Investigation of greenhouse gas emission from urban sewer systems in Xi’an
In general, there are four kinds of dominant functional areas in cities: residential areas, comprehensive service areas, schools, and business districts. Due to the diverse functions of these areas, the types of wastewater in these areas differ (such as toilet, kitchen, wash wastewater, and rain water) and the amounts of these types of wastewater change markedly under different weather conditions (as shown in Additional file 1: Figure S1). The wastewater flows from the outfall to branch sewer pipe, then to the sub-main sewer pipe, and finally to the main sewer pipe. Due to the different flow conditions in the three ranks of sewer, the transformation of diverse contaminants in different types of wastewater might change, which could further influence the characteristics of greenhouse gas generation.
To verify this hypothesis, the concentrations of CO2 and CH4 were monitored along the sewer under different conditions. As shown in Fig. 1, the production of greenhouse gases was highest in branch pipes, followed by the sub-main pipes and main pipes. Furthermore, the generation rates of greenhouse gases in separate sewer systems were higher than those in combined sewer systems. Based on the statistical results, the average concentrations of CO2 in branch pipe, sub-main pipe, and main pipe were 5916 mg/L, 2871 mg/L, and 1830 mg/L, respectively, and the average CH4 concentrations were 2937 mg/L, 1445 mg/L and 914 mg/L, respectively. In Xi’an, the lengths of branch pipes, sub-main pipes, and main pipes are 247,521 m, 219,506 m, and 371,873 m, respectively. The total amounts of CO2 and CH4 can then be calculated (15 t/day and 8 t/day, respectively). Because the influence factors of CO2 and CH4 as greenhouse gases are 1 and 23 [16], respectively, the total greenhouse gas amount in Xi’an can be calculated (188 t/day). The results indicated that the emission of greenhouse gases from urban sewer systems cannot be neglected. Due to the complex condition in real urban sewers, investigations of the greenhouse gas emission values should be further verified by analyzing the biological generation pathways (i.e., the degradation of organic matter).
Analysis of greenhouse gas generation pathway
To comprehensively understand the greenhouse gas generation pathways in sewer, a metagenome analysis was used in this study. Kyoto Encyclopedia of Genes and Genomes Pathway Database (KEGG) analysis identified key genes implicated in the methane and carbon pathways (as shown in Fig. 2). The results showed that various enzymes encoded by corresponding functional genes took part in CH4 and CO2 production in urban sewer system. Within the methane pathway, the genes coding for Methylcoenzyme M (Methyl-CoM; EC 2.8.4.1) had the highest abundance in the biofilm sample. In addition, methylamine, methanol, formate, and acetate utilized diverse enzymes, including EC 2.7.2.1, EC 23.1.101, and EC 1.2.99.5, to generate the Methyl-CoM and then promote the accumulation of methane in urban sewer system. The annotated formate dehydrogenase (EC 1.2.1.2) and glycine dehydrogenase (EC 1.4.4.2) played an important role in the CO2 producing pathway, and the relative abundances of microbial communities that decomposed carbohydrates, proteins, and lipids to CO2 were non-negligible in the sewer system. Furthermore, methanogens and fermentative bacteria were also relatively abundant in the sewer sediment. Therefore, CH4 and CO2 can easily be generated in sediment.
According to the metagenomics analysis mentioned above, the macromolecule contaminants of carbohydrates, proteins and lipids, micromolecule contaminants of methyl alcohol, methanoic acid, methylamine, and so on were the node sources for CO2 and methane generation, therefore, those contaminants were monitored for 3 years along branch sewer, sub-main sewer, and main sewer, respectively. The average monitoring results can represent full range of greenhouse gas emission under the effect of weather condition and different systems. As shown in Fig. 3, the concentrations of methylamine, methanol, formate, and acetate in sewage decreased and that of methane increased along the real urban sewer system. In addition, the carbohydrates and proteins in sewage decomposed along the sewer system, and could provide abundant substrates for CO2 generation. The degradation of organic matter in the combined system was generally similar to that in the separate sewer system. The concentrations of polysaccharides, proteins, and lipids in branch pipes were significantly higher than those in main pipes (for 112 mg/L, 42 mg/L, and 6 mg/L, respectively), indicating that these organic materials in sewage were degraded along the sewer. Previous studies showed that sediment was ubiquitous along the urban sewer and was contributed by the deposition of particulate matter from sewage [8, 17]. To determine the pollutant degradation in sediment, 10 meters of sediment was taken from the sewer. As shown in Fig. 4, under the flow of sewage, the concentration of diverse pollutants such as macromolecular pollutants (carbohydrate, protein, and lactic acid) and micromolecular pollutants (acetic acid, methylamine, and methyl alcohol) decreased in sediment. The results also showed that the concentration of the degradative mass in sediment was highest in branch pipes. For example, the concentration of protein was 531 mg/L in the sediment of branch pipes, and it decreased rapidly to 122 mg/L in the sediment of sub-main pipes. Although the convergence phenomenon occurred in main pipes, the concentration of protein (276 mg/L) was still lower than that in branch pipes, which was resulted from the significant degradation of pollutants in upstream pipes. Based on the high-throughput analysis mentioned above (as shown in Fig. 2), functional microbial communities such as Trichococcus, Cloacibacterium, and Paludibacter were abundant in sewer sediment. Their abilities on hydrolysis and acidification of pollutants were the main reasons for the generation of CO2 and methane [18,19,20]. Thus, it indicated that the contributions to greenhouse gas generation by sewage and sediment changed in the different levels of sewer, and this characteristic should be considered in the calculations of greenhouse gas emission in sewers.
Calculation of the greenhouse gas emission from sewer systems
Based on the monitoring results of node contaminants in sewers, the generation values of CO2 and methane can be calculated through the equations which are obtained from the contaminant transformation venation (as shown in Additional file 5: Figure S5 and “Methods” section). As shown in Fig. 5 and Additional file 2: Figure S2a, there were four transformation pathways that utilized carbohydrates, three transformation pathways that utilized proteins and three transformation pathways that utilized lipids during the CO2 generation process in sewer. All the transformation pathways induced the continuous accumulation of CO2 in the branch sewer (4.6 × 104 mg/day), sub-main sewer (2.2 × 105 mg/day), and main sewer (2.7 × 105 mg/day), therefore, the accumulation rates of CO2 gradually increased in the three levels of sewer, and the highest accumulation rate was found in the main sewer. It should be noted that at the confluence segment of the three levels of sewer, the generation rate of CO2 reached to a peak value and then maintained a steady state along the sewer. This phenomenon was especially significant at the beginning segment of branch sewer and might have been due to the high concentrations of carbohydrates, proteins, and lipids present in this segment, which provided abundant substrates for the generation of CO2. The substrates were consumed by the microbial communities in the rear segment. Therefore, the generation rate of CO2 gradually achieved a steady state. As shown in Fig. 5 and Additional file 2: Figure S2b, there were four transformation pathways (utilizing methyl alcohol, methanoic acid, methylamine, and acetic acid) for methane generation in sewer which induced the continuous accumulation of methane in the branch sewer, sub-main sewer, and main sewer. The highest generation rate of methane occurred in the main sewer (1.3 × 105 mg/day), and this was also observed for CO2 generation. In addition, the emission rates of greenhouse gas in sediment were higher than those in sewage in branch pipes. They were basically the same as those in sewage in sub-main pipes, but were much lower than those in sewage in main pipes. Due to the smaller pipe diameter of branch pipes, sediment formed easily in them, and the abundant pollutants provided a suitable environment for the generation of greenhouse gases. With the increased flow velocities and larger pipe diameters of the sub-main and main pipes, the sediment mass was gradually decreased. Therefore, the generation rates of greenhouse gas were lower than in sewage.
The total lengths of the three levels of urban sewer network in Xi’an were 247,521 m, 219,506 m, and 371,873 m, respectively. Therefore, the CO2 production in branch pipe, sub-main pipe, and main pipe in Xi’an was calculated to be 1.13 t/day, 4.85 t/day, and 10.12 t/day, respectively, and the methane production was 0.57 t/day, 2.43 t/day, and 4.96 t/day, respectively. The conversion indexes of CO2 and methane to greenhouse gas are 1 and 23, respectively [16], so the total greenhouse gas emissions in sewer was calculated to be 199 t/day, which was basically the same as the monitoring results. It indicated that the calculation methods were reasonable.
Effect of greenhouse gas emission from urban sewer systems
As mentioned above, the total greenhouse gas emission amount from the urban sewer network in Xi’an is 199 t/day. The total population of Xi’an is 8,705,600, and the total population of China is equivalent to 154 times that of Xi’an. Therefore, it can be estimated that the greenhouse gas emission from sewer systems in China is 30,685 t/day. Based on the characteristics of population distribution in China, the greenhouse gas emission amounts differ in different regions of China. As shown in Fig. 6a, the greenhouse gas emission amounts in Sichuan Basin (southwest of China) and eastern coastal cities are extremely high, but are comparatively low in the western regions. In addition, the per capita carbon footprint in Xi’an is approximately 2.483 t/year [21], which is 6.803 × 10−3 t/day. Based on this index, the total amount of greenhouse gas emission from the sewer systems in Xi’an is equivalent to the emissions from 29,300 people, and the amount of greenhouse gas emission in China is equivalent to the emissions from 4.51 million people which is basically equal to the entire population of New Zealand. As shown in Fig. 6b, the amount of greenhouse gas emission from urban sewers in China is close to the emission from the total population of New Zealand. In addition, the greenhouse gas emissions from the total population of New York City, London, and Tokyo can be calculated to be 58,171 t/day, 58,512 t/day, and 91,897 t/day, respectively.
Based on the concept of life cycle, Flysjö et al. [22] established a carbon footprint assessment framework and related standards for dairy farms, and used this method to calculate the carbon footprint for the production of 1 kg standard milk (FPCM) (1.34 kgCO2). The amount of milk produced in Australia in 2016 was 10 million tons, so the amount of greenhouse gas emissions can be calculated to be 36,749 t/day. Lee [23] established models to evaluate carbon emission and found a value of 379 kg/car for the four major manufacturing processes and material movements in the automobile production system; and the global car output in 2016 was 95 million, which led to the release of 98,585 t/day of greenhouse gases. Li [24] established a model for calculating carbon dioxide emissions from iron and steel enterprises and summarized the carbon dioxide emission from each process in Baosteel. The carbon emissions during this process are 1636 kg/t produced, and annual output of iron and steel is 42 million tons in Germany in 2016, so the total amount of greenhouse gas emissions is 188,691 t/day. The amount of greenhouse gases produced by the global urban sewer system is much higher than the amounts produced by the above industries, which indicates that the amount of greenhouse gas emitted from the urban sewer systems should not be underestimated and need much more attention. In addition, raising feasible strategies to mitigate the greenhouse gas emissions coming from sewer systems should be considered. Previous studies have demonstrated that air/pure oxygen injection could be adopted to mitigate sulfide generation in sewer systems [10, 25]. The formation of greenhouse gases occurred under the anaerobic environment which was the same as sulfate-reducing process, therefore, changing the condition of dissolved oxygen could also be considered to mitigate greenhouse gas emissions. Furthermore, the greenhouse gas generations were contributed by the metabolism of functional microbial pathways, which have been illustrated in this study. If the biocides such as biocide-free nitrous acid (FNA) are added in sewers, the biophase dynamics can be prohibited, and those methods were also proposed by many scholars to mitigate sulfide generation [26, 27]. However, changing the environment and adding the biocides in sewers are uneconomical and cannot guarantee long-term effect, therefore, the new feasible strategies to mitigate the greenhouse gas emissions should be continually explored.