Study On Optimization Of Pipe Materials Driven By The Scaling Influence Mechanism Of Leachate Transport Pipe System

Leachate is a by-product of municipal solid waste（MSW）landfill,which must be collected through a pipeline system and transported to the corresponding treatment facility for advanced treatment.During the process of collection and transportation,it is easy to induce scaling on the pipe wall,resulting in failure of the pipe system and thus posing a threat to the environment and public health.However,the scaling process of the pipe system is complex,the mechanism of the transportation pipe scaling is still unclear,and the anti-scaling performance of the existing drainage pipes is poor.Therefore,it is of great practical significance to study the selection and optimization of pipe materials under the guidance of the scaling mechanism of leachate pipes to ensure the safe operation and management of the leachate collection and transportation system.Existing research mainly focuses on the cause of leachate pipe scaling from a single-factor perspective,using empirical or semi-empirical models to explain the scaling mechanism.Limited by the correlation and interaction of influencing factors,the scaling mechanism cannot be fully explained,and it is difficult to scientifically and effectively guide the scale inhibition and optimal design of pipe systems.In this study,by designing a static scaling experiment and a multi-factor control of the dynamic scaling experiment of transportation pipelines that integrates"leachate water quality-transportation conditions-pipe surface properties",the key factors affecting scaling were identified,and the pipe scaling characteristics and the mechanism of multi-dimensional factors on scaling under different aged and different type wastes was discussed.The relationship between multiple factors was analyzed.Based on the system dynamics method,a scaling simulation model of leachate pipes coupled with multiple scaling behaviors,including chemical precipitation,organic matter degradation,particle deposition and stripping,and biofilm growth,was constructed to reveal the endogenous relationship between factors,factors and scaling,and the blocking risk of leachate pipes in different landfill scenarios was evaluated.The fluorine and silicon acrylate modified high-density polyethylene（HDPE）composites,named HPFS and HPS respectively,were prepared by bulk polymerization and melt blending,and the feasibility of the modified materials to alleviate the scaling of leachate pipes was compared and discussed.Moreover,based on the theory of sustainable design,a grey target decision-making model for quantitative comparison and selection of pipes considering function,cost and environment are constructed,and the application value of composite materials and common commercial pipes in the design of leachate transportation pipeline system is comprehensively compared.The main conclusions of this work are as follows:（1）The pipe scaling characteristics were different due to the great differences in the properties of leachate in different ages and different types of landfill waste.The average contents of organic and inorganic scales formed in different leachate were 29.6%and 70.4%respectively.The scale formed in MSW leachate and MSW-IBA leachates both contained living bacteria and dead bacteria,and in these two kinds of scales,the proportion of living bacteria was 55%and 27%,3%and 0.5%for dead bacteria,respectively.The growth and metabolism of bacteria in MSW leachate with a higher proportion of organic matter was much faster,and the scale in MSW-IBA leachate was thinner and denser.The immersion of HDPE pipes had a certain intervention effect on the concentration of calcium ion(Ca²⁺),chloridion,ammonia and pH value in the leachate,and caused a change of bacterial community in the leachate.The bacteria in the scale were mainly derived from the leachate,JG30-KF-CM45and Rhodothermaceae were enriched during the scaling process,and the unique Caldisericota was generated.The relative abundance of genes related to biofilm formation in the scale was higher than that in the leachate group,and the largest contribution was Gammaproteobacteria.The synergistic activity of chemotrophic and biodegradable functional microorganisms plays a key functional role in the scaling process.（2）The leachate qualities,transportation conditions and pipe properties had a certain impact on the pipe scaling,Ca²⁺concentration and flow velocity were the key factors affecting the scaling.Ca²⁺,volatile fatty acid（VFA）,pH,temperature,pipe surface roughness and pipe diameter had positive effects on the scaling formation,while the flow velocity directly related to the surface shear stress of the scale presented a negative effect on the scaling.The decrease of Ca²⁺content could significantly reduce the pipe scaling,and the increase of flow velocity made the scaling amount curves present a nonlinear fluctuation state.The scaling amount of rough surface with higher surface energy was significantly higher than that of lower free energy surface,but there is no linear correlation between the scaling amount and surface energy.Pipes with smaller diameters had less scale formation,but the scaling layer was much thicker and more prone to blockage.（3）Using the established system dynamics model,the scaling development trend of MSW and MSW-IBA leachate pipes during the 150 months operation period were simulated.Their scaling amount changes showed a trend of a rapid increase in the early stage and gradually stable in the late stage.The scaling amount of the MSW-IBA co-disposal landfill exhibited periodic fluctuations in the growth process and reached a stable state faster.Chemical precipitation and particle deposition played leading roles in scaling,the main component of scale is calcium carbonate（CaCO₃）,accompanied by a small amount of sodium chloride（NaCl）and magnesian calcite([Mg_0.03Ca_0.97]（CO₃）).The contribution rate of chemical precipitation to the scaling of MSW and MSW-IBA landfill pipelines is 74.1%and 83.9%,respectively,and the biological fouling contributes 25.9%and 16.1%to the pipe scaling of the two types of landfills,respectively.The thickness of the scale layer in MSW-IBA landfill pipes increased faster in the same operation time,which had a greater risk of blockage.（4）The corrosion current of polymer PFA was nearly 2 orders of magnitude lower than that of fluorine-free polymer,and it had good corrosion resistance.Composite HPS and HPFS had good hydrophobicity,thermal stability,and acid-base resistance.Due to their low surface energy and small surface roughness,HPS and HPFS could reduce the adhesion stress and nucleation sites of scale on their surfaces,resulting in 29.6%and 16.8%reduction in the amount of scale per unit surface compared with commercial HDPE pipes,showed excellent anti-scaling performance.After bacterial inoculation and culture,the bacterial adhesion on the surface of HPFS was reduced by 47.9%and 66.9%respectively compared with HDPE and PVC pipes,indicating HPFS had excellent inhibition of bacterial adhesion.HPS and HPFS showed adaptability to a high concentration and complex water environment and had potential engineering application prospects in leachate pipes and other wastewater transportation.（5）A comprehensive index system for pipe selection considering function,cost and environmental needs was established.Combined with experimental data and literature data,the results showed that HPS had excellent anti-scaling properties,and HPFS had good corrosion resistance.Four kinds of pipes,including HPS,HPFS,HDPE,and PVC,were sorted by using the established grey target decision model as follows:HPS was the best choice,followed by HPFS and HDPE,and PVC pipes are the worst,which indicated that the silicon modified composites prepared in this study had better comprehensive properties and the application potential in leachate transportation pipe systems.