A review on Co-Hydrothermal carbonization of sludge: Effect of process parameters, reaction pathway, and pollutant transport

Growing population results in an increase Sewage Sludge (SS) from Sewage Treatment Plants (STPs). Because sewage sludge contains several pathogens and hazardous metals, it must be treated before disposal. Over the past two decades, converting sludge to energy has drawn increased attention to an attempt to achieve a circular economy. Hydrothermal Carbonization (HTC) is a thermochemical process where organic feedstock is trans-formed into a valuable product known as hydrochar (HC). This paper discusses the feasibility of the Co-hydrothermal carbonization (Co-HTC) of SS with other substances, including food waste, agricultural waste, plastic waste, lower-ranked coal, etc. Because of the synergistic effect caused by the Maillard and Mannich re-actions, different feedstock compositions improve hydrochar properties. This article also highlights the syner-gistic effects of catalyst, residence duration, moisture content, and reaction temperature on hydrochar. Comparison of the results of proximate analysis, hydrochar yield (HCY), combustion behavior, and surface functional group between Sludge produced hydrochar (SS-HC) and hydrochar generated from Co-feedstock (Co -HC). Co-HTC increases heavy metal immobilization, which has a positive influence on the environment. Recy-cling process water and using it as a feedstock for anaerobic digestion (AD) reduces the potential for greenhouse gases while increasing energy value. Some innovative techniques and prospective ideas are suggested at the conclusion of this article.