污泥水热处理时磷的迁移转化机制及其调控研究

Sewage sludge represents an important secondary phosphorus (P) source due to its high P content. By hydrothermal treatment (HTT), both phosphorus and nitrogen in the sludge can be converted into dissolved phosphates and ammonia, which provides the ions for the struvite precipitation. Simultaneously, the sludge can be dewatered deeply and upgraded. However, less attention has been paid to P recovery from the sludge with HTT. And the conversion of phosphorus to dissolved phosphates was low in the existing researches. It is the key to improve its conversion by scientifically understanding the migration and transformation behaviors of P speciation in the HTT process and exploring its regulation mechanism. In this study, according to the properties of sewage sludge and its derived hydrochar, the optimization of P extraction condition and its extraction method will be made to characterize P speciation, based on chemical sequential extraction, liquid 31P NMR spectroscopy and XANES. The interaction between P speciation and heavy metal, EPS and surface adsorption of hydrochar is studied as well as the effects of HTT conditions on the phase of P speciation. Combining theoretical simulation (Visual MINTEQ, et al.), the mechanism about migration and transformation of P speciation under HTT conditions will be established. According to these, the factors which have dominant impacts on P conversion will be determined by principle component analysis to explore the regulation methods, which can improve P conversion. At the end, a new idea about P recovery from sewage sludge based on HTT will be proposed. The research achievements have great theoretical significance for the realization of new process coupling the P recovery and sewage sludge dewatering and upgrading.

高磷含量使污泥成为重要的二次磷资源。水热处理可使污泥中氮磷转化为铵离子和磷酸根离子，为以鸟粪石沉淀反应回收磷提供了离子条件，同时还实现污泥脱水提质。现有研究并未关注水热条件下磷回收问题，且水热处理时磷转化率偏低。科学认识水热条件下污泥中不同形态磷的迁移转化机理和建立促进磷转化的调控机制是提高其转化率的关键。本项目基于化学连续提取法、液态31P NMR和XANES，依据样品特性，优化磷的提取条件和提取方法，建立适合污泥及水热炭的磷形态综合定量方法；通过分析磷与胞外聚合物（EPS）成分、金属离子和水热炭吸附特性之间的相互作用以及水热条件对磷赋存方式的影响，结合理论模拟，阐明水热条件下污泥中磷的迁移转化机理；基于此，由主成分分析确定影响磷转化的关键因素，探索促进其转化的调控方法，实现基于污泥水热处理以鸟粪石形式回收磷这一新思路。研究成果有望为实现耦合磷回收的污泥脱水提质预处理新工艺提供理论依据。

高含水率是制约城市污水污泥处理处置的瓶颈之一，水热处理可以实现污泥深度脱水和提质。而污泥磷含量高，使其成为重要的二次磷资源。若能阐明水热条件下磷的迁移转化机制是实现从污泥中有效回收磷资源的前提。本项目首先考察了水热温度（160-280oC）、水热时间（30-120min）和溶液初始pH（3-13）对污泥中磷迁移转化特性的影响规律。发现水热条件下，污泥中绝大部分有机磷均转化为无机磷，且富集在水热焦中。因此研究水热处理过程中无机磷的转化特性对于实现从污泥中回收磷资源尤为关键。而且提高水热温度使得水热焦中非磷灰石无机磷（NAIP）和磷灰石磷（AP）含量均提高，并可促进非磷灰石无机磷转化为磷灰石磷。相比于水热温度，水热时间对污泥中磷的迁移转化影响较弱。碱性环境有利于水热焦中非磷灰石无机磷转化为磷灰石磷。其次，通过添加钙基添加剂（氧化钙和氯化钙）可以强化水热焦中非磷灰石无机磷转化为磷灰石磷。基于实验研究结果和吉布斯自由能最小化原理，借助热化学平衡模型，揭示了水热条件下含磷化合物的迁移转化路径；同时阐明了钙基添加剂调控污泥中磷定向转化为羟基磷酸钙（Ca5(PO4)3OH）的作用机制。在钙离子的作用下，污泥中磷酸铁、磷酸铝和磷酸镁等含磷化合物倾向于先转化为Ca3(PO4)2和CaHPO4；若同时存在碱性环境时，上述化合物则进一步转化为Ca5 (PO4)3OH。因此，碱性环境和供应充足的钙离子是强化水热处理过程中磷灰石磷生成的前提条件。随着氧化钙添加量的增加，水热焦中磷的有效性提高。本项目还通过柠檬酸将水热焦中磷释放至溶液中，并与水热处理废水中铵根离子结合形成鸟粪石结晶，氮磷回收率均大于50%，且鸟粪石结晶纯度大于95%，纯度较高，证实基于水热处理实现污泥中氮磷协同回收是可行的。本研究可为从污泥中有效回收磷资源提供理论和技术储备。