Chlorination is a widely used disinfection method applied in the last stages of sewage treatment. As a strong oxidizer, chlorine reacts with organic compounds, but can also lead to the formation of harmful chlorinated byproducts with negative effects on human health and the environment ^[1][7]. Several studies have reported the potency of chlorination in disinfection. In the work of Decol et al. ^[2][8], a 2.5-log reduction in E. coli was achieved, whereas Francy et al. ^[3][9] reported a 0.7- to 2.6-log reduction in a number of microbiological indicators such as E. coli (highest log reduction) and viruses. Chlorination does not affect the nutrient content of secondary effluent (in terms of N and P), making this method unsuitable when the treated wastewater is led to recipients prone to eutrophication. Pharmaceutical compounds, on the other hand, are prone to oxidation from chlorine, with Li and Zhang ^[4][10] testing the effects of the chlorination of compounds, i.e., sulfamethoxazole, ciprofloxacin, norfloxacin, tetracycline, trimethoprim, and erythromycin, reporting a reduction of 43 to 73%. Regarding the economic aspect of chlorination, it is considered one of the cheapest tertiary methods, with a treatment cost of 0.0003 to 0.006 EUR/m^{3 [5][6][7]}[11,12,13]. Finally, due to its minimum energy needs, chlorination leads to very low greenhouse gas (GHG) emissions, with Walsh and Mellor ^[8][14] reporting GHG as low as 0.004 kg CO₂ eq. m⁻³, whereas Pasqualino et al. ^[9][15] reported a slightly higher value of 0.07 kg CO₂ eq. m⁻³.

Ultraviolet technology, alone or combined with other degradation processes, has been extensively investigated as a tertiary treatment process. Zhang et al. examined the UV process coupled with H₂O₂ for secondary effluent treatment, achieving an ARG reduction equal to 3.48 log at pH 3.0 and 2.32 log at pH 7.0, whereas the cost was estimated at 0.296 USD/m^{3 [10]}[16]. The coupling of UV with H₂O₂ technology was studied in the removal of microcystin-RR by Qiao et al., with the reduction percentage of this pharmaceutical being 94.83% ^[11][17]. The combination of UV and the ozonation process by Chin and Bérubé in the treatment of surface water resulted in 50%, 80%, and 70% reductions in TOC, trihalomethane formation, and haloacetic acid formation, respectively ^[12][18].

A reduction of 100% for both diclofenac and bezafibrate was achieved when coupling UV irradiation with H₂O₂, as reported by De la Cruz et al., whereas the combination of three technologies UV/Fe²⁺/H₂O₂ resulted in a 100% elimination of diclofenac ^[13][19]. Moreover, Guo et al. examined the UV disinfection process over ARB and ARGs present in municipal sewage ^[14][20]. The reduction in bacteria resistant to erythromycin and tetracycline was found to be equal to 1.4 and 1.1 log, respectively.

Membrane filtration methods include a diverse group of processes, with the most common ones being pressure-driven membranes. During pressure-driven membrane filtration, a pressure difference is imposed on the two sides of a semi-permeable membrane, with the kinds of solutes permeating the membrane, further defining the membrane types. Membranes with a pore size on a scale of 1 μm (microfiltration, MF) typically reject suspended solids. Ultrafiltration (UF) has a smaller pore diameter and can reject larger dissolved molecules. Membranes with a pore size on a scale of 1 nm fall within the nanofiltration (NF) group and can reject smaller dissolved molecules (typically up to 200 Da) and divalent ions. Finally, reverse osmosis (RO) has no pores and separation occurs through the different diffusion rates of the solutes in the polymer of the membrane. RO membranes can even reject monovalent ions ^[15][21].

An integrated pilot unit combining UF, RO, and electrooxidation to manage municipal sewage was introduced by Urtiaga et al. ^[16][22]. All the target compounds, namely naproxen, ofloxacin, furosemide, bezafibrate, and fenofibric acid, were rejected with a percentage higher than 99%. Cheng et al. studied the efficiency of an anaerobic MF system in the removal of some antibiotic resistance bacteria (ARB) and their associated ARGs found in municipal sewage and the microbial load reduction was equal to 2–3 log units ^[17][23]. The elimination of ARB and ARGs present in secondary wastewater effluent by employing a TiO₂-modified polyvinylidene fluoride (PVDF) UF membrane was investigated by Ren et al. ^[18][24]. ARGs were removed at a rate of 98%.

Dolar et al. examined performance as it concerned the degradation of selected veterinary Phs present in pharmaceutical sewage using a laboratory and pilot scale RO/NF membrane treatment process ^[19][25]. The removal of TOC and COD was 70.8% and 35.4%, respectively, whereas the degradation percentage of the selected Phs ranged from 94% to 100% for the NF and RO membranes, respectively. Furthermore, Ho et al. examined palm oil mill effluent treatment by implementing graphene oxide (GO)/multi-walled carbon nanotube (MWCNTs) conductive membranes ^[20][26].

Treatment costs using membrane filtration can range from 0.4 to 1 EUR/m^{3 [21]}[27] and the produced emissions can range from 0.2 to 2.3 kg CO₂ eq. m⁻³, with the total emissions depending on the number of membrane steps and the required transmembrane pressure ^[22][23][28,29].

Constructed wetlands (CW) have been used since the 1950s as a waste management option ^[24][30]. The premise of this wastewater treatment method is based on the naturally occurring processes involving vegetation, soil, and microorganisms in a controlled environment ^[24][30].

Breitholtz et al. ^[25][31] achieved a BOD reduction of 40%, whereas reductions in 92 pharmaceuticals (Phs) ranged between 42 and 52%. The treatment of wastewater derived from residential areas was investigated over horizontal flow constructed wetlands (HFCW), vertical flow constructed wetlands (VFCW), and biofilters by Adrados et al. ^[26][32]. According to their study, the TN reduction ranged from 21 to 85%. Younger and Henderson ^[27][33] reported 41%, 59%, and 66% reductions in BOD, P-PO_4, and N-NH₄, respectively, employing an innovative full-scale mine water/sewage cotreatment CW for polluted mine waters. The yield of a vertical up-flow CW for swine wastewater was studied by Huang et al. ^[28][34] with remarkable results. Reductions in COD, TN, N-NH_4, and TP were 92.2%, 92.7%, 94.4%, and 97.8%, respectively, whereas the degradation of Phs ranged between 98.3% and 99.9%. The removal of various ECs (77.2%) and antibiotic resistance genes (ARGs) derived from landfill leachate was investigated by Yi et al. ^[29][35] by employing a full-scale horizontal subsurface flow CW. An integrated surface flow CW was employed over a 10-year period for the removal of ARGs found in domestic wastewater, as reported by Fang et al. ^[30][36]. The COD, BOD, TN, TP, and N-NH₄ degradations were 70.8%, 75.2%, 60.2%, 55.6%, and 61.3%, respectively. Chen et al. examined the ARG (85.8%) and antibiotic eliminations from wastewater derived from residential areas using mesocosm-scale horizontal subsurface flow CWs ^[31][37]. VFCWs were applied for the removal of ciprofloxacin HCl, oxytetracycline HCl, and sulfamethazine from swine sewage achieving elimination rates of 85%, 95%, and 73% for each of the aforementioned antibiotics, as found by Liu et al. ^[32][38]. The elimination of ARGs from municipal wastewater was studied by Nõlvak et al. ^[33][39] by employing horizontal subsurface flow CWs, achieving a 92% and 25% removal of BOD and TN, respectively. Chen et al. studied the effect on the degradation of antibiotics and ARGs of domestic sewage by employing six mesocosm-scale CWs ^[34][40]. The removal rates of COD, TOC, TN, and N-NH₃ were 80.2%, 80.3%, 54.7%, and 44%, respectively, whereas the total removal of all detected antibiotics was 98.6%. A 62% reduction in the rate of tet genes and a 90% average total removal rate of oxytetracycline and difloxacin antibiotics from swine sewage were achieved by applying a VFCW by Huang et al. ^[35][41].

Ledón et al. found a 90% reduction in the BOD rate from domestic wastewater by employing horizontal subsurface flow (HSSF) CW with the HSSF pretreatment cost being equal to 1903 USD/p.e, whereas the GHG emissions ranged between 3.8 and 4.7 kg CO_2-eq/kg for BOD₅ ^[36][42]. The coupling of microbial fuel cell (MFCs) technology with a conventional HSSF CW was investigated for municipal wastewater treatment by Corbella et al., achieving a remarkable 85% BOD reduction, where the cost of a conventional CW was estimated at 430 EUR/p.e ^[37][43]. Winery sewage treatment employing various scenarios of CW operations was examined by Flores et al., with the GHG emissions derived from the LCA being 1.3 kg CO_2-eq/m^{3 [38]}[44]. Significant variations in costs were observed via life cycle costing (LCC) analysis of two different small-scale WWTPs coupled with CW technology for the treatment of wastewater produced by a student residential building and its coffee shop located in Brazil ^[39][45]. The cost of the scenario implementing a mobile CW (2.42 × 10⁴ kg CO_2-eq) was found to be higher than that of the scenario using a decentralized VFCW (1.03 × 10⁴ kg CO_2-eq) for wastewater treatment by Lakho et al. ^[40][46]. Pan et al. investigated the treatment of wastewater produced by the residential area of Changzhou in China using a vertical subsurface flow CW system, achieving 96% and 83% reductions in BOD and N-NH₄, respectively, whereas the total GHG emissions were estimated at 38.83 kg CO_2-eq/d ^[41][47]. A subsurface flow CW system was implemented for blackwater and greywater treatment coming from a rural area in Southern Brazil and showed an impressive COD, BOD, TKN, N—NH_3, and total P reduction, as described by Lutterbeck et al. ^[42][48]. The LCA revealed GHG emissions of 1.33 × 10³ kg CO_{2 eq}. Finally, Garfí et al. examined the performance of a combined VFCW and HFCW system for the treatment of sewage produced by small rural areas ^[43][49]. The BOD reduction was 89%, the capital cost was 210.36 EUR/p.e., the operational and maintenance costs were 0.4 EUR/m³, and the GHG emissions were ca. 990 g CO₂/m³_water.

WWTPs based on microalgae have gained significant attention since they combine environmentally friendly tertiary treatment technology with enhanced biomass production ^[44][50]. Two different microalgae-based WWTPs were compared considering their performance in agro-industrial sewage treatment, as reported by Magalhães and co-workers ^[44][50]. Both proposed WWTPs were examined for wastewater treatment, as well as microalgae biomass production. The first system was a bubble column photobioreactor (PBR), whereas the second was a high-rate pond (HRP). The COD reduction of the former was 54.3%, whereas the reduction percentage of the latter was 47.7%. The removal rates of N-NH₄ and P were found to be complete in the case of the PBR and were 59.5% and 100%, respectively, in the case of the HPR, highlighting the superior performance of the PBR. Silambarasan et al. studied the performance of coupling microalgae (Scenedesmus sp. and Chlorella sp.) with lipid augmentation in order to remove nutrients from domestic sewage ^[45][51]. The reduction percentages of COD, TOC, TN, N-NH₄⁺, N-NO₃⁻, and PO₄³⁻ were estimated at 83%, 86%, 94%, 98%, 96%, and 95%, respectively.

Marangon et al. compared the environmental impact of two different scenarios for domestic sewage treatment ^[46][52]. According to the first scenario, a high-rate algal pond (HRAP) treatment system resulted in 0.1 kg CO_2eq GHG emissions, whereas following the second scenario based on a hybrid reactor formed by an HRAP and a BR, the emissions corresponded to 0.19 kg CO_2eq. Li and co-workers investigated the treatment of municipal sewage by a non-separated nutrient resource derived from municipal wastewater and integrated in order to facilitate biofuel production from microalgae ^[47][53]. The GHG emitted from the proposed process were 20,881 kg CO_2eq/y.

Ozonation was used for wastewater treatment purposes in 1906 in Paris, France ^[48][54]. In ozonation processes, ozone reacts with organic contaminants and degrades them but can form intermediary toxic products, and its low water solubility leads to low process efficiency ^[48][54]. This process has been examined in the literature as a tertiary treatment method, producing great results for the microbial load and the pharmaceuticals removed, but did not significantly affect the nutrient content of the secondary effluent. More specifically, Lamba and Ahammad ^[49][55], reported a 4 log reduction in coliforms, whereas Nasuhoglu et al. ^[50][56], Shi et al. ^[51][57], and Maniakova et al. ^[52][58], reported a 2.2- to 5.3-log reduction in coliforms, E. coli, Salmonella, and Enterococcus. Regarding pharmaceutical removal, Liu et al. ^[53][59] found that ozonation was capable of removing a range of pharmaceuticals by 5 to 80%. Antoniou et al. ^[54][60] also reported a removal of 70 to 100% for carbamazepine, naproxen, beclomethasone, and memantine. Regarding the treatment costs, 0.03 EUR/m³ has been reported in the literature ^[55][61], with a global warming potential of 0.025 to 0.3 kg CO₂ eq. m⁻³.

Among the various advanced oxidation processes (AOPs), the Fenton oxidation process has gained significant attention, mostly because of its low operational costs ^[56][62]. The photo-Fenton process was employed in order to be studied for the removal of sulfamethazine, resulting in complete degradation of this antibiotic, whereas the TOC reduction was 56%, as reported by Pérez-Moya et al. ^[56][62]. A 100% elimination of amoxicillin and an 81% TOC removal were obtained by implementing the photo-Fenton process as presented by Trovó et al. ^[57][63]. The degradation of 22 micropollutants present in municipal wastewater was investigated by employing a photo-Fenton (UV254/H₂O₂/Fe) process, as shown by De la Cruz et al. ^[13][19]. The average percentage concerning the removal of all 22 pollutants was 80%.

The solar photo-Fenton process was quite effective for ARB and ARG removal from urban sewage, as mentioned by Giannakis et al. ^[58][64]. Various Phs, namely ofloxacin, carbamazepine, flumequine, ibuprofen, and sulfamethoxazole, were found to be almost completely removed from municipal sewage when coupling NF and solar photo-Fenton technology, as reported by Miralles-Cuevas et al. ^[59][65]. Quite interesting is the work of Elmolla and Chaudhuri, who combined the photo-Fenton process with a sequencing batch reactor (SBR) to study performance concerning the treatment of antibiotic wastewater ^[60][66]. They achieved an 89% reduction in soluble COD.

Reductions in the Phs present in wastewater produced by a Spanish pharmaceutical industrial unit were investigated by considering the environmental impact of employing an LCA over heterogeneous and homogeneous Fenton processes, as reported by Rodríguez and co-workers ^[61][67]. The heterogeneous Fenton process exhibited lower GHG emissions of 0.04 kg CO_2eq. An LCA study was performed by Pesqueira et al. of various solar-based treatments including solar circumneutral photo-Fenton (SPF) ^[62][68]. The GHG emissions of the latter were estimated at 0.331 kg CO_2eq. The emitted GHG of 554 kg CO_2eq/1000 m³ as it concerns the operation of a solar photo-Fenton process at acidic pH for municipal wastewater treatment were found to be lower compared to systems operating at neutral pH, as presented by Gallego-Schmid and co-workers ^[63][69].

A semi-industrial solar photo-Fenton reactor was investigated by Foteinis et al. concerning the environmental sustainability of the proposed process over real wastewater effluent derived from a pharmaceutical laboratory ^[64][70]. The TOC reduction was found to be 79%, whereas the GHG emissions obtained from the LCA corresponded to 2.71 kg CO_2eq m⁻³. Photo-Fenton processes in compound parabolic concentrator-type solar reactors have attracted significant interest recently, which is mostly attributed to their enhanced performance over the degradation of recalcitrant pollutants ^[65][71]. As mentioned in the analysis performed by Belalcázar-Saldarriaga and co-workers of the above-mentioned process employed for the degradation of acid orange 52 dye (AO52), the obtained COD and TOC reduction percentages were 55% and 35%, respectively, and the GHG emitted from the system operation were 0.762 kg CO_2eq/m³ wastewater ^[65][71].