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Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants a
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Y. G. Ren , J. H. Wang , H. F. Li , J. Zhang , P. Y. Qi & Z. Hu
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Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, PR China b
School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, PR China c
Beijing Municipal Engineering Consulting Corporation, Beijing, PR China
Accepted author version posted online: 01 Aug 2012.Version of record first published: 02 Aug 2012.
To cite this article: Y. G. Ren, J. H. Wang, H. F. Li, J. Zhang, P. Y. Qi & Z. Hu (2012): Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants, Environmental Technology, DOI:10.1080/09593330.2012.696717 To link to this article: http://dx.doi.org/10.1080/09593330.2012.696717
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Environmental Technology iFirst, 2012, 1–11
Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants Y.G. Rena , J.H. Wangb , H.F. Lic , J. Zhanga,∗ , P.Y. Qia and Z. Hua a Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, PR China; b School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, PR China; c Beijing Municipal Engineering Consulting Corporation, Beijing, PR China
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(Received 1 January 2012; final version received 14 May 2012 ) Nitrous oxide (N2 O) and methane (CH4 ) are two important greenhouse gases (GHG) emitted from biological nutrient removal (BNR) processes in municipal wastewater treatment plants (WWTP). In this study, three typical biological wastewater treatment processes were studied in WWTP of Northern China: pre-anaerobic carrousel oxidation ditch (A+OD) process, pre-anoxic anaerobic-anoxic-oxic (A-A/A/O) process and reverse anaerobic-anoxic-oxic (r-A/A/O) process. The N2 O and CH4 emissions from these three different processes were measured in every processing unit of each WWTP. Results showed that N2 O and CH4 were mainly discharged during the nitrification/denitrification process and the anaerobic/anoxic treatment process, respectively and the amounts of their formation and release were significantly influenced by different BNR processes implemented in these WWTP. The N2 O conversion ratio of r-A/A/O process was the lowest among the three WWTP, which were 10.9% and 18.6% lower than that of A-A/A/O process and A+OD process, respectively. Similarly, the CH4 conversion ratio of r-A/A/O process was the lowest among the three WWTP, which were 89.1% and 80.8% lower than that of A-A/A/O process and A + OD process, respectively. The factors influencing N2 O and CH4 formation and emission in the three WWTP were investigated to explain the difference between these processes. The nitrite concentration and oxidation– reduction potential (ORP) value were found to be the dominant influencing factors affecting N2 O and CH4 production, respectively. The flow-based emission factors of N2 O and CH4 of the WWTP were figured out for better quantification of GHG emissions and further technical assessments of mitigation options. Keywords: wastewater treatment plant; nitrous oxide; methane; greenhouse gas emission; biological nutrient removal process
Introduction Our global average temperature was recognised to have risen at least 1◦ C and may possibly reach the highest temperature in the past 10 million years during the next 65 years [1]. The reason for this accelerated increase of global average temperature has been attributed mostly to the anthropogenic greenhouse gas (GHG) emissions [2]. And the anthropogenic GHG emissions are related to humanrelated activities, such as the production and consumption of fossil fuels, agricultural and industrial activities, landfills and wastewater treatment [3,4]. Wastewater treatment plants (WWTP) have been identified as one of the larger minor sources of GHG emissions [5]. These plants emit three major GHG – carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide (N2 O) – during the treatment processes [6]. Based on the IPCC [7] protocol, CO2 emission from wastewater treatment was not considered as net GHG emission because of its biogenic source and should be excluded from the GHG inventories. CH4 and N2 O are two important GHG, and for a 100-year period CH4 and N2 O were ∗ Corresponding
author. Email: [email protected]
ISSN 0959-3330 print/ISSN 1479-487X online © 2012 Taylor & Francis http://dx.doi.org/10.1080/09593330.2012.696717 http://www.tandfonline.com
estimated to have global warming potentials (GWP) of 25 and 310 expressed relative to CO2 , respectively. They account for more than 30% of the total radiative forcings due to well-mixed GHG [8–10]. The increasingly rigorous standards on nitrogen and organic loading removal from WWTP have significantly promoted the implementation of biological nutrient removal (BNR) processes in full-scale municipal WWTP in China. BNR processes employ anoxic and aerobic steps to remove nitrogen from wastewater by maintaining specific bacteria in activated sludge during the nitrification and denitrification process. Some studies have revealed that N2 O could be produced and released to the atmosphere from nitrification and denitrification processes in activated sludge [11,12]. During the anaerobic BNR process, CH4 was formed from the decomposition of complex organic matters activated by methanogens in the absence of oxygen, which could be emitted to the atmosphere under aerated conditions. In addition, the wasted sludge produced from the wastewater treatment process may be further biodegraded during
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storage in the sludge holding tanks, sludge concentration tanks and sludge drying ground under anaerobic conditions, resulting in CH4 production and emission [13,14]. Although there are many arguments about the potential of BNR processes for N2 O and CH4 generation and emission, few reports have quantified such emissions from full-scale BNR processes [13,15–17]. Most studies cited the N2 O and CH4 emission factors from WWTP as 7 g/person/year and 0.6 kg CH4 /kg BOD, respectively during BNR processes [7,18] in China. Both emission factors, however, were based on a limited data set and might not accurately reflect N2 O and CH4 released from China’s WWTP, considering treatment capacities, serving populations and water qualities, which are different from WWTP in western countries. Due to the significant difference in
operating parameters and broad diversity of BNR processes existing in China, it is useful to collect N2 O and CH4 data from China’s WWTP for better quantification of GHG emissions during wastewater treatment. This study established a database of N2 O and CH4 emissions from different WWTP using different activated sludge BNR processes through onsite sampling and laboratory testing, and identified the key factors affecting N2 O and CH4 emissions from activated sludge BNR processes. Materials and methods Field site N2 O and CH4 flux measurements were carried out at three full-scale municipal WWTP located in northern China
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Figure 1. Simplified wastewater treatment process diagrams of three WWTP. Note: a = WWTP I; b = WWTP II; c = WWTP III; AWTP I = advanced wastewater treatment process I; AWTP II = advanced wastewater treatment process II.
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Environmental Technology (WWTP I in Jinan and the other two in Qingdao). The WWTP I, WWTP II and WWTP III where N2 O and CH4 emissions were measured employed pre-anaerobic carrousel oxidation ditch (A+OD), pre-anoxic anaerobicanoxic-oxic (A-A/A/O) and reverse anaerobic-anoxic-oxic (r-A/A/O) processes, respectively. According to the report by Qiu [19], anaerobic-anoxic-oxic (A/A/O), sequencing batch reactor (SBR) and oxidation ditch (OD) were three major BNR processes used in China, which jointly comprised 65% of WWTP amounts and 54% of wastewater treatment capacity. These three types of BNR processes were used for high nitrogen and phosphorous removal efficiency in the three WWTP, and attained by a combination of anaerobic tanks, anoxic tanks and oxic tanks. The schematic diagrams of the BNR processes mentioned above are shown in Figure 1. N2 O can be produced during nitrification and denitrification steps in the oxic tanks and anoxic tanks, respectively [20] and CH4 can be produced by methanogenic bacteria under low oxygen condition in the anaerobic tanks and anoxic tanks [21]. The wasted activated sludge disposal process involved the wasted activated sludge from the final clarifier tanks flowing into sludge concentration tanks for thickening and then pumped to sludge centrifugal dewatering machines. Finally, the dewatered sludge was conveyed by screw conveyor to an open yard for drying and then transported outward for landfill or other disposal processes. Large amounts of CH4 may be formed and released to the atmosphere from sludge decomposition in the centrifugal dewatering machines and from the sludge drying ground in the absence of oxygen [22]. The wastewater treated in these three WWTP is mostly domestic wastewater with the influent chemical oxygen demand (COD) > 140 mg L−1 and total nitrogen (TN) > 55 mg L−1 (see Table 1). All the treated effluents meet the most stringent effluent standards in China: COD
0.15 mg L ) in the anoxic tank + of WWTP III due to the higher NH4 –N load caused by the anoxic tank in front of the anaerobic tank.
CH4 emissions from the three WWTP Experimental and theoretical studies indicated that CH4 was released from the anaerobic and anoxic biodegradation of the organic fractions in wastewater during the BNR process in WWTP [13,36]. In this study, CH4 emission was found to occur in each processing unit of the three WWTP. The measured CH4 emission flux (g m−2 day−1 ) range from each processing unit during the experimental period is presented in Figure 4 and a similar emission flux variation trend was determined for different processing units in different WWTP.
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CH4 emission flux Although there was relatively low CH4 emission flux and low dissolved CH4 concentration (see Figure 4a) in the final clarifier tanks, the study found an emission peak of CH4 from the following AWTP I in the WWTP II and there was also a high dissolved CH4 concentration in AWTP I. Although the wastewater treatment equipment had been partially updated in this plant, there was still a large amount of floating sludge in the wastewater during the BNR process, and it was hard to completely separate activated sludge from the wastewater in the final clarifier tanks because the WWTP II was old and could not fully adapt to the current water flow and quality. Activated sludge therefore existed in the network of the inclined plate clarifier tanks, and a large quantity of CH4 was produced under low DO conditions and released to the atmosphere, causing a high CH4 emission flux in the AWTP I of WWTP II. It has been reported that CH4 was generated in wastewater treatment plant in areas of high COD and low DO concentration, such as the influent lines, primary settling tanks, sludge holding tanks and sludge transfer lines [13]. The variation trend shown in Figure 4 was consistent with that finding. All the primary settling tanks, anaerobic tanks and anoxic tanks had high dissolved CH4 concentrations but relatively low CH4 fluxes because of their smooth wastewater surface. The CH4 emission fluxes in the oxic tanks were higher than in the anaerobic tanks and the anoxic tanks of WWTP II and WWTP III due to the CH4 accumulation along the wastewater flow and the subsequent release from the liquid phase caused by turbulent liquid flow initiated by aeration. The CH4 emission flux in the oxic tank was lower than in the anaerobic tank and the anoxic tank of WWTP I. The demand of controlling low aeration rate for the A+OD process, which relies on more consistent spatial DO profiles to promote SND in the oxic tank of WWTP I, may explain this phenomenon. Figure 4a showed that the CH4 emission fluxes in almost every processing unit of WWTP I was much lower than from WWTP II and WWTP III, possibly related to a low influent COD load in WWTP I (see Table 1).
Total annual CH4 fluxes The calculation of the total annual CH4 fluxes (kg yr−1 ) from each processing unit, taking into account the entire wastewater surface areas, is more valuable for better quantifying the CH4 emission from the three WWTP. The total annual CH4 flux range from each processing unit is shown in Table 4. The dominant CH4 sources were found to be different in the three WWTP. Similar to the total annual N2 O fluxes, the total annual CH4 fluxes also showed different variation trends from the CH4 emission flux because there was a large difference in wastewater surface area between the processing units. The total CH4 fluxes range were 3.19–8.09 × 104 kg yr−1 ,
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Y.G. Ren et al. (a) 160
WWTP I WWTP II WWTP III
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120 100 80 60 40 20 0 GC
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Figure 4. Comparison of CH4 fluxes (a) and dissolved CH4 concentrations of wastewater (b) from each processing unit of three WWTP (n = 15). Abbreviations are the same as Figure 2. The error bars shown are standard deviations (n = 15).
Table 4.
Total annual CH4 flux range from each processing unit of three WWTP (n = 15). WWTP I
Processing unit IPS GC PST PAT AnaT AnoT OT FCT AWTP I AWTP II SCT SSC SDG Total
WWTP II
WWTP III
Water surface area (m2 )
CH4 flux (kg yr−1 )
Water surface area (m2 )
CH4 flux (kg yr−1 )
Water surface area (m2 )
CH4 flux (kg yr−1 )
45 50 N/A N/A 2083 6540 17920 9156 648 N/A 400 0.25 400
304–1308 378–450 N/A N/A 1898–3979 6746–10,163 5421–7861 11570–48,065 450–1062 N/A 3500–5001 79–120 1637–3000 31,904–80,889
7.2 270 1032 680 1000 6390 7200 5664 2052 840 400 1 250
17–38 3364–9868 431–938 622–2514 901–2091 2659–5176 37534–66,141 1814–2743 18959–118,147 582–2184 6642–27,680 6–10 2483–3928 76,014–241,357
15 100 227 N/A 766 562 1675 1344 145 N/A 200 0.75 200
85–286 1432–3073 836–1973 N/A 650–1511 215–326 2790–4366 1002–1567 336–1814 N/A 879–5820 10–11 1774–3020 10,004–23,764
Note: N/A = not available; abbreviations same as Figure 2.
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Environmental Technology
Factors influencing CH4 emission The ORP parameter can reflect the DO concentration in an activated sludge reactor system and it is also an indicator of operational conditions such as over-aeration and under-aeration [37]. During the experiment period, it was clear that ORP had a positive correlation with DO concentration, which meant aerobic and anaerobic/anoxic environment could be easily determined by means of the ORP value [38,39]. All methanogens were strictly and obligatory anaerobic so large amounts of oxygen existed in wastewater would inhibit CH4 formation [40]. On the other hand, more CH4 would emit from wastewater due to the mechanical aeration. The CH4 emission flux could not reflect the real quantity of CH4 generated under different ORP values. Data was obtained from anaerobic tanks, anoxic tanks and oxic tanks in the three WWTP and Figure 5 shows the relationship between ORP value and dissolved CH4 concentration. Previous studies confirmed that when the ORP value was below +100 mV, the more ORP declined, the higher the activity level of methanogens and the greater the quantity of CH4 generated by anaerobic biodegradation of organic materials in wastewater [41]. Figure 5 is consistent with that finding and presented a big gap about the quantity of CH4 generated between the anaerobic/anoxic zone and aerobic zone. With respect to COD, CH4 production rate increased (see Figure 4) with influent COD load (see Table 1), which demonstrates that CH4 emissions have a significant relationship with organic carbon. Temperature was another factor that affects CH4 flux by influencing the activities of CH4 -oxidizing and CH4 -producing bacteria [42]. In this study, wastewater temperature was observed between 12– 24◦ C, no significant correlation was observed between CH4 emission and wastewater temperature.
70 Dissolved CH4 concentration (10-1 mg m-3)
7.60–24.14 × 104 kg yr−1 and 1.00–2.38 × 104 kg yr−1 from WWTP I, WWTP II and WWTP III, respectively. The WWTP II had the highest total annual CH4 flux due to its inefficient operating condition and more pretreatment and advanced treatment processes than the other two WWTP. Results also showed the total annual flux was influenced by different BNR process employed in these WWTP. The CH4 emissions accounted for approximately 0.714% ± 0.157, 1.262% ± 0.367 and 0.137% ± 0.035 of the total TOC removed from the A + OD process, AA/A/O process and r-A/A/O process, respectively (see Table 3). That means CH4 conversion ratio of the r-A/A/O process was the lowest among the three WWTP, which were 89.1% and 80.8% lower than that of the A-A/A/O process and A+OD process, respectively. Based on the treatment capacity (see Table 1), the flow-based CH4 emission factors were 4.99–12.66 × 10−4 , 2.08–6.61 × 10−3 and 5.48–13.02 × 10−4 g CH4 (L of wastewater)−1 for WWTP I, WWTP II and WWTP III.
9
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Anoxic/Anaerobic zone Aerobic zone
50 40 30 20 10 -150
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0 ORP value (mV)
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Figure 5. Relationship between dissolved CH4 concentration and ORP value (n = 15).
Conclusion Significant amounts of N2 O and CH4 were found to emit from the three WWTP. The mean total annual N2 O fluxes from WWTP I, WWTP II and WWTP III were 4.94 × 103 kg yr−1 , 4.44 × 103 kg yr−1 and 2.33 × 103 kg yr−1 , respectively. Flowbased emission factors were estimated to be 7.73 × 10−5 , 1.22 × 10−4 and 1.28 × 10−4 g N2 O (L of wastewater)−1 for WWTP I, WWTP II and WWTP III. The mean total annual CH4 fluxes from WWTP I, WWTP II and WWTP III were 3.76 × 104 kg yr−1 , 1.91 × 105 kg yr−1 and 1.79 × 104 kg yr−1 , respectively. Flowbased emission factors were estimated to be 5.88 × 10−4 , 5.22 × 10−3 and 9.82 × 10−4 g CH4 (L of wastewater)−1 for WWTP I, WWTP II and WWTP III. Some laboratory experimental studies indicated that both N2 O and CH4 emissions changed upon variation of − several BNR process factors including: NH+ 4 –N, NO2 –N, temperature, DO concentration, ORP and COD load. During the experimental period, it was confirmed that NO− 2 –N concentration and ORP value were the dominant influencing factors for the N2 O and CH4 emission in the full-scale municipal WWTP using different activated sludge BNR processes, respectively. Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (No. NCET-10-0554), National Natural Science Foundation of China (NO. 21177075) and National Natural Science Foundation of China (No. 21007032). We would like to thank Professor X.J. Bi from the School of Environmental and Municipal Engineering, Qingdao Technological University for his invaluable assistance in the gas sampling.
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Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants a
b
c
a
a
Y. G. Ren , J. H. Wang , H. F. Li , J. Zhang , P. Y. Qi & Z. Hu
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Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, PR China b
School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, PR China c
Beijing Municipal Engineering Consulting Corporation, Beijing, PR China
Accepted author version posted online: 01 Aug 2012.Version of record first published: 02 Aug 2012.
To cite this article: Y. G. Ren, J. H. Wang, H. F. Li, J. Zhang, P. Y. Qi & Z. Hu (2012): Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants, Environmental Technology, DOI:10.1080/09593330.2012.696717 To link to this article: http://dx.doi.org/10.1080/09593330.2012.696717
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Environmental Technology iFirst, 2012, 1–11
Nitrous oxide and methane emissions from different treatment processes in full-scale municipal wastewater treatment plants Y.G. Rena , J.H. Wangb , H.F. Lic , J. Zhanga,∗ , P.Y. Qia and Z. Hua a Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan, PR China; b School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan, PR China; c Beijing Municipal Engineering Consulting Corporation, Beijing, PR China
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(Received 1 January 2012; final version received 14 May 2012 ) Nitrous oxide (N2 O) and methane (CH4 ) are two important greenhouse gases (GHG) emitted from biological nutrient removal (BNR) processes in municipal wastewater treatment plants (WWTP). In this study, three typical biological wastewater treatment processes were studied in WWTP of Northern China: pre-anaerobic carrousel oxidation ditch (A+OD) process, pre-anoxic anaerobic-anoxic-oxic (A-A/A/O) process and reverse anaerobic-anoxic-oxic (r-A/A/O) process. The N2 O and CH4 emissions from these three different processes were measured in every processing unit of each WWTP. Results showed that N2 O and CH4 were mainly discharged during the nitrification/denitrification process and the anaerobic/anoxic treatment process, respectively and the amounts of their formation and release were significantly influenced by different BNR processes implemented in these WWTP. The N2 O conversion ratio of r-A/A/O process was the lowest among the three WWTP, which were 10.9% and 18.6% lower than that of A-A/A/O process and A+OD process, respectively. Similarly, the CH4 conversion ratio of r-A/A/O process was the lowest among the three WWTP, which were 89.1% and 80.8% lower than that of A-A/A/O process and A + OD process, respectively. The factors influencing N2 O and CH4 formation and emission in the three WWTP were investigated to explain the difference between these processes. The nitrite concentration and oxidation– reduction potential (ORP) value were found to be the dominant influencing factors affecting N2 O and CH4 production, respectively. The flow-based emission factors of N2 O and CH4 of the WWTP were figured out for better quantification of GHG emissions and further technical assessments of mitigation options. Keywords: wastewater treatment plant; nitrous oxide; methane; greenhouse gas emission; biological nutrient removal process
Introduction Our global average temperature was recognised to have risen at least 1◦ C and may possibly reach the highest temperature in the past 10 million years during the next 65 years [1]. The reason for this accelerated increase of global average temperature has been attributed mostly to the anthropogenic greenhouse gas (GHG) emissions [2]. And the anthropogenic GHG emissions are related to humanrelated activities, such as the production and consumption of fossil fuels, agricultural and industrial activities, landfills and wastewater treatment [3,4]. Wastewater treatment plants (WWTP) have been identified as one of the larger minor sources of GHG emissions [5]. These plants emit three major GHG – carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide (N2 O) – during the treatment processes [6]. Based on the IPCC [7] protocol, CO2 emission from wastewater treatment was not considered as net GHG emission because of its biogenic source and should be excluded from the GHG inventories. CH4 and N2 O are two important GHG, and for a 100-year period CH4 and N2 O were ∗ Corresponding
author. Email: [email protected]
ISSN 0959-3330 print/ISSN 1479-487X online © 2012 Taylor & Francis http://dx.doi.org/10.1080/09593330.2012.696717 http://www.tandfonline.com
estimated to have global warming potentials (GWP) of 25 and 310 expressed relative to CO2 , respectively. They account for more than 30% of the total radiative forcings due to well-mixed GHG [8–10]. The increasingly rigorous standards on nitrogen and organic loading removal from WWTP have significantly promoted the implementation of biological nutrient removal (BNR) processes in full-scale municipal WWTP in China. BNR processes employ anoxic and aerobic steps to remove nitrogen from wastewater by maintaining specific bacteria in activated sludge during the nitrification and denitrification process. Some studies have revealed that N2 O could be produced and released to the atmosphere from nitrification and denitrification processes in activated sludge [11,12]. During the anaerobic BNR process, CH4 was formed from the decomposition of complex organic matters activated by methanogens in the absence of oxygen, which could be emitted to the atmosphere under aerated conditions. In addition, the wasted sludge produced from the wastewater treatment process may be further biodegraded during
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storage in the sludge holding tanks, sludge concentration tanks and sludge drying ground under anaerobic conditions, resulting in CH4 production and emission [13,14]. Although there are many arguments about the potential of BNR processes for N2 O and CH4 generation and emission, few reports have quantified such emissions from full-scale BNR processes [13,15–17]. Most studies cited the N2 O and CH4 emission factors from WWTP as 7 g/person/year and 0.6 kg CH4 /kg BOD, respectively during BNR processes [7,18] in China. Both emission factors, however, were based on a limited data set and might not accurately reflect N2 O and CH4 released from China’s WWTP, considering treatment capacities, serving populations and water qualities, which are different from WWTP in western countries. Due to the significant difference in
operating parameters and broad diversity of BNR processes existing in China, it is useful to collect N2 O and CH4 data from China’s WWTP for better quantification of GHG emissions during wastewater treatment. This study established a database of N2 O and CH4 emissions from different WWTP using different activated sludge BNR processes through onsite sampling and laboratory testing, and identified the key factors affecting N2 O and CH4 emissions from activated sludge BNR processes. Materials and methods Field site N2 O and CH4 flux measurements were carried out at three full-scale municipal WWTP located in northern China
(a)
(b)
(c)
Figure 1. Simplified wastewater treatment process diagrams of three WWTP. Note: a = WWTP I; b = WWTP II; c = WWTP III; AWTP I = advanced wastewater treatment process I; AWTP II = advanced wastewater treatment process II.
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Environmental Technology (WWTP I in Jinan and the other two in Qingdao). The WWTP I, WWTP II and WWTP III where N2 O and CH4 emissions were measured employed pre-anaerobic carrousel oxidation ditch (A+OD), pre-anoxic anaerobicanoxic-oxic (A-A/A/O) and reverse anaerobic-anoxic-oxic (r-A/A/O) processes, respectively. According to the report by Qiu [19], anaerobic-anoxic-oxic (A/A/O), sequencing batch reactor (SBR) and oxidation ditch (OD) were three major BNR processes used in China, which jointly comprised 65% of WWTP amounts and 54% of wastewater treatment capacity. These three types of BNR processes were used for high nitrogen and phosphorous removal efficiency in the three WWTP, and attained by a combination of anaerobic tanks, anoxic tanks and oxic tanks. The schematic diagrams of the BNR processes mentioned above are shown in Figure 1. N2 O can be produced during nitrification and denitrification steps in the oxic tanks and anoxic tanks, respectively [20] and CH4 can be produced by methanogenic bacteria under low oxygen condition in the anaerobic tanks and anoxic tanks [21]. The wasted activated sludge disposal process involved the wasted activated sludge from the final clarifier tanks flowing into sludge concentration tanks for thickening and then pumped to sludge centrifugal dewatering machines. Finally, the dewatered sludge was conveyed by screw conveyor to an open yard for drying and then transported outward for landfill or other disposal processes. Large amounts of CH4 may be formed and released to the atmosphere from sludge decomposition in the centrifugal dewatering machines and from the sludge drying ground in the absence of oxygen [22]. The wastewater treated in these three WWTP is mostly domestic wastewater with the influent chemical oxygen demand (COD) > 140 mg L−1 and total nitrogen (TN) > 55 mg L−1 (see Table 1). All the treated effluents meet the most stringent effluent standards in China: COD
0.15 mg L ) in the anoxic tank + of WWTP III due to the higher NH4 –N load caused by the anoxic tank in front of the anaerobic tank.
CH4 emissions from the three WWTP Experimental and theoretical studies indicated that CH4 was released from the anaerobic and anoxic biodegradation of the organic fractions in wastewater during the BNR process in WWTP [13,36]. In this study, CH4 emission was found to occur in each processing unit of the three WWTP. The measured CH4 emission flux (g m−2 day−1 ) range from each processing unit during the experimental period is presented in Figure 4 and a similar emission flux variation trend was determined for different processing units in different WWTP.
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CH4 emission flux Although there was relatively low CH4 emission flux and low dissolved CH4 concentration (see Figure 4a) in the final clarifier tanks, the study found an emission peak of CH4 from the following AWTP I in the WWTP II and there was also a high dissolved CH4 concentration in AWTP I. Although the wastewater treatment equipment had been partially updated in this plant, there was still a large amount of floating sludge in the wastewater during the BNR process, and it was hard to completely separate activated sludge from the wastewater in the final clarifier tanks because the WWTP II was old and could not fully adapt to the current water flow and quality. Activated sludge therefore existed in the network of the inclined plate clarifier tanks, and a large quantity of CH4 was produced under low DO conditions and released to the atmosphere, causing a high CH4 emission flux in the AWTP I of WWTP II. It has been reported that CH4 was generated in wastewater treatment plant in areas of high COD and low DO concentration, such as the influent lines, primary settling tanks, sludge holding tanks and sludge transfer lines [13]. The variation trend shown in Figure 4 was consistent with that finding. All the primary settling tanks, anaerobic tanks and anoxic tanks had high dissolved CH4 concentrations but relatively low CH4 fluxes because of their smooth wastewater surface. The CH4 emission fluxes in the oxic tanks were higher than in the anaerobic tanks and the anoxic tanks of WWTP II and WWTP III due to the CH4 accumulation along the wastewater flow and the subsequent release from the liquid phase caused by turbulent liquid flow initiated by aeration. The CH4 emission flux in the oxic tank was lower than in the anaerobic tank and the anoxic tank of WWTP I. The demand of controlling low aeration rate for the A+OD process, which relies on more consistent spatial DO profiles to promote SND in the oxic tank of WWTP I, may explain this phenomenon. Figure 4a showed that the CH4 emission fluxes in almost every processing unit of WWTP I was much lower than from WWTP II and WWTP III, possibly related to a low influent COD load in WWTP I (see Table 1).
Total annual CH4 fluxes The calculation of the total annual CH4 fluxes (kg yr−1 ) from each processing unit, taking into account the entire wastewater surface areas, is more valuable for better quantifying the CH4 emission from the three WWTP. The total annual CH4 flux range from each processing unit is shown in Table 4. The dominant CH4 sources were found to be different in the three WWTP. Similar to the total annual N2 O fluxes, the total annual CH4 fluxes also showed different variation trends from the CH4 emission flux because there was a large difference in wastewater surface area between the processing units. The total CH4 fluxes range were 3.19–8.09 × 104 kg yr−1 ,
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Y.G. Ren et al. (a) 160
WWTP I WWTP II WWTP III
140
CH4 flux (g m-2 day-1)
120 100 80 60 40 20 0 GC
PST
PAT AnaT AnoT
-3 -1
OT
FCT AWTPIAWTPII SCT
SSC
SDG
The processing units of three WWTPs
(b)
WWTP I WWTP II WWTP III
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IPS
100 80 60 40 20 0 IPS
GC
PST
PAT
AnaT
AnoT
OT
FCT
AWTPI AWTPII SCT
The processing units of three WWTPs
Figure 4. Comparison of CH4 fluxes (a) and dissolved CH4 concentrations of wastewater (b) from each processing unit of three WWTP (n = 15). Abbreviations are the same as Figure 2. The error bars shown are standard deviations (n = 15).
Table 4.
Total annual CH4 flux range from each processing unit of three WWTP (n = 15). WWTP I
Processing unit IPS GC PST PAT AnaT AnoT OT FCT AWTP I AWTP II SCT SSC SDG Total
WWTP II
WWTP III
Water surface area (m2 )
CH4 flux (kg yr−1 )
Water surface area (m2 )
CH4 flux (kg yr−1 )
Water surface area (m2 )
CH4 flux (kg yr−1 )
45 50 N/A N/A 2083 6540 17920 9156 648 N/A 400 0.25 400
304–1308 378–450 N/A N/A 1898–3979 6746–10,163 5421–7861 11570–48,065 450–1062 N/A 3500–5001 79–120 1637–3000 31,904–80,889
7.2 270 1032 680 1000 6390 7200 5664 2052 840 400 1 250
17–38 3364–9868 431–938 622–2514 901–2091 2659–5176 37534–66,141 1814–2743 18959–118,147 582–2184 6642–27,680 6–10 2483–3928 76,014–241,357
15 100 227 N/A 766 562 1675 1344 145 N/A 200 0.75 200
85–286 1432–3073 836–1973 N/A 650–1511 215–326 2790–4366 1002–1567 336–1814 N/A 879–5820 10–11 1774–3020 10,004–23,764
Note: N/A = not available; abbreviations same as Figure 2.
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Environmental Technology
Factors influencing CH4 emission The ORP parameter can reflect the DO concentration in an activated sludge reactor system and it is also an indicator of operational conditions such as over-aeration and under-aeration [37]. During the experiment period, it was clear that ORP had a positive correlation with DO concentration, which meant aerobic and anaerobic/anoxic environment could be easily determined by means of the ORP value [38,39]. All methanogens were strictly and obligatory anaerobic so large amounts of oxygen existed in wastewater would inhibit CH4 formation [40]. On the other hand, more CH4 would emit from wastewater due to the mechanical aeration. The CH4 emission flux could not reflect the real quantity of CH4 generated under different ORP values. Data was obtained from anaerobic tanks, anoxic tanks and oxic tanks in the three WWTP and Figure 5 shows the relationship between ORP value and dissolved CH4 concentration. Previous studies confirmed that when the ORP value was below +100 mV, the more ORP declined, the higher the activity level of methanogens and the greater the quantity of CH4 generated by anaerobic biodegradation of organic materials in wastewater [41]. Figure 5 is consistent with that finding and presented a big gap about the quantity of CH4 generated between the anaerobic/anoxic zone and aerobic zone. With respect to COD, CH4 production rate increased (see Figure 4) with influent COD load (see Table 1), which demonstrates that CH4 emissions have a significant relationship with organic carbon. Temperature was another factor that affects CH4 flux by influencing the activities of CH4 -oxidizing and CH4 -producing bacteria [42]. In this study, wastewater temperature was observed between 12– 24◦ C, no significant correlation was observed between CH4 emission and wastewater temperature.
70 Dissolved CH4 concentration (10-1 mg m-3)
7.60–24.14 × 104 kg yr−1 and 1.00–2.38 × 104 kg yr−1 from WWTP I, WWTP II and WWTP III, respectively. The WWTP II had the highest total annual CH4 flux due to its inefficient operating condition and more pretreatment and advanced treatment processes than the other two WWTP. Results also showed the total annual flux was influenced by different BNR process employed in these WWTP. The CH4 emissions accounted for approximately 0.714% ± 0.157, 1.262% ± 0.367 and 0.137% ± 0.035 of the total TOC removed from the A + OD process, AA/A/O process and r-A/A/O process, respectively (see Table 3). That means CH4 conversion ratio of the r-A/A/O process was the lowest among the three WWTP, which were 89.1% and 80.8% lower than that of the A-A/A/O process and A+OD process, respectively. Based on the treatment capacity (see Table 1), the flow-based CH4 emission factors were 4.99–12.66 × 10−4 , 2.08–6.61 × 10−3 and 5.48–13.02 × 10−4 g CH4 (L of wastewater)−1 for WWTP I, WWTP II and WWTP III.
9
60
Anoxic/Anaerobic zone Aerobic zone
50 40 30 20 10 -150
-100
-50
0 ORP value (mV)
50
100
150
Figure 5. Relationship between dissolved CH4 concentration and ORP value (n = 15).
Conclusion Significant amounts of N2 O and CH4 were found to emit from the three WWTP. The mean total annual N2 O fluxes from WWTP I, WWTP II and WWTP III were 4.94 × 103 kg yr−1 , 4.44 × 103 kg yr−1 and 2.33 × 103 kg yr−1 , respectively. Flowbased emission factors were estimated to be 7.73 × 10−5 , 1.22 × 10−4 and 1.28 × 10−4 g N2 O (L of wastewater)−1 for WWTP I, WWTP II and WWTP III. The mean total annual CH4 fluxes from WWTP I, WWTP II and WWTP III were 3.76 × 104 kg yr−1 , 1.91 × 105 kg yr−1 and 1.79 × 104 kg yr−1 , respectively. Flowbased emission factors were estimated to be 5.88 × 10−4 , 5.22 × 10−3 and 9.82 × 10−4 g CH4 (L of wastewater)−1 for WWTP I, WWTP II and WWTP III. Some laboratory experimental studies indicated that both N2 O and CH4 emissions changed upon variation of − several BNR process factors including: NH+ 4 –N, NO2 –N, temperature, DO concentration, ORP and COD load. During the experimental period, it was confirmed that NO− 2 –N concentration and ORP value were the dominant influencing factors for the N2 O and CH4 emission in the full-scale municipal WWTP using different activated sludge BNR processes, respectively. Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (No. NCET-10-0554), National Natural Science Foundation of China (NO. 21177075) and National Natural Science Foundation of China (No. 21007032). We would like to thank Professor X.J. Bi from the School of Environmental and Municipal Engineering, Qingdao Technological University for his invaluable assistance in the gas sampling.
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