Antibiotic resistance genes and mobile genetic elements removal from treated wastewater by sewage-sludge biochar and iron-oxide coated sand

Abstract

Disinfection of treated wastewater in wastewater treatment plants (WWTPs) is used to minimize emission of coliforms, pathogens, and antibiotic resistant bacteria (ARB) in the environment. However, the fate of free-floating extracellular DNA (eDNA) that do carry antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) is overlooked. Water technologies are central to urban and industrial ecology for sanitation and resource recovery. Biochar produced by pyrolysis of sewage sludge and iron-oxide-coated sands recovered as by-product of drinking water treatment were tested as adsorbents to remove ARGs and MGEs from WWTP effluent. DNA adsorption properties and materials applicability were studied in batch and up-flow column systems at bench scale. Breakthrough curves were measured with ultrapure water and treated wastewater at initial DNA concentrations of 0.1-0.5 mg mL-1 and flow rates of 0.1-0.5 mL min-1. Batch tests with treated wastewater indicated that the adsorption profiles of biochar and iron-oxide coated sand followed a Freundlich isotherm, suggesting a multilayer adsorption of nucleic acids. Sewage-sludge biochar exhibited higher DNA adsorption capacity (1 mg g-1) and longer saturation breakthrough times (4 to 10 times) than iron-oxide coated sand (0.2 mg g-1). The removal of a set of representative ARGs and MGEs was measured by qPCR comparing the inlet and outlet of the plug-flow column fed with treated wastewater. ARGs and MGEs present as free-floating eDNA were adsorbed by sewage-sludge biochar at 85% and iron-oxide coated sand at 54%. From the environmental DNA consisting of the free-floating extracellular DNA plus the intracellular DNA of the cells present in the effluent water, 97% (sewage-sludge biochar) and 66% (iron-oxide coated sand) of the tested genes present were removed. Sewage-sludge biochar displayed interesting properties to minimize the spread of antimicrobial resistances to the aquatic environment while strengthening the role of WWTPs as resource recovery factories. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=159 SRC="FIGDIR/small/302018v1_ufig1.gif" ALT="Figure 1"> View larger version (48K): org.highwire.dtl.DTLVardef@bdc78borg.highwire.dtl.DTLVardef@177a537org.highwire.dtl.DTLVardef@1ffefdeorg.highwire.dtl.DTLVardef@1dd1457_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LISewage-sludge biochar and iron oxide coated sands were tested to adsorb DNA and cells. C_LIO_LIBiochar removed 97% of genes tested from environmental DNA of unfiltered effluent. C_LIO_LI85% of ARGs and MGEs of free-floating extracellular DNA were retained by biochar. C_LIO_LIBiochar is a WWTP by-product that can be re-used for public health sanitation. C_LI