Determination of greenhouse gas emission reductions from sewage sludge anaerobic digestion in China

In China, the amount of sludge generated by sewage plants has gradually increased since the year 2000 (Chen et al. 2012). However, owing to deficient policies, financing and technologies, a large amount of un-dewatered sludge has been discarded at random or buried non-standardly (Guo et al. 2012; Liu & Zhang 2013). In addition to increased environmental pollution risk to soil and water systems, the current situation of improper sludge disposal has led to considerable and unordered emissions of greenhouse gas (GHG). For instance, methane generated from sludge landfill or anaerobic digestion is known as a kind of clean fuel (Abbasi et al. 2012). Because the sludge is rich in decomposable organic matter, it is regarded as a significant source for GHG emissions (Majumder et al. 2014). Most biogas from landfilled sludge is lost to the atmosphere instead of being captured and reused. However, biogas that is reclaimed is commonly used to generate heat and electricity by combined heat and power plants or as fuel for vehicles (Tilche & Galatola 2008); therefore, biogas from anaerobic digestion leads to reduced carbon emissions and energy balance by supplying secondary biogas products (Weichgrebe et al. 2008; Komatsu et al. 2011; Niu et al. 2013; Remy et al. 2013).

In China, anaerobic digestion is generally only one process in the entire sludge treatment–disposal chain, which usually also includes thickening, dewatering or pyrolysis (Qiao et al. 2011). Nevertheless, anaerobic digestion remains an important method for achieving reduced carbon emissions, which could account for about 90% of the total GHG reductions resulting from all options of sludge treatment and disposal (Niu et al. 2013). Anaerobic digestion of sludge can be classified into mesophilic and thermophilic digestion. In China, most (about 80%) sludge anaerobic digestion reactors employ mesophilic technology. In such systems, biogas gathered from anaerobic digestion is dewatered and then subjected to desulfurization, after which it is used to generate electricity that is supplied back to the sludge plant. If abundant, surplus electricity can be merged into the local electrical grid, reducing the need to burn coal or natural gas. Nevertheless, in spite of anaerobic digestion being only a small proportion of the sludge treatment structure, the carbon debit and credit led by sludge anaerobic digestion and the amounts of these carbon reductions correlating with Certified Emission Reduction (CER) and financial support from international organizations still deserve to be investigated (Liu et al. 2014b).

Previous reports showed that the lowest carbon emission was from sludge anaerobic digestion among all sludge treatment technologies (Barber 2009). Additionally, the present GHG accounting guidelines, which assume that all carbon emission from sludge is biogenic, may lead to underestimation (Law et al. 2013). However, a certain proportion of organic carbon in sludge originates from fossil fuels, such as carbon in daily-used detergents. All the same, this portion of carbon in this study is tiny in the direct emitted carbon from sludge anaerobic digestion. Therefore, all direct carbon emissions in this study were assumed as biogenic.

Currently, there is sufficient information available to quantify the GHG emissions triggered by sludge anaerobic digestion. But detailed GHG qualification to anaerobic digestion with comparison to baseline scenario and its corresponding GHG reduction potential based on IPCC (Intergovernmental Panel on Climate Change) guidelines have been rarely reported. Therefore, this study was conducted to investigate GHG emissions from all units of a treatment system during sludge anaerobic digestion, which is combined with mechanical dewatering generally employed in China, with a focus on identification of direct and indirect GHG emission and determination of the role of these emissions in the carbon budget and reduction potential.

PE_elec,y, PE_fuel,y and PE_tran,y were calculated as described in the previous section.

Because the moisture content of sludge just leaving the sewage plant was 80%, dewatering was necessary to reduce the level to 60%. Therefore, indirect GHG emission caused by electricity consumption was mainly from sludge dewatering. Additionally, GHG emissions due to fossil fuel consumption were mainly from machines associated with landfill activities. According to Equations (2) and (3), PE_elec,y and PE_fuel,y were 0.0068 MgCO₂ MgWS⁻¹ (WS: wet sludge) and 0.0238 MgCO₂ MgWS⁻¹, respectively. In addition to these two component units, another indirect GHG emission source was sludge transportation. In this case, the distance from the sewage plant to the landfill site was assumed to be 20 km (average distance from sewage plant to landfill site obtained from statistical data both in Dalian and Xiamen city), and the approved loading per sludge truck was limited to 5 Mg. Therefore, the PE_tran,y was calculated as 0.0028 MgCO₂ MgWS⁻¹ according to Equation (4). Overall, the indirect GHG emissions were 0.0334 MgCO₂ MgWS⁻¹. With regard to direct GHG emissions, the PE_d,y was quantified as 0.6776 MgCO₂ MgWS⁻¹ based on the calculation according to Equations (5) and (6) and literature reported by Liu et al. (2014b).

During sludge anaerobic digestion, indirect GHG emissions were mainly associated with mechanical electricity, heat elevation and transportation to the digestion tank, as well as sludge transportation and loading on-site. According to Equations (3) and (4), the indirect GHG emissions caused by fossil fuel consumption (PE_fuel,y) and sludge transportation (PE_tran,y) were 0.0002 and 0.0033 (including biogas residue) MgCO₂ MgWS⁻¹, respectively. The GHG emissions from electricity use on-site differed owing to differences in temperature between northern and southern China. When sludge was pumped to the storage tank, the temperature was elevated to 35 °C, after which it was lifted to the anaerobic fermentor. The electricity use triggered by this process was 2.5 times higher in the Dalian sewage plant than in the Xiamen sewage plant. In detail, the electricity consumption data of northern and southern cities in China (Dalian and Xiamen) were statistically counted to 27.5 kWh MgWS⁻¹ and 10.1 kWh MgWS⁻¹ respectively (main electricity consumption units associated with sludge anaerobic digestion and their percentages shown in Table 2). According to Equation (2), the values of PE_elec,y were accordingly quantified as 0.0244 MgCO₂ MgWS⁻¹ for northern China (Dalian city) or 0.0074 MgCO₂ MgWS⁻¹ for southern China (Xiamen city). Overall, indirect GHG emissions from sludge anaerobic digestion were 0.0279 (northern China) and 0.0109 (southern China) MgCO₂ MgWS⁻¹.

All biogas generated from sludge anaerobic digestion was captured and then purified for reuse in the form of heat conversion to electricity. One of the assumptions was that there was no leakage emission of methane throughout the anaerobic digestion period. Therefore, the direct GHG emissions from anaerobic digestion (value of PE_a,y) were considered to be 0 MgCO₂ MgWS⁻¹. According to Equation (8) and biogas reclamation data in China, the value of BE_EN,y was 0.0383 MgCO₂ MgWS⁻¹. After anaerobic digestion treatment, the sludge volume decreased by half, after which further dewatering was carried out by solid–liquid separation.

The carbon debit consisted of indirect and direct GHG emission units, but the carbon credit only included replaceable emissions reduced by biogas from anaerobic digestion instead of natural gas. As shown in Table 3, total landfill emissions were calculated to be 0.711 MgCO₂ MgWS⁻¹, while those from anaerobic digestion were −0.0104 (northern China) or −0.0274 (southern China) MgCO₂ MgWS⁻¹. Consequently, a decrease in GHG emissions of 0.7214 (northern China) or 0.7384 (southern China) MgCO₂ MgWS⁻¹ can be achieved by simply not disposing of the sludge in landfills.

Anaerobic digestion is considered the optimum method for reduction of carbon emissions among currently available sludge treatment technologies (Wong et al. 2009; Fernandez et al. 2014). This is because biogas generated from sludge can be captured and reused as energy gas. The reduction in carbon emissions from reuse of this resource can offset the emissions associated with anaerobic digestion, including direct and indirect GHG emissions (Komatsu et al. 2011; Fine & Nadas 2012). In addition to biogas, the product of sludge anaerobic digestion also includes biogas slurry and biogas residue. Currently, biogas slurry is regarded as effluent that must be sent to the sewage plant for nitrogen and phosphorus removal, and cannot be reclaimed directly. Previously, biogas residue was directly sprayed onto farm soil after being dewatered to moisture content of 80%, without composting pretreatment prior to being amended to soil. However, there are problems associated with this approach, such as high salinity in the residue and relatively lower maturity after dewatering (Liu et al. 2014a). Therefore, stabilization and pretreatment are necessary for biogas residue prior to application to soil. Additionally, there is still debate regarding whether biogas residue after sludge anaerobic digestion should be included in the investigative frame. In this case, aerobic composting of biogas residue was not taken into consideration. Correspondingly, reuse of biogas residue, such as application as organic fertilizer to farmland or grassland, was usually ignored because the interaction between soil and amended residue compost is complicated and the amount of GHG emitted from soil or residue compost cannot be accurately quantified (Lopez-Valdez et al. 2011; Liu et al. 2014b). Subsequently, GHG emissions from the treatments of the biogas slurry and biogas residue were all not taken into consideration in this study. Ignoring this segment of the treatment chain will result in reduction in the carbon credit of replaceable emissions reduction, such as reuse of composted biogas residue to soil as organic fertilizer. Meanwhile, the carbon debit from treatment of biogas slurry and biogas residue will also be omitted. Therefore, to some extent, this omission poses a minor effect on total carbon emissions.

In China, the organic matter content in sludge is relatively low (Guo et al. 2009). Therefore, the potential production of biogas from sludge anaerobic digestion is not as high as in other countries. As a result, the proportion of anaerobic digestion in currently available sludge treatment plants in China is very low. In addition to the low organic matter content, the management level of the operational department responsible for sludge treatment did not also match process requirements. In other words, the management level in China is not high enough to develop stable operation of sludge anaerobic digestion. As shown in Table 3, the increase in carbon credit was all from reduction of replaceable emissions of biogas reuse, while the carbon debit reduction was mainly due to decreased fossil fuel consumption on-site and reduced direct GHG emissions. Especially, the distinct difference between anaerobic digestion and landfill is that the former allows effective collection of generated biogas and subsequent transformation into useable fuel (Batstone & Virdis 2014). However, the biogas, which is mainly made up of methane, generated slowly and unorderedly from landfill emitted into atmosphere. This emission is an important contribution to the carbon debit.

Overall, a total emission reduction of 0.7214 (northern China) or 0.7384 (southern China) MgCO₂ MgWS⁻¹ was achieved. This is a considerable GHG reduction, to some extent, and is important for obtaining CER, which enables access to financial support in the Clean Development Mechanism (Rogger et al. 2011). For example, the current sludge yield of Xiamen is 500 Mg daily. If this amount of sludge can be treated by anaerobic digestion rather than landfill, there will be a CER of about 13,480 MgCO₂ annually.

Defined boundaries of investigative cases can influence reported potential changes in GHG emissions (Vergara et al. 2011). For instance, the sludge just after effluent treatment or, if the sludge is to be dewatered to moisture content of 80%, the volume-reduced sludge can be set up as the boundary beginning point. There would be significant differences in GHG emissions differences between these two project boundaries. If dewatering was taken as the study's consideration, the corresponding electricity consumption will also increase, and this increase will lead to the entire carbon debit to increase, which is not as much as landfill. In China, the moisture content in sludge for landfill needs to be dewatered to 60% according to the limit value for sludge landfill. Therefore, the indirect GHG emission resulting from landfill will be more than from anaerobic digestion. For sludge treatment, anaerobic digestion is the preferential method of reducing the carbon footprint, despite not yet being widely applied in China. Additionally, anaerobic digestion is suitable for energy-saving and decreased carbon emissions. The positive effects of sludge anaerobic digestion also depend on reduced emissions owing to secondary product (biogas) replacement and low self-energy consumption during the treatment process.

This study was financially supported by the National Natural Science Foundation of China (41201585), Science and the Technology Project from the Ministry of Housing and Urban-Rural Development of China (2012-K7-2), Beijing Nova Program (Z121109002512061) and Beijing Excellent Talent Training Program (2013D012001000006).