Carbon Footprint Calculation And Optimization Of Rural Typical Toilet System

In order to improve the living environment in our country,we are vigorously promoting the rural toilets improvement work,but there are some problems in the process.For example,the application of flush toilets breaks the virtuous cycle of material and energy between people and the land,and may increase the difficulty of rural sewage treatment.In general,these problems are the manifestation of the contradiction between"hygiene"and"ecological"in rural toilet improvement work.In order to solve the problem of ensuring the safety of the ecological environment and achieving sustainable development while meeting the sanitation needs of rural toilets,there have been certain achievements,but these achievements lack systematic or whole-process thinking,and only focus on manure and its material cycle of nutrients elements.Therefore,looking at the ecology of the toilet system from a holistic perspective is a relatively blank aspect,which is worth studying.The carbon footprint analysis can judge the contribution of the target to the greenhouse effect,which is an effective ecological evaluation method.Carbon emissions,which are part of the carbon footprint,are the focus of attention of the international community and our country in the next few decades.In combination with the actual situation of resource consumption and greenhouse gas generation caused by rural toilet improvement work,there will be a significant impact in the rural population base of hundreds of millions.Based on the above background,this paper attempts to analyze the carbon footprint of typical rural toilet systems through the carbon footprint algorithm,identify the carbon emission characteristics of the toilet system in the process of construction and operation,and explore ways to reduce carbon emissions in the rural toilets improvement work,and provide guidance.The research conclusions are as follows:（1）Under the framework of the pan-ecological chain,the toilet system can be defined by borrowing the concept of a social-economic-natural composite ecosystem,which refers to a composite system that takes people as the center and connects biological elements,socioeconomic elements,and environmental elements through a variety of interactions such as material flow.On this basis,combined the functions of the components of the toilet system,the toilet system is divided into four units:toilet house,collection and storage,harmless treatment,ecological environment,and two stages of construction and operation.The composition and boundary of the rural toilet system are clearly defined.Lay the foundation for carbon footprint accounting.（2）According to the four-unit composition of the toilet system,the toilet house,collection and storage,harmless treatment,and ecological environment,as well as the two-stage model of construction and operation,the paper lists the activity data list of the urine-diverting toilet system,the double-pit alternate toilet system,the septic tank water flushing toilet system,the centralized treatment water flushing toilet system,and the decentralized treatment water flushing toilet system,and the carbon emission accounting is carried out.The results show that the carbon footprint of different toilet systems in the construction and operation stages is significantly different.In the construction phase,the septic tank water flushing toilet system has the lowest 457.89 kg CO₂e/cap,and the double-pit alternate toilet system has the highest 590.23 kg CO₂e/cap.Integral waste storage facilities and in-situ treatment of manure harmlessness are powerful factors that promote the lowest possible carbon footprint at the construction stage and the three building materials of brick,cement and sand may be the focus of carbon emission reduction during the construction phase.In the operation stage,the urine-diverting toilet system has the lowest carbon emission-4.77 kg CO₂e/（cap·a）,and the centralized water flushing toilet system has the highest carbon emission of 105.01 kg CO₂e/（cap·a）.The separated source of urine has played an important role,but based on the future development of carbon neutrality in the sewage treatment,the centralized treatment water flushing toilet system has a large room for emission reduction,and the selection of manure treatment and disposal methods and the selection of covering materials are also feasible emission reduction directions.（3）From the perspective of greenhouse gas generation,the N₂O production of the urine-diverting toilet system and the double-pit alternate toilet system is relatively serious,mainly from the composting process.The production of CH₄ and N₂O in septic tank water flushing toilet systems,centralized treatment water flushing toilet systems,and decentralized water flushing toilet systems are relatively serious,which come from septic tank treatment and clearing sludge composting respectively.Among them,the CH₄produced by the septic tank flushing toilet system corresponds to a significantly higher carbon dioxide equivalent.Combined with the smaller warming potential of CH₄,the corresponding production situation is very serious.In addition,the centralized treatment water flushing toilet system should also pay attention to the CH₄ generation in the sludge treatment process.（4）Based on the carbon footprint calculation of the typical toilet system during the construction and operation stages,it is believed that urine-diverting toilets should be the first choice for rural toilets improvement work.When using double-pit alternate toilets,the idea of source separation should be followed,and the ues of centralized collection sewer toilets be based on the implementation of sewage sludge energy recovery.The septic tank flushing toilets are relatively unsuitable to be strongly recommended in the process of rural toilet improvement work due to two reasons:the large amount of greenhouse gas production and the low emission reduction potential.Combined with the current rural toilet system construction and operation mode,the paper further gives 5emission reduction measures:（1）Reduce the wall thickness of the toilet house,apply low-carbon bricks,and obtain nearby materials;（2）Apply integral septic tank products and recommend a common septic tank for multiple neighboring households;（3）Construct toilets under the idea of source separation,and achieve separation of excrement and urine as much as possible;（4）The urine-diversifying toilet system and the double-pit alternate toilet system preferentially use natural plant-derived plant ash as mulch material,followed by agricultural waste such as straw powder;（5）The use of sanitary landfill should be avoided as far as possible for sludge treatment.Combining anaerobic digestion,land use or incineration is a better choice.For sewage treatment,water source heat pumps are recommended.The annual carbon emission reduction of the five optimized toilet systems is 1.48 million tons for the feces and urine separation toilet system,9.1 million tons for the double-pit alternate toilet system,11.89 million tons for the septic tank water flushing toilet system,62.2 million tons for the centralized treatment toilet system,and13.05 million tons for the decentralized treatment toilet system,which are basically in the same order of magnitude as the total carbon emissions of our country’s sewage treatment in 2015.Among them,the optimized centralized water flushing toilet system and the urine-diverting toilet system can achieve"carbon neutrality"in the operation phase.