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Domestic wastewater (DWW) contains inorganic and organic components that can be harmful to aquatic organisms. Traditional remediation approaches (physical, chemical and biological) can be used on-site or off-site to purify polluted domestic water (activated sludge, built-wetlands, stabilization ponds, trickling filters and membrane bioreactors), and each has its own advantages and limitations. Biosorption through microorganisms, bacteria (microbe-mediated remediation), fungi (mycoremediation) and algae (phycoremediation) has shown promising results in removing toxic chemicals and nutrients. The type of waste and its concentration, heterogeneity level and percentage of clean-up required; and the feasibility of the clean-up technique and its efficiency, practicability, operational difficulties, environmental impact and treatment costs are all factors that are to be considered when choosing a technique for domestic wastewater treatment (DWWT).
As is known to the world, water is “the elixir of life” and a valuable resource for agricultural, industrial and domestic purposes. However, the fact that we have only limited access to safe freshwater is also true 
. This rising water scarcity all over the world has stimulated the reuse of treated wastewater (WW). The global water use has escalated by a factor of six in the last hundred years and will be increasing slowly at a rate of 1% per year. Moreover, variability in rainfall patterns, the rapidly increasing population, urbanization and industrialization have aggravated the issue of water security 
The practice of protecting and maintaining drinking water quality came into effect several hundred years ago. Rapid advancements in the medical and scientific fields have resulted in the provision of basic sanitation services in both urban and rural areas. One of the first cities in the United States to get piped drinking water was Philadelphia. In 1801, drinking water began to flow via the mains of the Philadelphia Water Department. Interestingly, it was a major step by the public health protection system to connect the spread of diseases with centralized water systems 
. The use of different water treatment and purification techniques for domestic wastewater (DWW), such as filtration, well-maintained distribution systems and disinfectants under the aegis of Centre for Disease Control and Prevention (CDC), are some of the best practices of the 20th century (under the umbrella of infectious disease control).
Domestic wastewater (DWW) is the wastewater derived from household activities such as washing clothes and utensils; bathing; cleaning one’s hands, home and vehicles; defecation; and micturition. The DWW can be subcategorized as yellow- (containing urine), brown- (containing feces plus flushed water), black- (containing urine, feces, bacterial activity) and greywater (containing water from the kitchen, laundry, shower and handwashing) (Figure 1).
Figure 1. Water pollution: sources and emerging contaminants.
DWW contains millions of intestinal bacteria and a minority other organisms which further lead to threats to the population. Laundry WW, which is rich in detergents, phosphates and nitrates, causes foam formation and endangers the aquatic organisms of the freshwater ecosystem through eutrophication. Hence, the purification of DWW is crucial for the sustainability of water bodies and aquatic life 
As per the global database, there are more than fifty-eight thousand WWT plants in the world. Among these, there are more than sixteen thousand in the United States and eighteen thousand in Europe. In US alone, 62.5 billion gallons of wastewater (on an average 50 to 70 gallons is produced per person per day) is treated every day. The establishment of more sewage treatment plants (STPs) would serve as a solution the problem. The strategies for removal of pollutants from wastewater include conventional methods (sand filtration, coagulation/flocculation, precipitation, biodegradation, adsorption using activated charcoal), established methods (evaporation, oxidation, incineration, solvent extraction, membrane separation, membrane bioreactors, electrochemical treatment, ion exchange) and non-conventional methods (advanced oxidation, biosorption, bio/nanofiltration, biomass, adsorption onto nonconventional solids).
Treatment of sullage/greywater and conversion of sludge into various less harmful by-products can be performed by conventional processes. The conventional methods can be divided into preliminary, primary, secondary and advanced treatment processes. The basic objective of WWT is (i) removal of the biodegradable organic substances; (ii) removal of various nutrients, such as phosphates; (iii) destruction of pathogens; and (iv) prevention of water pollution to safeguard aquatic organisms. However, maintenance and monitoring, emerging contaminants, low efficiency and sludge treatment and disposal are the major limitations, as they increase the total cost of WWT.
Besides the conventional waste in sewage, non-conventional waste (emerging contaminants), such as industrial chemicals, pesticides, pharmaceuticals and personal care products, is increasing day by day 
. Effective removal of these emerging contaminants can be achieved through adsorption regimes 
. Removal of antibiotics is necessary, as they may destroy the existing microbial populations of natural water bodies. Photochemical destruction of antibiotics such as penicillin G (PENG) is a green and efficient advanced oxidation process that can be applied to treat wastewater containing non degradable antibiotics 
. Nanoparticles can also be used to trap and remove hazardous contaminants from wastewater systems. Magnetic-MXene has been established as an efficient nanoparticle-based WWT system, but further research is needed to increase the scale-up efficiency of all such methods 
. Wastewater from different sources contain heavy metal (HM) contaminants such as mercury (Hg), cadmium (Cd), chromium (Cr), nickel (Ni), copper (Cu) and lead (Pb) that are non-degradable and lead to biomagnification. These HM components can be effectively removed from waterflow by metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) by their highly organized structures with different organic groups. Although they can effectively trap and remove all HM components, they are specifically used to remove CdII
from aqueous media 
Thus, apart from conventional strategies, a blend of conventional and modern and innovative strategies can effectively mitigate the problem of WWT for sustainability in WWT regimes 
2. Water Contaminants
Water gets frequently contaminated from various sources, as shown in Figure 1
. Industrial, agricultural and domestic wastes severely contaminate water. Contaminated drinking water may contain several kinds of pathogens, such as bacteria, viruses and protozoa, which are potential threats to public health 
. Water supplied under indirect potable reuse (IPR) projects is most likely contaminated by viruses. Although a large number of particles may be present in municipal waste water (MWW) and most of them are susceptible to chlorine inactivation, the presence of excess particles in contaminated water needs careful management 
. MWW effluents infected by viruses may further cause serious human diseases, such as gastroenteritis and hepatitis. A proper analysis in the laboratory is required to monitor the presence and control of viruses. Waste produced by military facilities is more or less similar to the waste produced by the civilian residential communities or commercial facilities, and the disposal of the excreta and other organic wastes is crucial for effective WWT.
2.1. Characterization of Wastewater (WW)
Wastewater is any water whose quality has been degraded in terms of physical, chemical and biological composition by anthropogenic activity. This water possesses a wide range of contaminants at various concentrations. Routinely testing and monitoring the water quality is mandatory for eradicating the potential hazards. The majority of wastewater contains 99.9% water with relatively small amounts of suspended and dissolved organic and inorganic contaminants.
3. Process of Wastewater Treatment
3.1. Preliminary Treatment Plant
This process removes debris and coarse particles suspended in the wastewater. Unique facilities and equipment are required in this phase to separate rags, grit, foreign objects and other debris. If not done, it becomes difficult to deal with large substances during other subsequent operations. The preliminary treatment plant removes 25% of the organic load and almost all of the non-organic solids. The waste material is removed and disposed of in a landfill. The screening can be classified according to the use of fine and coarse screens. Coarse screens are used in preliminary treatment, whereas fine screens have been deployed as a substitute to sedimentation. Solids are also passed through each channel, so they convert into shredded matter through comminution. Grit chambers are used in a separate system which slows the velocity of water flow in order to remove the inert/inorganic materials 
. Economically, it prevents the operation problems in channels and pipes and reduces the formation/accumulation of excess sludge 
3.2. Primary Treatment Plant
The floating materials and settled organic and inorganic matter are removed during this process. Around 60% of grease and oil, 50% of BOD5 and 70% of suspended solids are oxidized at this stage. Some organic nitrogen and phosphorous and HMs are removed from the wastewater during primary sedimentation. The effluent obtained from primary sedimentation is referred to as primary effluent 
3.3. Secondary Treatment Plant
During this process, some of the residual solids and colloidal and biodegradable wastes are removed in an aeration tank, in which micro-organisms are exposed to wastewater. Microorganisms degrade it into an inorganic end-product. High-rate processes are the most applicable parts compared to low-rate processes because they maintain the high content of micro-organisms under controlled conditions. Mechanically, it is possible to treat bad water through trickling filters, activated sludge and a rotatory biological contactor 
3.4. Tertiary Treatment Plant
This stage of purification involves some extra steps that reduce organics, nutrients, turbidity, nitrogen, phosphorous, HMs, bacteria and viruses. The main purpose of this treatment plant is reuse or recycling of wastewater so that it can be used further for irrigation, etc. Purified water is then allowed to meet with water reservoirs.
At this stage of the water purification system, the final treatment is performed by using the chemical and physical methods. Chlorine and its derivatives are used as disinfectants during this stage. This treatment (chlorine treatment) varies according to the type of wastewater and other elements, such as pH, organic content and the type of effluent received. Other treatments, such as ozone or UV treatment, can be performed as a requirement for irrigation, and the reclaimed wastewater can then be used in urban areas.
This entry is adapted from the peer-reviewed paper 10.3390/w14213542