Article pubs.acs.org/est
Electrochemical Nutrient Recovery Enables Ammonia Toxicity Control and Biogas Desulfurization in Anaerobic Digestion Joachim Desloover, Jo De Vrieze, Maarten Van de Vijver, Jacky Mortelmans, René Rozendal, and Korneel Rabaey* Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, B-9000 Gent, Belgium S Supporting Information *
ABSTRACT: Organic waste streams can be valorized and reduced in volume with anaerobic digestion (AD). An often-encountered key issue however is the high ammonium (NH4+) content of certain waste streams. Ammonia (NH3), in equilibrium with NH4+, is a toxic compound to the methanogenic community, which limits the organic loading rate and endangers process stability. An electrochemical system (ES) linked to a digester could, besides recovering this nutrient, decrease NH3 toxicity through electrochemical extraction. Therefore, two digesters with and without ES attached in the recirculation loop were operated to test whether the ES could control NH3 toxicity. During periods of high ammonium loading rates, the methane (CH4) production of the ES-coupled reactor was up to 4.5 times higher compared to the control, which could be explained through simultaneous NH4+ extraction and electrochemical pH control. A nitrogen flux of 47 g N m−2 membrane d−1 could be obtained in the ES-coupled reactor, resulting in a current and removal efficiency of 38 ± 5% and 28 ± 2%, respectively, at an electrochemical power input of 17 ± 2 kWh kg−1 N. The anode also oxidized sulfide, resulting in a significantly lower H2S emission via the biogas. Lastly, limited methanogenic community dynamics pointed to a nonselective influence of the different operational conditions.
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INTRODUCTION Anaerobic digestion (AD) is a key technology for stabilization and valorization of organic waste streams.1,2 In short, this technology comprises a stepwise conversion of low-value organic compounds into biogas, a mixture of mainly methane (CH4) and carbon dioxide (CO2). Methane is an energy carrier and can be valorized through, for example, a combined heat and power unit, delivering electricity and heat. Next to biogas, AD also produces a nutrient-rich digestate that can be applied as a fertilizer in agriculture.3 Despite numerous advantages of the AD process, instability is an often-encountered problem that can lead to complete failure of the reactor. One of the key compounds causing instability is ammonia (NH3), especially when treating nitrogen-rich waste streams.4,5 Ammonia is a cell membrane-permeable molecule for which methanogens, executing the final step in AD, have a low tolerance.6 Moreover, acetoclastic methanogenesis is, in general, more susceptible to inhibition than hydrogenotrophic methanogenesis.6 This exposes digesters to a risk of process instability and limits the loading and thus biogas production rate.4 To avoid this, operators usually feed the digester at a lower and safer loading rate, acclimate the biomass, or co-digest with carbon-rich substrates to maintain a suitable carbon to nitrogen ratio.5 Other more advanced but also more expensive approaches are struvite precipitation, anammox, and the use of zeolites.5 In this study, we present an alternative to control NH3 toxicity and to maximize resource recovery from AD by coupling an electrochemical system (ES) to an anaerobic © XXXX American Chemical Society
digester. An ES has the attractive feature that an oxidation process (anode) is separated from a reduction process (cathode), typically by an ion selective membrane. By applying a current to this system, membrane electrolysis can take place during which ions can be extracted from anode to cathode or vice versa.7,8 In the context of anaerobic digestion, one can send the digestate through an anode compartment, enabling recovery of valuable nutrients such as ammonium (NH4+) and potassium (K+). Hence, by combining AD with ES technology, nutrients can be harvested while simultaneously lowering the risk for ammonia toxicity. We have recently demonstrated the proof of concept of an ES for nutrient recovery from liquid waste streams.8 Here, we studied the direct coupling of an ES to an upflow anaerobic sludge blanket (UASB) reactor treating molasses. We investigated whether the placement of an anode in the recirculation line had any negative effects on the digester and whether the ES could stabilize AD performance when exposed to toxic NH3 concentrations. Lastly, we also investigated in what manner the ES can affect the quality of the biogas generated. Received: October 1, 2014 Revised: December 13, 2014 Accepted: December 17, 2014
A
DOI: 10.1021/es504811a Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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MATERIAL AND METHODS Experimental Setup. Two cylindrical UASB reactors (2.3 L glass reactor with effective volume of 2 L) were constructed, serving as test and control reactors (Figure 1). These reactors
Table 1. Composition Tap Water-Diluted Molasses To Obtain Desired Loading Rate of 5 g COD L−1 d−1. parameter
value
unit
pH conductivity COD Kj-N TAN SO42‑ T-P Cl− TS VS VSS
5.7 ± 0.3 8.5 ± 1 11.7 ± 2.2 439 ± 19 16 254 80 156 11 9 0.8
mS cm−1 g L−1 mg L−1 mg N L−1 mg L−1 mg P L−1 mg L−1 g L−1 g L−1 g L−1
UASBs to compensate for any later addition of NH4Cl, when the performance was investigated under high nitrogen loading conditions. Both reactors were pH controlled (Dulcometer D1C, Prominent, Germany) with 1 M NaOH. The electrochemical process parameters of the ES were defined and calculated according to Desloover et al.8 The cathode compartments of the control and test setups were fed continuously with 6.4 g L−1 NaCl at 1 L d−1 (HRT of 4.8 h) with an internal recirculation rate of 2 L h−1. Experimental Plan. The experimental plan comprised four main phases (Table 2). During Phase I, the organic loading rate was gradually increased from 1 to 5 g COD L−1 d−1 (Phase Ia). After stable operation, the pH was stepwise (0.25 pH units per week) increased from 7 to 8 (Phase Ib) to shift the NH4+/NH3 equilibrium more to the direction of NH3 (ratio NH3/NH4+ = 0.11 at pH 8 and 34 °C). The free ammonia fraction was calculated according to Anthonisen et al.10 Next, the ES of the test setup was switched on at an applied current density of 10 A m−2 (relative to projected membrane surface area) to investigate the impact during low nitrogen loading. At day 120, the UASB of the test setup crashed due to clogging and subsequent malfunction of the pH controller. Hence, both the test and control reactors were cleaned, and the biomass of the control UASB was split over the test and control setups to initiate a second start-up (Phase IIIa) during which the organic loading rate and pH were maintained at 5 g COD L−1 d−1 and 8, respectively. Also, the ES of the test setup was stepwise increased from 5 to 10 A m−2. After a steady-state period (Phase IIIb), the effect of the ES was investigated under periodically increased nitrogen loading conditions (Phase IV). Therefore, the effect of an operational ES on the test setup was investigated during a period of increased nitrogen loading (Phase IVa), as well as a period during which the extra-added nitrogen was again removed from the feed (Phase IVb). Next, this operational procedure was repeated during a period where the ES of the test setup was switched off (Phases IVc and IVd). Finally, after an adaptation period where the ES was switched on again (Phase IVe), the effect of the ES was investigated during additional nitrogen loading up to 2 g N L−1 (Phases IVf and IVg) and where we also allowed the electrochemical cell to control the pH by taking advantage of the acidifying anode reaction. Chemical Analysis. Liquid samples of the influent and effluent streams as well as gaseous samples from the headspace were taken three times a week. Liquid samples were filtered (0.22 μm) and stored at 4 °C until further analysis.
Figure 1. Schematical overview of the experimental setup.
had an internal diameter of 5.4 cm and a total height of 900 cm. For the test reactor, an ES was coupled to the UASB for extraction of cations. This was done by inserting the anode compartment (5 cm × 20 cm × 2 cm) of the ES in the recirculation loop of the UASB reactor. The anode compartment was separated from the cathode compartment (5 cm × 20 cm × 2 cm3) by a cation exchange membrane (CEM, Membranes International, U.S.A.). The anode electrode used was an IrOx coated titanium mesh electrode 9 (12g m−2 of Ir/ Ta = 65/35), with a projected surface area of 5 cm × 20 cm (Magneto Special Anodes, The Netherlands), while the cathode electrode was a stainless steel mesh (5 × 20 cm2, mesh width 564 μm Solana, Belgium). Both electrodes were placed close to the CEM and separated by a polytetrafluoroethylene (PTFE) spacer with a projected surface area of 5 cm × 20 cm (turbulence promoter mesh, Electrocell, Denmark) to avoid direct contact. At the anode, water was oxidized to oxygen and protons, while at the cathode, water was reduced to hydrogen gas and hydroxyl ions. The ES was controlled galvanostatically by a VSP multipotentiostat (Biologic, France). The control reactor was also coupled to an ES in the recirculation loop. The ES was equipped with a CEM, but electrodes were omitted. Hence, the ES of the control setup was operated in open circuit (no anode and cathode), meaning that no current could be applied to the system and only diffusion driven processes could take place. Reactor Operation. The experiment was conducted under mesophilic conditions (34 ± 1 °C). The UASB reactors were inoculated with granular sludge from a full-scale UASB reactor (Brewery Van Steenberge, Belgium) and diluted with tap water to obtain an initial sludge concentration of 10 g of volatile suspended solids (VSS) L−1. The UASBs were fed every two hours with tap water-diluted molasses according to the desired loading rate and were operated at a hydraulic retention time (HRT) of 2 days. The characteristics of the diluted molasses to obtain a desired loading rate of 5 g COD L−1 d−1 are shown in Table 1 (raw composition molasses, Table S1, Supporting Information). Furthermore, an internal recirculation rate was applied over the UASB and anode compartment of 2 L h−1 to maintain an upflow velocity of 1 m h−1 in the digester. In order to maintain the same conductivity throughout the experimental period, 4.14 g L−1 NaCl was initially added to the feed of both B
DOI: 10.1021/es504811a Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Table 2. Overview of Experimental Plan phase
operation
period (d)
Ia Ib II IIIa IIIb IVa IVb IVc IVd IVe IVf IVg
start-up 1 gradual increase pH 7→8 switch on ES test setup start-up 2 + switch on ES test setup steady state add 1 g N L−1 to feed remove additional 1 g N L−1 from feed switch off ES test setup + add 1 g N L−1 to feed remove additional 1 g N L−1 from feed switch on ES test setup add 1 g N L−1 to feed add 2 g N L−1 to feed + electrochemical acidification (pH 8→7)
1−30 30−70 70−120 130−210 210−225 225−238 238−266 266−275 275−287 287−303 303−313 313−340
different parameters obtained during analysis with the StepOnePlus software V2.3 (Table S2, Supporting Information). Statistical Analysis. All statistical data analysis were performed with the statistical software R, version 3.0.2. for Windows. In the case of normally distributed data sets that were homoscedastic the regular t test was applied. In the case where the data was heteroscedastic, a Welch-modified t test was used. In the case where the data was not normally distributed, the Wilcoxon Rank Sum (Mann−Whitney U) test was applied.
Volatile suspended solids (VSS), Kjeldahl nitrogen (Kj-N), ammonium (NH4+), chemical oxygen demand (COD), pH, and conductivity were analyzed according to standard methods.11 Volatile fatty acids (VFA) were, after extraction in diethyl ether, analyzed with a DB-FFAP 12-3232 column (30 m × 0.32 mm × 0.25 μm; Agilent, Belgium) and a flame ionization detector (FID) gas chromatograph (GC-2014, Shimadzu, The Netherlands). The gas phase composition was analyzed with a compact GC (Global Analyzer Solutions, Breda, The Netherlands). The GC was equipped with two channels. In channel 1, a Porabond precolumn and Molsieve 5A column were used for CH4, O2, H2, and N2 measurement, and in channel 2 a Rt-Q-bond precolumn and column were used for CO2, N2O, and H2S analysis. Concentrations of gases were determined by means of a thermal conductivity detector and were reported at STP (standard temperature and pressure) conditions. Biogas production was measured with an in-house manufactured calibrated gas counter. Molecular Analysis. Total DNA was extracted from the sludge samples using the protocol of Vilchez-Vargas et al.12 DNA quality and quantity of the extracts was analyzed by means of a 1% agarose gel and a Nanodrop ND-1000 spectrophotometer (Isogen Life Science, IJsselstein, The Netherlands). Triplicate samples of a 100-fold dilution of the DNA samples were prepared to reach a final DNA concentration between 1 and 10 ng μL−1. Real-time PCR (qPCR) was performed on a StepOnePlus Real-Time PCR System (Applied Biosystems, Carlsbad, CA, U.S.A.). The reaction mixture of 15 μL was prepared using the GoTaq qPCR Master Mix (Promega, Madison, WI, U.S.A.) and consisted of 10 μL of GoTaq PCR Master Mix, 3.5 μL of nuclease-free water, and 0.75 μL of each primer (final concentration of 375 nM), to which 5 μL of template DNA was added. The qPCR program was performed in a two-step thermal cycling procedure that consisted of a predenaturation step of 10 min at 94 °C, followed by 40 cycles of 15 s at 94 °C and 1 min at 60 °C for total bacteria, using the P338F and P518r primers, as described by Ovreas et al.13 The qPCR program for the methanogenic orders Methanobacteriales and the families Methanosaetaceae and Methanosarcinaceae consisted of a predenaturation step of 10 min at 94 °C, followed by 40 cycles of 10 s at 94 °C and 1 min at 60 °C. For quantification of the Methanomicrobiales order an annealing temperature of 63 °C was used. The primers for the methanogenic orders Methanomicrobiales and Methanobacteriales and the families Methanosaetaceae and Methanosarcinaceae were described by Yu et al.14 Real-time PCR quality was evaluated by means of the
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RESULTS AND DISCUSSION ES Has a Temporal Effect on Digester during Low Nitrogen Loading Conditions. After the start-up phase (Phase Ia), the average CH4 production rate of the test and control reactors during Phase Ib was 949 ± 90 and 950 ± 134 mL CH4 L−1 d−1, respectively (Figure 2, Table S3, Supporting
Figure 2. CH4 production in function of time of the test and control setup during Phases Ia−II.
Information). Hence, gradually adapting the pH from 7 to 8 did not have a notable effect on methane production. Moreover, by operating at a high pH, a CH4 content up to 83% could be reached in both the test and control reactors (Table S3, Supporting Information). When the ES of the test setup was switched on at the start of Phase II, an initial decrease in CH4 production rate of about C
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Figure 3. CH4 production (A), VFA concentration (B), NH4+ concentration (C), and H2S content in the biogas (D) in function of time of the test and control setups during Phases IIIa−IVg. The labels presented in this figure account for all graphs.
performance (Figure 3A). Furthermore, the decline in CH4 production of the control reactor coincided with the accumulation of VFA up to 2700 mg COD L−1, whereas no VFA could be detected in the test reactor (Figure 3B). VFA accumulation is a strong sign of methanogen inhibition, and as described in other studies, this was most probably caused by the high ammonium content in combination with a high pH.6,15 The fact that the test reactor outperforms the control reactor can thus be explained by an on average 23% lower ammonium level in the test reactor caused by membrane electrolysis (Figure 3C, Table 3). By omitting the additional nitrogen from the feed (Phase IVb), the control reactor was able to partially recover over a period of 30 days (Figure 3A). Repeating this procedure with a nonworking ES of the test reactor (Phases IVc and IVd) resulted in a decrease in performance of both the test and control reactors (Figure 3A). The ammonium levels in both reactors were identical (Figure 3C), and also, this time not only VFA accumulation could be observed in the control reactor, but also in the test reactor (Figure 3B). These findings prove that ammonium extraction by the ES was essential to maintain constant increased methane production values under high nitrogen loading conditions. Most likely, the corresponding average NH3 concentration during Phase IVa in the test reactor (94 mg N L−1) did not reach a level that inhibited the methanogenic community,
20% could be observed. The temporary negative impact on the performance of the microbial community was probably due to a shock effect caused by the instantaneous oxygen and proton production by the anode reaction, in combination with a higher NaOH dosage to counteract acidification (Table S3, Supporting Information). The produced oxygen represented 7% of the COD loading rate at 10 A m−2 but was consumed because no O2 could be detected in the biogas. Moreover, the conductivity of the test (19.3 ± 2.7 mS cm−1) and control (21.1 ± 3.2 mS cm−1) reactors were not significantly different (Table S3, Supporting Information, p > 0.05), due to membrane electrolysis. After 40 days of operation, the test reactor recovered and reached again the performance of the control reactor (Figure 2). Electrochemical NH3 Toxicity Control during High Nitrogen Loading Conditions. After the crash of the test reactor and a second start-up (Phase IIIa), both reactors again reached equal performance (Phase IIIb, p > 0.05), with a CH4 production rate of 925 ± 94 and 848 ± 66 mL CH4 L−1 d−1 in the test and control reactors, respectively (Figure 3A, Table 3). When the nitrogen loading was increased by adding an additional 1 g N L−1 to the feed (Phase IVa), a 43% decrease in the CH4 production rate could be observed for the control reactor, while the test reactor was able to maintain its D
DOI: 10.1021/es504811a Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Table 3. Overview of Parameters in Test and Control Setups during Phases IIIa−IVca parameter UASB (with anode compartment) CH4 production (mL CH4 L−1d−1) efficiencyb (%) sCOD effluent (g COD L−1) total VFA (mg COD L−1) acetate (mg L−1) propionate (mg L−1) TAN (mg N L−1) conductivity (mS cm−1) pH (−) NaOH dosage (mL d−1) CH4 (%) H2S (%) Cathode TAN (mg N L−1) conductivity (mS cm−1) pH (−) Electrochemical N fluxc (g N m−2 d−1) NH4+ current efficiency (CE,%) NH4+ removal efficiency (RE,%) cell voltage (V) energy input (kWh kg−1 N)
Phase IIIa (n = 27)
Phase IIIb (n = 6)
Phase Iva (n = 6)
Phase IVb (n = 12)
Phase IVc (n = 4)
test
control
test
control
test
control
test
control
test
control
618 ± 173
626 ± 155
925 ± 94
848 ± 66
890 ± 51
571 ± 121
940 ± 64
740 ± 63
749 ± 147
551 ± 107
36 ± 10 ND
36 ± 9 ND
52 ± 2 0.96 ± 0.26
50 ± 2 2.04 ± 0.37
54 ± 3 0.97 ± 0.02
34 ± 8 2.82 ± 0.40
56 ± 5 0.93 ± 0.12
44 ± 4 3.71 ± 1.21
45 ± 10 3.30 ± 0.99
34 ± 7 4.45 ± 0.75
ND
ND
BDL
630 ± 306
BDL
1939 ± 704
BDL
1983 ± 429
1575 ± 817
3119 ± 730
ND ND ND 20.1 ± 2.1 7.9 ± 0.1 106 ± 30 82 ± 4 0.25 ± 0.15
ND ND ND 19.7 ± 2.8 7.9 ± 0.1 81 ± 10 84 ± 3 0.27 ± 0.06
BDL BDL 250 ± 30 16.0 ± 0.3 7.9 ± 0.1 142 ± 28 92 ± 2 0.05 ± 0.07
249 ± 95 370 ± 203 294 ± 20 17.5 ± 0.5 8.0 ± 0.1 73 ± 5 94 ± 1 0.22 ± 0.03
BDL BDL 823 ± 116 18.3 ± 1.3 7.9 ± 0.1 166 ± 16 89 ± 1 BDL
1218 ± 459 670 ± 259 1069 ± 51 17.8 ± 0.5 8.0 ± 0.1 83 ± 4 93 ± 1 0.22 ± 0.04
BDL BDL 302 ± 54 18.6 ± 1.9 8.0 ± 0.2 155 ± 15 91 ± 7 0.05 ± 0.08
1299 ± 293 609 ± 163 376 ± 69 20.0 ± 1.3 7.9 ± 0.1 80 ± 12 91 ± 3 0.19 ± 0.09
1133 ± 645 408 ± 147 1001 ± 199 22.3 ± 1.3 8.0 ± 0.1 95 ± 17 89 ± 1 0.51 ± 0.10
2519 ± 669 510 ± 71 1001 ± 195 21.6 ± 1.2 8.0 ± 0.1 86 ± 10 89 ± 2 0.23 ± 0.01
ND 17.0 ± 5.2 9.9 ± 2.2
ND 14.1 ± 3.8 7.9 ± 0.3
42 ± 21 22.4 ± 5.5 12.5 ± 0.1
10 ± 6 9.4 ± 2.7 7.8 ± 0.2
220 ± 20 20.6 ± 2.0 12.1 ± 0.1
38 ± 12 12.0 ± 1.1 7.6 ± 0.1
60 ± 15 24.0 ± 3.3 12.4 ± 0.1
23 ± 8 12.2 ± 1.4 7.7 ± 0.2
24 ± 11 12.3 ± 1.5 8.5 ± 0.3
44 ± 9 12.6 ± 2.3 7.9 ± 0.1
ND ND
ND ND
4±2 3±1
1±1 NA
19 ± 2 15 ± 1
3±1 NA
5±1 4±1
2±1 NA
2±1 NA
3±0 NA
ND
ND
14 ± 6
3±2
22 ± 3
3±1
17 ± 1
6±2
3±1
4±0
ND ND
ND ND
3.58 ± 0.65 307 ± 182
NA NA
4.72 ± 0.91 60 ± 6
NA NA
4.72 ± 0.91 251 ± 37
NA NA
NA NA
NA NA
a n = number of data points. ND: not determined. BDL: below detection limit. NA: not applicable. bCOD to CH4 conversion efficiency at STP conditions and relative to the amount of COD fed to the reactor. cRelative to the projected membrane surface area.
Figure 4. Relative abundance of methanogens (Methanomicrobiales, Methanobacteriales, Methanosarcinacea, and Methanosaetaceae) in the test (T) and control (C) reactors throughout the experimental period.
instantaneous recovery of the test reactor, while the control reactor seemed to reach an inhibited steady state,4 which was on average 43% lower in performance compared to the test reactor. Gradually switching on the ES of the test reactor from 5 to 10 A m−2 caused again a temporary negative effect, comparable
whereas inhibition might have occurred in the control reactor were a higher average NH3 concentration was present (118 mg N L−1). Indeed, reported inhibitory NH3 concentrations are within a range of 80−150 mg N L−1 and are dependent on the operational conditions and degree of adaptation.4,5 Removal of the extra-added NH4+ from the feed (Phase IVd) caused E
DOI: 10.1021/es504811a Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Table 4. Overview of Parameters in Test and Control Setups during Phases IVd−IVga parameter UASB (with anode compartment) CH4 production (mL CH4 L−1d−1) efficiencyb (%) sCOD effluent (g COD L−1) total VFA (mg COD L−1) acetate (mg L−1) propionate (mg L−1) TAN (mg N L−1) conductivity (mS cm−1) pH (−) NaOH dosage (mL d−1) CH4 (%) H2S (%) Cathode TAN (mg N L−1) conductivity (mS cm−1) pH (−) Electrochemical N fluxc (g N m−2 d−1) NH4+ current efficiency (CE,%) NH4+ removal efficiency (RE,%) cell voltage (V) energy input (kWh kg−1 N)
Phase IVd (n = 5)
Phase IVe (n = 7)
Phase IVf (n = 4)
Phase IVg (n = 12)
test
control
test
control
test
control
test
control
973 ± 87 58 ± 8 2.1 ± 0.7 780 ± 529 504 ± 326 260 ± 180 504 ± 128 21.7 ± 0.6 8.1 ± 0.1 72 ± 15 89 ± 0 0.27 ± 0.10
537 ± 39 33 ± 4 4.7 ± 0.2 3239 ± 281 2611 ± 237 545 ± 51 482 ± 121 23.6 ± 0.9 8.0 ± 0.1 82 ± 5 90 ± 1 0.22 ± 0.02
798 ± 136 50 ± 9 2.7 ± 0.8 1019 ± 386 840 ± 291 86 ± 43 284 ± 16 21.5 ± 2.6 8.0 ± 0.1 140 ± 43 89 ± 2 0.11 ± 0.03
507 ± 33 32 ± 3 4.8 ± 0.3 3735 ± 210 2834 ± 140 442 ± 84 341 ± 26 24.4 ± 2.3 8.0 ± 0.1 94 ± 24 91 ± 1 0.19 ± 0.03
786 ± 139 48 ± 7 3.5 ± 1.4 3052 ± 1493 2465 ± 1096 233 ± 182 703 ± 198 17.7 ± 0.9 8.0 ± 0.1 179 ± 26 93 ± 2 0.08 ± 0.02
458 ± 22 30 ± 3 5.7 ± 0.8 3291 ± 686 2552 ± 528 312 ± 96 1040 ± 120 21.3 ± 3.6 7.9 ± 0.1 88 ± 11 90 ± 1 0.18 ± 0.02
644 ± 188 40 ± 11 3.1 ± 1.0 1459 ± 1041 1204 ± 876 78 ± 53 1527 ± 92 23.6 ± 1.8 7.1 ± 0.2 109 ± 39 64 ± 11 0.12 ± 0.13
252 ± 87 16 ± 5 7.0 ± 0.4 6252 ± 1000 5168 ± 878 440 ± 35 1889 ± 113 27.0 ± 2.1 7.9 ± 0.1 82 ± 5 90 ± 3 0.19 ± 0.02
28 ± 5 12.1 ± 0.3 8.3 ± 0.2
44 ± 8 13.6 ± 3.2 7.9 ± 0.2
109 ± 15 30.1 ± 3.4 12.6 ± 0.2
45 ± 9 18.6 ± 4.5 8.1 ± 0.2
296 ± 51 25.6 ± 4.3 12.5 ± 0.3
99 ± 15 16.6 ± 5.0 7.8 ± 0.2
600 ± 101 18.6 ± 2.6 12.4 ± 0.2
155 ± 19 14.1 ± 1.3 7.9 ± 0.1
2±0 NA 5±1 NA NA
3±1 NA 8±1 NA NA
9±1 9±2 29 ± 5 1.69 ± 0.13 39 ± 9
3±1 NA 11 ± 3 NA NA
23 ± 3 18 ± 2 27 ± 4 3.40 ± 0.25 36 ± 4
7±2 NA 9±2 NA NA
45 ± 8 36 ± 6 27 ± 3 3.25 ± 0.10 18 ± 3
13 ± 1 NA 8±1 NA NA
a n = number of data points. ND: not determined. BDL: below detection limit. NA: not applicable. bCOD to CH4 conversion efficiency at STP conditions and relative to the amount of COD fed to the reactor. cRelative to the projected membrane surface area.
to Phase II. However, this time the recovery took only 16 days (Phase IVe). This shows that gradual increase of the current density is necessary to allow adaptation of the microbial community to the new conditions. The limited degree of change in the methanogenic community (Figure 4) indicates that adaptation of the microbial community took place through physiological responses rather than microbial community composition variation. During the final two phases (Phases IVf and IVg), the nitrogen loading was again increased by addition of 1 (Phase IVf) and 2 (Phase IVg) g N L−1 to the feed. In contrast to Phase IVa, the CH4 production rate of the test reactor started to decrease dramatically (Figure 3A), which can be explained by the fact that the methanogenic community was also stressed in the test reactor because residual VFA were present (Figure 3B). A further increase in the nitrogen loading (additional 2 g N L−1) led to a minimum CH4 production rate of 322 mL of CH4 L−1 d−1 at day 317, which was equal to the performance of the control reactor (Figure 3A). Clearly, the extraction of ammonium by the ES was insufficient to decrease the NH4+ and hence also the NH3 concentration below a toxic level (Figure 3C). Therefore, from day 317 onward, not only the extraction capacity of the ES was utilized but also the ability to acidify and hence control the pH of the reactor through the use of the acidifying anode oxidation reaction. Until day 317, the generated protons by the ES were counteracted by NaOH addition in order to operate the test and control setups at the same pH. As a result, NaOH addition to the test reactor was almost double of the control reactor whenever the ES was switched on (Tables 3 and 4). This, however, did not generate a significant (p > 0.05) difference in conductivity between both setups, as the base addition to the test reactor was compensated
by membrane extraction of the ES. By steadily decreasing the base dosage of the test reactor down to the level of the control reactor, the pH of the test reactor evolved to 7.1 (Table 4), and as such, the acidifying effect of the ES could be investigated. The test reactor completely recovered and reached a CH4 production rate of 856 ± 38 mL of CH4 L−1 d−1 during the last seven days of operation, while the control reactor remained at a 4.5 times lower CH4 production rate of 192 ± 10 mL of CH4 L−1 d−1. This coincided with a steep decrease in VFA below detection limit in the test reactor and VFA accumulation up to 7500 mg L−1 in the control reactor. From this, we can conclude that, next to electrochemical NH4+ extraction, also electrochemical pH control is a powerful tool allowing efficient NH3 toxicity control in this study. Moreover, electrochemical NH4+ extraction generates added value as it allows for recovery of this nutrient here under the form of a H2/NH3 gas mixture. This gas mixture can easily be separated via, for example, ammonia condensation, thus delivering a concentrated liquid ammonium stream and a purified H2 gas stream for injection in the anaerobic digester. Electrochemical Nutrient Recovery. The concept of electrochemical nutrient recovery has been demonstrated previously, both with an electrochemical 8 as with a bioelectrochemical system,16,17 whether or not in combination with power consumption or production. This concept was also applied in this study, especially during the final phase (Phase IVg). In this phase, the need for a high ammonium concentration in order to obtain a high extraction efficiency was again shown (Figure 4C). Here, NH4+ could be extracted at a flux of 47 ± 6 g N m−2 membrane d−1, resulting in a removal and current efficiency of 36 ± 6% and 27 ± 3%, respectively (Table 4). In terms of electrochemical power input, the ammonium could be extracted at 17 ± 2 kWh kg−1 N. This is F
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Environmental Science & Technology
the main acetoclastic methanogens relates to their filamentous structure and therefore crucial role in anaerobic granule formation.24 The persistence of the Methanobacteriales and Methanomicrobiales order points out that not only acetoclastic but also hydrogenotrophic methanogenesis, whether or not in combination with syntrophic acetate oxidation, could take place. The overall limited dynamics of the methanogenic community composition and abundance indicates that the methanogenic community was not selectively influenced by the operational conditions in the rapid succession of the different phases.
comparable to the results obtained in our previous study where we obtained a power input of 13 kWh kg−1 N at a current density of 10 A m−2 and 2 g N L−1 with digestate from a municipal solid waste digester.8 Simultaneously with the NH3, K+ is removed from the digestate, and upon extraction of the latter, the energy input could be spread over those two products. Ammonium is driven to a solids-free catholyte with high pH (Tables 3 and 4) and converted into volatile NH3. The NH3 can then be recovered from this stream through stripping and absorption technology8 but was not subject of this study. Extra Asset: Electrochemical Remediation of H2S. An interesting side observation during this study was the significantly lower H2S content in the biogas of the test reactor when the ES was switched on (Figure 3D, p < 0.05). Under these conditions, the H2S concentration in the biogas of the test reactor was often below the detection limit (0.01%, Tables 3 and 4), except for the period after the crash and cleaning of the reactors (Phase IIIa) when the ES was switched on. However, a lot of variation was observed due to the different phases. As such, H2S was higher during the period after the crash and cleaning of the reactors (Phase IIIa) and at the beginning of Phase IVg, when the pH shifted from 8 to 7. In contrast, the H2S concentration in the control setup was on average 0.20 ± 0.06% over the entire experimental period. The overall lower H2S content in the biogas of the test reactor when the ES was switched on was most likely caused by direct or indirect electrochemical oxidation of the dissolved sulfide species in the vicinity of the anode electrode. Electrochemical sulfide removal from domestic wastewater was recently studied in detail at Ir/ Ta-coated MMO-coated titanium anodes.18 The main mechanism was indirect sulfide oxidation to elemental sulfur, thiosulphate, and sulfate by the in situ produced oxygen at the anode. Most likely, this also took place in our setup, as a similar anode electrode was used. Next to plain electrochemical oxidation, also bioelectrochemical oxidation could take place as microorganisms were growing in the anode compartment.19 A yellow deposition was observed on the anode, indicating the production of elemental sulfur. However, due to the complex mixed liquor, it was not possible to analyze this. The presence of H2S constitutes a severe problem to any biogas conversion technology as it can cause corrosion.20 Hence, electrochemical H2S removal is a valuable asset next to NH3 toxicity control and nutrient recovery. In combination with a typically high reactor pH, the biogas has a low CO2 content as well, leading to a highly methane enriched biogas. Microbial Community Findings. Microbial community analysis was carried out to evaluate whether a differentiating impact could be observed due to the presence of the ES and through time. Real-time PCR analysis of the total bacteria and summation of the different methanogenic groups resulted in an overall coverage of the microbial community by the methanogens between 1% and 5% only (Figure S1, Supporting Information). This is in contrast to their crucial role in the anaerobic digestion process and anticipated activity, yet is in agreement with other lab- and full-scale AD installations in which the methanogenic community covered no more than 5% of the total microbial community.21−23 Analysis of the main different methanogenic populations in AD demonstrated the presence of Methanobacteriales, Methanomicrobiales, and Methanosaetaceae for at least 10% of the methanogenic community in each sample, irrespective of the treatment or time point (Figure 4). The assumed dominance of Methanosaetaceae over Methanosarcinaceae (below detection limit in every sample) as
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ASSOCIATED CONTENT
S Supporting Information *
Information as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +32 (0)9 264 59 76. Fax: +32 (0)9 264 62 48. E-mail:
[email protected]. Web page: www.labmet.Ugent.be. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.D. is supported by an Advanced Grant of the Industrial Research Fund at Ghent University (F2012/IOF-Advanced/ 094). K.R. is supported by a European Research Council Starter Grant ELECTROTALK. We acknowledge Tim Lacoere for the graphical abstract and Frederiek-Maarten Kerckhof for developing the R-script and statistical analysis.
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REFERENCES
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