Study On The Resourceful Disposal Model Of Urban Sewage Plant Sludge According To Local Conditions

With the acceleration of urbanization,the amount of urban sludge discharge in China continues to increase,and there is a significant gap in the construction and operation of sludge treatment and disposal facilities.Only environmental,economic,and social factors are considered in optimizing existing urban sludge treatment and disposal modes.The fuzziness and uncertainty of subjective evaluation and the unreasonable distribution of weight ratio will cause considerable errors in the evaluation results.This study used entropy theory and the entropy weight method to build a"based on local conditions"sludge treatment and disposal evaluation index system with compost,building materials,heat,and electric energy utilization as the core.Based on the geographic information system（Arc GIS）,a spatial analytic model（SRMUSRD）for resource utilization and optimal allocation of urban sludge was developed,which covered land use suitability evaluation,economic analysis and environmental impact of urban sludge,and provided a reference for the application of centralized and decentralized disposal technology of urban sludge.Taking the"local conditions-based"resource disposal of urban sludge in Xian as an example,the research on sludge disposal and resource utilization preparation in Xian in 2025 was carried out.The main conclusions are as follows:（1）The average particle size distribution of sewage sludge in Xi’an is 52.5-130.1um,and the Zeta potential is-10.2 to-35.7 m V.The mineral content of the sludge samples was dominated by Si0₂,Fe₂0₃and Al₂O₃,with a composition similar to that of clay;Heavy metal indicators such as arsenic（As）and cadmium（Cd）all meet the requirements of the standard"Sludge quality of urban sewage treatment plants"（GB24188-2009）;The average values of organic matter and total nutrient content of the nutrients were 388.3-579.1 g/kg and 25.5-48.3 g/kg respectively,with organic matter content higher than the land improvement standard of>10%and the landscaping standard of>25%.（2）Extracellular Polymeric Substances（EPS）of municipal wastewater treatment plant sludge are mainly composed of proteins and polysaccharide components.Bound EPS（B-EPS）is the main fugitive form and an essential source of organic matter in sludge composition.The capillary sanction time（CST）of municipal wastewater treatment plant sludge is 10-33.2 s-L/g,with good dewatering performance.（3）The average values of the sludge in the third,fourth,fifth,sixth and tenth sewage treatment plants are 11298k J/kg,11762 k J/kg,10734 k J/kg,9256 k J/kg,and 5424 k J/kg,respectively.The sludge from the sewage treatment plant meets the requirements of"sludge for separate incineration of sludge from municipal sewage treatment plant"（GB/T 24602-2009）>5000/3500 k J/kg.（4）When decision-making,the entropy weight system eliminates subjective and objective factors such as regional sludge production,gross domestic product（GDP）,building materials and thermo electric collaborative disposal.It can establish a"local"urban sludge-disposal technology optimization model according to local conditions.This model can reflect the priorities set by different stakeholder groups and effectively reflects the preferences of sludge disposal technologies.The sustainability evaluation index system of municipal sludge disposal technology is constructed from the perspectives of environmental impact,economic impact,entropy change,resource utilization rate and regional adaptability,including 5 standards and 15 sub-standards.The constructed index system considers environmental and economic sustainability and local conditions.It can more comprehensively evaluate the"sustainability"of sludge disposal technology,which is close to the actual needs of sludge management.The combination of the entropy weight method and entropy increase theory and the application of resource-based disposal mode according to local conditions can provide technical means for the optimization of urban sludge treatment and disposal technology in large and medium-sized cities in developing countries.（5）The analysis using the"localized"urban sludge-disposal technology preference model showed the priority of urban sludge treatment and disposal technologies in Xi’an.Brick-making cooperation（S6）>cement kiln Cooperation（S3）>composting（S5）>collaborative power generation（S4）>landfill（S1）>incineration disposal（S2）.Therefore,the combination of brick-making synergy（S6）,cement kiln synergy（S3）and composting（S5）is the core sludge disposal technology in the region.Supplemented by co-generation（S4）,it has the advantages of low construction investment,low environmental impact and high efficiency in resource utilization,which can achieve resourcefulness,harmlessness and reduction of urban sludge disposal in Xian.It is the preferred technology combination for regional urban sludge-disposal.（6）By integrating regional geographical information conditions,the shortest path optimization method,and transport and disposal economics analysis methods,a spatial analysis model for the resourceful use and optimal allocation of urban sludge（SMUSRD）was constructed,thus clarifying the applicability of urban sludge zoning management.The model integrates the regional geographic information conditions,the shortest path optimization method and the economic analysis method of transportation and disposal and defines the applicability of urban sludge zoning management.Based on the spatial analysis model of urban sludge-resource utilization and optimal allocation,it is determined that the total area suitable for composting sludge on non-irrigated cultivated land in Xi’an is 128134 ha,accounting for 12.7%of the regional area.This area is mainly distributed between 34.023°N and 109.5067°E,covering the north of Zhouzhi County,Fuyi district,and the west of Lantian County.Combined with the cultivated land area,which is suitable for composting sludge and the existing urban sludge volume level,taking the organic matter,nitrogen and phosphorus in composting sludge as the evaluation index,the appropriate sludge application area in Xi’an is 128134ha.The capacity of non-irrigated cultivated land to receive urban composting sludge is 1.922×106 T/a（moisture content 50%）.It is appropriate to apply composting sludge every 10 years according to the application rate of 15t/ha.After composting sludge was applied to non-irrigated farmland in Xi’an,the soil with heavy-metals Cd,Cr,Pb,Cu,Zn,Ni,as and Hg were 0.13-0.271,24.93-39.11,26.89-52.11,58.74-149.7,41.2-119.4,32.14-71.43,0.05-0.19and11.45-20.54 mg/kg.The maximum concentration increased by 4.23%,10.1%,29.8%,22.4%,21.0%,28.1%,7.7%and 4.9%compared with that before applying.However,the highest concentration values are within the range of soil environmental quality risk screening value,which shows that the land use of urban composted sludge is at a low environmental risk.（7）Based on the analysis of Xi’an’s social and economic development level from2020 to 2025,the average annual growth rate of Xi’an’s sewage treatment capacity is5%during the"14th five-year plan".Xi’an’s sewage treatment capacity will be 4.87million tons/day in 2025;The average sludge production rate of sewage treatment plants is 10t/10⁴m³.In 2025,the total sludge of sewage treatment plants in the city except Xi’an new area was 4880 tons/day（moisture content 80%）.The urban sludge zoning management measures optimized by the spatial analytical model of optimal allocation（SMUSRD）have realized the double unification of economic and environmental benefits of regional urban sludge disposal.