title: Removal of bacterial contaminants and antibiotic resistance genes by conventional wastewater treatment processes in Saudi Arabia: Is the treated wastewater safe to reuse for agricultural irrigation?
abstract: This study aims to assess the removal efficiency of microbial contaminants in a local wastewater treatment plant over the duration of one year, and to assess the microbial risk associated with reusing treated wastewater in agricultural irrigation. The treatment process achieved 3.5 logs removal of heterotrophic bacteria and up to 3.5 logs removal of fecal coliforms. The final chlorinated effluent had 1.8 × 10(2) MPN/100 mL of fecal coliforms and fulfils the required quality for restricted irrigation. 16S rRNA gene-based high-throughput sequencing showed that several genera associated with opportunistic pathogens (e.g. Acinetobacter, Aeromonas, Arcobacter, Legionella, Mycobacterium, Neisseria, Pseudomonas and Streptococcus) were detected at relative abundance ranging from 0.014 to 21 % of the total microbial community in the influent. Among them, Pseudomonas spp. had the highest approximated cell number in the influent but decreased to less than 30 cells/100 mL in both types of effluent. A culture-based approach further revealed that Pseudomonas aeruginosa was mainly found in the influent and non-chlorinated effluent but was replaced by other Pseudomonas spp. in the chlorinated effluent. Aeromonas hydrophila could still be recovered in the chlorinated effluent. Quantitative microbial risk assessment (QMRA) determined that only chlorinated effluent should be permitted for use in agricultural irrigation as it achieved an acceptable annual microbial risk lower than 10(-4) arising from both P. aeruginosa and A. hydrophila. However, the proportion of bacterial isolates resistant to 6 types of antibiotics increased from 3.8% in the influent to 6.9% in the chlorinated effluent. Examples of these antibiotic-resistant isolates in the chlorinated effluent include Enterococcus and Enterobacter spp. Besides the presence of antibiotic-resistant bacterial isolates, tetracycline resistance genes tetO, tetQ, tetW, tetH, tetZ were also present at an average 2.5 × 10(2), 1.6 × 10(2), 4.4 × 10(2), 1.6 × 10(1) and 5.5 × 10(3) copies per mL of chlorinated effluent. Our study highlighted that potential risks associated with the reuse of treated wastewater arise not only from conventional fecal indicators or known pathogens, but also from antibiotic-resistant bacteria and genes.
authors: Water Desalination and Reuse Center, Environmental Science and Engineering, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology (KAUST), 4700 King Abdullah Boulevard, Thuwal 23955-6900, Saudi Arabia.
authors: Nada
authors: N
authors: Al-Jassim
authors: Water Desalination and Reuse Center, Environmental Science and Engineering, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology (KAUST), 4700 King Abdullah Boulevard, Thuwal 23955-6900, Saudi Arabia.
authors: Mohd Ikram
authors: MI
authors: Ansari
authors: Water Desalination and Reuse Center, Environmental Science and Engineering, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology (KAUST), 4700 King Abdullah Boulevard, Thuwal 23955-6900, Saudi Arabia.
authors: Moustapha
authors: M
authors: Harb
authors: Water Desalination and Reuse Center, Environmental Science and Engineering, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology (KAUST), 4700 King Abdullah Boulevard, Thuwal 23955-6900, Saudi Arabia. Electronic address: peiying.hong@kaust.edu.sa.
authors: Pei-Ying
authors: PY
authors: Hong
mesh_terms: D057914:Agricultural Irrigation / Q000592:standards*; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001419:Bacteria / Q000187:drug effects / Q000235:genetics / Q000302:isolation & purification; D004352:Drug Resistance, Microbial / Q000235:genetics; D008969:Molecular Sequence Data; D012329:RNA, Bacterial / Q000235:genetics; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D059027:Recycling; D018570:Risk Assessment; D012529:Saudi Arabia; D017422:Sequence Analysis, DNA; D014865:Waste Disposal, Fluid / Q000379:methods*; D062065:Wastewater / Q000382:microbiology*; D018508:Water Purification / Q000592:standards*
chemical_list: D000900:Anti-Bacterial Agents; D012329:RNA, Bacterial; D012336:RNA, Ribosomal, 16S; D062065:Wastewater
keywords: Antibiotic resistance; Conventional activated sludge processes; Microbial contaminants; Microbial risk; Water reuse