hydrophobicity, chemical structure, and compound polarity are likely to be important factors affecting the removal of NSAIDs in MBR systems. Understanding to
what extent and how each property affects the removal of NSAIDs would help better
design and operate MBR-based WWTPs for controlling NSAIDs-bearing waste
streams.
Hydrophobicity is a major factor affecting the sorption of NSAIDs by MBR. Of
the many NSAIDs present in wastewater, some are highly hydrophobic and can be
readily removed by MBR treatment via biosorption. For example, 80% of
nonylphenol was eliminated in a pilot-scale MBR process treating landfill leachate
[24], largely due to the high hydrophobic nature of nonylphenol (log D ¼ 6.19 at
pH 8). In a laboratory-scale study, the removal of hydrophobic compounds (log
D > 3.2), such as amitriptyline 17β-estradiol, androsterone, and simvastatin, by
MBR was greater than 85% at pH 8 [19]. However, in the same study, the authors
found that less than 20% removal was achieved for hydrophilic and moderately
hydrophobic compounds (log D < 3.2).
The chemical structure of the NSAIDs can be another major factor affecting their
removal by MBR. Compounds with simple chemical structures (e.g., the absence of
a branched alkyl chain) are likely easily degraded, whereas compounds with complex structures, or with toxic functional groups (e.g., halogens and nitro group), have
a higher resistance to biodegradation, resulting in incomplete degradation [24]. In
addition, simple structure (e.g., not containing multiple rings) compounds with
chloride groups (e.g., diclofenac) are less removable by MBR [25]. Cirja et al.
[24] reported a decrease in the degradation rate of aromatic compounds when the
number of nitro and chlorine groups increases. Therefore, MBR can represent a
promising technology, but further research on the removals of NSAIDs in relation to
MBR-based water reclamation processes is highly desired.
2.1.2 Enzyme-Based Process
The enzymatic treatment process is at the border of traditional chemical and biological processes, where enzymes are the biological catalysts of chemical reactions. The
use of enzymes purified from various plant and microbial sources for wastewater
treatment has been actively studied in recent years. Enzymatic treatment processes
have various advantages over conventional biological and chemical processes such
as high substrate specificity, effective degradation of recalcitrant xenobiotic compounds, high reaction rate, and biodegradability in discharged water [26–29]. Despite
all these advantages, the deployment of enzyme-based technologies in wastewater
treatment is impaired by their relatively high production costs, limited scalability,
sensitivity to inhibitors, and low stability under harsh environmental conditions [30].
Among the different oxidative enzymes of interest for wastewater treatment,
laccases are the most studied [30]. Laccases are multicopper oxidases produced in
fungi, bacteria, and some algae, which can oxidize phenols and similar substrates
and have been shown to degrade NSAID compounds [9, 31, 32]. Laccases catalyze
the ring cleavage of aromatic compounds using oxygen as an electron acceptor. Kim
Contemporary Methods for Removal of Nonsteroidal Anti-inflammatory Drugs in. . .
221
what extent and how each property affects the removal of NSAIDs would help better
design and operate MBR-based WWTPs for controlling NSAIDs-bearing waste
streams.
Hydrophobicity is a major factor affecting the sorption of NSAIDs by MBR. Of
the many NSAIDs present in wastewater, some are highly hydrophobic and can be
readily removed by MBR treatment via biosorption. For example, 80% of
nonylphenol was eliminated in a pilot-scale MBR process treating landfill leachate
[24], largely due to the high hydrophobic nature of nonylphenol (log D ¼ 6.19 at
pH 8). In a laboratory-scale study, the removal of hydrophobic compounds (log
D > 3.2), such as amitriptyline 17β-estradiol, androsterone, and simvastatin, by
MBR was greater than 85% at pH 8 [19]. However, in the same study, the authors
found that less than 20% removal was achieved for hydrophilic and moderately
hydrophobic compounds (log D < 3.2).
The chemical structure of the NSAIDs can be another major factor affecting their
removal by MBR. Compounds with simple chemical structures (e.g., the absence of
a branched alkyl chain) are likely easily degraded, whereas compounds with complex structures, or with toxic functional groups (e.g., halogens and nitro group), have
a higher resistance to biodegradation, resulting in incomplete degradation [24]. In
addition, simple structure (e.g., not containing multiple rings) compounds with
chloride groups (e.g., diclofenac) are less removable by MBR [25]. Cirja et al.
[24] reported a decrease in the degradation rate of aromatic compounds when the
number of nitro and chlorine groups increases. Therefore, MBR can represent a
promising technology, but further research on the removals of NSAIDs in relation to
MBR-based water reclamation processes is highly desired.
2.1.2 Enzyme-Based Process
The enzymatic treatment process is at the border of traditional chemical and biological processes, where enzymes are the biological catalysts of chemical reactions. The
use of enzymes purified from various plant and microbial sources for wastewater
treatment has been actively studied in recent years. Enzymatic treatment processes
have various advantages over conventional biological and chemical processes such
as high substrate specificity, effective degradation of recalcitrant xenobiotic compounds, high reaction rate, and biodegradability in discharged water [26–29]. Despite
all these advantages, the deployment of enzyme-based technologies in wastewater
treatment is impaired by their relatively high production costs, limited scalability,
sensitivity to inhibitors, and low stability under harsh environmental conditions [30].
Among the different oxidative enzymes of interest for wastewater treatment,
laccases are the most studied [30]. Laccases are multicopper oxidases produced in
fungi, bacteria, and some algae, which can oxidize phenols and similar substrates
and have been shown to degrade NSAID compounds [9, 31, 32]. Laccases catalyze
the ring cleavage of aromatic compounds using oxygen as an electron acceptor. Kim
Contemporary Methods for Removal of Nonsteroidal Anti-inflammatory Drugs in. . .
221