pH-gradient real-time aeration control for nitritation community selection in a non-porous hollow fiber membrane biofilm reactor (MBfR) with dilute wastewater

Chemosphere 90 (2013) 2320–2325

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Technical Note

pH-gradient real-time aeration control for nitritation community selection in a non-porous hollow fiber membrane biofilm reactor (MBfR) with dilute wastewater Po-Heng Lee a,b,c,⇑, Samuel F. Cotter b, Silvia C. Reyes Prieri c, Dinu Attalage c, Shihwu Sung b,⇑ a b c

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Department of Civil, Construction, and Environmental Engineering, Iowa State University, 394 Town Engineering Building, Ames, IA 50010, USA Department of Environmental Engineering, Inha University, Namgu, Yonghhyun dong 253, Incheon, Republic of Korea

h i g h l i g h t s " We developed a O2-based membrane biofilm reactor for nitritation from dilute waster. " A precise O2 supply is provided with the non-porous membrane and aeration control. 

þ

" Over 88% NO2 accumulation efficiencies by providing 1.5 mol O2/mol NH4 –N fed. " This O2 supply rate was confirmed by mass balance and rate performance analyses.

a r t i c l e

i n f o

Article history: Received 28 July 2012 Received in revised form 19 October 2012 Accepted 22 October 2012 Available online 24 November 2012 Keywords: Nitritation Non-porous hollow fiber membrane Anammox Membrane biofilm reactor (MBfR) Real-time aeration control

a b s t r a c t Nitritation (ammonium to nitrite) as a pre-treatment of Anammox (anaerobic ammonium oxidation) is a key step for an energy-efficient nitrogen-removal alternative from dilute wastewaters, e.g. anaerobicallytreated sewage, with which limited study has achieved sustainable nitritation at ambient temperature and short hydraulic retention times. To this end, pH-gradient real-time aeration control in an oxygenbased membrane biofilm reactor was observed at 20 °C in the sequencing batch mode. An optimum oxygen supply via diffusion for ammonium-oxidizing bacteria (AOB) was established, but nitrite-oxidizing bacteria (NOB) could be inhibited. The system achieved nitrite accumulation efficiencies varying from 88% to 94% with the aeration control. Mass balance and rate performance analyses indicate that this aeration control is able to supply an oxygen rate of 1.5 mol O2 mol1 ammonium fed, the benchmark oxygenation rate based on stoichiometry for nitritation community selection. Microbial analyses confirmed AOB prevalence with NOB inhibition under this aeration control. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Autotrophic nitrogen removal, coupling nitritation (ammonium to nitrite) with anaerobic ammonium oxidation (Anammox), offers a cost-effective nitrogen-removal alternative. Its advantages include: (1) 60% less aerating-energy utilization and 90% sludge reduction compared to the conventional nitrogen removal process, nitrification followed by denitrification, (2) no organic source needed for denitrification, organics can instead be converted into energy in the form of methane via anaerobic treatment, and (3)

⇑ Corresponding authors. Address: Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Tel.: +852 2766 6067; fax: +852 2334 6389 (P.-H. Lee), tel.: +1 515 294 3896; fax: +1 515 294 8216 (S. Sung). E-mail addresses: [email protected] (P.-H. Lee), [email protected] (S. Sung). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.10.061

reduction of N2O emission, a powerful green house gas (van Dongen et al., 2001; Fux and Siegrist, 2004; Kampschreur et al., 2009; McCarty et al., 2011). Based on these advantages, there has been increasing interest in application of the process from concentrated wastewaters as well as extending its application to dilute wastewaters with ambient temperature (Kartal et al., 2010; Ma et al., 2011; De Clippeleir et al., 2011; Winkler et al., 2012; Hendrickx et al., 2012), since efficient anaerobic treatment of organics from low-strength wastewaters, e.g. sewage, for biogas recovery has been demonstrated (Kim et al., 2011; McCarty et al., 2011; Stuckey, 2012). Studies on dilute wastewater autotrophic nitrogen removal have all indicated the significance of stable nitritation performance at ambient temperature. Previously, stable nitritation has been maintained by raised temperature, high free ammonia concentration, high free nitrous acid concentration, and high pH (Villaverde et al., 1997; van

P.-H. Lee et al. / Chemosphere 90 (2013) 2320–2325

Dongen et al., 2001; Vadivelu et al., 2007; Park and Bae, 2009), demonstrating these control approaches are unsuitable for dilute wastewaters with ambient temperature, such as anaerobicallytreated sewage. Nevertheless, direct limited dissolved oxygen (DO) control or indirect limited DO control via regulation of the aeration duration according to the pH profile in the sequencing batch reactor (SBR) operation of nitritation for denitrification (shortcut path of the conventional nitrogen removal process) has been shown to successfully produce nitrite from domestic wastewaters (Yang et al., 2007; Guo et al., 2009). However, these studies treating the presence of moderate bCOD (biodegradable chemical oxygen demand) levels from sewage-like synthetic or domestic wastewaters have facilitated limited DO dosages for the selective inhibition of nitrite oxidizing bacteria (NOB) and eliminated the sludge bulking problem of low DO level operation due to the involvement of heterotrophically aerobic and anoxic activities. As for the nitritation of efficient anaerobically-treated sewage with little biodegradable organics, limited DO supply is very difficult when air spargers are used. When insufficient oxygen is supplied, incomplete ammonium oxidation would occur. When over aeration occurs, then NOB may dominate, causing complete oxidization of ammonium to nitrate (Kwak et al., 2012). Due to this challenging aeration control issue, Zhang et al. (2012) reported laboratory results of unstable nitritation performance with raw sewage. Besides, the sludge bulking problem resulting from limited DO supply with dilute wastewater causes biomass washout (Blackburne et al., 2008). To the best of our knowledge, no work has demonstrated sustainable nitrite accumulation with dilute ammonium-containing organic-free wastewater at ambient temperature. In this study, we developed a non-porous membrane biofilm reactor (MBfR) integrated with pH-gradient real-time aeration control for nitritation with a synthetic dilute-ammonium organic-free wastewater at 20 °C. The goal of this study was to determine if this aeration control in the MBfR could serve as a pivotal operating parameter for enriching ammonium oxidizing bacteria (AOB) and inhibiting NOB as a pre-treatment step for Anammox from low strength ammonium-containing wastewaters. Moreover, mass balance and performance rate analysis based upon stoichiometry were used to estimate its feasibility of providing the optimum oxygen dosage as a selection pressure for sustainable nitrite accumulation of dilute ammonium-containing organic-free wastewaters. 2. Materials and methods 2.1. Reactor setup Fig. 1 is a schematic diagram of the MBfR used in this study. It is comprised of an acrylic constructed reservoir and an external non-porous silicone hollow-fiber membrane (HFM) module with

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effective volumes of 700 and 500 mL, respectively. The total effective volume was 0.97 L. The module consisted of sixty 122 cm-long HFMs (Gilbrane silicone HFM, Fuji System Corporation, Japan) with the outer diameter and wall thickness of 2.0 and 0.25 mm, respectively, which provided a total surface area of 4597 cm2. Each end (the oxygen supply end and the sealed end) of the HFM bundle was connected with a pressure gauge (DPG5600B, Omega Engineering, Connecticut, OH, USA) to measure the oxygen pressure drop by the HFM bundle so that the oxygen supply rate could be estimated. Pure oxygen was supplied to the inside of the HFMs and diffused through the HFMs to the liquid solution in the HFM module. A recirculation pump (Masterflex I/P 7591-00, Thermo Fisher Scientific, Waltham, MA, USA) was operated at 50 mL min1 for liquid recirculation between the reservoir and the HFM module. A mixer (Eastern Mixer Brand, Clinton, CT, USA) in the reservoir provided a mixing speed of 90 rpm. The influent feed was flushed with nitrogen gas, and the influent tank was sealed to achieve an anaerobic starting condition. DO and pH probes (Mettler Toledo, Columbus, OH, USA) connected with a real-time programming aeration duration controller using LabVIEW 8.5.1, National Instruments, USA software were installed at the half-way point in the reservoir. The reactor was operated at 20 °C. 2.2. Inoculum and synthetic wastewater The MBfR was inoculated with activated sludge (3900 mg L1 mixed liquor suspended solids) from the wastewater treatment plant in Boone, Iowa. The synthetic wastewater used contained (in g L1) 0.28 NH4HCO3, 0.5 KHCO3, 0.025 KH2PO4, 0.3 CaCl22H2O, 0.2 MgSO47H2O, and 1 mL each of trace element solutions I and II each liter of tap water (van de Graaf et al., 1995). 2.3. pH-gradient real-time aeration control and operational strategy The system was operated in the SBR mode. Each cycle consisted of 15 min feeding, aeration reaction, and 15 min decanting. Following successful biofilm cultivation, the reactor was operated without aeration control as a control test. Followed by the operation periods using the aeration control, the operation pattern and aeration programing command are shown in Fig. 2. In these periods, only the duration of aeration reaction was variable: aeration terminated when the pH slope was zero for 10 min. However, the aeration duration would be approximately constant when feeding an unchanging influent ammonium concentration at a constant oxygen supply pressure under quasi-steady-state condition. During the cultivation period, the mixed liquor in the HFM module was not withdrawn from the decanting stage in the SBR operation cycle in order to keep the microbial inoculum for the formation of a nitrifying biofilm on the membrane surface. With

Fig. 1. Schematic diagram of non-porous silicone hollow-fiber membrane biofilm reactor (MBfR).

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Fig. 2. The MBfR operation pattern.

this approach, the attachment of a thicker biofilm on the membrane surface was accomplished within approximately 40 d. Afterwards, the mixed liquor both from the HFM module and the reservoir withdrawn in the decanting stage of the SBR cycle was applied to the entire course of the experiment since the active nitrifying biomass was acclimated and immobilized on the membrane surface. 2.4. Experimental procedure During phase I operation, the reactor was operated without aeration control at a fixed aeration duration of 13.5 h and an oxygen supply pressure of 24.1 kPa as a control test. During phase II, aeration control was performed and maintained with the same oxygen supply pressure as that of phase I to assess its feasibility for sustainable nitritation. After observing stable nitritation performance in the system, oxygen supply pressures of 75.8 and 103.4 kPa were supplied in phases III and IV to investigate the effect of oxygen supply pressure. The oxygen supply pressures applied to the MBfR were selected based on the practical ranges as performed by Sahinkaya et al. (2011). 2.5. Analytical procedures DO and pH values were measured online and recorded. Ammonium was measured by an ion-selective electrode (Thermo Fisher Scientific, Waltham, MA, USA) and total and volatile suspended solids (TSS and VSS) were analyzed according to Standard Methods (APHA, 1996). Nitrite and nitrate were determined by an ionchromatograph (DX 500, Dionex, Sunnyvale, CA, USA) equipped with a column (Allsep Anion IC, Deerfield, IL, USA) and a conductivity detector (CD20, Dionex, Sunnyvale, CA, USA). 2.6. Microbial analysis The total genomic DNA was extracted by using the phenol/chloroform extraction procedure described by Cheng and Jiang (2006). Real-time PCR reactions were performed in optical-grade 96-well PCR plates in an ABI Prism 7900 Sequence Detection System (ABI, Foster City, CA, USA), and the primers used in this study for Nitrosomonas AOB, Nitrobacter NOB, and Nitrospira NOB were according to the previous studies (Mintie et al., 2003; Siripong and Rittmann, 2007; Poly et al., 2008). The DNA concentration was determined using an ND-1000 NanoDrop spectrophotometer (NanoDrop Technologies, Wilmington, DE, USA).

OSR ¼

K DPAt d

ð3Þ

where K is the oxygen permeability constant by pure oxygen via the silicone HFM at 20 °C and 101.3 kPa, 49.0  109 (mg O2 kPa1 cm1 d2, provided by the manufacturer, Fuji System, Japan) in this study provided by the manufacturer; d is the thickness of the HFM (cm); DP is the pressure drop between the opening and sealed end (kPa); A is the surface area of the HFM (cm2); and t is the aeration time (d). The oxygen consumption rate (OCR, mg d1) is estimated in Eq. (4) from the influent and effluent concentrations of nitrite and nitrate based upon the stoichiometric equations of Eqs. (1) and (2):

OCR ¼ Q i

  32 ½1:5ðNO2e  NO2i Þ þ 2ðNO3e  NO3i  14

ð4Þ

 where Qi is the influent flow rate (L d1), NO 2 and NO3 (mg N L1) represent nitrite and nitrate concentrations, respectively; the subscripts i and e represent influent and effluent stream concentrations, respectively; the ratio of 32/14 represents the ratio of the formula weights for O2 and N. The coefficients of 1.5 and 2 represent the mol of oxygen consumption per mol of ammonium to nitrite and per mol of ammonium to nitrate, respectively, determined from Eqs. (1) and (2). Eq. (4) is an empirical equation. The key assumptions are the following: (1) ammonium removal is only either through nitritation or nitratation, and nitrogen transformation to N2O or other more oxidized species are neglected; (2) denitrification and Anammox reactions are not significant, since there are no potential electron donors in the influent and no enriched Anammox seed in the reactor; (3) no other additional oxygen consumption, besides ammonium oxidation to nitrite and/or nitrate, occurs, because no other reduced compounds, such as hydrogen sulfide, were contained in the synthetic wastewater used for this study. Under the assumptions above, a benchmark oxygenation rate (BOR, mg d1) is defined to represent the oxygen requirement associated only for nitritation via AOB as governed by Eq. (1):

BOR ¼ Q i

  32 ½1:5ðNHþ4i Þ 14

ð5Þ

NHþ4 þ 1:5O2 ! NO2 þ H2 O þ 2Hþ

ð1Þ

In this study, of particular interest was to evaluate whether the aeration control could automatically deliver an optimum oxygen amount only for AOB but not for NOB under various reactor operating conditions. For this, a ratio of OSR-to-OCR approaching one with no DO detected in the bulk solution of the reactor would provide evidence of the dominant reactions in the operating process matching well with the assumptions of ammonium conversion either to nitrite or further to nitrate for OCR, Eq. (4), development. In this case, BOR, Eq. (5), derived from these assumptions could be tenable to represent oxygen demand only for AOB reaction; thus, the practicality of using the aeration control as a system operational parameter for nitrite accumulation could be projected by a ratio of OSR-to-BOR close to one.

NO2 þ 0:5O2 ! NO3

ð2Þ

3. Results and discussion

2.7. Mass balance and rate equations The stoichiometric reactions for nitritation by AOB and nitratation (oxidation of nitrite to nitrate) by NOB are presented in the following equations:

These two reactions were used to develop DO and nitrogen transformation mass balance rate equations for the MBfR operating under quasi-steady-state conditions. The oxygen supply rate (OSR, mg d1) by the HFM to the bulk solution of the MBfR was then determined as:

3.1. pH-gradient real-time aeration control effect Fig. 3 shows typical variation of DO, pH, and concentrations of   NHþ 4 ; NO2 ; and NO3 (a) without pH-gradient real-time aeration

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reaches zero, is an essential controlling parameter for nitritation selection in low-strength ammonium organic-free wastewaters, as also reported by Yang et al. (2007) and Guo et al. (2009) treating domestic wastewaters with moderate organic presence.

control at the fixed aeration duration of 13.5 h on day 45 and (b) with pH-gradient real-time aeration control on day 96 in the SBR MBfR. Under both operating conditions, a drop in pH occurred from the oxidation of ammonium into nitrite (nitritation) resulting from acid production. When the pH slope was approximately at zero, the ammonium concentration had been depleted and the nitrite concentration peaked, indicating the end of nitritation. When excess aeration continued, the pH level remained stable, instead of a pH increase caused by CO2 stripping from bicarbonate using air spargers as reported by Yang et al. (2007) and Guo et al. (2009), was maintained due to the completion of nitritation with no acid produced in the bubbleless oxygen supply. Meanwhile, DO increased dramatically because the oxygen demand for nitratation only requires one third of that for nitritation (Fig. 3a). However, without aeration control, high DO appeared with available nitrite after the completion of nitritation causing nitrate to be the dominant effluent compound and suggesting the prevalence of NOB. As for the condition under aeration control (Fig. 3b), DO was not supplied for nitratation after the pH level reached zero. Accordingly, nitrite was the dominant product during the whole SBR cycle under a quasi-steady-state condition, suggesting that NOB was inhibited due to DO deficiency. A summary of the MBfR performance under quasi-steady-state operation (based on the effluent parameters and aeration duration being appropriately stable) is shown in Table 1. The observed average nitrite accumulation efficiencies (NAE) without (phase I) and with aeration control (phase II) were 89 and 2%. Thus, the termination of aeration, when the pH slope

3.2. Oxygen supply pressure effect Table 1 shows the average performance for phases II–IV of the MBfR at various oxygen supply pressures under aeration control. The observed average NAEs in phases II–IV were between 88% and 94% at hydraulic detention times (HRTs) varying from 8.8 to 4.5 h. This corresponds to the required aeration duration reduction accompanying higher oxygen supply pressure. This confirms higher oxygen supply pressure in a practical application range is more favorable for system effectiveness in terms of shorter operational HRT. 3.3. Microbial community analysis The biofilm samples collected from the HFM surfaces of the MBfR under steady-state conditions of phases I, II, and IV were used for real-time PCR analyses. The microbial analysis results from phases I to II (without and with aeration control) showed that the Nitrosomonas AOB population increased by almost five times, while the Nitrobacter NOB population decreased by about 93%, and that of Nitrospira NOB stayed the same. Furthermore, the results from phases I to IV (at oxygen supply pressures from 24.1

7.8 40

4

7.6

3

7.4

2

10

7.2

1

0

7.0

30

20

DO (mg/L)

50

5

(a)

pH DO Ammonium Nitrite Nitrate

pH

Nitrogen compounds (mg/L as N)

8.0

0 0

2

4

6

8

10

12

14

16

Time (hr) 7.8

(b)

pH DO Ammonium Nitrite Nitrate

7.6

0.20

0.15

30 7.4

0.10

7.2

0.05

20

DO (mg/L)

40

pH

Nitrogen compounds (mg/L as N)

50

10

0

0.00

7.0 0

2

4

6

8

Time (hr)   Fig. 3. Typical variation of DO, pH, and concentrations of NHþ 4 ; NO2 ; and NO3 (a) without pH-gradient real-time aeration control at the fixed aeration duration of 13.5 h (on day 45) and (b) with pH-gradient real-time aeration control (on day 96) in the SBR hollow-fiber MBfR.

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Table 1 Reactor performance and operating conditions of non-porous silicone hollow-fiber membrane biofilm reactor. Phase

I

II

III

IV

pH-gradient real-time aeration control Aeration duration/HRT (h) Duration (d) Number of samples DO (mg L1) Oxygen supply pressure (kPa) DP (kPa) Qi (L d1) 1 Influent NHþ 4 (mg N L ) 1 (mg N L ) Effluent NHþ 4 1 Effluent NO 2 (mg N L ) 1 Effluent NO 3 (mg N L ) VSS (mg L1) TSS (mg L1) NHþ 4 removal efficiency (%) Nitrite accumulation efficiency (%) 1 1 rammonium: NHþ d ) 4 removal rate (mg N L 1 1 rnitrite: NO d ) 2 production rate (mg N L  1 1 rnitrate: NO3 production rate (mg N L d ) OSR (mg d1) OCR (mg d1) BOR (mg d1) OCR/OSR OSR/BOR

No 13.5/14.3 47 9 Varied 24.1 1.2 ± 0.1 2.01 52 ± 4 0±1 1±2 48 ± 3 6±4 9±6 96 ± 3 3±2 108 2 100 963 463 370 0.48 2.60

Yes 8.1/8.8 ± 0.3a 52 10 ND 24.1 1.1 ± 0.2 3.28 48 ± 3 0±1 40 ± 2 5±1 3±3 7±6 97 ± 2 89 ± 7 162 135 17 541 541 556 1.00 0.97

Yes 4.9/5.3 ± 0.2a 50 11 ND 75.8 3.6 ± 0.3 5.52 53 ± 4 0±1 42 ± 3 6±3 4±3 5±4 98 ± 1 88 ± 6 297 235 34 1005 958 1016 0.95 0.99

Yes 3.9/4.5 ± 0.3a 49 9 ND 103.4 5.2 ± 0.5 6.41 52 ± 2 1±1 46 ± 2 10 ± 2 3±2 4±5 96 ± 2 94 ± 5 344 304 20 1199 1133 1178 0.94 1.02

Hydraulic retention time (HRT); dissolved oxygen (DO); oxygen pressure drop in the membrane (DP); influent flow rate (Qi); Not detectable (ND): the detection limits are under 0.01 mg L1 for DO and 0.1 mg N L1 for nitrogen compounds; total and volatile suspended solids (TSS and VSS). a Aeration duration/HRT was controlled based on the pH-gradient real-time aeration control.

to 103.4 kPa with aeration control) indicated that Nitrosomonas increased more than 23 times, Nitrobacter was absent, and Nitrospira remained the same. The observation of more complete trait of Nitrospira sp. (population remain) than Nitrobacter sp. (population reduce) under an oxygen limited environment in this study has also been reported in a trickling filter biofilm (Schramm et al., 1996; Okabe et al., 1999) and a membrane-aerating biofilm (Schramm et al., 2000). In sum, these outcomes suggest that the aeration control could enrich AOB and inhibit NOB with positive impact by raising the oxygen supply pressure varied from 24.1 to 103.4 kPa.

during the entire experimental run for 196 d in this study. With this design, which is intended for application to dilute wastewater, the fouling issue should be relatively mild as observed in this study. Even so, a periodic weak acid cleaning of the membrane should solve this problem, as addressed in an MBfR study treating concentrated sulfide wastewater (Sahinkaya et al., 2011). However, for practical application, application of this weak acid cleaning approach should also consider how to retain most active AOB biofilm, and thus, require further study with the current reactor.

3.4. Mass balance and rate performance analysis

The MBfR integrated with the pH-gradient real-time aeration control was shown to be a suitable solution for nitrite accumulation as a pre-treatment of Anammox from dilute wastewater with little- or non-biodegradable organic presence, e.g. efficient anaerobically-treated sewage. Mass balance and rate performance analysis indicates that this aeration duration control is able to supply an oxygen rate reaching near the BOR of 1.5 (mol O2 mol1 ammonium fed), conforming to nitritation stoichiometry for AOB community selection. Microbial analyses confirmed the dominance of ammonium oxidizers over nitrite oxidizers.

Mass balance and rate performance under various operating conditions are shown at the bottom of Table 1. The ratios of OCR-to-OSR varied between 0.94 and 1.00 with an average of 0.97 between phases II and IV. This close agreement suggests that the nitritation reaction listed in Eq. (1) prevailed in the MBfR using the aeration control. As for the ratio of OCR-to-OSR in phase I, the low value (0.49) might be mainly associated with excess aeration after the end of nitritation without aeration control in the MBfR, which was noted in Fig. 3a. Of significant interest is that effective NAEs were only achieved under aeration control with an OSR/ BOR ratio close to 1.0 during phases II–IV of operation, indicating that the system is able to supply an oxygen rate reaching near the BOR of 1.5 (mol O2 mol1 ammonium fed) based upon Eq. (1) for nitritation community selection, as also shown by the microbial findings in the previous section. In sum, all of the results from this study support that the aeration control could deliver the limited DO for sustainable nitrite accumulation from dilute wastewater. 3.5. Membrane fouling concern in the MBfR system The membrane fouling problem caused by the attached biomass (e.g. AOB) should be considered in order to attain efficient diffusive oxygen transfer in the MBfR, even though no fouling occurred

4. Conclusions

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