Circular economy (CE) is defined as a system in which the value of products, materials and resources is maintained for as long as possible, minimizing their consumption and the generation of waste. Within the CE framework, anaerobic digestion (AD) represents an attractive technology, as it uses waste to produce biogas as renewable energy and stabilizes the sewage sludge for land application. In this way, this technology contributes to "closing the loop"between energy consumption, food production and the disposal of the subsequent waste. However, these potential benefits may be limited by negative impacts related to the land disposal of the stabilized sewage sludge. For example, the presence of micropollutants (MPs) in the input sludge and the inability of current AD methods to remove them are recognized potential risks for human health and for the environment. The aim of this review is to provide an overview of the different MPs present in the raw sewage sludge and stabilized sewage sludge (biosolids) with AD, to assess their potential adverse effects, and to highlight possible remediation strategies. This review will focus on three important groups: pharmaceuticals and personal care products (PPCPs), metallic trace elements, and polycyclic aromatic hydrocarbons (PAHs). The resulting toxicity of the biosolids will depend on the operational characteristics of AD and on the physicochemical properties of the different MPs. These two factors ultimately determine their final concentration, their persistence and bioaccumulative potential, and the formation of metabolites, which sometimes can be more toxic than the corresponding parental compounds.

This work was supported by ANID/FONDAP/15130015. M. Venegas thanks to National for Research and Development (ANID) for her Scholarship Program National for Research and Development (ANID)/Scholarship Program 2016-21160100 for supporting her Ph.D studies at the Universidad de Concepción, Chilen.