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Review

Status of Research on Greenhouse Gas Emissions from Wastewater Collection Systems

by
Dongmei Gu
1,
Yiwen Liu
1,
Weigao Zhao
1,
Shuntian Qiu
1,
Nuo Cui
2,
Xinyue Hu
2 and
Peng Zhao
1,*
1
School of Environmental Science and Engineering, Tianjin University, Tianjin 300354, China
2
North China Municipal Engineering Design & Research Institute Co., Ltd., Tianjin 300381, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(14), 2512; https://doi.org/10.3390/w15142512
Submission received: 24 May 2023 / Revised: 30 June 2023 / Accepted: 3 July 2023 / Published: 9 July 2023
(This article belongs to the Special Issue Green and Low Carbon Development of Water Treatment Technology)
Browse Figure
Versions Notes

Abstract

:
Wastewater collection systems (WCSs) not only play an indispensable role in urban life but also significantly contribute to greenhouse gas (GHG) emissions. Based on extensive literature research, this study (1) summarizes current research on the production mechanisms, influencing factors, control techniques, and quantitative estimates of GHGs emitted from WCSs and (2) presents initial estimates of total GHG emissions from WCSs in China. A variety of factors affect GHG production, but standard methods are still lacking to quantify GHG emissions from WCSs. China’s WCSs emit approximately 3.86–15.35 Mt of CO2eq annually (equivalent to 5.1–20.2% of the GHG emissions from wastewater treatment). Thus, GHG emissions from WCSs are significant and deserve attention. Control of GHGs in WCSs can be achieved through the application of chemical agents, while the construction of a green stormwater infrastructure can further facilitate GHG reductions. This review provides valuable insights for policymakers to consider in future policy planning related to GHG reduction and the improved operation of WCSs. Future research should focus on quantifying the impacts of various factors and accumulating field data on GHGs in various regions to facilitate the development of standardized calculation methods.

1. Introduction

In recent decades, the impacts of global climate change have become increasingly prominent. Many countries around the world, including China, are making efforts to address and mitigate this issue, including carbon emission reduction, which requires reducing emissions of greenhouse gases (GHGs). To promote more effective global carbon emission reduction, it is essential to quantify the GHG emissions of various industries. Consequently, extensive research is underway to determine the GHG emissions associated with different industries [1,2,3,4,5,6]. Among these, wastewater collection systems (WCSs) not only play an indispensable role in urban life but also significantly contribute to GHG emissions.
According to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories published by the Intergovernmental Panel on Climate Change (IPCC), the waste management industry is one of the main sectors contributing to GHG emissions, along with the energy industry, agriculture, forestry, and other land uses. Accordingly, waste management has the potential to significantly reduce GHG emissions. WCSs are an important constituent of the waste management industry, delivering sewage from users to wastewater treatment plants (WWTPs). The 2006 IPCC Guidelines stated that the sewers of WCSs are “not a source of CH4/N2O” and CO2 “should not be included in national total emissions” [7], which suggests that GHG emissions from WCSs can be ignored. However, many studies have shown that WCSs are an important source of GHGs [8,9,10,11,12,13]. Therefore, the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories pointed out that sewers are a “likely source of CH4/N2O. However, insufficient data exist to quantify emission factors that address the variation in sewer type and operational conditions” [14].
As the amount and scale of China’s WCSs are substantial, the quantity of GHG emissions they collectively discharge may also be very large and should be quantitatively estimated. In 2019, China proposed the goal of limiting peak carbon emissions before 2030 and achieving carbon neutrality by 2060. It is urgently necessary to quantify GHG emissions in WCSs to achieve this goal.
Based on an extensive literature review, we summarize here the status of research on GHG emissions direct from wastewater in WCSs, including production mechanisms, influencing factors, control techniques, quantitative estimates, and modeling status. The overall research goal was to summarize the current status of research on GHGs in WCSs, demonstrate the feasibility of quantifying GHG emissions in WCSs, and evaluate the potential for GHG reduction. Additionally, this study also aims to provide direction and suggestions for future research and serve as a reference for decision-makers.

2. Greenhouse Gases in Wastewater Collection Systems

2.1. Greenhouse Gases

GHGs refer to gases that can cause global warming, including CO2, CH4, N2O, and other trace gases [15]. Discussions of climate change tend to focus on CO2, as it is the main GHG produced by burning fossil fuels, industrial production, and land use changes [16]. However, CH4, N2O, and other trace gases also make significant contributions to global climate change. In fact, although the amounts of CH4 and N2O emitted are much smaller than that of CO2, their global warming potentials (GWPs) are much larger than that of CO2. GWP, a metric used in policies, integrates the radiative forcing of a substance over a chosen time horizon relative to that of CO2, which is assigned a value of 1. The GWPs of CH4 and N2O are 29 and 265, respectively, as assessed by the IPCC over a 100-year period [15]. In 2016, CO2 was the largest contributor to global warming, accounting for 74.4% of total emissions, followed by CH4 at 17.3%, N2O at 6.2%, and other gases at 2.1% [16].
The atmospheric concentrations of CO2, CH4, and N2O have all increased since 1750 due to human activity. In 2011, the concentrations of these GHGs were 391 ppm, 1803 ppb, and 324 ppb, respectively, exceeding pre-industrial levels by about 40%, 150%, and 20%, respectively [17]. The global average temperature has risen by more than 1 °C since pre-industrial times due to the increased concentrations of GHGs [16]. To reduce the risks and impacts of climate change, the Paris Agreement proposed maintaining the global average temperature rise well below 2 °C above pre-industrial levels and striving to limit it to 1.5 °C above pre-industrial levels [18]. During the 26th Conference of the Parties to the United Nations Framework Convention on Climate Change (COP26) in 2021, the United States and the European Union launched the “Global Methane Pledge” initiative, which has been signed by more than 100 countries.

2.2. Wastewater Collection Systems

Urban WCSs refer to the series of wastewater collection pipelines located underground in urban developed areas. WCSs generally receive domestic sewage from residences, as commercial and industrial sewage generally requires pretreatment to meet water quality standards before discharging. As an important component of urban infrastructure, the healthy operation of WCSs affects people’s lives. In the past few decades, China has experienced many urban environmental problems due to rapid urbanization and insufficient attention to environmental protection, in which WCSs play a role. In 2019, China’s Ministry of Housing and Urban–Rural Development, the Ministry of Ecology and Environment, and the National Development and Reform Commission jointly issued the “Three-Year Action Plan (2019–2021) for Improving the Quality and Efficiency of Urban Wastewater Treatment”, which aims to solve existing problems in WCSs. Investigations have found significant problems in WCSs in China, such as cross-connections between storm sewers and sewage pipelines, leaking pipes, and high water level operation [19]. Apart from causing inconvenience in urban life, these problems also have adverse effects on carbon emissions.

2.3. Greenhouse Gases in Wastewater Collection Systems

GHGs emitted from WCSs include CH4, CO2, and N2O. During long-term operation, inorganic and organic particles and microorganisms in the wastewater settle at the bottom of the pipelines. The microorganisms colonize the sediments (mainly on the surface, to a depth of about 2 cm) and the pipeline walls, and grow, forming a biofilm with a thickness of several hundred to several thousand microns after several months [20,21,22,23,24,25]. A cross-section of a wastewater collection pipeline is shown in Figure 1a. The biofilms are mainly composed of large amounts of inorganic substances (such as water and inorganic salts) and some organic substances (such as microorganisms and extracellular polymers), with bacteria playing a dominant role in the biofilm [10]. The formation of biofilms is influenced by many factors, such as the pipeline operation mode, sewage characteristics, and water flow shear force. During the transport of the sewage from the users to the WWTP in pipelines, the microorganisms in the biofilm degrade and utilize the organic substances in the sewage, decreasing the concentration of organic matter. The effect of this biological action is significant over the long term during long-distance sewage transport [26]. Zan et al. found that biological processes in two laboratory-scale sewer reactors consumed 25% and 30% of the total COD, respectively [23]. In a laboratory-scale WCS, CH4 production accounted for ~70% of the total soluble COD (sCOD) loss [9].
The degradation of pollutants in sewage by microorganisms reduces the concentration of organic matter in the sewage, resulting in a lower organic matter concentration in the influent of the downstream WWTP [27]. To remove other pollutants such as nitrogen and phosphorus in the biological treatment process of the WWTP, additional carbon sources need to be added to the sewage, increasing sewage treatment costs. In addition, harmful gases such as H2S, CH4, and N2O are released during transport in WCSs. H2S causes pipeline corrosion, odor, and other problems, while CH4 and N2O are GHGs that contribute to climate change [7]. Moreover, high CH4 concentrations also present an explosion hazard [28,29].

2.4. Mechanism of Greenhouse Gas Production in Wastewater Collection Pipelines

Li et al. provide a detailed summary of the mechanism of CO2, CH4, and N2O production in the WCS biofilm (Figure 1b) [20]. In wastewater collection pipelines, organic substances in the wastewater are typically degraded into volatile fatty acids (VFAs), such as methanol, acetic acid, and propionic acid, as well as CH4, CO2, and H2 by fermentative bacteria and hydrogen-producing acetogenic bacteria. These VFAs, CO2, and H2 are then utilized by methanogenic archaea (MA) to produce CH4. N2O is a byproduct of microbial nitrification and denitrification processes [20,30,31]. During nitrification, NH4+ in the sewage is oxidized to NO2− by microorganisms. Oxidation of the intermediate product NH2OH and reduction in NO, which is the product of NH2OH, generate N2O [32]. During denitrification, the intermediate product N2O is produced during the conversion of nitrite to nitrogen. In gravity flow WCSs, some of the generated CO2, CH4, and N2O evaporate into the gas phase and enter the atmosphere from exhaust ports. The production of CH4 in the sediments in WCSs can be divided into three stages: hydrolysis, fermentation, and methane production [12]. The fermentable COD in the wastewater can be degraded into easily fermentable COD after hydrolysis, which is then utilized by microorganisms to produce H2, acetic acid, and propionic acid. MA then primarily use acetic acid and H2 to produce CH4 (Figure 1c).

3. Factors Affecting Greenhouse Gas Production in Wastewater Collection Systems

3.1. Factors Affecting CH4 Production

The production and emission of CH4 in many natural and artificial anaerobic environments (such as wetlands, paddy fields, landfills, animal waste, and wastewater and sludge treatment systems) has been widely studied [9], whereas research on wastewater collection pipelines is relatively limited [33]. However, existing studies suggest that CH4 production mainly originates from microbial activity in the biofilm on the pipeline wall and in the biofilm on sediments in WCSs.

3.1.1. Biofilm

The factors affecting CH4 production in the pipeline biofilm are shown in Table 1. Conditions favoring higher CH4 production include higher temperature and pressure, lower nitrate and sulfate concentrations, higher COD concentration and biomass, neutral solution, longer hydraulic retention time, and higher A/V of the pipeline biofilm. Since higher nitrate concentration and pH inhibit CH4 production, they are often used to suppress CH4 generation in WCSs [34,35].

3.1.2. Sediment

The factors affecting CH4 production in the sediment are similar to those in the pipeline biofilm, with some differences (Table 2). The presence of wastewater inflow, sediment, and microorganisms produces CH4 in WCSs. These conditions are typical of WCSs and can lead to CH4 emissions once there is an accumulation of sediment.

3.2. N2O

Since the production of N2O in WCSs has only recently been studied, research is scant. Factors affecting N2O production in WCSs include the carbon–nitrogen ratio, organic loading rate, oxygen environment (aerobic, anaerobic, or hypoxic), pH, hydraulic retention time, substrate concentrations (NH4+-N and NO3−-N) and intermediates (NO2−-N, NO, and free nitrous acid), as well as the abundance and activity of N2O-producing microorganisms [31,46].

4. Control of Greenhouse Gases in Wastewater Collection Systems

4.1. CO2

Little research has been conducted on controlling CO2 in WCSs. Possible reasons for this include: (1) CO2 has a relatively low GWP compared to other GHGs, resulting in a less severe greenhouse effect; (2) CO2 emissions in WCSs are typically low (see Section 5.1); and (3) CO2 emissions from WCSs are not included in national total emissions calculations. Zhang et al. found that the CO2 flux in three laboratory-scale sewer reactors can be reduced by 60%, 33%, and 99% after dosing with 30 mg N/L of nitrate, 30 mg N/L of nitrate plus 0.6 mg/L of DO, and 30 mg N/L of nitrate plus 50 mg/L of CaO2, respectively [47].

4.2. CH4

Research on controlling CH4 in WCSs is often associated with controlling H2S, due to their simultaneous occurrence and high concentrations [27,34,44,48,49,50,51,52,53,54], which present significant hazards. The methods employed to control CH4 production primarily focus on inhibiting the activity of methanogenic bacteria, rather than directly converting the already produced CH4. Some methods for controlling CH4 in WCSs are outlined below.

4.2.1. O2

The inhibition of CH4 production by O2 is attributed to its toxic effects on MA in biofilms [55]. Ganigue et al. observed that the percentage of damaged biofilm increased from 25.7% to 56%, 66%, 73%, and 70% after oxygenation periods of 1, 2, 3, and 6 h, respectively [56]. During the initial oxygenation stage (15–25 mg O2/L), the CH4 concentration in the two reactors decreased by 15% and 47%, respectively. However, after 13 days, the CH4 concentration recovered to its original level due to O2 consumption and other factors. These findings allayed concerns about potential increases in N2O production upon introducing O2, as the accumulation of N2O in the liquid phase remained negligible, with concentrations consistently below 0.01 mg N/L [56].

4.2.2. Nitrate

Nitrate can significantly suppress CH4 production [35,57,58]. This inhibition may be due to changes in redox conditions caused by the presence of nitrate or chemical oxidation of CH4 by nitrate [57]. It may also result from competition for sCOD between heterotrophic nitrate-reducing bacteria and sulfate-reducing bacteria, leading to insufficient sCOD availability for MA located deep within the biofilm [55]. Jiang et al. observed that, after approximately 4 weeks of dosing, CH4 production in the reactor stabilized at around 21% of the original level for the remaining 5 months [35]. One concern regarding the addition of nitrate is the generation of N2O. However, the presence of N2O is only temporary. After the depletion of nitrate salts, denitrifying bacteria further reduce N2O, resulting in negligible levels of N2O production [35,58].

4.2.3. Nitrite (Free Nitrous Acid)

Nitrite has been found to effectively inhibit CH4 production [48]. The presence of free nitrous acid damages enzymes, cell membranes, cell walls, and nucleic acids, thereby inhibiting the activity of MA [59]. Mohanakrishnan et al. observed a reduction in CH4 production to less than 10% of the original capacity by adding 20–140 mg N/L of nitrite to the biofilm in laboratory reactors [44]. However, continuous exposure to nitrite likely stimulated the growth of heterotrophic nitrite-reducing bacteria in the biofilm, leading to the production of N2O [44]. Complete inhibition of CH4 production has been achieved by intermittently applying nitrite at a concentration of 100 mg/L for 33 h over 3 d in a rising main pipeline spanning 1 km. CH4 production remained at negligible levels for at least 3 months [51].

4.2.4. Free Ammonia

Free ammonia (FA) exerts inhibitory and antimicrobial effects on various microorganisms. When present at inhibitory levels, FA can passively diffuse through the cell membrane into the cytoplasm, causing proton imbalance and potassium deficiency [60]. High concentrations of ammonia have adverse effects on microorganisms in anaerobic digestion, particularly on MA [52]. This can lead to the inhibition of CH4 production through intracellular pH alterations and other mechanisms [61]. Urine dilution (equivalent to 154 mg N/L) has been found to rapidly decrease CH4 production to less than 20% of the original capacity with 24 h of continuous exposure. This reduced yield was maintained at below 50% of the original capacity for 40 d [49]. Additionally, this method does not impact the total nitrogen load on downstream wastewater treatment plants, as the separation of urine waste is typical of urban wastewater.

4.2.5. pH

CH4 control can be achieved through moderate pH elevation. Inhibition of MA growth may be attributed to an increase in the concentration of free ammonia under elevated pH conditions [50]. At pH significantly deviating from the optimal range of 7.0–7.5, MA must allocate energy to maintain intracellular balance rather than engage in synthetic metabolism [50]. Gutierrez et al. found that exposing sewer biofilms to pH 9.0 for 2 h was sufficient to limit CH4 production to below 25% for several weeks. Prolonged elevation of the pH to levels between 8.6 and 9.0 can hinder CH4 production activity in sewer biofilms, resulting in almost no CH4 production [50].

4.2.6. Iron Salts

Iron salts have been shown to effectively inhibit the activity of MA in the upper layer of sewer sediment. This inhibition can be attributed to the toxicity of Fe3+ or the high oxidation-reduction potential (ORP) induced by Fe3+ [62]. Cao et al. observed that iron salts inhibited 21% of CH4 production in sewer sediment [62]. Furthermore, Zhang et al. found that the addition of Fe3+ significantly suppresses CH4 production activity in sewer biofilms, with inhibition rates ranging from 52% to 80% [63].

4.2.7. Ferrate

The bactericidal action of Fe(VI) is primarily attributed to oxidation. The strong oxidative effect of Fe(VI) inflicts damage on cell walls, cytoplasm, the genome, and other vital microbial organelles, resulting in the rapid death of microorganisms [64]. Furthermore, ferrate can disrupt extracellular polymeric substances produced by microorganisms in sewer biofilms, leading to a loosened biofilm structure [65]. Dosing with Fe(VI) can cause significant detachment of the biofilm in the reactor, further inhibiting biofilm recovery [66]. Yan et al. found that following exposure to Fe(VI), the abundance of the key gene mcrA, responsible for CH4 production, was only 13.4% of the pre-dosing level. With the addition of Fe(VI), CH4 production is completely suppressed; model predictions suggest that recovery to 50% of the pre-dosing level would require 44.5 d [66].

4.2.8. Other Substances

Guo et al. found that one-time dosing of polyoxometalates combined with a low level of free nitrous acid could suppress 80% of methanogenic activity for 30 d [67]. Sun et al. demonstrated that the use of electrodes with anode potential control can help control methane emissions under simulated sewer conditions [68]. Gao et al. found that after upstream natural pulsed ventilation, CH4 concentration was reduced by 42.29% and 35.70% in the upstream and downstream sewer pipes, respectively [69]. There are a number of liquid-phase biological treatment products on the market that claim to have the ability to control CH4, but the results of tests conducted on three of them showed that none of these biological products have any significant effect [70].

4.3. N2O

N2O production in WCSs is typically low, often at the µg level [31,46,47]. Thus, the harm from N2O is significantly lower than that from CH4. As a result, compared to the extensive research conducted on CH4, there has been limited research specifically on reducing N2O emissions. Some methods used to inhibit CH4 and H2S in sewers may even increase N2O emissions (see Section 4.2) [47]. However, numerous studies have been conducted on controlling N2O emissions in other emission-intensive areas, such as nitrate production [71], soil emissions, and denitrification in wastewater treatment plants. Increasing soil temperature and humidity may reduce N2O production [72]. Similarly, reducing the concentration of nitrite and ammonium substrates during wastewater denitrification or enhancing the anaerobic denitrification process with an external carbon source can minimize N2O emissions [73]. Online control strategies have been developed for sewage treatment plants to optimize oxygen utilization in aeration zones, thereby minimizing N2O emissions [74].

5. Quantitative Estimates of Greenhouse Gases in Wastewater Collection Systems

5.1. CO2

Regarding CO2 in WCSs, the 2006 Guidelines stated “these are of biogenic origin and should not be included in national total emissions” and the 2019 Refinement Guidelines retained the same conclusion [7,14].
Among GHGs in WCSs, CO2 has a relatively small impact on the environment because of its low production and its smaller GWP compared to CH4 and N2O. This, along with the conclusions of the IPCC, has resulted in little research on its production in WCSs. Chen et al. found that the amount of CO2 released after sediment contacts sewage was about three times that of CH4 in the same experiment [33]. Similarly, Willis et al. continuously measured gas-phase CH4 and CO2 concentrations at a pressure pipeline manhole and found that the CO2 concentration was about 3500 ppm, five times that of CH4 [11]. Regarding the quantification of CO2, Jin et al. found that CO2 emission in the Xi’an WCS was 16.1 t/d, twice the CH4 emissions [75]. However, considering that the GWP value of CH4 is 28 times that of CO2, the GHG effect of CO2 in WCSs is comparatively small.

5.2. CH4

Regarding CH4 in WCSs, the IPCC’s 2006 Guidelines concluded that “Wastewater in closed underground sewers is not believed to be a significant source of CH4” [7]. However, since then, research has begun to focus on CH4 emissions in WCSs. Based on more recent studies [33,75,76,77], the IPCC’s 2019 Refinement Guidelines changed its assessment to state that “wastewater in closed underground sewers likely generate CH4, but there are insufficient data available to quantify the emissions from these collection systems” [14].
Currently, CH4 emissions from WCSs are not considered in total emission statistics for various countries. However, in recent years, studies of CH4 in WCSs have shown that CH4 produced in WCSs may be a very important source of GHGs, with emissions comparable to those of WWTPs [9]. Additional studies have reported that CH4 emissions from global urban WCSs are higher than those from the Italian dairy industry and global automobile production industry [75]. Some studies have attempted to quantify CH4 emissions in local pipeline segments. For example, Guisasola et al. estimated the mass of CH4 released into the atmosphere from a rising main pipeline on the Gold Coast of Australia to be approximately 40–250 t CH4/y based on measurements of the CH4 concentration in the pipeline (about 4.0–22.0 mg/L) [9]. Shah et al. estimated CH4 emissions from a pumping station in Stone Mountain Park, Georgia, USA to be 9.06 t CO2eq/y (i.e., 0.394 t CH4/y) through continuous monitoring of the CH4 concentration during the pumping cycle [78]. Guo et al. measured CH4 emissions from a simple WCS and found that emissions from this simple system were 21.9 t CH4/y [79]. For larger regions, Willis et al. used instantaneous CH4 concentration measurements from 65 pumping stations in Decatur County, Georgia, USA to quantify the county’s CH4 emissions from its WCS, estimated at ~1194 t CO2eq of CH4 per year (i.e., 52 t CH4/y) [11]. Jin et al. used data from Xi’an’s WCS to estimate CH4 emissions of 7.96 t CH4/d and based on relative population, calculated China’s CH4 emissions to be ~1226 t CH4/d [75].
Many studies have attempted to calculate GHG emissions from entire wastewater treatment systems (sewage collection and treatment) or water systems (clean water and wastewater) using a life cycle approach [80,81]. However, they did not quantify all GHGs emitted from the WCSs [82], even in more recent publications [76,83], and frequently provide qualitative descriptions. Some studies have attempted to quantify GHG emissions from WCSs using others’ research data, but only included CH4 and ignored CO2 and N2O. For example, Eva et al. [45] used data from the SeweX model to quantify CH4 emissions from the Grabels town WCS (population 5200) in southern France, estimating its CH4 emissions as 22.25 kg CO2eq/d and considering it to represent the system’s GHG emissions [82]. Kyung et al. used CH4 membrane productivity measured by Foley et al. to calculate CH4 emissions from the urban WCS in Daegu, South Korea (population 1.5 million) as 35,100 t CO2eq/y and considered it to represent the system’s GHG emissions [84]. In summary, there are currently only limited quantitative studies on CH4 emissions from WCSs.

5.3. N2O

Similar to CH4, the IPCC’s description changed from being “not a source of N2O” in the 2006 Guidelines to “Likely source of N2O. However, insufficient data exist to quantify emission factors that address the variation in sewer type and operational conditions” in the 2019 Refinement Guidelines [7,14]. The IPCC provides emission factors for N2O in different receiving waters, wastewater treatment processes, and sludge treatment processes, but not for N2O emissions from WCSs [14].
N2O, as the third most important long-lived greenhouse gas after CO2 and CH4, has been studied in WWTPs and aquatic receiving environments in wastewater systems, but studies of N2O in urban WCSs are limited [30,76]. Some studies have suggested that microbial sediments are the main source of N2O [85]. Currently, there is no standard method for quantifying N2O emissions from WCSs [31].
As mentioned above, some studies attempting to quantify GHG emissions in WCSs only included CH4 emissions and not N2O emissions [79,84], although there have been some studies on N2O. Short et al. measured the N2O concentration in raw sewage from three large Australian cities’ WWTPs and obtained an N2O emissions factor of 1.39–1.84 g N2O/person·y for the WCS [31]. Shah et al. estimated that a 5.31 km long pressure pipeline in Australia emitted about 1.83 kg N2O per year by measuring the N2O concentration at the pipeline’s final discharge point [78].

6. Modeling

Due to the complexity and size of a sewer system, it is not feasible to detect pipeline problems or monitor pipeline operation by field sampling or installation of multiple devices, which requires a lot of time, manpower, and financial resources. It is more efficient and economical to detect pipeline problems and monitor pipeline operation by establishing a model for the pipeline.
Jia et al. have summarized the modeling of gas emissions in WCSs in detail [86]. Currently, most available models are for CH4, there are fewer models for CO2, and no models for N2O. Most models for CH4 have been developed for predicting production or understanding the gas production process and they have been validated or calibrated using experimental or on-site sampling data. These models include regression models, empirical models, and kinetic models. Although many models have been applied to practical problems, the corresponding prediction accuracy is generally moderate and sometimes low, especially when dealing with larger WCSs. Here, we briefly describe several prediction models for CH4.
Foley et al. developed a simple empirical model (1) for estimating CH4 production in pressure pipelines based on their research on two rising main pipelines in Australia. In this model, the methane concentration in the sewage is positively correlated with the HRT of the sewage and the A/V ratio of the pipeline [38]:
c(CH4) = 5.24 × 10−5 × (A/V × HRT) + 0.0015
where c(CH4) (kg/m3) is the concentration of methane, A (m2) is the biofilm surface area, V (m3) is the volume of the wastewater, and HRT (h) is the hydraulic retention time.
Chaosakul et al. proposed a similar empirical model (2) for predicting CH4 production in gravity sewer pipelines based on their research on a gravity WCS in Thailand. This model is a modification of model (1) with a temperature correction term [36]:
C(CH4) = 6 × 10−5 × (A/V × HRT) × 1.05T−20 + 0.0015
where T ( °C) is temperature.
Liu et al. found that the CH4 production in sediment was not significantly related to the sulfate concentration (5–30 mg S/L), but increased significantly with an increase in fermentable COD, thus obtaining a simple semi-kinetic model (3) from a detailed sediment model [12]:
r(CH4) = k × (SF)0.5
where r(CH4) (g CH4/m2·d) is CH4 production per unit biofilm area, k is the rate constant of CH4 production, and SF is the fermentable COD concentration in the sewage.
Sun et al. used laboratory reactor data to propose an empirical Equation (4) that describes the dependence of the maximum CH4 production rate on the long-term average sulfate and sCOD concentrations [39]:
K(CH4) = a × SSO42− + b × SsCOD + c
where K(CH4) (g COD/m2·d) is the maximum CH4 production rate, SSO42− (mg S/L) is the long-term mean sulfate concentration in sewage (application range 5–30 mg S/L), SsCOD (mg COD/L) is the long-term mean sCOD concentration in the sewage (application range 100–500 mg COD/L), and a, b, and c are constants (a = −0.41 ± 0.02, b = 0.078 ± 0.001, and c = 1.62 ± 0.46).
Xu et al. studied microorganisms and wall shear stress by establishing a pilot-scale laboratory reactor and developed a simple empirical model (5) [37]:
Q(CH4) = YCH4/X × X × θT−20 × HRT
where
X = −0.0485F2 + 0.161F − 0.0142,
F = 0.21941 + 0.44146 I + 1.73331 n Ȓ 0.52041 v + 0.13167 nI Ȓ 0.24688 vI Ȓ 1.47281 nv + 0.23833 I2 + 0.12750 n2 + 1.88828 v2,
and Q(CH4) (mg/L∙d) is the CH4 production, YCH4/X (mg CH4/kg biomass) is the productivity coefficient, X (kg) is the biomass, θ is the temperature coefficient, T ( °C) is temperature, F (Pa) is the wall shear force, I (‰) is the conduit slope, n is the pipeline filling degree, and v is the flow rate of the sewage.
In addition to the simpler models described above, Guisasola et al. developed a detailed biofilm production model for H2S and CH4 prediction [87] and Liu et al. developed a detailed sediment production model for H2S and CH4 prediction [12]. Both models predict CH4 production based on mechanistic principles and are relatively accurate. However, due to their large size, they are difficult to apply to the simulation of large WCSs.

7. Discussion

As China’s WCSs are collectively vast, their combined GHG emissions are considerable. Table 3 provides a rough calculation of the annual CH4 and N2O emissions from China’s WCSs, based on data from China’s Urban Construction Statistical Yearbook 2020, considering China’s urban population (1.4839 billion), the length of its WCSs (813,000 km), and its sewage discharges (67.5124 billion m3/year). The method used to calculate China’s CH4 and N2O emissions was as follows:
  • Emission data from the literature [84] were multiplied by the ratio of the length of China’s pipeline system to the studied pipeline to calculate emissions.
  • Emission data from the literature [11,75] were multiplied by the ratio of China’s urban population to that of the studied areas to calculate emissions.
  • Emission factors for CH4 [31] and N2O [88] were used to calculate the emissions of these GHGs based on the emission factors multiplied by China’s urban population.
  • Liquid-phase CH4 concentration data provided in [8,89] were used to calculate CH4 emissions based on the liquid-phase CH4 concentration multiplied by China’s volume of sewage discharge.
Based on these estimates, China’s WCSs emit approximately 114,957–525,337 t CH4/y and 2419 t N2O/y, equivalent to 3.86–15.35 Mt of CO2eq (CH4 and N2O) annually. In the Third National Communication on Climate Change of the People’s Republic of China [90], released in December 2018, China’s total greenhouse gas emissions from wastewater treatment in 2010 were 76 million tons of CO2eq. The estimated GHG emissions from WCSs in our study were approximately 5.08–20.20% of those from wastewater treatment and 0.04–0.16% of China’s total emissions. As CH4 emissions from wastewater treatment in 2010 were 2.194 Mt and N2O emissions were 96,000 t, it can be estimated that CH4 and N2O emissions from WCSs accounted for 5.2–23.9% and 2.5% of those from wastewater treatment, respectively. It is evident that GHG emissions from WCSs are substantial and warrant inclusion in national emission calculations, rather than being disregarded.
WCSs have significant potential for GHG reductions, through two methods. First, CH4 production can be reduced using chemical treatment. As detailed in Section 4.2, by carefully selecting appropriate CH4 control agents and dosing times, CH4 inhibition of 15–90% can be achieved. According to the preliminary calculations in this study, this could result in a potential reduction of 17,243–472,803 t CH4/y, equivalent to 0.78–21.55% of the CH4 emissions from wastewater treatment in 2010. Second, emissions can be reduced through the construction of “sponge cities”, an innovative urban development model in China, aimed at effectively managing urban rainwater [91]. Sponge cities utilize green infrastructure to establish a sustainable urban hydrological cycle, facilitating natural accumulation, infiltration, and purification of rainfall–runoff [92]. As a result, the volume of rainwater and pollutants entering the urban drainage system is significantly reduced, minimizing carbon emissions associated with WCS operation [83]. In newly constructed sponge cities, the separation of rainwater and wastewater can reduce the impact and pollution load of rainwater. Additionally, the sponge city transformation of older urban areas can also alleviate the impact and pollution load caused by rainwater on the combined sewer system. Su et al. demonstrated that sponge city drainage systems, compared to traditional urban drainage systems, can achieve an average carbon emission reduction of 49% in integrated urban drainage systems [83]. A case study conducted in Dongying, China estimated that a green-infrastructure-based urban drainage system reduced GHG emissions by 25.9–68.7% compared to traditional urban drainage systems [92].
There are various factors that affect GHG production in WCSs. However, correlations between them are difficult to establish because authors seldom report all parameters [46]. It is found that research on GHG emissions in WCSs primarily comes from laboratory-scale reactor studies [35,37,39,44,47,48,49,50,52,53,54]. There is limited research on the field data in WCSs [9,11,13,36,75] and these field studies are scattered and limited to only one or two cities in a few countries, such as China, the United States, Australia, and Thailand. This limited scope hampers an in-depth exploration of the potential variability in GHG emissions among different regions or within different types of urban areas.
Nonoptimal operation of urban WCSs may exacerbate GHG emissions. The production of CH4 and N2O in the pipeline system mainly occurs in the biofilm on the damp pipeline walls and sediment surface biofilm. High water level operation could increase the wet circumference of the pipeline, providing more area for microorganisms in WCSs and thus producing more CH4 and N2O. Pipeline leakage allows organic matter in the sewage to enter the soil and be utilized by anaerobic microbes in the soil to produce more CH4 and N2O, while infiltration can cause problems such as high water level operation. Rainwater and sewage cross-connections can cause high-organic wastewater to enter the stormwater pipeline, resulting in the formation of biofilms and subsequent release of CH4 and N2O.

8. Conclusions

Here, we presented a comprehensive overview of the current status of research on GHG emissions from WCSs. The majority of research has focused on CH4 emissions, while investigations into CO2 and N2O emissions in WCSs are scarce. Since GHGs in WCSs primarily originate from biofilms and sediments, controlling GHG emissions mainly involves the addition of chemical agents to inhibit biological activity in sediments and biofilms, thereby reducing GHG production. China’s WCSs emit approximately 3.86–15.35 Mt of CO2eq annually (equivalent to 5.1–20.2% of that of wastewater treatment). While the emissions of GHGs in WCSs are substantial and cannot be ignored, standardized methods for quantifying these emissions are currently lacking. Existing quantification methods in the literature mainly rely on field measurements or laboratory reactor studies to estimate overall emissions. Future research should focus on (1) the various factors that influence GHG production in WCSs to enhance our understanding of these factors and provide quantitative descriptions of their impacts, and (2) the accumulation of field data on GHGs in various regions and different types of urban areas to gain insights into the potential variability among them. Ultimately, the goal is to establish a comprehensive, accurate, and practical quantification method for GHGs in WCSs.
The results of this study confirm substantial GHG emissions from WCSs and significant potential for GHG reduction in WCSs. It is crucial for policymakers to take these findings into account when formulating relevant policies concerning urban development, WCS upgrades, and overall infrastructure planning. Such considerations will contribute to the realization of more sustainable urban planning and infrastructure development.

Author Contributions

Conceptualization, D.G. and P.Z.; methodology, D.G. and P.Z.; validation, formal analysis, investigation, data curation, D.G.; resources, N.C. and X.H.; writing—original draft preparation, D.G.; writing—review and editing, P.Z. and Y.L.; visualization, P.Z.; supervision, W.Z. and S.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology of The People’s Republic of China, project number 2021YFC3001400, and the Ministry of Housing and Urban–Rural Development of The People’s Republic of China, project number 2022-K-162.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Water 15 02512 g001 550
Figure 1. (a) Wastewater collection pipeline cross-section; (b) CH4 and N2O generation mechanisms in biofilm (VFAs—volatile fatty acids); (c) CH4 generation mechanisms in sediment.
Figure 1. (a) Wastewater collection pipeline cross-section; (b) CH4 and N2O generation mechanisms in biofilm (VFAs—volatile fatty acids); (c) CH4 generation mechanisms in sediment.
Water 15 02512 g001
Table
Table 1. Factors influencing CH4 production in the pipeline biofilm.
Table 1. Factors influencing CH4 production in the pipeline biofilm.
FactorEffects
TemperatureProduction of CH4 is higher in summer than in winter [8,13,36].
Flow velocityProduction of CH4 in biofilms in gravity pipelines has an optimum flow rate (water flow shear force ≈ 1.45 Pa) [37]. An increase in the flow rate increases the generation of CH4 in sediments when the flow rate is less than 0.31 m/s [38].
CODProduction of CH4 increases with an increase in sCOD [23,33,39,40].
BiomassProduction of CH4 in biofilms increases with an increase in biomass [37].
Pipeline typeProduction of CH4 in pressure pipelines is higher than in gravity pipelines [25,41,42].
HRT and A/VThe concentration of CH4 in solution is positively correlated with HRT and the A/V ratio of the pipeline biofilm [9,10,13,36].
NO3−Increasing the nitrate concentration inhibits the production of CH4 [35].
SO42−Production of CH4 decreases with an increase in sulfate concentration (>40 mg S/L) [23,33,40]; however, another study found that changes in sulfate concentration (5–30 mg S/L) did not significantly affect methane production [12,39].
pHThe optimal pH is about 7, and an increase in pH inhibits the production of CH4 [34].
DODO reduces CH4 production by affecting the anaerobic environment of microorganisms in sewage. However, studies have shown that DO is completely consumed above the surface biofilm and sediments, so anaerobic conditions are widespread throughout the biofilm and sediment, even if the DO in the water is high [38,43,44].
Table
Table 2. Factors influencing CH4 production in the sediment.
Table 2. Factors influencing CH4 production in the sediment.
FactorEffects
MicroorganismsMicroorganisms in sediments are the key producers of CH4; deactivated sediments have low production of CH4 [33].
WastewaterFresh sediments with no sewage can produce only a small amount of CH4, while the organic matter in sewage significantly increases sediment CH4 emissions [33].
BiomassThe rate of methane generation is relatively insensitive to the concentration of MA in the sediments. Lower concentrations of MA in sediments can lead to deeper penetration of the substrate, allowing the MA in deeper sediments to use the substrate to produce CH4, resulting in relatively small changes in the overall methane production rate [38].
Sediment typeThe location of the sediment, the age of a WCS, and the characteristics of the wastewater discharged into a WCS account for the production of sediments with differing physical and biological properties, affecting CH4 production rates [38,45].
Sediment depthCH4 production in sediments is a surficial process, mainly occurring at depths of 0–2 cm. Due to the limited permeability of fermentable COD, CH4 production in deeper layers of the sediment (2–3.5 cm) is very low, consistent with the distribution of MA in sediments with depth. Therefore, CH4 production is largely unaffected by the total sediment depth since substrate penetration into sediments is relatively shallow, up to a few millimeters [38].
Table
Table 3. Rough calculations of annual CH4 and N2O emissions from China’s wastewater collection systems.
Table 3. Rough calculations of annual CH4 and N2O emissions from China’s wastewater collection systems.
GHGSourceWCS LocationLiterature DataLiterature
Results		Emissions in
China
CH4[84]Daejeon, South Koreapipeline length, 1940 km1254 t CH4/y 1525,337 t CH4/y
[75]Xi`an, Chinapopulation,
8,705,600		7.96 t CH4/d495,232 t CH4/y
[11]Dekalb County, USApopulation,
600,000		56.86 t CH4/y140,623 t CH4/y
[88]New South Wales,
Australia 2		population77.47
gCH4/person·y		114,957 t CH4/y
[8]Queensland, Australia 3concentration4.50 mg/L303,806 t CH4/y
[89]Ontario, Canadaconcentration2.1–3.0 mg/L141,776–202,537 t CH4/y
N2O[31]New South Wales,
Australia 4		population1.63
gN2O/person·y		2419 t N2O/y
Notes: 1 The result given by [84] was 35,100 t CO2eq/y. A conversion coefficient of 28 was used to convert CO2 to CH4. 2 Calculation based on the CH4 data for three WWTPs. 3 Calculation based on the average CH4 concentration in sewage in an 1100 m long pressure pipe section. 4 Calculation based on the N2O data for the inlets of three WWTPs.
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Gu, D.; Liu, Y.; Zhao, W.; Qiu, S.; Cui, N.; Hu, X.; Zhao, P. Status of Research on Greenhouse Gas Emissions from Wastewater Collection Systems. Water 2023, 15, 2512. https://doi.org/10.3390/w15142512

AMA Style

Gu D, Liu Y, Zhao W, Qiu S, Cui N, Hu X, Zhao P. Status of Research on Greenhouse Gas Emissions from Wastewater Collection Systems. Water. 2023; 15(14):2512. https://doi.org/10.3390/w15142512

Chicago/Turabian Style

Gu, Dongmei, Yiwen Liu, Weigao Zhao, Shuntian Qiu, Nuo Cui, Xinyue Hu, and Peng Zhao. 2023. "Status of Research on Greenhouse Gas Emissions from Wastewater Collection Systems" Water 15, no. 14: 2512. https://doi.org/10.3390/w15142512

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