An advanced anaerobic expanded granular sludge bed (AnaEG) for the treatment of coal gasification wastewater

Abstract

A state-of-the-art advanced anaerobic expanded granular sludge bed (AnaEG) was developed for the anaerobic treatment of coal gasification wastewater (typical industrial wastewater with poor biodegradability and high toxicity). Three batch tests were conducted to check the efficiency of the reactor. The reactor was run for 330 days using acidification, with a final hydraulic retention time (HRT) of 48 hours. With an influent concentration of chemical oxygen demand (COD) of 1400 mg L⁻¹, 320 mg L⁻¹ total phenol , and 150 mg L⁻¹ volatile phenol, the effluent COD, total phenol, and volatile phenol could be decreased to 800 mg L⁻¹, 200 mg L⁻¹ and 40 mg L⁻¹, respectively. The AnaEG shows a COD removal efficiency of 50%, with a loading rate of 0.806 (kg COD per m³ per day), a removal rate of 0.357 (g COD per day), total phenol removal efficiency of 50%, and a volatile phenol removal efficiency of 80%. Besides being able to remove 70–95% of organic matter from the wastewater, this technology generates less sludge. Finally, scanning electron microscopy (SEM) revealed long-chain filamentous bacteria; coccus and rod-shaped bacteria were the dominant microorganisms. A CH₄ production rate of 227.23 (ml CH₄ per L per day) during a loading rate of 626.25 (mg COD per L per day) and a removal rate of 87.68 (mg COD per L per day) was observed. The significant reduction in the amount of sludge produced therefore reduces the sludge management cost. It does not have to reflux; this equipment is simple as it requires less accessory equipment and lower power consumption, and produces a high quality effluent. All the results demonstrate that the AnaEG could be used efficiently for the treatment of coal gasification wastewater containing high COD and phenol concentration.

---

1. Introduction

For organically polluted industrial waste streams, anaerobic wastewater treatment is considered the most cost-effective solution¹ and due to increasing energy prices it has gained interest. Anaerobic treatment processes are known for their unique ability to convert highly objectionable waste into useful products.² Treatment of industrial wastewater in anaerobic bioreactors has grown and has become especially important since the introduction of the Upflow Anaerobic Sludge Blanket (UASB) reactor more than 30 years ago.³ UASB reactors are still the most common worldwide, even though a variety of additional anaerobic bioreactor designs have now been developed.⁴

Expanded Granular Sludge Blanket (EGSB) and Internal Circulation (IC) reactors are currently replacing more conventional UASB systems due to improved performance and economic efficiency.⁴ EGSB systems have a comparable design to UASB reactors, but contain an expanded granular sludge bed allowing more circulation and interaction between the microorganisms and the organic compounds in the sludge granules.

The UASB system dominated industrial wastewater treatment worldwide in the 1980’s and before 1997. Of all the anaerobic reactors that were sold worldwide between 2002 to 2007, 52% were EGSB type reactors, while only 34% were UASB reactors.^4–6

The present study combines the advantages of UASB and EGSB technology. The simplicity and high treatment efficiency of the AnaEG reactor in comparison to the EGSB reactor are shown in Table 1. The AnaEG reactor is a hybrid reactor and is the third generation of high efficiency anaerobic biological processes invented⁷ by our research group. It is the next step in an advanced anaerobic systems. The AnaEG reactor is state-of-the-art technology for the treatment of organic or biological waste.

Table 1

	AnaEG™	EGSB
Power consumption	The grade of sludge expansion depends upon the influent and the biogas production, and is automatically adjusted by the loading rate. A circulating pump is not required to maintain the expansion. Therefore, the power consumption is reduced.	The reactor requires a recirculation pump to adjust and maintain the expansion.
Removal efficiency & adaptability to shock-load	The AnaEG reactor is a plug-flow reactor, so its adaptability to shock-load is greater, and the organic matter removal rate is comparatively higher (generally, it is above 90%).	It is a complete mixed reactor, its adaptability to shock-load is lower, and the organic matter removal rate is comparatively lower (generally, it is below 70–75%).
Organic loading	The AnaEG reactor can pick up a load of up to 50 000 mg L^−1 COD.	It generally cannot pick up more than 10 000 mg L^−1 COD.

---

First of all, in the anaerobic treatment process (AnaEG), the hydrolysis acidification and methanogenic processes are placed in one unit as seen in Fig. 1. The bottom section (1/5 – 1/3 of the height of the sludge bed) is mainly in acidogenesis conditions, while above it, the upper section is the methanogenesis zone; the effluent of the anaerobic reactor does not need to be recycled back to the influent to maintain a high upward velocity, and the wastewater flows upward in a plug flow pattern. Meanwhile the organic matter in the wastewater is decomposed by the acidogenesis to methanogenesis process in an upward direction, which achieves a two-phase anaerobic process in one reactor. An alkalinity requirement in operation may not be needed, and the desired pH range is relatively wide, generally between 6 and 9.

Fig. 1 Schematic diagram of the experimental apparatus of an anaerobic reactor (AnaEG).

During well-dispersed upward influent flow, the organic matter is degraded and biogas (mainly methane) is produced, which causes the granular sludge bed to expand. The grade of expansion increases from the bottom to the top, and creates the fluidization condition at the upper section. Therefore, in our process there is no bad odour in the hydrolysis pool as the process takes place in an enclosed reactor, significantly saving the land usage area as the treatment plant is smaller, with a strict control system over the influent pH.

The biggest feature is that it has no water cycle. Water in the reactor is pushed forward by the streaming flow pattern. The reactor can be divided into three parts: the inlet area, reactor area, and gas–solid–liquid separation area. Wastewater enters the reactor from the reactor base and flows through the reactor in an upward direction. As it flows through the reactor, organic matter is biodegraded in an anaerobic process. Organic acid and methane gas are formed in two different layers in the AnaEG™ reactor.

The AnaEG™ treatment system removes 70–95% of the organic matter from the wastewater. A simple aerobic process easily removes the remaining organic matter.

It was designed to overcome the shortcomings of the existing anaerobic reactor and it was then employed for a case study to treat the coal gasification wastewater (CGW) which is characterized by a complicated composition, high concentrations of pollutants and high toxicity.^8–13 The organic components present in the coal gasification wastewater mainly include volatile phenols, polyphenols, polycyclic aromatic hydrocarbons (PAHs), aromatic and heterocyclic compounds, most of which are toxic, mutative, carcinogenic, teratogenic, and may produce long-term adverse effects in the environment.^9,14–19 Therefore in China, coal gasification has become an efficient way to provide clean energy in recent years, yet it is also a growing contributor of pollution in china.^18,20,21

In this paper, AnaEG was employed to evaluate its effectiveness and feasibility for the treatment of coal gasification wastewater. The effect of the AnaEG operating parameters were investigated in detail. The variations of pH and methane production were also determined. Finally, the morphological and microbial structure of the anaerobic bacteria were studied during the course of coal gasification wastewater treatment at a stable state. This study will not only help to understand the process and performance of the AnaEG for treatment of coal gasification wastewater treatment, but also contribute to providing information for treatment of other high strength industrial wastewater by AnaEG.

2. Materials and method

2.1 Characteristics of coal gasification wastewater

The coal gasification wastewater was received from the Coal Long Hua Harbin Coal Chemical Industry Co. Ltd, Harbin, China. The raw water had a pH of 8.5–9, COD concentration of 4400 mg L⁻¹, biochemical oxygen demand (BOD) concentration of 700 mg L⁻¹, 950 mg L⁻¹ total phenol (TP), 450 mg L⁻¹ volatile phenol (VP), and an ammonia concentration of 300 mg L⁻¹. In addition, the BOD/COD ratio was 0.15. The high concentration of ammonia and phenol had an inhibitory effect on the biological treatment^22,23 but the AnaEG treatment has the capability to effectively treat a wide range of wastewater and is particularly efficient for problematic wastewater with very high organic content.

2.2 Pilot scale anaerobic reactor

The experiments were performed in the apparatus as shown in Fig. 1. It consisted of an inlet, an outlet zone, a control unit, a three phase separator, a gas dome, an expanding granular sludge layer, a water dispensing unit, and a main body for wastewater treatment. The main body was made of transparent rigid Plexiglas with an inner diameter of 100 mm and a height of 1500 mm. The effective volume of the reactor was 13.4 L, and its shape was cylindrical. It does not have to reflux; this equipment is simple as it requires less accessory equipment and lower power consumption, and produces a high quality effluent. The grade of sludge expansion in the AnaEG depends upon the influent and the biogas production, and is automatically adjusted by the loading rate. A circulating pump is not required to maintain the expansion. Therefore, power consumption is reduced. It is a plug-flow reactor, so its adaptability to shock-load is greater. The top phase separation zone is a three phase separation (gas–liquid–solid separation). The reactor was operated under mesophilic conditions (35 °C) and the temperature was maintained by the recycling of hot water by a thermostatic water bath. The CO₂, H₂S and other acid gases will be absorbed by the base liquid.

2.3 Pre-start up operating conditions

The inoculated sludge of anaerobic reactor was taken from digested sludge of sewage treatment plant in Lu village (Wuxi, Jiangsu province), China. The inoculation quantity was about 40% of anaerobic reactor effective volume. The tap water was added in order to lower the COD of the coal gasification wastewater.

2.4 Start-up and operating conditions

The inoculated anaerobic granular sludge was made in our own laboratory (inoculum: anaerobic reactor effective volume of 40%). The main aim of the start-up was to develop the most appropriate microbial culture for wastewater treatment, so a certain amount of glucose was added for the activation of the anaerobic microorganisms. After that, the amount of glucose was gradually reduced until ultimately no glucose was added.

The AnaEG reactor was run for 330 days. Start-up was divided into three stages. The first start-up ran for 87 days, the HRT was 96 h, the wastewater flow efficiency was controlled at 3.4 L per day and according to the amount of glucose it was divided into five phases. Gradually the amount of glucose was reduced, in order to make the anaerobic microorganisms grow and adapt the coal gasification wastewater environment. The second stage was to reduce the HRT and stability (88–200 days) and it was run for 110 days. The HRT was reduced from 96 h to 48 h to achieve stable running for about 60 days. The third stage was the second start-up with a stabilization phase (201–333 days). The HRT was 48 h and it was run for 133 days.

2.5 Analytical methods

Biogas production was measured daily with a wet glass flow meter making corrections for atmospheric pressure and temperature. The methane concentration was determined by GC2010A gas chromatography (Shimadzu, Japan) with a stainless steel column (300 cm × 0.3 cm) packed with active carbon (30–60 mesh) using thermal conductivity detection (TCD).

The effluent and influent pH values were measured using a pH meter; COD, volatile phenol, and total phenol analyses were carried out according to the standard procedures.²⁴ The concentrations of total phenols and volatile phenols were measured by a titration method.²⁵ To further understand the nature of the wastewater, a gas chromatography mass spectrometer (GCMS 2.0, Shimadzu, Japan) was used. SEM was carried out using a Hitachi TM3000 Tabletop Microscope.

3. Results and discussion

3.1. Pre-start up operational stage of the anaerobic reactor

The pre-start up stage was run for 148 days. During the operation (1–39 days and 40–62 days) tap water was used as the dilution water. For 63–104 days the reactor showed a 40–45% removal efficiency for COD, 50–55% for total phenol, and 70–85% for VP. When the reactor operated from 105–145 days, the COD removal was about 45%, total phenol removal was 30%, and VP removal efficiency was 50–55% (aerobic effluent was taken as the dilution water). TP was controlled at 300–350 mg L⁻¹, and the pH was maintained at 7.0–7.5 for the further stage of anaerobic influent. Thus when the raw water was diluted with tap water, the anaerobic reactor performance was excellent.

3.2. Start-up stage of AnaEG

For 87 days, the anaerobic reactor first start-up stage was divided into five phases of operation according to the amount of glucose. The HRT remained at 96 h. The operating time of each stage and the glucose amount are shown in Table 2 and the degradation trends of COD and COD removal changes, TP removal, VP removal and changes in pH are shown in Fig. 2.

Table 2 Anaerobic reactor operating conditions

Stage	First start-up					Increasing system loading and stable running			Second start-up and stable running
	Phase I	Phase II	Phase III	Phase IV	Phase V
Time (days)	1–34	35–54	55–65	66–76	77–87	88–117	118–142	143–200	201–222	223–333
HRT (h)	96	96	96	96	96	72	48	48	48	48
Amount of glucose added (mg COD per L)	1000	800	500	300	0	0	0	0	0	0
COD conc. (mg L^−1)	2340–2500	2180–2240	1940–2010	1710–1760	1350–1440	1400–1410	1270–1420	1360–1370	1210–1330	1000–1210
Total phenolic conc. (mg L^−1)	290–320	310–330	290–320	310–330	300–310	320–330	270–310	300–310	240–250	230–250
Volatile phenol conc. (mg L^−1)	150–160	140–150	130–140	130–140	140–150	130–140	140–150	140–150	120–140	110–130

---

Fig. 2 Anaerobic reactor initialization phase degradation trends. (a) COD degradation trends, (b) COD removal efficiency, (c) total phenol, (d) volatile phenol and (e) pH variations.

In the first phase of the start-up stage for 34 days, due to the higher seeded sludge activity (adding a sufficient amount of metabolites as glucose), initially the total COD removal was higher at 73%, CGW COD removal was 54%, total phenol removal efficiency was about 40%, and VP removal efficiency was about 20%. However with the extended operational time, highly unfavourable toxicity of the coal gasification wastewater effluent hinders the removal efficiency of COD, total phenols and volatile phenol. The total COD removal efficiency decreased to about 40%, coal gasification wastewater COD removal efficiency dropped to about 10%, total phenol removal efficiency decreased to about 20%, and removal of volatile phenol decreased to about 20%.

For the second phase (35–54 days), the amount of glucose was reduced to 800 mg COD per L, and the COD and phenol removal efficiency gradually increased. The total COD removal was 51.5%, coal gasification wastewater COD removal efficiency was 24%, TP removal efficiency was 34%, and VP removal efficiency was 37.8%. When the amount of glucose was further reduced to 500 mg COD per L in the third phase (55–65 days), a brief transient inadaptability of the anaerobic microorganisms occurs, but they soon resumed their activity. The total COD removal was 47%, coal gasification wastewater COD removal efficiency slightly increased to 29%, total phenol removal efficiency was 24%, and volatile phenol removal efficiency was 27%.

In the fourth phase (66–76 days), the amount of glucose was further reduced to 300 mg COD per L, COD removal efficiency was reduced after showing a brief upward trend, the total COD removal efficiency was 47%, compared with the previous glucose lowering, coal gasification wastewater COD removal efficiency from increased 29% to 37%. After a brief decline, phenol removal also showed a gradual upward trend, the TP removal efficiency was 30%, and VP removal efficiency was 30%. In the fifth phase (77–87 days) glucose was not added so the microorganism could behave well as the removal of COD was significantly reduced, but after a short period of time, this was reversed and the removal of organic matter gradually increased.

Fig. 2(e) and Table 2, show that in the first start-up stage the reactor runs in the normal range of basic pH values. The system only observed a pH value less than 6.8 four times, but after a timely adjustment of the reactor the pH quickly returned to the normal range.

After 87 days of operation, the successful completion of the first start-up stage takes place. The HRT was 96 h from the start till the end of the stage. With the decrease in glucose amount, coal gasification wastewater COD removal reached a gradual upward trend, indicating that the anaerobic microorganisms continued to be domesticated. Effluent COD was about 680 mg L⁻¹, and COD removal efficiency was about 50%; total phenol concentration was about 170 mg L⁻¹ with a removal efficiency of about 44%; volatile phenol concentration was about 40–60 mg L⁻¹, with a removal efficiency of about 50–70%

3.3. Increasing system loading and stabile running stage (88–200 days)

At this stage, the reactor was running for 123 days. Since the removal ratio of the organic compounds in the anaerobic systems had achieved relatively high levels, two increasing loads were carried out at this stage. The first start-up stage of 96 h HRT was reduced to 78 h, running for 30 days (88–117 days). In the second loading stage (118–142 days) the HRT was further shortened to 48 h, close to the optimum operating parameter obtained from a shaker test. From 143–200 days, the loading was maintained and the reactor performed stably. Fig. 3(a) and Table 2 shows that the influent of COD was between 800–900 mg L⁻¹ (fluctuates). COD removal efficiency was in decreasing trend but still more than 30% in first increase of loading. This indicates that the increase of loading had some effect on anaerobic microorganisms but did not have a serious impact on the activity of anaerobic microorganisms it can be overcome by acclimatization. The COD removal ratio was in between 30–40%. During the stable operation for 53 days, the effluent of anaerobic process exhibited a decreasing trend. Between 47 and 38 days, the effluent COD was 800 mg L⁻¹ or less.

Fig. 3 Anaerobic system loading trends during the stable running phase. (a) COD degradation, (b) total phenol degradation and (c) volatile phenol degradation.

From Fig. 3(b) and (c) and Table 2, it can be seen that the TP concentration was 200–260 mg L⁻¹ and the VP fluctuated between 100–140 mg L⁻¹ after increasing the loading. During the stable running the TP and VP concentrations showed a degradation trend. At the first half stage, the TP concentration was maintained at 170–200 mg L⁻¹, and the VP concentration was 80–130 mg L⁻¹. In the latter half, the TP concentration was only 150 mg L⁻¹ and the VP concentration was 20–40 mg L⁻¹, with removal efficiencies of 50% and 70%, respectively.

3.4. Biogas and methane production

Fig. 4(a) shows the biogas production (CH₄ 60%, CO₂ 40%, H₂S <1%). With the continuous reduction of the glucose amount (Table 2), the biogas production was gradually reduced and biogas generation was less than 0.4 L per day when glucose was not added. Fig. 4(b) shows the methane production rate during the course of the first start-up stage of the reactor. The reactor showed a CH₄ production rate of 227.23 (ml CH₄ per L per day) during a loading rate of 626.25 (mg COD per L per day) and a removal rate of 87.68 (mg COD per L per day). The increase of CO₂ in the biogas indicates that the acidifying microorganisms are prevailing compared to the methanogens, which may lead to volatile fatty acid (VFA) accumulation. The coal gasification wastewater (poor biodegradability) has a low BOD/COD ratio (0.15), a high concentration of phenol, ammonia and other refractory materials which exhibit an inhibitory effect on the anaerobic bacteria, which results in a low biogas production rate.

Fig. 4 (a) Biogas production and (b) methane production.

3.5. Anaerobic reactor second start-up and stable running stage

During the wastewater treatment, there may be circumstances where, for various reasons waste water treatment system had to stop running. In order to understand the influence of stopping the anaerobic reactor, we stopped the entire wastewater treatment system for 10 days, and after 10 days the second start-up was carried out. The hydrolysis acidification process was adopted in the anaerobic treatment system. We maintained the previous load of 48 h HRT in the second start-up. Fig. 5 and Table 2 shows that during the first 10 days of the second start-up stage, as the reactor was stopped for more than one week, it had some adverse effects the removal ratios of COD, TP, and VP. The removal efficiencies were very low, 10–20%, but had a tendency to recover.

Fig. 5 Anaerobic reactor second start-up and stable running. (a) COD degradation trends, (b) total phenol degradation trends and (c) volatile phenol degradation trends.

As shown in Table 2 and Fig. 5(a), the COD removal ratio increased to more than 30% after 10 days of recovery and after 20 days, the COD removal ratio recovered to 40%, and showed an increasing trend. The effluent COD concentration was maintained at about 500–600 mg L⁻¹. As shown in Fig. 5(b) and Table 2, on the 15th day of the second start-up stage, the TP removal ratio had recovered to 40–50%, and the TP effluent concentration was about 100–130 mg L⁻¹. Degradation of volatile phenols and total phenols were roughly the same, as shown in Fig. 5(c) and Table 2. At the 18th day its removal was restored to about 70%, and the VP effluent concentration was about 15 mg L⁻¹. The maximum COD, total phenol, and volatile phenols removal rates were found to be 0.483 g COD per L per day, 0.08335 g per L per day, 0.063 g per L per day, respectively, for a total loading rate of 0.6835 kg COD per m³ per day, total phenol loading rate of 0.151 kg per m³ per day and volatile phenol loading rate of 0.0725 kg per m³ per day. Thus, it can be considered that as the second anaerobic reactor start-up takes about 22 days, its anaerobic effluent will meet the water requirements of the influent of subsequent aerobic biological treatment.

3.6. Microbiological analysis of the anaerobic sludge

The microbial composition of anaerobic bioreactors is rather complex.³ In order to observe the microbial structural characteristics in the anaerobic reactor, the internal and external structures of the sludge were analyzed through SEM after 128 days of running, as shown in Fig. 6. As can be seen from the SEM, there are mainly some micrococci; filamentous bacteria can be seen intertwined randomly throughout the cross-section. Fig. 6(a and b) show the slimeous zoogloeal colonies, but the internal structure of the bacteria is not obvious. According to the SEM (×2000 times), Fig. 6(c and d) show that there are long-chain filamentous bacteria, coccus and rod-shaped bacteria, presenting typical shapes of acid-producing bacteria. Meanwhile, there are some filamentous bacteria and bacillus intertwined together. Many colonies consisting of cocci and bacillus were also observed.

Fig. 6 Anaerobic sludge scanning electron micrographs (a)–(d) of filamentous bacteria and bacillus intertwined together.

3.7. Advantages of the AnaEG reactor

The AnaEG reactor has several key benefits.

• It is capable of treating a wide range of wastewater (even with high organic content).

• It has a high removal rate for organics.

• It does not require oxygen supply in the treatment process.

• It produces a large amount of methane – generates up to 10 000 kJ of energy for every 1 kg of COD removed.

• Significant savings in energy costs.

• Significant reduction in the amount of sludge produced – lower sludge management costs as the removed COD was mainly converted into CH₄ in the AnaEG reactor, while the amount of COD which is utilized for the multiplication of anaerobic bacteria generally accounts for 10% of the COD removed. During more than 200 days of continuous running, the sludge quantity in the bioreactor does not show an obvious increase.

• Effectively reduced odour production as the process takes place in an enclosed reactor.

• Increased capacity for organic volumetric load.

• It combines perfect principles of modern reactor engineering theory and anaerobic microbiology.

Conclusions

Anaerobic treatment plays an important role both in the treatment and optimization treatment efficiency of coal gasification wastewater. The results show that the start-up time was 90 days when we add glucose as a co-metabolite and the second start-up time was 20 days without metabolites. During the stable running of the anaerobic reactor, its COD removal efficiency reaches 50%, the TP removal ratio also reached 50% and the TVP removal efficiency reached up to 80%. The SEM result shows that the anaerobic sludge has long-chain filamentous bacteria, and coccus and rod-shaped bacteria, presenting typical shapes of acid-producing bacteria and methanogens leading to efficient methanogenic activity. The AnaEG reactor has distinct advantages over other systems currently available because of its compact design, which occupies a relatively small footprint. It boasts the ability to work on higher organic loading rates and variable hydraulic loads. The system also does not emit noise or odour, and the biogas produced can be captured and converted into energy, hence reducing the emission of greenhouse gases to the atmosphere.

Acknowledgements

The authors express their gratitude to the School of Environmental Science and Engineering, Shanghai Jiao Tong University for providing the research facilities.

References

1. J. B. VanLier, F. P. VanderZee, N. C. G. Tan, S. Rebac and R. Kleerebezem, Water Sci. Technol., 2001, 44, 15–25.
2. P. L. McCarty, Water Sci. Technol., 2001, 44, 149–156.
3. K. Roest, Ph.D, Wageningen University, 2007.
4. R. J. Franklin, Anaerobic digestion for sustainable development 2001, vol. 1, pp. 2–8.
5. J. B. V. Lier, Water Sci. Technol., 2008, 57, 1137–1147.
6. N. Musee and L. Lorenzen, Water SA, 2012, 31, 131–142.
7. Z. Zhang, Anaerobic treatment method for organic wastewater, ZL 2009 1 0050787.1, 7 May 2009–20 April 2011.
8. D. Bamelis, Rev. Metall., 1992, 132–145.
9. H. Gai, Y. Jiang, Y. Qian and A. Kraslawski, Chem. Eng. J., 2008, 138, 84–94.
10. D. Jenkins, Water Sci. Technol., 1992, 25, 215–230.
11. W. Keith and J. Antil, Steel Times Int., 1991, 26–35.
12. W. Wang and H. Han, Bioresour. Technol., 2012, 103, 95–100.
13. C. F. Yang, Y. Qian, L. J. Zhang and J. Z. Feng, Chem. Eng. J., 2006, 117, 179–185.
14. S. H. Hosseini and S. M. Borghei, Process Biochem., 2005, 40, 1027–1031.
15. Y. Li, G. Gu, J. Zhao and H. Yu, Process Biochem., 2001, 37, 81–86.
16. Z. Wang, X. Xu, J. Chen and F. Yang, J. Environ. Chem. Eng., 2013, 1, 899–905.
17. T. Felföldi, A. J. Székely, R. Gorál, K. Barkács, G. Scheirich, J. András, A. Rácz and K. Márialigeti, Bioresour. Technol., 2010, 101, 3406–3414.
18. W. Wang, H. Han, M. Yuan, H. Li, F. Fang and K. Wang, Bioresour. Technol., 2011, 102, 5454–5460.
19. Z. J. Yu, Y. Chen, D. C. Feng and Y. Qian, Ind. Eng. Chem. Res., 2010, 49, 2874–2881.
20. K. Z. Li, R. Zhang and J. C. Bi, Int. J. Hydrogen Energy, 2009, 357, 2722–2726.
21. C. S. Yang, Int. Energ. J., 2010, 15, 35–40.
22. L. Amor, M. Eiroa, C. Kennes and M. C. Veiga, Water Res., 2005, 39, 2915–2920.
23. W. Zixing, X. Xiaochen, G. Zheng and Y. Fenglin, J. Hazard. Mater., 2012, 235–236, 78–84.
24. APHA (American Public Health Association), AWWA (American Water Works Association), WEF (Water Environment Federation), Standard Methods for the Examination of Water and Wastewater, 20 edn, 1999.
25. F. S. Wei, W. Q. Qi, Z. G. Sun, Y. R. Huang and Y. W. Shen, Water and Wastewater Monitoring and Analysis Method, China Environmental Science Press, Beijing, 4th edn, 2002.

---