Characteristics Of Wasted Aqueous Phase Products From Hydrothermal Carbonization And Its Utilization In Paddy Field

Hydrothermal carbonization（HTC）technology,as a rapidly developing waste biomass resource disposal technology,has been applied to environmental remediation,soil improvement,carbon sequestration,and emission reduction as a solid product.However,the resourceful disposal technology of the liquid phase by-products（HAP）of HTC has yet to be solved.It has been found that during the HTC process,a large number of nutrients will enter the liquid,while the process will also produce more humus;this indicates that HAP has the potential for resource utilization.In contrast,as a conventional and widespread artificial ecosystem,paddy fields have a powerful ability to degrade and use organic fractions and nutrients in eutrophic water bodies.Based on this,this study proposes using the paddy field ecosystem for the resource utilization of HAP.In this study,three biomass materials（food waste,sewage sludge and pig manure）were selected for HTC at three temperatures（180℃,220℃and 260℃）and the HAP produced was systematically analyzed.The effects of raw material and preparation temperature on the fugacity of nitrogen,phosphorus and potassium,the content of（heavy）metals and the organic fraction in the HAP were first investigated.The experiments were carried out in two consecutive seasons in2020 and 2021.In this study,two types of HAP,food waste and sewage sludge,prepared at 180℃and 260℃,were selected for return to the paddy in the 2020 rice season to explore the effects of different HAP on the paddy system.In 2021,HAP obtained from food waste and sewage sludge prepared at 220℃was used as the experimental material and two different addition levels were set in the treatment to explore the effect of the application level on the paddy system.The main results of this study are as follows.（1）Basic characteristics of HAP.The p H of HAP is influenced by the raw material,with sludge-generated HAP being weakly alkaline and food waste and pig manure HAP being acidic or weakly acidic.Meanwhile,the p H value of HAP could be increased by increasing the preparation temperature.The HTC process causes a large amount of carbon and nitrogen from the biomass to the liquid phase,resulting in total organic carbon and total nitrogen in the HAP that can reach 5.41 g L^-1-9.68 g L^-1 and 1.94 g L^-1-3.10 g L^-1 respectively.HTC is an artificial and rapid humification process.HAP fluorescence excitation-emission matrix（EEM）revealed that the prominent fluorescence peaks were located in the characteristic peaks of humic acids,with food waste,sewage sludge,and pig manure accounting for 86.0%-90.4%,84.9%-87.8%,83.5%-85.3%of the total fluorescence material.While feedstocks with more cellulose（e.g.,food waste）had more humic content.Furthermore,increasing the preparation temperature promoted the conversion of humic substances,thus reducing their humic content.Analysis of HAP by electrospray ionisation-Fourier transform ion cyclotron resonance mass spectrometry（ESI-FT-ICR-MS）revealed that the lignin derivative/carboxylic rich alicyclic molecule（CRAM）-like structures increased continuously content of the molecule could reach 44.8%-76.2%and that the higher HTC temperature（260℃）would partially degrade the lignin,resulting in a reduction of the lignin derivative content.Analysis of the heavy metal content of HAP showed that the heavy metal content in HAP did not exceed the standard for agricultural biogas slurry.The risk of heavy metal contamination was manageable when returned to the field as a liquid fertilizer.（2）Disturbance of field surface water in paddy ecosystems due to HAP.The entry of HAP into the paddy field was found to increase the concentration of NH₄⁺-N and COD in the field surface water and reduce the dissolved oxygen in the field surface water in the short term.However,after seven days of purification treatment in the paddy wetland,both N and organic matter in the HAP were effectively absorbed or degraded.The degradation of organic matter in field water is mainly a reduction of humus,with more efficient degradation of SHAP.HAP application influenced the effect of HAP application on the paddy ecosystem,and there was no increase in the risk of heavy metal pollution.（3）HAP application affected soil nutrient content;HAP application caused a short-term decrease in soil p H;HAP application increased soil total organic carbon content by 4.0%-32.1%and 6.1%-10.0%in 2020 and 2021respectively,with HAP from food waste increasing soil total organic carbon content even more.The potential for HAP from sewage sludge to enhance soil N is even more tremendous.（4）The application of HAP affected soil DOM and soil microbial diversity.The impact of HAP on soil DOM was mainly to increase its humus content,with the three soil humus contents increasing by 2.59-4.10,045-1.75 and 0.34-6.36 times in 2021,respectively.The HAP of food waste with higher cellulose content enhanced soil humus more.The application of HAP also increased the percentage of CHON molecules in the soil DOM,increasing the soil N content.As HAP is a lignin-derived substance,the entry of HAP increased soil lignin derivatives by 16.8%-44.4%,thus increasing the carbon content.HAP had no significant effect on the overall diversity abundance of the microbial community.Still,it grew some bacteria expression associated with organic matter degradation and soil contamination,such as Asteraceae,Acetobacterales,KD4-96（Chloroflexi phylum）and Rhodopseudomona in response to the changes in the soil.（5）Paddy ecosystems use HAP to achieve stable or increase yields.It was found that the partial replacement of nitrogen fertilizer with HAP could meet the growing demand for nitrogen and potentially promote or stabilize rice growth of rice under the condition of reduced nitrogen fertilizer application in paddy systems.In particular,increases in rice seed yield of 1.2%-34.1%were achieved in the 2020 rice season and 1.2%to 8.1%in the 2021 rice season.The impact of different feedstocks on rice production varies,and sludge HAP increases rice grain yield more significantly.（6）Paddy ecosystems using HAP,no increased risk of ammonia emissions.HAP by lowering the p H of the paddy soil and water system during the application period,promoting soil nitrogen fixation and rice nitrogen uptake,resulting in a 8.09%-27.68%reduction in NH₃volatilization in the paddy field（P>0.05）.The reduction effect of HAP on NH₃ volatilization varied among different raw materials,with sludge HAP having a better reduction effect than food waste HAP.In summary,this study shown that the HTC process caused a large number of nutrients and organic fractions to enter the liquid phase product,which had the potential to be returned to the field as a liquid fertilizer.Multiple positive effects could be achieved by applying HAP in rice ecosystems and partially replacing chemical fertilizers.This study provides new ideas for the resourceful use of HAP and NH₃ reduction in paddy fields.In the future,an in-depth analysis of the impact mechanisms of the relevant processes should be conducted,the environmental risks of HAP farmland applications should be thoroughly investigated,and its comprehensive economic and environmental benefits should be evaluated to provide a low-cost and high-efficiency response strategy for the resource utilization of liquid by-products of HTC technology.