Romsan Madmananga,
Zhen Heb and
Tongchai Sriwiriyarat
*c
aEnvironmental Science Program, Faculty of Science, Burapha University, Chonburi, Thailand
bDepartment of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
cDepartment of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi, Thailand. E-mail: sriwiri@buu.ac.th
First published on 11th October 2018
The acute effects of acrylamide and its biodegradation products on the respiration activities of microbes during wastewater treatment are not well understood. Herein, unacclimatized mixed culture bacteria and Enterobacter aerogenes from two aerobic treatment systems, Activated Sludge (AS) and Integrated Fixed Film Activated Sludge (IFAS) both of which were sequencing batch reactors (SBR), were studied for their response to acrylamide. Respiration activities and biodegradation rates were determined by both the OxiTop respirometer and batch studies. The experimental results revealed that E. aerogenes in the AS system quickly removed both acrylamide and acrylic acid without the need of an acclimation period, but required two hours for removing acrylic acid in the IFAS system. The mixed culture bacteria in both AS and IFAS systems required 2 hours to acclimatize with acrylamide and 1 hour for acrylic acid, respectively. Acrylic acid was initially polymerized to produce acrylic acid polymer or reacted with ammonia to form acrylamide, resulting in the reduced acrylamide biodegradation rate. Both E. aerogenes and mixed culture bacteria from the AS systems could simultaneously biodegrade acrylamide and acrylic acid whereas only acrylamide was biodegraded by both cultures in the IFAS systems due to the limitation of acrylic acid diffusion. The results also indicated that ammonia inhibited the acrylamide biodegradation by both E. aerogenes and mixed culture bacteria from the AS systems. The unacclimatized E. aerogenes and mixed culture bacteria from the AS systems showed superior performances compared to the ones from the IFAS systems.
It has been reported that acrylamide is lethal to most microorganisms due to its inhibitory effect on sulfhydryl proteins.3,4 However, several laboratory studies have reported the success of using several microbial genera including Arthrobacter sp., Nocardia sp., Bacillus sp., Xanthomonas sp., Rhodopseudomonas sp., Ralstonia sp., Geobacillus sp., Pseudomonas sp., Rhodococcus sp., K. georgiana, and E. aerogenes5–9 to degrade acrylamide in aquatic and soil environment. Among them, E. aerogenes was reported to be capable of degrading acrylamide at a concentration as high as 5000 mg AM L−1 in the culture media.5 E. aerogenes could adapt their metabolisms including the evolution of genes for encoding the amidase and other synthesis proteins for the deamination of acrylamide during the acclimation period.4 Some of these studies compared the acrylamide biodegradation efficiencies between microorganisms cultured as free cells and immobilized cells. It was found that the free cells of Pseudomonas aeruginosa began to degrade acrylamide after an incubation period of 24 h, but the immobilized cells removed acrylamide within 24 h.8 Another study found that the immobilized cells of Rhodococcus sp. could degrade 64 mM of acrylamide within 3 h, but free cells needed a longer period than 24 h to remove it.7 On the other hands, Buranasilp and Charoenpanich5 reported that free cells of E. aerogenes degraded acrylamide within 1 h after incubation, but immobilized cells of E. aerogenes took over 6 h after cultivation to degrade acrylamide. It appears that immobilized cells and free cells of bacteria in the culture media required different acclimation time for adapting them with acrylamide and this warrants further investigation.
As a result of acrylamide biodegradation, ammonia and acrylic acid (AA) are produced by the deamination reaction with amidase as a catalyst.6,8–10 For wastewater treatment practices, additional carbon oxidation of acrylic acid and biological nitrogen removal (BNR) of ammonia would be required to meet the effluent standards. Jangkorn et al.11 studied the acrylamide biodegradation by E. aerogenes and mixed culture bacteria in the SBR activated sludge (AS) treatment system. They found that both E. aerogenes and mixed bacteria culture could completely mineralize acrylamide at 200 mg AM L−1. Free ammonia nitrogen (FAN) from the acrylamide biodegradation accumulated in the SBR systems and inhibited the acrylamide biodegradation and nitrification. Acrylic acid from the acrylamide biodegradation is typically considered as a readily biodegradable substrate and can be rapidly oxidized by various microorganisms under an aerobic condition.12 Larson et al.13 reported that mixed culture bacteria that were acclimatized with a mixture of acrylic acid polymers in the AS system degraded completely both acrylic acid monomer and dimer. However, the biodegradability of acrylic acid decreases proportionally with the increase of molecular weight of acrylic acid polymer. These previous findings suggest that both acrylamide biodegradation products including ammonia and acrylic acid are possible to interfere with the acrylamide biodegradation.
To implement the acrylamide biodegradation successfully in the biological wastewater treatment system, it is necessary to increase the capacity of systems and to minimize the amount of ammonia. Integrated Fixed Film Activated Sludge (IFAS), a hybrid system of both suspended growth (free cells) and attached growth (immobilized cells), has widely been accepted as an alternative method to sustain nitrification at low temperatures14,15 and to enhance the capacity and stability of an activated sludge system.16 With effective nitrification, the FAN inhibition can be minimized, resulting in the enhancement of acrylamide biodegradation. Nevertheless, as reported previously, the acclamation periods were required differently between free cells and immobilized cells of unacclimatized microorganisms in the culture media. It is important to investigate the acclimation period of unacclimatized bacteria in both AS and IFAS for acrylamide biodegradation. In this study, the acute effects of acrylamide and its biodegradation products including ammonium and acrylic acid on the unacclimatized E. aerogenes and mixed culture bacteria in the conventional AS and IFAS sequencing batch reactor (SBR) wastewater treatment systems were evaluated with the respirometric activities and biodegradation studies. Specifically, the acclimation periods, respirometric activities and biodegradation rates of both unacclimatized bacteria from both biological wastewater treatment processes were reported.
All four SBR systems were fed with synthetic wastewater containing total chemical oxygen demand (TCOD) of 445 ± 30 mg COD L−1 and total Kjeldahl nitrogen (TKN) of 42.3 ± 2.5 mg N L−1 resulting in the carbon to nitrogen ratio (C/N ratio) of 10.5. The synthetic wastewater was prepared with 12 g of sucrose (Commercial Grade, Wangkanai, Thailand), 24 g of CH3COONa (Industrial Grade of 58.8%, Sinoway International, China), 2.0 g of K2HPO4 (Food Grade of 99.2%, Young Jin Chemical, South Korea), 4 g of KH2PO4 (ACS Grade, VWR Chemicals, EC), 20 g of NaHCO3 (Food Grade of 99.5%, Tianjin Soda Plant, China), 9 g of NH4Cl (Industrial Grade of 99.5%, Tianjin Soda Plant, China), 2.8 g of MgCl2 (Industrial Grade of 47%, Dead Sea Works, Ltd., Israel), and 1.6 g of CaCl2 (Food Grade of 74.0%, Young Jin Chemical, South Korea) in a 40 L tap water. The total suspended solids (TSS) concentrations and pH values of the synthetic wastewater were 83.8 ± 9.2 mg SS L−1 and 7.2 ± 2.7, respectively.
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To determine the respirometric activities of E. aerogenes and mixed culture bacteria in response to the synthetic wastewater, acrylamide, and acrylic acid additions, both E. aerogenes and mixed culture bacteria were collected from the AS-1, AS-2, IFAS-1, and IFAS-2 SBR systems at the end of reacting period in each cycle to obtain the total sludge volume of 1 L after achieving the steady state conditions. Each type of sludge was washed three times with distilled water to remove remaining substrates and subsequently was diluted with distilled water to obtain 2 L sludge samples (50% dilution factor). The sludge was transferred into four 1 L OxiTop bottles (0.5 L each). Subsequently, all OxiTop bottles were injected with three different substrates including synthetic wastewater that was fed to the SBR systems, acrylamide (Acrylamide PAGE, GE Healthcare Bio-Sciences, USA), and acrylic acid (ACS Grade 99%, Sigma-Aldrich, Netherlands) at the concentrations of 400 mg COD L−1, 400 mg AM L−1 (5.63 mM), and 400 mg AA L−1 (5.55 mM), respectively. Acrylic acid contained 180–200 ppm of hydroquinone monomethyl either (MEHQ) or methoxyphenol as a polymerization inhibitor. Ammonium at a concentration of about 50 mg N L−1 was added as a nitrogen source for synthetic wastewater and acrylic acid. Two different ammonium concentrations of 50 and 500 mg N L−1 were added with acrylamide to evaluate the inhibitory effects of ammonia on the acrylamide biodegradation as reported by Jangkorn et al.11 Furthermore, 20 BioPortz media containing the sludge were randomly taken from the IFAS-1 and IFAS-2 SBR systems and added into the OxiTop bottles. Other nutrients similar to the synthetic wastewater were also supplemented for acrylamide and acrylic acid. Two mL of N-allylthiourea (C4H8N2S) (98%, Alfa Aesar, UK) at a concentration of 5 g L−1 were added to each bottle to inhibit nitrification; therefore, the effects of ammonia on acrylamide biodegradation could be evaluated. To minimize the difference in temperature between sample and incubator, all chemicals and sludge samples were allowed to acclimatize with the room temperature of ∼28 °C. The OxiTop system was incubated in the incubator controlled at the temperature of 28 °C, which was the same operating temperature of SBR systems, for a time period of 5 days. The pressure data were recorded at 20 min time interval by a handheld remote controller and then were transferred to the personal computer (PC) via a cable and a software called Achat OC (version 2.03).
The biodegradations of acrylamide and its products including ammonia and acrylic acid were also conducted in parallel with the respirometric evaluations in four closed and stirred bottles for each type of microorganisms from the AS-1, AS-2, IFAS-1, and IFAS-2 systems. Each sludge was prepared according to the procedures described above and then transferred to the bottles. The experiments were setup in similar to the OxiTop bottles and were run in the same incubator as the OxiTop system at the temperature of 28 °C. The samples were collected at different time intervals from the bottles for parameter analyses including COD, acrylamide, acrylic acid, ammonium, nitrite, nitrate, pH, MLSS, and MLVSS.
For the determination of attached biomass in the BioPortz media, two BioPortz media were randomly sampling from the IFAS systems. The biomass was removed out from the media with a high-pressurized water jet produced from a syringe into a beaker. After mixing the samples to achieve homogenous liquid, the samples were collected for further MLSS and MLVSS analyses. The MLSS and MLVSS concentrations were calculated to determine the attached biomass per BioPortz media so that the biomass residing in the 510-BioPortz media of IFAS SBR systems could be determined. Equivalent MLSS and MLVSS concentrations of attached biomass were then calculated by dividing the amount of biomass by the volume of reactors. The combination of both suspended and attached biomass indicated the total MLSS and MLVSS in this study. A high performance liquid chromatography (HPLC) (Varian 9050) was used to quantitatively determine acrylamide and acrylic acid. The HPLC was equipped with a UV spectrophotometric detector (JENWAY 6305) operating at 254 nm and a Nova-Pack C18 (4 μm 60 Å) guard pak insert column (Waters, Ireland) in a reversed system using 50% deionized water and 50% acetonitrile as a mobile phase. A 60 μL injection loop of filtered sample was injected into the HPLC system to deliver 20 μL volume to obtain the peak areas as shown in Fig. 2 with a run time of 5 min. at room temperature under a constant flowrate of 1 mL min−1. Acrylamide and acrylic acid standards were used to determine both acrylamide and acrylic acid concentrations. The first and second peaks were identified as acrylic acid and acrylamide, respectively.
System | Inocula | MLSS mg L−1 | MLVSS mg L−1 | Equivalent MLSS mg L−1 | Equivalent MLVSS mg L−1 | Total MLSSmg L−1 | Total MLVSSmg L−1 |
---|---|---|---|---|---|---|---|
AS-1 | E. aerogenes | 913 ± 54 | 903 ± 54 | — | — | 913 ± 54 | 903 ± 54 |
IFAS-1 | E. aerogenes | 1233 ± 60 | 1223 ± 60 | 3188 | 1887 | 4420 ± 60 | 3110 ± 60 |
AS-2 | Mixed culture | 855 ± 42 | 845 ± 42 | — | — | 855 ± 42 | 845 ± 42 |
IFAS-2 | Mixed culture | 1668 ± 127 | 1613 ± 97 | 4361 | 2601 | 6028 ± 127 | 4214 ± 97 |
All four SBR systems achieved the similar COD removal efficiencies of about 80% within the reacting period of 10 h, resulting in the effluent COD concentrations of about 80 mg COD L−1. With regarding to nitrification, all four systems could completely nitrify ammonium resulting in the ammonium removal efficiencies of nearly 100%. Nitrite was not detectable in all SBR systems, indicating that nitrite was rapidly converted to nitrate nitrogen. The effluent nitrate concentrations of AS-1, IFAS-1, AS-2, and IFAS-2 were 33.2 ± 8.3, 29.6 ± 7.0, 34.1 ± 5.9, and 29.3 ± 7.4 mg N L−1, respectively. E. aerogenes in both the AS-1 and IFAS-1 systems could heterotrophically nitrify the ammonium nitrogen.11,24 Mixed culture bacteria in the AS-2 and IFAS-2 systems contained nitrifiers that autotrophically removed the ammonium nitrogen.
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Fig. 3 Pressure drops during 5 day incubation period of (a) E. aerogenes (AS-1) and (b) mixed culture bacteria (AS-2). |
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Fig. 4 Pressure drops during 5 days incubation period of (a) E. aerogenes (IFAS-1) and (b) mixed culture bacteria from the IFAS-2 systems. |
It appears that the pressures in four OxiTop bottles initially increased with time during the first 1–2 hours, reaching different maximum pressures depending on the substrates. The maximum pressure of 9 hPa was observed after the incubation period of 1 h for synthetic wastewater. It is expected that E. aerogenes had already been acclimatized with synthetic wastewater in the AS-1 system because the systems reached the steady state condition; therefore, it is likely that the increase in pressure was resulted from the temperature. After the temperature equilibrium, there was a linear decrease in pressure with time due to the microbial metabolism consuming oxygen and producing carbon dioxide. E. aerogenes required no acclimation period to synthetic wastewater. The results from the linear regression analyses revealed that there were two different slopes on the straight line, likely related to the degradation of acetate and sucrose as carbon sources in the synthetic wastewater. It was observed that the negative pressure of 14 hPa remained constant for one hour during the 7th and 8th hour after the addition of synthetic wastewater, indicating a short lag phase in this period. It is evident that E. aerogenes exhibited the diauxic growth pattern with the presence of two carbon sources.25 After the linear decrease of pressure, it appears that E. aerogenes consumed oxygen at a slower rate, suggesting that less biodegradable substrate was available or endogenous respiration occurred. It is expected that oxygen in the gaseous phase was not depleted; otherwise, the pressure drop must remain constant over a period of time. Table 2 lists the oxygen uptake rates (OURs) and the specific oxygen uptake rates (SOURs) of E. aerogenes for biodegrading the synthetic wastewater.
Experiments | SBR system | E. aerogenes | Mixed culture bacteria | ||||||
---|---|---|---|---|---|---|---|---|---|
OUR mg O2 L−1 h−1 | SOUR mg O2 (g VSS)−1 h−1 | OUR mg O2 L−1 h−1 | SOUR mg O2 (g VSS)−1 h−1 | ||||||
1 | 2 | 1 | 2 | 1 | 2 | 1 | 2 | ||
a Ammonium as a nitrogen source at the concentration of about 50 mg N L−1.b Ammonium as a nitrogen source at the concentration of about 500 mg N L−1; 1 and 2 indicated the first and second OUR and SOUR rates. | |||||||||
Synthetic wastewatera | AS | 5.81 | 5.97 | 10.75 | 11.05 | 4.09 | 5.99 | 7.05 | 10.33 |
Acrylamidea | 2.05 | — | 3.80 | — | 1.90 | — | 3.28 | — | |
Acrylamideb | 1.54 | — | 2.86 | — | 1.56 | — | 2.69 | — | |
Acrylic acida | 4.97 | — | 9.21 | — | 3.83 | 6.68 | 6.60 | 11.51 | |
Synthetic wastewatera | IFAS | 3.34 | — | 0.94 | — | 2.70 | — | 1.13 | — |
Acrylamidea | 1.37 | — | 0.38 | — | 1.32 | — | 0.55 | — | |
Acrylamideb | 1.40 | — | 0.39 | — | 1.48 | — | 0.62 | — | |
Acrylic acida | 2.28 | 3.56 | 0.64 | 1.00 | 1.66 | 2.35 | 0.70 | 0.98 |
According to the results in Fig. 3(a), the highest pressure was detected for acrylic acid at 40 hPa during the first two hours of incubation period, which was much higher than the maximum pressure as a result of temperature difference in the OxiTop bottle containing synthetic wastewater (9 hPa). It is noted that acrylic acid can be easily polymerized and then a large amount of heat can be generated due to the exothermic polymerization of acrylic acid. Even though the MEHQ as a polymerization inhibitor was added into acrylic acid to prevent polymerization, some contamination or excessive heat can cause polymerization.26 The results indicate that pressure in the OxiTop bottle containing acrylic acid increased tremendously as compared with synthetic wastewater; therefore, the acrylic acid must have been polymerized. After the temperature equilibrium, the pressures decreased linearly in the similar fashion to the synthetic wastewater. It can be interpreted that E. aerogenes did not require any acclimation time for acrylic acid biodegradation. However, the OUR and SOUR of acrylic acid by E. aerogenes in Table 2 suggest that the rates were slightly lower than the synthetic wastewater.
Acrylamide was reported as a toxic compound to most microorganisms;3,4 therefore, the experiments spiked acrylamide to two OxiTop bottles with different ammonium concentrations for evaluating the respirometric activities. The maximum pressures of 15 hPa and 18 hPa were found at the incubation period of 2 hours for acrylamide with ammonium concentrations of about 50 and 500 (mg N) L−1, respectively. Higher increases in pressure of acrylamide than synthetic wastewater indicate that a certain process occurred in the systems in addition to the temperature difference; otherwise, the increases in pressure must be the same as the synthetic wastewater. It is possible that acrylamide was biodegraded and then acrylic acid was produced and polymerized. A linear reduction in pressure with time was found during a period between 2nd hour and 65th hour as a result of microbial oxygen consumption. It appears that E. aerogenes from the AS-1 system did not require the acclimation period to acrylamide. The calculated OUR and SOUR in Table 2 indicate that the rates were considerably lower than the rates of E. aerogenes for biodegrading the synthetic wastewater or acrylic acid. It was evident that the FAN reduced the OUR and SOUR of E. aerogenes as compared with acrylamide supplemented with lower ammonium concentration. However, the inhibition effects of ammonia on the acclamation period of E. aerogenes was not found in this experiment.
Fig. 3(b) shows the respirometric activities of mixing culture bacteria from the AS-2 system degrading different substrates in the OxiTop bottles. The pressures increased during the initial incubation time of 2–3 hours to 12, 24, 24, and 42 hPa when degrading synthetic wastewater, acrylamide with ammonium concentration of 50 mg N L−1, acrylamide with ammonium concentration of 500 mg N L−1, and acrylic acid, respectively. The pressure dropped immediately after the temperature equilibrium, suggesting that the acclimation period was not required for mixed culture bacteria to degrade synthetic wastewater. The results revealed that the pressures remained constant for 1 hour for the acrylic acid biodegradation and for 2 hours to degrade acrylamide with both ammonium concentrations of 50 and 500 mg N L−1. Thus, the mixed culture bacteria required an acclimation period to biodegrade acrylamide or acrylic acid. The FAN inhibited the acrylamide biodegradation of mixed culture bacteria, resulting in lower biodegradation rates. There was no difference in terms of the inhibition effects of the FAN on the acclimation period. The OUR of mixed culture bacteria for the acrylic acid biodegradation was less than that of E. aerogenes during the first 25 hours, but subsequently the rate greatly increased. It was reported that acclimatized mixed culture bacteria completely biodegraded both acrylic acid monomer and dimer, and the biodegradability of acrylic acid decreased as the molecular weight increased.13 Therefore, it is suggested that the unacclimatized mixed culture bacteria required time to reduce the polymerized acrylic acid.
When the mixed culture bacteria was immobilized in the BioPortz media (IFAS-2 system), the time to reach temperature equilibrium was longer than the suspended growth of mixed culture bacteria (AS-2 system) (Fig. 4b). After reaching temperature equilibrium, the mixed culture bacteria from the IFAS-2 system required 1 hour to acclimatize with acrylic acid and acrylamide with ammonium concentration of 50 mg N L−1. The FAN resulted in longer acclimatization period of 3 hours for the mixed culture bacteria from the IFAS-2 system to biodegrade the acrylamide. It appears from the OURs and SOURs (Table 2) that the inclusion of the attached growth of mixed culture bacteria into the suspended biomass was not superior to biodegrade the synthetic wastewater, acrylamide, and acrylic acid over the suspended growth system.
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Fig. 5 Acrylic acid and acrylamide concentrations in the batch reactors containing E. aerogenes and mixed culture bacteria from the AS and IFAS systems (AA: acrylic acid and AM: acrylamide). |
After the initial removal of acrylic acid due to the acrylamide formation, acrylic acid was continuously removed during the incubation period of 50 hours at the biodegradation rates of 5.47 (R2 = 0.99) and 4.54 (R2 = 0.96) mg AA L−1 h−1 by E. aerogenes from the AS-1 and IFAS-1 systems, respectively. The mixed culture bacteria from the AS-2 system had a biodegradation rate of 5.11 (R2 = 0.95) mg AA L−1 h−1 in a similar fashion to the synthetic wastewater, because acrylic acid is a readily biodegradable organic compound and can be rapidly oxidized under an aerobic condition.12 The mixed culture bacteria in the IFAS-2 reactor could not degrade acrylic acid efficiently as compared with other systems, this result is in good agreement with the respirometric activity of mixed culture bacteria from the IFAS-2 system. The OUR of IFAS-2 for acrylic acid biodegradation was much less than other systems (Table 2). As shown in Fig. 5, acrylamide began to decrease at the 50th hour, after the depletion of acrylic acid. In contrast, acrylamide was not consumed by the mixed culture bacteria from the IFAS-2 system because acrylic acid was still available in the solution. Thus, acrylic acid is a preferable substrate than acrylamide for biodegradation by both E. aerogenes and mixed culture bacteria because acrylamide can be toxic to microorganisms3,4 and acrylic acid is a readily biodegradable organic compound.12
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Fig. 7 Time courses of residual COD concentrations of (a) E. aerogenes (AS-1), (b) E. aerogenes (IFAS-1), (c) mixed culture bacteria (AS-2), and (d) mixed culture bacteria (IFAS-2). |
It is evident from Fig. 6(b) that E. aerogenes immobilized in the BioPortz media did not degrade acrylamide at the same rate as the suspended growth of E. aerogenes. The diffusion of substrates could be limited due to the mass transfer resistance in the media. As shown in Table 3, the acrylamide biodegradation rate and removal efficiency were 1.36 mg AM L−1 h−1 (R2 = 0.981) and 41.6%, respectively. It is obvious that E. aerogenes from the IFAS-1 system did not enhance the acrylamide biodegradation. The OUR obtained from the respirometric evaluation was 1.37 mg O2 L−1 h−1, which was approximately the same as the acrylamide biodegradation rate, suggesting that only acrylamide was degraded. In addition, it was found that about 31.3% of ammonium was removed during the first incubation period of 6 hours, suggesting that acrylic acid produced from the acrylamide biodegradation was used to form acrylamide, resulting in the reduced acrylamide biodegradation rate. As acrylamide was not considerably degraded, less amount of acrylic acid was accumulated in the system; thereby, the acrylic acid concentrations remained constant at the average concentration of 16.9 ± 2.47 mg AA L−1. Fig. 7(b) confirms the explanations, as the SCOD concentrations did not decrease during the first 45 hours of incubation period.
Experiments | SBR system | NH4+ mg N L−1 | AM removal efficiency (%) | AM biodegradation rate mg AM L−1 h−1 | R2 |
---|---|---|---|---|---|
E. aerogenes | AS-1 | 50 | 73.7 | 2.05 | 0.978 |
Mixed culture bacteria | AS-2 | 72.3 | 2.09 | 0.978 | |
E. aerogenes | IFAS-1 | 41.6 | 1.36 | 0.981 | |
Mixed culture bacteria | IFAS-2 | 31.2 | 1.06 | 0.982 | |
E. aerogenes | AS-1 | 500 | 51.7 | 1.18 | 0.909 |
Mixed culture bacteria | AS-2 | 41.7 | 1.14 | 0.977 | |
E. aerogenes | IFAS-1 | 42.5 | 1.28 | 0.988 | |
Mixed culture bacteria | IFAS-2 | 22.4 | 0.88 | 0.975 |
The unacclimatized mixed culture bacteria from the AS-2 system biodegraded acrylamide slowly during the first incubation period of 15 hours (Fig. 6(c)). The results supported the respirometric evaluation that the suspended growth of mixed culture bacteria required a few hours to acclimatize with acrylamide. The acrylamide biodegradation rate during the first 65 hours was 1.79 mg AM L−1 h−1 (R2 = 0.978), which was approximately equal to the OUR of 1.90 mg O2 L−1 h−1 as listed in Table 2. This implied that minimal acrylic acid was degraded; thus, the SCOD concentrations did not decrease as illustrated by Fig. 7(c). Subsequently, the acrylamide biodegradation rate was increased to 2.85 mg AM L−1 h−1 (R2 = 0.984). Ammonium increased gradually as a result of acrylamide biodegradation; thereby, the acrylamide formation from the reaction between acrylic acid and ammonia did not occur.
Fig. 6(d) shows that acrylamide concentrations decreased linearly with time due to the acrylamide biodegradation by the mixed culture bacteria from the IFAS-2 system with the biodegradation rate of 1.06 mg AM L−1 h−1 (R2 = 0.982). According to the OUR of 1.32 mg O2 L−1 h−1 in Table 2, it is implied that acrylic acid was minimally degraded. A minimal amount of ammonium decreased during the first 4 hours, indicating that acrylamide was formed from the reaction of acrylic acid and ammonia during this period. The respirometric activities indicated that the acrylic acid produced from the acrylamide biodegradation was polymerized. Therefore, acrylic acid from the acrylamide biodegradation in this test was polymerized and used to form acrylamide. Subsequently, ammonium increased in the solution at the production rate of 0.34 mg N L−1 h−1 (R2 = 0.932), indicating that acrylamide was not produced from the reaction of acrylic acid and ammonia. Acrylic acid was slowly degraded by the mixed culture bacteria from the IFAS-2 system as illustrated by Fig. 5; therefore, acrylic acid was accumulated over a period of 86 hours. Thus, the SCOD concentrations as shown in Fig. 6(d) remained constant for a period of about 5 days. Afterward, acrylic acid was gradually biodegraded at the slow biodegradation rate resulting in the reductions of SCOD concentrations.
The experimental results in Fig. 6(a) and (c), the OURs and SOURs in Table 2, and the acrylamide biodegradation rates in Table 3 support the conclusion that the acrylamide biodegradations by E. aerogenes and the mixed culture bacteria from the AS-1 and AS-2 systems were inhibited by the high concentration of ammonium. The experimental results were in good agreement with the previous findings from Jangkorn et al.11 The effects of ammonia inhibition on the acrylamide biodegradation by E. aerogenes and the mixed culture bacteria from the IFAS-1 and IFAS-2 systems were minimized. It is possible that the diffusion of acrylamide and other substrates into the BioPortz media were limited in the IFAS systems as a result of mass transfer resistances.
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