Abstract:

Decoupling of temperature and pressure in hydrothermal process was achieved via external nitrogen gas pressurization. Effects of organic matters and initial pressure on the phosphorus transformation during subcritical hydrothermal conversion of sewage sludge were investigated, and the feasibility of vivianite formation was verified simultaneously. The results show that initial pressure can enhance the enrichment of phosphorus in hydrochar. It promotes the decomposition and conversion of glucose and protein into reducing substances, which favors the formation of vivianite confirming by its strong intensity of diffraction peaks. Phosphorus is preferred to enrich in hydrochar and further enhanced by increasing initial pressure. The increase of total phosphorus (TP) content from 31.45 mg/g (0.1 MPa) to 39.68 mg/g (1.0 MPa) is observed. At hydrothermal temperature of 110℃, increasing the initial pressure promotes the conversion of Fe(Ⅲ)-P to Fe(Ⅱ)-P. The Fe(Ⅱ)-P content in the hydrochar (1.0 MPa) is 6.31 times that of raw sludge. Increasing the initial pressure promotes the decomposition of organic compounds such as carbohydrates and proteins in sludge. The organic acids in the process water lower its value of pH, while the increasing $\mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}-\mathrm{N}$ concentration is observed due to the degradation of proteins. The increase in redox potential of process water indicates that reducing substances such as organic acids are consumed, which promotes the reduction of trivalent iron with the increase of Fe(Ⅱ)-P content in hydrochar. The characteristic diffraction peaks of vivianite are observed in the XRD patterns of hydrochar samples under the pressurized conditions, indicating that it is feasible for the formation of vivianite during the decoupling temperature-pressure hydrothermal conversion of sewage sludge. This study provides a new way for the phosphorus recovery from sewage sludge.

Key words: decoupled temperature and pressure, redox potential, reduction, waste treatment, recovery

摘要：

利用外源氮气加压的方式实现了水热过程温度-压力解耦，考察了亚临界水热过程有机物组成和初始水热压力对污水污泥中磷形态转化的影响，同时验证了该过程中生成蓝铁矿的可行性。污泥模化物水热实验表明，初始水热压力强化了磷富集于水热炭中，促进了葡萄糖和蛋白质转化为还原性物质，从水热炭中观察到较强的蓝铁矿X射线衍射峰。城市污泥水热实验表明，水热处理有利于磷富集于水热炭中，压力强化了磷向水热炭迁移，总磷（TP）浓度从31.45 mg/g（0.1MPa）提升到39.68 mg/g（1.0 MPa）。水热温度110℃时，增加初始水热压力促进了污泥中Fe(Ⅲ)-P向Fe(Ⅱ)-P转化，1.0 MPa水热炭中Fe(Ⅱ)-P浓度为原始污泥的6.31倍。提高初始水热压力促进了污泥中碳水化合物和蛋白质等有机物分解形成有机酸，使得水热液pH降低；蛋白质转化导致水热液中$\mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}-\mathrm{N}$浓度升高。同时，水热液氧化还原电位升高，说明有机酸等还原性物质被消耗，促进了三价铁还原，致使水热炭中Fe(Ⅱ)-P浓度升高，有利于蓝铁矿生成。增压条件下，污泥水热炭XRD衍射图中观察到了蓝铁矿的特征峰，说明采用温度-压力解耦水热方法可以生成蓝铁矿。本研究为污泥磷资源回收提供了新思路。

关键词: 温度-压力解耦, 氧化还原电位, 还原, 废物处理, 回收