A new scheme for low-carbon recycling of urban and rural organic waste based on carbon footprint assessment: A case study in China
Introduction
During 2010-2019, the concentrations of greenhouse gases, such as CO2, CH4, and N2O in the atmosphere increased by 5%, 3%, and 2%, respectively. This resulted in human-induced warming of 0.9 to 1.3 °C relative to the pre-industrial era, posing great challenges to the 1.5 °C warming target set by the Paris Agreement1,2. China accounts for 30% of global carbon emissions, and its share has increased in recent years. To counteract global warming, the Chinese government proposed at the 75th session of the UN General Assembly the ambitious goal of “striving to peak CO2 emissions by 2030 and achieving carbon neutrality by 2060”3. Chinese solid waste industry emits more than 76.6 million tons of carbon, accounting for about 7% of the country’s carbon emissions4,5. Urban-rural organic waste (UROSW), which accounts for more than 60% of total solid waste6, has a high water content, rich in organic carbon and nutrients such as nitrogen and phosphorus7,8. Under current disposal models dominated by incineration and landfilling (Fig. 1), they produce large amounts of carbon dioxide, methane and nitrous oxide9. This makes it a major contributor to carbon emissions. The International Solid Waste Association (ISWA) estimates that recycling solid waste to achieve carbon offsets can reduce global carbon emissions by 20%10. Therefore, it is of great significance to realize a transition from UROSW disposal involving high carbon emissions to low carbon utilization to achieve carbon neutrality11. Unfortunately, the path to this transition is not yet clear.
The transformation of organic waste from harmless disposal to resource utilization.
Some resource utilization measures (Fig. 1) have been taken to try to reduce carbon emissions, including dehydration and drying, anaerobic digestion, and aerobic composting12,13,14. The calorific value of the organic waste after dehydration and drying is higher, which can reach more than 8000 kJ kg−1, and can be prepared into biomass fuel by the hot pressing method instead of coal15,16. Anaerobic digestion can decompose the biodegradable organic matter in organic waste into biogas mainly composed of CH4 and CO217. Biogas is a clean energy source that can offset the use of fossil fuels18. Aerobic compost can humify organic waste to form organic fertilizer through microbial reaction19. Using organic fertilizer instead of chemical fertilizer in farmland can achieve carbon sequestration20. However, the effect of treating and utilizing various organic wastes separately is poor, and the energy consumption is high, which may not be able to effectively reduce emissions21. Therefore, the collaborative treatment and utilization of organic waste has been widely studied. Some areas have formed an organic solid waste recycling system with “sludge drying incineration – garden waste resource treatment – combined heat and power generation – kitchen waste anaerobic fermentation – biogas utilization” as the core22. The system improves waste utilization efficiency, reduces energy consumption and greatly reduces carbon emissions by realizing the cascade and recycling of matter and energy between different organic waste treatment projects22. In addition, in view of the limited degradation capacity of anaerobic fermentation and the difficulty in disposing of secondary products, some cities have developed a treatment and utilization mode of anaerobic fermentation and aerobic compost21. In this model, biogas produced by anaerobic digestion of organic waste is used for heating of the project system through a boiler to produce steam, and biogas residue and solid residue are dehydrated by thin layer dryer and then subjected to aerobic fermentation to produce organic fertilizer for garden greening soil21. This enables the full chain utilization of organic waste, while also reducing carbon emissions throughout the life cycle23.
In recent years, aiming at the problems of the long time required for aerobic fermentation of single organic waste, poor fertilizer efficiency, and high greenhouse gas emission, multi-material collaborative aerobic composting measures have been developed24,25,26. For instance, the synergistic composting of ST and BR reduces the maturation time by 30 days27. The combination of FA, ST, and algal sludge in composting can decrease microcystin content in the pile by 99.5%28. The composite composting of GW and FA reduces nitrogen loss by approximately 57%, N2O emissions by 13.2%, and CH4 emissions by 25.7%, promoting the humification of organic matter29. In addition, biochar production from organic waste has great potential in reducing greenhouse gas emissions and enhancing organic carbon sequestration30,31. This is because pyrolysis can fix part of the carbon in the organic waste in the biochar32. This carbon can remain in the soil for more than 100 years33. The addition of biochar in aerobic fermentation systems can reduce CH4 emissions by 20.8%, N2O emissions by 50.3%, and NH3 emissions by 16.2%34,35 due to the porous structure and surface functional groups in biochar, which can improve the retention of nutrients and promote changes in the microbial community in reactors36. In addition, biochar can play a huge role in sludge dewatering. It can make the sludge flocculants expand and aggregate rapidly, form large particle settlements, and play the role of skeleton structure when the sludge is compressed37. It can also reduce the specific filtration resistance of activated sludge through hydrophobicity, and effectively remove tryptophan protein and humus in soluble EPS and loosely bound EPS38. The calorific value of sludge cake can be increased and waste recycling can be realized by using biochar instead of lime as a skeleton auxiliary for sludge dewatering15. The pyrolysis of straw to biochar also generates bio-gas and bio-oil as co-products39,40, which can be used to generate electric power through an integrated pyrolysis and electricity generation system. The energy generated by the system can be used to replace fossil fuels, and therefore reduce GHG emissions41.
However, at the regional scale, for a variety of urban and rural organic wastes, the resource utilization path aiming at optimal carbon emission reduction is still unclear. Therefore, we predicted the carbon emissions of various types of urban organic waste and rural organic waste under separate resource utilization scenarios. Based on the optimal scheme, a model of urban-rural synergistic utilization based on fertilizer and biochar was constructed. Then, its environmental and economic benefits were evaluated from a full lifecycle perspective, hoping to explore the direction for low-carbon utilization of organic waste.
Results
High yield of urban-rural organic waste
Figure 2a shows that China’s MOSW is mainly distributed in the eastern coastal areas (SD, JS, ZJ, GD) and SC five provinces. Each of them has more than 4 million tons, together accounting for 41% of the national total (Table S1). The output of UOSW is nearly three times that of MOSW, with the highest production in NM, HEN, SC, HB, which have developed animal husbandry, each producing over 100 million tons (Fig. 2b). Since 2010, the production of both MOSW and UOSW has increased in line with population growth, reaching 87.41 million tons and 2.42 billion tons, which are 1.03 and 1.73 times higher than those in the EU, respectively42, by 2021 (Fig. 2c and d). This poses a serious threat to the environment43. The MOSW is mainly municipal sludge, which reaches 43.34 million tons; UOSW is dominated by FA, accounting for more than 80%. Based on the population data from 2010 to 2021, a Logistic population model was utilized to forecast the population size in China for the next nine years, with a goodness of fit (R2 of 0.9636) for the prediction model. Building upon this, we simplified the forecasting process by using a per capita waste generation coefficient (assuming a constant coefficient) to predict the future production of UROSW. The results show that the growth rate of UROSW slowed down after 2021 and the output of MOSW will reach 90.67 million tons by 2030 with a growth rate of 3.7% (Fig. 2c). Meanwhile, ST is about 457 million tons and FA is 2.1 billion tons with an increase of 4% (Fig. 2d). It can be predicted that the output of organic waste will not change dramatically in the future, but the huge base will also bring severe challenges to the “Dual Carbon Goal”.
a: Municipal organic solid waste (MOSW); b: Rural organic solid waste (UOSW); c: Forecasted generation of MOSW; d: Forecasted generation of UOSW.
Carbon emissions in the baseline scenario
Figure 3a shows MFA results for elemental carbon in baseline scenario (BS). China’s UROSW contains a total of about 130 million tons of C, of which 43% is buried in landfills and wasted, 31% is fixed in the soil, and 26% is emitted into the atmosphere in the form of CO2 or CH4 (Text S1). This suggests that the carbon sequestration and utilization rate is low. Figure 3c shows that in BS, the indirect carbon emission of UROSW plays an important role, and more than 87% is caused by the consumption of electric energy, heat energy and chemical agents. Among them, the heat consumption emits 90.8 Mt CO2e yr−1(Fig. 3c), while the carbon compensation value in this scenario is only 22.4 Mt CO2e yr−1 (Fig. 3d), which is mainly brought about by incineration power generation (70%). Therefore, it is very important to save energy reduce consumption, and improve resource utilization. These factors contribute to China’s UROSW’s total carbon emissions of approximately 690 Mt CO2e yr−1, accounting for 6% of China’s total carbon emissions in 2021, which is nearly five times of the solid waste industry during the peak carbon period in the United States44. The main greenhouse gas is methane, which is the result of large amounts of organic waste being dumped in sanitary landfills. At the provincial level, UROSW’s carbon emissions in most provinces are around 20 Mt CO2e yr−1, while Sichuan, Henan, Hunan, Shandong, Yunnan, and Inner Mongolia are all above 40 Mt CO2e yr−1 due to their high UROSW production (Fig. 4a and b). Such large carbon emissions pose a serious obstacle to achieve carbon neutrality.
a Carbon flow in the BS scenario; b Carbon flow in URIRP scenario; c Indirect carbon emissions due to material and energy consumption in three scenarios, EP stands for electricity, TE stands for thermal energy, Oil stands for diesel oil, MT stands for materials and chemicals, ED represents terminal pollutant treatment; d Carbon offsets brought about by products in three scenarios, Gas represents biogas, Oil represents bio oil, Ofe stands for organic fertilizer.
a, b: BS; c, d: MS2 and CS3, respectively; e, f: URIRP.
The carbon reduction potential of URIRP
Using Jiangsu Province as an example, we explored the pros and cons of mitigation scenarios. Text S2 and Fig. S5 show that among the two mitigation scenarios of MOSW, MS2 has the best carbon emission reduction effect, reaching 78%, which is 24 percentage points higher than MS1. Among the three mitigation scenarios in UOSW, CS3 has the lowest carbon emission, with a carbon emission reduction rate of 18%, which is 1.2 percentage points higher than CS1 and 1.6 percentage points higher than CS2. Under the optimal scheme MS2 and CS3, the consumption of electric energy, heat energy, and chemicals is reduced, and the output of biogas and bio-oil is increased, which realizes energy saving and emission reduction (Fig. 3c and d). Among it, synergetic aerobic composting of UOSW greatly reduces the carbon intensity of DM and BR (−8.5 Mt CO2e yr−1), as well as the heat energy consumption of DM, BR and GW, and it can also reduce CH4 emissions compared with sanitary landfills, which is consistent with the results of previous studies45. The use of sludge biochar reduces the amount of SS dewatering agents used, thereby reducing indirect carbon emissions (−10 Mt CO2e yr−1) (Fig. 3c). The tar and biogas generated by ST pyrolysis also increase the carbon compensation by 14 Mt CO2e yr−1 (Fig. 3d). These factors reduce the total carbon emissions of the whole country to 320 Mt CO2e yr−1, among which the carbon emissions of high-carbon-emission areas such as Sichuan, Henan, Hunan and Inner Mongolia are below 30 Mt CO2e yr−1, and the carbon emissions of other provinces are below 20 Mt CO2e yr−1 (Fig. 4c and d). The main greenhouse gas becomes CO2.
Analysis of the MFA for C in Fig. 3b shows that compared with BS, although the pyrolysis of ST and the incineration of bio-oil and biogas in URIRP increased the emission of 7.14 million tons of C, UOSW and GW were all used to cooperate with aerobic fermentation to produce organic fertilizer, and 93.35 million tons of carbon was fixed in the soil. This resulted in no reduction in carbon emissions in the URIRP scenario, but a 130% increase in carbon going into the soil relative to BS. In addition, compared to MS2 and CS3, the URIRP further reduces GW power and pharmaceutical carbon emissions (-0.8 Mt CO2e yr−1) through the combined composting of GW and UOSW and increases the carbon compensation amount of organic fertilizer (Fig. 3c and d). In the end, the main indirect carbon emissions of the URIRP are reduced by 87% compared with BS and 5% compared with MS2 and CS3, and the main carbon compensation amount is increased by 48% compared with BS1 and 12% compared with MS2 and CS3. At the provincial level, the carbon emissions of Sichuan, Henan and Inner Mongolia were reduced to 23 Mt CO2e yr−1, 25 Mt CO2e yr−1 and 26 Mt CO2e yr−1, respectively, and the total UROSW was further reduced to 282 Mt CO2e yr−1 (Fig. 4e and f). Therefore, the URIRP has advantages over MS2 and CS3, and a large-scale implementation of the URIRP can further reduce greenhouse gas emissions.
Environmental and economic benefits of URIRP
Implementing a long-term URIRP could be effective in mitigating global warming. Figure 5a shows that the carbon reduction by 2030 could offset 6-8% of carbon emissions from the power sector, 47.5% of carbon emissions from the metal processing industry, or 44% of carbon emissions from agriculture. The URIRP could also mitigate air pollution, contributing to China’s short-term air quality goals by 2030. The URIRP would reduce annual particulate, SO2, and NOX emissions by 21% (12 Gg), 27% (14 Gg), and 2% (840 Gg), respectively, as compared to BS1 (Fig. 5b). These reductions in air pollutants can be attributed to the following: (1) reduced direct incineration of GW, BR, and DM; (2) avoidance of sanitary landfilling of UROSW, and use of carbon-based fertilizer to reduce the demand for chemical fertilizer; and (3) production and utilization of dry SS, biogas, and bio-oil to reduce the use of coal and diesel. In addition, URIRP program can increase the content of organic matter in cultivated land in China by 0.25‰, and the recovery rates of N and P can be increased by 23% and 0.1%, respectively, through returning organic fertilizer to the field (Table S2). By reducing direct incineration, co-composting, and utilization of ash residue for building, the secondary solid waste production of the URIRP was reduced by 33% (840,000 tons). LCA showed that the processes in URIRP have little impact on the environment except for CNS and NS (Fig. 5b and c). From the perspective of WC(AE), LU, and OD, the COF even has a beneficial impact (Text S3). The endpoint damage assessment showed that the URIRP reduced human health damage, ecosystem damage, and resource consumption losses by 17.6% (9.9 GPt), 17.9% (0.6 GPt), and 19.8% (0.2 GPt) compared to BS1 and decreased them by 3.4% (1.7 GPt), 4% (0.1 GPt), and 7% (0.07 GPt) compared to MS2 and CS3.
a: Carbon emissions reduction of URIRP; b: Life cycle assessment and pollutants reduction rates of URIRP; c: Environmental and economic benefits of URIRP.
An economic analysis of the URIRP was conducted and compared with BS (Table S3). In BS, it would cost $25.42 billion to treat UROSW nationwide. With the exception of the $597 million profit from the bio-oil and biogas in KW treatment, the other UROSW requires government subsidies. Secondly, large amounts of FA were landfilled and inefficient composted bringing huge costs. In addition, SS treatment costs reached $1.8 billion, mainly due to dewatering agents and electricity. In the URIRP, the utilization of UROSW can lead to a profit of $8.78 billion. Among them, ASR and sludge char as conditioners reduced the cost of SS dewatering by 70%. The dried SS generated a profit of $51 million from incineration to generate electricity. Second, in the URIRP, the profit from KW output was increased ($13 million) by producing carbon sources and nutritive soil through the use of biogas slurry and KW impurities, respectively. In addition, the coupling of synergetic composting and ST carbonization can produce high-value biochar-based fertilizer ($230·t−1) and pyrolysis oil ($570·t−1). The pyrolysis gas produced can essentially meet the heat energy required for carbonization, which limits the carbonization cost to approximately $25·t−1. When the cost of composting is less than $15·t−1, the income from composting and carbonization is greater than the cost, resulting in a profit of $14.4 billion (Table S4). Compared with the BS, the URIRP shows great economic benefits and sustainable development potential.
Discussion
Potential advantages of the URIRP
Although UROSW is a pollutant and an important source of carbon emissions in China, it has resource attributes and can be converted into bio-fertilizers and bio-energy through resource utilization technologies. Our analysis indicates that URIRP achieves a low-carbon (reducing over 50%) and high-value utilization (increased 3.5 times) of organic waste by reducing incineration and landfilling, leveraging the synergistic treatment of MOSW and UOSW, and coupling aerobic composting, anaerobic digestion, and carbonization processes. Although the energy utilization model based on anaerobic digestion, supplemented by aerobic composting and the production of fuel rods studied by Portuguese scholars can achieve zero carbon emissions of organic waste management, it is not economically sustainable, and the treatment scale is limited, which is not suitable for developing countries like China, which produces a large amount of waste46. In addition, the large-scale implementation of the URIRP can improve rural environmental pollution control via the construction of industrial parks for the integrated utilization of urban and rural organic waste to promote the integrated development of urban-rural ecological construction. At the same time, the promotion of the URIRP can increase the recycling rate of UROSW by 21% and enhance the recovery of nitrogen and phosphorus, promoting the development of a circular economy in China. The production of biogas, biomass fuel, and bio-oil can reduce coal consumption by 8.4 million tons and contribute to improved air quality. Second, China’s vast farmland can absorb a large number of biochar-based organic fertilizers in URIRP, which can not only replace part of the use of chemical fertilizers, reduce TN non-point source pollution of about 520,000 tons, but also improve the soil quality of cultivated land.
The field experiments (Table S5 and Fig. S1) showed that compared with chemical fertilizers, the application of organic fertilizer can improve the quality of rice, increasing the amylose and crude protein contents of rice (35% and 10% improvement, respectively) and increasing the nutrient content of rice cultivation soil after planting (N increased by 37.5%, P increased by 16.7%). The promotion and implementation of the URIRP can increase the organic matter content of cultivated topsoil in China by 2.5‰ and the organic carbon content by 39%47, equivalent to 7.66 billion tons of CO2 fixed in the soil. Additionally, improvements to soil quality can effectively control soil-borne diseases48 and increase crop yield by 27%49. Secondly, the conversion of UROSW into high-value commodities such as biochar-based fertilizer, bio-oil, and biogas improves the revenue capacity of solid waste treatment enterprises, ensuring that the period implementation of the URIRP is economically sustainable. If the long-term implementation of the URIRP is considered to improve soil, atmosphere and water environments and promote the positive cycling of energy and materials between urban and rural environments, the social benefits introduced by the URIRP scenario will be further improved. It can be seen that URIRP can achieve sustainable management of organic waste, with significant environmental and economic benefits, providing a reference solution for low-carbon circular utilization of waste in the Taihu Lake region, China, and even the world.
Suggestions to promote the URIRP
The implementation of the project on a national scale requires the following measures. First, we need to overcome the decentralized distribution of UROSW and the seasonal supply of ST (DM) in the URIRP, forming a classified collection, centralized storage, and unified transportation system for urban waste and agricultural and forestry waste. Compared to developed countries, China currently has a lower capacity for waste classification and collection (e.g., organic waste separation rate lower than 40%)50. It is necessary to establish a construction layout consisting of “one organic waste decentralized collection station per village, one organic waste centralized collection station per town, and one organic waste utilization center per city (county).” Second, it is necessary to improve the mechanism of solid waste charging and treatment and systematically formulate treatment prices according to costs. We explored the establishment of financial subsidies for organic waste resource products and encourage local governments to provide subsidies on the basis that the price of products is determined by the market. Third, for normative implementation of URIRP, it is necessary to formulate the classification and management norms for solid waste in terms of source management, the technical guidelines for multisource solid waste collaborative composting, the management methods for straw collection, storage and transportation from the field, and the technical guidelines for pyrolysis coupled with power generation, eta. Fourth, we should continue scientific and technological innovation, develop biomass pyrolysis-coupled power generation technology, improve energy efficiency and reduce the price of biochar-based fertilizer to promote the enthusiasm of farmers to use. Fifth, it is necessary to build a policy support system, incorporate the organic waste treatment center into the ecological and environmental infrastructure construction planning, give clear policy support in land use, environmental assessment, approval and other aspects, and manage the relevant electricity and land use according to agricultural production projects.
Furthermore, on the path of international promotion of URIRP, other regions can adjust the material ratios and process settings of each treatment unit in URIRP according to their own circumstances, in order to achieve the optimal operational efficiency that suits their country’s needs. The main focus of this study is to provide a perspective, rather than a rigid solution, on how to achieve synergistic circular utilization of UROW. It aims to offer valuable insights for other developing countries with similar circumstances to China.
Supplement to the URIRP
Although the URIRP can help achieve carbon utilization across China via UROSW recycling, the carbon emissions in the Inner Mongolia, Sichuan and Henan regions remain above 20 Mt CO2e yr−1. The main reason is that FA production in these areas is enormous, resulting in more CH4 and NOX produced through aerobic compost. Therefore, these provinces should develop closed and efficient aerobic fermentation equipment to optimize process parameters and develop efficient fermentative bacterial agents based on functional microorganisms to further reduce CH4 and N2O emissions45. Our study shows that the carbon emissions of UROSW increase with an increase in population. Further reductions in carbon emissions require reductions in waste production at the source and concerted efforts by other sectors to reduce waste production (e.g., reductions in carbon emission factors from power, various energy sources and materials). Therefore, together with the URIRP, methods to reduce food waste and sewage discharge as well as improve lake ecology to reduce cyanobacterial production deserve further research. This could be where the additional potential for carbon reduction lies. Due to the limited examination of regions beyond Jiangsu Province in China, the limitations of this study are inevitable. Theoretically, implementing URIRP nationwide could reduce carbon emissions by 58% during the UROSW management process. However, due to regional economic and technological disparities, the actual potential for carbon emission reduction may be higher in developed regions and lower in underdeveloped regions. Therefore, we recommend that future studies evaluate the URIRP at more local scales (cities and counties) to refine our estimates and explore the effectiveness of the URIRP in other parts of the globe.
Conclusions
UROSW production in China has exceeded 2.5 billion tons, generating 690 Mt CO2e yr−1 GHG emissions. A separate urban and rural treatment mode (MS2 and CS3) can reduce the carbon emissions of UROSW by 53.6%. The proposed URIRP can further reduce carbon emissions to 283 Mt CO2e yr−1 through synergistic and integrated utilization of MOSW and UOSW. Based on projected production of UROSW, URIRP would reduce emissions of 410 Mt CO2e yr−1 by 2030, offsetting 6% of emissions in power sector or 46.1% of emissions in agricultural sector. Importantly, a large-scale implementation of URIRP can facilitate resource recovery and utilization rate of UROSW by 21% with alleviated environmental pollution, thereby reducing annual emissions of particulate matter, SO2, NXO, and solid waste by 21%, 27%, 2%, and 33%, respectively. Application of biochar-based organic fertilizer can improve the quality of cultivated land and enhance the organic matter content of soil by 0.25‰, leading to improved yield and quality of crops. The overall recovery rate of nitrogen and phosphorus was increased by more than 20%. Moreover, the economic analysis indicates that URIRP would achieve an annual benefit of $8.8 billion by substituting the existing high cost of organic solid waste treatment. Therefore, URIRP can realize sustainable management of UROSW to accelerate carbon neutrality, and provide a reference scheme for low-carbon recycling of organic waste for other developing countries with similar conditions as China.
Methods
Baseline and mitigation scenarios
The UROSW studied in this paper included municipal sludge (SS), kitchen waste (FW), garden waste (GW), wheat and rice straw (ST), faeces of livestock animals (FA), biogas residue (BR) and cyanophyllous algal mud (DM). These are typical urban and rural organic wastes in China, covering the vast majority of China’s organic wastes51,52. The Taihu Lake region (Fig. 6) is one of China’s economically developed areas and serves as the first demonstration zone for integrated urban-rural organic waste management established by the government. Many waste management policies and initiatives in China are piloted in this region before nationwide implementation once the experience becomes mature. Therefore, using the current state of UROSW management in this region as a benchmark scenario is crucial for studying the future patterns of organic waste management and utilization in China. According to “The research report on the construction of urban and rural organic waste treatment and utilization demonstration zone around Taihu Lake in Jiangsu Province”, the current situation is as follows (Fig. 7). SS is basically dehydrated to a moisture content of 80% and then burned with coal. KW is mainly treated by anaerobic digestion, and the secondary product biogas slurry is treated as sewage. GW is sent to waste incineration plants for direct incineration, and the utilization rate of heat energy is low. 92.4% of ST is returned to the fields, causing serious soil burden and diseases of pests. BR and DM are mainly disposed of by incineration after drying, which has high energy consumption. The FA is dominated by aerobic compost, with another 25% going to landfill.
Geographical location of the study area.
a: MOSW; b: UOSW.
Based on BS, two mitigation strategies (MS1, MS2) were set up for urban organic solid waste (MOSW), and three mitigation strategies (CS1, CS2, CS3) were set up for rural organic solid waste (UOSW). These strategies are based on more advanced and potential technologies already in demonstration engineering, and are based on improving the recycling level of UROSW over time, technological advances and relevant government requirements. As shown in Fig. 7, in MS1, drying measures were taken to increase the calorific value of SS and GW. The organic residue after KW screening was used for feeding actinomycetes. The byproduct biogas slurry was sent to a sewage treatment plant, and the biogas was purified and fed into a natural gas pipeline network. In MS2, the biochar produced by sludge pyrolysis is used to condition the sludge so that the water content of the sludge after press filtration is reduced to 60%15, and GW was transformed into aerobic composting. Both MS1 and MS2 are processed and utilized in the vein industrial park. UOSW was synergetic and treated with aerobic fermentation to prepare organic fertilizer in CS1. In CS2, the field separation rate of ST was increased to 8%, and the added ST was used to produce biochar. In CS3, biochar-based fertilizer was prepared by mixing biochar with GW, ST, FA, BR, DM. The specific scenario elements can be found in Text S4.
In order to make full use of the characteristics of various wastes and realize the synergetic treatment and utilization of UROSW, we built URIRP as shown in Fig. 8 (Text S5) according to the research conclusions of Text S6. The water content of press-filtered SS can be reduced to 55% after conditioning of SS biochar (pyrolysis temperature: 500 °C) and aluminum salt enhancers. The dewatered SS was dried to 20% moisture content (8300 kJ·kg−1) and then sent to thermal power plants to replace coal in power generation53. KW underwent anaerobic fermentation after oil removal, CH4 (+20%) production was increased via the addition of conductive materials (magnetite, etc.), purified biogas was used as natural gas, and biogas slurry (COD of 5000 mg·L−1) was used as a carbon source for the sewage treatment plant. A part of the ST was pyrolyzed (800 °C) into biochar, which was combined with BR, GW, FA, DM, and the remaining ST for aerobic fermentation to obtain biochar-based organic fertilizer (biochar: 10%) for use on farmland. The tar and dry distillation gas produced were transported to thermal power plants for incineration to recycle heat. The electric energy from the power plant was supplied to the organic solid waste treatment plant in the park, and the heat energy was sent to the sludge drying plant. The slag generated was used as building material54. This mode realizes the combined treatment of organic solid waste and the recycling of electricity, gas, and heat and turns waste into resources. The specific scenario elements can be found in Text S5.
Conceptual model of synergetic low-carbon recycling of UROSW (URIRP).
Yield and forecast for UROSW
Provincial spatial estimation of UROSW statistics in Chinese cities was carried out. The SS data are derived from internal government statistical reports, and the KW estimates are based on Eq. 1 in the “FW Processing Technical Specification (CJJ-184-2012).” GW is estimated by multiplying the area of urban parks by the coefficient of pollution production. The ST and FA data are from the National Bureau of Statistics. The BR data are converted according to the KW output. DM data comes from blue-green algae refloatation stations around major lakes. All data are presented in Table S6 and Table S7. UROSW is mainly produced from human daily activities. Therefore, we use population number as an intermediate parameter to predict UROSW production. China has promised to reach peak carbon emissions in 2030, so this study uses the Logistic Population Model (Eqs. 2 and 3) to predict the population from 2024 to 203055,56. The population forecast was based on data from the China Statistical Yearbook for the years 2010 to 2021 (Table S8). The predicted parameters (N0, Nm, r) and goodness of fit (R2) are shown in Table S9. The production of UROSW was estimated based on historical empirical coefficients between population and organic solid waste generation, assuming that the coefficients remain constant over time.
where Mc is the daily yield of FW in an urban area, kg d−1; R refers to the residential population of the city; and m is the base of per capita FW yield, kg p−1 d−1. Nt is the residential population in year t; NM is the maximum residential population that the natural environment can accommodate; N0 is the resident population corresponding to the initial time t0; and r is the ratio of the growth of the residential population to the total population per unit time. The parameters in the model were calculated in Matlab.
Methods of carbon footprint analysis
Figure 7 and S2 describe the boundary for material flow analysis (MFA) and carbon emissions. We draw on the method of Graedel57 to consider the whole flow of C from collection to resource utilization in UROSW. The model involves treatment and utilization of KW (KWD), dewatering, drying, and incineration of SS (SDI), coproduction of biochar-based fertilizer (COF) with UROSW, aerobic composting of FA and biochar (CNS), aerobic composting of FA alone (NS) and return of straw to fields (STT). Common emission factor methods are used to determine material flow58. For the sake of study, only the main materials in the process are considered, and the distribution of UROSW is assumed to be annular with respect to the utilization point (Text S7). When biochar, straw, and organic fertilizer are returned to fields, we assume that carbon, nitrogen, and phosphorus enter the soil. The carbon, nitrogen, and phosphorus that are incinerated and disposed of in landfills are considered waste. The balance formula is shown in Eq. S1.
There are three types of carbon accounting processes59. The first category is indirect carbon emissions from resource or energy investment and secondary pollutant treatment, including electricity, heat investment, and purchases of various chemical agents. The calculation method involves the multiplication of their application volume by their respective carbon emission coefficients (Table S10). The second type is the uncontrolled emissions of greenhouse gases generated during incineration, anaerobic fermentation, and aerobic composting processes, calculated using the mass conservation method and emission factor method (Text S8). The third category is carbon compensation through the use of UROW resources, mainly including incineration power generation, organic fertilizer replacement fertilizer, and bio-oil recovery. Conduct sensitivity analysis on the key parameters used in the carbon footprint to determine the impact of changes in these parameters on the final carbon emissions.
Sensitivity analysis of material flow
Conduct sensitivity analysis on the key parameters used in the carbon footprint to determine the impact of changes in these parameters on the final carbon emissions. The impact of major parameter changes on the final carbon footprint is shown in Table S11. The results of sensitivity analysis showed that the parameters that had the greatest influence on the final carbon emission were the emission factors of CO2 and CH4 in aerobic compost. Due to changes in these two parameters, the final greenhouse gas emissions can increase by 126% and 37%, respectively. This is because composting is the main way UROSW is treated and utilized. Due to the differences in working conditions, the emission of CO2 and CH4 in the composting process fluctuated by more than 150%60. However, changes in these parameters did not affect the final conclusion, and the carbon emissions of URIRP were always much lower than BS, MS2 and CS3. The reason is that the carbon loss rate consumption of composting cannot be higher than that of incineration, and the product quality of collaborative composting is higher than that of single-material composting, which can effectively replace chemical fertilizers to achieve carbon compensation24. Additionally, the efficiency of anaerobic digestion methane production has a significant impact on carbon compensation, which indicates that improving the ability of anaerobic fermentation methane production is an effective measure for carbon emission reduction.
Assessment of multiple attributes
Fig. S3 shows the methodological framework for multiattribute assessment, including life cycle assessment (LCA), environmental assessment, and economic evaluation. The characteristics of UROSW and the basic data for analysis are shown in Table S12, and Table S14. The LCA was carried out in SimaPro using the ReCiPe 2016 methodology (Text S9). The method includes 22 midpoint impact classes and 3 endpoint damage classes (Fig. S4). The environmental benefit analysis is based on MFA and LCA results, including reductions in atmospheric pollutants61, greenhouse gases and solid waste, as well as the recovery of nitrogen and phosphorus and the improvement of soil quality through organic fertilizer instead of chemical fertilizer application to fields. Economic performance was analyzed based on the economic indicators in Table S4. We calculated the cost of UROW processing and estimated the profits from the final product based on the investigation (the construction and maintenance costs of the plants are not considered). The treatment cost of UROSW is based on the amount that the pollution-producing agency needs to pay to the treatment agency.
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