methaneemissionsfromafull-scaleaaowastewatertreatmentplant.pdf

Methane emissions from a full-scale A/A/O wastewater treatment plant

Jinhe Wang ^a,b, Jian Zhang ^a,⇑, Huijun Xie ^c, Pengyu Qi ^a, Yangang Ren ^a, Zhen Hu ^a

^a Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, 27 Shanda Nanlu, Jinan 250100, PR China

^b School of Municipal and Environmental Engineering, Shandong Jianzhu University, Fengming Road, Jinan 250101, PR China

^c Environmental Research Institute, Shandong University, 27 Shanda Nanlu, Jinan 250100, PR China

a r t i c l e i n f o

Article history:
Received 31 July 2010
Received in revised form 17 October 2010
Accepted 20 October 2010
Available online 23 October 2010

Keywords:
Greenhouse gas
Methane emission
Anaerobic/anoxic/oxic
Wastewater treatment plant

a b s t r a c t

Methane (CH ) emissions from a full-scale anaerobic/anoxic/oxic (A/A/O) wastewater treatment plant (WWTP) (Jinan, China) were investigated during spring and summer of 2010. Results showed that the major emission sources of $\mathrm { C H } _ { 4 }$ performed the following descending order: anaerobic tanks, oxic tanks, aerated grit chambers and sludge concentration tanks. The total annual fluxes of $\mathrm { C H } _ { 4 }$ emissions from the Jinan WWTP were $1 . 6 9 \times 1 0 ^ { 4 } \mathrm { k g } \mathrm { y r } ^ { - 1 }$ , with the emission factors of per capita emissions of 11.3 g CH₄ person $^ 1 \mathrm { y r } ^ { - 1 }$ and flow-based emissions of 1.55 - $1 0 ^ { - 4 } \mathrm { { g } \ C H _ { 4 } \ ( L }$ of wastewater)¹. The estimated source strength of methane for all WWTPs in China was $6 . 2 \ : \mathrm { G g } \ : \mathrm { y r } ^ { - 1 } ( 1 \mathrm { G g } = 1 0 ^ { 9 } \ : \mathrm { g } ) .$ The most significant factors influencing methane emissions were dissolved oxygen concentration in aerated grit chamber and oxic tank and water temperature in high density settler tanks.

Introduction

Methane $\left( \mathrm { C H } _ { 4 } \right)$ is an important greenhouse gas (GHG) with a global warming potential of 25 over a 100-year period. It accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases (Shindell et al., 2009). Since 1750, global average atmospheric concentrations of methane have increased by 150% from approximately 700–1751 ppb by volume (ppbV) in 2002 (Dlugokencky et al., 2003). It is estimated that more than 50% of global methane emissions are related to humanrelated activities, including landfills, natural gas and petroleum systems, wastewater treatment, agriculture, and certain industrial processes (IPCC, 2007).

Extensive studies have been performed on the production and emission of methane from many natural and anthropogenic anaerobic systems, e.g. wetlands, rice paddies, landfills, animals and sludge treatment systems (Khalil et al., 2000; Mikaloff Fletcher et al., 2004). The management and treatment of domestic and industrial wastewater have been identified as an important source of $\mathrm { C H } _ { 4 }$ (Bousquet et al., 2006). During the anaerobic wastewater treatment processes, methane is a byproduct of anaerobic decomposition activated by methanogenic bacteria and subsequently released to the atmosphere by surface diffusion and mechanical aeration, which could be collected for use as fuel after purification in some wastewater treatment plants. In addition, the sludge produced from wastewater treatment may be further biodegraded under anaerobic conditions, resulting in methane emissions (Metcalf and Eddy et al., 2002; Carneiro et al., 2008; Yoshida et al., 2009; Zhu et al., 2009).

Post-consumer waste is a small contributor to global GHG emissions (<5%), with landfill $\mathrm { C H } _ { 4 }$ accounting for >50% of current emissions. Secondary sources of emissions are wastewater $\mathsf { N } _ { 2 } \mathsf { O }$ and $\mathrm { C H } _ { 4 } ,$ but this sector can positively contribute to GHG mitigation at low cost and promote sustainable development through proper regulation of waste management (IPCC, 2007). However, only a few quantitative data exist regarding the generation of $\mathrm { C H } _ { 4 }$ in sewer systems and wastewater treatment other than the anaerobic digestion of sewage sludge. The lack of quantitative data describing $\mathrm { C H } _ { 4 }$ emissions from modern wastewater treatment plants (WWTP) will limit further technical assessments of mitigation options. Guisasola et al. (2008) first studied the methane formation in sewer systems, found that a significant amount of methane can be produced in sewer systems, and this production is positively correlated with the hydraulic retention time of wastewater. Czepiel et al. (1993) first measured methane emissions from one small WWTP in Durham, USA and calculated its emission factors.

With the increasingly stringent effluent standards implemented in China, the biological nutrient removal (BNR) processes have been widely employed in large-scale municipal WWTPs. The anaerobic/ anoxic/oxic (A/A/O) process is one of the most popular BNR techniques in China’ WWTPs. The $\mathsf { A } / \mathsf { A } / 0$ process consists of anaerobic and aerobic steps, where large quantities of CH could generate and emit under certain conditions. However, so far there are no literature reports of methane emissions from China’s WWTPs. Besides, the typical full-scale municipal WWTP in China has a capacity greater than $1 \times 1 0 ^ { 5 } \mathrm { m } ^ { 3 }$ day¹ and serving population than $5 \times 1 0 ^ { 5 }$ much larger than the WWTP involved in Czepiel et al. (1993) both in treatment capacity (4000 m³ day¹) and serving population (12,500). Therefore the emission factors derived from Durham plant might not accurately reflect the $\mathrm { C H } _ { 4 }$ release from China’s WWTPs. China is one of the largest greenhouse gas emitters with the largest population in the world, but China’s GHG emission data collection work lagged far behind the developed countries and large amounts of emission data used presently were referred to foreign reports (IPCC, 2007). Therefore it is important to collect $\mathrm { C H } _ { 4 }$ emission data and calculate the emission factors from China’s WWTPs for better quantification of GHG emissions and further technical assessments of mitigation options. In this study, methane emissions from a typical full-scale A/A/O WWTP in China were measured, and the major emission sources were identified. Factors influencing the $\mathrm { C H } _ { 4 }$ fluxes were also investigated.

Bioresource Technology 102 (2011) 5479–5485

一

Bioresource Technology

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⇑ Corresponding author. Tel./fax: +86 0531 88363015.

E-mail address: zhangjian00@sdu.edu.cn (J. Zhang).

Methods

2.1. Field site

The field sampling experiment was carried out at a full-scale tertiary municipal WWTP in Jinan, a metropolis in northern China. The WWTP has a treatment capacity of $3 \times 1 0 ^ { 5 } \mathrm { m } ^ { 3 } \mathrm { d a y } ^ { - 1 }$ and a serving population of about 1,500,000. The detailed process configuration is shown in Table 1. The wastewater treated in this plant is mostly domestic sewage with an average influent COD of 200 mg L¹ and an average suspended solids concentration (SS) of $1 5 0 \mathrm { m g } \mathrm { L } ^ { - 1 }$ . The effluent from this WWTP can meet the most stringent effluent standards in China: $\mathrm { C O D } < 5 0 \mathrm { m g } \mathrm { L } ^ { - 1 } , \mathrm { S S } < 1 0 \mathrm { m g } \mathrm { L } ^ { - 1 }$ , total nitrogen (T-N) <15 mg L¹. The wastewater composition and organic loading rate on the WWTP changed slowly with respect to seasonal changes. The WWTP’s nutrient removal efficiency is high and large quantity of methane might be produced as an intermediate product during the biological nutrient removal process under anaerobic conditions.

A/A/O process is used in this plant for high nitrogen and phosphorus removal, achieved by a combination of anaerobic tanks, anoxic tanks and oxic tanks. The A/A/O process can be divided into three stages. Firstly, the wastewater and external returned activated sludge flow into the anaerobic tanks, with agitators to prevent sedimentation of material suspended in the wastewater. Secondly, the wastewater flows into the anoxic tanks with propellers to control water flow, where denitrification and nitrogen removal occur. Methane may be produced by methanogens due to the low oxygen concentration and low nitrite/nitrate concentration during the anaerobic and anoxic processes (Mohanakrishnan et al., 2008; Banihani et al., 2009; Jiang et al., 2010). Finally, the wastewater enters the oxic tanks with aeration equipment, where nitrification and phosphorus removal occur and ${ \tt B O D } _ { 5 }$ in the wastewater undergoes further decomposition by aerobic bacteria presented in this unit. The dissolved methane in the wastewater may be stripped out from the liquid phase by mechanical aeration during this stage (Crittenden, 2005). In order to enhance the phosphorus and SS removal efficiency, a set of high density settler tanks and high efficiency fiber filter beds are employed respectively following the final clarifier tanks. The treated water moves to the UV disinfection unit and is subsequently released into the receiving river. The schematic wastewater treatment process is depicted in Fig. 1 and the detailed sludge flow is omitted in the figure. The sludge treatment process is simple: the waste activated sludge discharged from final clarifier tanks first enters sludge concentration tanks for thickening, and then is transferred to sludge centrifugal dewatering machines. The dewatered sludge is transferred by screw conveyor to an open yard for drying and subsequently transported outward for landfill or other disposal. In the centrifugal dewatering machines and sludge drying ground, large quantity of methane may be produced from sludge in the absence of oxygen (Zhu et al., 2009).

2.2. Sampling and analysis

Methane flux was measured from each processing unit of the Jinan WWTP, i.e. influent pump station, aerated grit chambers, anaerobic tanks, anoxic tanks, oxic tanks, final clarifier tanks, high density settler tanks, high efficiency fiber filter bed, sludge concentration tanks, sludge screw conveyor and sludge drying ground. The sampling was conducted jointly by two groups of students on Monday, Wednesday and Friday of each week from March to June, at approximately the same time of each sampling day. The specific field sampling time was from 8 a.m. to 5 p.m., and the sampling order was in accordance with the direction of wastewater flow. The spatial variability of measured methane fluxes from each processing unit was examined previously. Results showed that the methane flux changed gradually with the dissolved oxygen along the wastewater flow in each wastewater processing unit. In the wastewater processing units with large spatial variability, gas samples were collected from multiple points to obtain the mean value of methane fluxes. The sampling point positions were determined by the dissolved oxygen change and the water surface area. For the export of the sludge screw conveyor, gas samples were collected three times at each sampling day to obtain the mean value of methane emissions. Similarly, the methane fluxes were measured from three evenly distributed sampling points in the rectangular sludge drying ground. The number of sampling points and the sampling frequency are shown in Table 1. The water surface can be divided into two categories: aerated liquid surfaces (in aerated grit chambers and oxic tanks) and nonaerated liquid surfaces (in the remaining wastewater treatment units). The export of sludge screw conveyor belongs to aerated surface because high gas flow was measured here. The sludge surface in sludge drying ground belongs to nonaerated surface. The following sampling methods were first described by Czepiel et al. (1993).

(Table 1 Wastewater treatment process configuration and the number of sampling points or sampling frequency.)

Process unit	Water surface area/m-²	Volume/m-³	Number of sampling points or sampling frequency	Main equipment
Influent pump station	45	450	1	Lift pump
Aerated grit chambers	350	1225	2 3	Jet aerator
Anaerobic tanks	6400	16,000		Propeller
Anoxic tanks	6300	44,600		Propeller
Oxic tanks	9400	65,800	4	Micropore aerator, propeller
Final clarifier tanks	12,800	38,400	3 2	Propeller
High density settler tanks	1020	5200		N/A
High efficiency fiber filter bed	840	8200	2	Fiber Filters
Sludge concentration tanks	2900	8700	2	Sludge-scraping device
Sludge screw conveyor	0.25 (export)	15	3 times daily	Screw conveyor
Sludge drying ground	250 (sludge area)	250	3	N/A

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2.2.1. Nonaerated surface sampling

An emission isolation flux hood technique was used to measure fluxes from nonaerated wastewater surfaces. The diameter and volume of the cylindrical stainless steel flux hood was 0.41 m and ${ 3 0 } \mathrm { L } ,$ respectively, with a sampling area of $0 . 1 3 \mathrm { m } ^ { 2 } .$ . The flux hood was floated on the liquid surface with a thermocouple probe mounted inside to measure the enclosed air temperature. Floats made from swim ring were fastened to the flux hood sides, and the flux hood was manually held in place during sampling to minimize hood movement due to surface turbulence. The specific sampling site was 1 m from the bank of each water processing unit or was set under the center of the raised walkway across the unit tank if available. A battery-operated blower circulated the air within the hood and through a closed loop of tube from which headspace air samples were withdrawn. Samples were collected in 60-mL polypropylene syringes at 2-min intervals for 10 min. The methane flux measurement from the nonaerated sludge surface of drying ground was also carried out by this emission isolation flux hood technique. The methane flux, F (mass area¹ time¹), from the liquid surface in the static flux hood was calculated by

$$ E = ( V / A ) \rho ( d c / d t )\tag{ð1Þ} $$

where V is the volume of the flux hood, A is the enclosed surface area, q is the density of the methane gas at the temperature recorded in the hood, and dc/dt is the linear increase of the methane concentration in the hood during the sampling period.

2.2.2. Aerated surface sampling

A bag technique was used to measure methane fluxes from aerated liquid surfaces. A 40 L polyethylene sample bag, fastened to the inside of a plastic support frame, was used to collect gas samples from the agitated surface of the aerated grit chambers and the oxic tanks. The open base of the frame allowed gas to be collected from $0 . 0 9 \mathrm { m } ^ { 2 }$ of the water surface. To collect samples, the bag was first collapsed and emptied of ambient air, and then the plastic support frame was immersed several inches into the water. Dissolved methane within the wastewater, transferred to the mechanically distributed air by diffusion, then filled the collection bag. Gas samples were withdrawn from the bag in 60-mL polypropylene syringes. When collecting sample from the export of sludge screw conveyor, the plastic support frame was fixed to the export directly, the sample bag was filled soon by the high gas flow. The gas flux, F (mass $\mathsf { a r e a } ^ { - 1 } \mathsf { t i m e } ^ { - 1 } )$ , was calculated by

$$ F = \rho c Q / A\tag{ð2Þ} $$

where $\rho$ is the density of the methane at the temperature recorded in the sample bag, c is the sample gas concentration, Q is the total diffuse air flow, and A is the total surface area.

2.2.3. Dissolved gas sampling

To collect samples of gas dissolved in wastewater, the headspace gas method was used as described by Kimochi et al. (1998). 30 mL each of water and argon gas were sealed into a 50-mL syringe, and 1 mL of 20 mM mercury (II) chloride was added to prevent biological methane oxidation. After vigorous shaking, the syringe was left at room temperature for 1 h without moving it. The resulting gas phase in the syringe was collected as a gas sample. Then the dissolved $\mathrm { C H } _ { 4 }$ concentration could be calculated based on Henry’s Law using equilibrated headspace methane concentration.

2.2.4. Analytical methods of gas and water samples

To determine dominant factors affecting $\mathrm { C H } _ { 4 }$ production and emission, water samples were collected at the sampling site. Dissolved oxygen concentration $( { \mathsf { D O } } ) ,$ pH and water temperature were measured in situ. COD, $\mathrm { T } - \mathrm { N } , \mathrm { N H } _ { 4 } - \mathrm { N } , \mathrm { N O } _ { 3 } ^ { - } - \mathrm { N }$ and $\mathsf { N O } _ { 2 } ^ { - } - \mathsf { N }$ were measured in accordance with the standard methods for examination of water and wastewater (APHA, 2005). A gas chromatograph equipped with a flame ionization detector (FID–GC) and stainless steel packed columns of Porapak Q was used to measure the $\mathrm { C H } _ { 4 }$ concentration of the gas samples (Crill et al., 1991). Relationships between $\mathrm { C H } _ { 4 }$ emissions and water quality parameters were examined using linear regression techniques (Montgomery et al., 2007).

Results and discussion

3.1. Methane fluxes from unit operation

Methane can be produced during anaerobic and anoxic processes in wastewater treatment plants due to low dissolved oxygen concentration. With a low water solubility (Speight, 2004), CH emission from wastewater is relatively fast. The field measurements found that $\mathrm { C H } _ { 4 }$ emission occurred in every processing unit. The measured daily methane flux range from each unit during the experimental period is presented in Table 2. Since a similar flux variation trend was obtained among different processing units at different sampling days, measurement data on May 7th, which is shown in Fig. 2, was used to facilitate the following analysis.

(Methane flux range of each processing unit.)

Processing unit	Flux range $( \mathbf { g } \mathbf { m } ^ { - 2 } \mathbf { d a y } ^ { - 1 } )$
Influent pump station	4.34-6.82
Aerated grit chamber	6.19-18.87
Anaerobic tanks	2.13-3.44
Anoxic tanks	0.11-0.18
Oxic tanks	1.12-2.75
Final clarifier tanks	0.06-0.11
High density settler tanks	1.78-3.02
High efficiency fiber filter bed	0.06-0.12
Sludge concentration tanks	1.07-1.89
Sludge screw conveyor	631-978
Sludge drying ground	0.11-0.23

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3.1.1. Sludge screw conveyor

The maximum of methane flux occurred at the export of sludge screw conveyor, which was $8 2 3 \mathrm { g } \mathrm { m } ^ { - 2 } \mathrm { d } \mathsf { a } \mathrm { y } ^ { - 1 }$ . This result can be explained by the mechanism of $\mathrm { C H } _ { 4 }$ production. The screw conveyor connects sludge centrifugal dewatering machine and sludge drying ground. When the centrifuge is working, large amounts of $\mathrm { C H } _ { 4 }$ is produced due to high temperature and anoxic conditions within the machine and then the methane flows into the screw conveyor and emits to atmosphere at the export to the sludge drying ground (Crittenden, 2005). Because of the export area is very small $( 0 . 2 5 \mathrm { m } ^ { 2 } )$ and air flow is large $( 5 \mathrm { L } \operatorname* { m i n } ^ { - 1 } )$ , the methane flux reached the maximum, which is far greater than that from other sampling units.

3.1.2. Pump station and aerated grit chambers

The methane fluxes from the aerated grit chambers and influent pump station were the second $( 1 5 . 8 0 \mathrm { g } \mathrm { m } ^ { - 2 } \mathrm { d } \mathbf { a } \mathbf { y } ^ { - 1 } )$ and third highest $( 5 . 8 4 \mathrm { g } \mathrm { m } ^ { - 2 } \mathrm { d } \mathbf { a } \mathbf { y } ^ { - 1 } ) ,$ , respectively. It has been reported that a lot of CH was produced during the anaerobic biological nutrient decomposition processes in the sewer systems (Guisasola et al., 2008). The generated $\mathrm { C H } _ { 4 }$ could subsequently be released from liquid phase in pump station by water turbulence and in aerated grit chambers by intensive mechanical aeration.

3.1.3. Anaerobic/anoxic/oxic tanks

The methane fluxes from the anaerobic/anoxic/oxic process were 2.93, 0.15 and $1 . 3 3 \mathrm { g } \mathrm { m } ^ { - 2 } \mathrm { d } \mathbf { a } \mathbf { y } ^ { - 1 }$ , respectively. The relatively higher fluxes from anaerobic tanks and oxic tanks were caused by the methane formation due to low dissolved oxygen concentration $( < 0 . 2 \mathrm { m g } \mathrm { L } ^ { - 1 } )$ during anaerobic process and the intensive mechanical aeration stripping of dissolved methane from water during oxic process, respectively. The methane flux from anoxic tanks was relative low for lack of the above two advantages.

3.1.4. Final clarifier, high density settler and high efficiency fiber filter

The methane fluxes from final clarifier tanks, high density settler tanks and high efficiency fiber filter bed according to the water flow were 0.08, 2.58 and $0 . 1 0 \mathrm { g } \mathrm { m } ^ { - 2 } \mathrm { d } \mathbf { a } \mathbf { y } ^ { - 1 }$ , respectively. After the adequate mechanical aeration stripping of dissolved methane in oxic tanks, the methane flux from the following final clarifier tanks was lowest due to the smooth water flow. Surprisingly, the $\mathrm { C H } _ { 4 }$ flux from high density settler tanks was much higher than that from final clarifier tanks, maybe because of the water turbulence in this unit. Further detailed research will be carried out to explain this phenomenon. Though water agitation also existed in high efficiency fiber filter bed, the methane flux from this unit was very low due to the high methane emissions from the previous high density settler tanks.

3.1.5. Sludge concentration tanks and sludge drying ground

The methane flux of 1.51 g m² day¹ from the sludge concentration tanks came from both the dissolved methane in the waste activated sludge and regenerated methane under anaerobic conditions in this unit. The methane flux from sludge drying ground was very low at $0 . 1 8 \mathrm { g } \mathrm { m } ^ { - 2 } \mathrm { d } \mathbf { a } \mathbf { y } ^ { - 1 }$ , because the dewatered sludge was simply air dried for several hours and then trucked away, and only a fraction of methane dissolved in sludge was emitted to the atmosphere.

From the above analysis, it could be found that the methane fluxes from some processing units were relatively low and accounted for less than 10% of that from aerated grit chambers, but they could not be neglected because the total annual fluxes calculation of the WWTP needed to multiply the large surface area of these processing units, as shown in Table 1.

3.2. Dissolved methane concentration

Dissolved $\mathrm { C H } _ { 4 }$ concentration in each wastewater treatment unit was measured at roughly 1-week interval at the same sampling point as the gas flux measurement during the experimental period. The dissolved gas analyses from different sampling days showed a similar flux variation trend among the wastewater processing units, which was quite different from the methane emission fluxes variation trend as shown above. A typical dissolved methane concentration comparison measured on May 7th is presented in Fig. 3. The concentration of dissolved $\mathrm { C H } _ { 4 }$ in the influent pump station and aerated grit chambers were still much higher than that in other wastewater treatment units, which was consistent with and thus led to the higher $\mathrm { C H } _ { 4 }$ fluxes from these two units. The dissolved $\mathrm { C H } _ { 4 }$ concentrations in anaerobic tanks and anoxic tanks were relatively high due to the methane formation under conditions of lower dissolved oxygen in these two units, which could also explain the higher dissolved $\mathrm { C H } _ { 4 }$ concentrations in sludge concentration tanks. Although the $\mathrm { C H } _ { 4 }$ flux from oxic tanks was relatively high, the dissolved $\mathrm { C H } _ { 4 }$ concentration in this unit was lowest, because the sufficient aeration had stripped most of the $\mathrm { C H } _ { 4 }$ dissolved in wastewater. From the dissolved methane concentration difference between the anoxic tanks and oxic tanks, the methane emitted from the anoxic tanks could be roughly calculated, which was $1 0 7 1 \mathrm { g } \mathrm { d a y } ^ { - 1 }$ . The methane released from the same tanks calculated from the measured flux and water surface area was $9 4 5 \mathrm { g } \mathrm { d } \mathsf { a y } ^ { - 1 }$ . The difference between the two methods

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