Prediction of municipal solid waste treatment and disposal in high cold and high altitude area based on system dynamics: a case study of Lhasa

At present, the high cold and high altitude areas of China continue to face challenges in achieving high rates of resource utilization for the municipal solid waste (MSW). In this paper, for the first time, a prediction method based on system dynamics (SD) for the treatment and disposal of municipal solid waste in high cold and high altitude areas is proposed and applied to Lhasa City. The model simulate and predict the proportion of treatment methods (including landfill, incineration, kitchen waste disposal, recycling) and the rate of resource utilization (other disposal methods except landfill) of municipal solid waste under Natural Tendency and regulation schemes (Strengthened Municipal Solid Waste Classification Scheme and Municipal Solid Waste Classification and Treatment Scheme). The results show that strengthening municipal solid waste classification and treatment can significantly enhance the resource utilization rate of municipal solid waste in Lhasa, with an expected rate of 100% by 2026. This outcome has the potential to promote sustainable and healthy development of society, economy and environment of a city.


Introduction
In recent years, with economic development, population growth and the improvement of people's consumption level, the production of municipal solid waste in China is growing rapidly (Wei 2018).In 2020, the removal volume of MSW in China reached 235.117 million tons (National Bureau of Statistics of China 2021b), an increase of 1.49 times compared with 158.048 million tons in 2010 (National Bureau of Statistics of China 2011a).A large amount of MSW, if not treated in time, will cause a series of ecological and environmental problems (Pang et al 2004, Yang and Wang 2008, Wang 2013, Duan 2014, Xie 2016), such as destroying the soil ecosystem and eventually leading to poor vegetation growth, causing water pollution, air pollution, threatening human health.
In order to reduce the environmental pollution caused by the improper treatment of MSW and promote the sustainable development of society, based on the principles of reduction, harmless and resources, the main treatment and disposal methods of MSW in China include sanitary landfill, composting and incineration (Zhang et al 2011, Tian et al 2013, Zhang and Huang 2021).Landfill is the oldest MSW treatment method with mature technology, simple management and low operating cost.But landfill takes up a lot of land, and the usage proportion is gradually declining (Hu et al 2022).Composting is a treatment method that slowly oxidizes degradable organic matter in MSW by controlling aerobic or anaerobic conditions and using microorganisms existing in nature, and finally generates CO 2 , H 2 O and residual solid matter.The residual solid material can be used to make fertilizer as a soil amendment.The method of composting can realize the harmless and resource utilization of MSW (Wang et al 2020).Incineration refers to the pyrolysis and oxidation of combustible components in MSW through high temperature in the incinerator, while generating heat energy for power generation.Incineration can reduce capacity by 80% ∼ 90% of garbage (Lu 2013) and achieve the purposes of reduction, harmless and resource utilization of MSW.
Many scholars at home and abroad have studied the evaluation of MSW treatment and disposal technology (Manfredi and Christensen 2009, Ji et al 2011, Xie et al 2015, Chen et al 2018, Cao et al 2019) and optimization of treatment programs (Zhou et al 2012, Minoglou and Komilis 2013, Huang et al 2018, Li et al 2021).However, these studies did not involve the improvement and dynamic prediction of the harmless treatment rate, and resource utilization rate of MSW, and did not provide theoretical and data support for policy makers.The System Dynamic method is a 'policy laboratory', which pays attention to the interconnected feedback relationship between variables and the internal structure of a system and has no strict requirements on the accuracy of parameters.It is a kind of simulation method combining qualitative and quantitative analysis, which is suitable for complex, dynamic and nonlinear systems.At present, SD has been widely used in the field of MSW, such as predicting the amount of MSW generation (Xi et al 2008, Kollikkathara et al 2010, Tang et al 2019), explaining the production process and recycling process of domestic waste (Zulkipli et al 2016), evaluating MSW management policies and measures (Bi and Wang 2008, Sukholthaman and Sharp 2016, Giannis et al 2017).Domestic scholars started to use System Dynamics (SD) to carry out research on MSW management late, and the research is few, and mainly concentrated in the economically developed cities.The research on the high cold and high altitude areas is blank.
The high cold and high altitude areas are sensitive to climate change and the anti-interference ability of ecosystem is weak.They are typical ecologically fragile areas and serve as ecological security barriers for China and the whole world.As a typical high cold and high altitude city, Lhasa has a rapid increase in the production of MSW in recent years with the rapid advancement of urbanization, leading to great pressure of treatment and environmental burden.Based on the method and principle of SD, this study uses VENSIM software to build a MSW treatment and disposal system model in the high cold and high altitude area, taking Lhasa City as an example.By adjusting the relevant parameters to achieve the setting of schemes, and then predict the development trend of different schemes of MSW treatment and disposal.Finally, put forward optimization suggestions for policy makers according to the simulation results, and provide theoretical support and practical guidance for the treatment and disposal of MSW in high cold and high altitude areas.
The rest of the paper is divided into three sections.Section 2 focuses on the construction of Lhasa MSW treatment and disposal system model, including the overview of the research area, the introduction to the data sources, types and process of establishing mathematical relationships of the main variables.Section 3 introduces the setting of control schemes and the dynamic simulation results and analysis of different schemes.The final section provides a concise conclusion to the paper.

Construction of Lhasa MSW treatment and disposal system model
In this paper, the system dynamics simulation model of Lhasa municipal solid waste treatment and disposal is constructed.The simulation interval spans from 2010 to 2030, in which 2010-2020 is the simulation period, and 2021-2030 is the dynamic simulation period.Based on the historical data of Lhasa municipal solid waste treatment and disposal from 2010 to 2020, this paper debugs the structure and parameters of the model.The model takes into account multidimensional factors, including social, economic and environmental factors.

Analysis of the characteristics of the social economic environmental system in Lhasa
As shown in Figure 1, Lhasa, which is the capital of the Tibet Autonomous Region, is situated in the southeast of Tibet.It is the smallest prefecture-level city in Tibet and serve as the political, economic, cultural, scientific and educational center of Tibet.
Since the reform and opening up, Lhasa has experienced rapid urbanization and a significant increase in the urban population.The permanent population of Lhasa was 559,423 in 2010 and 867,891 in 2020, with an average annual growth rate of 4.49%.In addition, the floating population of Lhasa is also increasing rapidly.In 2010, Lhasa received 4,134,200 tourists from home and abroad (Lhasa Municipal Bureau of Statistics 2011).In 2020, despite the impact of the global COVID-19 epidemic, Lhasa still received 20,080,300 tourists (Lhasa Municipal Bureau of Statistics 2021), 4.86 times that of 2010.
Over the past few years, Lhasa's economy has grown at a rapid pace.In 2010, the GDP of the region was 17.891 billion yuan (Lhasa Municipal Bureau of Statistics 2011), and in 2020 it was 67.816 billion yuan (Lhasa Municipal Bureau of Statistics 2021), with an average annual growth rate of about 14.25%.
Lhasa, renowned for its beautiful scenery and rich history, has earned the nickname 'City of Sunshine'.Lhasa's greatest strength, greatest responsibility, greatest potential and greatest contribution lie in its ecological environment (Tu 2022).However, due to the particularity of its geographical location, its ecological environment is very fragile, and it is a typical ecologically fragile city in the high cold and high altitude area.

Data source
The data of this study came from China Statistical Yearbook, Tibet Statistical Yearbook, Lhasa Statistical Bulletin, official websites of government departments, and field investigations of local governments and MSW treatment enterprises.The data sources of main variables are shown in table 1.

Stock and flow diagram
By analyzing the current situation of MSW treatment and disposal in Lhasa, this study comprehensively considers social, economic, environmental and other factors to determine the stock, flow and auxiliary variables.Figure 2 shows the stock and flow diagram of Lhasa MSW treatment and disposal system, with 98 variables in total, and main variables and their types are shown in table 2. In this study, the trend extrapolation method, regression analysis method, assignment method, experience method and other methods are comprehensively used to analyze the existing data in combination with local planning and referring to the research results of other scholars, so as to realize the quantification of the Lhasa MSW treatment and disposal system model.The prediction of MSW production is the premise of treatment and disposal prediction.In this study, per capita consumption expenditure and urban family size are selected as the key variables affecting per urban residents' MSW production.The three relationships are obtained by fitting the historical data: Per urban residents MSW production ¢  The fitting result shows multiple R = 0.999 and p-value of significance F is 0.000108, proving that there is a high correlation between the three variables.Historical data showed that per rural residents' MSW production is about 0.222 tons year −1 .
Before 2018, urban (namely, Chengguan District, Dazi District, Duilong Deqing District, Qushui County and the streets under their jurisdiction within 30 km radius of the central city) MSW was only treated harmlessly in landfills.In 2018, Lhasa became the first city in the Tibet Autonomous Region to build and operate the MSW incineration power plant.Combined with this situation, two treatment methods of centralized incineration and landfill are introduced into the model.Due to anaerobic digestion of kitchen waste is simple in operation and management, mature in technology, environmentally friendly and with high economic benefits, centralized anaerobic digestion of urban kitchen waste is added to this model.According to the logical relation, the mathematical relations of treatment capacity of different treatment methods are constructed, such as

Amount of urban kitchen waste disposal, -
That is, when the amount of urban MSW excluding the amount of urban recyclables recovered and the amount of urban kitchen waste disposal is less than the capacity of urban MSW incineration, the amount of urban MSW incinerated is equal to the remaining amount of MSW.Otherwise, the amount of urban MSW incinerated is equal to the capacity of urban MSW incineration.
The treatment capacity of each treatment facility is converted according to the treatment scale corresponding to the actual investment in fixed assets.Taking the centralized incineration of urban MSW as an example, when the fixed asset investment was 460 million yuan, the incineration capacity was 255,500 tons year −1 (data of Lhasa municipal solid waste incineration power plant), the relationship between the two variables is shown in the formula (2-3).

Capacity of urban MSW incineration
Investment in fixed assets of urban MSW incineration 0.0006 2 3 ( -) = For rural (the more distant suburban counties include Mozhusongka County, Dangxiong County, Nimu County, Linzhou County and the villages and pastoral areas under their jurisdiction) MSW, Lhasa only uses simple landfill treatment at present.Due to the vast and dispersed population in rural areas, the quantity of MSW is relatively small and widely distributed.In this model, distributed pyrolysis incineration treatment is added to reduce the transport distance and cost of rural MSW.Rural kitchen waste only accounts for 13.77% of the total amount of rural MSW (Dan and Bu 2012), decentralized treatment cannot meet the requirements of normal operation equipment.Therefore, centralized anaerobic fermentation treatment of rural kitchen waste is increased.
In this study, the treatment and disposal methods of Lhasa MSW include landfill, incineration, anaerobic fermentation of kitchen waste and recycling of recyclables.Taking landfill as an example, the formula for the proportion of each treatment method is as follows:

Proportion of landfill
Total landfill amount Total amount of MSW 2 4 / ( -) = Since landfill has no material conversion and no economic benefits, it is not used as a resource treatment method in this study.Therefore

Amount of MSW utilization
Total amount of MSW Total landfill amount 2 5 ( -) = -

Resource utilization rate of MSW
Amount of MSW utilization Total amount of MSW 2 6 / ( -) =

Results and discussion
3.1.Results and discussion of natural tendency 3.1.1.Historical test In order to verify the accuracy, validity and reliability of the model, it is necessary to test the model, including dimensional test, operational test and historical test.'Model check' and 'Unit check' are displayed as 'Model is ok' and 'Units are A.O.K' respectively, which proves that this model has passed the dimensional test and operation test.The actual data of three variables, namely, the output of urban household waste, the output of rural household waste and the total output of household waste (lacking statistical data in some years), are selected for historical test.The mean absolute percentage error of each variable is less than 7%, indicating a small difference with the actual values, which proves that the model has high accuracy and can be used to predict the dynamic trend of MSW treatment and disposal in Lhasa.

Simulation results and analysis of natural tendency
This study is mainly based on 'The 14th Five-Year Plan for the national economic and social development of Lhasa and the outline of the long-term goals for 2035' to set parameters.Set the 'GDP growth rate' for 2021-2030 at 8%. 'Urbanization construction factor' is set at 1.1 (1 represents the current level, greater than 1 means promoting urbanization construction, and less than 1 means inhibiting urbanization construction) to promote the improvement of urbanization rate.'Proportion of education investment in GDP' is set at 9.5% to achieve the expected goal of '13.6 years of average schooling of the working population by 2025'.
Figure 3 shows the simulation results of the proportion of each treatment method and resource utilization rate of Lhasa MSW in 2021-2030 under Natural Tendency.The proportion of waste recovery remains relatively stable throughout the forecast period, hovering around 9%.However, the proportion of recyclables in Lhasa's MSW is as high as 56% (Dan and Han 2012).It must be noted that the amount of recyclables recovered in Lhasa is significantly lower than the amount of recyclables produced.This indicates a weak recycling and resource utilization capacity of recyclables in Lhasa.The government needs to establish a recycling system for recyclables and formulate incentive policies to encourage related enterprises to actively use recyclables.The incineration capacity in 2021-2025 is 276,000 tons day −1 .In 2026, due to the completion and operation of the second phase of the Lhasa Municipal Solid Waste Incineration Plant, the incineration treatment capacity will increase to 547,500 tons day −1 , so the proportion of incineration will be increase overall.However, after 2026, the production of MSW will increase rapidly, the incineration capacity will be insufficient, and more waste will be landfill, so the proportion of incineration will decline year by year.
Due to the production of MSW increases year by year after 2021, the treatment capacity of incineration remains unchanged and cannot meet the treatment demand for the amount of remaining urban MSW (the amount of remaining urban MSW = the production of urban MSW-the recycled amount of urban recyclables) (figure 4), the resource utilization rate of MSW decreases year by year.Due to the second phase of the incineration power plant will be put into operation in 2026, the resource utilization rate will further increase to 89.4%.However, after 2027, the amount of remaining urban MSW is still greater than the incineration capacity of urban MSW (figure 4).Therefore, with the increase of MSW production and limited incineration capacity, the resource utilization rate decreases year by year (figure 3).With the increase of rural MSW production, the storage capacity of rural MSW landfill will be full in 2027 (figure 4), which cannot meet the demand for harmless treatment of rural MSW.Therefore, it is necessary to strengthen the classification of MSW and reduce the amount of remaining MSW.It is also important to improve the treatment and disposal methods of MSW and make up the shortcomings, in order to reduce the amount of landfilling and extend the service life of landfill.

Regulation schemes
Strengthened MSW Classification Scheme: Lhasa is one of the first 46 cities in China to pilot garbage classification.The classification of municipal solid waste and commercial tourism waste has been promoted since 2017.In 2020, the Lhasa Municipal People's Government promulgated the Measures for the Classification and Management of Municipal Solid Waste in Lhasa.However, due to the lack of publicity and education, lack of supervision and residents' weak awareness of garbage classification, the effect of garbage classification is little, which affects the proportion of MSW treatment and the resource utilization rate.This scheme aims to improve the classification rate of MSW by adjusting the publicity and education factors and the supervision and constraint factors to strengthen the publicity and education, strengthen the supervision and punishment in this study.'Supervision and restraint factor' is adjusted from 0 in 2020 to 1 year by year (0-1 indicates that the supervision restraint level of MSW classification is from weak to strong).According to the status quo of publicity and education of garbage classification in Lhasa, 'Propagandism and education factor' is set at 0.1, and it is adjusted from 0.1 to 1 year by year (0-1 indicates that the propagandism and education level of garbage classification is from weak to strong).
MSW Classification and Treatment Scheme: The Measures for the Classification and Management of Lhasa Municipal Solid Waste put forward specific requirements for the classification and disposal of Lhasa MSW:(1) The recyclables shall be recovered, sorted and reused.(2) Kitchen waste shall be utilized as resources and treated harmlessly.(3) Hazardous waste shall be treated harmlessly according to the requirements of hazardous waste.(4) Other refuse shall be disposed of harmlessly by means of incineration, sanitary landfill, etc Currently, the classification and treatment facilities in Lhasa are incomplete, and the classification and treatment in rural areas are blank.Under the premise of strengthening MSW classification, this scheme estimates the treatment scale of each treatment facility based on the amount of MSW generation, the amount of kitchen waste classification and the amount of recyclables.Then the classification treatment is realized by adjusting the input of fixed assets.

Simulation results and analysis of regulation schemes
Figure 5 shows the simulation results of the proportion of each treatment of Lhasa MSW under Strengthened MSW Classification Scheme.As a result of measures taken to improve MSW classification, the proportion of recovery has steadily increased, reaching 37% by 2030, an increase of about 4.1 times compared to Natural Tendency.It can be seen that the measures taken have a significant impact on the classification effect.
As can be seen from figure 6, about 22,000 tons of kitchen waste will be separated in 2022 and about 60,000 tons of kitchen waste will be separated in 2025 under Strengthened MSW Classification Scheme.Therefore, a new kitchen waste treatment plant is considered in MSW Classification and Treatment Scheme.To meet the demand of urban kitchen waste treatment, it is recommended to increase fixed assets investment by 90 million yuan in 2022 and 200 million yuan in 2025.
In 2024, about 900 tons of rural kitchen waste will be separated under Strengthened MSW Classification Scheme, which is insufficient to meet the requirements of decentralized disposal.Therefore, it is suggested to  invest 1.25 million yuan in the fixed assets of centralized anaerobic fermentation of rural kitchen waste in 2024 to meet the treatment demand.Excluding the separated kitchen waste and recyclables, about 55,000 tons of rural MSW will be left in 2022 and 68,000 tons in 2027.It is suggested to invest 42.5 million yuan in 2022 and 12.5 million yuan in 2027 for rural distributed pyrolysis incineration.
Figure 7 shows the simulation results of the proportion of Lhasa MSW treatment methods in 2021-2030 under MSW Classification and Treatment Scheme.With an increase in the proportion of recovery and separate treatment of kitchen waste, the incineration capacity can meet the needs of the remaining amount of MSW.After 2026, there will be little need for landfilling, which will extend the service life of landfills and reduce the land loss and environmental pollution caused by landfill.As can be seen from figure 8, the resource utilization rate of MSW in Lhasa has been improved under Strengthened MSW Classification Scheme.Taking 2030 as an example, it will increase by about 1.24 times compared with Natural Tendency.This is due to a significant increase in the proportion of recovery resulting from strengthened garbage sorting, which has increased from less than 10 percent to 37 percent (figure 5).Therefore, the resource utilization rate of MSW will be further improved under MSW Classification and Treatment Scheme, and it will basically reach 100% after 2026.Taking the simulation results in 2030 as an example, the rate will increase by about 1.37 times compared with Natural tendency, and by about 1.11 times compared with Strengthened MSW Classification Scheme.After classification and treatment, kitchen waste is treated separately, and the incineration capacity can meet the disposal needs of the amount of garbage that remove the recyclables and kitchen waste, without the need for landfill.
According to the above analysis, if Lhasa only expands the Municipal Solid Waste Incineration Power Plant in 2026, it still cannot meet the demand for MSW treatment and disposal, and cannot greatly improve the resource utilization rate.It is very necessary to strengthen garbage classification and treatment, which can maximize the harmless treatment and resource utilization of MSW, and meet the requirements of circular economy and ecological civilization construction.

Conclusion
With the growth of population, the improvement of people's living standards and the rapid development of economy in Lhasa, the production of MSW has increased significantly, which has resulted in certain problems of treatment and disposal.Improper treatment of MSW can not only pollute the environment, harm human health, but also restrict the sustainable development of the city.Based on the system dynamics, this paper constructs the simulation model of Lhasa municipal solid waste treatment and disposal.By adjusting the relevant variables in the model and setting different control schemes, the proportion of each treatment method and resource utilization rate of Lhasa MSW in 2021-2030 are predicted, and the policy measures more suitable for Lhasa MSW treatment and disposal are put forward: (1) The MSW classification system should continue to be vigorously promoted.(2) It is necessary to improve the treatment and disposal mode of MSW and make up for the shortcomings of treatment.(3) Make clear the management subject and specific requirements of MSW treatment and disposal, and strengthen the construction of the system.
There are many limitations in the research process: Lhasa Municipal solid waste treatment and disposal system is a complex feedback system, but some feedback relationships are difficult to quantify, and more indepth research is needed in the future, such as how the economic benefits generated by solid waste treatment affect population growth.The construction of this model is based on the actual situation of Lhasa, so its applicability to other high-cold and high-altitude cities needs to be improved.Further research is needed to improve and expand the model, so as to provide reference for the treatment of domestic waste in high-cold and high-altitude areas around the world.

Figure 2 .
Figure 2. Stock and flow diagram of MSW treatment and disposal system.

Figure 3 .
Figure 3. Proportion of treatment methods and resource utilization rate of MSW from 2021 to 2030 under Natural Tendency.

Figure 4 .
Figure 4. Residual capacity of rural landfills, amount of remaining urban MSW and incineration capacity of urban MSW from 2021 to 2030 under Natural Tendency.

Figure 5 .
Figure 5. Proportion of treatment methods of MSW from 2021 to 2030 under Strengthened MSW Classification Scheme.

Figure 6 .
Figure 6.Amount of rural left MSW, amount of urban/rural kitchen waste from 2021 to 2030 under Strengthened MSW Classification Scheme.

Figure 7 .
Figure 7. Proportion of treatment methods of MSW from 2021 to 2030 under MSW Classification and Treatment Scheme.

Figure 8 .
Figure 8. Resource utilization rate of MSW from 2021 to under Tendency and regulation schemes.

Table 1 .
Data sources of main variables.Growth of fixed asset investment in urban kitchen waste treatment field investigations of local governments Growth of fixed asset investment in rural kitchen waste treatment field investigations of local governments Growth of fixed asset investment in urban MSW incineration field investigations of local governments Growth of fixed asset investment in rural MSW incineration field investigations of local governments Annual investment in fixed assets of urban MSW landfills field investigations of local MSW treatment enterprises Annual investment in fixed assets of rural MSW landfills field investigations of local MSW treatment enterprises Capacity of urban kitchen waste disposal field investigations of local MSW treatment enterprises Capacity of rual kitchen waste disposal field investigations of local MSW treatment enterprises Capacity of urban MSW incineration field investigations of local MSW treatment enterprises Capacity of rural MSW incineration field investigations of local MSW treatment enterprises Figure 1.Geographic location of Lhasa in Tibet Autonomous Region.

Table 2 .
Main variables and their type.