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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20
A model for methane production in sewers a
a
Thitirat Chaosakul , Thammarat Koottatep & Chongrak Polprasert
b
a
Environmental Engineering and Management, School of Environment, Resources and Development, Asian Institute of Technology, Bangkok, Thailand b
Department of Civil Engineering, Faculty of Engineering, Thammasat University, Bangkok, Thailand Published online: 26 Jun 2014.
Click for updates To cite this article: Thitirat Chaosakul, Thammarat Koottatep & Chongrak Polprasert (2014) A model for methane production in sewers, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:11, 1316-1321, DOI: 10.1080/10934529.2014.910071 To link to this article: http://dx.doi.org/10.1080/10934529.2014.910071
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Journal of Environmental Science and Health, Part A (2014) 49, 1316–1321 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.910071
A model for methane production in sewers THITIRAT CHAOSAKUL1, THAMMARAT KOOTTATEP1 and CHONGRAK POLPRASERT2 1
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Environmental Engineering and Management, School of Environment, Resources and Development, Asian Institute of Technology, Bangkok, Thailand 2 Department of Civil Engineering, Faculty of Engineering, Thammasat University, Bangkok, Thailand
Most sewers in developing countries are combined sewers which receive stormwater and effluent from septic tanks or cesspools of households and buildings. Although the wastewater strength in these sewers is usually lower than those in developed countries, due to improper construction and maintenance, the hydraulic retention time (HRT) could be relatively long and resulting considerable greenhouse gas (GHG) production. This study proposed an empirical model to predict the quantity of methane production in gravity-flow sewers based on relevant parameters such as surface area to volume ratio (A6 V) of sewer, hydraulic retention time (HRT) and wastewater temperature. The model was developed from field survey data of gravity-flow sewers located in a peri-urban area, central Thailand and validated with field data of a sewer system of the Gold Coast area, Queensland, Australia. Application of this model to improve construction and maintenance of gravity-flow sewers to minimize GHG production and reduce global warming is presented. Keyword: Empirical model, global warming, gravity-flow, greenhouse gas, hydraulic retention time, methane production, mitigation measure, sewer, slope, validate.
Introduction Global warming is an important issue in the world today because it can cause climate changes resulting in severe flooding or drought. Among the greenhouse gas (GHG) responsible for climate change such as methane (CH4), carbon dioxide (CO2) and nitrogen dioxide (N2O), CH4 has a global warming potential 21–23 times more powerful than CO2, by weight.[1] Due to rapid industrialization and population growth, the atmospheric concentrations of GHG have more than doubled since pre-industrial time, although there have been observed fluctuations with respect to the rate of increase.[2–4] The differences in sewer and sewage treatment systems in developing countries, as compared to developed countries, cannot be overemphasized. Generally, wastewater in developed countries is directly discharged to sewer systems and to central treatment plants. Whereas wastewater in developing countries comprising about 85% of the world’s Address correspondence to Thitirat Chaosakul, Environmental Engineering and Management, Asian Institute of Technology, School of Environment, Resources and Development, P.O. Box 4, Klong Luang, Bangkok, 12120 Thailand; E-mail: aliz.ck@ gmail.com Received December 29, 2013. Color versions of one or more of the figures in this article can be found online at www.tandfonline.com6 lesa.
population [5] are inhabited, is mostly discharged to onsite treatment systems such as septic tanks or cesspools. The effluent of these septic tanks or cesspools, which is partially treated, is further discharged into nearby storm sewers or combined sewers for further treatment or disposal to nearby water courses. Due to improper construction and maintenance, the storm sewers usually have inadequate slope or leakages, resulting in stagnation of wastewater flow leading to long hydraulic retention time and occurrence of anaerobic conditions in the sewer systems. These conditions, especially under tropical conditions, enable the formation of GHG in the sewers which can be released to the atmosphere and contributing to global warming. Previous research on environmental aspects of sewers focused mostly on hydrogen sulfide (H2S) production.[6–8] Lahav et al.[6] used hydraulic theory to predict H2S emission rate in sewers. Parkhurst and Pomeroy[7] estimated H2S emission rate based on oxygen transfer and wastewater pH. Sharma et al.[8] described the dynamics of H2S production in sewer systems associated with pH. As the problems of global warming have become more serious, interests in CH4 formation in sewers have also grown. CH4 produced in sewers was reported by Foley et al.,[9] Guisasola et al.,[10] and Guisasola et al.[11] to be a significant source of GHG emission to the atmosphere. Foley et al.[9] developed an empirical model for estimating CH4 production in sewer systems using the field data of dissolved CH4 concentrations; while Guisasola
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Methane production in sewers et al.[10,11] also developed kinetic models for assessing CH4 formation in sewers considering the biological processes, which involved interactions of sulfate-reducing bacteria and methanogenic archaea. Their models showed the fitted correlation between CH4 production with pipe area to volume (A6 V) ratio and HRT in fully pressurized sewers in Australia. However, there is not much research done on CH4 emission from combined sewers in developing countries especially under tropical climate. A model for CH4 emission from these combined sewers is therefore necessary in assessing the extent of GHG emission and could be used as a tool in the design and operation of combined sewers in developing countries to help minimize the global warming problems. This study aimed to develop an empirical model for predicting CH4 emission from sewers located in tropical areas of developing countries. The specific objectives of the research are: 1. To develop an empirical model for predicting CH4 emission from sewers based on relevant parameters such as A6 V, HRT and temperature, using field data collected from gravity-flow combined sewers located in central Thailand.
Fig. 1. Field sampling site of a peri-urban area, central Thailand.
2. To validate the developed model with field data of CH4 production in a pressurized sewer system of a city in Queensland, Australia. 3. To propose some measures for reducing CH4 production in gravity-flow combined sewers in developing countries.
Materials and methods This section describes the peri-urban area used in the field survey to collect the data on: CH4 production and wastewater characteristics in gravity-flow sewers, flow velocity, water level and temperature. Field sampling The Rattanakosin Village was chosen as the study area because it represents a typical peri-urban area in central Thailand (Figure 1). The mean annual temperature is 28.6 C and the annual precipitation is 1,604 mm. As such, seasonality can be important when characterizing a sewer system which usually has a high rainfall intensity for 1–2 h in a day during the rainy season. The village covers an
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area of 0.93 km2 and has a population of about 8,000. Each household is equipped with a septic tank or cesspool that receives toilet flushing wastewater, and the septic tank or cesspool effluent is discharged into the gravity-flow combined sewer system. The grey water (i.e., wastewaters from washing, bathing and cleansing) is discharged directly into the gravity-flow sewers. The village consists of 3 main gravity-flow sewers, each with an approximate length of 1 km and a diameter of 1 m. The sewer slopes are relatively flat at about 0.00038. Samples of wastewater and CH4 gas were collected in the sewers at the 3 sampling points (Figure 1) during the dry and raining periods. The sampling frequency was every 3 hours for an extended period of 2–3 days. A Sigma 910 area-velocity flow meter (Model 910, Hach, Loveland, CO, USA) was installed at Sampling point 1 in the sewer to measure the flow velocity, flow rate and wastewater depth (Figure 2). The flow meter recorded the flow velocity and water level at 5-min intervals. The wastewater HRT in the sewer was defined as volume per flow rate (V6 Q), while the wetted perimeter of the sewer was determined from the wastewater depth. The surface area (A) of the biofilm growth responsible for CH4 production was calculated by multiplying the wetted perimeter with the sewer length. Temperature and dissolved oxygen (DO) concentrations of the wastewater were measured on site using a monitoring meter (YSI Inc., 6920, Yellow Springs, OH, USA). The wastewater in the sewer was collected in 120-mL plastic bottles for analysis of its major characteristics (Chemical Oxygen Demand, COD; total solids, TS; volatile solids, VS) at the laboratory of the Asian Institute of Technology, Pathumthani, Thailand. The analytical methods follow those outlined in the APHA Standard Methods.[12] A Thermo Scientific MIRAN SapphIRe Portable Ambient Analyzer (205B-XL, Thermo Fisher Scientific Inc.,
Waltham, MA, USA) was used to measure CH4 concentration in the headspace of the sewer.The MIRAN SapphIRe is a filter-based, single beam spectrophotometer with user selectable wavelengths in single or multiple gas mode that compensate for interfering gases with similar absorption bandwidths. The detection limits for CH4 are 1.5 ppm. Chemical filters were used in the field to calibrate the MIRAN SapphIRe instrument (Franklin, MA, USA). After a warm-up time of 18 secs, the MIRAN SapphIRe sampling tube was inserted down about 30 cm through a hole in the closed manhole cover to sample the headspace gas of the sewer (Figure 2). The concentrations of CH4 in the headspace were calculated using Henry’s law.
Results and discussion The field data collected from the sewers of the Rattanakosin Village, central Thailand, are shown in Table 1. Depending on the wastewater flow, rainfall and sewer slope, the wastewater HRTs during the dry period were found to be in the range of 22–31 h, while they were 3–12 h during the wet period. The wastewater depth during the dry and wet periods was approximately 0.4–0.5 m (the wastewater depth during the wet period was controlled by pumping to avoid wastewater overflow and flooding). The average COD concentration of wastewater during the dry period was found to be 175 mg L¡1, and during the wet period it was diluted to be 60 mg L¡1. The average TS and VS concentrations of wastewater during the dry period were found to be 410 and 266 mg L¡1, yet during the wet period they were 437 and 372 mg L¡1, respectively. The DO levels in the wastewater sewers during the dry and wet periods were in the range of 0.79–1.21 and 1.17–2.08 mg L¡1, respectively. The concentrations of CH4 and mass CH4 emission per unit volume of
Manhole
MIRAN SapphIRe 205B-XL
Ground elevaon
Sigma 910 meter
Water level Invert elevaon
Fig. 2. Installation of the Sigma 910 flow meter and gas sampling in sewer.
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Methane production in sewers Table 1. Field data collected from Rattanakosin Village, Central Thailand (average of 3 sampling points).
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CH4 Condition
Time
HRT (hr)
Water depth (m)
Temp ( C)
COD (mg L¡1)
ppm
kg6 m3
Dry period
18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00
22.00 22.00 22.00 26.71 26.71 26.71 26.71 28.20 28.80 31.43 31.43 31.43 31.43 31.43 31.43 27.90 3.64 24 27 30 0 3 6 9 12 0 3 0 3 6 0 3 6 9 0 3 6 9 12 7.77 8.72
0.49 0.49 0.50 0.50 0.50 0.50 0.50 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.50 0.01 0.42 0.40 0.39 0.38 0.38 0.37 0.36 0.35 0.34 0.42 0.42 0.43 0.42 0.42 0.41 0.41 0.48 0.45 0.46 0.46 0.45 0.45 0.41 0.04
33.3 33.0 33.0 33.1 33.1 33.2 33.9 33.6 33.0 33.2 33.2 33.2 33.2 33.3 34.2 33.3 0.3 29.9 30.1 29.7 30.0 30.0 30.4 30.6 31.2 30.5 30.6 30.3 30.4 29.7 30.0 30.9 30.9 31.4 29.7 28.8 29.0 29.2 30.3 30.2 0.7
161.25 166.25 183.75 172.50 181.25 171.25 187.50 202.50 188.75 178.75 187.50 173.75 153.75 152.50 168.75 175.33 13.81 42.16 129.39 44.32 64.86 40.00 72.43 52.97 46.49 48.65 46.49 35.68 61.62 33.51 70.27 76.76 40.00 70.27 84.32 55.14 57.32 57.30 61.62 58.71 21.03
18,000 16,020 16,070 16,500 14,200 14,500 19,000 23,000 20,000 18,250 17,500 16,000 13,500 16,200 19,000 17,183 2,469 1,220 175 370 14,600 18,300 16,100 170 140 360 1,500 1,800 15,600 18,400 14,500 65 450 440 990 1,120 14,000 18,300 19,000 7,164 8,084
0.0102 0.0090 0.0087 0.0085 0.0084 0.0094 0.0114 0.0137 0.0115 0.0107 0.0100 0.0092 0.0080 0.0104 0.0123 0.0101 0.0016 0.0036 0.0001 0.0003 0.0090 0.0105 0.0108 0.0001 0.0001 0.0002 0.0009 0.0011 0.0101 0.0108 0.0103 0.0000 0.0001 0.0002 0.0005 0.0007 0.0095 0.0107 0.0114 0.0046 0.0050
Average S.D Wet period
18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00
Average S.D S.D. D Standard deviation.
wastewater during the dry period were 13,500–23,000 ppm and 0.0076–0.0131 kg m¡3, respectively; while during the wet period, due to dilution effects and short HRT, they were 65–19,000 ppm and 0.00004–0.0114 kg m¡3, lower than those of the dry period. The relatively low CH4 concentrations and emissions that occurred during particular periods were likely due to the pumping operation to control flooding. Because of these man-made fluctuations, the CH4 data during the wet period were not used in development of the CH4 emission model.
Model development Because, in principle, CH4 production in sewers is dependent on wastewater temperature, the amount of biofilm or A and HRT,[13] an empirical model for CH4 emission from sewers could be written as: MCH4 D g QT ¡ 20 A HRT
(1)
where MCH4 is the mass CH4 emission from a sewer, kg; g
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Chaosakul et al.
is the specific rate of CH4 emission, kg m¡2-h¡1; Q is the temperature coefficient D 1.05;[13] T is the temperature of sewer wastewater, C; A is the surface area of biofilm growth, m2; and HRT is the wastewater retention time in sewer, h. To determine the mass CH4 emission per unit volume of wastewater, the term CCH4 can be used: A HRT V
(2)
where CCH4 is the mass CH4 emission per unit volume of wastewater, kg m¡3, and A6 V is the surface area to volume ratio of sewer, m¡1. Equation (2) is similar to the model previously proposed by Foley et al.,[9] except it includes the temperature coefficient, which is an important factor for methane production, especially under tropical conditions prevalent in most developing countries. The value of g was determined by fitting with the field data of CH4 emissions during the dry periods from the gravity-flow sewers located in central Thailand (Table 1). Figure 3 illustrates the relationship between CH4 emissions and ([A6 V] HRT) in gravity-flow sewer, using the dry period data shown in Table 1. The best fitted value of g determined from these data was found to be 6 £ 10¡5 kg m¡2h¡1 and the CH4 emission model can be written as: CCH4 D 6 £ 10
¡5
1:05
. T ¡ 20 /
A HRT C 0:0015 V (3)
The R2 value of Eq. (3) was found to be rather low, being 0.06, probably because: (1) due to location of the sampling points (Figure 1), the (A6 V)HRT data were limited to within the range of 85–125 h m¡1, (2) there were leakages of CH4 gas through manholes to the atmosphere due to volatilization, and (3) diurnal variation of the climatic 0.0160
Methane Emission (kg/m3 ww)
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CCH4 D g QT ¡ 20
Proposed model Foley data (2009) Dry period data
0.0140
R2 = 0.06
0.0120
conditions such as ambient temperatures which varies from 25 to 35 C during night and day times, respectively, which affected the extent of CH4 volatilization and DO concentrations in the sewer wastewater.[13] However, Eq. (3) was validated with CH4 emission data of pressurized sewers in Queensland, Australia where average temperature of sewer wastewater was 24 C.[9] As shown in Figure 3, Eq. (3) was found to fit well with Foley et al.[9] data, having R2 value of 0.76. Because the Foley et al.[9] data came from pressurized sewer system with much less interferences from climatic conditions than the gravity-flow combined sewers of central Thailand, there was less fluctuation of CH4 emission data resulting in satisfactory prediction of the CH4 emission using Eq. (3). Due to the long HRT of about 28 h during the dry period (Table 1), the gravity-flow sewers in central Thailand were found to emit more CH4 than the pressurized sewers in Australia whose HRT was about 4 h (Figure 3). However, more research should be done to improve Eq. (3) to better predict CH4 emissions from gravity-flow combined sewers. Application of CH4 emission model The developed form of Eq. (3) indicates that CH4 emission from sewers depend mainly on A6 VHRT, and due to high temperature, gravity-flow sewers located in tropical area can emit more CH4 than close sewers located in temperate climate. Because A6 V normally depends on wastewater flow rate and may not be practical to change, the only parameter that can be adjusted to reduce CH4 emissions is reducing HRT. There are several methods to reduce HRT of sewer wastewater such as by designing the sewer slope at least 0.002 and maintaining the wastewater flow velocity to be at least 0.6 m6 s to achieve self-cleansing.[13] The concerned authority should make inspections to ensure proper sewer construction and operation6 maintenance. Another possible way to minimizing CH4 production is routine cleansing of the sewers to avoid excessive sludge accumulation. The above measures will lead to the emission reduction of CH4 and also CO2 to the atmosphere, hence minimizing GHG emissions.
0.0100
Conclusions
0.0080 0.0060 0.0040 R2 = 0.76 0.0020 0.0000 0
20
40
60
80
100
120
140
[A/V] • HRT (h/m)
Fig. 3. Methane concentration (kg m¡3) vs (A6 V £ HRT) (h m¡1) for gravity-flow sewer.
Wastewater sewers can emit GHG and contributing to global warming problems. This study developed an empirical model to predict CH4 emission from wastewater sewers based on important parameters such as: A6 V, HRT and temperature. The specific rate of CH4 emission was determined from field data of gravity-flow combined sewers receiving partially treated domestic wastewater located in central Thailand. The developed model was satisfactorily validated with field data of a pressurized sewer system of a
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Methane production in sewers city of Queensland, Australia. Due to the relatively long HRT of the gravity-flow sewers in Thailand, their mass CH4 missions per unit volume were found to be much higher than those of the pressurized sewer system in Australia. Measures to reduce CH4 emissions from gravity-flow sewers are proposed, i.e., designing sewer slope to be at least 0.002 and maintaining the self-cleansing velocity in the sewers. Routine cleansing of the sewers to avoid excessive sludge accumulation should be undertaken also. The developed model could be applied to estimate the amount of CH4 emission from sewers and used as a planning tool in the design and operation of wastewater sewers to minimize GHG emission and reduce global warming.
[4]
[5]
[6]
[7] [8]
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[9]
References [1] Intergovernmental Panel on Climate Change (IPCC). 2006 IPCC Guidelines for national greenhouse gas inventories. Eggleston, H.S., Buendia, L., Miwa, K., Ngara, T., Tanabe, K., Eds.; Prepared by the National Greenhouse Gas Inventories Programme, Institute for Global Environmental Strategies (IGES): Kanagawa, Japan, 2006; Vol. 5, Chap. 6. [2] Wuebbles, D.J.; Hayhoe, K. Atmospheric methane and global change. Earth-Sci. Rev. 2002, 57, 177–210. [3] Bousquet, P.; Ciais, P.; Miller, J.B.; Dlugokencky, E.J.; Hauglustaine, D.A.; Prigent, C.; Van der Werf, G.R.; Peylin, P.; Brunke, E-G.; Carouge, C.; Langenfelds, R.L.; Lathiere, J.; Papa, F.; Ramonet, M.; Schmidt, M.; Steele, L.P.; Tyler, S.C.; White, J.
[10] [11]
[12]
[13]
Contribution of anthropogenic and natural sources to atmospheric methane variability. Nature 2006, 443(28), 439–443. Rigby, M.; Prinn, R.G.; Fraser, P.J.; Simmonds, P.G.; Langenfelds, R.L.; Huang, J.; Cunnold, D.M.; Steele, L.P.; Krummel, P. B.; Weiss, R.F.; O’Doherty, S.; Salameh, P.K.; Wang, H.J.; Harth, C.M.; Muhle, J.; Porter, L.W. Renewed growth of atmospheric methane. Geophys. Res. Lett. 2008, 35, L22805. Population Reference Bureau (PRB). World Population Data Sheet. Author: Washington, DC, 2013. Available at www.prb.org (accessed Nov 2013). Lahav, O.; Sagiv, A.; Friedler, E. A different approach for predicting H2S(g) emission rates in gravity sewers. Water Res. 2006, 40, 259–266. Parkhurst, J.D.; Pomeroy, R.D. Oxygen absorption in streams. J. Sam. Eng., Div., ASCE 1972, 98, 101–124. Sharma, K.R.; Yuan, Z.; Haas, D.; Hamilton, G.; Corrie, S.; Keller, J. Dynamics and dynamic modelling of H2S production in sewer systems. Water Res. 2008, 42(10–11), 2527–2538. Foley, J.; Yuan, Z.; Lant, P. Dissolved methane in rising main sewer systems: field measurements and simple model development for estimating greenhouse gas emissions. Water Sci. Technol. 2009, 60(11), 2963–2971. Guisasola, A.; Haas, D.; Keller, J.; Yuan, Z. Methane formation in sewer systems. Water Res. 2007, 42(6–7), 1421–1430. Guisasola, A.; Sharma, K.R.; Keller, J.; Yuan, Z. Development of a model for assessing methane formation in rising main sewers. Water Res. 2009, 43, 2874–2884. American Public Health Association (APHA). Standard Methods for the Examination of Water and Wastewater, 21th ed. APHA, American Water Works Association, Water Environment Federation: Washington, DC, 2005. Metcalf, L.; Eddy, H.P. Wastewater Engineering: Treatment, Disposal, Reuse, 4th ed.; McGraw-Hill, Inc.: New York, 2003.
Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20
A model for methane production in sewers a
a
Thitirat Chaosakul , Thammarat Koottatep & Chongrak Polprasert
b
a
Environmental Engineering and Management, School of Environment, Resources and Development, Asian Institute of Technology, Bangkok, Thailand b
Department of Civil Engineering, Faculty of Engineering, Thammasat University, Bangkok, Thailand Published online: 26 Jun 2014.
Click for updates To cite this article: Thitirat Chaosakul, Thammarat Koottatep & Chongrak Polprasert (2014) A model for methane production in sewers, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:11, 1316-1321, DOI: 10.1080/10934529.2014.910071 To link to this article: http://dx.doi.org/10.1080/10934529.2014.910071
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Journal of Environmental Science and Health, Part A (2014) 49, 1316–1321 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.910071
A model for methane production in sewers THITIRAT CHAOSAKUL1, THAMMARAT KOOTTATEP1 and CHONGRAK POLPRASERT2 1
Downloaded by [Selcuk Universitesi] at 18:14 20 December 2014
Environmental Engineering and Management, School of Environment, Resources and Development, Asian Institute of Technology, Bangkok, Thailand 2 Department of Civil Engineering, Faculty of Engineering, Thammasat University, Bangkok, Thailand
Most sewers in developing countries are combined sewers which receive stormwater and effluent from septic tanks or cesspools of households and buildings. Although the wastewater strength in these sewers is usually lower than those in developed countries, due to improper construction and maintenance, the hydraulic retention time (HRT) could be relatively long and resulting considerable greenhouse gas (GHG) production. This study proposed an empirical model to predict the quantity of methane production in gravity-flow sewers based on relevant parameters such as surface area to volume ratio (A6 V) of sewer, hydraulic retention time (HRT) and wastewater temperature. The model was developed from field survey data of gravity-flow sewers located in a peri-urban area, central Thailand and validated with field data of a sewer system of the Gold Coast area, Queensland, Australia. Application of this model to improve construction and maintenance of gravity-flow sewers to minimize GHG production and reduce global warming is presented. Keyword: Empirical model, global warming, gravity-flow, greenhouse gas, hydraulic retention time, methane production, mitigation measure, sewer, slope, validate.
Introduction Global warming is an important issue in the world today because it can cause climate changes resulting in severe flooding or drought. Among the greenhouse gas (GHG) responsible for climate change such as methane (CH4), carbon dioxide (CO2) and nitrogen dioxide (N2O), CH4 has a global warming potential 21–23 times more powerful than CO2, by weight.[1] Due to rapid industrialization and population growth, the atmospheric concentrations of GHG have more than doubled since pre-industrial time, although there have been observed fluctuations with respect to the rate of increase.[2–4] The differences in sewer and sewage treatment systems in developing countries, as compared to developed countries, cannot be overemphasized. Generally, wastewater in developed countries is directly discharged to sewer systems and to central treatment plants. Whereas wastewater in developing countries comprising about 85% of the world’s Address correspondence to Thitirat Chaosakul, Environmental Engineering and Management, Asian Institute of Technology, School of Environment, Resources and Development, P.O. Box 4, Klong Luang, Bangkok, 12120 Thailand; E-mail: aliz.ck@ gmail.com Received December 29, 2013. Color versions of one or more of the figures in this article can be found online at www.tandfonline.com6 lesa.
population [5] are inhabited, is mostly discharged to onsite treatment systems such as septic tanks or cesspools. The effluent of these septic tanks or cesspools, which is partially treated, is further discharged into nearby storm sewers or combined sewers for further treatment or disposal to nearby water courses. Due to improper construction and maintenance, the storm sewers usually have inadequate slope or leakages, resulting in stagnation of wastewater flow leading to long hydraulic retention time and occurrence of anaerobic conditions in the sewer systems. These conditions, especially under tropical conditions, enable the formation of GHG in the sewers which can be released to the atmosphere and contributing to global warming. Previous research on environmental aspects of sewers focused mostly on hydrogen sulfide (H2S) production.[6–8] Lahav et al.[6] used hydraulic theory to predict H2S emission rate in sewers. Parkhurst and Pomeroy[7] estimated H2S emission rate based on oxygen transfer and wastewater pH. Sharma et al.[8] described the dynamics of H2S production in sewer systems associated with pH. As the problems of global warming have become more serious, interests in CH4 formation in sewers have also grown. CH4 produced in sewers was reported by Foley et al.,[9] Guisasola et al.,[10] and Guisasola et al.[11] to be a significant source of GHG emission to the atmosphere. Foley et al.[9] developed an empirical model for estimating CH4 production in sewer systems using the field data of dissolved CH4 concentrations; while Guisasola
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Methane production in sewers et al.[10,11] also developed kinetic models for assessing CH4 formation in sewers considering the biological processes, which involved interactions of sulfate-reducing bacteria and methanogenic archaea. Their models showed the fitted correlation between CH4 production with pipe area to volume (A6 V) ratio and HRT in fully pressurized sewers in Australia. However, there is not much research done on CH4 emission from combined sewers in developing countries especially under tropical climate. A model for CH4 emission from these combined sewers is therefore necessary in assessing the extent of GHG emission and could be used as a tool in the design and operation of combined sewers in developing countries to help minimize the global warming problems. This study aimed to develop an empirical model for predicting CH4 emission from sewers located in tropical areas of developing countries. The specific objectives of the research are: 1. To develop an empirical model for predicting CH4 emission from sewers based on relevant parameters such as A6 V, HRT and temperature, using field data collected from gravity-flow combined sewers located in central Thailand.
Fig. 1. Field sampling site of a peri-urban area, central Thailand.
2. To validate the developed model with field data of CH4 production in a pressurized sewer system of a city in Queensland, Australia. 3. To propose some measures for reducing CH4 production in gravity-flow combined sewers in developing countries.
Materials and methods This section describes the peri-urban area used in the field survey to collect the data on: CH4 production and wastewater characteristics in gravity-flow sewers, flow velocity, water level and temperature. Field sampling The Rattanakosin Village was chosen as the study area because it represents a typical peri-urban area in central Thailand (Figure 1). The mean annual temperature is 28.6 C and the annual precipitation is 1,604 mm. As such, seasonality can be important when characterizing a sewer system which usually has a high rainfall intensity for 1–2 h in a day during the rainy season. The village covers an
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area of 0.93 km2 and has a population of about 8,000. Each household is equipped with a septic tank or cesspool that receives toilet flushing wastewater, and the septic tank or cesspool effluent is discharged into the gravity-flow combined sewer system. The grey water (i.e., wastewaters from washing, bathing and cleansing) is discharged directly into the gravity-flow sewers. The village consists of 3 main gravity-flow sewers, each with an approximate length of 1 km and a diameter of 1 m. The sewer slopes are relatively flat at about 0.00038. Samples of wastewater and CH4 gas were collected in the sewers at the 3 sampling points (Figure 1) during the dry and raining periods. The sampling frequency was every 3 hours for an extended period of 2–3 days. A Sigma 910 area-velocity flow meter (Model 910, Hach, Loveland, CO, USA) was installed at Sampling point 1 in the sewer to measure the flow velocity, flow rate and wastewater depth (Figure 2). The flow meter recorded the flow velocity and water level at 5-min intervals. The wastewater HRT in the sewer was defined as volume per flow rate (V6 Q), while the wetted perimeter of the sewer was determined from the wastewater depth. The surface area (A) of the biofilm growth responsible for CH4 production was calculated by multiplying the wetted perimeter with the sewer length. Temperature and dissolved oxygen (DO) concentrations of the wastewater were measured on site using a monitoring meter (YSI Inc., 6920, Yellow Springs, OH, USA). The wastewater in the sewer was collected in 120-mL plastic bottles for analysis of its major characteristics (Chemical Oxygen Demand, COD; total solids, TS; volatile solids, VS) at the laboratory of the Asian Institute of Technology, Pathumthani, Thailand. The analytical methods follow those outlined in the APHA Standard Methods.[12] A Thermo Scientific MIRAN SapphIRe Portable Ambient Analyzer (205B-XL, Thermo Fisher Scientific Inc.,
Waltham, MA, USA) was used to measure CH4 concentration in the headspace of the sewer.The MIRAN SapphIRe is a filter-based, single beam spectrophotometer with user selectable wavelengths in single or multiple gas mode that compensate for interfering gases with similar absorption bandwidths. The detection limits for CH4 are 1.5 ppm. Chemical filters were used in the field to calibrate the MIRAN SapphIRe instrument (Franklin, MA, USA). After a warm-up time of 18 secs, the MIRAN SapphIRe sampling tube was inserted down about 30 cm through a hole in the closed manhole cover to sample the headspace gas of the sewer (Figure 2). The concentrations of CH4 in the headspace were calculated using Henry’s law.
Results and discussion The field data collected from the sewers of the Rattanakosin Village, central Thailand, are shown in Table 1. Depending on the wastewater flow, rainfall and sewer slope, the wastewater HRTs during the dry period were found to be in the range of 22–31 h, while they were 3–12 h during the wet period. The wastewater depth during the dry and wet periods was approximately 0.4–0.5 m (the wastewater depth during the wet period was controlled by pumping to avoid wastewater overflow and flooding). The average COD concentration of wastewater during the dry period was found to be 175 mg L¡1, and during the wet period it was diluted to be 60 mg L¡1. The average TS and VS concentrations of wastewater during the dry period were found to be 410 and 266 mg L¡1, yet during the wet period they were 437 and 372 mg L¡1, respectively. The DO levels in the wastewater sewers during the dry and wet periods were in the range of 0.79–1.21 and 1.17–2.08 mg L¡1, respectively. The concentrations of CH4 and mass CH4 emission per unit volume of
Manhole
MIRAN SapphIRe 205B-XL
Ground elevaon
Sigma 910 meter
Water level Invert elevaon
Fig. 2. Installation of the Sigma 910 flow meter and gas sampling in sewer.
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Methane production in sewers Table 1. Field data collected from Rattanakosin Village, Central Thailand (average of 3 sampling points).
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CH4 Condition
Time
HRT (hr)
Water depth (m)
Temp ( C)
COD (mg L¡1)
ppm
kg6 m3
Dry period
18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00
22.00 22.00 22.00 26.71 26.71 26.71 26.71 28.20 28.80 31.43 31.43 31.43 31.43 31.43 31.43 27.90 3.64 24 27 30 0 3 6 9 12 0 3 0 3 6 0 3 6 9 0 3 6 9 12 7.77 8.72
0.49 0.49 0.50 0.50 0.50 0.50 0.50 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.50 0.01 0.42 0.40 0.39 0.38 0.38 0.37 0.36 0.35 0.34 0.42 0.42 0.43 0.42 0.42 0.41 0.41 0.48 0.45 0.46 0.46 0.45 0.45 0.41 0.04
33.3 33.0 33.0 33.1 33.1 33.2 33.9 33.6 33.0 33.2 33.2 33.2 33.2 33.3 34.2 33.3 0.3 29.9 30.1 29.7 30.0 30.0 30.4 30.6 31.2 30.5 30.6 30.3 30.4 29.7 30.0 30.9 30.9 31.4 29.7 28.8 29.0 29.2 30.3 30.2 0.7
161.25 166.25 183.75 172.50 181.25 171.25 187.50 202.50 188.75 178.75 187.50 173.75 153.75 152.50 168.75 175.33 13.81 42.16 129.39 44.32 64.86 40.00 72.43 52.97 46.49 48.65 46.49 35.68 61.62 33.51 70.27 76.76 40.00 70.27 84.32 55.14 57.32 57.30 61.62 58.71 21.03
18,000 16,020 16,070 16,500 14,200 14,500 19,000 23,000 20,000 18,250 17,500 16,000 13,500 16,200 19,000 17,183 2,469 1,220 175 370 14,600 18,300 16,100 170 140 360 1,500 1,800 15,600 18,400 14,500 65 450 440 990 1,120 14,000 18,300 19,000 7,164 8,084
0.0102 0.0090 0.0087 0.0085 0.0084 0.0094 0.0114 0.0137 0.0115 0.0107 0.0100 0.0092 0.0080 0.0104 0.0123 0.0101 0.0016 0.0036 0.0001 0.0003 0.0090 0.0105 0.0108 0.0001 0.0001 0.0002 0.0009 0.0011 0.0101 0.0108 0.0103 0.0000 0.0001 0.0002 0.0005 0.0007 0.0095 0.0107 0.0114 0.0046 0.0050
Average S.D Wet period
18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00
Average S.D S.D. D Standard deviation.
wastewater during the dry period were 13,500–23,000 ppm and 0.0076–0.0131 kg m¡3, respectively; while during the wet period, due to dilution effects and short HRT, they were 65–19,000 ppm and 0.00004–0.0114 kg m¡3, lower than those of the dry period. The relatively low CH4 concentrations and emissions that occurred during particular periods were likely due to the pumping operation to control flooding. Because of these man-made fluctuations, the CH4 data during the wet period were not used in development of the CH4 emission model.
Model development Because, in principle, CH4 production in sewers is dependent on wastewater temperature, the amount of biofilm or A and HRT,[13] an empirical model for CH4 emission from sewers could be written as: MCH4 D g QT ¡ 20 A HRT
(1)
where MCH4 is the mass CH4 emission from a sewer, kg; g
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is the specific rate of CH4 emission, kg m¡2-h¡1; Q is the temperature coefficient D 1.05;[13] T is the temperature of sewer wastewater, C; A is the surface area of biofilm growth, m2; and HRT is the wastewater retention time in sewer, h. To determine the mass CH4 emission per unit volume of wastewater, the term CCH4 can be used: A HRT V
(2)
where CCH4 is the mass CH4 emission per unit volume of wastewater, kg m¡3, and A6 V is the surface area to volume ratio of sewer, m¡1. Equation (2) is similar to the model previously proposed by Foley et al.,[9] except it includes the temperature coefficient, which is an important factor for methane production, especially under tropical conditions prevalent in most developing countries. The value of g was determined by fitting with the field data of CH4 emissions during the dry periods from the gravity-flow sewers located in central Thailand (Table 1). Figure 3 illustrates the relationship between CH4 emissions and ([A6 V] HRT) in gravity-flow sewer, using the dry period data shown in Table 1. The best fitted value of g determined from these data was found to be 6 £ 10¡5 kg m¡2h¡1 and the CH4 emission model can be written as: CCH4 D 6 £ 10
¡5
1:05
. T ¡ 20 /
A HRT C 0:0015 V (3)
The R2 value of Eq. (3) was found to be rather low, being 0.06, probably because: (1) due to location of the sampling points (Figure 1), the (A6 V)HRT data were limited to within the range of 85–125 h m¡1, (2) there were leakages of CH4 gas through manholes to the atmosphere due to volatilization, and (3) diurnal variation of the climatic 0.0160
Methane Emission (kg/m3 ww)
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CCH4 D g QT ¡ 20
Proposed model Foley data (2009) Dry period data
0.0140
R2 = 0.06
0.0120
conditions such as ambient temperatures which varies from 25 to 35 C during night and day times, respectively, which affected the extent of CH4 volatilization and DO concentrations in the sewer wastewater.[13] However, Eq. (3) was validated with CH4 emission data of pressurized sewers in Queensland, Australia where average temperature of sewer wastewater was 24 C.[9] As shown in Figure 3, Eq. (3) was found to fit well with Foley et al.[9] data, having R2 value of 0.76. Because the Foley et al.[9] data came from pressurized sewer system with much less interferences from climatic conditions than the gravity-flow combined sewers of central Thailand, there was less fluctuation of CH4 emission data resulting in satisfactory prediction of the CH4 emission using Eq. (3). Due to the long HRT of about 28 h during the dry period (Table 1), the gravity-flow sewers in central Thailand were found to emit more CH4 than the pressurized sewers in Australia whose HRT was about 4 h (Figure 3). However, more research should be done to improve Eq. (3) to better predict CH4 emissions from gravity-flow combined sewers. Application of CH4 emission model The developed form of Eq. (3) indicates that CH4 emission from sewers depend mainly on A6 VHRT, and due to high temperature, gravity-flow sewers located in tropical area can emit more CH4 than close sewers located in temperate climate. Because A6 V normally depends on wastewater flow rate and may not be practical to change, the only parameter that can be adjusted to reduce CH4 emissions is reducing HRT. There are several methods to reduce HRT of sewer wastewater such as by designing the sewer slope at least 0.002 and maintaining the wastewater flow velocity to be at least 0.6 m6 s to achieve self-cleansing.[13] The concerned authority should make inspections to ensure proper sewer construction and operation6 maintenance. Another possible way to minimizing CH4 production is routine cleansing of the sewers to avoid excessive sludge accumulation. The above measures will lead to the emission reduction of CH4 and also CO2 to the atmosphere, hence minimizing GHG emissions.
0.0100
Conclusions
0.0080 0.0060 0.0040 R2 = 0.76 0.0020 0.0000 0
20
40
60
80
100
120
140
[A/V] • HRT (h/m)
Fig. 3. Methane concentration (kg m¡3) vs (A6 V £ HRT) (h m¡1) for gravity-flow sewer.
Wastewater sewers can emit GHG and contributing to global warming problems. This study developed an empirical model to predict CH4 emission from wastewater sewers based on important parameters such as: A6 V, HRT and temperature. The specific rate of CH4 emission was determined from field data of gravity-flow combined sewers receiving partially treated domestic wastewater located in central Thailand. The developed model was satisfactorily validated with field data of a pressurized sewer system of a
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Methane production in sewers city of Queensland, Australia. Due to the relatively long HRT of the gravity-flow sewers in Thailand, their mass CH4 missions per unit volume were found to be much higher than those of the pressurized sewer system in Australia. Measures to reduce CH4 emissions from gravity-flow sewers are proposed, i.e., designing sewer slope to be at least 0.002 and maintaining the self-cleansing velocity in the sewers. Routine cleansing of the sewers to avoid excessive sludge accumulation should be undertaken also. The developed model could be applied to estimate the amount of CH4 emission from sewers and used as a planning tool in the design and operation of wastewater sewers to minimize GHG emission and reduce global warming.
[4]
[5]
[6]
[7] [8]
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[9]
References [1] Intergovernmental Panel on Climate Change (IPCC). 2006 IPCC Guidelines for national greenhouse gas inventories. Eggleston, H.S., Buendia, L., Miwa, K., Ngara, T., Tanabe, K., Eds.; Prepared by the National Greenhouse Gas Inventories Programme, Institute for Global Environmental Strategies (IGES): Kanagawa, Japan, 2006; Vol. 5, Chap. 6. [2] Wuebbles, D.J.; Hayhoe, K. Atmospheric methane and global change. Earth-Sci. Rev. 2002, 57, 177–210. [3] Bousquet, P.; Ciais, P.; Miller, J.B.; Dlugokencky, E.J.; Hauglustaine, D.A.; Prigent, C.; Van der Werf, G.R.; Peylin, P.; Brunke, E-G.; Carouge, C.; Langenfelds, R.L.; Lathiere, J.; Papa, F.; Ramonet, M.; Schmidt, M.; Steele, L.P.; Tyler, S.C.; White, J.
[10] [11]
[12]
[13]
Contribution of anthropogenic and natural sources to atmospheric methane variability. Nature 2006, 443(28), 439–443. Rigby, M.; Prinn, R.G.; Fraser, P.J.; Simmonds, P.G.; Langenfelds, R.L.; Huang, J.; Cunnold, D.M.; Steele, L.P.; Krummel, P. B.; Weiss, R.F.; O’Doherty, S.; Salameh, P.K.; Wang, H.J.; Harth, C.M.; Muhle, J.; Porter, L.W. Renewed growth of atmospheric methane. Geophys. Res. Lett. 2008, 35, L22805. Population Reference Bureau (PRB). World Population Data Sheet. Author: Washington, DC, 2013. Available at www.prb.org (accessed Nov 2013). Lahav, O.; Sagiv, A.; Friedler, E. A different approach for predicting H2S(g) emission rates in gravity sewers. Water Res. 2006, 40, 259–266. Parkhurst, J.D.; Pomeroy, R.D. Oxygen absorption in streams. J. Sam. Eng., Div., ASCE 1972, 98, 101–124. Sharma, K.R.; Yuan, Z.; Haas, D.; Hamilton, G.; Corrie, S.; Keller, J. Dynamics and dynamic modelling of H2S production in sewer systems. Water Res. 2008, 42(10–11), 2527–2538. Foley, J.; Yuan, Z.; Lant, P. Dissolved methane in rising main sewer systems: field measurements and simple model development for estimating greenhouse gas emissions. Water Sci. Technol. 2009, 60(11), 2963–2971. Guisasola, A.; Haas, D.; Keller, J.; Yuan, Z. Methane formation in sewer systems. Water Res. 2007, 42(6–7), 1421–1430. Guisasola, A.; Sharma, K.R.; Keller, J.; Yuan, Z. Development of a model for assessing methane formation in rising main sewers. Water Res. 2009, 43, 2874–2884. American Public Health Association (APHA). Standard Methods for the Examination of Water and Wastewater, 21th ed. APHA, American Water Works Association, Water Environment Federation: Washington, DC, 2005. Metcalf, L.; Eddy, H.P. Wastewater Engineering: Treatment, Disposal, Reuse, 4th ed.; McGraw-Hill, Inc.: New York, 2003.
