Study On The Performance Of Municipal Solid Waste Incineration (MSWI) Fly Ash (FA) Bricks Based On Bacteria Mineralization To Recycle MSWI FA

With the continuous development of the economy and the continuous improvement of people’s living standards,the output of garbage has increased year by year,and many cities in China have suffered from the"garbage siege"for a long time.Incineration was the main means of harmless and reduced waste disposal.During the incineration process,heavy metal elements such as Cr,Pb,Hg,Cd,Cu,Zn and Ba were concentrated in the MSWI FA through volatilization and condensation.MSWI FA was classified as hazardous waste by the state because it is rich in heavy metals,dioxins and other harmful substances.At present,the main method of MSWI FA disposal in China is the co-processing of cement kilns.The traditional cement production process is accompanied by a large amount of greenhouse gas emissions,which is not in line with the national low-carbon and environmental protection development concept.In this paper,the application of microbial mineralization technology to the consolidation of MSWI FA and the stabilization of heavy metals was proposed to achieve the purpose of recycling MSWI FA and preparing microbial MSWI FA bricks（MMBs）.This paper conducted experimental research on the basic physical and chemical properties of MSWI FA,the interaction between MSWI FA and microorganisms,the hydration characteristics of MSWI FA,the performance research and optimization research of MMBs.The main work and conclusions are as follows:（1）Research on preparation and basic mechanical properties of microbial municipal solid waste incineration fly ash bricksThe preparation and compressive strength optimization of MMBs were carried out,and the influence of MSWI FA content,bacterial concentration and calcium ion concentration in nutrient solution on the compressive strength of MMBs was analyzed.The results showed that with the increase of MSWI FA content（30%-50%）,optical density value of bacterial solution at a wavelength of 600 nm(OD₆₀₀)（0-1.20）and calcium ion concentration in nutrient solution（0 mol/L-1.10 mol/L）,the compressive strength of MMBs showed a trend of first increase and then decrease.It was found that the proportion of MSWI FA and slaked lime in the powder material may affect the formation of hydration products;the encapsulation effect of biological organic matter on MSWI FA had two sides,low concentration bacterial solution was beneficial to the improvement of compressive strength,high concentration bacterial solution was not conducive to the formation of MSWI FA hydration products;high concentrations of calcium ions inhibited bacterial activity.When the content of MSWI FA was40%of the solid mass,the OD₆₀₀ value of the bacterial solution was 0.6 and the calcium ion concentration was 0.3 mol/L,the compressive strength of MMBs specimens in each single factor test group reached the peak value.The MMBs with a size of 100 mm×100 mm×50mm were prepared,and the compressive strength was measured to be 33.90 MPa,the density was 1937.40 kg/m3,and the leaching concentration of heavy metals was lower than the specification limit（GB 16889-2008）.（2）Research on coupling relationship between microorganisms and municipal solid waste incineration fly ash and municipal solid waste incineration fly ash hydration productsThe basic physical and chemical properties of municipal solid waste incineration fly ash,the interaction between fly ash and microorganisms,the chemical composition and mineral composition of municipal solid waste incineration fly ash hydration products were studied.It was showed that CaO,Al₂O₃ and SiO₂ were the main chemical components of MSWI FA and the material basis of pozzolanic,and MSWI FA contains a lot of endogenous calcium,which can provide sufficient calcium source for microbial-induced calcium carbonate deposition;MSWI FA environment with high alkalinity and heavy metal toxicity was not conducive to microorganisms to maintain their activity;the concentration of heavy metal ions in the acetic acid leachate of fly ash was reduced after mixing with bacteria water suspension and urea aqueous solution.,traces of heavy metal elements such as Pb,Cu and Zn were captured in the EDS spectrum of biological calcite deposits,and the mineralizing bacteria had the ability to convert heavy metal ions into heavy metal carbon;the hydration products of MSWI FA included hydrocalumite（Ca₄Al₂O₆Cl₂·10H₂O）,calcium aluminate hydrate（Ca₃Al₂O₆·x H₂O）,gismondine（CaAl₂Si₂O₈·4H₂O）and other active minerals at the trace level.（3）Research on optimization of physical and mechanical properties of microbial municipal solid waste incineration bricksThe effects of bacteria-water suspension（BWS）OD₆₀₀,urea aqueous solution（UAS）concentration,carbonation and slaked lime dosage on the properties of MMBs were studied.It was showed that with the increase of the BWS OD₆₀₀（0–1.2）and the UAS concentration（0.3 mol/L–1.1 mol/L）,the flexural and compressive strengths of MMBs increased first and then decreased;carbonation can greatly improve the flexural（103%）and compressive strength（54%）of MMBs;With the increase of slaked lime content（0%-20%）,the flexural and compressive strengths of MMBs increased continuously.It was found that the encapsulation of MSWI FA by bacteria inhibited the formation of MSWI FA hydration products,hindering the utilization of MSWI FA as a calcium source,and inhibiting the formation of calcite,this dual inhibitory effect was enhanced with the increase of BWS OD₆₀₀and UAS concentration;the contribution of mineralized product calcite to brick strength was greater than that of MSWI FA hydration product;granular gel-state carbonized calcium carbonate partially filled the pores of 4000 nm-25000 nm,making up for the deficiency of the smaller calcite particles of mineralized products,so the flexural and compressive strength of MMBs after carbonization curing is further improved;the water-cured MSWI FA brick body realized the solidification and stabilization of some heavy metals through the physical encapsulation effect of MSWI FA hydration products,and the coordination effect and the interlayer substitution effect of hydrocalumite,the application of microbial mineralization technology and carbonization technology further reduced the leaching concentration of heavy metal ions in MMBs,the heavy metal leaching concentration of MMBs was lower than the limit specified in GB 18598-2019 and GB 16889-2008;slaked lime did not fully react,the unreacted slaked lime and the hydrated MSWI FA harden together to form a consolidated brick body,hardened hydrated lime provided a new source of strength for MMBs.