Abstract

The biodegradation of polyacrylamide (PAM) includes the hydrolysis of amino groups and cleavage of the carbon chain; however, the effect of molecular weight on the biodegradation needs further investigations. In this study, biodegradation of low molecular weight PAM (1.6 × 106 Da) was evaluated in two aerobic (25 °C and 40 °C) and two anaerobic (35 °C and 55 °C) reactors over 100 days. The removal of the low molecular weight PAM (52.0–52.6%) through the hydrolysis of amino groups by anaerobic treatment (35 °C and 55 °C) was much higher than that of the high molecular weight (2.2 × 107 Da, 11.2–17.0%) observed under the same conditions. The molecular weight was reduced from 1.6 × 106 to 6.45–7.42 × 105 Da for the low molecular weight PAM, while the high molecular weight PAM declined from 2.2 × 107 to 3.76–5.87 × 106 Da. The results showed that the amino hydrolysis of low molecular weight PAM is easier than that of the high molecular weight one, while the cleavage of its carbon chain is still difficult. The molecular weights of PAM in the effluents from the two aerobic reactors (25 °C and 40 °C) were further reduced to 4.31 × 105 and 5.68 × 105 Da by the biofilm treatment, respectively. The results would be useful for the management of wastewater containing PAM.

INTRODUCTION

Polyacrylamide (PAM), particularly high molecular PAM (>107 Dal), with a viscosity of over 58 centipoise (cp), has been increasingly used for enhanced oil recovery in oilfields (Sheng 2014). Produced water generated during enhanced oil recovery contains high concentration of PAM, which should be removed before discharge (Yan et al. 2016). There have been a lot of studies focusing on the biodegradability of PAM. Some studies found that high molecular weight PAM was refractory to biodegradation (Guezennec et al. 2015), and pretreatment with advanced oxidation technologies such as Fenton was required to improve the biodegradability of PAM (Pi et al. 2015). This recalcitrance of PAM to biodegradation was supposed to be related to its high molecular weight, which makes it inaccessible to microbial attack (Bao et al. 2010).

A previous study showed that PAM with a molecular weight of 2.2 × 107 Da was removed with the maximum PAM removal rate of 24.35% by the expanded granular sludge bed reactor (Song et al. 2018). Our previous study showed that both the aerobic and anaerobic biological treatments could lead to the reduction of the molecular weight of PAM from 2.2 × 107 Da to 3.76–5.87 × 106 Da (Song et al. 2017). The molecular weight of PAM would be reduced to a level of 106 Da in produced water due to the shear and chemical degradation under the high pressure and temperature conditions in the underground environment (Xiong et al. 2017). Therefore, the biodegradability of residual PAM with low molecular weight in produced water should also be considered from the perspective of environmental safety in case of discharge into the natural environment. On the other hand, it was found that the PAM resistance to biodegradation results from not only the high molecular weight but also the molecular structure containing the amide group (Suzuki et al. 1978). Therefore it is interesting to know the effect of molecular weight on the hydrolysis of amino groups and cleavage of the carbon chain during the biodegradation of PAM.

In this study, synthetic wastewater containing the low molecular weight PAM (1.4 × 106 Da) was treated in two aerobic (25 and 40 °C) and two anaerobic (35 and 55 °C) reactors over 100 days, which were previously used to treat wastewater containing the high molecular weight PAM (2.2 × 107 Da) (Song et al. 2017). Starch was added into the synthetic wastewater to ensure sufficient growth of biomass, and sodium chloride (NaCl, 4,000 mg/L) was used to increase the total dissolved solids (TDS) concentration to simulate the produced water. The effluents from the two aerobic sludge reactors were further treated using the biofilm reactors. The performance of the biological treatment was evaluated by analyzing the changes in both molecular weight and molecular structure of PAM. The molecular weight was measured by using flow field-flow fractionation with multi-angle light scattering (FFF-MALS) and the molecular structure was characterized by using an attenuated total reflectance–Fourier transform infrared (ATR-FTIR) instrument. This study will provide useful information for the management of wastewater containing PAM, particularly the enhanced oil recovery produced wastewater.

MATERIALS AND METHODS

Preparation of the synthetic wastewater

The PAM polymer (1.4 × 106 Da, 24% hydrolyzed) was acquired from SNF SAS ZAC de Milieux, France. The PAM solution (200 mg/L) was prepared by adding 2.4 g of PAM powder into a bucket containing 12 L of tap water, and stirring with a mechanical stirrer for 4 days at 200 rpm. The synthetic wastewater contained PAM (200 mg/L), starch (206 mg/L), NH4Cl (23.92 mg/L), NaH2PO4 (4 mg/L), NaCl (4,000 mg/L), and micronutrient (Table S1, supplementary material). Sodium chloride (NaCl; typical for produced water) was used to increase the solution's TDS concentration.

Operation conditions of the biological wastewater treatment systems

Similar to our previous study (Song et al. 2017), two aerobic reactors (named as Aerobic 25 °C and Aerobic 40 °C) and two anaerobic (named as Anaerobic 35 °C and Anaerobic 55 °C) were used to treat the synthetic wastewater. The high molecular weight PAM was firstly used as the inlet for these reactors, followed by the low molecular weight PAM. The effluent of the two aerobic reactors were used as the influent of the two biofilm reactors to further treat the effluents from Aerobic 25 °C and Aerobic 40 °C, respectively. Two biofilm systems were maintained at 25 °C (Biofilm 25 °C) and 40 °C (Biofilm40 °C), and the detailed information about the biofilm reactors is shown in the supplementary material (Figure S1). The process flow diagram is shown in Figure 1. All the reactors were operated for 100 days, with a hydraulic retention time (HRT) of 2 days for each reactor.

Figure 1

Flow diagram of the treatment process (*the effluent of the two aerobic reactors was used as the influent of the two biofilm reactors).

Figure 1

Flow diagram of the treatment process (*the effluent of the two aerobic reactors was used as the influent of the two biofilm reactors).

Characterization of wastewater and PAM samples

Wastewater samples were taken from the influent and effluent twice a week for nitrogen (total nitrogen, ammonia, nitrite, and nitrate nitrogen), PAM concentration, chemical oxygen demand (COD), and viscosity analyses after filtration. Nitrogen and COD were measured by the standard methods (APHA 2005). The PAM concentration was determined by the turbidity method (Guan et al. 2013).

The PAM removal was defined by the PAM concentration removal ratio. The PAM concentration was defined based on chlorination of the amide group under acidic conditions, giving a product which can be measured by the UV-vis spectrum. The PAM removal ratio was calculated according to the following formula: 
formula
where C(PAMinfluent) and C(PAMeffluent) are the influent and effluent PAM concentration, respectively.

A Brookfield DV3 T viscometer was used to determine the viscosity of water samples at 25 °C. Because the PAM was dissolved in a solution of water, the addition of methanol could precipitate the PAM. PAM characterization in the influent and effluent samples was undertaken by ATR-FTIR analysis (Avatar 360, Thermo Nicolet, USA) to analyse the functional group of PAM by precipitating PAM from wastewater through the addition of methanol followed by vacuum-drying.

The influent and effluent wastewater samples were centrifuged at 10,000 rpm for 10 min to remove the suspended solids, and then stored at 4 °C before analysis. The PAM molecular weight was then determined by using FFF-MALS (Wyatt Technology Corp., Santa Barbara, CA, USA). The detector flow rate was set at 0.8 mL/min. The signal was collected by using ASTRA 4.73 software. Deionized water with a pH of 3 was used as the mobile phase, and the regenerated cellulose membrane with a cut-off of 10,000 Da was used. The accuracy of method was validated using PAM standards (1–9 × 106 Da). The detailed procedure of FFF-MALS analysis is shown in Table S2.

Microbial analysis

Sludge samples were collected from the biological reactors, centrifuged at 10,000 rpm for 10 min, and then stored at 4 °C. A FastDNA® SPIN kit for soil (Qbiogene, Solon, OH, USA) was used to extract DNA, and an aliquot (50 ng) of purified DNA from each sample was used as a template for amplification of 16S rRNA genes. The primers 515F and 907R for the V4–V5 hypervariable regions of bacteria 16S rRNA genes were used to amplify the genomic DNA, which was transferred to Novogene Co. Ltd in Beijing for library-construction and pair-end sequencing using the Illumina HiSeq 2500 PE250 system.

Pairs of reads from the original DNA fragments were merged by FLASH (Magoč & Salzberg 2011), and then were filtered using QIIME quality filters. Polymerase chain reaction chimeras were cut by UCHIME (Edgar et al. 2011). The taxonomic classification of the sequences was performed by the Ribosomal Database Project Classifier at the bootstrap cut-off of 80%. The results were submitted to the National Center for Biotechnology Information sequence read archive, and the range of accession numbers is SAMN06626872–06626966.

Statistical analysis

In this study the plots were generated by Origin 8.0 (OriginLab, USA). Hierarchical cluster and principal component (PCA) analyses were conducted by using R 3.5.1 (http://www.r-project.org/). Pearson's correlation was implemented by SPSS 12, and values of p < 0.05 were regarded as significant.

RESULTS AND DISCUSSION

Performance of PAM (1.4 × 106 Da) biodegradation in different biological systems

The wastewater treatment performance over the whole experimental period (100 days) is summarized in Table 1 and Figure S3 (supplementary material). The Anaerobic 55 °C and Anaerobic 35 °C exhibited relatively high PAM removals (52.6% and 52.0%, respectively), while the Aerobic 25 °C and Aerobic 40 °C exhibited quite poor performances (25.1% and 8.6%, respectively). It should be noted that in spite of the higher temperature, the Aerobic 40 °C exhibited much lower PAM removal than the Anaerobic 35 °C, indicating that thermal hydrolysis should not be the main reason for the removal of PAM.

Table 1

Characteristics (average ± SD) of influent and effluent from four reactors treating PAM with a molecular weight of 1.4 × 106 and 2.2 × 107 Daa

PAM molecular weightParameterInfluentAerobic 25 °C effluentAerobic 40 °C effluentAnaerobic 35 °C effluentAnaerobic 55 °C effluent
1.4 × 106 Da COD (mg/L) 620.1 ± 13.5 251.7 ± 22.0 255.5 ± 71.7 250.5 ± 14.9 248.2 ± 12.8 
PAM concentration (mg/L) 199.8 ± 7.4 149.6 ± 16.5 182.7 ± 31.9 96.0 ± 4.8 94.7 ± 4.2 
COD/PAMb 1.60c 1.68 1.40 2.61 2.62 
N-value (mg/L)d 29.94e 23.31 27.71 19.89 19.09 
2.2 × 107 Da COD (mg/L) 619.2± 97.6 264.2 ± 32.0 272.8 ± 35.3 262.4 ± 25.2 257.0 ± 9.8 
PAM concentration (mg/L) 207.6 ± 19.6 179.5 ± 9.3 188.8 ± 27.8 184.3 ± 13.2 172.4 ± 6.1 
COD/PAM 1.60 1.47 1.45 1.42 1.49 
PAM molecular weightParameterInfluentAerobic 25 °C effluentAerobic 40 °C effluentAnaerobic 35 °C effluentAnaerobic 55 °C effluent
1.4 × 106 Da COD (mg/L) 620.1 ± 13.5 251.7 ± 22.0 255.5 ± 71.7 250.5 ± 14.9 248.2 ± 12.8 
PAM concentration (mg/L) 199.8 ± 7.4 149.6 ± 16.5 182.7 ± 31.9 96.0 ± 4.8 94.7 ± 4.2 
COD/PAMb 1.60c 1.68 1.40 2.61 2.62 
N-value (mg/L)d 29.94e 23.31 27.71 19.89 19.09 
2.2 × 107 Da COD (mg/L) 619.2± 97.6 264.2 ± 32.0 272.8 ± 35.3 262.4 ± 25.2 257.0 ± 9.8 
PAM concentration (mg/L) 207.6 ± 19.6 179.5 ± 9.3 188.8 ± 27.8 184.3 ± 13.2 172.4 ± 6.1 
COD/PAM 1.60 1.47 1.45 1.42 1.49 

aParts of this table are adapted from Song et al. (2017).

bThe COD/PAM value is the ratio of the COD to PAM concentration.

cThe empirical value of initial PAM sample in aqueous solution (excluding the addition of starch).

dN-value means the difference between the total nitrogen and the free nitrogen (the sum of the ammonia, nitrite, and nitrate nitrogen).

eThe empirical value of initial PAM sample in aqueous solution (excluding the addition of nitrogen).

The two aerobic and two anaerobic reactors exhibited quite a similar COD removal performance. The influent COD was 620.1 mg/L, and the average effluent COD concentrations were around 250 mg/L, with the Aerobic 40 °C exhibiting a little higher value (255.5 mg/L). The contribution of PAM to COD was about 320.0 mg/L, indicating that the remaining COD of the effluent should be mainly accounted for by residual PAM. Therefore, mineralization of PAM might have not happened during biological wastewater treatment.

Mw and Mn are the weight-average and number-average molecular weight, respectively. The polydispersity index is defined by the ratio between the Mw and Mn. The Mw, Mn and Mw/Mn of the inlet and effluent PAM determined by FFF-MALS are shown in Table 2. The Mw of PAM was reduced from the initial 14.03 × 106Da to 8.50 × 105, 9.31 × 105, 7.42 × 105 and 6.46 × 105 Da in the effluents of Aerobic 25 °C, Aerobic 40 °C, Anaerobic 35 °C, and Anaerobic 55 °C, respectively. The Mn of PAM exhibited a similar trend, and the polydispersity index increased from 1.25 to 1.34–1.60, suggesting that the molecular weights were distributed in a wider range after biological treatment (Alasonati et al. 2010).

Table 2

Average viscosity and molecular weight of wastewater samples from four reactors and combined with biofilm using the PAM of 1.4 × 106 Da

ParameterInfluentAerobic 25 °C effluentBiofilm 25 °C effluentAerobic 40 °C effluentBiofilm 40 °C effluentAnaerobic 35 °C effluentAnaerobic 55 °C effluent
Viscosity (cp) 1.22 ± 0.02 1.11 ± 0.02 1.08 ± 0.02 1.11 ± 0.02 1.10 ± 0.02 1.12 ± 0.03 1.10 ± 0.03 
Mw (105 Da) 14.04 ± 1.92 8.50 ± 1.79 4.31 ±0.78 9.31 ± 1.15 5.68 ± 0.29 7.42 ± 1.5 6.45 ± 1.26 
Mn (105 Da) 11.21 ± 1.52 5.31 ± 1.23 3.03 ± 0.66 6.19 ± 0.68 3.34 ± 1.01 5.17 ± 1.17 4.81 ± 1.10 
Mw/Mn 1.25 1.60 1.42 1.50 1.70 1.44 1.34 
ParameterInfluentAerobic 25 °C effluentBiofilm 25 °C effluentAerobic 40 °C effluentBiofilm 40 °C effluentAnaerobic 35 °C effluentAnaerobic 55 °C effluent
Viscosity (cp) 1.22 ± 0.02 1.11 ± 0.02 1.08 ± 0.02 1.11 ± 0.02 1.10 ± 0.02 1.12 ± 0.03 1.10 ± 0.03 
Mw (105 Da) 14.04 ± 1.92 8.50 ± 1.79 4.31 ±0.78 9.31 ± 1.15 5.68 ± 0.29 7.42 ± 1.5 6.45 ± 1.26 
Mn (105 Da) 11.21 ± 1.52 5.31 ± 1.23 3.03 ± 0.66 6.19 ± 0.68 3.34 ± 1.01 5.17 ± 1.17 4.81 ± 1.10 
Mw/Mn 1.25 1.60 1.42 1.50 1.70 1.44 1.34 

The viscosity changed from 1.22 cp in the inlet to 1.14, 1.15, 1.12 and 1.10 cp in the effluents of Aerobic 25 °C, Aerobic 40 °C, Anaerobic 35 °C, and Anaerobic 55 °C, respectively, which was in accordance with the molecular weight results (p < 0.05). Since starch contributed little to the solution viscosity, the solution viscosity could be mainly due to PAM in the aqueous solutions (Song et al. 2017). Both the molecular weight and viscosity analytical results indicated that the average polymer size was reduced, with the Anaerobic 55 °C exhibiting the best and the Aerobic 40 °C the poorest performances.

Effect of molecular weight on the biodegradation of PAM

The different molecular weight PAMs have an effect on the performance of the four reactors, and the effluent COD and PAM concentration of the four reactors fed with the low molecular weight PAM were lower in comparison with the high molecular weight PAM. Previous studies have demonstrated that the biodegradation of PAM comprised two parts: the hydrolysis of the amino group and the cleavage of the main carbon backbone (Dai et al. 2015; Guezennec et al. 2015). It was found that the resistance to biodegradation of PAM results from not only the high molecular weight but also the molecular structure containing the amide group (Suzuki et al. 1978). The removal of PAM did not mean the mineralization of PAM. The influent and effluent total organic carbon (TOC) was determined; however, the decrease of the TOC was mainly caused by the consumption of the starch. The effluent TOC was equal to the TOC contributed by the influent PAM, indicating the mineralization of PAM was little. The Mw and viscosity results indicated that the average polymer size was reduced.

Nitrification existed in the aerobic reactors, and most of the ammonia nitrogen was transformed into the nitrate. To examine the possible decomposition of the amino group, the N-value was calculated to determine the residual organic nitrogen in PAM. The N-value is defined as the difference between the total nitrogen and free nitrogen (the sum of the ammonia, nitrite and nitrate nitrogen) (Kay-Shoemake et al. 1998a). The effluent of the Aerobic 40 °C exhibited the highest N-value (27.71 mg/L), while the Anaerobic 55 °C exhibited the lowest one (19.09 mg/L). So the changes of the N-value could be used as a proof of the hydrolysis of the amino group, which was in accordance with a previous study showing the availability of nitrogen source from PAM for microbial growth (Haveroen et al. 2005). The N-values of the anaerobic reactors were lower than those of the aerobic reactors, indicating the higher hydrolysis degree of the amino group in PAM under the anaerobic condition.

The PAM concentration was defined based on chlorination of the amide group under acidic conditions, giving a product which can be measured by UV-vis spectroscopy. The organic nitrogen was calculated based on the difference between the total nitrogen and the free nitrogen (the sum of the ammonia, nitrite, and nitrate nitrogen). Although there are some differences between the two methods, the removal of organic nitrogen and PAM indicated the hydrolysis of amino groups.

Although the Anaerobic 55 °C could remove 52.5% of PAM from wastewater, it only reduced the molecular weight (Mw) from 1.4 × 106 Da to 6.45 × 105 Da. Thus, comparing to the main carbon backbone, the amino group was more easily attacked by the microorganisms, which is in accordance with previous findings (Kay-Shoemake et al. 1998b; Haveroen et al. 2005).

As shown in Figure 2, the removal of the low molecular weight PAM (1.6 × 106 Da, 52.0–52.6%) by anaerobic treatment was much higher than that of the high molecular weight PAM (2.2 × 107 Da, 11.2–17.0%) observed under the same conditions. The molecular weight was reduced from 1.6 × 106 to 6.46–9.31 × 105 Da for the low molecular weight PAM, while the high molecular weight PAM declined from 2.2 × 107 to 3.76–5.87 × 106. The hydrolysis of the amino group of the low molecular weight PAM was easier; however, the cleavage of its carbon chain was still difficult.

Figure 2

The difference of concentration (a) and molecular weight (b) between the two molecular weight PAMs in two anaerobic reactors.

Figure 2

The difference of concentration (a) and molecular weight (b) between the two molecular weight PAMs in two anaerobic reactors.

Effects of the extension of aerobic treatment duration by adding the biofilm reactors

The effluent of the two aerobic reactors was used as the influent of the two biofilm reactors. As shown in Table 2, the Mw in the effluents of the aerobic reactors could be further decreased to 4.31 × 105 and 5.68 × 105 Da, and the total PAM removal improved to 48.3% and 46.7%, after biological treatment using the Biofilm 25 °C and Biofilm 40 °C, respectively. The cumulative weight fraction of PAM is shown in Figure 3(a), which illuminates the variation in molecular weight. The viscosity was also further reduced to 1.08 and 1.10 cp, respectively. Previous studies have demonstrated that temperature (lower than 55 °C) and shear rheology could not affect the PAM molecular weights (Song et al. 2017). Therefore, the decrease in viscosity and molecular weight should be mainly caused by the biological treatment, suggesting that PAM could be decomposed further by extending the biological treatment duration.

Figure 3

Cumulative molar mass by FFF-MALS (a) and ATR-FTIR spectra (b) of inlet, aerobic 25 °C effluent, and biofilm 25 °C effluent.

Figure 3

Cumulative molar mass by FFF-MALS (a) and ATR-FTIR spectra (b) of inlet, aerobic 25 °C effluent, and biofilm 25 °C effluent.

ATR-FTIR spectra of the PAM samples collected from the Aerobic 25 °C and Biofilm 25 °C effluents are shown in Figure 3(b). The bands at 1,615, 2,815, and 3,400 cm−1 could be assigned to the -C=O, -CHR-CHR-, and -NH2 groups in PAM, respectively (Bao et al. 2010; Wen et al. 2010; Song et al. 2017). The heights of the three peaks decreased after the aerobic treatment and the following biofilm treatment, suggesting that decomposition of the amino group and the main carbon backbone occurred simultaneously.

Extension of HRT could promote the degradation of PAM; however, mineralization of PAM requires a much longer time than what could be tolerated for wastewater treatment. The biofilm reactors were operated under aerobic condition, so the denitrification did not happen in the biofilm reactors. The nitrite and nitrate have little effect on the PAM removal in the biofilm reactors. Therefore, further investigations on the complete removal of this contaminant from wastewater are required.

Microbial community of the bioreactors for treating the wastewater with PAM at 1.4 × 106 Da

MiSeq analysis was used to characterize the microbial communities of the sludge samples collected from the reactors, and the bacterial reads (average length) were 51,699 (371 bp), 55,451 (373 bp), 51,286 (373 bp), 44,665 (373 bp), 47,284 (372 bp) and 46,431 (372 bp) in the Aerobic 25 °C, Aerobic 40 °C, Biofilm 25 °C, Biofilm 40 °C, Anaerobic 35 °C, and Anaerobic 55 °C, respectively. The distribution of bacteria at the phylum level is shown in Figure S4. The Aerobic 25 °C was dominated by Bacteroidetes (41.9%), Proteobacteria (24.9%), Planctomycetes (10.5%), and Chloroflexi (9.0%), and the Aerobic 40 °C with Chloroflexi (35.0%), Proteobacteria (23.4%), Planctomycetes (10.7%), and Bacteroidetes (20.9%). The dominant phyla of the Anaerobic 35 °C were Chloroflexi (20.1%), Proteobacteria (20.1%), Bacteroidetes (25.0%), and OP8 (12.0%), and of the Anaerobic 55 °C were Chloroflexi (37.2%), Proteobacteria (15.6%), Nitrospirae (8.3%) and Bacteroidetes (7.9%).

The dominating genera in the four aerobic reactors (two suspended and two biofilm) and two anaerobic ones are shown in Figure S5(a) and S5(b), respectively. The Aerobic 25 °C was dominated by Flavobacterium (18.1%), Dok59 (9.0%), Methylibium (3.5%), Planctomyces (3.3%), and Luteolibacter (3.0%), while the Aerobic 40 °C was dominated by Caldilinea (29.3%), Planctomyces (4.7%) and Rhodoplanes (1.4%). The Biofilm 25 °C was dominated by Phormidium (9.8%), Caldilinea (5.0%), and Hyphomicrobium (1.2%), which were quite different from those of the Aerobic 25 °C. The Biofilm 40 °C was dominated by Caldilinea (11.2%), Phormidium (5.5%), and Planctomyces (1.3%), which were in accordance with those in the Aerobic 40 °C. Flavobacterium, Dok59, and Caldilinea in the two aerobic suspended reactors were reported to be associated with the removal of starch (Li et al. 2015; Xin et al. 2016). The decrease in the abundance of Caldilinea, Dok59, and Flavobacterium in the two biofilm systems might be due to the fact that most of the starch had already been degraded in the previous suspended reactors.

The dominant bacterial genera in the Anaerobic 35 °C were Bacteroides (19.6%) and C1_B004 (1.7%), while the Anaerobic 55 °C reactor was dominated by Caloramator (4.6%), Anaerolinea (1.7%) and Thermodesulfovibrio (8.2%). Bacteroides, Anaerolinea and Caloramator were reported to be responsible for the hydrolysis of starch (Song et al. 2017). As shown in Figure S5(a) and S5(b), the dominating bacterial genera in the two aerobic and two anaerobic reactors were quite similar to the results obtained for the same systems treating synthetic wastewater containing high molecular weight PAM (Song et al. 2017). However, it is not still confirmed whether the genera for the degradation of starch are also responsible for the degradation of PAM. On the other hand, the reported PAM-degrading bacteria, including Acinetobacter, Bacillus, Clostridium and Pseudomonas, were also found in the reactors (Table S3). Further efforts are required to identify the PAM-degrading bacteria.

CONCLUSIONS

The hydrolysis of the amino group of the low molecular weight PAM (1.4 × 106 Da) was much higher than that of the high molecular weight PAM (2.2 × 107 Da), with anaerobic treatment exhibiting better performances. Although the reduction in molecular weight for the low molecular weight PAM from 1.6 × 106 to 6.46 × 105–9.31 × 105 Da was similar to that of the high molecular weight PAM, the cleavage of its carbon chain was still difficult. Mineralization of both high and low molecular weight PAM was very low, as indicated by similar residual COD of 248.2–255.5 mg/L in the effluents in spite of quite variable PAM removals. The molecular weights of PAM in the effluents from the two aerobic suspended reactors (25 °C and 40 °C) were further reduced to 4.31 × 105 and 5.68 × 105 Da by the biofilm treatment, and the total PAM removals increased to 48.3% and 46.7%, respectively.

DISCLOSURE STATEMENT

No potential conflict of interest is reported by the authors.

ACKNOWLEDGEMENTS

This study was supported by National Natural Scientific Foundation of China (No. 21590814) and the Ministry of Science and Technology, People's Republic of China (2012AA063401).

SUPPLEMENTARY MATERIAL

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.109.

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Supplementary data