Resourcelization Study Of Residual Sludge Hydrothermal Liquid As Carbon Source For Wastewater Biological Denitrification

The residual sludge is produced along with the sewage treatment process,and has the dual characteristics of energy and pollutant.Hydrothermal carbonization is a treatment technology for sludge resourceization and reduction,and the application of hydrothermal resourceization of residual sludge from wastewater treatment plants in wastewater treatment can realize a concept of"treating water with mud and mud with water".In this study,the hydrothermal treatment of residual sludge is used as a carbon source for biological denitrification of wastewater.Firstly,we investigated the effect of different pretreatment methods on the effectiveness of hydrothermal sludge as a carbon source for biological denitrification of wastewater;secondly,we separated the components of hydrothermal sludge by soil drenching,and used the retention effect of soil to intercept and adsorb large organic matter and nitrogen and phosphorus substances in hydrothermal sludge,and learned the physicochemical properties of hydrothermal sludge and drenching sludge,and compared the effectiveness of hydrothermal sludge and drenching sludge as a carbon source for denitrification,and the effect of applying hydrothermal sludge on the physicochemical properties of soil.The effects of hydrothermal solution on soil physicochemical properties were compared.The following conclusions were drawn.（1）Pretreatment affects the properties of hydrothermal liquid and its efficacy for biological denitrification of wastewater.H₂SO₄ catalysis and ultrasonication can promote the dissolution of organic matter from hydrothermal process,and the best dissolution of hydrothermal process sludge was achieved after pretreatment with 144W power ultrasonication;H₂SO₄ catalysis contributes to the hydrolysis of proteins in hydrothermal liquid,and the combination with ultrasonication can further promote the hydrolysis,resulting in a decrease in the percentage of organic nitrogen in hydrothermal liquid.In addition,the hydrothermal liquor catalyzed by 0.05 mol/L H₂SO₄ alone,240W power ultrasound alone and combined pretreatment of both had better organic biochemical properties;however,using it as a carbon source for denitrification,the H₂SO₄-catalyzed sludge hydrothermal liquor had only 44%removal of nitrate nitrogen,which was similar to the denitrification effect of the hydrothermal liquor without any pretreatment（49%）,while The sludge hydrothermal solution treated with combined ultrasound and H₂SO₄ achieved 73%and 66%removal of nitrate nitrogen,respectively.Therefore,pretreatment with combined ultrasound or acidification prior to sludge hydrothermal treatment can enhance the utilization effectiveness of sludge hydrothermal liquor as a carbon source for denitrification and denitrification.（2）Effect of soil drenching on hydrothermal liquid properties.By studying the effects of different hydrothermal time and temperature on the organic matter leaching and biochemical properties of sludge hydrothermal solution,the optimized hydrothermal conditions were obtained:time of 3 h and temperature of 180℃.The hydrothermal solution prepared under this condition was drained through different soils with different water-soil ratios,and the draining conditions were determined according to the retention effects of SCOD,N,P,protein and polysaccharide:water-soil mass ratio 1.2:1,and acidic red soil was selected.Under this drenching condition,76%of SCOD was retained in the hydrothermal solution,mainly protein and humus,and the protein concentration decreased from 1421.6 mg/L to 144.3 mg/L,humus decreased from 659.8 mg/L to 116.1 mg/L,and the molecular weight decreased by 80%,organic nitrogen and ammonia nitrogen decreased by 87%and 95%,which effectively reduced the content of large molecule organic matter and nitrogen and phosphorus in the drenching solution.The content of macromolecular organic matter and nitrogen and phosphorus load were effectively reduced.（3）Hydrothermal liquid and hydrothermal leachate were used as carbon sources for denitrification.After domestication and stabilization,the denitrification efficiency of dewatering solution as carbon source（over 95%）was better than that of hydrothermal solution（76%-97%）,and the NO₃^--N removal rate and removal rate of single batch denitrification using dewatering solution were higher than that of hydrothermal solution.In addition,the TN removal effect（more than 59%）was better than that of hydrothermal solution（more than 40%）due to the lower ammonia and organic nitrogen load introduced by the leachate.Examination of the relative abundance of functional genes in denitrifying sludge revealed that hydrothermal and drench liquids caused down-regulation of narG and napA but up-regulation of nirS,nirK and cnorB.In addition,the increase in EPS content of sludge caused by hydrothermal liquid and drench solution may be related to the stimulating effect of aromatic compounds contained in hydrothermal liquid and drench solution on denitrifying sludge.（4）Effect of hydrothermal liquid drenching on soil physicochemical properties.The soil was drenched with hydrothermal liquid until the soil was saturated,and continued to be drenched for 2 months to keep the soil wet weight constant.Soil pH increased with the increase of total hydrothermal liquid watering;soil organic carbon content showed an overall increasing trend,but decreased due to microbial decomposition when the amount of hydrothermal liquid applied was small;total nitrogen and phosphorus content in soil showed a significant increase in general,but total nitrogen showed a decreasing pattern,which may be related to the increased microbial activity in soil and denitrification when organic carbon coexisted.In addition,the exchangeable potassium content of the soil increased slig LCy.Thus,it can be seen that hydrothermal solution by soil leaching can not only improve the denitrification efficiency of leaching solution,but the part retained by soil also has the potential to enhance the soil quality of poor soil.