Biological nitrification and denitrification of asimulated high ammonia landfill leachate using4-stage Bardenpho systems: system startup andacclimation

P. Ilies and D.S. Mavinic

Abstract: This research investigated the nitrogen removal capability of two biological nitrification systems, with pre-and post-denitrification, when treating a landfill leachate characterized by high ammonia concentrations and low levelsof biodegradable organics. The recycle ratios of the systems were set so that, at an average influent flow of 10 L/d, theactual hydraulic retention time of the first anoxic reactor was about 1.5 h for one system and 1.7 h for the other sys-tem. The systems also operated at a first aerobic reactor actual hydraulic retention time of 3 and 3.4 h, respectively.Methanol was used as a supplementary organic carbon source for denitrification. High leachate ammonia concentrationswere simulated by artificially increasing influent ammonia to about 2200 mg N/L. This paper presents an overview ofinitial startup and acclimation, as well as some of the direct and indirect effects of methanol addition on process per-formance. The reported data were collected during two runs at incrementally increasing influent ammonia concentra-tions. During the first run to reach 2200 mg N/L, methanol loading rates were increased concomitantly with ammonialoading rates, to match expected aerobic NOx production, using a CH3OH:NOx of about 20:1. This resulted in methanolcarry-over into the first aerobic zone, enhanced aerobic heterotrophic growth, and further inhibition of the nitrifyingpopulation, already inhibited by recycling through the elevated free ammonia levels of the first anoxic zone. Whenthese systems were allowed to adapt up to 14 days, rather than 7 days, initially, to each incremental ammonia increase,and with methanol loading rates subsequently changed to yield CH3OH:NOx of only 5:1, the influent ammonia concen-tration was increased to approximately 2200 mg N/L within 88 days from the start of the second run, without any in-hibitory problems. The timing and levels of ammonia and methanol loading rate increases, with respect to each otherand to the corresponding previous loading rate increase, played an important role in system stability and the onset ofnitrification failure.

97Key words: biological treatment, high ammonia leachate treatment, denitrification, methanol, nitrification.

Rsum : Cette recherche a tudi la capacit denlvement dazote de deux systmes de nitrification biologiques, avecune pr- et une post-dnitrification, lors du traitement dcoulements de dcharge charactris par des concentrationsleves en ammoniaque et de bas niveaux de produits organiques biodgradables. Les taux de recyclage des systmesont t calibrs de sorte que, un coulement moyen deaux traiter de 10 L/j, la priode relle de rtention hydrau-lique du premier racteur anoxique a t environ 1,5 h pour un systme, et 1,7 h pour lautre systme. Les systmesont galement fonctionn avec une rtention hydraulique du premier racteur arobique de 3 h, et de 3,4 h, respective-ment. Le mthanol a t utilis comme source de carbone organique supplmentaire pour la dnitrification. Desconcentrations dcoulements de dcharge leves en ammoniaque ont t simules en augmentant artificiellementlammoniaque de leau traiter jusque 2200 mg N/L environ. Cet article prsente une vue densemble de la mise enroute initiale et de lacclimatation, ainsi que de certains des effets directs et indirects de lajout du mthanol sur la per-formance du processus. Les donnes enregistres ont t collectes pendant deux passages des concentrations en am-moniaque deaux traiter graduellement croissantes. Pour atteindre 2200 mg N/L pendant le premier passage, les tauxde chargement du mthanol ont t augments simultanment avec les taux de chargement de lammoniaque, afin decorrespondre avec la production arobique de NOx prvu, et ce en utilisant un CH3OH:NOx denviron de 20:1. Ceci aeu comme consquence le report du mthanol dans la premire zone arobique, une augementation de la croissance a-robique htrotrophe, et davantage dinhibition de la population nitrifiante, dj inhib par le recyclage par les niveauxlevs dammoniaque libres de la premire zone anoxique. Quand ces systmes ont t permis de sadapter jusqu

Can. J. Civ. Eng. 28: 8597 (2001) 2001 NRC Canada

85

DOI: 10.1139/cjce-28-1-85

Received March 13, 2000. Revised manuscript accepted August 28, 2000. Published on the NRC Research Press Web site onJanuary 31, 2001.

P. Ilies and D.S. Mavinic.1 Environmental Engineering Group, Department of Civil Engineering, The University of BritishColumbia, 2324 Main Mall, Vancouver, BC V6T 1Z4, Canada.

Written discussion of this article is welcomed and will be received by the Editor until June 30, 2001.1Author to whom all correspondence should be addressed (e-mail: dsm@civil.ubc.ca).

14 jours, plutt que 7 jours initialement, chaque accroissement dammoniaque par tape, et avec des taux de charge-ment en mthanol chang ultrieurement pour un rendement CH3OH:NOx de 5:1 seulement, la concentration en ammo-niaque de leau traiter a grimp jusqu approximativement 2200 mg N/L dans les 88 jours depuis le dbut dudeuxime passage, sans problmes inhibiteurs. La synchronisation et les niveaux des augmentations du taux de charge-ment de lammoniaque et du mthanol, par rapport lun et lautre et par rapport laugmentation prcdente du tauxde chargement, ont jous un rle important dans la stabilit du systme et dans lchec de la nitrification consequante.Mots cls : traitement biologique, traitement des coulements de dcharge lev en ammoniaque, dnitrification, mtha-nol, nitrification.

[Traduit par la Rdaction] Ilies and Mavinic

Introduction

One of the most significant environmental concerns asso-ciated with sanitary landfilling is leachate production.Leachate is mainly the result of the infiltrating water thatpasses through the solid waste fill and facilitates transfer ofcontaminants from a solid phase (waste) into a liquid phase(infiltrating water). Leachate generation and composition aredetermined by the quality of the solid waste, the biologicaland chemical processes occurring in the waste fill at any onetime, the amount of precipitation and percolation, and localenvironment.

The two major landfill leachate constituents of environ-mental concern are organic carbon and nitrogen (EPA 1993).Recently, methanogenic-state landfill leachates (Pohland etal. 1985), characterized by low organic carbon concentra-tions, high ammonia-N levels, and a large spectrum of met-als, have become a major concern globally (Azevedo et al.1995; Mavinic 1998; Robinson and Gronow 1998). A treat-ment method priority for this particular type of leachate istotal nitrogen removal.

Biological nitrification and denitrification is one of themost feasible, effective, and relatively low cost methods ofremoving nitrogen from municipal and industrial discharges(EPA 1995; Metcalf & Eddy, Inc. 1991). Treatability of mu-nicipal landfill leachates, using combined stage nitrifica-tion/denitrification processes, has been the focus of anongoing research program at The University of British Co-lumbia (UBC) for the last 10 years. In response to environ-mental concerns regarding rising ammonia-N concentrationsin leachates, the most recent studies (Azevedo 1993; Ilies1999; Shiskowski 1995) investigated the treatment of highstrength leachates, containing up to 2200 mg N/L of ammo-nia, using continuous flow, complete mix, suspended-growth, single sludge, activated sludge systems. One of theprocess configurations examined was nitrification, with pre-and post-denitrification, generally referred to as a 4-stageBardenpho process (Ilies 1999; Shiskowski 1995).

Nitrification is the autotrophic, sequential oxidation ofammonium ion (NH4+), first to nitrite (NO2) and then to ni-trate (NO3), while denitrification is the heterotrophic,anoxic two-step conversion of nitrate, first to nitrite and thento gaseous nitrogen compounds (EPA 1993). When treatinghigh ammonia landfill leachates characterized by low biode-gradable organic levels, a supplementary source of organic

carbon is required to ensure adequate denitrification. Metha-nol is an excellent source of supplementary organic carbonand has been used successfully for such purposes, especiallyin landfill leachate treatment (Azevedo 1993; Robinson et al.1998; Shiskowski 1995).

This paper presents the results of biological treatment of amethanogenic landfill leachate, when using a nitrificationprocess, with pre- and post-denitrification. The data werecollected during two attempts at incrementally increasingammonia concentrations in the influent leachate up to about2200 mg N/L and subsequent treatment. Since the prime ob-jective of the research was the investigation of the processperformance in treating this high strength leachate, thisstudy did not explicitly explore bacterial responses to pro-gressively increased methanol loadings. However, the datacollected during the early stages of operation are noteworthyand may contribute to a better understanding of the bacterialactivity occurring within this particular type of system, espe-cially when treating high ammonia wastes. Furthermore, inconcert with previously reported work (Azevedo et al. 1995;Shiskowski and Mavinic 1998), the findings of this studymight help to establish the framework for startup and opera-tion of systems treating high ammonia/low organic carbonliquid wastes from any source. A detailed description of theentire research work and its findings are available elsewhere(Ilies 1999).

Experimental setup and operation

Process configuration and system designA pre- and post-denitrification process configuration was

used in the present study. The process configuration, gener-ally known as a 4-stage Bardenpho process, is a continuousflow, complete mix, suspended-growth, single sludge, acti-vated sludge process relying on a sequence of anoxic andaerobic zones for biological nitrification and denitrification(Fig. 1). Two identical, parallel, laboratory-scale systemswere run during this study. Both systems consisted of ananoxic reactor (Anoxic #1), an aerobic reactor (Aerobic #1),a second anoxic reactor (Anoxic #2), followed by a secondaerobic reactor (Aerobic #2), and a clarifier. The average de-sign and operating parameters of the systems are summa-rized in Table 1.

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Based on previous work and to maintain consistency(Shiskowski 1995), the systems were operated at an Aerobic#1 solids retention time (SRT) of 20 days and a sludge recy-cle ratio, i.e., external recycle or clarifier to Anoxic #1 recy-cle, of about 3:1. Throughout the study, one system (System#1) used a mixed-liquor recycle ratio, i.e., internal recycle orAerobic #1 to Anoxic #1 recycle, of about 4:1, while theother system (System #2) used an internal recycle ratio of3:1. These ratios were set so that an average first anoxic(Anoxic #1) actual hydraulic retention time (AHRT) of 1.5 hfor System #1 and 1.7 h for System #2 could be maintainedduring the entire study period. The first aerobic (Aerobic #1)AHRT was about 3 h for System #1 and 3.4 h for System#2. These values, although seemingly close numerically,were reflective of attempts at system optimization, based onprevious research (Shiskowski and Mavinic 1998).

A temperature-controlled room of the Environmental Lab-oratory, Department of Civil Engineering, UBC, was usedfor the system setup. The ambient temperature was main-tained at about 20C during this phase of the study. In a fol-low-up phase, operating temperatures were loweredconsiderably, concurrently with maintaining high influentammonia levels. The results of this phase will be the subjectof a separate manuscript.

Leachate feedThe leachate (herein called the base leachate) used in this

study originated from the Burns Bog Landfill, located inDelta (near Vancouver), British Columbia. The landfill, stillin use today, began operation in 1966 (Guo 1992). Theleachate being generated at this time has the typical charac-

teristics of a methanogenic landfill leachate or a nitrogen-based leachate (i.e., low levels of readily biodegradable or-ganic matter and high concentrations of ammonia-N). Thebasic composition of this leachate is shown in Table 2. Theleachate flow rate was set at about 9 L/d, so that the totalflow into the first anoxic reactor, including chemical addi-tion flows, could be maintained at approximately 10 L/d(Table 1). Increased ammonia-N concentrations in the leach-ate (herein called the simulated leachate) were achieved bypumping an ammonium chloride (NH4Cl) solution to thefirst anoxic reactor of each system. The desired increase inthe influent concentration was achieved by changing theconcentration of the ammonium chloride solution.

Chemical feedA phosphorus solution was also fed to the first anoxic re-

actor of each system. Solution concentrations were adjustedso that a minimum concentration of 2 mg P/L of biologicallyavailable orthophosphate, considered sufficient for nitrifica-tion and denitrification (Manoharan et al. 1992), was assuredin all reactors. Tribasic sodium phosphate (Na3PO412H2O)was used to prepare the phosphorus solution. A methanol so-lution (CH3OH) was added to both anoxic reactors of eachsystem, to provide the organic carbon levels required fordenitrification. Theoretically, 3.7 g COD/g NO3-N and 2.3 gCOD/g NO2-N are required for complete denitrification,when using methanol (EPA 1993). Azevedo (1993) used aCH3OH:NOx of 5:1, as g COD/g N, while Carley (1988)used one of 6.5:1. In this study, methanol loadings were pro-gressively increased to match anoxic denitrification require-ments in concert with stable nitrification. Finally, sodium

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Fig. 1. Pre- and post-denitrification system configuration.

bicarbonate (NaHCO3) solution was also fed into the firstaerobic reactor of each system, to provide sufficient alkalin-ity for nitrification. Bicarbonate solutions were pumped intothe reactors by pH/pump controllers, whenever the continu-ously monitored Aerobic #1 pH levels dropped below thepH set point of 7.5 a value recommended for best nitrifi-cation (EPA 1993; Metcalf & Eddy, Inc. 1991). The solutionconcentration was near saturation (i.e., 80 g NaHCO3/L) inorder to minimize the effect of the volumetric addition onthe Aerobic #1 AHRT. Sodium bicarbonate solution flow in-creased eventually to 1.8 L/d.

MonitoringSubmerged oxidationreduction potential (ORP) elec-

trodes, connected to pH/mV meters, continuously monitoredthe oxidationreduction potential within the Anoxic #1 reac-tor of both systems. The ORP values of the Anoxic #2 reac-tors were measured daily. Once denitrification wasestablished, the ORP values fluctuated between about 10and 250 mV. Dissolved oxygen (DO) levels in the Aerobic#1 reactors were continuously monitored using submersibleDO probes. The DO levels in the Aerobic #2 reactors of the

systems were measured on a daily basis. A minimum DO of2 mg/L was ensured. Leachate, mixed liquor, and effluentpH levels were measured on a daily basis using an Ag/AgClcombination pH electrode connected to a portable pH meter.The probe was submerged directly into the leachate from thefeed bucket, reactor mixed liquor, and supernatant of theclarifier. The leachate, mixed liquor, and effluent super-natants were sampled two to three times per week and peri-odically analyzed for total ammonia (NH4+ + NH3), nitrite +nitrate (NOx), nitrite (NO2), orthophosphate, total and vola-tile suspended solids (TSS and VSS), chemical oxygen de-mand (COD), biochemical oxygen demand (BOD5), andalkalinity. The analysis of various constituents and the mea-surements of other parameters of interest were performed inaccordance with the Standards Methods for the Examina-tions of Water and Wastewater (APHA 1993) and the corre-sponding instrument instruction manuals.

It should be noted that, in this paper, the terms ammonia,ammonia-N, and total ammonia refer to the sum of ammo-nium ion (NH4+) and free ammonia (NH3), i.e., unionizedammonia or molecular ammonia. Other works reported inthe literature refer to the sum as ammoniacal-N or, simply,NH4. The two constituents coexist in an equilibrium regu-lated by pH and temperature (EPA 1993). The higher the pHand (or) the temperature, the higher is the fraction of freeammonia, assuming constant total ammonia concentration.

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ParameterSystem #1(mean value)

System #2(mean value)

Anoxic #1 volume (L) 5 5Aerobic #1 volume (L) 10 10Anoxic #2 volume (L) 5 5Aerobic #1 volume (L) 10 10Clarifier volume (L) 4 4System volume (L) 35* 35*Influent flow (L/d) 10 10External recycle ratio 3:1 3:1Internal recycle ratio 4:1 3:1

Aerobic #1 wasting (L/d) 0.5 0.5Aerobic #1 SRT (d) 20 20Anoxic #1 nominal HRT (h) 12 12Aerobic #1 nominal HRT (h) 24 24Anoxic #2 nominal HRT (h) 12 12Aerobic #2 nominal HRT (h) 24 24Clarifier nominal HRT (h) 9.6 9.6System nominal HRT (h) 84 84Anoxic #1 actual HRT (h) 1.5 1.7Aerobic #1 actual HRT (h) 3 3.4Anoxic #2 actual HRT (h) 3 3Aerobic #2 actual HRT (h) 6 6Clarifier actual HRT (h) 2.4 2.4Total ammonia input (mg N/L) 2302300 2302200BOD5/COD input (range) 0.050.5 0.050.5

*1 L is estimated for pumps and tubing in each system.Refers to total flow into Anoxic #1 reactor, including chemical feed

flows (base leachate 9 L/d, NH4Cl 0.4 L/d, Na3PO4 0.3 L/d,CH3OH 0.3 L/d).Mean values before the first and after the last incremental increase inthe influent ammonia concentration.

Based on data collected during the experimental period discussed in thepresent paper.

Table 1. System design and operating parameters.

Parameter*Concentrationrange

Meanconcentration

COD (mg/L) 207440 311BOD5 (mg/L) 2584 46Ammonia (mg N/L) 50310 149Nox (mg N/L) 0.001.71 0.53NO2 (mg N/L) 0.000.58 0.20O-PO4 (mg P/L) 0.003.40 0.32Suspended solids (mg/L) 49150 81Alkalinity (mg CaCO3/L) 10202290 1513pH (pH units) 7.307.81 7.53Total aluminum (mg/L)

OperationThis research was conducted in four phases, over a period

of 311 days. Nitrification and denitrification of the baseleachate was achieved during the base leachate phase. Themain purpose of the ammonia loading phase was to accom-plish nitrification and denitrification of the progressively in-creased influent ammonia concentration. During pH phase,denitrification performance was thought to be improved bydecreasing the anoxic pH levels. The temperature phase ex-amined the overall system performance under progressivelydecreased ambient temperature. As noted earlier, the presentpaper discusses mainly the results of the of the ammonialoading phase.

The base leachate treatment, i.e., influent ammonia con-centrations of up to about 300 mg N/L, and the two runswith increasing influent ammonia concentration proceededfor 185 days. By day 64 of the base leachate treatment pe-riod, both systems generated ammonia-free effluents, withNOx concentrations of less than 2 mg N/L. On day 66, thefirst increase in the influent ammonia concentration was per-formed. From this day on, the dates were renumbered asphase day 1 to 120 to reflect operational changes. Sinceboth systems had similar responses to operational changes,illustrative figures for only System #1 are presented forbrevity.

As noted earlier, influent ammonia levels were artificiallyincreased by adding ammonium chloride solutions into thefirst anoxic reactor of both systems. Ammonium chloride so-lution concentrations were adjusted to generate incrementalincreases of about 300 mg N/L in the influent leachate. Thissimulated leachate, with increasing ammonia levels of ap-

proximately 400, 700, 1000, 1300, 1600, 1800, and 2300 mgN/L, was progressively fed to both systems (Fig. 2).

Results and discussion

The first run at incrementally increasing the incoming am-monia concentration was conducted from phase day 1 to 27as per the procedure reported in previous works (Azevedo1993; Shiskowski 1995). The first four incremental increasesin the influent ammonia concentration, i.e., from about400 mg N/L up to about 1300 mg N/L, were followed in aday or two by increases in the methanol loadings to theanoxic reactors. The systems were allowed to adapt to eachnewly increased ammonia loading for about 7 days.

Based on the findings of the base leachate treatment pe-riod and assuming that most of the ammonia entering thefirst aerobic reactors would be converted to nitrate,CH3OH:NOx were initially kept at about 20:1, so that or-ganic carbon was not limiting for denitrification require-ments. On the 27th day of this phase, strong nitrificationinhibition (Fig. 3) and ammonia accumulation (Fig. 2) wereobserved within both systems. Speculating that the systemswould not recover under a continuing ammonia concentra-tion of about 1300 mg N/L (Shiskowski 1995), the ammo-nium chloride addition was halted and the methanol loadingwas discontinued for several days; the latter was then read-justed to meet base leachate denitrification requirements.

Both nitrification and denitrification processes within thetwo systems recovered within 4 days. On phase day 32, thesecond run with increase in the influent ammonia concentra-tion began. A couple of operational procedures were changed.

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Fig. 2. System #1: anoxic and aerobic total ammonia [NH4+ + NH3].

First, the systems were allowed to adjust to the increasedammonia loading for a longer period of time than during thefirst attempt, i.e., about 1014 days; second, only after thisprocedure were anoxic methanol loadings increased to meetdenitrification requirements. Such changes were made be-cause nitrification inhibition during the first run was attrib-uted mainly to methanol breakthrough into the first aerobicreactors (Azevedo 1993; Carley 1988; Hanaki et al. 1990),thus increasing reactor COD and BOD levels. In addition,after a detailed examination of the data collected during thisphase, the conclusion reached was that the instability of thetwo systems may have been also determined by several otherinterconnected factors, as discussed later.

Ammonia and NOx levelsBoth systems responded very well to the first two incre-

mental increases in the influent ammonia concentration, per-formed on phase days 1 and 7. The virtually 0 mg N/LAerobic #1 ammonia levels showed that most of the ammo-nia was removed in the Aerobic #1 reactors (Fig. 2). TheAnoxic #1 ammonia levels increased from the 20 mg N/L ofthe base leachate treatment period to about 50 mg N/L. Thecorresponding increase in Aerobic #1 NOx levels (Fig. 3)demonstrated that, except for ammonia assimilation, the am-monia was removed mainly by nitrification, i.e., Aerobic #1NOx levels were 50 mg N/L concomitantly with Anoxic #1ammonia levels of same value. In addition, Anoxic #1 and#2 NOx levels were practically zero, reflecting efficientdenitrification. With the exception of the nitrification inhibi-tion and system failure period, the above relationship wasobserved during the entire phase.

During the failure period, ammonia accumulation gradu-ally increased within both systems. The highest ammonialevels were reached in the Anoxic #1 reactors where, in ad-dition to the influent concentrations, ammonia was returnedby both, internal and external recycle lines (Fig. 2). System#1, with Anoxic #1 ammonia levels over 1500 mg N/L andAerobic #1 NOx levels practically zero, was more affectedby nitrification inhibition than System #2, with Anoxic #1ammonia levels of only 350 mg N/L and a considerably de-creased (but still present) Aerobic #1 nitrification. Thehigher Anoxic #1 and Aerobic #1 AHRTs of System #2were the main reason for the better performance of System#2, when both systems were under virtually identicalchanges in loading rates. As soon as ammonium chlorideand methanol additions were stopped, a peak in NOx levelsoccurred in both systems, with the existing ammonia beingconverted to nitrites and nitrates by a rapidly recoveringpopulation of nitrifiers. The NOx concentrations were similarin all reactors (Fig. 3), since methanol loading rates were ad-justed to meet only base leachate denitrification require-ments, i.e., organic carbon levels within the anoxic reactorswere insufficient to meet denitrification requirements of theincreased NOx concentrations.

Most of the incoming ammonia was removed within theAerobic #1 reactor of both systems, when sufficient timewas ensured between successive influent ammonia concen-tration increases. Any ammonia escaping treatment in theAerobic #1 reactor was removed by bacterial assimilationwithin the Anoxic #2 reactors and occasional nitrificationwithin the Aerobic #2 reactor. The effluent in both systemswas virtually ammonia free, from the beginning until the

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Fig. 3. System #1: anoxic and aerobic NOx.

end, i.e., from phase day 32 to 120, of the second run atincrementally increasing the influent ammonia concentra-tion. Since methanol loading rates were increased in a moreconservative manner, i.e., the anoxic reactors were fed meth-anol at CH3OH:NOx ratios estimated to match denitrificationrequirements of each antecedent influent ammonia concen-tration and not the actual one, elevated NOx levels were ob-served within all reactors on several occasions (Fig. 3).However, subsequent to each methanol loading increase, theNOx levels decreased rapidly, as a result of responsivedenitrification.

Alkalinity and methanol loadingsAccording to basic concepts (EPA 1993), the nitrification

process destroys alkalinity at a theoretical ratio of 7.14 gCaCO3 per g NH4+-N nitrified, while the denitrification pro-cess produces alkalinity at a theoretical ratio of 3.57 gCaCO3 per g NO3-N reduced. Therefore, in a pre-denitrification process, theoretically, half of the nitrificationalkalinity requirements can be provided by the denitri-fication process. In addition to the alkalinity produced bydenitrification, alkalinity requirements in this study were en-sured by both the natural alkalinity of the landfill leachate ofabout 1500 mg CaCO3/L and sodium bicarbonate additioninto the first aerobic reactors. Alkalinity loading rates intoboth systems increased progressively, in response to higherAerobic #1 nitrification rates (Fig. 4), reflective of higher in-fluent ammonia levels. Once methanol loading rates wereadjusted to match (to the extent possible) the increased NOxloading rates, the rise in alkalinity decreased slightly, a di-

rect result of increased alkalinity recovery throughdenitrification.

Increased alkalinity loading also meant an increase in so-dium bicarbonate solution flow into Aerobic #1 reactor,hence, decreased reactor AHRTs. Therefore, the higher theammonia loading into the aerobic reactor, the higher was thealkalinity loading, resulting in lower AHRT. If nitrificationalkalinity requirements had not been partially provided bythe denitrification process occurring within the antecedentanoxic reactor, it is speculated that Aerobic #1 AHRTsmight have eventually become insufficient to ensure ade-quate nitrifier growth (Hanaki et al. 1990). Still, during thisphase, surges in alkalinity loadings (hence, fluctuations inAerobic #1 AHRTs) continuously threatened the nitrificationprocess. Along with other factors, such as excess methanolloading and potential nitrite accumulation before bacterialacclimatization (Turk and Mavinic 1989), decreased Aerobic#1 AHRT contributed to nitrification inhibition and systemfailure, during the first run at increasing influent ammoniaconcentrations.

Throughout this phase of study, anoxic methanol loadingswere also increased to match aerobic NOx production andmeet anoxic NOx removal requirements (Fig. 5). Methanolincreases made almost concomitantly with influent ammoniaincreases, based on CH3OH:NOx of about 20:1, establishedduring treatment of base leachate, resulted in methanol car-ryover into the Aerobic #1 reactors and subsequent nitrifi-cation inhibition. Nitrification inhibition was attributed, inpart, to enhanced heterotrophic bacterial growth (Hanaki etal. 1990). However, both systems responded well to metha-

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Fig. 4. System #1: aerobic nitrification rate and alkalinity loading.

nol loading increases made after allowing additional time forthe establishment of a reasonably stable nitrification withinthe Aerobic #1 reactor, i.e., consistent NOx loadings into theanoxic reactors. In addition, the methanol increases weremade under lower CH3OH:NOx and in accordance with pre-vious studies (Azevedo 1993; Carley 1988).

The methanol loadings into the Anoxic #1 and #2 reactorsof System #1 were, for the most part, equal (Fig. 5), as bothreactors received estimated equal NOx loads, i.e., the internalrecycle line, that returned NOx into Anoxic #1 reactor, had aflow rate of about 40 L/d, theoretically equal to the NOx-laden overflow from the first aerobic reactor into Anoxic #2reactor. However, the methanol loadings into Anoxic #2 re-actor of System #2 were higher than the loadings into theAnoxic #1 reactor of that system, since the second anoxicreactor received higher NOx loads, i.e., the internal recycleflow rate was about 30 L/d, while the first aerobic overflowinto the second anoxic reactor was about 40 L/d. By the endof this phase, the COD to NOx removed ratios of theAnoxic #1 reactors of the systems were about 5:1, while theAnoxic #2 ratios were about 5:1 for System #1 and about10:1 for System #2. The higher Anoxic #2 ratio of System#2 was a direct result of decreased denitrification rateswithin that reactor.

pH and free ammonia levelsThe progressive increase in denitrification, within both

systems, resulted in higher anoxic pH levels, an expected re-sult (Fig. 6). Anoxic pH increases, from pH levels of about 8up to as high as 9.8 in the Anoxic #2 reactor of System #1,

resulted in increased free ammonia levels within allanoxic reactors (Benefield et al. 1982; EPA 1993). Thefree ammonia concentrations within the reactors were alsocontrolled by the reactor total ammonia concentrations atany given time. Even though Anoxic #1 pH levels were al-ways lower than the Anoxic #2 pH levels because of dilutionby recycle flows, the Anoxic #1 free ammonia concentra-tions (e.g., about 40 mg N/L, at the end of this phase) werealways higher than those in Anoxic #2 (Fig. 6). This was re-flective of the fact that the total ammonia concentrationswithin the first anoxic reactor were always higher than thetotal ammonia concentrations within any other reactor of thesystem. However, the higher Anoxic #2 pH level, i.e., pHsof over 9.6, compared to the maximum recommended pH =8 for optimal denitrification (EPA 1993; Metcalf & Eddy,Inc. 1991), seemed to have a more harmful influence ondenitrification performance within the respective reactorsthan the elevated Anoxic #1 free ammonia levels (e.g.,percentage denitrification of 93% and 76%, respectively, forSystem #1 and of 52% and 43%, respectively, for System#2, at the end of this phase (where reactor %denitrification = [(NOx in NOx out) / NOx in] 100)).

The continuous recycle of the nitrifiers through the ele-vated free ammonia levels of the Anoxic #1 reactor deter-mined the inhibition of Nitrobacter, the nitrite oxidizers.Consequently, progressive nitrite accumulation (within bothsystems) occurred after each incremental increase in the in-fluent ammonia concentration (Fig. 7). However, thenitrifiers did acclimatize to some extent and, by the end of thephase, were very well able to handle the Anoxic

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Fig. 5. System #1: anoxic denitrification rate and methanol loading.

#1 40 mg N/L of free ammonia, confirming the work ofTurk and Mavinic (1989). Nitrification processes within theAerobic #1 reactors of both systems were not as affected byNitrobacters inhibition to free ammonia. Percentage nitri-fication of the respective reactors eventually became 100%(where reactor % nitrification = [(NOx out NOx in)/totalammonia in] 100). Furthermore, the occasional ammoniarelease due to cell lysis was immediately nitrified, resultingin percentage nitrification of over 100%. The NO2/NOx ra-tios eventually stabilized to a value of approximately 0.60.8 in both systems. By the end of this phase (i.e., day 120),System #1 had Aerobic #1 nitrite levels of about 250300 mg N/L, with corresponding NOx levels of 400 mg N/L;System #2 had Aerobic #1 nitrite levels of about 400450 mgN/L, with corresponding NOx levels of about 600 mg N/L.With the exception of the nitrification inhibition period(when increases in methanol loadings were made withoutconsidering the possible nitrite accumulation within the sys-tems and concurrent reduced methanol requirements), theoverall performance (nitrification/denitrification) of eithersystem did not seem to be greatly affected by nitrite accu-mulation. On the contrary, high NO2/NOx ratios determinedlower CH3OH:NOx removed, i.e., ratios of about 5 to 1; thisresulted in decreased methanol requirements and additionsinto the anoxic reactors of both systems, again confirmingprevious work (Turk and Mavinic 1989).

Volatile suspended solids levelsReactor volatile suspended solids (VSS) concentrations

were influenced by several variables, including influent am-

monia concentration, methanol loading rate, actual hydraulicretention time, internal and external recycle flows, and per-formance of the reactor itself.

Reactor VSS concentrations began to steadily increase inresponse to the first four influent ammonia concentration in-creases. During the initial nitrification inhibition period,VSS levels became more erratic, with peaks in the Aerobic#2 VSS levels of about 5000 mg/L for System #1 and5100 mg/L for System #2. These high reactor VSS levels re-sulted from increased ammonia concentrations reaching theAerobic #2 reactors, due to nitrification inhibition within thefirst aerobic reactor.

During the second run at incrementally increasing influentammonia concentrations, System #1 VSS concentrations in-creased to about 4800 and 3600 mg/L, respectively, in theAnoxic #1 and #2 reactors and to about 4000 and5200 mg/L, respectively, in the Aerobic #1 and #2 reactors.System #2 VSS levels increased to approximately 4000 and3800 mg/L, respectively, in the Anoxic #1 and #2 units andto 3200 and 3600 mg/L, respectively, in the Aerobic #1 and#2 units. Enhanced cell synthesis within the Anoxic #1 reac-tors might have been one reason for these VSS levels beinghigher than Anoxic #2 VSS levels. Higher VSS concentra-tions in Aerobic #2 than in Aerobic #1 were possibly the re-sult of the longer hydraulic retention time (i.e., AHRT of6 h). Differences of up to 1600 mg/L of VSS between thetwo systems were common, most likely due to the slightlydifferent operating conditions.

The VSS to TSS ratios were, for the most part, approxi-mately 0.80 in System #1 reactors and about 0.76 in System

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Ilies and Mavinic 93

Fig. 6. System #1: anoxic free ammonia and pH.

#2 reactors. Occasional lower ratios occurred mainly duringthe most unstable aspects of this phase. The final effluentVSS concentration fluctuated between about 10 and 50 mg/L,during satisfactory performance of system, with peaks of ashigh as 328 mg/L, during nitrification inhibition.

Overall system performanceBy day 120 of this phase, both System #1 and #2 were

generating ammonia free effluents, but with average NOxconcentrations of about 80 and 250 mg N/L, respectively,based on an influent ammonia concentration of over2200 mg N/L (Fig. 8). Except for the nitrification inhibitionperiod noted earlier, system ammonia removals were consis-tently around 100%, while total inorganic nitrogen removalswere erratic because of surges in NOx removal rates (Fig. 9).However, on the last day of this phase (day 120), the twosystems had percentage total inorganic nitrogen removals of95% and 82%, respectively. The higher removal in Sys-tem #1 compared to System #2 was the result of better over-all denitrification performance.

The overall performance of this particular treatment sys-tem was influenced, at any one time, by the nitrogen conver-sion capability of each sequenced reactor. About 1020% ofthe incoming ammonia was removed in the Anoxic #1 reac-tor, mainly through bacterial assimilation. However, most ofthe influent ammonia was removed in the Aerobic #1 reactorby nitrification and some bacterial assimilation, rather thanby any air stripping, since the reactor free ammonia levelswere virtually zero. Aerobic #1 ammonia removals reachedvalues of over 90%, even though some of the incoming am-monia escaped treatment and was removed by bacterial as-

similation and nitrification in the subsequent reactors. Ni-trites and nitrates were converted to nitrogen gas mainly inthe anoxic reactors of each system. On the last day of thisphase, Anoxic #1 percentage denitrification was 93% forSystem #1 and 52% for System #2, while Anoxic #2 per-centages were 76% and 43%, respectively. These values in-dicate that the hydraulic configuration (employing therecycle rates outlined) utilized in System #1 was adequatefor the treatment of this strength of landfill leachate. How-ever, relatively high effluent NOx levels point to the need forfurther hydraulic manipulation.

Summary and conclusions

Both 4-stage Bardenpho treatment systems initially expe-rienced nitrification inhibition at incoming ammonia-N lev-els of about 1300 mg N/L and under methanol loading ratesthat were increased concomitantly, to match expected aero-bic NOx production by using an initial CH3OH to NOx ratioof 20:1. After allowing sufficient extra time, about 1014 days instead of 7, for bacterial adaptation to each incre-mental ammonia-N increase, and subsequently using lowerCH3OH:NOx (about 5:1), both operating systems generatedammonia-N free effluents after each loading rate increase.Within 88 days from the start of the second run at increasinginfluent ammonia concentration, both treatment systems pro-duced ammonia-N free effluents with average NOx concen-trations of about 80 (System #1) and 250 mg N/L (System#2), respectively, when treating simulated landfill leachatewith a concentration of over 2200 mg N/L of ammonia-N.Although changes in methanol loading rate played an impor-

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Fig. 7. System #1: anoxic and aerobic nitrite [NO2].

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Ilies and Mavinic 95

Fig. 8. System #1: effluent total ammonia and NOx.

Fig. 9. System #1: system % ammonia and % total inorganic nitrogen removal.

tant role in the performances of the two systems, severalother concomitant and (or) consequent changes in operatingparameters may have also contributed to process instabilityat any time.

Initial nitrification inhibition was postulated to be a resultof several influential factors that, together, generated imme-diate and undesirable responses in bacterial performance.One factor was an increased ammonia-N concentrationreaching the reactors. Increased ammonia-N concentration inthe Aerobic #1 reactor stimulated nitrification and, conse-quently, triggered increased sodium bicarbonate flows;hence resulting in up to 3% decreases in reactor hydraulicretention time. A decrease in aerobic hydraulic retentiontime may have temporarily hindered the growth rate ofnitrifiers. In addition, increased Anoxic #1 free ammonialevels, associated with increased pH levels, had further sup-pressed the downstream nitrifying bacterial population by re-cycling the nitrifiers through the elevated free ammoniaconcentrations of the Anoxic #1 reactor. This led to the ex-posure of the bacterial population to nitrite accumulation.

Another influential factor in this study was methanol load-ing rate increases, initiated prior to allowing sufficient timefor the establishment of a sustained NOx production, follow-ing each ammonia-N increase. In addition, the methanolloading adjustments were made without considering theprobable nitrite accumulation, i.e., lower organic carbon re-quirements for nitrite reduction. Therefore, the excess meth-anol in the Anoxic #1 reactor carried over to the Aerobic #1reactor, leading to enhanced aerobic heterotrophic growthand further nitrifier inhibition. Both the timing and theamount of ammonia-N and methanol loading rate increases,with respect to each other, and to the corresponding previousloading rate increase, were decisive and critical in this treat-ment process train.

Based on the observed behaviour of the bacterial popula-tions of the 4-stage Bardenpho systems, treating methano-genic-state landfill leachate, it can be concluded thatchanges in ammonia and methanol loading rates generatechain-reaction responses within this type of system. A cor-relation between dosing level and timing of methanol addi-tion, in response to changes in the influent ammoniaconcentration, was evident; it also requires proper under-standing and control, since it may directly or indirectly af-fect the system stability, at any time. Further research,targeted to investigate incipient operating stages of this par-ticular type of system, treating very high ammonia liquidwastes, could determine the critical points for nitrificationand denitrification inhibition and suggest a means of pre-venting and (or) minimizing process failure. This should beundertaken prior to any scale-up attempts.

Acknowledgments

The authors would like to acknowledge the excellent tech-nical assistance provided by the staff of the EnvironmentalEngineering Laboratory, Department of Civil Engineering,The University of British Columbia. This research was sup-ported by a Natural Sciences and Engineering ResearchCouncil (NSERC) of Canada grant to the second author.

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