**1. Introduction**

Rapid urbanization and industrialization, together with new contaminants arising from many different sources, make it necessary to move forwards with research to face future challenges regarding water pollution.

Conventional wastewater treatment plants (WWTPs) are designed primarily to remove organic matter and nutrients. For this reason, in many cases, these treatments are inefficient for the removal of specific pollutants, with the consequent risk that this entails.

Emerging contaminants (ECs) represent a wide group of potentially hazardous compounds that have been classified into various categories, including pharmaceutical and personal care products (PPCPs), per- and poly-fluoroalkyl substances (PFAS), flame retardants, surfactants, endocrine-disrupting chemicals (EDCs), and microplastics (MPs) [1]. Recently, it has been highlighted that chemical and plastic pollution has surpassed global boundaries, resulting in harmful effects, not only for the environment, but also for the humanity [2].

Furthermore, large amounts of sludge—the main residue originating from WWTPs are produced every year. The management of this waste leads to high costs, both in economic and environmental terms, so exploring alternatives for its valorisation is essential. Residue reduction and reuse of wastewater seem to be an excellent option to achieve a circular economy, lessening the environmental impacts associated with wastewater treatment processes. In addition, different efforts to valorise agricultural by-products to treat wastewater have been conducted, which also meets sustainability goals.

In view of the current situation, this Special Issue aimed to compile the latest research on relevant wastewater treatment concerns, such as EC occurrence and removal in WWTPs, novel technologies for wastewater and sludge treatment, sludge valorisation, transformation of agricultural residues into materials for wastewater treatment, etc.

Of the papers submitted to this Special Issue, eight were finally accepted and published. These articles can be organized into three different topics, as synthetized below.
