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Wastewater Treatment Approaches for the Removal of Antidepressant Residues

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Radu C. Racovita and Maria D. Ciuca

Submitted: 17 January 2024 Reviewed: 17 January 2024 Published: 28 February 2024

DOI: 10.5772/intechopen.1004333

Wastewater Treatment - Past and Future Perspectives IntechOpen
Wastewater Treatment - Past and Future Perspectives Edited by Başak Kılıç Taşeli

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Wastewater Treatment - Past and Future Perspectives [Working Title]

Prof. Başak Kılıç Taşeli

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Abstract

Pharmaceuticals are a major class of contaminants of emerging concern for wastewaters and natural waters alike. Among pharmaceuticals, antidepressants are the most rapidly increasing subclass, as more and more people are dealing with depression in their fast-paced and challenging everyday lives. As psychotropic medications, residual antidepressants in water must be carefully monitored and maintained below levels, where human health may be endangered. Moreover, aquatic life forms were shown to be seriously affected when such pollutants entered their natural habitat, in terms of locomotory, reproductive functions, or social behavior. Therefore, modern wastewater treatment plant technologies should incorporate solutions for removing antidepressant residues. This chapter summarizes recent efforts toward this goal and covers a wide range of proposed treatment approaches. Firstly, adsorptive methods are presented, whether based on classic, yet effective adsorbents like activated carbon or silicates, or modern alternatives such as ion-exchange resins or molecularly imprinted polymers. Secondly, extractive methods are considered, although currently impractical due to lack of both cheap and environmentally-benign solvents. Thirdly, advanced oxidation processes are surveyed, including ozone treatment, ultraviolet, gamma radiation, and electrochemical options, some of which, alone or in combination, may yield complete mineralization of antidepressant toxicants. Lastly, biological treatment with microorganisms is discussed, which may be highly specific, but usually does not enable a complete mineralization.

Keywords

  • contaminants of emerging concern
  • antidepressants
  • wastewater
  • adsorption
  • extractive methods
  • advanced oxidation processes
  • biological treatment

1. Introduction

Contaminants of Emerging Concern (CECs) are pollutants whose occurrence in the environment has been investigated more only in the last 20 years due to the lack of readily available, sensitive analytical methods that can detect their relatively low levels (typically in the order of μg/L) in environmental samples. These contaminants are widespread in both aquatic and terrestrial environments and include anthropogenic and naturally occurring chemicals, food additives, pharmaceuticals and personal care products (PPCPs), metabolites and other transformation products of PPCPs, illicit drugs, and engineered nanomaterials. CECs are not currently regulated in drinking water supplies and, therefore, are not regularly monitored in the environment. However, many of these contaminants have the potential to cause adverse ecological and/or human health effects even at low concentration levels [1].

In the European Union (EU) member states, a watch list of CECs is elaborated within national monitoring programs, as required by the Water Framework Directive (WFD) [2]. This list presents emerging substances requiring further attention due to their high frequency of occurrence, the expected risk for human health and/or aquatic life, and/or for the lack of available monitoring techniques. To date, the WFD has designated, for the EU, 45 substances or compound classes as priority pollutants [3]. However, the means for managing these substances remain a challenge. The sources of PPCPs in the environment are mostly represented by anthropic activities, coming from municipal wastewater treatment plants (WWTPs) and hospital wastewater discharges, illicit drugs, and municipal sewage [4]. In addition, animal agriculture and aquaculture are also important sources, especially for antibiotics that are administered both therapeutically and sub-therapeutically, and for hormones that are excreted naturally or synthetic hormones that are used to regulate the reproductive system and animal growth. These compounds have been detected worldwide in WWTP effluents [5, 6], surface water [7, 8, 9], drinking water [10, 11], and groundwater [12].

Over the past two decades, due to continuously growing demands, challenges, and pressures experienced in their professional and daily lives by modern people, a particular class of pharmaceuticals started to be prescribed and consumed more and more across European countries and other developed countries, such as Canada, Australia, New Zealand, South Korea, or Israel [13]. This class is represented by antidepressants. The graph below (Figure 1) shows how the defined daily dosage (DDD) per 1000 inhabitants has increased continuously over the past years in several EU and European Economic Area (EEA) countries according to the latest statistics released by the Organization for Economic Co-operation and Development (OECD). In this plot, the DDD is defined as “the assumed average maintenance dose per day for a drug used on its main indication in adults” [13]. While countries such as Iceland, Portugal, Sweden, and the UK appear to be the highest European consumers of antidepressants, in fact, all European societies have experienced some increase in the prescription and consumption of these pharmaceuticals, somewhat commensurate with their annual healthcare spending per capita [14]. A study from 2015 concluded that the average percentage of the general population of the 27 EU member states who reported using antidepressants regularly was 7.2%, ranging from a high 15.7% in Portugal to a low 2.7% in Greece [14]. These percentages have increased since and will only continue to grow, especially after the recent COVID-19 pandemic measures enforced by governments worldwide, which only augmented depression symptoms through human isolation and limited social interaction [15, 16, 17].

Figure 1.

Trends in consumption of antidepressant drugs in twelve EU/EEA countries over the past two decades according to statistics released by OECD.

Along with high prescription and consumption rates comes a correlated increase in contamination levels with such drugs and their metabolites of municipal waters or wastewaters from pharmaceutical companies or hospitals [18, 19]. Although it has been estimated that, overall, these types of drugs make up only 4% of all pharmaceuticals detected in the environment [20], the reported levels of contamination of wastewaters with antidepressant residues range from the limit of detection (~0.1 ng/L) up to about 3000 ng/L in WWTP raw influents and a few tens or hundreds of ng/L in final effluents [19], which are concerning levels given the toxic effects of these chemical compounds for both humans and aquatic species.

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2. Antidepressants and their impact on human health and aquatic life

Antidepressant drugs are typically classified based on their biochemical mode of action and on some structural features (Figure 2) into (a) selective serotonin reuptake inhibitors (SSRI), which are the most highly prescribed and encountered, for example, fluoxetine (FLU), sertraline (SER), paroxetine (PAR), fluvoxamine (FLA), and citalopram (CIT); (b) serotonin-noradrenaline reuptake inhibitors (SNRI), like venlafaxine (VEN) and duloxetine (DUL); (c) tricyclic antidepressants (TCA), including imipramine (IMI), clomipramine (CLO), trimipramine (TRI), amitriptyline (AMI), and doxepine (DOX), which are usually employed in major depressive disorders [21]; (d) monoamine oxidase inhibitors (MAOI), such as phenelzine, isocarboxazid, or selegiline; and (e) atypical antidepressants, like bupropion (BUP), mianserin (MIA), and trazodone, among others [22]. Regardless of their particular class, the administration of antidepressants to patients is always done under careful supervision by medical practitioners, often in combination with psychotherapy and with regular monitoring of blood levels for an informed adjustment of dosages in the long term [23].

Figure 2.

Classes of antidepressants.

Therefore, intoxications may occur upon ingestion of uncontrolled concentrations of antidepressants by people. Many studies have shown that antidepressants can cause health problems ranging from mild, such as dry mouth, fatigue, nausea, headaches, and vomiting, to very serious, for example, cardiac dysrhythmias, sexual dysfunctions, seizures, and even death [24]. The fatal toxicity index (FTI), i.e. the number of deaths per million prescriptions, varies from a low 2.0 to 6.2 in the case of SSRI and atypical antidepressants to around 13.5 for MAOI and a high 34.1 in the case of TCAs, which are the most concerning antidepressant class [24].

When WWTP effluents arrive eventually in natural water reservoirs, remnant antidepressant residues can affect fish and other forms of aquatic life. Several studies revealed various negative consequences of antidepressant ingestion by fish: reduction of food intake, reduction of swimming velocity and distance covered; activation of locomotory organs irrespective of the presence or absence of a predator, modification of courtship behavior, which in turn affects population dynamics, weight loss, and reduction of hatching period [19, 25]. Aquatic invertebrates were also shown to be affected even by trace amounts of antidepressants. Spawning and larval release in bivalves, as well as locomotion and fecundity in snails, were reduced. Among crustaceans, antidepressants affected the photo- and geotactic behavior of amphipods, the aggression of crayfish, and the reproduction and development of daphnids [26].

As such, effective technologies for minimizing antidepressant residues in wastewaters must be developed and implemented in the immediate future.

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3. Overview of water treatment approaches for the removal of antidepressant residues

The current approaches for water treatment to remove residual antidepressant drugs are, broadly, the same as for other types of CECs. Treatment options for the removal of CECs from drinking water as well as wastewater include physical-chemical separation [27], coagulation and sorption processes [28], ozone-based and other advanced oxidation processes (AOPs) [29, 30], including ultraviolet (UV) [31, 32, 33] or gamma radiation treatment [34], biological and enzymatic treatment [35, 36], ultrasonic degradation [37], ultrafiltration (UF) and nanofiltration (NF) through membranes [37], electrochemical oxidation [38], and reverse osmosis (RO) using special membranes [34, 39].

Each of these methods has its pros and cons when applied to PPCPs. Physical-chemical treatments such as coagulation or flocculation were generally found to be unable to effectively remove PPCPs. On the other hand, although AOPs can be effective for the removal of some pharmaceuticals, these processes can lead to the formation of oxidation intermediates that have occasionally been found to be even more toxic than the parent compounds, thus raising concerns about their presence in the environment and the necessity to assess their ecological risks along with their parent compounds [40, 41]. Adsorption processes do not add undesirable by-products and have been found to be superior to other techniques for wastewater treatment in terms of simplicity of design and operation, and insensitivity to toxic substances [42]. Among several materials used as adsorbents, activated carbon (AC) has been used for the removal of different types of PPCPs, but its use is sometimes restricted due to high costs [43]. Furthermore, once the AC has been exhausted, it can be regenerated for further use, but regeneration processes have additional costs, may result in a loss of carbon, and the regenerated product may have a lower adsorption capacity after a few rounds of regeneration in comparison to the non-recycled AC [44]. This has triggered attempts by various scientists and engineers to prepare low-cost alternative adsorbents, which may replace AC in pollution control by adsorption and may overcome its economic disadvantages [42]. Recently, natural materials that are available in large quantities from agricultural activities have been evaluated as low-cost and environmentally-friendly sorbents. Moreover, the utilization of these waste materials as such or after some minor treatment as adsorbents is receiving attention because they represent unused secondary resources and would otherwise cause serious disposal problems [45]. When comparing biological treatment technologies, waste stabilization ponds, trickling filters, and activated sludge supplemented with a biofilter system for estrogenic and aryl hydrocarbon receptor, the waste stabilization ponds showed greater or similar removal efficiency as activated sludge with biofilters, and both were much more efficient than the trickling filter, which had the lowest removal efficiency [46]. For enhanced performance, biological treatment has been combined sometimes with ultrasound and membrane filtration [47]. Electro-oxidation (anodic oxidation) performed using high cell voltages in wastewater treatment can involve either only reactive oxygen species (like HO· or H2O2) or, for chloride-rich waters, also chlorine species (Cl2, HClO/ClO−, ClO2−), which further improve the oxidative action against pharmaceutical contaminants [34].

For removal of antidepressant drug residues from wastewater and other contaminated waters, treatment options considered to date can be classified into adsorptive, extractive, oxidative (O3 and/or other AOP), and biological degradation processes (Figure 3).

Figure 3.

Methods for removal of antidepressant residues from wastewater.

3.1 Adsorptive methods

Adsorbents used to date for the removal of antidepressants from aqueous media include cation-exchange resins [48], activated carbon, silicates, and titanium dioxide nanoparticles [49], or molecularly imprinted polymers [50].

Choi et al. [48] screened six different cation-exchange resins for their adsorption efficiencies with respect to tricyclic antidepressants. Three of these had sulfone functional groups (Amberlite IR-120, Dowex 50WX4-200, and Dowex HCR W2) and the other three had carboxyl functionalities (Amberlite IRC-86, Dowex MAC-3, and Amberlite IRP-64). Using one-point check tests with AMI as a model pharmaceutical contaminant, they showed that Dowex 50WX4-200 had the highest uptake of AMI. Aqueous pH was shown to have an important effect on adsorption efficacy, specifically a negative effect above 7, where TCAs, having pKa values around 9–10, tend to partially deprotonate and become insoluble in water. However, pH values between 3 and 6.5 did not inflict any significant changes on adsorption capacity, therefore, an almost neutral pH of 6.5 was chosen for further measurements. Using adsorption isotherm models for the experimental results, the maximum adsorption capacities of the resin were determined for a series of four TCAs (AMI, CLO, IMI, and its demethylated derivative desipramine DES) and ranged from 2.53 to 3.76 mmol/g. The removal efficiency of DES from real wastewater samples containing other miscellaneous cations was also probed and, interestingly, it was weaker in the case of filtered wastewater (56.66%) as compared to unfiltered (77.99%). The latter was explained based on an additional adsorptive contribution from the activated sludge found in unfiltered wastewater [48].

Activated charcoal was also used successfully as an adsorbent for IMI, exhibiting a maximum adsorptive capacity of 610 mg/g (2.17 mmol/g) at pH 8.5 and 372 mg/g (1.33 mmol/g) at pH 6.8 according to the Langmuir adsorption model. Unlike with cation-exchange resins, the deprotonated form (electrically neutral) of IMI adsorbed better on the carbonaceous substrate than the protonated one [51]. In a different study, AMI and its demethylated derivative, nortriptyline (NOR), were removed from water using activated carbon, diosmectite (a natural silicate), and TiO2 nanoparticles [49]. Activated carbon performed best, with specific adsorption capacities of 3.75 and 1.87 mmol/g for AMI and NOR, respectively. Nonetheless, diosmectite performed very well also, with specific capacities of 3.30 and 1.82 mmol/g, respectively, while TiO2 nanoparticles showed only 2.97 and 0.79 mmol/g maximum adsorption capacities for AMI and NOR. The mechanism of adsorption was also investigated and concluded to be a combination of (1) electrostatic interaction between positively charged antidepressant molecules and the negative surface charge of the adsorbents and (2) π-π stacking interactions between the delocalized aromatic rings of antidepressants and carbon structures on the surface of activated carbon, thus explaining why this particular material performed better. Nevertheless, the cost of production of activated carbon is quite high [52], however, several options exist for its cost-effective regeneration such as chemical [53], biological [54], and electrochemical regeneration [44].

One of the most recent developments in adsorptive materials for environmental remediation is represented by molecularly imprinted polymers (MIPs), which are polymers that had been imprinted during polymerization with a desired template to enable highly selective recognition sites onto their surface, which is why they are sometimes referred to as artificial antibodies or synthetic receptors [50]. Gornik et al. prepared such MIPs using methacrylic acid as monomer, either alone or co-polymerized with methyl methacrylate or 2-hydroxymethyl methacrylate. Although adsorptive capacity was lower than for activated carbon (e.g. 72.6 mg/g or 0.24 mmol/g for SER), primarily due to much smaller specific surface area of the polymer materials (27.4–193.8 m2/g) compared to the highly porous AC (1400 m2/g), the main advantage of these MIPs was represented by their higher selectivities and affinities for their target contaminants. The MIPs selectively removed SSRI antidepressants from water, like SER, FLU, PAR, and CIT, leaving behind interfering related molecules such as BUP and the structurally related drug, bupivacaine. Furthermore, the polymers required regeneration after longer intervals compared to AC, which is advantageous in terms of cost of operation.

Although the best-performing adsorbents can have high initial costs of production, the available technologies for their regeneration and reuse make adsorptive methods a highly relevant option for the removal of antidepressant residues from contaminated water.

3.2 Extractive methods

In general, liquid-liquid extraction methods are only of historical significance as most organic immiscible solvents that were tried over the years are not green solvents and would actually augment the contamination of wastewaters with their own residues, of even greater concern than antidepressants themselves. Kocoglu et al. [55] tested the extraction efficacy of halogenated and non-halogenated organic solvents, such as dichloromethane, chloroform, and ethyl acetate, using SER as a model antidepressant molecule. Ethyl acetate yielded the best results, especially when 2% (v/v) ethanol was added to it, however, ethyl acetate presents a plethora of toxic effects [56] and this method is not suitable for wastewater treatment.

Alternative, greener solvents were tested in other studies for antidepressant extraction from water. Thus, water-in-oil microemulsions, prepared using various homologous carboxylic esters and Brij 30, a non-ionic, environmentally-benign surfactant, were used to extract four antidepressants (AMI, DOX, IMI, and CLO) from contaminated water with extraction efficiencies ranging from 67.2 to 99.5% at nearly neutral pH and low ionic strength [57]. Propyl acetate-based microemulsions performed best, maintaining extraction yields between 88 and 99.3% even at high concentrations of antidepressant contaminants (up to 100 mg/L). The presence of other ions in the water (high ionic strength) or a more basic pH of 9 produced further increases in extraction yields, from 93 to nearly 100%, revealing that these extraction solvents remained very promising even with more diverse contaminant loadings of wastewaters. Still, the use of organic ester solvents in the composition of the microemulsions, even though in smaller proportions, remains an inconvenient factor, as is also the cost of the non-ionic surfactant.

Ionic liquids, with their well-known advantages of negligible volatility, high chemical and thermal stability, excellent solvation ability, and non-corrosive nature, were also proposed as green extraction solvents for antidepressants [58]. Zawadski et al. [59] used aqueous biphasic systems based on either tetrabutylammonium or tetrabutylphosphonium salts and phosphate buffers to extract AMI from pharmaceutical waste. The observed extraction efficiencies ranged from 94 to 100%, and they were improved when the pH was raised from 6.6 to 9.6, however, they remained virtually unchanged when further raised to 13.2, proving that advanced alkalization is not necessary in this case. Nevertheless, the high costs currently associated with ionic liquids make these solvents only promising candidates for future technologies.

Given the toxicity associated with classic organic solvents and the high costs of modern green solvents, extractive methods do not represent currently a viable method of decontamination for wastewaters containing antidepressant residues.

3.3 Advanced oxidation processes (AOPs)

UV irradiation alone, whether UVA, UVB, or UVC, has proven itself to be a suitable technique for water decontamination when polluted by antidepressants or their derivatives [60]. UVC treatment has been most effective, as shown by recent studies of photo-degradation of tricyclic antidepressants like AMI and CLO [61] or TRI [62] in water with efficiencies above 90%, while the combination of UVA + UVB applied to DES in water yielded a very long half-life of 36 hours [63] and UVA irradiation alone was only able to degrade less than 30–40% of both DOX and VEN even after 114 days of irradiation [64]. The performance of the latter was only improved significantly when used in combination with a photocatalyst, TiO2, or with Fe2+ and H2O2, i.e. a photo-Fenton process, which led to 75% degradation of IMI in 4 hours and complete degradation in 24 hours [65]. In general, for increased effectiveness, UV treatment tends to be combined with some other type of advanced oxidation process.

Ozone (O3) has proven to be highly effective at degrading antidepressant residues from wastewater. Mendez-Arriaga et al. [66] performed a study of oxidative disposal of FLU from aqueous environment using O3, O3 + TiO2, O3 + H2O2 (PEROXONE process), and O3 + TiO2 + H2O2, with and without simultaneous UV irradiation. The effectiveness varied, leading to partial or complete photo-mineralization of FLU to yield CO2 and inorganic ions like NO2−, NO3−, and F−. Under dark conditions, no depletion of FLU occurred at either acid or basic pH. Methods involving TiO2 were only effective in alkaline medium, because adsorption onto the TiO2 surface was a mandatory pre-condition for degradation and cationic antidepressant species in acid environment were repelled by the positively charged surface of the oxide catalyst. UVA (360 nm) irradiation was also not able to degrade FLU in acidic medium, but after alkalization, the concentration of FLU decayed by ca. 15% after 60 min of illumination. The extent of photo-degradation doubled when combining UVA and O3 treatment with a small amount of H2O2. The heterogeneous photo-assisted process UVA + O3 + TiO2 led to 50% mineralization in 60 minutes. The presence of H2O2 enhanced photo-mineralization further, such that the hybrid configuration UVA + O3 + H2O2 + TiO2 attained 97% mineralization, suggesting it to be the best combination for wastewater treatment against such recalcitrant drug pollutants.

An alternative to UV radiation is represented by ionizing radiation, namely gamma (γ) radiation. Bojanowska-Czajka et al. investigated the γ-ray-inflicted degradation of SER and CIT in surface water, including in the presence of commonly encountered radical scavengers such as CO32−, NO3−, and humic acid [67]. As γ radiation source, 60Co was used, with radiation doses between zero and 500 Gy. At concentration levels of 1 mg/L, both antidepressants were fully degraded by a dose of 100 Gy, while at 10 mg/L, SER needed 200 Gy and CIT 400 Gy for complete mineralization, which for a dose rate of 2.15 kGy of the radiation source translated into only 11 minutes of irradiation, a much shorter time than in the case of UV radiation. When 10 mg/L each of CO32−, NO3−, and humic acid were added to simulate radical scavengers that may be commonly encountered in such media, the degradation of SER was almost unaffected at a concentration 1 mg/L and 100 Gy absorbed dose. The degradation of CIT at the same level of concentration was unaffected by nitrates, but carbonates and humic acid did have an impact. For an absorbed dose of 50 Gy, the degradation efficiency was 70% in the presence of CO32− and 50% with humic acid. Carbonates continued to inhibit somewhat the degradation of CIT even at 200 Gy, where an 80% efficiency was attained. Interestingly, 100% degradation of CIT was achieved with humic acid additive at a dose of only 150 Gy, better than in the absence of added radical scavengers.

Electro-oxidation (anodic oxidation) has also been reported as a suitable technique for destruction of antidepressant residues found in water. Depending on the simultaneous presence or absence of chloride anions in the same aqueous environment, electro-oxidation processes can be with or without participation of active chlorine species [34]. Various reactive oxygen species (ROS) are generated from water discharge at the anode surface, e.g. H2O2, O3, and physisorbed OH radical, which is the strongest oxidant. Anodes that are typically used in this type of water treatment are Pt, IrO2, or RuO2, known as “active” electrodes, which tend to oxidize water molecules to strongly bound, chemisorbed “active oxygen” or metal superoxide species, as well as PbO2, SnO2, and boron-doped diamond (BDD), considered “non-active” electrodes, where water splitting only leads to physisorbed OH radicals, which interact so weakly with the anode surface that they easily oxidize organics to their complete mineralization. BDD is usually the most potent among them [34]. Melin et al. investigated the degradation of 130 mg/L AMI by three different electrochemical AOPs, where active OH radicals were generated by simple anodic oxidation (AO), an electro-Fenton (EF), and a photo-electro-Fenton (PEF) process, respectively [68]. For AO, at a fixed current density of 100 mA/cm2, they also compared the efficiency of different electrodes, such as Pt, RuO2, and BDD, the latter yielding the best results, namely 76% mineralization after 360 minutes, as revealed by total organic carbon (TOC) analysis. Only 21 and 27%, respectively, mineralization was achieved with Pt and RuO2, and even this was mainly the outcome of the presence of Cl− in the system, as the AMI hydrochloride salt was employed. Under these conditions, at pH less than 3.3, the main oxidant is Cl2, while at higher pHs, hypochlorite ClO− takes over [68]. Using the BDD anode, Melin et al. also studied the influence of current density and obtained within 360 minutes degrees of mineralization of 50, 64, 76, and 83% with applied currents of 33.3, 66.7, 100, and 150 mA/cm2, respectively. By adding 0.5 mmol/L Fe2+ catalyst to the medium, conditions for an electro-Fenton (EF) process were created, as H2O2 is formed anyways by OH radical coupling during AO. Indeed, the EF process yielded 78% mineralization faster, within 240 min of electrolysis at 100 mA/cm2 using the BDD anode. Interestingly, doubling the concentration of catalyst did not improve mineralization, because iron hydroxide species are formed, limiting the generation of OH radicals in the bulk and thus causing a decrease of the efficiency of the EF process [68]. It was, however, the PEF process which allowed the best performance for mineralization of AMI due to the synergistic action of highly reactive physisorbed OH onto BDD, homogeneous OH in the bulk, and UVA (360 nm) radiation. After 360 min of treatment, maintaining 100 mA/cm2 current density and 0.5 mmol/L Fe2+, 95% mineralization was achieved. Using gas chromatography and high-performance liquid chromatography coupled with mass spectrometry analysis, the authors of the study were also able to evidence some of the intermediates formed from AMI along its mineralization pathway, namely polyaromatic hydrocarbons and their oxidized derivatives, such as dibenzosuberone and 5-dibenzosuberenone, and carboxylic acids like succinic, malic, oxalic, and formic acid [68].

3.4 Biological treatment

The biological treatment of wastewaters containing antidepressant drug residues involves the employment of microorganisms, which typically produce enzymes that are responsible for the breakdown of the structurally complex, toxic pharmaceutical molecules into simpler, less toxic products. The biological transformation of pharmaceutical residues in WWTP occurs via two principal mechanisms: co-metabolism, where the drug pollutant is degraded by some of the enzymes secreted by the microbial communities found in the sewage sludge, or by sole substrate degradation, in which the target compound constitutes the sole source of carbon and energy for the microbes [69]. Depending on the availability of oxygen and its actual usage by microorganisms, drug biotransformation mechanisms can also be classified as aerobic or anaerobic.

The efficacy of biodegradation of pharmaceutical pollutants depends primarily on their solubility in the wastewater. If the solubility is low (hydrophobic drug molecules), then they will be retained in the sewage sludge, providing more time for microbial degradation, whether by action of catabolic enzymes or for utilization as single carbon source. Highly soluble drug micropollutants, on the other hand, may evade the biodegradation process, leaving the WWTP after a very short period of residence [69].

Gornik et al. performed a biotransformation study of the most commonly prescribed antidepressant, SER, using two different approaches: (1) batch biodegradation and sorption and (2) flow-through pilot wastewater treatment bioreactor [70]. For the batch experiments, sorption to the activated sludge was the true leading removal process for SER, around 90%, and only secondary was the biodegradation, highly influenced also by the presence of other carbon sources easily biodegradable by the microbial population. Identified products of biodegradation included norsertraline, sertraline ketone, hydroxyl-sertraline, and traces of a few others, i.e. biological degradation did not yield complete breakdown to inorganic reaction products. The highest removal effectiveness achieved with the flow-through bioreactor was 94%. When real wastewaters from two WWTP and one psychiatric hospital were subjected to the flow-through bioreactor treatment process, SER removal efficiencies ranged from 77 to 81%, but, once again, in addition to the small percentage of leftover SER, eight other structurally complex transformation products could still be identified in the resulting effluents [70].

Mycoremediation of wastewater containing 2.5 mg/L each of SER, PAR, FLU, CIT, CLO, VEN, and MIA was attempted in a different study using Pleurotus ostreatus [71]. To separate fungal biodegradation from mere adsorption on fungal biomass, control experiments were also performed, where the fungus was autoclaved for 15 min at 120°C to inactivate all its enzymes. Based on combined adsorption+enzymatic degradation, removal efficiencies above 90% were attained after 96 hours of fungal biomass exposure for SER, PAR, CLO, and MIA, about 85% was attained in the case of FLU, and 50% and 22% only for CIT and VEN, respectively. When considering only the enzymatic contribution from the fungus, CIT and VEN were essentially unaffected, MIA and CLO exceeded 80% degradation, while PAR, SER, and FLU were degraded in proportion of 50, 48, and 24%, respectively [71]. The authors of the study interpreted these findings in terms of the degree of structural similarity between each antidepressant and lignin monomeric structures, which represent the natural substrates of the fungal enzymes. Thus, lignin-like multi-aromatics such as SER, CLO, MIA, PAR, and FLU showed the highest degradation efficiencies, slightly lower in the case of FLU because of its three electron-withdrawing fluorine atoms causing a reduced aromaticity of one of the benzene rings. A similar reason may have caused the increased resistance of CIT, while VEN may have not been degraded significantly because it contains a single aromatic ring and was thus not recognized as a substrate by the specific enzymes of the fungus [71]. Overall, the results suggested that certain microorganisms will perform better when dealing with a specific type of environmental drug pollutant.

Table 1 summarizes the comparative efficiencies of the various methods of removal of antidepressant residues from wastewater discussed in this chapter.

Wastewater treatment methodSpecific conditionsAntidepressant removedEfficiency of removal (%)Reference
AdsorptionCation-exchange resinsAMI, CLO, IMI, DES57–78[48]
Activated carbonAMI, NOR17–37[49]
IMI37–61[51]
DiosmectiteAMI, NOR16–31[51]
TiO2 nanoparticlesAMI, NOR7–22[51]
Molecularly imprinted polymersSER, FLU, CIT, PAR70–100[50]
BUP55–90[50]
ExtractionEthyl acetateSER86–95[55]
Propyl acetate+Brij 30 + Water microemulsionAMI, DOX, IMI, CLO88–100[57]
Tetrabutyl-ammonium/ phosphonium salt+phosphate bufferAMI94–100[59]
Advanced oxidation processesUVA irradiationDOX, VEN30–40[64]
FLU15[66]
UVA + UVB irradiationDES50[63]
UVC irradiationAMI, CLO88–100[61]
TRI92[62]
O3 + TiO2 + UVAFLU50[66]
O3 + H2O2 (PEROXONE) + UVAFLU70[66]
O3 + TiO2 + H2O2 + UVAFLU97[66]
Gamma (γ) radiationSER, CIT80–100[67]
Electro-oxidation using BDD anodeAMI76[68]
Electro-Fenton (EF)AMI78[68]
Photo-electro-Fenton (PEF)AMI95[68]
Biological degradationActivated sludgeSER77–81[70]
Fungal adsorption+ enzymatic degradationSER, PAR, CLO, MIA, FLU, CIT, VEN22–98[71]

Table 1.

Efficiencies of removal of various antidepressant residues from wastewater by currently available treatment methods.

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4. Conclusions

In conclusion, antidepressant drug residues have become of greater concern every year when considering wastewater treatment technologies, because of their continuously increasing presence in the society and implicitly in the environment. Their occurrence should not be neglected in light of recent evidence suggesting they can severely affect aquatic ecosystems and, at higher levels, even human health.

Although they have only started to emerge, several wastewater treatment options are available nowadays to deal with this new class of drug pollutants, ranging from classic methods like sorption onto activated carbon, silicates, or ion-exchange resins, to advanced oxidation processes using ozone and other reactive oxygen species generated either photo-, electro-, or radio-chemically, and ending with modern technologies such as biological degradation by fungi or other microorganisms with high affinity for such organic contaminants.

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Acknowledgments

This work was supported by a grant from the Ministry of Research, Innovation and Digitization, CNCS-UEFISCDI, project number PN-III-P1-1.1-TE-2021-1216, within PNCDI III.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Radu C. Racovita and Maria D. Ciuca

Submitted: 17 January 2024 Reviewed: 17 January 2024 Published: 28 February 2024

© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.