Treatment of high-strength ethylene glycol waste water in an expanded granular sludge blanket reactor: use of PVA-gel beads as a biocarrier
© The Author(s) 2016
Received: 24 October 2015
Accepted: 24 May 2016
Published: 23 June 2016
Industrial-scale use of polyvinyl alcohol (PVA)-gel beads as biocarriers is still not being implemented due to the lack of understanding regarding the optimal operational parameters. In this study, the parameters for organic loading rate (OLR), alkalinity, recycle rate, and addition of trace elements were investigated in an expanded granular sludge blanket reactor (EGSB) treating high-strength ethylene glycol wastewater (EG) with PVA-gel beads as biocarrier. Stable chemical oxygen demand (COD) removal efficiencies of 95 % or greater were achieved, and continuous treatment was demonstrated with appropriate parameters being an OLR of 15 kg COD/m3/day, NaHCO3 added at 400 mg/L, a recycle rate of 15 L/h, and no addition of trace elements addition. A biogas production yield rate of 0.24 m3/kg COD was achieved in this study. A large number of long rod-shaped bacteria (Methanosaeta), were found with low acetate concentration in the EGSB reactor.
KeywordsAnaerobic Cost Methanosaeta Methanosarcina
Ethylene glycol is widely used as a raw material in industrial processes, and many of these processes discharge high-strength ethylene glycol wastewater (EG). Usually a biological process is suggested in treating EG, and good removal performance was achieved with an influent chemical oxygen demand (COD) range between 1000 and 3000 mg/L (Hassania et al. 2014). Despite this, an anaerobic treatment method is preferred due to its simplicity, reduced sludge production and lower power consumption. The formation of microbial granules is a key factor for successful operation of an anaerobic reactor; however, granule formation when treating EG fails to occur (Hulshoff Pol et al. 2004), thus EG treatment plants generally operate with a reduced organic loading rate (OLR).
Polyvinyl alcohol (PVA)-gel beads are thought to be suitable candidate carriers (Wenjie 2008a; Wenjie et al. 2011; Khanh et al. 2011). Wenjie et al. (2009) used cultivated PVA-gel beads to seed a lab-scale anaerobic fluidized bed reactor treating corn steep liquor, and a removal efficiency of 91 % was achieved at an OLR of 27.5 kg-COD/m3/day. Wenjie et al. (2011) used PVA-gel beads in an upflow anaerobic sludge blanket (UASB) reactor to treat EG, and successful treatment performance was achieved with addition of sufficient trace elements. The results obtained from the aforementioned studies indicated that PVA-gel beads could display good performance in EG treatment. However, industrial-scale implementation has not occurred due to the lack of understanding of the optimal operational parameters, which are necessary for design.
In this study, PVA-gel beads were used in an expanded granular sludge blanket (EGSB) reactor to evaluate their effectiveness as biocarriers in treating EG. The parameters of EGSB, such as OLR, alkalinity, recycle rate, and trace elements, were investigated for the purpose of further application of this method.
Inoculation and feeding media
New PVA-gel beads with an average diameter of 3–4 mm were used. At first, in total 2.5 L of digester sludge and 0.95 L of PVA-gel were mixed in one tank with a sequence batch mode. Corn steep liquor (CSL) was used as feed.
After one month, the PVA-gel beads turned yellow in color compared to the white color of unused ones. This means that PVA-gel beads have good biomass affinity. The PVA-gel beads were then separated from the tank and introduced into the EGSB reactor whereby they contributed to about one quarter of the reactor volume. The reactor was started by using EG as influent with an initial COD concentration of about 500 mg/L.
COD, suspended solids (SS), volatile suspended solids (VSS), volatile fatty acids (VFAs), alkalinity: Filtered COD (1 µm) were measured by the closed reflux colorimetric method (APHA 1995). SS and VSS in effluent and sludge samples were measured in accordance with Standard Methods (APHA 1995). Alkalinity levels in effluent samples were determined by titration (APHA 1995). VFAs were quantified by using a CTO-10AS liquid chromatograph (Shimadzu, Japan).
Biogas collection and analysis: Biogas was collected through GSS and the volume was measured using an inverted measuring cylinder containing tap water with the pH lowered to 3 using 1 N H2SO4. Biogas analyses were performed using a GC-14B gas chromatograph (Shimadzu, Japan).
PVA-gel characteristics: the settling velocity of PVA-gel/granules was measured by the method of Wenjie et al. (2008). The amount of sludge (biomass or solids) attached to PVA granules (g VSS/g PVA gel) was determined by the wet weight difference from an average of 30 pairs of new (unused) and granulated PVA-gel.
Scanning electron microscopy (SEM): samples were first washed in a 0.1 M phosphate buffer solution (pH 7.4) for 5 min. The samples were then hardened for 90 min in a 2.5 % glutaraldehyde solution prepared with the buffer solution. Next, the samples were washed in the buffer solution three times for 10 min each and then fixed for 90 min in a 1.0 % OsO4 solution prepared with the buffer solution. After washing the samples three times for 10 min each in the buffer solution, they were dewatered for 10 min each in serially graded solutions of ethanol at concentrations of 10, 30, 50, 70, 90, and 95 %. SEM observations were conducted using a scanning electron microscope (JEOL, JSM-5310LV, Japan).
Methanogenic activity: the methanogenic activity tests were conducted on the PVA-gel beads from the UASB reactor using the method outlined by Wenjie et al. (2011). The tests were performed in one 3.9 L reactor with CSL substrates of equal COD (5 g/L) in order to obtain total methanogenic activity.
Results and discussion
Summary of conditions used during operation of the EGSB reactor treating EG
Recycle rate (L/h)
Trace nutrients (ml/L)
2.1 ± 0.1
523 ± 35
7.56 ± 0.28
4.5 ± 0.1
1121 ± 36
7.51 ± 0.24
9.8 ± 0.3
2453 ± 64
8.11 ± 0.25
12.0 ± 0.0
3004 ± 0
8.12 ± 0.00
14.6 ± 0.3
3640 ± 87
8.02 ± 0.30
13.4 ± 0.7
3360 ± 183
8.22 ± 0.29
13.2 ± 2.7
3304 ± 686
8.16 ± 0.25
15.2 ± 0.7
3807 ± 176
7.84 ± 0.27
15.0 ± 1.2
3743 ± 289
7.62 ± 0.15
14.0 ± 1.0
3497 ± 256
7.79 ± 0.21
14.0 ± 0.2
3503 ± 46
7.89 ± 0.23
13.1 ± 1.0
3283 ± 261
7.68 ± 0.17
The reactor was operated continuously for a period of 133 days, during which time the OLR was maintained at 12–15 kg COD/m3/day. In phases 6 and 7, the recycle rate was reduced to 20 L/h, and from phase 8, this value was reduced again to 15 L/h. In order to maintain the expanded PVA bed, a recycle rate of 15 L/h must be applied. Thus, no further reduction of recycle rate was carried out in the study. In order to determine the appropriate quantity of added chemicals, the added trace nutrients were reduced to 3.75 ml/L in phase 7 and to 0 in phase 11. In phases 8, 9, 10 and 12, the amounts of NaHCO3 added in the influent were 1000, 800, 600, and 400 mg/L, respectively.
The matured black PVA-gel beads had an average settling velocity of 168 m/h (5 cm/s) and 0.83 g VSS/g PVA gel. Compared with the results of the UASB reactor, the settling velocity decreased following the biomass attachment.
Comparison of UASB and EGSB reactors treating different substrates
Influent COD (mg/L)
COD removal efficiency (%)
Biomass attached (g MLSS/g PVA-gel)
Compared to the results of the UASB, the cultivated PVA-gel could function well with a shorter HRT and higher OLR due to the higher amount of biomass attached to the PVA-gel beads. Furthermore, the specific bacteria Methanosarta was confirmed in the EGSB reactor, which indicated that the mixing condition was better in the EGSB reactor, and the COD level in the reactor was lower.
According to the COD concentration profiles throughout the depth of the reactor, only about 40 mg/l COD was removed from the upper part of the reactor (above 50 cm). It can be concluded that the EG was most degraded by the PVA-gel layer. At the end of the experiment, the concentrations of the added trace nutrients and NaHCO3 were 0 and 400 mg/L, respectively. Thus, it can be concluded that the PVA-gel in this study can function effectively as a biocarrier to retain enough sludge in the reactor due to the dilution effect.
By using PVA-gel cultivated in one batch mode, it was possible to start an EGSB to treat EG. The COD removal efficiency could reach a value higher than 95 % with an OLR of 15 kg COD/m3/day. The influent COD was about 3500 mg/L, which was almost the same as real wastewater for this application. Biogas production was about 0.3 m3/kg COD with a methane concentration of 77 %. Compared with the results of the UASB reactor, the amount of biomass attached to the PVA-gel in the EGSB reactor was greater. SEM analysis experiment also showed that the inner part of the PVA-gel beads in the EGSB reactor was also filled by bacteria due to the high recycle rate.
YJ carried out the SEM and FISH studies, and drafted the manuscript. DW participated in the design of the study and performed the statistical analysis. WZ conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.
This research was supported by the Guangxi Natural Science Foundation (2014 GXNSFBA118265), the project of high level innovation team and outstanding scholar in Guangxi colleges and universities. The authors thank the thesis of “Application of PVA-Gel beads as biomass carrier for anaerobic wastewater treatment” by Wenjie Zhang, Kumamoto University.
The authors declare that they have no competing interests.
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