Biochar from co-pyrolysis of biological sludge and sawdust in comparison with conventional filling media for the treatment of domestic-textile wastewater in vertical flow constructed wetlands

Constructed wetlands (CWs) are low-cost, low environmental impact, and simple-technology systems for wastewater treatment, which exploit remov-al mechanisms active in natural ecosystems and have proved to be very effi-cient for the removal of conventional water quality macroparameters. How-ever, recalcitrant organic matter such as organic micropollutants (OMPs) of-ten remain unremoved in this kind of systems, unless hydraulic retention times of several days are used, which are unfeasible in the management of wastewater treatment plants, thus representing a limitation in the use of CWs, particularly considering the wide diffusion of OMPs in all kinds of wastewater, including domestic ones. A possible solution to these issues is the use of alternative filling materials to those traditionally used in CWs (i.e., sand and gravel), providing a higher surface area and therefore sup-posed to be able to host higher amounts of microorganisms and to provide a larger adsorption capacity against organic matter recalcitrant to degradation. This study aimed at investigating the removal efficiency of lab-scale verti-cal flow (VF-CWs) filled with biochar produced by co-pyrolysis of a mix-ture of sawdust and sewage sludge 70/30 (w/w), in comparison with sys-tems filled with coarse sand, gravel, and cobblestones. VF-CWs were fed for eight months with a real effluent wastewater from an activated sludge bio-logical treatment operating in a mixed domestic-industrial textile context and providing treated wastewater for reuse purposes. VF-CWs influent and effluent wastewater were monitored for conventional water quality parame-ters and absorbances at 254 nm and 420 nm, the latter as rapid and reliable screening parameters for investigating the removal efficiency of residual dyes and OMPs absorbing in the UV and visible regions. Biochar-based VF-CWs provided a statistically significant improvement in chemical oxy-gen demand (COD, Δ≈22%) and ammonia (Δ≈35%) removal, as well as in the reduction of UV-Vis absorbances (Δ≈32-34% and Δ≈28% for 254 and 420 nm, respectively), compared to gravel-filled microcosms. The higher removal of organic matter, measured through COD and UV-Vis absorbance, was mainly attributed to the well-known adsorption properties of biochars, while for nitrogen the biological mechanisms played a predominant role.