Analysis of methane emissions from digested sludge

Wastewater and sewage sludge treatment systems of the future have to fulfill the requirements of health protection, water pollution control and resource conservation. Thereby, resource conservation includes questions of resource efficiency, meaning minimization of resource consumption (for example, energy and operating materials as well as minimization of environmental impact), resource recovery, meaning utilization of resources contained in wastewater or sewage sludge, in particular water, nutrients and energy (Schaum 2016).

From the energetic point of view, anaerobic sewage sludge treatment (digestion) becomes more important, as electricity and heat can be generated by using biogas. The used sewage sludge is then assessed as climate-neutral in terms of ‘renewable raw materials’. On the other hand, methane emissions from sewage sludge treatment get little attention and so far there are only few – highly variable – measurement data published (Foley et al. 2015); see Table 1.

Basically, methane emissions on wastewater treatment plants (WWTPs) without digestion will be identified mainly in the influent area, that is in areas with anaerobic processes (Foley et al. 2015). On WWTPs with digestion, the findings will shift towards the digestion plant, especially its upstream and downstream structures/facilities, see Table 1. Becker et al. (2012) identified sludge digestion as a main emitter of methane, whereby approximately 50% of the overall methane emissions were measured at the heads of the digester.

Table 1 shows that there are only few published data regarding methane emissions from wastewater treatment, and those that exist show a large range of data from 0.09 to 307 CH₄/(PE·a) (all without digestion). For the entire field of digestion (digestion, thickener/stack container, combined heat and power plant (CHP)), a range between 65 and 281 g CH₄/(PE·a) can currently be identified; mean value: 162 ± 87 g CH₄/(PE·a). Below, the main emission sources are listed (Schaum et al. 2015):

• Dissolved methane in digested sludge, discharged together with the digested sludge from the digestion unit.

• Residual gas potential that originates in the post-thickener and emits into the atmosphere.

• Methane slip, emitting from biogas utilization in the CHP.

The focus of this research will be on the measurement and assessment of dissolved methane emission from digestion:

• Development and validation of an analysis method for dissolved methane.

• Analysis of dissolved methane depending on different operation conditions (for example, meso-/thermophilic, gas injection, digester feeding).

Another factor affecting gas solubility is the salt concentration. In a saturated sodium chloride solution, only a small percentage of methane dissolves compared to pure water. This effect can be used in the measurement of methane concentrations in digested sludge. The saturation concentration of sodium chloride at 20 °C is approximately 360 g/L, whereby the influence of temperature on the solubility is relatively low (Lautenschläger 2001). Including the Sechenov constant, which describes the solubility, of 0.15 m³/kmol, assessed according to Hermann et al. (1995) and Schumpe (1993), only 10% of the methane is dissolved in comparison to pure water. Although the Sechenov constant only moderately depends on temperature, Hermann et al. (1995) state a validity range of 298.15 ± 5 K.

In Figure 1, the theoretical solubility of methane as a function of pressure, temperature, salt concentration and methane concentration in the gas phase is illustrated. At an assumed pressure of 2,000 hPa (digester height of 10 m at an air pressure of 1,000 hPa), the concentration of dissolved methane will be approximately 20–27 mg/L CH₄ at 37 °C and 28–35 mg/L CH₄ at 20 °C, depending on the salt concentration and the methane concentration in the gas phase. It becomes clear, as well, that by adding sodium chloride the solubility of methane decreases.

It is essential to consider that the theoretical approaches are based on the presented boundary conditions. Potential impacts of sludge behavior during digestion (total solids concentration, methane production, oversaturation, etc.) are not accounted for.

Method validation was carried out via laboratory tests. A fully mixed reactor (reactor volume 12 L) was used in an experiment with pure water, where pressure (1,000–1,540 hPa), temperature (0.6–37 °C), salt concentration (sodium chloride (NaCl), 0–1 mol/L NaCl) and gas quality (99.5 vol-% CH₄ and a gas mixture of 60 vol-% CH₄ and 40 vol-% CO₂, both from Linde) were varied. The measured dissolved methane concentrations were measured and compared to the theoretically expected concentrations. All assessments are based on multiple measurements (at least three-fold determination).

The investigation on the concentration of dissolved methane in digested sludge was carried out at seven industrial-scale municipal WWTPs (hydraulic retention time (HRT) always >20 d), see Table 2.

In addition, at WWTP #3 and WWTP #7, the influence of mixing during digestion was investigated by means of gas injection (via lances).

A semi-industrial test plant with a digested sludge volume of 1 m³ was used to investigate the influence of digested sludge feeding on the solubility of methane (HRT 20 d, mixing via stirrer). Thereby, the effects of continuous feeding of raw sludge were compared to those of discontinuous feeding. In addition, the dissolved chemical oxygen demand (COD) was analyzed (cuvette test; Hach).

Characteristic sewage sludge parameters were analyzed in the collected sewage sludge samples: total solids (TS) (DIN 2001), total volatile solids (TVS) (DIN 2012) and the chemical parameters pH-value (Knick Elektronische Messgeräte) and electrical conductivity (WTW).

The challenge in measuring the concentration of dissolved methane in digested sludge is to avoid gas exchange with the surrounding air: as soon as the digested sludge leaves the digester, methane stripping into the atmosphere occurs. Furthermore, one has to consider the content of TS in the digested sludge when sampling, due to its impact on the sampling method to be selected (diameter of tubes, etc.).

Several methods have been developed for water/wastewater, e.g. Daelman et al. (2012), Foley & Lant (2009), Alberto et al. (2000). However, there is no unification or even standardization. For the field of digestion for example, sewage sludge, method modifications are necessary for measurements in industrial-scale digestion plants, in particular regarding the connection to piping systems (circulation pipes) in the range of DN 100–200, pressure conditions in the pipes of approximately 1,800–2,600 hPa and the content of TS of 2–3% TS.

The vacuum degassing method according to Daelman (2014), Daelman et al. (2012) and Gal'chenko et al. (2004) was modified and applied for the measurement of dissolved methane concentrations in digested sludge. The method is based on headspace gas chromatography in combination with vacuum degassing and salting out for setting a new state of equilibrium.

The sampling of digested sludge (4–6 L and, respectively, 0.4–0.5 L containers/flasks) took place directly at the circulation pipe of the digestion unit (digestion units with gas injection, as well, are equipped with circulation pipes for heating the unit). A vacuum container (12 L and, respectively, 1 L containers/ flasks) was used to avoid methane stripping into the atmosphere. To shift the solubility of methane, sodium chloride (NaCl) was added (addition of 400 g NaCl/L for a saturate solution) to strip the dissolved methane, as well as a 50% sodium hydroxide solution (approximately 20 ml/L sample volume) to inhibit microbiological activity. Following a reaction time of 30–60 min (Schaum et al. 2015), during which the container, or flask, was shaken at intervals, sampling of the gas phase was done via a septum. The sampled gas was analyzed by means of gas chromatography (flame ionization detector (FID); Agilent).

Then, the weight of the vacuum container or the glass flasks was determined. Subsequently, the gas space was completely filled with a saturated NaCl solution (density NaCl ≈ 1.2 kg/L at T = 20 °C). The gas phase volume was calculated via the total weight of the container and the specific density of the NaCl solution.

By recording pressure and temperature as well as gas and sludge volume, one can calculate the concentration of dissolved methane in sludge via the ideal gas law. The determination of the theoretical concentration of dissolved methane follows Henry's Law. The decisive pressure and temperature were measured directly in the pipes via a portable measuring instrument (Greisinger). Furthermore, processing and constructional parameters (circulation, height of the sludge level, etc.) were taken into account.

As part of the method development, the second step was to validate the experimental setup and procedure, whereby the following results were obtained:

• Multiple determinations (15 individual measurements, WWTP #1): mean: 36.9 mg/L; standard deviation: 3.4 mg/L (with NaCl), which provided confirmation for the high reproducibility of the experimental setup;

• Variations in the volumes of digested sludge samples: Figure 3(b) demonstrates the excellent correlation between variation in sample volume and dissolved methane. The concentration was 39.7 ± 1.9 mg/L CH₄.

• Variations in reaction times: measurements with differing reaction times demonstrate that the solubility equilibrium was achieved relatively spontaneously (Schaum et al. 2015). Therefore, a reaction time of 30–60 min was chosen for subsequent experiments.

Table 3 presents the experimental results, which demonstrate the following dependencies:

• In accordance with the theoretical considerations, using salt (NaCl) produced a clearly higher methane concentration, because the shift in solubility led to the complete stripping of the methane dissolved in the sludge according to the theoretical solubility equilibrium (Hermann et al. 1995; Sander 2014); see also Figure 1. Approximately only 65%, on average, of the dissolved methane were detected without the use of NaCl (measurements at 37 °C).

• In two WWTPs (WWTP #1 and #2), a comparison between measured (with NaCl) and calculated methane concentration yields values that are approximately 50% higher for the measured concentrations.

Supersaturation, meaning the measured methane concentration is higher than that calculated on the basis of theory, has also been observed in anaerobic sewage treatment plants, whereby the degree of supersaturation varies strongly; supersaturation ratios are between 1.9 and 6.9 (Hartley & Lant 2006). Data for digested sludge are not yet available. Currently, and on the basis of recent results from anaerobic sewage treatment, it is only possible to speculate on the possible causes of supersaturation and the relevant mechanisms of action. For instance, sludge flocs and/or the related production or distribution of gas (micro gas bubbles) may lead to a shift in the solubility equilibrium. Further laboratory experiments that allow variations in defined boundary conditions will investigate possible effects on the solubility of methane.

By contrast, the other WWTPs that were investigated show close agreement between the measured and calculated concentrations.

Basically, calculating the concentration of the dissolved methane with Henry's Law provides a good first approximation for determining this concentration in digested sludge. The most sensitive factor in the determination of the dissolved methane concentration is the pressure that has to be applied, in particular because of possible measurement errors caused by the sampling location (e.g. pipeline layout, pump, etc.). This sensitivity is related to the fact that pressure is directly proportional to the dissolved methane concentration. Other factors, such as temperature, conductivity or methane concentration in the gas phase can, by contrast, be assessed exactly during data analysis.

• The mesophilic/thermophilic comparison reveals that methane has a low solubility in digested sludge, which is in accordance with theoretical calculations.

An influence of the pH, which ranged between 7.1 and 7.4 for all samples, as well as of the conductivity, which ranged between 6.3 and 9.8 mS/cm, was not observed. The TS concentration in the investigated samples ranged from 2 to 3% (mean: 2.8%) with a TVS of 55–62% (mean: 59%); technical process conditions produced an outlier of 71%. A dependency on the dissolved methane concentration was not observed.

Due to the even feeding of digester A with raw sludge, a continuous gas production with comparable concentrations of dissolved methane and dissolved COD were observed within a period of 10 h, as had been expected.

Caused by impact feeding with raw sludge, a significant increase in gas production immediately following the feeding of digester B was observed. With a certain time lag, hydrolysis leads to an increase in the concentration of dissolved COD, which subsequently is degraded again by conversion to biogas (Lensch et al. 2016). The dissolved methane concentration, as well as the supersaturation, correlates directly with the concentration of dissolved COD; see Figure 5.

Comparable effects were observed when analyzing the dissolved methane concentrations in the digester of WWTP #3. The digestion unit in WWTP #3 consists of three reactors operated in series. The measurements show that the dissolved methane concentrations decrease from reactor 1 to reactor 3 (digester 1: 29 mg/L CH₄; digesters 2/3: 11–16 mg/L CH₄). The two effects suggest that with the increase in volume load, both the concentration of dissolved methane and the supersaturation are increased. Parallel investigations of the Technische Universität Darmstadt, Institute IWAR show comparable effects in the anaerobic wastewater treatment via upflow anaerobic sludge blanket (UASB) and internal circulation (IC) reactors. The dissolved methane concentration correlates with the sludge bed (via reactor height) and the substrate gradient.

The energetic use of sewage sludge is an important step in the generation of electricity and heat within a WWTP. For a holistic approach, greenhouse gas emissions have to be considered. With anaerobic treatment, methane emissions are of particular concern.

The vacuum salting out degassing method can be applied to analyze methane dissolved in digested sludge. Method validation shows a high correlation between measured and calculated data (based on Henry's Law).

Measurements of dissolved methane concentrations at different WWTPs reveal a concentration range of approximately 17–37 mg CH₄/L. The average concentration of dissolved methane in mesophilic digested sludge was approximately 29 mg CH₄/L, which corresponds to an estimated yearly specific load of approximately 14–21 g CH₄ per PE; under the following assumption: amount of digested sludge per day 40–50 g TS/(PE·d), TS concentration 25–30 g TS/L (Schaum 2015). This result is in the lower range of the latest literature research on methane emissions in the field of sewage sludge treatment of 65–281 g CH₄/(PE·a), see Table 1. However, one has to consider that the concentration of dissolved methane in digested sludge represents only one factor. Other factors include methane emissions from storage devices, as well as the methane slip during combustion in coupled heat and power plants (Schaum et al. 2015). Compared to the total digester gas production of about 2,800 g CH₄/(PE·a), the proportion of the dissolved methane is <1% (Schaum 2015).

The dissolved methane emissions of 14–21 g CH₄/(PE·a) corresponds to a global warming potential of 476–714 g CO₂ equivalent/(PE·a), considering a global warming potential of methane of 34 according to the Intergovernmental Panel on Climate Change (IPCC) (2013). For comparison: with the current German electricity mix, the global warming potential of the total electricity consumption of a municipal WWTP is about 8,400 g CO₂ equivalent/(PE·a) (Schaum 2015).

Vacuum degassing installations are able to abate emissions caused by dissolved methane. This technology has been installed in WWTP #5, with the aim of improving dehydration of sewage sludge. Initial studies demonstrate similar dissolved methane concentrations for the operation data of the full-scale vacuum degassing plant and the laboratory-scale results presented here. Nevertheless, the exhaust gas has to be incinerated to avoid further emissions.

Furthermore, the investigation shows that digester operation, in particular digester feeding, impacts the concentration of dissolved methane. Via respective operational measures, e.g. operation of several digesters, preferably operated in series rather than in parallel, concentrations of dissolved methane can be reduced.

In summary, the experimental results demonstrate the need for further measurements of dissolved methane in digested sludge. The focus should be on the interaction between biogas, water, and sludge flocs under various boundary conditions (pressure, temperature, circulation, etc.).