Resource Utilization Of Hydrothermal Carbonization Liquid From Excess Sludge

With the rapid development of urban sewage treatment technologies,the discharge of excess sludge is increasing and the traditional methods have long been unable to meet the requirements of sludge treatment.The hydrothermal carbonization technology isn?t restricted by water content of raw materials and has the advantages of simple preparation process,and biochar obtained can be used as soil amendment and fuel.Thus,hydrothermal carbonization technology is considered to be an ideal method for preparing biochar with high-moisture biomass.While the hydrothermal carbonization treatment is used to obtain biochar with good performance,the treatment of its liquid product is also important yet often ignored.Therefore,it is necessary to study resource utilization of hydrothermal carbonization liquid from excess sludge in depth.This paper focuses on the resource utilization of organic and nutrient components in liquid product of excess sludge by hydrothermal carbonization treatment.The excess sludge of the sewage treatment plant was treated by hydrothermal carbonization technology,and the best conditions for the release of organic and nutrients were explored.Under the optimal conditional,utilization of hydrothermal carbonization liquid（HCL）was circulated.Finally,the nutrient of HCL is recycled by struvite precipitation,and the supernatant liquor after recycling nutrient as supplemental carbon source was added to the denitrification reactor of the low carbon nitrogen ration wastewater treatment plant to improve the biological denitrification performance.Firstly,potential impacts of temperature,reaction time and the ration of material and water on hydrothermal carbonization of excess sludge were studied through a orthogonal test,in which the affecting order of the factors and optimal process conditions were determined by the values of COD and NH₄⁺-N.The results showed the best conditions for hydrothermal carbonization:reaction temperature 240°C,reaction time 180 min and the ration of material and water 1:10.Under the optimum process conditions,excess sludge was used as raw material,and the liquid product of hydrothermal carbonation was recycled as a solvent.After 10 cycles,the concentrations of COD,TN and NH₄⁺-N of the hydrothermal carbonized liquid were 220g/L,22.95 g/L and 13.4 g/L respectively.Afterwards,according to the NH₄⁺-N and PO₄^3-concentrations of liquid product of the hydrothermal carbonization after the 10 cycles,the mole ratio of Mg²⁺,NH₄⁺and PO₄^3-is adjusted by adding Mg²⁺ions and PO₄^3-ions to explore.The best condition for recovering nutrient components in the form of struvite precipitation were determined by the single factor test.The results showed that the maximum recovery rate of ammonia nitrogen was up to 97%under the condition of[Mg²⁺]:[NH₄⁺]:[PO₄^3-]=1:1:1 and pH 9.Finally,the effect of the supernatant after recycle of nutrient components by struvite precipitation on the denitrification process was investigated.In the control experiment,the nitrate removal rate of simulated wastewater was 78%.When the system COD/TN was adjusted to 4,5,6 and 7 by adding hydrothermal carbonization liquid in the simulated wastewater,the removal rates of nitrate nitrogen were 82.37%,87.47%,100%and 83.9%,respectively.Thus,the optimal addition amount of the hydrothermal carbonation liquid as a supplementary carbon source was determined,and getting the best COD/TN ratio was 6.Compared with the control,the removal rate of nitrate nitrogen in the system COD/TN=6 was increased by 28.2%.By adding methanol and the hydrothermal carbonation liquid into the reactor as supplementary carbon source,the feasibility of the hydrothermal carbonation liquid as biological nitrogen removal of wastewater was explored.the removal rate of nitrate nitrogen were 84.3%and 79.7%for the system of adding the same volume of methanol to hydrothermal carbonization liquid and the system of adding methanol to the COD/TN=6.Compared with the group added methanol,the removal rate of nitrate nitrogen in the system containing hydrothermal carbonization liquid was 18.6%and 25.4%higher,respectively.Therefore,the hydrothermal carbonization liquid of excess sludge is a kind of supplemental carbon source which is superior to methanol,and can replace methanol to strengthen the biological nitrogen removal efficiency of wastewater treatment plants.To sum up,the excess sludge is treated by proper hydrothermal carbonization technology,and the dual effect of pollution control and resource utilization is realized.