10.2 Fate of antibiotic-resistant bacteria in treatment plants

Antibiotic-resistant bacteria in wastewater pose a serious challenge. These microbes enter treatment plants from hospitals, homes, and farms, surviving various processes and potentially spreading resistance genes to other bacteria.

Conventional treatment methods reduce resistant bacteria, but some persist. Advanced techniques like membrane bioreactors and disinfection offer better removal. However, treatment plants can become reservoirs for resistance, potentially releasing these bacteria into the environment.

Antibiotic-Resistant Bacteria in Wastewater Treatment

Antibiotic resistance in wastewater treatment

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• Antibiotic-resistant bacteria enter wastewater treatment plants through various sources
  • Hospital effluents contain high levels of antibiotics and resistant bacteria (MRSA, VRE)
  • Domestic sewage includes antibiotics and resistant bacteria from household use (triclosan, ciprofloxacin)
  • Agricultural runoff carries resistant bacteria from livestock and aquaculture (tetracycline, streptomycin)
• Survival of antibiotic-resistant bacteria in treatment processes depends on several factors
  • Type of treatment process influences removal efficiency (activated sludge, membrane bioreactors)
  • Operating conditions affect bacterial survival (temperature, pH, hydraulic retention time)
  • Presence of selective pressures promotes resistance (antibiotics, heavy metals)
• Persistence of antibiotic-resistant bacteria can occur through gene transfer mechanisms
  • Vertical gene transfer passes resistance from parent to daughter cells during cell division
  • Horizontal gene transfer spreads resistance between different bacterial species
    • Conjugation transfers genetic material via cell-to-cell contact (plasmids, transposons)
    • Transformation involves uptake of free DNA from the environment (cell lysis, extracellular DNA)
    • Transduction transfers genetic material via bacteriophages (viral vectors)

Effectiveness of bacterial removal methods

• Conventional activated sludge process reduces antibiotic-resistant bacteria
  • Typically achieves 1-3 log unit reduction (90-99.9% removal)
  • Removal efficiency varies based on operating conditions and bacterial species (E. coli, Enterococcus)
• Membrane bioreactors (MBRs) demonstrate higher removal efficiency
  • Can achieve 4-6 log unit reduction (99.99-99.9999% removal)
  • Smaller pore sizes and longer sludge retention times improve removal (ultrafiltration, nanofiltration)
• Disinfection processes provide additional reduction of antibiotic-resistant bacteria
  • Chlorination, UV irradiation, and ozonation are common disinfection methods
  • Effectiveness depends on dosage, contact time, and bacterial species (Pseudomonas, Acinetobacter)
  • Some antibiotic-resistant bacteria may develop resistance to disinfectants (chlorine-resistant E. coli)

Treatment plants as resistance reservoirs

• Wastewater treatment plants provide favorable conditions for the exchange of antibiotic resistance genes
  • High bacterial density and diversity facilitate gene transfer (activated sludge, biofilms)
  • Presence of selective pressures promotes the development and spread of resistance (antibiotics, heavy metals)
• Antibiotic resistance genes can persist in treatment processes and be transferred to other bacteria
  • Horizontal gene transfer mechanisms enable the spread of resistance (conjugation, transformation, transduction)
  • Formation of biofilms on surfaces within treatment plants provides a stable environment for gene exchange (pipes, tanks)
• Treated effluent and biosolids can contain antibiotic resistance genes
  • Release into the environment may contribute to the spread of antibiotic resistance (rivers, lakes, soil)

Environmental risks of resistant bacteria

• Discharge of treated effluent into receiving water bodies poses risks
  • Potential for antibiotic-resistant bacteria to persist and spread in aquatic environments (rivers, lakes)
  • Risk of exposure to humans through recreational activities or consumption of contaminated water or seafood (swimming, fishing)
• Land application of biosolids can introduce resistant bacteria to soil
  • Antibiotic-resistant bacteria and resistance genes may persist in soil (manure, compost)
  • Potential for uptake by crops or transfer to soil microorganisms (lettuce, tomatoes)
  • Risk of exposure to humans through consumption of contaminated crops or contact with soil (gardening, farming)
• Airborne transmission of antibiotic-resistant bacteria is possible
  • Aerosolization of antibiotic-resistant bacteria during treatment processes (aeration tanks)
  • Potential for inhalation exposure to workers or nearby communities (wastewater treatment plant employees)
• Ecological impacts of releasing resistant bacteria into the environment
  • Alteration of microbial communities in receiving environments (aquatic, terrestrial)
  • Potential for transfer of antibiotic resistance genes to environmental bacteria (soil, water)