**4. Available technologies for wastewater treatment**

Generally, conventional wastewater treatment plants are not constructed to eliminate emerging contaminants. The occurrence of ECs in the environment affects public health, marine life and produces resistant bacteria, neurotoxin effects, endocrine interruption, and tumors. To eliminate these organic pollutants from water, several techniques such as membrane filtration, coagulation-flocculation, solvent extraction, ion exchange, catalytic oxidation, electrochemical oxidation, and precipitation, etc. have been tested (**Figure 4**). However, these techniques are less effective, very expensive and do not eliminate the contaminants completely from polluted water which makes this issue more challenging for the researchers. Besides these techniques, adsorption and photocatalytic degradation are considered potential approaches to remove wastewater contaminants [26].

**Figure 4.** *Various treatment methods used for wastewater.*

### **4.1 Membrane filtration**

Membrane technology is a physical method implemented to eliminate emerging contaminants from aquatic system. Membranes are formed from substances having filtering properties such as specific surface charge, pore size and hydrophobicity to remove suspended contaminants. Membrane filtration is categorized as ultra-filtration (UF), nano-filtration (NF), microfiltration (MF), forward osmosis (FO) and reverse osmosis (RO). Major membrane processes including forward osmosis, membrane refinement and electro-dialysis of the membrane have the ability to reduce emerging contaminants upto greater than 99% but still have not been executed on large scale [27].

The ultrafiltration technique works at low pressure for the removal of colloidal, suspended or dissolved pollutants depending on the membrane and pollutant type. UF has pore size in range of 0.001–0.1 μm which is larger than dissolved hydrated metals ions, thus easily pass through it. Polymer enhanced ultrafiltration (PEUF) and Micellar enhanced ultrafiltration (MEUF) processes were studied to enhance the removal efficiency of metal ions such as copper, zinc, chromate, arsenate, cadmium, nickel, serinium, and organics like phenol, o-cresol, etc.

Microfiltration has pore size ranges from 0.1 to 10 μm and is commonly operated at atmospheric pressure but cannot effectively remove contaminants of size greater than 1 μm. Reverse osmosis and forward osmosis depend on the osmotic pressure gradients and use semi-permeable membrane to efficiently remove dissolved particles up to 1 nm from water. Nanofiltration membrane possess small pore size ranges from 1 to 10 nm and have high competency for removal of ECs based on type of membrane and contaminant. NF can be used for removal of pharmaceuticals and natural hormones such as anti-inflammatory drugs, sulfonamide and fluoroquinolone antibiotics, testosterone, estradiol, and progesterone [28].

### **4.2 Coagulation-flocculation**

Coagulation-flocculation process is effective for the elimination of larger colloidal or suspended particles of disperse dyes colored wastewater. Coagulation is a procedure in which dye solution systems are dispersed to form flocs and agglomerates while in flocculation aggregated flocs are joined to form larger agglomerates which settle down due to gravity [29]. Coagulation/flocculation is economically feasible and simply operated and commonly used in textile industries to purify wastewater. In this method, coagulants like lime (Ca(OH)2), ferric sulfate (Fe2(SO4)3∙7H2O), aluminum sulfate (Al2(SO4)3∙18H2O), and ferric chloride (FeCl3∙7H2O), combine with the pollutants and remove them by electrostatic interactions or sorption. Use of aluminum sulfate (Al2(SO4)3 for removal of pharmaceuticals such as betaxolol, chlordiazepoxide, bromazepam, warfarin and hydrochlorothiazide by coagulation-flocculation has been reported. This technique diminishes suspended matter, soluble dyes, colloidal particles and coloring agents from wastewater [30].

### **4.3 Solvent extraction**

Solvent extraction is widely used technique for the elimination of organic and inorganic pollutants discharged into wastewater from various industries. It is based on three major operations. First is the extraction/transferring of solute particles to solvent from water. Secondly, the separation of solute from solvent and the third stage is the solvent recovery stage. Solvent extraction is mostly operated for exclusion of

phenols, creosols and other phenolic acids from contaminated water containing low quantity of solute arising from petroleum processing plant, coke-oven plants in the steel and plastics manufacturing [31].

### **4.4 Adsorption**

Adsorption is one of the most efficient techniques used for treating wastewater due to its simple design, high competence and ease of operation, capital cost, easy recovery, adaptability and technical feasibility without producing harmful byproducts. This technique is not new but is recognized throughout the world because of removal capacity and regeneration of adsorbents. This technique has been broadly applied for both organic inorganic toxins from household and industrial wastewater [32]. Various research efforts have been devoted to discover low-cost adsorbents having large surface area and excellent binding capacity to enhance their adsorption efficiency. Different types of adsorbents, e.g., peat, bamboo dust, chitosan, silica gel, activated carbon,, fly ash, zeolites, metal organic frameworks nano-adsorbents for example carbon nanotubes and graphene have been applied for elimination of emerging contaminants [33, 34]. Activated carbon is widely used as traditional adsorbent because of highly porous surface area, convenient pore composition and thermo stability for removal of dyes and pharmaceutical products, e.g., 17β-estradiol, 17α-ethynylestradiol, bisphenol A, and fluoroquinolonic Caffeine from wastewater [35].

### **4.5 Advanced oxidation process**

Advanced oxidation processes (AOPs) have been introduced as proficient technology in wastewater treatment. AOPs are based on the generation of hydroxyl (OH) or sulfate radicals for oxidation of ECs while sometimes ozone and UV irradiation are used for enhanced removal efficiency. AOPs methods efficiently remove biologically injurious or non-degradable compounds such as pesticides, aromatics, petroleum essentials and volatile organic compounds (VOCs) rather than transferring these to another phase. AOPs are applicable for the removal of many organic contaminants at the same time without producing any hazardous substance in water, as OH˙ is reduced to form H2O as byproduct. AOPs include ozonation (O3), hydrogen peroxide (H2O2), electrochemical oxidation, Fenton process, UV light and photocatalytic process [36].

### *4.5.1 Non-photochemical processes*

**Ozonation:** Ozone is an extremely efficient oxidizing agent and has the potential for elimination of organic and inorganic compounds from industrial effluents. It is a complex oxidation method. Pre-oxidation processes give significant development in biological degradation while post-oxidation process improves effluent quality. The limitation of ozonation is low solubility, stability and short half-life. O3/H2O2 and catalytic ozonation have been investigated for generation of hydroxyl radical which efficiently removed organic pollutants such as antibiotics, antiphlogistics, beta blockers, lipid regulators and their metabolites, natural estrogen estrone, antiepileptic drug carbamazepine and musk fragrances in wastewaters effluents.

**Electrochemical method:** Electrochemical procedure is generally applied for removal of toxic contaminants from textile effluents by direct or indirect oxidation. This procedure is commonly applied for elimination of ECs like dye by using either

mercury electrode, graphite rod, boron doped diamond electrode**,** platinum foil or titanium/platinum as anode while SS304 is used as cathode in textile sewage treatment. This process is cost effective as minute amount of chemical is required and stability is easily attained by manipulating the electric current**.**

**Fenton process:** Reaction between ferrous iron and hydrogen peroxide is termed as Fenton's reaction. Fenton method is used for removal of organic pollutants like phenols, reactive dyes and pesticides. Fenton process is of low cost as no energy is required for activating H2O2, environmental friendly, easy to control and efficient for elimination of organic pollutants.

### *4.5.2 Photolytic chemical process*

### *4.5.2.1 Homogeneous photolytic chemical process*

**Ultraviolet lamp (UV)**: In this process oxidizing agent like H2O2 is initiated by UV process to produce OH˙ and can degrade micropollutants efficiently which can be affected by various parameters such as pH, structure of dye, composition of effluent and intensity of UV radiation. Generally, UV process occurs at standard wavelength of 254 nm at low pressure. A pilot plant with UV/H2O2produced hydroxyl radical to treat effluent, achieved 98% removal of mecoprop and diclofenac. O3/H2O2/UV processes were examined in treatment of textile effluent to achieve complete degradation [36].

**Photo-fenton process:** In this process, formation of hydroxyl radical is improved by UV light in the presence of Fe and competently degrades wastewater effluents. Fenton process and photo-fenton process are similar but in the later process mineralization is much better. Removal of numerous ECs like pharmaceuticals, beta blockers and pesticides excluding triclosan by photo-fenton process is enhanced significantly (95–100%).

## *4.5.2.2 Heterogeneous photolytic chemical process*

Mostly semiconductor consists of two energy bands, high conduction band and low energy valence band and these two are separated by band gap. In heterogeneous processes, semiconductor sensitized photolytic chemical oxidation produces OH radical. Adsorption of photon having energy (≥band gap energy of catalyst) is needed for photocatalytic reaction to occur. ZnO, strontium titanium trioxide and TiO2 have been utilized extensively as photocatalysts for commercial application. Photo-catalysis is commonly used for dyestuff degradation from textile wastewater. Photocatalytic process enhances efficiency in the presence of H2O2 up to 100% for numerous pollutants such as bisphenol A, pesticides, pharmaceuticals [37].

## **4.6 Application of nanotechnology for ECs removal**

Nanomaterials are generally defined as the materials having at least one dimension smaller than 100 nm. Nanomaterials have higher density and larger surface area resulting in increasing adsorption efficiency, surface reactivity, and resolution mobility. Current investigation in the exploitation of nanomaterials has facilitated the application of nanotechnology in wastewater treatment via adsorption, AOPs and filtration. Nanomaterials have been reported to effectively eliminate emerging contaminants from wastewater. A variety of nanomaterials

have been reported for wastewater treatment (**Figure 5**) such as zerovalent metal nanoparticles, metal-oxide nanoparticles, carbon nanomaterials and nanocomposites [38].
