Greenhouse gas emissions from stabilization ponds ...

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Greenhouse gas emissions from stabilization ponds in subtropical climate a

a

b

a

I.Y. Hernandez-Paniagua , R. Ramirez-Vargas , M.S. Ramos-Gomez , L. Dendooven , F.J. b

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Avelar-Gonzalez & F. Thalasso a

Departamento de Biotecnología y Bioingeniería, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (Cinvestav), Av. IPN 2508, San Pedro Zacatenco, 07360 México City, México b

Departamento de Fisiología y Farmacología, Universidad Autónoma de Aguascalientes, Av. Universidad 940, 20131 Aguascalientes, Ags. México Published online: 23 Oct 2013.

To cite this article: I.Y. Hernandez-Paniagua, R. Ramirez-Vargas, M.S. Ramos-Gomez, L. Dendooven, F.J. Avelar-Gonzalez & F. Thalasso , Environmental Technology (2013): Greenhouse gas emissions from stabilization ponds in subtropical climate, Environmental Technology, DOI: 10.1080/09593330.2013.848910 To link to this article: http://dx.doi.org/10.1080/09593330.2013.848910

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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.848910

Greenhouse gas emissions from stabilization ponds in subtropical climate I.Y. Hernandez-Paniaguaa , R. Ramirez-Vargasa , M.S. Ramos-Gomezb , L. Dendoovena , F.J. Avelar-Gonzalezb and F. Thalassoa∗ a Departamento

de Biotecnología y Bioingeniería, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (Cinvestav), Av. IPN 2508, San Pedro Zacatenco, 07360 México City, México; b Departamento de Fisiología y Farmacología, Universidad Autónoma de Aguascalientes, Av. Universidad 940, 20131 Aguascalientes, Ags. México

Downloaded by [Thalasso Frederic] at 13:48 24 October 2013

(Received 24 April 2013; accepted 19 September 2013 ) Waste stabilization ponds (WSPs) are a cost-efficient method to treat municipal and non-toxic industrial effluents. Numerous studies have shown that WSPs are a source of greenhouse gas (GHG). However, most reports concerned anaerobic ponds (AP) and few have addressed GHG emissions from facultative (FP) and aerobic/maturation ponds (MPs). In this paper, GHG emissions from three WSP in series are presented. These WSPs were designed as anaerobic, facultative and aerobic/maturation and were treating agricultural wastewater. CH4 fluxes from 0.6 ± 0.4 g CH4 m−2 d−1 in the MP, to 7.0 ± 1.0 g CH4 m−2 d−1 in the (AP), were measured. A linear correlation was found between the loading rates of the ponds and CH4 emissions. Relatively low CO2 fluxes (0.2 ± 0.1 to 1.0 ± 0.8 g CO2 m−2 d−1 ) were found, which suggest that carbonate/bicarbonate formation is caused by alkaline pH. A mass balance performed showed that 30% of the total chemical oxygen demand removed was converted to CH4 . It has been concluded that the WSP system studied emits at least three times more GHG than aerobic activated sludge systems and that the surface loading rate is the most important design parameter for CH4 emissions. Keywords: carbon dioxide; mass balance; methane; nitrous oxide; stabilization ponds

1. Introduction Waste stabilization ponds (WSPs) are a method for the treatment of municipal and non-toxic wastewater. A WSP is considered a low-cost, low-energy, low-maintenance, highly efficient and sustainable technology, very attractive for wastewater treatment in both developed and developing countries.[1] In Mexico, 16% (729) of the wastewater treatment plants are WSPs, treating 13 m3 s−1 of wastewater.[2,3] Despite several advantages, WSPs are responsible for greenhouse gas (GHG) emissions; mainly methane (CH4 ), carbon dioxide (CO2 ) and nitrous oxide (N2 O). Table 1 shows some CO2 and CH4 emission data, reported in the literature for WSPs and anaerobic lagoons, which are in many ways similar to anaerobic WSPs. As shown, these systems emit GHG with a global average of 85 and 86 g m−2 d−1 , for CO2 and CH4 , respectively. In CO2 equivalent units, this CH4 emission corresponds to 2.1 kg CO2 eq. m−2 d−1 . On the other side, Huttunen et al.[14] reported N2 O emissions ranging from −130 to 170 mg N2 O m−2 d−1 in natural ponds, whereas Singh et al.[21] reported a range from 0 to 0.5 mg N2 O m−2 d−1 in an urban shallow pond. According to these data, CH4 is responsible for most of GHG emission in ponds, with more than 90% of total emissions, in CO2 equivalent units. Reports found in the literature are mostly related to ponds located in temperate or continental regions and ∗ Corresponding

author. Email: [email protected]

© 2013 Taylor & Francis

relatively little attention has been given to GHG emissions from WSPs in subtropical climates. Anaerobic conditions promote methanogenesis, so anaerobic ponds (AP) have been investigated intensively and only a few studies have addressed GHG emissions from facultative (FP) and aerobic/maturation ponds (MPs). Even less studies report N2 O emissions from WSPs. Therefore, there is need to quantify GHG emissions, including N2 O, from different WSP operated under similar conditions, to compare emissions from AP, FP and MPs, and to investigate the factors that control these emissions. The objectives of this study were (a) to determine CH4 , CO2 and N2 O fluxes from a series of AP, FP and MPs in subtropical climate and (b) to link GHG emissions to wastewater characteristics. 2.

Materials and methods

The WSP system studied was located in the State of Aguascalientes, Mexico (Lat. 21.9664, Long. −102.3738, Figure 1). The climate in the area is classified as subtropical, with dry mild winters, warm summers and with an average yearly temperature of 18.5◦ C. The WSP system contained three ponds; an AP, a FP and a MP. Each pond included baffles to ensure a successive descending and ascending flow and to control the flow pattern while avoiding channelling.

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I.Y. Hernandez-Paniagua et al. Table 1.

Emissions of GHG from ponds and lagoons as reported in literature. GHG fluxes (mg m−2 d−1 )

Systema

Climate


AeP Tropical AeP Temperate AP Temperate AP Temperate AP Temperate AP Mediterranean AP Mediterranean AP Mediterranean AP Maritime AP Equatorial NP Boreal NP Boreal NP Boreal NP Boreal AL Temperate AL Temperate AL Temperate Mean rangesb Global averagec Global averaged a AeP:

CO2

CH4

Reference

−5,039–9,898 – 5,422–333,988 5,578–366,300 6,051–19,233 – – – 11,368 – −1.5–110 – −43,027–98,064 – – 11,787–294,695 11,964–404,518 −1,025–218,099 85,699 85,699

−78–956 14–1,296 16,428–378,571 18,571–442,857 12,985–34,814 7,707–64,028 26,885 1,428–52,857 9,070 587,331 2–73 −7–1,080 −4–3,420 1–500 800–6,180 9,286–234,148 9,258–314,285 5,457–102,505 86,118 2’027,962

Silva et al.[4] Stadmark and Leonardsson [5] Park and Craggs [6] Craggs et al.[7] Toprak [8] Picot et al.[9] Toprak [10] Paing et al.[11] Heubeck and Craggs [12] Yacob et al.[13] Huttunen et al.[14] Dove et al.[15] Roulet et al.[16] Weyhenmyer [17] Sharpe et al.[18] Safley and Westerman [19] Safley and Westerman [20]

Aerobic pond; AP: Anaerobic pond; NP: Natural pond; AL: Anaerobic lagoon.

b Mean estimated from averages minimums and maximums. c Arithmetic mean of all data. d Arithmetic mean of all data in CO equivalent units. 2

Figure 1.

Location and map of the study site, including the WSP system configuration within the site. Numbers show the sampling points.

Environmental Technology Table 2.

Operational parameters of the system studied.

Parameter


Depth Area Total volume Hydraulic retention time Hydraulic surface loading Organic loading to the first pond

Units m m2 m3 d m3 m2 d−1 kg BOD5 ha−1 d−1

AP

FP

MP

2 1.7 0.7 122.5 211.8 214.3 245 360 150 14.5 21.3 8.9 0.14 77.5

0.08 19.7

0.08 12.9

The operational parameters of the system studied are listed in Table 2. The WSP system was receiving a constant flow of wastewater at 16.9 ± 0.7 m3 d−1 from an experimental farm of the Autonomous University of Aguascalientes. The wastewater contained human sewage combined with organic sewage from the experimental farm and a small dairy facility. The WSP was visited four times in one-year period: in June 2008, September 2008, February 2009 and June 2009. A total of eight sampling points were defined to measure physicochemical parameters and GHG emissions; four in AP, two in FP and two in MP (Figure 1). Additionally, influent and effluent water samples from each pond were taken only for physicochemical analysis. Each sampling point was located at mid-width of the ponds and the same sampling points were used during the entire experiment. Water samples were taken at different depths (i.e. each 0.5 m) using a horizontal 2.2 L Van Dorn bottle (WILDCO, USA). Samples were transferred to amber glass bottles and kept on ice until analysis. Temperature, pH, dissolved oxygen (DO), oxidation reduction potential and conductivity were measured in situ using a calibrated multiparametric probe (556 MPS, YSI, USA). Biological oxygen demand (BOD5 ), chemical oxygen demand (COD), − − ammonium (NH+ 4 ), nitrate (NO3 ), nitrite (NO2 ), phosphate 3− (PO4 ), total phosphorus (TP) and sulphates (SO2− 4 ) were measured according to standard methods.[22] Total nitrogen (TN), total carbon (TC) and total organic carbon (TOC) were measured with a TC and nitrogen analyser (Shimadzu Vcsn equipped with a TN1 module). CH4 , CO2 and N2 O fluxes were determined at each sampling point, four times during the year of studies. To investigate possible diel variations in the CH4 and CO2 fluxes, diurnal and nocturnal measurements were performed in two of the campaigns (September 2008 and February 2009). Diurnal and nocturnal campaigns started by 9am and 11pm, respectively. Fluxes were measured with cylindrical static chambers (St. Louis et al.[23]; Johansson et al.[24]; Huttunen et al.,[14]), and designed according to St. Louis et al.[23]; inner diametre 0.14 m; height 0.20 m. Static chambers were covered with aluminium foil to reduce heating and avoid solar radiations during measurements. Gas samples were taken at time 0, 20, 40 and 60 min and

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transferred to 25 ml amber glass vials. For that purpose, vials were placed in a water bucket and filled upside-down to displace water by the gas sample before being closed by rubber stoppers and aluminium caps. CH4 concentration in the vials was determined with an Agilent Technology 4890D gas chromatograph (GC) fitted with flame ionization detector (FID) and a Porapak-Q column. The N2 O concentration was determined in a similar GC fitted with an electron capture detector (ECD) and a GS-Q column (J&W Scientific). The CO2 concentration was determined with a SRI 8610C GC fitted with a thermal conductivity detector (TCD) and a packed column (Alltech CTRI). Gas standards were injected before and after sample analysis. GHG fluxes (g m−2 h−1 ) were determined from the slope of the gas concentration increase versus time. To accept data trends, two criteria were used, according to Duchemin et al.;[25] (i) that the initial concentration was nearly equal to ambient atmospheric concentration and (ii) that the correlation coefficient (R2 ) from the linear regression analysis was >0.95 for CH4 and >0.90 for N2 O and CO2 . It should be noted that over the sampling time (≤1 h) no decline of the concentration increase was observed, as previously reported by Livingston et al.[26] and that all flux measurements were done in triplicate to allow data interpretation if a data trend was rejected. The statistical analyses were performed using the software SPSS 19.0® for windows.[27]

3. Results and discussion 3.1. Water quality and WSPs operation The WSP system received an influent with COD, BOD5 and TN concentrations of 951 ± 40, 561 ± 35 and 148 ± 38 g m−3 , respectively. In situ characterization showed that no DO was found in AP, at any depth. In FP, DO was only found near the surface (3.8 ± 0.7 mg L−1 at the surface and 0.5 ± 0.6 mg L−1 at 0.5 m of depth). In MP, large variations of DO concentration were observed. In June 2008 and June 2009, surface concentrations were close to 8 mg L−1 , with one reading unexplainably as high as 17 mg L−1 , in June 2009. In September 2008 and February 2009 surface concentrations were below 2 mg L−1 . Bottom DO concentrations in MP were below 0.8 mg L−1 in all the campaigns. Redox potential confirmed anaerobic conditions in AP, FP and MP (data not shown). An alkaline pH was observed, increasing linearly from 7.5 ± 0.4 at the influent of AP to 8.5 ± 0.2 at the effluent of MP (R2 = 0.97). Figure 2(a) shows the COD, BOD5 and TN average profiles observed along the WSP system using averaged data of one-year characterization. The WSP system removed 74 ± 3% of the COD influent and 76 ± 4% of the BOD5 influent. The COD and BOD5 profiles showed a similar shape with constant COD/BOD5 ratio of 2.0 ± 0.2 from the influent to the effluent. Despite relatively long hydraulic residence times and influent BOD5 within previously reported values,[28]

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I.Y. Hernandez-Paniagua et al.


Figure 2. One-year average profiles of (a) COD (-•-), BOD5 (-◦-) and TN (--) concentrations and (b) CH4 (-•-), N2 O (--) and CO2 (-◦-) fluxes observed in the WSP system as a function of the hydraulic residence time in the WSP system. Error bars show standard deviations. Table 3.

Nitrogen, phosphorus and sulphur compounds concentration in WSP system.

Pondsite

TNa (g m−3 )

NH+ 4-N (g m−3 )

NO− 2 -N (g m−3 )

NO− 3 -N (g m−3 )

PO2− 4 -P (g m−3 )

TPb (g m−3 )

SO2− 4 -S (g m−3 )

AP-2 AP-3 AP-4 AP-5 FP-7 FP-8 MP-10 MP-11

141 ± 69 128 ± 72 117 ± 61 115 ± 60 105 ± 47 91 ± 44 64 ± 52 51 ± 39

59 ± 13 53 ± 14 53 ± 12 53 ± 13 46 ± 14 40 ± 12 20 ± 13 16 ± 13

0.2 ± 0.4 0.3 ± 0.4 0.1 ± 0.2 0.9 ± 1.8 0.1 ± 0.1 0.3 ± 0.5 0.6 ± 0.8 0.7 ± 1.0

1.2 ± 1.4 1.1 ± 1.3 1.2 ± 1.3 1.9 ± 1.0 1.9 ± 0.9 1.4 ± 1.4 1.4 ± 1.1 1.6 ± 0.2

25 ± 7 25 ± 6 22 ± 18 29 ± 1 28 ± 4 25 ± 1 24 ± 1 18 ± 6

67 ± 35 65 ± 37 73 ± 43 65 ± 40 64 ± 40 57 ± 32 54 ± 34 40 ± 30

18 ± 1 19 ± 1 5±2 2±1 NDc ND ND ND

a TN:

Total nitrogen.

b TP: Total phosphorus. c ND: Not detected.

the removal efficiency was lower than those observed in other WSPs with similar wastewater characteristics.[29] Table 3 shows the nitrogen, phosphorus and sulphur concentration in the WSP system. In AP, TN concentration decreased while in FP, despite longer hydraulic residence time, a lower amount of TN was removed. A sharp TN decrease was observed in MP (Table 3, Figure 2(a)). In both AP and FP, 44 ± 3% of TN was ammoniacal nitrogen (NH3 –N) while this percentage decreased to 12 ± 4% in MP. These results combined with strict anaerobic conditions in AP suggest that the decrease in TN was mainly due to assimilative immobilization, while nitrification was probably taking place in MP. Influent wastewater contained a TP and sulphate concentrations of 77 ± 18 and 18 ± 2 g m−3 , respectively. No significant TP removal was observed (p > 0.05), whereas a complete sulphate removal was observed in AP, probably due to anaerobic conditions promoting sulphate reduction.

3.2. GHG emission The CH4 fluxes ranged from 7.0 ± 1.0 in AP to 0.6 ± 0.4 g CH4 m−2 d−1 in MP (Figure 2(b)). No significant difference was found between diurnal and nocturnal measurements (p > 0.05, Figure 3). The absence of diel variation of CH4

Figure 3. Diel average fluxes of (a) CH4 and (b) CO2 measured during the campaigns carried out in September 2008 and February 2009. Error bars show standard deviations.

flux is in accordance to that observed by Silva et al.[4] in algal-based ponds. However, the same authors reported diel variation in ponds covered by duckweed (Lemnoideae). In the present work, no aquatic plant was present in any of the ponds. As seen in previous studies,[4,24] CH4 fluxes shown spatial variation with the largest emissions in the


Environmental Technology AP and the lowest in the MP. Compared with CH4 , CO2 fluxes were low; ranging from 1.0 ± 0.8 g to 0.2 ± 0.1 CO2 m−2 d−1 . These low CO2 fluxes are probably explained by the absorption of the CO2 into the pond water, as seen by Park and Craggs [6] and Craggs et al.[7] Indeed, an increase in inorganic carbon was observed along the system (data not shown) and the pH was alkaline over the entire characterization, which promotes carbonate/bicarbonate formation.[30] Similar to CH4 , no diel variation of CO2 fluxes was observed (Figure 3(b)). This may be explained by the low photosynthetic activity due to low chlorophyll-a concentration (not detected in the present work). Indeed, it has been shown that in the presence of photosynthetic activity, photoperiods influence strongly CO2 flux in constructed wetlands.[31] The N2 O fluxes varied from 0.12 10−4 to 9.5 10−4 g N2 O m−2 d−1 , with the highest N2 O fluxes found in FP. Similar to the other GHG measured, no diel variation of N2 O fluxes was observed (p > 0.05). In wastewater plants, N2 O can be produced during both nitrification and denitrification, but it remains unclear whether nitrifying or denitrifying microorganisms are the main source of N2 O emissions.[32] From the results obtained, no clear explanation can be found on the reason why N2 O fluxes were mostly found in FP. Ammonia, nitrite and nitrate mass balances were not significant and any conclusion on the nitrogen process or on a possible mechanism of N2 O emission would be arguable. However, it should be pointed −3 out that an abrupt NO− 2 uptake (from 0.93 to 0.07 g m ) was observed in FP. N2 O production combined with NO− 2 uptake can be explained either by anaerobic denitrification or by nitrifier denitrification.[33] Complementary research would be necessary to confirm which route might be involved. Taking into account the global warming potential of the GHG, CH4 was by far the most important GHG emitted by the WSP system. The CH4 emission was 45 g CO2 eq m−2 d−1 or 99% of total GHG emissions. The average CH4 emission from AP (5.33 ± 1.13 g CH4 m−2 d−1 ) was lower than most emissions previously reported for AP (average of 86 ± 92 g CH4 m−2 d−1 , Table 1). Despite that the previous studies were performed under very different

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conditions (ponds designs, loading rates and climatic conditions), it can be noticed that even with a complete COD conversion to CH4 of the WSP presented here, a total CH4 emission of 14 g CH4 m−2 d−1 would have been observed. This value would be still significantly inferior to previously reported emissions (Table 1). It might be due to the relatively low AP loading rate (33 ± 2 g BOD m−3 d−1 ), which is within typical loading rates received by WSP in Mexico [2,34] but was significantly lower than the design values recommended by Mara and Pearson (300 g BOD m−3 d−1 [35]). On the contrary, CH4 emissions in MP (0.67 ± 0.43 g CH4 m−2 d−1 ) were similar to average emissions observed in aerobic ponds (0.87 ± 0.75 g CH4 m−2 d−1 , Table 1). It should be noted that any increase in the loading rate would probably increase GHG emission. As it will be shown hereafter, a sigmoidal correlation was found between CH4 fluxes and COD and, additionally, Gonzalez-Valencia et al.[36] have shown that in aquatic ecosystems, an increase in the loading rate increases exponentially methane emission. The effect of the measured parameters on GHG fluxes was also investigated. Surprisingly, no significant effect of the temperature on GHG emissions was found, since no significant differences were observed between fluxes measured in winter and summer (p > 0.05). Regarding CH4 fluxes, a sigmoidal correlation was found between CH4 fluxes and COD (R2 = 0.98, Figure 4(a)) and BOD (R2 = 0.94, Figure 4(b)) concentrations. A sigmoidal relationship was also found between CH4 fluxes and TOC, TC and TN, but with poorer correlation coefficient (R2 from 0.89 to 0.93, results not shown). No correlation was observed between CH4 fluxes and nitrogenous, phosphorous or sulphurous compounds concentration. CO2 fluxes were inversely and linearly correlated to pH (R2 = 0.70), which is in accordance with pH dependency of CO2 equilibrium.[30] CO2 fluxes were also asymptotically proportional to COD, BOD, TOC, but not with any nitrogenous, phosphorus or sulphurous compounds concentration. These results show that carbonaceous compounds were governing CH4 and CO2 emissions. Due to the relatively low level of N2 O fluxes, no correlation was observed with any of the measured wastewater parameters.

Figure 4. One-year average CH4 fluxes as a function of COD (a) and BOD (b) observed in the WSP system. Error bars show standard deviations.

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Figure 5. COD mass balance over the WSP system. Values show percentage of the influent COD while values in brackets are percentages of the removed COD for a given pond.

3.3. Mass balance A COD mass balance over the entire WSP system was determined with influent COD considered as 100%. Figure 5 shows that 30% of the influent COD was converted into CH4 while 47% of the influent COD was removed by other processes. Remembering that anaerobic processes are conservative in terms of COD while in aerobic processes COD reduction equals oxygen consumed, it can be assumed that 47% of influent COD was removed by aerobic processes and/or biomass accumulation in the ponds. A closer analysis of COD mass balance in each pond showed that anaerobic and aerobic COD removal accounted for 40% and 60% of the total COD removal, respectively, regardless of the pond system. The observed similarity between COD mass balance in AP, FP and MP suggests that in the WSP system tested, the design of the ponds had little or no effect on GHG emissions. This became more apparent from the surface loading rate (BA , g COD m−2 d−1 ), defined as the loading rate per unit of pond surface. A linear correlation was observed between BA , expressed in COD units, and CH4 flux measured in the three ponds; flux (g m−2 d−1 ) = 1.15 BA − 4.9, R2 = 0.99. These results indicate that the surface loading rate is the most important design parameter, not only for operational purposes as previously reported [35] but also for CH4 emissions. As mentioned, methane emission, as well as COD removal, was proportional to BA , resulting in the same proportion of aerobic versus anaerobic removal processes in all ponds. This is the result of a combination of complex phenomena, including mass transfer, photosynthesis, sedimentation and multiple biodegradation kinetics, that cannot be solved from the global mass balance presented here. However, it has been shown that sedimentation of organic matter to an anaerobic biomass bed is an import removal mechanism in WSP.[37] Therefore, a possible and partial explanation might be that the same proportion of COD was

removed through anaerobic processes, after sedimentation to the sediment layer. These results confirm previous reports (Table 1) that WSP systems emit large amounts of CH4 . During the year of characterization, the average CH4 emission was 3.16 kg CO2 eq per kg of COD removed, or 2.32 kg CO2 eq m−3 of wastewater treated. In a comparison with activated sludge systems, the emissions reported here are about twice larger than those reported by Flores-Alsina et al.[38] who determined a range of GHG emissions from 1.0 to 1.1 kg CO2 eq m−3 in activated sludge systems. Moreover, Flores-Alsina et al. [38] were taking into account not only the activated sludge process itself, but also sludge processing and disposal (mostly done by anaerobic digestion), energy consumption and chemicals (considered as negligible). On the other hand, Keller and Hartley [39] reported a GHG emission of 0.91 kg CO2 eq kg−1 COD removed, taking into account fossil fuels consumption. Therefore, it can be estimated that the WSP system presented here emits at least three times more GHG than activated sludge processes. The WSP system characterized in the present work is similar to those generally used in Mexico. According to a national survey,[2,3,34] 13 m3 s−1 of wastewater are treated by WSPs, nationwide. With an average BOD of 0.66 kg m−3 , similar to influent BOD of the WSP system presented in this work, the total amount of treated BOD by WSPs reaches 917 ton d−1 , as an estimate. Assuming the same BOD removal efficiency and conversion to CH4 as was observed in this study (76% removal efficiency and 7 g CH4 g−1 BOD conversion), the total methane production in Mexico by WSPs would be 118 ton of CH4 per day or 1.1 Mt CO2 eq per year. This value represents 0.5% of the total CH4 emissions reported, nationwide.[40]

4. Conclusions In this study the performance of a pond system integrated by an anaerobic, facultative and MP in a subtropical climate was tested. No significant differences on diurnal and nocturnal CO2 and CH4 fluxes were seen. CH4 was by far the most important GHG emitted by the system studied in this work and accounted for 99% of the total CO2 equivalent emissions. A correlation was found between CH4 emissions and COD and BOD concentrations. Regardless of the pond system, 40% of the COD removal was the result of anaerobic and 60% of aerobic processes. Considering the latter, subtropical WSPs are responsible for the release to the atmosphere of significant amounts of GHG, regardless of the pond design, as previously observed in temperate and boreal climates. Acknowledgements The authors are thankful to Romina Martinez-Guerrero, Victoria T. Velazquez, Marco Luna-Guido and Joel Alba-Flores for technical assistance.

Environmental Technology Funding This work was supported financially by the CONACYT and the Ministry of Environment and Natural Resources (SEMARNAT) through project [grant No. 23661]. I.Y. Hernandez-Paniagua and R. Ramirez-Vargas received grant-aided support from the Mexican National Council of Science and Technology (CONACYT (scholarship number 209848 and 219393, respectively).


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