacity of 2 mg L−1. With therelatively low average phosphate concentrations (about 3.6 mg L−1)present in the influent wastewater, bacterial assimilation alone wouldbe sufficient for obtaining an effluent quality that fulfills dischargelimits. Nevertheless, between days 90 and 100 the P removalsignificantly exceeds the 2 mg L−1 (Fig. 2) that could be assimilatedbased on the averageMLSS concentration increase, whereas accordingto Fig. 3 between days 90 and 100 the biomass production decreased.This indicates that EBPR occurred. To corroborate the EBPR activity,
Fig. 2. Biological nutrient (C, N and P) removal evolution during the experimental study.
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batch test were carried out to measure the specific activity of PAOsand DPAOs (Fig. 4), which showed a clear increase in Puptake rates(Table 3) between the unacclimated sludge at day 70 (day 0 of thesecond period) and acclimated sludge at day 100 (day 30 of thesecond period). It should be noted that this experimental period isonly reflecting the biological nutrient removal during the start-up
Fig. 3. Evolution of the MLSS, permeability and LMH during the experim
of an MBR pilot plant, with relatively low biomass concentrations.When the biomass concentration reaches a desired value (10,000–12,000 mg L−1), the low F/M ratio results in a low biomass yield andthus decreased capacity of P removal through assimilation andconsequently P removal through polyphosphate-accumulating organ-isms will become more important.
ental period. Period I (from 0 to 59) and period II (from 70 to 118).
Fig. 4. Specific phosphate release (positive slope; measured as the increase in liquor PO43−-P concentration normalized per g of VSS) and uptake (negative slope) rates using acetate
as carbon source, presented in A) 70-day sludge (unacclimated) and in B) 100-day sludge (30-day acclimated sludge).
Table 4Specific and theoretic denitrification via DPAOs.
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The test shows that the PAO activity was higher than DPAOs inbatch test. However the Puptake is significantly different after 30 daysof operation in both PAO and DPAOs tests. Table 3 summarises theresults of the PAOs and DPAOs test.
The different rates showed that the PAOs and DPAOs activitiesincrease when the sludge is 30 days-acclimated. The Puptake rates aresignificantly different between the beginning of the second period(day 70) (6.24 and 2.26 mg P·g−1VSS h−1 for PAOs and DPAOs,respectively) and at day 100 (8.00 and 3.29 mg P·g−1VSS h−1 forPAOs and DPAOs, respectively). At day 70 the relative contribution ofDPAOs, calculated as the ratio of DPAOs:PAOs at day 70 (0.36) waslower than at day 100 (0.41), indicating that the DPAOs activityincreased faster than the PAOs activity. Whereas for long DPAOs havebeen largely overlooked as significant contributors to EBPR, they infact play a significant role in the process, which is even stimulatedwhen operating an MBR with UCT configuration as reflected in thehigher DPAO:PAO ratio.
Moreover, in UCT-based wastewater treatment plants denitrifica-tion takes place simultaneously under anoxic conditions in twodifferent ways: on one hand, conventional denitrification is carriedout by heterotrophic population and on the other hand denitrificationis carried out by DPAOs [14]. Therefore, the relative contribution ofDPAOs to the overall denitrification process was estimated (Table 4).
Table 3Comparison between Prelease and Puptake rates at days 70 and 100 for PAOs and DPAOs.
Parameter Units Day 70 Day 100
Test PAO Prel. rate mg P·g−1VSS h−1 4.37 4.94Pupt. rate mg P·g−1VSS h−1 6.24 8.00
Test DPAO Prel. rate mg P·g−1VSS h−1 3.97 4.17Pupt. rate mg P·g−1VSS h−1 2.26 3.29
% DPAO PUR ratio[4] – 0.36 0.41
Using the N balances we can estimate that the denitrification viaDPAO was 18% at day 75 (using the P test at day 70 as inoculum) and33% at day 100 (30 days after the second start-up). These values aredirectly related with the DPAOs activities obtained in the batch testand the MLSS in the anoxic reactor. Although, the specific Puptakeincreased slightly, a larger effect is observed from the increment of theMLSS concentration.
4. Conclusions
The experimental study has demonstrated that an MBR pilot plantwith UCT configuration is able to achieve high percentages of BNR,i.e. 95% of COD removal, more than 90% of N removal and more than80% of P removal. Moreover, the EBPR efficiencies increased after30 days of operation from 6.2 mg P·g−1VSS h−1 at day 70 to 8.0 mgP·g−1VSS h−1 at day 100. Also, the DPAOs activity increased from2.3 mg P·g−1VSS h−1 to 3.3 mg P·g−1VSS h−1. Moreover, the DPAOsactivity increased faster than PAOs activity, as the ratio between PAOsand DPAOs increased from 0.36 at day 70 to 0.41 at day 100.
Units Day 75a Day 100
Influent N mg N·L−1 35 60Flow L h−1 125 150Denitrification mg N-NOx
−·L−1 30 50mg N-NOx
−·h−1 3750 7500Specific denitrification rate DPAO mg N- NOx
−·VSSg−1 h−1 1.07 1.54VSS in anoxic reactor g L−1 0.9 2.4Anoxic reactor volume L 320 320Denitrification by DPAO mgN-NOx
−·h−1 662 2534% Denitrification via DPAOs % 18 33
a Using the specific denitrification rate of DPAOs obtained at day 70 with the morestabilised reactor values obtained from day 75.
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Acknowledgements
This researchwas funded by the SpanishMinistry of Education andScience ((MYCT-DPI2006-15707-C02-01), (CTM2009-14742-C02-01)and (CONSOLIDER-CSD2007-00055)). The authors would like tothank Mónica Roldán (Servei de Microscòpia of UAB) for herassistance with the FISH analysis, the Consorci de la Costa Brava andthe members of Castell d'Aro WWTP. Finally, the authors are gratefulto Ariadna Cabezas, Gemma Rustullet, Íngrid Ferrer and Sara Gabarrón(LEQUiA-UdG) for their support during the experimental study.
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