As the by-product of wastewater treatment, a large amount of sludge is generated annually. Turning this waste into a resource is a feasible and sustainable strategy to reduce potential environmental risks and recover energy. As a way to realize the international goal of carbon dioxide emission peak and carbon neutrality, producing biochar from sludge has gained worldwide attention. This review evaluates recent progress in synthesis techniques for biochar of sludge origin. Different preparation techniques and their key affecting factors are compared and described. The obtained sludge-derived biochar could be employed for eliminating aqueous pollution or purifying wastewater, mainly through adsorption and catalytic reactions. The removal of various pollutants by sludge-derived biochar and its related mechanisms are discussed and summarized in detail. This review will be conducive for a comprehensive understanding of recent progress in sludge-derived biochar study and for guiding the purposeful production of biochar, as well as for developing wastewater purification technology based on sludge-derived biochar.

Numerous studies have unequivocally demonstrated that biochar and, to a lesser degree, earthworms can independently improve soil fertility and crop productivity, although information about their co-application effects on soil characteristics is limited. In this review, (1) earthworm biomarkers and underlying influencing factors, as well as the changes in the amended soil quality in response to co-application of earthworms and biochar are presented, (2) the functional interactions between earthworms and biochar in soil are summarized; (3) the principles governing the synergetic effects of biochar and earthworms on soil quality enhancement are probed; and (4) alternative strategies to optimize the efficacy of earthworm and biochar amendments are provided. It is noteworthy that while low doses of biochar can have a positive effect on various earthworm biomarkers, including growth and reproduction, restoration of the intestinal environment, and the mitigation of cellular organelle toxicity and genetic damage, high biochar dosages can yield adverse effects. Conversely, earthworms play a crucial role in distributing biochar particles deeper into the soil matrix, bolstering carbon sequestration potential, and enhancing the persistence and efficiency of biochar utilization. Moreover, earthworms stimulate the production of soil extracellular enzymes by microorganisms, which are pivotal to the processing, stabilization, and decomposition of soil organic matter, as well as nutrient cycling in terrestrial ecosystems. Additionally, they enhance the binding affinities of these enzymes to biochar. Significantly, changes in earthworm biomarkers in response to biochar integration are predominately governed by biochar properties and dosage, contact time, and soil type. Graphical Abstract