Pharmaceuticals in wastewater pose a challenge for treatment plants. Removal mechanisms include sorption, biodegradation, and volatilization. Various treatment processes, like activated sludge and membrane filtration, show different levels of effectiveness in eliminating these compounds.
Current technologies have limitations, including incomplete removal and formation of transformation products. Operational parameters like sludge retention time and pH play crucial roles. Integrating different treatment processes can enhance overall removal efficiency, but economic constraints remain a concern.
Removal Mechanisms and Treatment Processes for Pharmaceuticals
Removal mechanisms in wastewater treatment
- Sorption involves hydrophobic interactions between pharmaceuticals and suspended solids or sludge, as well as electrostatic interactions between charged pharmaceuticals and sludge surfaces, influenced by pharmaceutical properties (octanol-water partition coefficient, pKa)
- Biodegradation occurs through microbial transformation of pharmaceuticals by bacteria in activated sludge, following aerobic and anaerobic degradation pathways, dependent on pharmaceutical structure, microbial community, and environmental conditions
- Volatilization transfers volatile pharmaceuticals from liquid to gas phase, influenced by Henry's law constant and aeration rates, generally less significant compared to sorption and biodegradation
Effectiveness of treatment processes
- Biological processes
- Activated sludge effectively removes biodegradable pharmaceuticals, with removal efficiency varying depending on sludge retention time (SRT) and hydraulic retention time (HRT)
- Membrane bioreactors (MBRs) achieve higher removal efficiency compared to conventional activated sludge due to longer SRTs and higher biomass concentrations enhancing biodegradation
- Physical processes
- Membrane filtration, including nanofiltration and reverse osmosis, can effectively remove a wide range of pharmaceuticals, with removal efficiency depending on membrane properties (pore size, surface charge) and pharmaceutical characteristics (molecular weight, polarity)
- Activated carbon adsorption, using granular activated carbon (GAC) and powdered activated carbon (PAC), can adsorb pharmaceuticals, with adsorption capacity influenced by carbon properties, pharmaceutical properties, and competition with other organic compounds
- Chemical processes
- Advanced oxidation processes (AOPs), such as ozonation, UV/H2O2, and Fenton processes, can degrade pharmaceuticals through oxidation reactions, with effectiveness depending on pharmaceutical reactivity, oxidant dose, and water matrix composition
- Chlorination can transform some pharmaceuticals but may lead to the formation of disinfection byproducts, having limited effectiveness compared to AOPs
Limitations of current technologies
- Incomplete removal occurs as conventional treatment processes may not achieve complete removal of all pharmaceuticals, with some pharmaceuticals being resistant to biodegradation or having low sorption affinity
- Formation of transformation products can result from biological and chemical processes transforming pharmaceuticals into new compounds, which may have different environmental fate and toxicity compared to parent compounds
- Operational and economic constraints arise as advanced treatment technologies (MBRs, AOPs) have higher energy and chemical requirements, with retrofitting existing treatment plants being costly and challenging
- Variability in pharmaceutical loads and composition, including fluctuations in concentrations and types in wastewater influent, as well as seasonal variations and changes in consumption patterns, can affect removal efficiency
- Analytical challenges exist in detecting and quantifying trace levels of pharmaceuticals in complex wastewater matrices, with a lack of standardized analytical methods and reference materials for some pharmaceuticals
Role of operational parameters
- Sludge retention time (SRT)
- Longer SRTs allow for the development of a more diverse microbial community
- Higher SRTs can improve the biodegradation of slowly degradable pharmaceuticals
- Hydraulic retention time (HRT)
- Longer HRTs provide more contact time between pharmaceuticals and microorganisms or sorbents
- Increasing HRT can enhance removal efficiency but may require larger treatment volumes
- Aeration and mixing ensure optimal contact between pharmaceuticals and microorganisms or sorbents, with proper aeration also promoting the volatilization of some pharmaceuticals
- pH and temperature affect the ionization state and solubility of pharmaceuticals, influencing their removal, with temperature impacting microbial activity and biodegradation rates
- Dosing of chemicals and sorbents, such as optimizing the dose of coagulants, flocculants, or activated carbon, can improve pharmaceutical removal while balancing the trade-off between removal efficiency and economic feasibility
- Integration of treatment processes, combining different technologies (biological treatment followed by advanced oxidation), can enhance overall removal efficiency, with optimizing the sequence and operating conditions of integrated processes maximizing pharmaceutical removal