of organics (Wang et al., 2018).
Traditionally, biological methods have been used in the management of pharmaceutical effluent. Pharmaceutical compounds and their metabolites undergo both aerobic and anaerobic decomposition in wastewater treatment processes (Hou et al., 2019; Patel et al., 2019). For instance, microbial cultures can be used to degrade pharmaceutical compounds. Environmental biodegradation is particularly important for the degradation of pharmaceutical compounds, especially when wastewater treatment plants are ineffective (Patel et al., 2019). Microorganisms can biotransform pharmaceutical compounds through metabolic biodegradation or via co-metabolic processes with other compounds. Because certain strains of algae, bacteria, and fungus biodegrade pharmaceutical compounds, biodegradation can be achieved using algal, bacterial, and fungal cultures.
2.3.4.2 Advanced Removal Methods
Except for some very polar hydrophilic pharmaceuticals such as iodinated contrast media and sulfamethoxazole, most pharmaceuticals can be removed by adsorption of activated carbon (Patel et al., 2019; Wu et al., 2019). Furthermore, pharmaceuticals can also be oxidized through advanced oxidation processes, of which ozonation is the most effective. Due to large pore sizes, low pressure membrane technologies such as ultrafiltration (UF) and microfiltration (MF) cannot effectively retain pharmaceutical compounds. Nonetheless, some hydrophobic pharmaceuticals can briefly adsorb onto the surfaces of MF and UF membranes providing a short-term decrease in concentration. The integration of MF or UF in membrane bioreactor systems does not result in additional removal of pharmaceuticals. On the other hand, high pressure membrane technologies such as RO and NF are effective in separating many pharmaceutical compounds from water (Motsa et al., 2014). The major drawback, however, is that brine with high concentrations of pharmaceutical compounds are difficult to manage post removal. RO, micro-, nano-, and UF have applied in the removal of pharmaceuticals. These are cross-flow filtration systems that use a selective semipermeable membrane. In RO, the pressure gradient between the permeate and feed sides of the membrane is the driving force, while ion repulsion is the major operational force in ultra- and nano-filtration since they use charged membranes (Motsa et al., 2014). To effectively remove pharmaceutical compounds, these physical techniques can be improved by integrating them with electrochemical advanced oxidation. While MF and UF can remove a significant level of pharmaceutical compounds, NF and RO exhibit higher removals (Colburn et al., 2019). Generally, NF has a lower energy demand since it uses lower operating pressures. Other methods like riverbank filtration and soil-aquifer treatment, which are natural processes with long retention times, can be included as extra treatment stages. The removal processes involved in riverbank filtration and soil-aquifer treatment are mainly biotransformation and adsorption.
2.3.4.2.1 Advanced Oxidation Processes
Highly reactive free radicals such as HO•, HO2•, and O2•, generated from hydrogen peroxide or ozone in combination with a catalyst or UV or γ radiation, can be used to degrade pharmaceuticals in water (Wang et al., 2019). Advanced oxidation processes use oxidants such as O3 separately, O3 integrated with O3/H2O2, UV/H2O2, Fe2+/H2O2, and Fenton degradation. The photon-initiated carbon-halogen bond breakage and superior HO• generation make these processes more effective than O3 on its own (Patel et al., 2019). Consequently, high removals of a range of pharmaceutical compounds from aqueous solution have been achieved. In addition, UV or visible radiation can produce reactive excited states of the pharmaceutical compounds. The major limitation of advanced oxidation processes, however, is the formation of toxic byproducts which can persist in the treated water (Patel et al., 2019).
2.3.4.2.2 Photolysis
There are two classes of photolytic treatments, namely, indirect and direct photolysis. Whereas direct photolysis involves the decomposition of pharmaceuticals following direct absorption of UV radiation, in indirect photolysis intermediate species such as reactive excited states and free radicals are generated through photosensitization or use of a photocatalyst (Patel et al., 2019). Subsequently, chemical reactions occur resulting in degradation. The efficiency of photolysis is influenced by the intensity and frequency of radiation, quantum yield, the chemistry of the pharmaceutical compounds, formation of the oxidant, and the composition of the aqueous system (Wu et al., 2019). Other photolysis processes are highly influenced by pH. A range of pharmaceutical compounds are degraded through indirect photolysis. Semiconductors such as TiO2 have been used as photocatalysts for the photodegradation of pharmaceuticals (Yahya et al., 2018; Cunha et al., 2019). The photocatalyst is activated by excitation of a valence band electron using light, to form an electron-hole pair. The holes thus generated exhibit high oxidation potentials, and can generate HO• from water on the surface of the photocatalyst. In general, TiO2-based photodegradation results in high removals and considerable mineralization of pharmaceutical compounds.
2.3.4.2.3 Ozonation
Ozone is an electrophilic and a strong oxidant, which can be used directly or indirectly. When ozone is used directly without irradiation or a catalyst, the process is called ozonation, whereas when in combination with a catalyst or photoactivation, it is classified as an advanced oxidation process. Pharmaceutical compounds with electron-rich functional groups such as double, and triple bonds, and aromatic structures or at certain nitrogen, oxygen, phosphorus, or sulfur moieties undergo direct ozonation (Patel et al., 2019). The formation of HO• occurs when ozone and OH- react in aqueous solution. However, ozonation is not 100% effective, thus the need to combine it with UV irradiation or other oxidants such as H2O2. In fact, wastewater treatment plants usually apply ozonation to degrade complex molecules to smaller biodegradable compounds. Although ozonation can be used with unstable effluent flows, it has high construction and maintenance costs. Moreover, the generation of zone is energy intensive, and the mass transfer of O3 from the gaseous phase increases operational costs. Like photolysis, ozonation is sensitive to the physico-chemical properties of water such as alkalinity, organic matter content, pH, the presence of other ions, suspended matter, and temperature.
2.3.4.2.4 O3/UV/H2O2 as an Oxidant
To enhance the performance of ozonation, photocatalysts, UV or visible radiation, and H2O2 can be integrated in advanced oxidation processes. On its own, UV radiation can photodegrade some pharmaceuticals, but will not effectively remove all xenobiotics. Under UV irradiation, H2O2 generates HO• radicals, which are broad-spectrum oxidants capable of photodegrading pharmaceuticals (Sharma et al., 2019). These advanced oxidation processes successfully remove a range of pharmaceuticals including refractory organics (Patel et al., 2019). In addition, H2O2 can be applied in enhancing ozonation through perozonation, via mechanisms similar to O3/UV, augmented with H2O2.
2.3.4.2.5 Fenton Process
With the capacity to work under both homogeneous and heterogeneous conditions, Fenton’s reagent is a strong oxidant made up of Fe2+ and H2O2 in solution (Meng et al., 2018). In heterogeneous processes, the catalyst is attached on a heterogeneous substrate, and the reactions are driven by the generation of HO• radicals (Tsoumachidou et al., 2017). The Fenton process can degrade pharmaceuticals such as sulfachlorpyridazine, trimethoprim, and tetracycline (Hou et al., 2019).
2.3.4.2.6 Adsorption
The adsorption process involves transportation of the solute in the bulk liquid phase, followed
