Research On Experiment And Evaluation Of Sewage Sludge Used As Cover Material In MSW Landfill

To meet the standard of sewage sludge safe disposal in big cities,this article takes municipal sewage sludge as the research object by a multi-disciplinary research of crossing technology which combines environmental science, chemical engineering, geotechnical engineering, etc. The validity and feasibility of sewage sludge used as cover material in MSW landfill is carried out by a series experimental study on physicochemical properties and geotechnical engineering characteristics for sludge, which can provide decision support for ecological security disposal of municipal sludge.The chemical properties and microscopic characteristics of sludge and properties of eroded sludge are first studied in this paper. Compound in sludge is similar to it in clay, so it can replace clay as cover material in MSW landfill. HAS, rapid hardening cement and the quick lime are used to modify sludge as modifiers. Then chemical properties such as water content, organic, bulk density, heat value are monitored, while mechanical properties and microscopic characteristics of sludge are researched. The principal results are as follows:(1) Based on microscopic characteristics of sludge, sludge component is similar to clay. Modified sludge can replace clay as landfill cover material. The combustion process of sewage sludge in an air atmosphere consisted of three stages, that is, dehydration, volatile devolatilization and fixed carbon combustion. The shape of sludge is irregular loose body, which has much pore on surface filling with a number of small grains and holding a lot of floc, forming an irregular structure. Based on properties of eroded sludge, shear strength is relatively small, fluidity is relatively large, and it easily has landslide phenomenon, when sludge is eroded in first phase. Particles are tended to be stable because flocculation of colloidal particles in last phase.(2) The physicochemical property of modified sludge is shown that bulk density of modified sludge does not change significantly. Heat value of modified sludge is smaller than raw sludge. Organic content of modified sludge can meet requirement of cover material in MSW landfill. Optimization analysis of water content is shown that it can meet requirement when rapid hardening cement content reaches 10% and curing time is 14 days. Based on the result of shear test, when modifier is HAS, rapid hardening cement and the quick lime, content reaches 15%, it can meet safety factor of demand, and safety factor of rapid hardening cement is the largest.(3) The microscopic characteristic of modified sludge is shown that there are a lot of connected pores and capillary pores with High Surface Area. So it can keep good filtration and adsorption performance, that is, modified sludge has filtration and adsorption of heavy metal performance, and thus it is able to prevent landfill from landslide and prevent heavy metal from inflow water. With addition of modifier, specific surface area increases gradually, of which surface area of the quick lime is relatively large, content reaches 10%, average pore size and total pore volume reach maximum. Based on analysis of porosity, porosity of HAS modified sludge is the largest. Rapid hardening cement takes the second place; the quick lime is the smallest.(4) Evaluation and analysis for sewage sludge used as cover material in MSW landfill shows that heavy metal of leaching toxicity reaches the standard in raw sludge and modified sludge, except heavy metal Cd in raw sludge. Sewage sludge used as cover material in MSW landfill doesn't influent agricultural landfill after cover. Some Heavy metal of plant in both sludge and clay is more than it in natural conditions. Bermuda grass has strong accumulation and enrichment for Cu. White Clover has strong accumulation and enrichment for Cd, Ni, and Fe. Tall fescue has certain accumulation and enrichment for Cd, Fe. Enrichment of heavy metal in plant has recovered heavy metal in sludge because high heavy metal in sludge. COD leaching toxicity of modified sludge accords with standard of cover material. The quick lime has relatively high pH as modifier, and it can't meet demand. Both HAS and rapid hardening cement as modifier can accord with standard of cover material.By above research results can provide theoretical basis and technical parameters for resource utilization and disposal of sludge. It also can provide technical support for performance evaluation of sewage sludge used as cover material in MSW landfill. It can solve the problem of difficult source of clay and natural soil. This research has important guiding significance and value.