Research On The Transformation And Recovery Of Phosphorus Based On Low-temperature Thermal Treatment Of Sewage Sludge

Municipal sewage sludge is a complex semi-solid waste produced during the process of municipal sewage disposoal.It is esay to spoil and deteriorate due to the high content of water and organic matter.Sewage sludge is enriched with many contaminants such as organic contaminants,heavy metals,pharmaceuticals,parasites,and pathogens.Inappropriate disposal of sewage sludge will inevitably cause secondary pollution of soil,water and air,seriously threatening the ecological environment and human health.On the other hand,it contains a large amount of nutrient elements and organic matter with high heat value,which has the potential for energy and resource recovery.In particular,sewage sludge is enriched in more than 50%of phosphorus(P)in the sewage,which is a huge P resource pool.Therefore,the reduction,harmless disposal and resource utilization of sludge have received extensive attention.The thermal treatment methods have become a research hotspot of sludge disposal because of its advantages such as reducing the volume,eliminating pathogenic microorganisms,decomposing organic pollutants,and recycling energy.Meanwhile,thermal treatment of sewage sludge will increase the P content in the solid product.However,at present,there is a lack of thermal treatment method aiming at recycling P from sludge,and there is insufficient research on the P speciation transformation mechanism during thermal treatment process and the recovery or modification of P in the thermal product.This paper aims to achieve the high-efficiency recovery and utilization of P and energy in sewage sludge,and design a low-temperature combustion method that utilizes its own heat value and reduces fly ash output.An in-depth and comprehensive study was conducted on the low-temp combustion characteristics of sludge,the distribution characteristics of various phase products,the influence of thermal treatment methods and conditions on P migration,the pathway of P transformation during low-temp combustion,and the methods of P release and recovery from thermal products.Besides,the P transformation during the low-temp co-combustion of sludge with biowaste and the characteristics of co-combustion products was systematically studied.The result of this paper provides technical support and theoretical basis for clean and efficient resource utilization of sewage sludge and other organic solid wastes.First,three methods namely fluidized bed incineration,fixed bed pyrolysis and low-temp combustion were used to study the effects of thermal treatment methods and conditions on P content and enrichment efficiency of the solid product.Thermal treatment can increase P content of the product to more than 85 mg/g.High temp incineration is not conducive to improve the P enrichment efficiency.When the incineration temperature is higher than 800?,the P enrichment efficiency is less than 75%.The P bioavailability increases with oxygen concentration,and the bioavailable P content in pure oxygen low-temp combustion ash is 48.7 mg/g.Low-temp combustion and pyrolysis can reduce the leaching of heavy metals in the sludge.Low-temp combustion can also convert heavy metals in the sludge from an easily leachable form(acid extractable fraction and reducible fraction)to a stable form(reducible fraction).Low-temp oxidation is suitable for the recovery and utilization of P in sewage sludge.The effect of particle size,moisture content and air flow rate on peak combustion temp,spreading rate of combustion front,and gas and liquid product components were studied on the low-temp combustion device.The combustion spreading rate increases with air flow rate when it is less than 10.4 mm/s.Beyond this range,the increase of convection heat transfer slows down the spread rate.The yield of liquid products reduced with increasing the air flow rate.Increasing the moisture content reduces the peak temp and spreading speed.The low-temp combustion cannot be self-sustained when the moisture content is higher than 11%.The increase of moisture content is not conducive to the complete combustion of sludge,which improve the yield of tar.Increasing the partical size of sludge fuel reduces the peak temp,and the spreading rate increase first and then decreases.When the particle size is wider than 1.0 cm,the spreading rate is affected by the specific surface area and the oxidation reaction rate.When it is less than 1.0 cm,the spreading rate is dominated by the bulk density affects.Tar yield increases with the particle size,but less effect on the relative content of organic compounds in the tar.The P migration and transformation mechanism during low-temp thermal treatment of sludge was studied.The content of inorganic P in the sludge exceeds 80%.Pyrolysis promotes the conversion of organic P into inorganic P,and converts orthophosphate monoester and pyrophosphate into orthophosphate.Increasing the pyrolysis temperature promote the concersion of non-apatite inorganic phosphorus to apatite inorganic phosphorus,the AP content increases with the temperature.Low-temp combustion converts the organic P and polyphosphate to inorganic P and orthophosphate,respectively.Specifically,vivianite and P-Ferrihy are converted into Fe PO₄ and Ca-P compounds,which promote the formation of Ca-P compounds in the product and increase the AP content.Deepening the degree of oxidation converts Fe PO₄ to Al-P or Ca-P compounds,and improves the stability and bioavailability of phosphorus in sludge ash.The P release and recovery of low-temp combustion ash were investigated.P in the low-temp combustion ash is recovered in the form of high-purity struvite(Mg NH₄PO₄)by using the method of"acid extraction-CER purification-MAP crystallization".When the concentration of HCl used to extract P from low-temp ash is not less than 0.4 M,the liquid-solid ratio has little effect on the P extraction efficiency.When the L/S ratio is not higher than 20 ml/g,the HCl concentration has little effect on the extraction efficiency of metal elements.Approximately 97%of P was extracted from low-temp ash at HCl concentration of 0.8 M and liquid/solid ratio of 20.0 ml/g.More than90%of impurities could be detached by CER.The macroscopic parametric equation Thomas model could clearly describe the adsorption characteristics of Fe in P-rich leachate by CER.Optimum purification conditions for P-rich leachate by cation exchange column method were determined.Approximately 84 wt%of total P in low-temp ash was recovered as struvite crystal which had low concentrations of heavy metals and could be eco-friendly for agricultural application.At last,this study systematically characterized the P transformation during low-temp co-combustion of sewage sludge with different biowastes at varied mixing ratios,revealed the transformation mechanism of P speciation both from the molecular and mineralogical aspects.In general,the introduction of biowaste increased the contents of volatile and high heating value and reduced the ash content of fuel blends,which contributed to the efficient,stable,and sufficient combustion of the sludge-biowaste blends.Organic P and pyrophosphate in the fuel blends transformed into orthophosphate during combustion.The released orthophosphate reacted with metal cations to form P-containing phases such as Fe PO₄,Al PO₄,Ca₃(PO₄)₂,and Mg₃(PO₄)₂.The relative abundance and transformation pathways of different P species were dependent on the fuel blend composition such as the type and concentration of dominate metal cations.Al and Fe phosphate minerals can transform to Ca-P phases with increasing mass fraction of biowaste in the fuel blends,which contributed to more Ca/Mg source.High Ca concentration biowastes used for blending can lead to more Ca-associated P in ash products,which has a significant contribution to P availability.