Methane is a powerful greenhouse gas and a source of energy. Recovering this gas means lower greenhouse gas emission and potential reduction of energetic costs. The lack of full-scale results, the use of different methodologies to detect dissolved methane (d-CH4) and the fact that no process to remove d-CH4 from anaerobic effluents is energetically or economically viable at full-scale urged a different approach to the problem. To avoid methodological interference and facilitate comparison of results the Standard Test Method number D8028-17 published by ASTM International can be used to determine d-CH4. The use of real anaerobic reactor effluent also helps results to be compared. In this study, 80 samples from a full-scale anaerobic reactor showed an average concentration of dissolved methane of 14.9 mg·L−1, meaning an emission of 229 kg of CO2 eq·h−1 and an average of 113.5 kW wasted. Using spray nozzles, an alternative to the methods being researched, the average methane recovery was 11.5 mg·L−1 of CH4, an efficiency of 81.6%, meaning 177 kg of CO2 eq·h−1 emissions avoided and 87.9 kW of recoverable energy.

  • Use of standard method to detect dissolved methane.

  • Use of real Upflow Anaerobic Sludge Blanket (UASB) reactor effluent.

  • Simple alternative method to recover dissolved methane.

  • High efficiency.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Since the beginning of the industrial revolution, human activities have changed the natural gas balance in the atmosphere and caused an increase of approximately 150% in the atmospheric concentration of methane (CH4) (Gupta & Goel 2019).

This increase in the amount of atmospheric methane caused by human activities has unbalanced the natural equilibrium affecting the climate worldwide because methane traps heat 28 times more effectively than carbon dioxide over a 100-year timescale and therefore is held responsible for about 23% of the climate change in the 20th century (NASA 2020).

Among anthropogenic sources, wastewater treatment and their related activities account for around 9% of global environmental methane emissions (Lee et al. 2021).

Since municipal wastewater is the most common type of wastewater sourcing from our daily life and biological treatments are worldwide used for treatment of organic matter present in wastewater, biological treatment of municipal wastewater is one of the main sources of methane emission in wastewater treatment (Kong et al. 2021).

Biological wastewater treatment can take place either with oxygen (aerobic treatment) or in the absence of oxygen (anaerobic treatment). In aerobic treatment, microorganisms use dissolved oxygen to convert organic components into carbon dioxide and biomass. Conversely, anaerobic treatment processes produce biogas, a small amount of excess sludge and have low energy requirements. Biogas is a mixture of its principal components, methane and carbon dioxide, with traces of hydrogen sulphide, nitrogen, and oxygen (Jung & Pauly 2011).

Since biogas is produced in anaerobic processes and methane is the main element in biogas, main emission sources should be expected from this kind of treatment.

In anaerobic treatment, methane is present in the effluent as dissolved methane (d-CH4) and is subsequently released without control to the environment (Crone et al. 2016)

Many authors have reported on anaerobic effluents that are ‘supersaturated’ with d-CH4 (Souza et al. 2011).

Souza et al. claimed that between 36 and 40% of all methane produced in an Upflow Anaerobic Sludge Blanket (UASB) reactor stayed dissolved in the effluent indicating that d-CH4 concentrations can be higher than those predicted based on Henry's law (Souza et al. 2012).

However, there are still few papers with empirical d-CH4 data from real UASB reactors. Most data are calculated and/or obtained from laboratory-size reactors that sometimes use synthetic wastewater.

Besides the lack of data, methods used to determine the amount of d-CH4 in the effluent were not standardized. As an example, Cabral et al. (2020) adapted a method to determine d-CH4 based on the methods described by Alberto et al. (2000) and Hartley & Lant (2006), while Gupta & Goel (2019) developed two methods to estimate the concentration of d-CH4 gas in the liquid phase. The first method was based on gas and liquid phase equilibrium of gases (Henry's law), and the second was based on the effect of salt on gas solubility in liquid and was based on the salting-out method of Gal'chenko et al. (2004) with some modifications (Gupta & Goel 2019; Cabral et al. 2020).

Despite being a powerful greenhouse gas, methane is also a renewable source of energy and recovering d-CH4 is an imperative as volatilization to the atmosphere represents both a substantial increase in greenhouse gas emissions and loss of energy (Marín et al. 2019; Pfluger et al. 2020).

Among the methods described in the literature to recover d-CH4 there is the use of membranes, but a recent review on the advances in anaerobic membrane bioreactor technology for municipal wastewater treatment has shown that this technology still needs to cope with membrane fouling, process configuration, process temperature, sewage sulphate concentration and sewage low organics concentration (Vinardell et al. 2020).

Even addressing recent efforts and future prospect of membrane contactors, the literature shows that fouling and wetting are still inherent problems of this technology and although hydrophobic coatings to avoid wetting and the use of patterns on membrane surfaces, the development of anti-fouling membranes, and the use of solvent-based membrane contactor to avoid fouling are being tested; membrane separation for d-CH4 recovery from anaerobic effluents still needs to develop its economic viability and process safety (Song et al. 2018; Lee et al. 2021).

Another possibility to avoid CH4 emissions is to oxidize anaerobic reactor effluents in post treatments using the Integrated Fixed Film Activated Sludge (IFAS) system, the management of microbial resource in an oxidizing reactor or the compost biofilter (Huete et al. 2018; Gupta & Goel 2019; Allegue et al. 2020).

Biogenic capture with the downflow hanging sponge (DHS) reactor is also an alternative to avoid d-CH4 emissions (Nurmiyanto & Ohashi 2019).

Aerated-based processes such as microaeration, spray aeration, packed columns, tray aerators, diffused aeration and membrane contactors are also methods to solve the d-CH4 problem (Heile et al. 2017; Cabral et al. 2020).

The critics to these technologies state that the use of biological oxidation leads to energy loss and aerated-based processes still have flooding, foaming and channelling problems with which to cope (Cao et al. 2020).

Analysing the alternatives, no biological and/or mechanical process to remove d-CH4 from anaerobic effluents has demonstrated energetic or economic viability at full scale and none is ready for mainstream wastewater treatment. Economical, technical, social and regulatory barriers must be faced before the widespread implementation to the use of biogas from anaerobic digestion becomes a reality (Pfluger et al. 2020).

Glória et al. reviewing the literature also found no feasible alternative to collect d-CH4 from UASB effluents and proposed a solution using stripping and a vacuum chamber. The vacuum chamber worked properly but the stripping method efficiency achieved only 30%. They used the turbulence caused by liquid falling and the result was directly affected by fall height (Glória et al. 2016).

To avoid this dependence on fall height, the use of spray nozzles for stripping could be a solution since they are suited for gas–liquid separation (Lobato et al. 2013; Rosa et al. 2016).

Spray nozzles are widely used in agriculture and their technology is well known.

As the liquid stream passes through a spray nozzle, droplets are created, increasing the surface area between the liquid and the gas phases. According to the two-phase theory, the mass transfer from the liquid phase to the gas phase occurs in the phase boundary and thus the higher the interface area the easier it is for the gas to leave the liquid phase (Glória et al. 2016).

Considering the shortcomings of the methods proposed to attempt d-CH4 removal from anaerobic effluents, the present study aims to determine the amount of methane dissolved in a real UASB effluent (from the wastewater treatment plant in Pirassununga) using a standardized methodology and propose a different approach to recover d-CH4, by a mist spray nozzle to increase the amount of energy recovered from an anaerobic based wastewater treatment while avoiding methane emissions.

Waste water treatment plant (WWTP) characteristics

Pirassununga's WWTP was designed to serve a city of 107.000 inhabitants (1,818 m3·h−1). The first and second stages of the WWTP are currently operating receiving the effluents of approximately 70,000 inhabitants with an average inflow of 549 m3·h−1.

After the preliminary treatment in which there is a bar screening, a 6 mm screening, a grit removal and a grease and oil flotation tank, the wastewater flow is split and sent to two UASB reactors divided into four equal cells each. The cell dimensions are 13.0 m × 12.75 m × 5.0 m and the hydraulic detention time is 7.8 hours.

The UASB effluent is then sent to two tricking filters, followed by two secondary clarifiers and after the chlorination tank is disposed into the Laranja Azeda river.

Raw material

Effluent from the UASB treating sanitary sewage at a full-scale WWTP in Pirassununga, Brazil, was used in this study.

The characteristics of the UASB influent and effluent and the removal efficiency of Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD) and total solids during this study are given in Table 1.

Table 1

Average UASB influent and effluent characteristics of Pirassununga's WWTP during the whole study

COD (mg·L−1)
COD removed (%)BOD (mg·L−1)
BOD removed (%)T. S. (mg·L−1)
T. S. removed (%)
InflowOutflowInflowOutflowInflowOutflow
Number of samples 11 44  11 10  11 44  
Average value 733 188.2 74.33 469.8 100.2 78.7 639.9 272.8 57.4 
Standard deviation 51.1 37.2  57.2 19.0  94.6 67.2  
COD (mg·L−1)
COD removed (%)BOD (mg·L−1)
BOD removed (%)T. S. (mg·L−1)
T. S. removed (%)
InflowOutflowInflowOutflowInflowOutflow
Number of samples 11 44  11 10  11 44  
Average value 733 188.2 74.33 469.8 100.2 78.7 639.9 272.8 57.4 
Standard deviation 51.1 37.2  57.2 19.0  94.6 67.2  

BOD, Biochemical Oxygen Demand; COD, Chemical Oxygen Demand; T. S., Total Solids.

As effluent characteristics vary during the day and day by day, data in Table 2 show the average UASB effluent characteristics during sampling.

Table 2

Average UASB effluent characteristics of Pirassununga's WWTP during sampling

Flow (m3·h−1)BOD (mg·L−1)COD (mg·L−1)Total solids (mg·L−1)pHTemperature (K)
549 103 158 361 7.3 303 
Flow (m3·h−1)BOD (mg·L−1)COD (mg·L−1)Total solids (mg·L−1)pHTemperature (K)
549 103 158 361 7.3 303 

BOD, Biochemical Oxygen Demand; COD, Chemical Oxygen Demand.

Sampling procedure

To detect the amount of d-CH4, 40 mL vials were submerged 10 cm under the surface of the UASB reactors, filled up to the top with the effluent, closed with a cap with PTFE/Silicone septa under the waterline so that no gas was inside the vial as determined in method number D8028-17 by ASTM International. The samples were kept refrigerated in a Consul Facilite Frost Free refrigerator at 5 °C until analysis (sample vials) (ASTM 2017).

The same sampling procedure was applied to detect d-CH4 after the effluent passed through the spray nozzle.

Recovery of d-CH4

The experimental system to recover d-CH4 is shown in Figure 1.

Figure 1

Experimental system used for d-CH4 recovery using spray nozzle in a vacuum chamber.

Figure 1

Experimental system used for d-CH4 recovery using spray nozzle in a vacuum chamber.

Close modal

The liquid effluent of the UASB reactor was pumped through a Claber 91258, 360° micromist sprayer in a vacuum chamber. The vacuum chamber was a hollow acrylic reel closed at both ends by acrylic caps and the −0.5 bar pressure inside, measured by the Rücken vacuum gauge, was created by a Prismatec vacuum pump model 131 with ¼ HP. Liquid samples were then collected from the UASB effluent before and after passing through the nozzle for each round.

This procedure was repeated three times on 3 different days and 41 samples were obtained after the effluent went through the nozzle and nine samples (three each round) were taken before the nozzle. The temperature of the effluent was measured using an Alla France Total Immersion thermometer with a range from 263.2 K to 423.2 K placed 10 cm below the waterline in the UASB just before pumping.

Analytical methods

The method used in this research was based on the Standard Test Method for Measurement of Dissolved Gases Methane, Ethane, Ethylene, and Propane by Static Headspace Sampling and Flame Ionization Detection (GC/FID published as number D8028-17 by ASTM) (ASTM 2017).

To create the headspace, 40-mL vials were injected with pure nitrogen gas for 2 minutes, to assure a known atmosphere inside the vial (headspace vials). After this procedure, 20 mL of the liquid from the sample vials were collected with a syringe and injected in the headspace vials creating a 20-mL headspace.

The vials with 20-mL headspace were put in a Marconi shaker at 25 °C and 200 rpm for 1 hour to achieve equilibrium between the gas and the liquid phase.

Analyses of the headspace gas phase were carried out using the GC-2014 Shimadzu gas chromatograph equipped with a with flame ionization detector (FID), HP-PLOT/Q column (30 m×0.53 mm×40 μm), and using helium as gas carrier. Sample volumes of 0.1 mL at room temperature and pressure were injected in the chromatograph.

Standard gas was supplied by São Carlos Gases certified by Industria Brasileira de Gases with 49.96% methane and 50.04% CO2.

The number of gas moles in the standard gas can be calculated using the ideal gas Equation (1):
formula
(1)
where P is the atmospheric pressure (P=0.925 atm in Pirassununga), V the volume of the gas injected (0.0001 L), R the universal gas constant (0.082 atm·L·mol−1·K−1) and T is the ambient temperature (K).

The moles of CH4 in the standard are calculated as ng (moles of the standard gas injected in the chromatograph) multiplied by the percentage of CH4 in the standard gas (49.96%).

Knowing the number of moles of CH4 in the standard and the area created by the standard and the sample in the chromatogram, the number of methane moles in the sample can be calculated using Equations (2) and (3):
formula
(2)
formula
(3)
where RF is the response factor, A standard is the area created by the CH4 in the standard in the chromatogram, n standard is the number of moles of CH4 in the standard gas, A sample is the area created by the CH4 in the sample in the chromatogram, and n sample is the number of moles of CH4 in the sample.

Energy

The energy wasted and able to be recovered was calculated using Equation (4):
formula
(4)
where E is energy in kWh, F is the flow rate in m3·h−1, CCH4 is the CH4 concentration in kg·m−3 and ICV is the inferior calorific value of CH4 in kWh·kg−1.

Results for d-CH4

There are still few data from real full-scale UASB reactors. Centeno-Moura et al., in 2020, collecting data from the literature found only three references from full-scale UASB reactors as shown in Table 3 (Centeno-Mora et al. 2020).

Table 3

D-CH4 concentrations reported in the literature for UASB reactor–treated sewage

ReferenceScale of the UASB reactorT (°C)D-CH4 (mg·L−1)
Souza et al. (2011) a Pilot 24–25 19.6–22.0 
Demob 19.2 
Full 20.0 
Nelting et al. (2015)  Full 20–25 6.0–10.0 
Matsuura et al. (2015)  Pilot 10–28 18.4 
Cookney et al. (2016) a Bench 25 25.4 
Glória et al. (2016) a Demo 22 15.0 
Santo (2017)  Demo 25 20.2 
Huete et al. (2018)  Full 42 12.2 
Marinho (2018)  Pilot 21.9 23.9 
Demo 15.9–21.0 
This study Full 25 14.89 
ReferenceScale of the UASB reactorT (°C)D-CH4 (mg·L−1)
Souza et al. (2011) a Pilot 24–25 19.6–22.0 
Demob 19.2 
Full 20.0 
Nelting et al. (2015)  Full 20–25 6.0–10.0 
Matsuura et al. (2015)  Pilot 10–28 18.4 
Cookney et al. (2016) a Bench 25 25.4 
Glória et al. (2016) a Demo 22 15.0 
Santo (2017)  Demo 25 20.2 
Huete et al. (2018)  Full 42 12.2 
Marinho (2018)  Pilot 21.9 23.9 
Demo 15.9–21.0 
This study Full 25 14.89 

aAdapted from Crone et al. (2016).

bDemonstration scale: 14-m3 reactor operating for an extended time period.

For Pirassununga's WWTP UASB effluent, after 10 rounds of sampling, collected on 10 different days, the average d-CH4 of 80 samples resulted in 14.9 mg·L−1, using the methodology described. The box plot of the results is presented in Figure 2 in which the dispersion of the results for each set of sampling, the median, the mean and the outliers are shown side by side.

Figure 2

Box plot of d-CH4 in Pirassununga's WWTP UASB effluent for each round (each collecting day in which eight samples were collected).

Figure 2

Box plot of d-CH4 in Pirassununga's WWTP UASB effluent for each round (each collecting day in which eight samples were collected).

Close modal

Comparing with the data in Table 3 for full-scale UASBs we determined that the data in this research were within the interval from 6 to 20 mg·L−1 obtained in the other studies.

In 2021, Gao et al. and Lee et al. considered a range from 10 to 25 mg·L−1 for d-CH4 in their studies (Gao et al. 2021; Lee et al. 2021).

This means the results obtained in this study are in agreement with the ones found in the literature.

Using the data from Sander (2015) at the Max Plank Institute for Chemistry site and correcting the data for the temperatures at the time each sample was collected, the average saturation value for d-CH4 would be 23.4 mg·L−1. This shows there was no supersaturation as this value corresponds to 64% of the saturation value (Sander 2015).

The flow rate at the time each round was performed, in m3·h−1 is presented in Table 4.

Table 4

Flow rate of the WWTP at the time of the sampling in m3·h−1

Round12345678910
Flow (m3·h−1697 559 711 520 639 326 419 511 640 469 
Average flow 549 m3·h−1         
Round12345678910
Flow (m3·h−1697 559 711 520 639 326 419 511 640 469 
Average flow 549 m3·h−1         

The average flow rate of the UASB reactors during the sampling was 549 m3·h−1 this means an average rate of d-CH4 of 8.2 kg·h−1.

As the Inferior Calorific Value (ICV) of methane is 13.9 kWh·kg−1, this means that 113.5 kW is lost in the effluent of the WWTP. The average electric consumption of the whole WWTP during the months of the sampling was 45,6 kW.

Considering that the best electric power generator can only convert 40% of the energy supplied into electric power, d-CH4 alone was almost enough to supply all the energy required by the WWTP.

Another issue to consider is that methane is not pure and, depending on biogas composition, further treatment should be applied before use.

On the environmental side, according to United States Environmental Protection Agency (USEPA), a typical passenger vehicle emits about 4.6 metric tons of carbon dioxide per year, which means 0.5 kg·h−1 (EPA 2018).

As the Global Warming Potential (GWP) of methane is 28. This means that the d-CH4 in Pirassununga's WWTP is equivalent to 431 typical passenger vehicles emissions per hour.

These results lead to the next phase of this paper, that is, how much of d-CH4 could be recovered using spray nozzles.

CH4 recovered

Average results after 3 rounds (41 collected samples) are shown in Table 5 and Figure 3 shows D-CH4 distribution before and after the nozzle for each round.

Table 5

Efficiency of the mist spray nozzle

Average – dissolved methane (mg·L−1)
Number of samples Before the nozzleAfter the nozzleAverage removed by nozzleEfficiency (%)
Overall average 41 14.2 2.6 11.5 81.6 
Average – dissolved methane (mg·L−1)
Number of samples Before the nozzleAfter the nozzleAverage removed by nozzleEfficiency (%)
Overall average 41 14.2 2.6 11.5 81.6 
Figure 3

D-CH4 before and after the nozzle for each round (each collecting day in which eight samples were collected).

Figure 3

D-CH4 before and after the nozzle for each round (each collecting day in which eight samples were collected).

Close modal

The samples were collected at a pressure of 0.925 atm and an average temperature of 303 K, and analysed at a pressure of 0,925 atm and an average temperature of 298 K. Under these conditions, the average amount of d-CH4 before the mist nozzle was 14.2 mg·L−1 and, after the nozzle 2.6 mg·L−1, this means an average rate of d-CH4 recovered of 6.3 kg·h−1 and a removal efficiency of 81.6%.

This result also shows that the mist nozzle could prevent the emission of 177 kg of CO2 eq·h−1 and recover 87.9 kW.

The effect of using the mist nozzle would be the same as reducing 334 typical passenger vehicles from the car fleet.

Gao et al., comparing the efficiencies of the methods to recover d-CH4, found the results in Table 6 here adapted to include the nozzle results.

Table 6

Dissolved CH4 removal by nozzles and other technologies

TechnologyDriving forceFeed sourceRecovery (%)Reference
Spray aerator Air Ground water 45–55 Heile et al. (2017)  
Packed column Air Ground water 99.6 Heile et al. (2017)  
PDMSa HFMCb Nitrogen Synthetic CH4 solution 40.8–92.6 Cookney et al. (2016)  
PPc HFMC Nitrogen Synthetic CH4 solution 63.3–98.9 Cookney et al. (2016)  
PDMS HFMC Vacuum EGSB effluent 72.4 Henares et al. (2017)  
HFMC Vacuum UASB effluent 71–97 Bandara et al. (2011)  
HFMC Vacuum AnMBRd effluent 5–65 Wongchitphimon et al. (2017)  
FOe Osmotic pressure UASB effluent 94% Gao et al. (2021)  
Spray nozzle Vacuum UASB effluent 81.6 This study 
TechnologyDriving forceFeed sourceRecovery (%)Reference
Spray aerator Air Ground water 45–55 Heile et al. (2017)  
Packed column Air Ground water 99.6 Heile et al. (2017)  
PDMSa HFMCb Nitrogen Synthetic CH4 solution 40.8–92.6 Cookney et al. (2016)  
PPc HFMC Nitrogen Synthetic CH4 solution 63.3–98.9 Cookney et al. (2016)  
PDMS HFMC Vacuum EGSB effluent 72.4 Henares et al. (2017)  
HFMC Vacuum UASB effluent 71–97 Bandara et al. (2011)  
HFMC Vacuum AnMBRd effluent 5–65 Wongchitphimon et al. (2017)  
FOe Osmotic pressure UASB effluent 94% Gao et al. (2021)  
Spray nozzle Vacuum UASB effluent 81.6 This study 

aPDMS, Polydimethylsiloxane.

bHFMC, Hollow Fiber Membrane Contactor.

cPP, Polypropylene.

dAnMBR, Anaerobic Membrane Bioreactor.

eFO, Forward Osmosis.

The results for the spray nozzle are above or within the range of the results obtained by most technologies and are lower than the packed column and the forward osmosis (FO).

The packed column technology increases the effective mass transfer area when the liquid passes through the packing materials that facilitate the mass transfer. Spray nozzles in the top of the column ensure a uniform distribution over the cross-section.

The problems associated with packed column technology are flooding, channelling, and wall flow, limiting the operation ranges of liquid and gas flows (Lee et al. 2021).

As to the FO technology, the FO membrane has a pore size range between 0.25 and 0.37 nm and is 2–3 orders of magnitude smaller than microfiltration and ultrafiltration membranes. As the dynamic diameter of CH4 is 0.38 nm, the size of the pore avoids the passage of CH4 through the membrane. Gao et al., however, observed that membrane fouling was the main drawback. (Gao et al. 2021).

So, the efficiency of spray nozzles compared to other systems is among the best and it does not have flooding, channelling, wall flow and fouling problems associated with the technologies been researched.

The amount of d-CH4 in a UASB reactor effluent is not negligible. Supersaturated or not, this gas is a source of energy and has high GWP. This study offers more real data to support further research and shows the importance of taking d-CH4 into account when designing an anaerobic-based treatment plant so that it could be more energetically efficient, reducing the cost of its operation and the environmental footprint of the whole process. The average value of d-CH4 in this study is within the range found in the literature, but as this is a wide range, further studies with real wastewater using a standardized methodology could narrow this range so that results could reflect the real amount of CH4 in the effluent of anaerobic wastewater treatment. The importance of this value relies on the need for more efficient energy plants and more accurate greenhouse gas emission data.

The use of hydraulic spray nozzles, a known technology, was able to recover a significant amount of methane that could be used for energy generation and reduce the emission of this highly hazardous gas. The efficiency of this recovery system option is within the range of other alternative methods. As no method is still viable, every option to recover d-CH4. must be considered so that the best option for each individual situation can be chosen.

The authors declare no conflict of interest.

The authors gratefully acknowledge the Brazilian National Council for Scientific and Technological Development (CNPq) for the Research Productivity Grant PQ: 309442/2018-4 for the third author and the support of SAEP – Pirassununga's Water and Wastewater Service and Environmental Biotechnology Laboratory, Faculty of Animal Science and Food Engineering (FZEA), University of São Paulo (USP), Pirassununga/SP, Brazil.

All relevant data are included in the paper or its Supplementary Information.

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