simultaneous nitrification denitrification in an aerobic mbr

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SIMULTANEOUS NITRIFICATION DENITRIFICATION IN AN AEROBIC MBR TREATING HIGH STRENGTH PET FOOD WASTEWATER Chandresh Acharya1, G. Nakhla2, A. Bassi2 1 Stantec Consulting, 800-171 Queens Avenue, London, ON N6A 5J7, Canada 2 The University of Western Ontario, Dept. Of Chemical & Biochemical Engg., London, ON N6A 5B9, Canada

ABSTRACT This paper discusses nitrogen removal mechanisms in an aerobic membrane bioreactor (MBR) treating high strength pet food wastewater characterized by oil & grease, COD, BOD5, TSS, TKN, NH4-N concentrations of 2800 mg/L, 25000 mg/L, 10000 mg/L, 4500 mg/L, 1650 mg/L and 1300 mg/L respectively. An MBR operating at SRT >25 days achieved mixed liquor total suspended solids (MLTSS) concentrations >30000 mg/L with a mean particle size of 50 µm. Potentials for various nitrogen removal mechanisms in the MBR were evaluated. Even at DO of 1.3 mg/L and oxygen reduction potential (ORP) > +90, the MBR removed > 200 mg/L of influent nitrogen by simultaneous nitrification denitrification (SND) through nitrite pathway. Contrary to the general diffusion limitation hypothesis, this paper infers the nitrification and denitrifications to take place in separate sludge particles. Mass balance revealed cell synthesis, stripping and SND to remove 14%, 17% and 21% of the influent nitrogen respectively. KEYWORDS membrane bioreactor; simultaneous nitrification denitrification, high strength wastewater; diffusion limitations INTRODUCTION Compared to conventional sludge systems, MBRs are known to operate at longer sludge residence times (SRT), low food to microorganism ratios (F/M) and reduced hydraulic residence times (HRT). Longer SRTs and higher biomass concentrations tend to result in larger sludge flocs in aeration tanks [1]. Oxygen diffusion limitations are hypothesized to form anoxic zones in the core of flocs larger than 40 µm [2]. Despite the fact that DO>1 mg/L, and ORP>+50 are known to completely inhibit denitrification, Bertanza [3] achieved SND even at steady ORP of +150 to 200 mv attributing to diffusion limitation mechanisms, and indicated possibilities of nitrite pathway of nitrogen removal. While SND has been widely explored in the activated sludge processes and MBRs, SND in a highly loaded MBRs (COD loadings of 11.5 kg COD/m3.d) operating at DO>1 mg/L has not been demonstrated. This work focused on identification and quantification of nitrogen removal mechanisms in an MBR treating high strength wastewater from a pet food industry located in southern Ontario. The wastewater is characterized by oil & grease, COD, BOD5, TSS, TKN, NH4-N concentrations of 2800 mg/L, 25000 mg/L, 10000 mg/L, 4500 mg/L, 1650 mg/L and 1300 mg/L respectively (Table 1). This paper also presents a modified diffusion limitation concept responsible for SND in the systems treating high strength wastewaters.

Copyright ©2006 Water Environment Foundation. All Rights Reserved

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Table 1- Raw Wastewater and DAF effluent Characteristics Raw wastewater

DAF Effluent

Parameters

TSS (mg/l) VSS (mg/l) TCOD (mg/l) SCOD (mg/l) TBOD5 (mg/l) Ammonia- N (mg/l) Phosphate (mg/l) O & G (mg/l)

Range

Average

Range

Average

17311 - 61700 15180 - 59800 74925 - 154100 13125 - 18450 80000 600-2000 500-830 20000-38800

36857 34383 96660 16757 80000 1500 665 30000

1150-12866 1100-10966 10025- 50500 6860-19980 8000-16000 640 - 2100 340-1059 600 -6000

2510 2290 23400 18870 10500 1360 650 2890

EXPERIMENTAL CONDITIONS Laboratory-scale MBRs were operated under varying HRT and temperature conditions. The wastewater treated in the experiment was generated from a rendering facility in St. Mary’s, Ontario, producing intermediates for pet food from byproducts of the meat processing industry. The wastewater is characterized by particularly high concentrations of O&G which would be detrimental to any biological treatment system. The wastewater generated at this facility undergoes physical separation of solids and O&G from the liquid using a dissolved air flotation (DAF) chamber. The high variability in the wastewater generated is depicted in Table 1. The variation in wastewater characteristics is a function of various operations undergone at the facility. In the experiments conducted in this study the DAF effluent was employed. EXPERIMENTAL SET-UP The experimental set up for the treatment of DAF effluent consisted of a 20 L storage tank followed by a 25 L activated sludge reactor (Figure 1). All tanks were made from stainless steel, provided with a glass window, and scaled to monitor the level. The system was initially inoculated with sludge taken from a local municipal treatment plant. The reactor was aerated using compressed air through air diffusers. Two Zenon membrane modules (ZW-1, pore opening of 0.04 μm, surface area of 0.047m2) were employed in the reactor to retain solids in the reactor and to draw permeate only as effluent. The storage tank was filled with DAF effluent every 2 days. The system was operated over a span of 490 days. The system was operated at different HRTs from 6.3 to 2.5 days at 18oC. This study focuses only on 2.5 days HRT operation over a period of 50 days from day 440 to day 490. An SRT of 25 days was maintained by direct sludge wasting from the reactor. A dissolved oxygen (DO) concentration of 1.3 mg/L was maintained in the MBR, while ORP during steady state remained around + 90 mv. Throughout the operation, pH in the MBR remained 8.2. Membranes trans-membrane pressure (TMP) was observed and membranes were cleaned twice a week by water flushing and soaking in 200-ppm solution of Sodium Hypochlorite (NaOCl).


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DAF Pretreated Wastewater

Membrane Permeate / Final Effluent

Storage Tank

Bioreactor

Ultra filtration Membrane Waste Sludge

Air

Figure 1-Experimental set up: submerged membrane bioreactor system ANALYTICAL METHODS Samples of the influent, membrane permeate (effluent) and mixed liquor were collected (1 to 2 samples per week) and analyzed for total suspended solids (TSS), volatile suspended solids (VSS) or mixed liquor volatile suspended solids (MLVSS), BOD5 and alkalinity using standard methods [5]. COD, total kjeldal nitrogen (TKN) and ammonia-N were analyzed using HACH Odyssey Analyzer and COD heating reactor with standard HACH testing kit. HPLC (Waters 515 HPLC Pump, Waters 432 Conductivity detector) was employed to determine nitrite–N, nitrate-N and phosphorous. All ammonia, nitrate, and nitrite results reported here are as nitrogen. For analysis of soluble parameters, samples were filtered through 0.45-μm filter (Wheaton). Oil & Grease (O&G) analyses were performed by commercial laboratories. Dissolved Oxygen (DO) was measured with a portable DO meter (YSI Dissolved Oxygen Meter Model 50). An extensive set of analysis including reactor gas space ammonia measurement was performed on daily basis for a period equivalent to 4 HRTs (10 days) during the 2.5 days HRT run. MBR was covered appropriately to aid gas space ammonia measurements. RESULTS AND DISCUSSION MBR achieved excellent contaminant removal (Table 2) at loading rates>11.0 kgCOD/m3.d, which is around 5 times higher than conventional activated sludge systems. Distinctive performance of the MBR can be attributed to the robust biological system retained by the membrane filtration system. Following sections of the result and discussion focuses on evaluation of such biological systems and the MBR performance.


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Table 2- MBR performance at 2.5 days HRT DAF effluent

Parameters TSS (mg/L) VSS (mg/L) TCOD (mg/L) SCOD (mg/L) TBOD5 (mg/L) SBOD5 (mg/L) NH3–N (mg/L) NO3-N(mg/L) NO2-N(mg/L) TKN (mg/L) Alkalinity(mg/L) O & G (mg/L) MLTSS (mg/L) MLVSS (mg/L)

2,510 ± 4,000 2,290 ± 3,600 23,400 ± 10,000 18,870 ± 2,200 10,500± 2,200 7,980 ± 1,300 1,360 ± 200 9±3 0 1800 ± 400 4,150 ± 600 2,890 ± 3,000 -

MBR Performance (HRT: 2.5 d, SRT: 25 d) Effluent % Removal concentrations 0 0 580 ± 200 97 15 ±4 99.8 780 ± 80 42 8±6 11 0 870 ± 100 52 2,070 ± 400 50.1 15 ± 4 99.5 32,800 ± 4,200 26,300 ± 3,600

MBR MIXED LIQUOR At steady state of all experimented HRT, SRT combinations (only 2.5 days HRT, 25 days SRT is discussed here), the system mixed liquor total suspended solids (MLTSS) stabilized such that F/M remained 0.4 gCOD/ gVSS.d almost all the time. It should be noted that, F/M ratios of 0.5 gCOD/ gVSS.d and 0.1 gCOD/ gVSS.d are typical to non-nitrifying and nitrifying activated sludge systems [6]. With complete retention of the solids, the system was operated at MLTSS of 32,000 mg/L, and SRT of 25 days. Endeavors to further increase the sludge edge reduced the membrane fluxes to an inoperably low level. Contrary to the submerged membrane operability limitations found in this study, Scholz and Fuchs [7] operated a cross flow membrane at 48,000 g/L in oily wastewater treatment. Membrane manufacturers generally relate the reduced membrane fluxes to a lower filterability of the sludges. Time to filter (TTF) test was conducted to study the sludge filterability. From TTF, the MBR mixed liquor was found to have around 90% lower filterability compared to that of typical municipal sludges. Due to the lower filterability of the MBR sludge and operations at higher MLTSS, membrane fluxes were only 30% of 20 L/m2.hr (LMH) specified by the membrane manufacturer. Higher sludge concentrations and longer sludge are known to affect sludge floc sizes [1] resulting in complex contaminant removal mechanisms [1, 2, 3]. Figure 2 shows results of the particle size distribution analysis conducted over a period of 15 days. The mean particle size found to be 50 μm, which is higher than 14.8 μm at 5 days SRT and to 30.6 μm at 40 days SRT observed by Huang et al. [1].


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MBR PERFORMANCE SOLIDS REMOVAL With inherent advantage of 0.04 μm pore size membranes used, the MBR completely retained all the solids within bioreactor, and generated solids free effluent (Table 2). As anticipated, the ultrafiltration effluent contained soluble contaminants only, and no organic nitrogen or phosphorous. Day 37

Day 39

Day 47

Day 48

Day 40

Day 44

Volume % .

8 6 4 2 0 0

1

6

23

91

363

1459

Particle Sizes (micro meter) Figure 2- Particle size distribution COD & BOD REMOVAL Despite markedly high variability in influent (DAF effluent), the system COD and BOD removal efficiency (Figure 3) mainly remained around 97% and 99.8% respectively. Where high O&G concentrations pose a challenge to any biological system, O&G removal efficiency of more than 99% corroborates the high contaminant removal capability of the system, which is similar to that reported in literature treating fuel oil and lubricating oil at loading rate of 0.26–0.54 g hydrocarbons g-1MLVSS d-1 [7]. Consistent contaminant removal at COD loadings as high as 11.5 kg COD/m3.d is noteworthy. Membrane Contribution to COD & BOD Removal Around 7 to 37 % of the soluble COD was removed across membrane due to retention of particles larger than the membrane pore size (not shown here). This finding is similar to that of Huang et al. [1], who observed additional 10 to 20% COD removal across the membranes. AMMONIA REMOVAL As indicated in Table 2, around 42% ammonia and 65.2 % TKN was removed across the MBR. Variations in the effluent ammonia concentrations followed those in the influent ammonia (Figure 4), i.e. the ammonia removal is proportional to the influent ammonia concentrations. Considering the facts that the variations in influent ammonia (Figure 4) closely aligned with the


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influent COD variations (Figure 3), and effluent COD remained consistently 700 mg/L in the reactor, and larger mean particle sizes, free ammonia stripping, chemical precipitation, and losses by SND were identified as other potential nitrogen removal mechanisms. STRIPPING At the bioreactor temperature of 18 oC and pH of 8.2, around 5% of the total measured ammonia will be unionized ammonia [8], i.e. the MBR mixed liquor will have around 40 mg/L of free or unionized ammonia. To determine stripping of the free ammonia in an aeration tank with diffused aerators, the general stripping model for diffused aeration systems [6] was used. Using the gas flow rate Qg equivalent to the total air supplied to the bioreactor diffusers and that used for the membrane aeration (measured as 170 L/m), and unit less Henry’s constant (Hu) of 5.12 x10-4 for ammonia [6], the stripping model can be reduce to equation 2.


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N stripped = Qg * Hu * CL*

(2)

Where, Nstripped is ammonia nitrogen removal by stripping and CL* is free ammonia concentration in the reactor. Using equation 2, nitrogen removal by stripping is found to be around 320 mg/L or 17% of the influent TKN. Influent

Effluent

Ammonia (mg/L)

2000 1600 1200 800 400 0 0

10

20

30

40

50

Time (days)

Figure 4 Ammonia variations across MBR NITROGEN REMOVAL BY PRECIPITATION pH higher than 8 is favorable for struvite formation [9]. The systems struvite formation potential was verified experimentally. Two aerated batch tests were conducted on the raw wastewater with magnesium addition at a moalr Mg:P ratio of 1:1, and without Mg at pH of 8.5 and 10. Samples without Mg addition exhibited no reduction in soluble phosphorous, which discounted the possibilities of struviation or nitrogen removal by precipitation in the MBR. NITROGEN REMOVAL BY SND In order to verify possibilities of SND, the systems nitrification and denitrification potentials were assessed. A batch test conducted using the MBR mixed liquor. The mixed liquor was isolated and aerated for 2 days to ensure complete organic degradation, which revealed nitrification rates of 8 to 10 mg/L.hr (Figure 5)i.e. 7.3 to 8.3 mg N/gVSS.day or 6000 mg/day, which is equivalent to 25% of the influent TKN. It should be noted that all nitrified nitrogen was converted into nitrites. Conversion into nitrite is conceivable, as the free ammonia concentration of 40 mg/L in the MBR is sufficient to inhibit and subsequently eliminate the presence of nitrobacter species. The influent contained around 7600 mg/L of readily biodegradable VFAs, which is sufficient to denitrify almost all nitrified nitrogen. Pochana and Keller [2] observed occurrence of SND in an aerobic reactor operating at DO of 0.8 mg/L and particles sizes of > 40 μm. SND in such larger particle systems was attributed to diffusion limitations mechanism. As indicated in the earlier part of the discussion, the MBR mixed liquor volume weighted particle size was between 50 to 88 μm, pointing to the potential for simultaneous nitrification denitrification.


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Concentrations (mg/L)

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nitrite

y = -10.15x + 230.1 R2 = 0.9941

250

ammonia

200 150 100

y = 8.74x + 18.72 R2 = 0.75

50 0 0

1

2

Time (hrs)

3

4

5

Figure 5: MBR maximum nitrification rate DIFFUSION LIMITATION HYPOTHESIS Pollutants concentration gradients across the biological floc, hypothesized by other researchers [10], are shown in Figure 6(a). DO diffusion limitations in a large sized particle form aerobic and anoxic zones in the periphery and the cores respectively. With such hypothesis, it was believed that the ammonia will be converted in to nitrate/nitrite in the aerobic zone where concentration of nitrate/nitrite will be highest in the reactor system. Such nitrates or nitrites will than be denitrified in the core of the same particle. Apart from DO concentration requirements, impacts of readily biodegradable COD/BOD are also different in nitrification and denitrification processes. Focusing on these impacts, the single particle SND hypothesis can be reviewed and modified as follows.

Concentration BOD

Anoxic Zone

Ammonia, DO

Nitrifying Aerobic Zone

Nitrites, Nitrates Radius

Figure 6 (a) Concentration profile in general diffusion limitation hypothesis


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Girardi [11] regards high concentration of readily biodegradable BOD to be inhibitory for nitrification process. On other hand, 3 to 5 mg/L of BOD would be required for denitrification of each mg/L of nitrates/nitrites, i.e. in order for denitrification to take place, sufficient BOD needs to diffused from the bulk phase to the core of the particle through the particle aerobic zone. Specifically in the high strength wastewaters, such BOD diffusion through particle periphery could inhibit nitrification, which would then prevent the SND. Based on these arguments, the authors believe that the nitrification and denitrification would take place in two different type of particle, i.e. one with an aerobic nitrifying zone and the other with aerobic heterotrophic zone. In such a case concentration profile in the denitrifying particles would change as shown in the Figure 6(b). Due to lake of nitrification in the denitrifying particle, nitrite/nitrate concentration would be maximum in the bulk phase which is contrary to that shown in Figure 6(a).

Concentration BOD

Anoxic Zone

Dissolved Oxygen Non- Nitrifying Aerobic Zone

Nitrites, Nitrates Ammonia Radius

Figure 6 (b) Concentration profile in modified diffusion limitation hypothesis QUANTIFICATION OF SND Since nitrogen gas is the major constituent of the air leaving the bioreactor, nitrogen gas generated by SND can not be measured distinctly. Hence, the conventional approach of nitrogen balance across the MBR was used to determine actual nitrogen losses. Nitrogen entering the MBR is transformed into various forms through various physical and biochemical processes within bioreactor as shown in Figure 7. Considering constituents entering and leaving the system, with all terms measured experimentally in Equation 3, NH4 SND is equivalent to unaccountable nitrogen losses or nitrogen removed by SND. NH4 SND = TKNin – TKNout - Nstripped - Ncell

(3)

Where, NH4 SND is ammonia nitrogen removal through SND, TKNin and TKNout are TKN in MBR influent and effluent respectively, and all other terms are as described earlier.


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MBR Cell synthesis NH3-N

NH4 +-N NH4 +-N

p ON

s ON Free NH3 -N

s ON

Stripping

NH3 in gas

N in liquid leaving the system

NH3-N + s ON

SND

s ON N from cell decay

N in biomass

Nitrification

Insoluble N2 gas NOx

Figure 7: Nitrogen transformation across MBR Ammonia in the gas leaving the system was measured by Dragger ammonia detection tubes. Accuracy of the ammonia measurement in the gas phase was insured by verification of the instrument and the method used. Nstripped was found experimentally using ammonia concentration in the gas phase and air flow rate to the system. Ncell values were found using Equation1. The mass balance in equation 3 revealed that, the SND removed 21 % of influent nitrogen, and proved to be one of the prominent nitrogen removal mechanisms. Cell synthesis and stripping accounted for 31 and 13% of influent TKN respectively. 3780 mg nitrogen removed per day by SND was equivalent to around 63% of the maximum nitrification capacity of the 1st stage. SUMMARY AND CONCLUSIONS • • • • •

The MBR achieved excellent COD, BOD5 and TSS removal of 97%, 99.8%, and 100%. The mean particle size was found to be 50 μm in the MBR operating at MLTSS of 32,000 mg/L and SRT of 25 days. The MBR sludge filterability found to be around 10 times lower than the typical municipal sludges. At MLTSS of 30,000 mg/L, membrane fluxes were only 30% of that specified by the membrane manufacturers. MLTSS exceeding 50,000 mg/L almost completely stopped the permeate generation. At steady state. SND removed around 21% of total influent nitrogen, with additional 14 % removed by cell synthesis and 17% by stripping. Experimental verifications proved that the SND occur through nitriting pathway. A modified diffusion limitation hypothesis was presented. Nitrification and denitrification are conceptualized to occur in separate sludge particles considering different impacts of readily biodegradable BOD on both processes.

ACKNOWLEDGEMENT Financial support for this study provided by CRES Tech (CRESTech Project # WR02WST93) is gratefully appreciated.


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[2]

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[3]

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[8]

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Stratful I.; Scrimshaw M.; Lester J. (2001), Conditions influencing the precipitation of magnesium ammonium phosphate, Wat. Res. Vol. 35, no.17, 4191-4199.

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