Upgrading residues from wastewater and drinking water treatment plants as low-cost adsorbents to remove extracellular DNA and microorganisms carrying antibiotic resistance genes from treated effluents

Highlights: • Sewage-sludge biochar and iron-oxide-coated sands were tested to adsorb DNA and microorganisms carrying resistances. • Up to 97% of genes tested were removed from the total environmental DNA of unfiltered effluent. • Up to 85% of ARGs and MGEs of free-floating extracellular DNA were retained. • Sewage-sludge biochar displayed the highest adsorption efficiencies. • By-products from wastewater and drinking water treatment plants can be re-used for sanitation. Wastewater treatment is challenged by the continuous emergence of chemical and biological contaminants. Disinfection, advanced oxidation, and activated carbon technologies are accessible in high-income countries to suppress them. Low-cost, easily implementable, and scalable solutions are needed for sanitation across regions. We studied the properties of low-cost absorbents recycled from drinking water and wastewater treatment plant residues to remove environmental DNA and xenogenetic elements from used water. Materials characteristics and DNA adsorption properties of used iron-oxide-coated sands and of sewage-sludge biochar obtained by pyrolysis of surplus activated sludge were examined in bench-scale batch and up-flow column systems. Adsorption profiles followed Freundlich isotherms, suggesting a multilayer adsorption of nucleic acids on these materials. Sewage-sludge biochar exhibited high DNA adsorption capacity (1 mg g⁻¹) and long saturation breakthrough times compared to iron-oxide-coated sand (0.2 mg g⁻¹). Selected antibiotic resistance genes and mobile genetic elements present on the free-floating extracellular DNA fraction and on the total environmental DNA (i.e., both extra/intracellular) were removed at 85% and 97% by sewage-sludge biochar and at 54% and 66% by iron-oxide-coated sand, respectively. Sewage-sludge biochar is attractive as low-cost adsorbent to minimize the spread of antimicrobial resistances to the aquatic environment while strengthening the role of sewage treatment plants as resource recovery factories.