**3. Sewage treatment using nanoparticles**

Nanoparticles are materials in which the structural components lie within sizes ranging from 1 and 100 nm in at least one dimension [12]. Because of this special nature, nanoparticles highly differ in their properties mechanical, electrical, optical, and magnetic in comparison with other materials. In recent years, nanoparticles have found applications in many fields, such as catalysis [13], medicine [14], sensing [15], and biology [16]. Specifically, due to their nano sizes and huge surface areas available for chemical and biochemical reactions, High mobility of nanoparticles in solutions, nanoparticles are being used in water and Sewage treatment extensively. Sewage s contain toxic metal ions like [Hg(II), Pb(II), Cr(III), Cr(VI), Ni(II), Co(II), Cu(II), Cd(II), Ag(I), As(V), and As(III)] [17]. Numerous Conventional chemical and physical techniques like adsorption, precipitation, ion exchange, reverse osmosis, electrochemical treatments, membrane filtration, evaporation, flotation, oxidation and biosorption processes are being expansively used for the removal of such toxins. These conventional techniques do offer good amounts of toxin removal but to treat huge volumes of Sewage, there is an emerging need to search for new alternatives. In the current scenario, widely used nanoparticles for water and Sewage treatment largely comprise zero-valent metal nanoparticles, metal oxide nanoparticles, carbon nanotubes (CNTs), and nanocomposites.

Current research trends show the use of various zero-valent metal nanoparticles, viz.; Fe, Zn, Al, and Ni, in sewage treatment. Silver nanoparticles (Ag NPs) which come under the category of Zero-Valent Metal Nanoparticles are extremely lethal to microorganisms and hence show a strong antibacterial effect against various microorganisms, including viruses [18], bacteria [19], and fungi [20]. These can adhere to the bacterial cell wall of the microorganisms and increase the permeability of the cell walls [21]. They can easily penetrate through the cell walls resulting in structural changes in the cell membrane leading to the death of the cells [22]. Further, when Ag NPs come in contact with bacteria, they generate free radicals. They can damage the cell membrane and are considered to cause the death of cells. Magnetic nanoparticles can also be utilized in water treatment to remove heavy metals, sediments, chemical effluents, charged particles, bacteria and other pathogens. Lower operating costs, Lower energy requirements, lesser sludge discharge, Reduction in the number of pesticides and VOCs (organic chemicals), and Reduction of heavy metals, nitrates and sulfates, color, tannins, and turbidity are some of the many advantages of Sewage treatment using nanoparticles.

#### **3.1 Carbon nanomaterials**

Carbon nanomaterials (CNMs) are sheets made of graphene which are rolled up in the form of cylinders having diameters as minor as 1 nm. Carbon nanomaterials possess exceptional structures, and a great capacity to adsorb a wide range of contaminants like dichlorobenzene, ethyl benzene, Zn2+, Pb2+, Cu2+, and Cd2+, dyes, electrical properties, fast kinetics, large surface area, rich porous structures [23]. All these contribute to their diverse applications in advantages in sewage treatment processes.

The nanocomposite is an emerging field of nanomaterials. Nanocomposites are prepared through the chemical deposition of nZVI on CNTs. The resulting adsorbent has huge potential for rapid and effective removal of nitrate components presenting sewage water. These nanocomposites also have good magnetic properties due to which, the adsorbent can be easily separated from the solution by using a magnetic field [24]. Many such nanocomposites can be fabricated creating a network on polyimide supports. These offer advantages of being nontoxic, long-term stable and low-price materials. In concept, ideal composites for physical applications should be continuous, bulk immobile materials in which the nano reactivity is acquired by impregnating a parent material structure with nanoparticles [25]. Still, much research is underway for creating nanocomposites which can serve sewage treatment in a costefficient manner.
