Research Article pubs.acs.org/journal/ascecg
In-Situ Stability Control of Energy-Producing Anaerobic Biological Reactors through Novel Use of Ion Exchange Fibers Yu Tian, Arup K. SenGupta, and Derick G. Brown* Department of Civil & Environmental Engineering, Lehigh University, 1 West Packer Avenue, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Anaerobic biological treatment of high-strength organic industrial wastes is preferred over aerobic treatment as it produces a methane-rich biogas, has much lower energy requirements, and produces significantly less biosolids. Process stability and reactor failure are of concern, however, for waste streams that exhibit large variations in organic loading, which can cause detrimental pH fluctuations, and that have the potential for accidental input of toxic metals. Here, we demonstrate for the first time that the use of ion exchange fibers (IXFs) can provide passive resilience to these failure modes, without requiring operator oversight or reactive process control via chemical addition. IXFs have the advantage of rapid kinetics due to their small size, and they can be readily inserted and withdrawn as woven mats or porous pillows. This approach is demonstrated here using the weak-acid IXF FIBAN X-1 and the strong-base FIBAN A-1. FIBAN X-1 passively stabilized anaerobic reactors by (i) buffering pH fluctuations resulting from organic overloading due to both an increase in organic concentration and a decrease in hydraulic residence time and (ii) moderating shock-loads of copper and nickel. FIBAN X-1 also retained ∼95% of its exchange capacity after one year of operation in anaerobic reactors, demonstrating its long-term performance. In addition, FIBAN A-1 stabilized anaerobic reactors to input of chromate. These results demonstrate that IXFs can be used to passively stabilize anaerobic biological reactors from upset and failure and that this technology can be used to enhance energy recovery from high-strength organic waste streams. KEYWORDS: Anaerobic, Ion exchange fiber, Methane, Metal toxicity, Methanogens, Organic overload
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INTRODUCTION High-strength organic waste streams, such as those produced in agricultural, food-processing, beverage, pharmaceutical, and chemical industries, contribute to a diverse portfolio of renewable energy sources. The biological oxygen demand (BOD) of these organic wastewaters can range from 0.15 to over 100 g/L (e.g., see the Supporting Information, Table S1). Aerobic biological processes, with their proven ability to reduce BOD and their stability to upset, are commonly used to treat these industrial wastes. This treatment comes at a cost, however, with significant energy demands for the input of oxygen into the system and the production of substantial amounts of biosolids that must be dewatered and disposed. The use of anaerobic biological processes is particularly attractive for treatment of these waste streams; as anaerobic systems produce energy in the form of methane, they have much smaller operational energy requirements and are typically net energy positive, and they produce much smaller amounts of biosolids as compared to aerobic treatment (Figure 1). Anaerobic reactors also have the advantage that the BOD loading can be 5−10 times greater than that with aerobic reactors, mainly because of the inherent oxygen transfer limitations in aerobic systems.1,2 The trade-off is that anaerobic reactors can be © 2017 American Chemical Society
susceptible to operational failure due to the buildup of volatile acids, causing acidification and inhibition of methanogenesis, and the input of toxic chemicals. The key concern here is that the methanogenic archaea have low cell yields and doubling times on the order of days, compared to hours typical of aerobic microorganisms, and this can require a long recovery time after a failure.1−3 Active operator oversight and control of these failure modes must be considered when designing and operating anaerobic systems.2,4−14 There are many microbial processes involved in anaerobic treatment (e.g., Figure S2), and these failure modes mainly affect the final steps. Here, facultative heterotrophs produce volatile acids, which are converted to acetic acid by acetogenic bacteria. The acetic acid is then converted to methane and carbon dioxide by methanogens. The acid-forming bacteria have a higher growth rate and wider operational pH range than the methanogens. When these processes are operating at steady-state, the rate of organic acid production is balanced by the uptake rate of the methanogens, and the reactor pH is Received: July 19, 2017 Revised: September 4, 2017 Published: September 6, 2017 9380
DOI: 10.1021/acssuschemeng.7b02435 ACS Sustainable Chem. Eng. 2017, 5, 9380−9389
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Figure 1. Example net biological stoichiometric yields and energy from aerobic and anaerobic treatment of 1 kg of the carbohydrate lactose. Aerobic treatment requires energy for the input of oxygen, while anaerobic treatment produces energy in the form of methane and creates a smaller mass of cells for disposal. Aerobic energy presented as equivalent energy of natural gas, with the range representing different electricity generation methods. See the Supporting Information for details.
Figure 2. (A) IXF FIBAN X-1 utilizes iminodiacetate functional groups attached within a polyacrylonitrile fiber and demonstrates a high buffer capacity due to its weak-acid functional groups and speciation across a wide pH range.30,31 (B) SEM image of FIBAN X-1 fibers. (C) SEM image of a FIBAN X-1 fiber cross section. (D) SEM image of a FIBAN A-1 fiber cross section.
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REACTOR STABILIZATION USING ION EXCHANGE FIBERS As an alternative to operator monitoring and reactive chemical addition, we introduce here a sustainable and passive proactive control approach through novel use of ion exchange fibers (IXFs) to maintain stable operation in anaerobic biological reactors. Conceptually, this is an in situ process control scheme where IXFs, without any operator monitoring or external intervention, (i) maintain neutral pH by removing excess H+ generated during stress periods of increased organic loading, and (ii) mitigate accidental input of toxic metals. For pH control, once the normal feed conditions are restored, the IXFs release H+ back into solution; i.e., they self-regenerate within the system. After an accidental input of metals, the IXFs can be allowed to slowly regenerate over time after the input of toxic metals ceases, leading to reversal of the metal concentration gradient in the IXF and slow desorption of the toxic metals,24 or they can be removed and replaced, as necessary. Spherical weak-acid ion exchange resin beads have been examined for their ability to stabilize anaerobic biological reactors. The ion exchange process showed promise; however, two identified shortcomings with spherical resin beads were slow kinetics and a difficulty in retaining them in anaerobic reactors.25 Advances in the development of ion exchange materials have led to the production of very thin IXFs, on the
maintained near neutral. The organic loadings in industrial wastewaters, however, can vary greatly because of batch operations and the production schedule of the facility.15−17 Reactor instability and failure can occur when the organic acid production exceeds the capacity for acetate utilization by the methanogens, resulting in the pH falling below the operational range of the methanogens. This is a concern with variable industrial wastewaters, and preventing this failure mode requires active monitoring of the pH and alkalinity within the reactor and reactive control via chemical addition or the use of more complex systems that separate the acidification process from methanogenesis.6,18 Another concern with industrial waste streams is the presence of toxic metal inhibitors, such as copper, nickel, zinc, and chromium.3,4,19−23 While pretreatment is warranted for known inputs of toxic metals, the concern is accidental releases of heavy metals during facility cleaning and maintenance operations or the downward drifting of pH from the neutral zone causing dissolution of heavy metal precipitates within the reactor and flow distribution system. Even though heavy metals can cause the failure of the anaerobic biological processes, these systems do not normally have any control mechanism to prevent their input. 9381
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ACS Sustainable Chemistry & Engineering order of 50 μm diameter, and these offer faster kinetics due to their increased surface area per unit mass and reduced internal diffusion distances as compared to those of resin beads with diameters on the order of 300−1000 μm.26−29 At the same time, IXFs allow practical installation in anaerobic reactors, where they can be suspended from the roof or walls of the reactor as woven mats or as porous pillows, and their submergence into the reactors can be varied without any major difficulty. In this study, we demonstrate that IXFs can stabilize anaerobic biological reactors and provide resilience to pH upset and toxic metal loading. We examined the weak-acid IXF FIBAN X-1 (Figure 2), which contains the iminodiacetate functional group and buffers H+ across a wide pH range.30,31 FIBAN X-1 in equilibrium with wastewater at a relatively high alkalinity will be mostly in the sodium or calcium form. Hydrogen ions have much higher affinity than calcium or sodium, and during an organic overloading event, FIBAN X-1 promptly removes H+ through exchange with Na+ and/or Ca2+; e.g., see the following equation:
order of Cu2+ > Ni2+ > Zn2+ > Mn2+ > Ca2+ > Mg2+, providing a means to remove toxic heavy metals that may be accidentally input into the reactor; e.g., see the following equation: R−CH 2N(CH 2COO)2 Ca + Cu 2 + → R−CH 2N(CH 2COO)2 Cu + Ca 2 +
Chromate inhibition of the anaerobic microorganisms is also a concern, and to address input of this anion, the strong-base IXF FIBAN A-1 was also examined in this study. The ability of IXFs to stabilize anaerobic biological reactors to both pH fluctuations during organic overload events and toxic metal inputs was demonstrated in this study by stressing operating bench-scale reactors with single and repeat organic overloading events and input of toxic metals. The reactors’ methane generation, pH, and chemical oxygen demand (COD) were monitored over time to determine the effects of each event on reactor operation and recovery. The long-term viability of the fibers within anaerobic biological reactors was also demonstrated over one year of operation. Overall, the results provided below demonstrate that IXFs can passively stabilize anaerobic biological reactors and provide a means to enhance sustainable energy generation from high-strength industrial organic waste streams.
R−CH 2N(CH 2COO)2 Ca + 2H+ → R−CH 2N(CH 2COOH)2 + Ca 2 +
(1)
The FIBAN X-1 fibers will then regenerate as the alkalinity and pH increase during reactor recovery from the organic overloading event; e.g., see the following equation:
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MATERIALS AND METHODS
Microbial Culture. The microbial culture was obtained from the anaerobic digester at the Bethlehem, PA, wastewater treatment plant and maintained in a completely mixed 8 L reactor (termed the mother reactor, see Figure S2). The mother reactor was step-fed once daily with a 35 day hydraulic residence time (HRT) at a temperature of 35 °C, and it was used to provide the seed microbial culture for use in the individual experiments. The anaerobic growth media is provided in Table S2. Alkalinity was added to the growth media as NaHCO3 at a concentration of 4 g/L, and lactose was provided as the carbon and energy source at a concentration of 10 g/L. The reactor was monitored for pH and methane generation to ensure stable operation over time. Methane generation was continuously measured using a liquid-displacement gas trap with an in-line NaOH CO2-scrubber (Figure S2). Ion Exchange Fibers. The weak-acid IXF FIBAN X-1 and strongbase IXF FIBAN A-1 were obtained from the Institute of Physical Organic Chemistry, Belarus. FIBAN X-1 is approximately 30 μm in diameter (Figure 2). It contains the iminodiacetate functional group attached to a polyacrylonitrile (PAN) fiber matrix, and the proton exchange capacity is 0.49 mequiv H+/g fiber (Figure S3). FIBAN A-1 is approximately 50 μm in diameter (Figure 2) and consists of the trimethylamine functional group attached to a fiber consisting of polypropylene grafted with copolymer of styrene and divinyl-benzene. Two additional fibers without ion exchange capacity were also examined to identify any effects on reactor stability due to retention of biomass within the fiber mat. These included glass fiber (Fisher Scientific) and PAN fiber (Bluestar Dongda Chemical Co., Ltd.) with the same diameter as FIBAN X-1. Organic Overload Experiments. The organic overloading experiments were conducted using 100 mL media bottles modified with gas and liquid sampling ports added to the cap (Figure S2). The reactors were seeded with the microbial suspension from the mother reactor and then step-fed with anaerobic growth media, with the lactose and alkalinity concentrations varied depending on the experiment. The reactors were mixed on a magnetic stirrer in an incubator at 35 °C, and pH and methane generation were measured as described above. Soluble COD was measured in the reactors by centrifuging samples at 9600g for 1 min, followed by filtering the supernatant through a 0.2 μm cellulose nitrate membrane filter and measuring the solute using USEPA method 8000.
R−CH 2N(CH 2COOH)2 + Ca(HCO3)2 → R−CH 2N(CH 2COO)2 Ca + 2H 2O + 2CO2
(3)
(2)
In this manner, the FIBAN X-1 fibers buffer pH when acid production increases and self-regenerate as the alkalinity recovers, and this cyclic process is depicted in Figure 3. Similarly, FIBAN X-1 takes up cations with a preference in the
Figure 3. Dissipation of protons and cationic heavy metals in an anaerobic biological reactor stabilized by the presence of FIBAN X-1, which contains the iminodiacetate functional group. The shaded chemicals depict biological inhibitors that are mitigated by the iminodiacetate through protonation and metal uptake during overloading events. 9382
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Figure 4. Results of experiments 1 and 2, showing reactor performance to organic overloading stress events, depicted by the gray shading. The stress was initiated as a cessation of influent alkalinity and a 3-fold increase in influent lactose concentration, with prestress influent lactose concentrations of 20 and 15 g/L for experiments 1 and 2, respectively. Horizontal dashed lines are the theoretical methane generation rates under prestress loading conditions, calculated using the biological stoichiometric methane yield (see Figure 1 and the Supporting Information). Four organic overload experiments (1−4) were conducted to examine the ability of FIBAN X-1 to stabilize anaerobic reactors to organic overloading events, with the events initiated through both an increase in influent organic concentration (single and repeat events) and a decrease in hydraulic residence time (HRT). Each experiment consisted of test reactors containing no fiber and different fiber mass loadings, and the reactors were initially run to steady-state conditions (stable pH and methane production) under the experiment’s baseline loading, which required approximately 20 days. The reactors were stressed after approximately 2 weeks of steady-state operation by a 3fold increase in the influent lactose concentration and a reduction in alkalinity (experiments 1−3), or a 3-fold decrease in the HRT (experiment 4). At the end of the overloading event, the reactors were returned to their baseline loadings. The specific conditions for each organic overload experiment are provided in Table S3. Metal Overload Experiments. Four different metal loading experiments (5−8) were conducted to demonstrate that FIBAN X-1
and FIBAN A-1 can reduce the effects of copper, nickel, and chromate loadings on anaerobic reactor operation. These experiments were set up and operated similarly to the organic overload experiments. The growth media was the same as the mother reactor, and the reactors were step-fed at an HRT of approximately 30 days. Reactor pH, methane generation, and aqueous metal concentration were measured over time, with the metal concentration determined by atomic adsorption spectroscopy (PerkinElmer AAnalyst 200). The specific conditions for each metal loading experiment are provided in Table S4. Aging of FIBAN X-1. The IXF exchange capacity during aging in the microbial suspension was examined over 1 year of operation by placing 0.2 g of FIBAN X-1 in 100 mL anaerobic reactors operating under the same conditions as the mother reactor. The fibers were removed at 4, 8, and 12 months and placed into 100 mL of anaerobic growth media at a pH of 6.5. Nickel was added to each reactor at a concentration of 300 mg/L as NiCl2·6H2O, and the equilibrium 9383
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ACS Sustainable Chemistry & Engineering aqueous concentration of nickel was determined by atomic adsorption spectroscopy.
levels in the reactor containing FIBAN X-1, while other reactors did not recover. A second overloading event was applied to the FIBAN X-1 reactor from days 68 to 75, and the reactor demonstrated improved performance during this second event, with higher pH, higher methane generation, and lower COD as compared to those of the first event. This is likely due to a higher concentration of methanogens present after growth during the first organic overloading event and demonstrates that the IXF pH buffering can help maintain active biomass and enhance reactor resilience during sequential organic overloading events. Examination of the first two experiments shows that a reactor with 5 g/L of FIBAN X-1 was resilient to an increase in influent lactose from 15 to 45 g/L, but not to an increase from 20 to 60 g/L, demonstrating a relationship between the organic overloading event and requisite IXF mass concentration on reactor resilience. The impact of FIBAN X-1 mass concentration on reactor resilience was examined in experiment 3, and the results are provided in Figure 5. The reactors reached and maintained steady-state conditions after ∼17 days of operation, and the organic overloading event was conducted from days 36 to 46. The three stressed reactors all demonstrated a decrease in pH during the stress period, with the magnitude of the pH drop decreasing with increasing mass of FIBAN X-1 present in the reactor. Concurrently, the methane production rates at the
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RESULTS AND DISCUSSION IXF Stabilization of Reactor Performance during Organic Overloading Events. Results from the first organic overloading experiment are provided in Figure 4. The reactors reached and maintained steady-state conditions by 20 days of operation. As seen here (dashed line in Figure 4) and in the upcoming experiments, the baseline methane generation rates were in good agreement with those predicted using the theoretical biological stoichiometric yield of 0.20 g of CH4 produced per g of lactose consumed, even with the assumptions inherent in these calculations (see Figure 1 and the Supporting Information). The organic overloading stress event was applied from days 35 to 39, and the pH dropped in all reactors during the stress period. The methane production rates initially increased because of the increase in lactose loading, and they then decreased as the pH decreased in the reactors, indicating that the methanogenic archaea could not keep up with the increase in acid production. COD simultaneously increased by over an order of magnitude during the stress period. While the reactors in this initial experiment all failed from this severe stress (methane production ceased), there was a difference between reactors with no fiber, neutral PAN and glass fibers, and FIBAN X-1. The reactors with FIBAN X-1 demonstrated a resilience to the organic overloading, with methane production approximately 2.8× higher at the end of the stress period compared to that of the reactor with no fiber, and approximately 1.8× greater than those of the reactors with the neutral fibers. The enhanced performance with reactors containing the neutral fibers compared to that with the reactor with no fibers can likely be attributed to a higher biomass concentration and longer solids retention time due to observed biomass floc held within the fiber mat.32 This suggests that the benefits observed with FIBAN X-1 are pH buffering via ion exchange, supplemented by biomass retention in the fiber mat. For the second experiment, the influent lactose concentration was decreased to 15 g/L, and the 3-fold concentration increase during the overloading event was maintained. The reactors reached and maintained steady-state conditions after approximately 20 days, and the first of two organic overloading events was applied from days 36 to 43 (Figure 4). Similar to experiment 1, the methane production rates initially increased because of the increased organic loading rate, and they then decreased as the reactor pH decreased. Concurrently, the COD concentrations increased during the stress event. As with the first experiment, there was an observable difference between the reactors with no fiber, neutral PAN and glass fibers, and FIBAN X-1. Upon the cessation of the first overloading event, the reactor with FIBAN X-1 had an elevated methane production rate, while methane production ceased in the reactors without FIBAN X-1. The associated pH values in the reactors containing no fiber, PAN fiber, and glass fiber were 4.9, 5.1, and 5.5, respectively, and the pH in the FIBAN X-1 reactor was 6.5, demonstrating that FIBAN X-1 buffered the reactor pH and maintained it in the optimal range of methanogens. This facilitated use of the increased substrate concentration in the reactor, which was observed as the higher methane production rate and the lower COD concentration compared to those of the other reactors. Upon cessation of the stress period, the pH, COD, and methane production rate returned to the prestress
Figure 5. Results from experiment 3 showing effects of FIBAN X-1 mass loading on reactor stabilization to an organic overloading event, initiated by a 3-fold increase in influent lactose concentration, depicted by the gray shading. Horizontal dashed line is the theoretical methane generation rate under prestress loading conditions, calculated using the biological stoichiometric methane yield (see Figure 1 and the Supporting Information). 9384
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ACS Sustainable Chemistry & Engineering beginning of the stress period increased in all three reactors because of the increased organic loading rate, and they then decreased over time as the pH decreased in the reactors. The reactor with no fiber failed at day 46 (methane generation ceased), and the pH in the reactor had decreased to approximately 5.5, whereas the pH in the reactors with 2 and 10 g/L of FIBAN X-1 had decreased to approximately 6.1 and 6.6, respectively. The methane generation in the 2 g/L reactor was moderately lower during the stress period, and it remained elevated in the 10 g/L reactor. Upon cessation of the stress period, the two reactors containing FIBAN X-1 recovered, whereas the reactor without fiber did not. The rise in methane production rate above the steady-state value in the 2 g/L reactor from days 56 to 67 suggests that the methanogens used the excess acetate remaining in the reactor once the pH rose above approximately 6.4. These experiments demonstrated that FIBAN X-1 provided resilience to organic overloading initiated as an increase in substrate concentration. The effects of an increase in organic loading via a reduction in the HRT were examined in the fourth experiment. This form of overloading simultaneously increases the organic loading rate and decreases the solids retention time, both of which stress the reactors. The reactors reached steadystate operation at ∼16 days, and the organic overloading was conducted as a decrease in HRT from 15 to 5 days during experiment days 21−27. As seen in Figure 6, the reactor with 5 g/L FIBAN X-1 showed excellent resilience to the organic overloading event, with a stable pH, elevated methane production, and slight increase in reactor COD. The reactor with 2 g/L of FIBAN X-1 showed an initial increase in methane generation; however, the pH buffering was not sufficient to maintain conditions for the methanogenic archaea to thrive during this stress event, and the reactor ultimately failed. This can be seen in Figure 6, where the pH declined through the stress period and leveled off at approximately 5.5. The methane generation correspondingly decreased to zero while the COD increased. Similar results are seen with the reactor with no fiber, with the reactor failure occurring more rapidly than with the 2 g/L FIBAN X-1 reactor. Relationship between Normalized Methane Generation and pH. These organic overloading results demonstrate that FIBAN X-1 can provide resilience to single and repeat events, because of both an increase in influent organic concentration and a decrease in HRT, and that the reactor stability is a function of the amount of IXF present in the reactor. This approach is readily scaled-up, and given an estimation of magnitude, duration, and frequency of overloading events for a specific organic waste stream, it should be possible to determine the mass of IXFs necessary to buffer the pH through bench-scale experimentation. To do this, the minimum pH necessary to allow reactor recovery from organic overloading events must be known. Two main factors can affect methane production during an organic overloading event. The first factor is positive, where the increase in organic substrate loading can result in a higher methanogen growth rate and an increase in methane production. The second factor is negative, where the increase in organic acid production can lower the pH and inhibit methane production. The purpose for introducing IXFs into the reactor is to passively mitigate the negative factor and maintain a suitable pH for methanogenic activity, allowing the positive factor to dominate and methane production to
Figure 6. Results from experiment 4 showing the performance of reactors undergoing an organic overloading event, initiated as a 3-fold reduction in HRT from 15 to 5 days, depicted by the gray shading. Horizontal dashed line is the theoretical methane generation rate under prestress loading conditions, calculated using the biological stoichiometric methane yield (see Figure 1 and the Supporting Information).
increase. The interrelationship between these two factors is seen in the experimental results presented in Figures 4−6. To elucidate this relationship across the different experiments, the methane production rates (mL/d) from the data in Figures 4−6 were normalized by dividing them by their associated average methane production rate during the prestress steady-state reactor operation. These normalized production rates are plotted as a function of reactor pH in Figure 7, and examination of this figure shows three distinct operational regions. The first region is above pH 6.4, where, except for two data points shown as the hollow circles, the reactors operated at or above the steady-state values for methane generation in this region. It is in this region where 9385
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with FIBAN X-1 because of exchange onto the iminodiacetate functional groups. This was confirmed using SEM−EDX (Figure S4), showing nickel measured across the FIBAN X-1 fiber. The reactor with FIBAN X-1 recovered from the nickel loading. In contrast, the pH and methane production in the reactor without fiber continued to decline, with pH stabilizing at approximately 5.6 and methane production ceasing. This suggests that the nickel impacted the methanogens more than the acid-producing bacteria, leading to acid accumulation and inhibition of methanogenesis due to both the higher nickel concentration and the low pH. Results with copper loading in Figure 8b are similar to the nickel results, where the two reactors with FIBAN X-1 recovered from the copper loading, and the reactor without FIBAN X-1 failed. One difference compared to the nickel experiment is that, because of the low solubility of copper in the neutral pH range, the aqueous concentration of copper was similar between the three reactors during the first loading event. The results suggest that the copper impacted the methanogens to a greater extent than the acidogens, resulting in acid accumulation and a rapid pH drop, and that the FIBAN X-1 mitigated the copper loading through pH control. The methane production was initially depressed, and it then increased above the baseline values as the pH recovered, indicating that the methanogens used excess acetate that accumulated while methanogenesis was impacted by the copper loading. The FIBAN X-1 reactors were allowed to recover, and then a second copper loading event was applied. The reactors with FIBAN X-1 recovered more quickly as compared to their response to the first event. In the first cycle, the days required for pH and methane production to recover in the reactor with 2 g/L FIBAN X-1 were 49 and 19 days, respectively, whereas, in the second cycle, they were 39 and 10 days. Similarly, in the reactor with 8 g/L FIBAN X-1, the corresponding data in the first cycle were 38 and 16 days, and in the second cycle they were 28 and 4 days. This enhancement during the second loading event is likely due to acclimation of the methanogens to the presence of the copper during the first cycle.4,33 Overall, both experiments 5 and 6 suggest that nickel and copper impacted the methanogens to a greater extent than the acidforming bacteria, as observed in the decrease in pH, and that FIBAN X-1 mitigated the impact of the metal loadings through combination of metal exchange and pH buffering. Results from the chromate loading experiment with the strong-base IXF FIBAN A-1 (experiment 7) are shown in Figure 8c. The aqueous chromium concentration decreased with increasing FIBAN A-1 mass loading because of exchange onto the trimethylamine functional groups, and this was confirmed using SEM−EDX (Figure S4). The chromate input resulted in cessation of methane production for the reactor with no fiber; a rapid decline in production in the reactor with 2 g/L FIBAN A-1; and a gradual decline in production in the reactor with 6 g/L FIBAN A-1. Correspondingly, there was a minor drop in pH in the two reactors with FIBAN A-1 and a larger drop in the reactor without fiber, indicating that chromate impacted the methanogens to a greater extent than the acidforming bacteria, similar to the observations with nickel and copper. All the reactors began to recover after the end of the shock period, and the reactors with FIBAN A-1 showed enhanced methane production as the pH recovered and as the methanogens utilized the accumulated acetic acid. Conversely, the reactor with no fiber was not able to recover quickly enough
Figure 7. Relationship between solution pH and normalized methane production for all data points in Figures 4−6. Methane production is normalized by dividing the daily methane production rate by the average steady-state methane production. The black rectangle highlights the data from the steady-state operation of the reactors, and the horizontal lines delineate the upper and lower bounds of the steady-state methane production. Vertical lines depict regions of stable operation, transition, and reactor failure. See the text for further discussion.
FIBAN X-1 maintained the pH within the operational range of methanogens, allowing them to utilize the increase in substrate concentration. The second region is below pH 6, where the reactors were inhibited and the methane production rate was below the steady-state range. In this region, most measurements showed no methane production as the pH dropped below 5.5. The hollow symbols in this region are values from experiment 2 where the pH dropped rapidly over the course of the 24 h measurement period associated with the data points, and the cumulative methane production did not directly correspond to the final pH measured at the end of this period (i.e., the methane would have been generated while the pH was higher, earlier in the period). The range between pH 6.0 and 6.4 was a transition region where the methane generation rates spanned above and below the steady-state operational range. It is in this region that the reactors were tending to failure, and for the stresses applied in this study, recovery occurred only in reactors containing a sufficient mass of FIBAN X-1 to arrest the decline in pH. These results indicate that a sufficient mass loading of IXF should be identified in bench-scale testing that maintains pH ≥ 6.4 for the system at hand, as this allows for continued operation during organic overloading events. Additionally, these experimental results suggest that the pH should not be allowed to decrease below 6.0 during an organic overloading event, as this resulted in continued pH decline and reactor failure. IXF Stabilization of Reactor Performance during Metal Loading Events. Results from the first metal loading experiment (experiment 5) are shown in Figure 8a. The addition of nickel resulted in an immediate decline in methane production and slight drop in pH in both reactors. As expected, the aqueous nickel concentration was much lower in the reactor 9386
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Figure 8. Results of experiments 5−7, showing IXF stabilization of the anaerobic reactors to inputs of nickel, copper, and chromate, respectively, depicted by the gray shading. Horizontal dashed lines represent the theoretical volume of methane generated during a sampling period under prestress loading conditions, calculated using the biological stoichiometric methane yield (see Figure 1 and the Supporting Information).
to overcome the decline in pH, and it subsequently failed as the pH continued to decline. The effects of both FIBAN X-1 and neutral glass fiber on reactor performance during a nickel exposure were compared in experiment 8, with the goal to demonstrate any effects of biomass retention by the fibers. The results, summarized in Figure S5, showed large, nearly identical, reductions in methane generation for the reactors with no fiber and with 8 g/L of glass fiber, while methane generation in the reactor with 8 g/L of FIBAN X-1 was not impacted by the nickel exposure. There was a slight decrease in aqueous nickel concentration in the reactor with the glass fiber as compared to the reactor with no fiber. Visual inspection showed biomass being retained within the fiber mass, suggesting that the small reduction in aqueous nickel concentration with the glass fiber reactor was due to sorption on this additional biomass. The reactor with FIBAN X-1 showed a much lower aqueous nickel concentration, similar to that seen in experiment 5. Finally, the pH values in all reactors stayed above 6.6 during the experiment, indicating that it was the presence of nickel, not the pH, that caused the observed reduction in methane production. These results suggest that while the presence of fiber, whether IXF or neutral fiber, may help to retain biomass (as also indicated in the
organic overloading experiments), only the IXF mitigated the input of nickel on reactor performance. Overall, the results indicate that the input of toxic metals impacted the methanogens to a greater extent than the acidforming bacteria. This was observed as a decrease in pH, and this decrease in pH then further impacted the methanogens and resulted in decreased reactor performance and reactor failure. The presence of FIBAN X-1 and FIBAN A-1 reduced these toxicity and pH effects and allowed the reactors to recover to the metal loading events. As with the organic overloading experiments, this approach is readily scaled-up, and given a particular waste stream, it should be possible to determine the mass of IXF necessary to mitigate a specified metal input through bench-scale experimentation. Long-Term Operation of FIBAN X-1 in Anaerobic Biological Reactors. For an IXF to remain viable, its exchange capacity must not decline significantly during prolonged operation within a microbial reactor. This was examined by quantifying the nickel exchange capacity with FIBAN X-1 after operation in test reactors for up to 1 year. Unused FIBAN X-1 absorbed nickel with a capacity of 1.16 mequiv/g fiber. During the 1 year exposure period, a change in color of the fibers was observed, from white to an orangebrown (Figure S6). The available ion exchange capacity, 9387
DOI: 10.1021/acssuschemeng.7b02435 ACS Sustainable Chem. Eng. 2017, 5, 9380−9389
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ACS Sustainable Chemistry & Engineering however, was only reduced by ∼5% during the 12 months of operation inside the biological reactor. The affinity of nickel on the iminodiacetate functional group is comparable to those of zinc and cadmium, and less than that of copper. Thus, the viability of the process with nickel as the target toxic metal assures its effectiveness with other commonly encountered toxic metals. This observation confirms that access to the IXF ion exchange sites remained essentially unaffected in an intense biological medium and indicates that the fibers may be able to last years before needing replacement. This study demonstrated a novel approach using ion exchange fibers to stabilize anaerobic biological reactors to operational fluctuations without the need for active control and chemical addition. Specifically, this study demonstrated that IXFs satisfy three essential requirements for improving anaerobic reactor stability: 1. IXFs can buffer pH fluctuations due to variations in organic loading. 2. IXFs can dissipate shock-loads of heavy metals through selective sorption and pH control. 3. IXFs are compatible with an aqueous phase with high solids content and retain their exchange capacity after prolonged operation; additionally, these fibers have the benefit that they can be formed as woven mats or porous pillows, and can be readily inserted and withdrawn as required. This approach is readily scaled-up, and given an estimation of magnitude, duration, and frequency of loading events for a specific waste stream, it should be possible to determine the mass of IXF necessary to stabilize a given system through bench-scale and pilot-scale experimentation.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02435. Tables and figures of supporting data including background data, growth media composition, experimental conditions, additional experimental results, and biological stoichiometric calculations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 610-758-3543. Fax: 610-758-6405. E-mail: dgb3@ lehigh.edu. ORCID
Derick G. Brown: 0000-0001-7311-3135 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially supported by the Pennsylvania Infrastructure Technology Alliance (PITA). REFERENCES
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DOI: 10.1021/acssuschemeng.7b02435 ACS Sustainable Chem. Eng. 2017, 5, 9380−9389
