Study On The Preparation And Adsorption Properties Of Hydrochars From Sewage Sludge By Microwave-assisted Hydrothermal Carbonization

With the development of industry and social progress,as well as people’s higher and higher requirements for the environment,the amount of urban sewage is rising.As an inevitable companion of sewage treatment,the production of sludge increases year by year.According to statistics,in 2021,the total amount of urban sewage treatment reached 208million m³/d,and the dry sludge production reached 1.42×10⁷tons.Sludge treatment and disposal costs often reach half of the entire sewage treatment plant operating costs.How to achieve efficient,safe and economical treatment and disposal of sludge has become one of the research hotspots in the field of environmental protection.On the one hand,as a potential resource,sludge has a certain recycling value.On the other hand,a large amount of pollutants accumulate in sludge,and the safe disposal is also worth further study.In recent years,thermochemical treatment methods,such as pyrolysis,incineration,gasification,liquefaction and hydrothermal carbonization,have been applied in the research of sludge resource utilization.Hydrothermal carbonization is a heat treatment technology that converts biomass into hydrochars at relatively low reaction temperature.hydrochars has been widely used in soil improvement/remediation agents,adsorbents,catalyst carriers and solid fuels.Researchers have carried out many researches on the hydrothermal carbonization of sludge,such as process parameter optimization,co-hydrothermal carbonization,migration and transformation of nutrient elements and pollutants,etc.Compared with traditional heating,microwave heating has higher thermal effect.In addition,microwave heating has a certain"non-thermal effect"in many biomass thermochemical processes.Therefore,microwave heating has also been introduced into the hydrothermal carbonization of lignocellulose and algae.The composition and structure of sludge and lignocellulose and algae are obviously different,and the characteristics of microwave hydrothermal carbonization are bound to be different.Therefore,the further study of the characteristics of microwave hydrothermal carbonization of sludge can provide a new idea for the development of sludge hydrothermal carbonization technology,but also can broaden the application field of microwave hydrothermal carbonization.In this study,microwave-assisted heating was introduced into the hydrothermal carbonization process of sludge,and the following three aspects were studied.（1）The main parameters of sludge microwave hydrothermal carbonization process were optimized,including reaction temperature,reaction time,solid-liquid ratio and catalyst,etc.The results show that the optimal parameters of microwave-assisted sludge hydrothermal carbonization are as follows:reaction temperature 230℃,reaction time30min,solid-liquid ratio 0.03g/m L.The hydrochars yield decreased with the increase of reaction temperature and time,but increased with the increase of solid-liquid ratio.The addition of Ca O as catalyst further increased the hydrochars yield,while H₃PO₄decreased the hydrochars yield.Compared with raw sludge,hydrochars shows higher aromaticity,carbonization,porosity and polarity,but has lower calorific value and poor combustion performance.At the same time,the main nutrients（N/P/K）were fixed in the hydrochars,and Ca O as a catalyst further promoted the recovery of P and K in the hydrochars.All heavy metals were enriched in hydrochars,and the leaching content of heavy metals decreased.When H₃PO₄catalyst was added,the total content of Zn and Cd in hydrochars decreased,but the leaching amount increased.（2）The feasibility of process water cycle as a medium for sludge hydrothermal carbonization was discussed.The results showed that the yield of hydrochars increased with the increase of the number of water recircuation in the process,and the highest yield was 76.42wt.%at the fifth recircuation.The hydrochars produced by process water recirculation always has low content of C,H,O and N,and low calorific value.Aromaticity and polarity increased,but carbonization decreased.The contents of TP and TK in hydrochars are higher than those in sludge raw materials.Although the contents of SP and SK are increased during the recycling process,they are still lower than those in raw materials,which indicates that hydrochars is a good slow-release fertilizer.At the same time,recycling can effectively reduce the content of heavy metals except Zn in hydrochars.（3）Methylene blue solution（MB）was used as the treatment object to simulate the alkaline organic dye waste solution to investigate the adsorption capacity of sludge hydrochars obtained under microwave conditions.The results show that the optimal adsorption amount of hydrothermal carbon（HC）without catalyst is 8g/L.The optimal adsorbent dosage of hydrochars（HC-Ca O）with Ca O as catalyst is 10g/L,and the removal rates of MB by two kinds of hydrochars increase with the increase of adsorbent dosage.The optimal adsorbent content of HC was 120mg/L.The optimal adsorbent content of HC-Ca O was 160mg/L,and the removal rates of MB decreased with the increase of adsorbent content.The removal rate of MB continued to increase with the reaction time,and the change trend of removal rate tended to be flat after 60 minutes.Reaction p H and temperature have little effect when other conditions are optimal.The adsorption process of MB by hydrochars is more consistent with the quasi-second-order kinetic model,indicating that the adsorption process in this experiment is chemical adsorption.The correlation between the two kinds of hydrochars and Langmuir adsorption model is the highest,and it is inferred that the related adsorption process is approximately monolayer adsorption.In this paper,the preparation of hydrochars from sludge assisted by microwave and its adsorption performance were studied.The results showed that microwave hydrothermal carbonization could obtain high yield and high quality hydrochars,which also had certain feasibility and great potential in the adsorption of alkaline dyes.