| Food waste refers to various food wastes produced in industrial and residential activities and it is an important part of municipal solid waste.Its chemical components mainly include starch,protein,hemicellulose,cellulose,etc.,among which starch and protein have a relatively high proportion.Food waste has the characteristics of high organic matter content,high moisture content,low calorific value,and high salt content.The classified collection and disposal of food waste has brought a series of challenges to the existing food waste disposal methods.After the waste is sorted and collected,the original disposal methods mainly based on mixed incineration and landfill cannot effectively utilize the materials and energy in the food waste.High water content reduces the energy utilization efficiency of food waste incineration.Bio-composting takes a long time and the metabolic process of bacteria is difficult to control accurately.There are also problems such as odor,leachate pollution,and salt damage.The hydrothermal conversion technology reacts in a high-temperature and highpressure aqueous solution environment,without dehydration pretreatment of food waste.High-temperature treatment can also completely kill germs.Hydrothermal conversion has high added value product and good adaptability to raw materials.Organic matter such as sugars and proteins in food waste can be hydrolyzed and repolymerized into high value-added solid-phase products during the hydrothermal conversion process,such as for fuel and environmental treatment.In this study,using food waste as raw material,the hydrothermal conversion process for organic fertilizer production was explored.The hydrothermal conversion process of typical components of food waste and the characteristics of the humus product were analyzed.The optimal hydrothermal conversion conditions for the production of organic fertilizer from food waste were studied.At the same time,the composition of humus and nutrients(nitrogen,phosphorus and potassium)were regulated to improve the quality of organic fertilizer products.As for the "salt damage" problem of food waste organic fertilizer products,this study analyzed the distribution of salts and heavy metals and evaluated their environmental risk.Preliminary assumptions and simulations were made on the process flow of food waste hydrothermal conversion.Comprehensive experiments and simulation data were used to compare and evaluate the life cycle assessment of hydrothermal conversion technology with traditional waste incineration and bio-composting technologies,so as to intuitively understand the environmental impact of process of hydrothermal conversion.The main research content of this paper is as follows:Taking the typical components of food waste(starch,protein,hemicellulose,and cellulose)as the research objects,the hydrothermal conversion laws of typical components at different temperatures were studied and the interaction between starch and protein was studied.The results showed that starch could be hydrolyzed into glucose below 200 °C.Hemicellulose was very unstable and its hydrolysis priority was very high in the hydrothermal process.Its hydrothermal product was closest to coal.Protein could be hydrolyzed at about 170 °C.Amino acids were unstable in the hydrothermal process and were prone to further degradation.Cellulose could not be completely hydrolyzed even at 230 °C.The content of humic acid in the single-component hydrothermal product was extremely low.The humic substance in the product at 230 ℃ was mainly humin.There was an obvious interaction between starch and protein.The reaction path of glucose was significantly changed.The content of humic acid in the solid phase product of the co-hydrothermal conversion of the two was greatly increased while the concentration of monosaccharide,protein,and amino acids in the water phase product were significantly reduced.According to the statistics of the physical components of food waste in China,the ratio of cabbage,pomelo peel,rice and chicken intestines were used to simulate food waste.The influence of key parameters on the hydrothermal conversion process,product humus and nutrient elements were studied and analyzed.The formation mechanism of humus was explored.The results showed that temperature had a more significant effect on the hydrothermal conversion process and product properties than residence time.Food waste had the highest humic acid content at 200 ℃and 30 min residence time.During the formation of hydrothermal solidphase products,the proportion of small molecular weight fulvic acid gradually decreased while the proportion of highly polymerized humic acid and humin gradually increased.At about 200 ℃,the condensation of polysaccharides,protein hydrolyzed monomers,various aromatic acids and phenolic substances was the main process of forming humic acid.Different temperatures lead to different degrees of carbonization and polymerization of the products.Excessively high temperatures could lead to intensified carbonization and polymerization of organic matter.Hydrothermal solid phase products existed in the form of large molecular weight aromatic clusters,which increased the content of humin and were not conducive to the formation of humic acid.During the hydrothermal conversion process,more than 80% of the potassium elements were transferred to the water phase.About 50% of the phosphorus elements and 60% of the nitrogen elements remained in the solid phase products.The regulation of the proportion of different humus and the form of nutrients(nitrogen and phosphorus)in the hydrothermal solid phase product was studied by adjusting the pH value of the initial feed water.The results showed that the acidic additives accelerated the hydrolysis of protein and reduced the recovery rate of nitrogen elements.It also strengthened the aromatization and greatly promoted the formation of humin.Although the yield of solid phase products was slightly increased,the quality of fertilizers declined significantly.Under alkaline conditions,more protein is retained in the solid-phase product.Phosphorus exists in the form of inorganic salts and the carbon content of humic acid has increased at the same time.Compared with the solid-phase product under neutral conditions,the carbon content of humic acid increased by 27.87 %,the carbon content of humin was reduced by 34.37%.The migration rules of inorganic salts and the form changes of heavy metals in food waste during hydrothermal conversion were analyzed.The environmental risk of inorganic salts and heavy metals was evaluated.The analysis of the ash content of the food waste in this study showed that the simulated food waste with vegetables,fruits,rice and meat had a low ash content,which lead to a low phosphorus recovery of the solid phase.The migration law of inorganic salts during the hydrothermal process had been proved.After hydrothermal conversion,most of the soluble ions were transferred to the water phase.The solid phase product had better nutritional elements performance and the conductivity was lower than that of traditional bio-composting products and the lower limit(4 m S/cm)of organic fertilizer environments,reflecting lower risk of sodium ions and salt.Heavy metals Pb,Cu,Ni,Cd and Zn were complexed by humic acid during hydrothermal conversion and exist in a more stable form.Hydrothermal conversion could generally reduce the environmental risk of heavy metal elements.This paper preliminarily conceived the process of food waste hydrothermal conversion to produce organic fertilizer.The Aspen Plus software was used to simulate the process flow.Based on the simulation and experimental data,the hydrothermal conversion technology was assessed by life cycle assessment together with incineration technology and biological aerobic composting disposal technology.The results showed that the steam energy consumption of the system was much higher than electricity energy consumption.The life cycle assessment results showed that the hydrothermal conversion technology was superior to incineration and aerobic composting in terms of comprehensive standardized environmental impact performance.Among the input materials,the environmental burden caused by a large amount of steam consumption and the discharge of organic acids in the water phase was very limited.So the hydrothermal conversion technology is a clean and efficient means of food waste treatment. |
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