Water treatment is any process that improves the quality of water to make it appropriate for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.
Water is the most crucial compound for life on Earth, and having drinkable water is a key worldwide concern for the twenty-first century. All living things require clean, uncontaminated water as a basic requirement. Water covers more than 71 percent of the earth’s surface, but only around 1% of it is drinkable according to international standards due to various contaminations. Waste water discharge from industries, agricultural pollution, municipal wastewater, environmental and global changes are the main sources of water contamination [1]. Even trace levels of heavy metals, dyes, and microbes are hazardous to human health, aquatic systems, and the environment [2]. According to a Food and Agriculture Organization assessment from 2007, absolute water scarcity will affect 1.8 billion people living in countries, and water stress might affect two-thirds of the global population.
To address water scarcity issues, it is required to recover water from current wastewater or develop alternate water sources for human consumption [3].
Domestic and industrial wastewater are the two types of wastewater. Domestic wastewater contains sewage, bacteria, viruses, hazardous and non-toxic organisms, sanitary outputs, rubbish, detergents, and other solid and liquid discharges from non-manufacturing processes [4].
Water contamination is primarily caused by the discharge of untreated wastewater from enterprises. The effluent from various enterprises, which contains varying levels of contaminants, is dumped into rivers or other water resources. The wastewater may have a high proportion of organic and inorganic contaminants at the initial discharge. Industries generate wastewater as a result of fabrication processes, processes dealing with paperand pulp, textiles, chemicals, and from various streams such as cooling towers, boilers, and production lines .
Elimination of hazardous chemicals from the water, many treatment procedures have been applied [5]. The selection of wastewater treatment systems is contingent on a number of factors:
(1)The degree to which a method is necessary to raise the waste water quality to a permissible level;
(2) the control method's flexibility;
(3) the process's cost;
(4) the process's environmental compatibility [3].
Heavy metals in wastewater have become a serious environmental issue in recent years, owing to the high damage they pose to ecosystems and human health even at extremely low concentrations. Heavy metal pollution is a substantial environmental burden due to its flexibility, accumulation, non-biodegradability, and persistence. Its effluent is discharged into the environment by industries such as paper, Insecticides, tanneries, metal plating, mining operations, and so on. This effluent is non-biodegradable and poisonous or damaging to human physiology and other biological systems [6].
Organic and inorganic pollutants are two types of pollutants found in wastewater, each with a different spectrum of dangerous values.
In the treatment of organic pollutants, biological, physical, and chemical methods are commonly used. However, these approaches are ineffective against inorganic pollutants such as heavy metals. Heavy metal decomposition is a serious concern due to properties such as solubility, oxidation-reduction characteristics, and complex formation [7]. Heavy metal is defined as an element with an atomic weight of between 63.5 and 200.6 and a specific gravity larger than 5.0 [8].
Heavy metals in open waters cause aquatic life to perish, oxygen deficiency, and algae blooms. When heavy metals are discharged into rivers, they are transformed into hydrated ions, which are far more hazardous than metal atoms. The enzymatical processes is disrupted by these hydrated ions, and absorption is accelerated. As a result, heavy metals must be removed in order to reduce public risk [9].
Chemical approaches are used in addition to physical and biological measures to reduce the discharge of pollutants and waste water into water bodies. Different chemical procedures for the conversion into final products or the removal of pollutants are used for the safe disposal of contaminants [3].
Pre-chlorination for algae control and arresting biological growth.
Aeration along with pre-chlorination for removal of dissolved iron when present with relatively small amounts of manganese.
Disinfection for killing bacteria, viruses and other pathogens, using chlorine, ozone and ultra-violet light.
Physical techniques of water/waste water treatment rely on physical phenomena to complete the removal process, rather than biological or chemical changes [3].
Most common physical techniques are :
Sedimentation is one of the most important main wastewater treatment procedures. Gravity settling is a method of separating particles from a fluid. The particle in suspension remains stable in quiescent conditions due to the decrease in water velocity throughout the water treatment process, following which the particles settle by gravitational force [10][11]. For solids separation that is the removal of suspended solids trapped in the floc.
Filtration is the technique of removing pollutants based on their particle size is known as filtration. Pollutant removal from waste water permits water to be reused for a variety of purposes. The types of filters used in the procedure differ depending on the contaminants present in the water. Particle filtration and Membrane filtration are the two main forms of waste water filtration [12].
Degasification is the process of removing dissolved gases from a solution . The law of Henry's law states that the amount of dissolved gas in a liquid is proportionate to the partial pressure of the gas. Degasification is a low-cost method of removing carbon dioxide gas from waste water that raises the pH of the water by removing the gas [3].
Coagulation for flocculation. Coagulation is followed by the flocculation of the unstable particles into bulky floccules to enhance particle size [13]. The addition of coagulants destabilizes colloidal suspensions by neutralizing their charges, resulting in the aggregation of smaller particles during the coagulation process[14].
Coagulant aids, also known as polyelectrolytes – to improve coagulation and for more robust floc formation.
Polyelectrolytes or also known in the field as polymers, usually consist of either a positive or negative charge. The nature of the polyelectrolyte used is purely based on the source water characteristics of the treatment plant.Biological Wastewater Treatment PlantThese will usually be used in conjunction with a primary coagulant such as ferric chloride, ferric sulfate, or alum.
Chemical precipitation is a common process of eliminating heavy metals from inorganic wastewater. The dissolved metal ions are transformed to the insoluble solid phase by a chemical interaction with a precipitant agent such as lime after the pH is adjusted to basic conditions (pH 11) [15].
Membrane filtration has gotten a lot of attention for inorganic effluent treatment since it can remove not only suspended solids and organic components, but also inorganic pollutants such heavy metals. For heavy metal removal, several forms of membrane filtration, such as ultrafiltration, nanofiltration, and reverse osmosis, can be used depending on the particle size that can be maintained. [17].
Ion exchange is a reversible ion exchange process in which an insoluble substance (resin) takes ions from an electrolytic solution and releases additional ions of the same charge in a chemically comparable amount without changing the resin's structure [18][19].
Adsorption is a mass transfer process in which a substance is transported from the liquid phase to the surface of a solid/liquid (adsorbent) and becomes physically and chemically bonded (adsorbate). Adsorption can be classified into two forms based on the type of attraction between the adsorbate and the adsorbent: physical and chemical adsorption, commonly known as physisorption and chemisorptions. [20][21].
Activated carbons (ACs) are effective adsorbents for a wide variety of contaminants. The adsorptive removal of color, aroma, taste, and other harmful organics and inorganics from drinking water and wastewater is one of their industrial applications [22].
Both a high surface area and a large pore size can improve the efficiency of activated carbon. Activated carbon was utilized by a number of studies to remove heavy metals and other types of contaminants from wastewater. The cost of activated carbon is rising due to a shortage of commercial activated carbon (AC). Because of its high surface area, porosity, and flexibility, activated carbon has a lot of potential in wastewater treatment [23].
This is the method by which dissolved and suspended organic chemical components are eliminated through biodegradation, in which an optimal amount of microorganism is given to re-enact the same natural self-purification process [24]. Through two distinct biological process, such as biological oxidation and biosynthesis, microorganisms can degrade organic materials in wastewater. Microorganisms involved in wastewater treatment produce end products such as minerals, carbon dioxide, and ammonia during the biological oxidation process. The minerals (products) remained in the wastewater and were discharged with the effluent. Microorganisms use organic materials in wastewater to generate new microbial cells with dense biomass that is eliminated by sedimentation throughout the biosynthesis process [25].
Bioremediation is a biological treatment method in which microorganisms breakdown or transform hazardous contaminants in wastewater to a less toxic or non-toxic state. The following technologies can be used to bioremediate wastewater, which can be done by autotrophs or heterotrophs; Phytoremediation, Rhizofiltration, Bioaugmentation, and Biostimulation. Autotrophhs are organisms that can fix carbon and use inorganic chemicals (carbon dioxide) in wastewater to make organic compounds such as fats, proteins, and carbohydrates. Heterotrophs will use the organic substances produced for their growth and development. Heterotrophs are organisms that are unable to fix carbon and rely on organic molecules from wastewater as a source of energy.
Slow sand filtration using a biofilm to metabolize organic matter, adsorb soluble components and entrap particulates
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^Wang, Lawrence K.; Vaccari, David A.; Li, Yan; Shammas, Nazih K. (2005), "Chemical Precipitation", Physicochemical Treatment Processes, Totowa, NJ: Humana Press, pp. 141–197, retrieved 2021-11-12
