Wastewater and its Treatment Techniques: An Ample Review

The prompt population increment, rapid urbanization of the cities and fast development of the industries leads the generation of much pollution in our atmosphere. Among the other pollutants; water pollution is one of the important issues for addressing seriously. Such pollution not only affects human health but also harmful for agriculture and the earth. Drinking safe and clean water is one of the rising problems around the globe. Various developed countries are utterly working for providing safe and clean water by treating the water. Such countries not only ground water treatment but also proper treatment of wastewater. There are various conventional and non-conventional wastewater treatment techniques. This research work highlights the issues of water, wastewater treatment and few conventional and nonconventional wastewater treatment techniques. More than 50 latest and critical articles relating to the subject area of this research work had been reviewed and also few web blogs has been used to address the challenges in the wastewater treatment. On the basis of reviewed material in this research work, it is concluded that drinking water shortage increased to scary level and needs serious address. Also non-conventional wastewater treatment techniques are more feasible than the conventional techniques. Among the other non-conventional techniques, the constructed wetlands are more beneficial for the wastewater treatment.


Introduction
Presently, environmental laws have turned out to be severe towards economy, health and lessening of pollution. The pollution is the source of release of numerous organic and inorganic elements into the environment. Among the other pollution sources; domestic, industrial and agricultural water is also. But yet; water is one of the utmost essential substance on earth. It is quite important for domestic usage, agriculture crafting, industrial operations and routinely entire animals and plants need water to live. On the off chance that there is no water there would be no life on earth. About 71% of the Earth's surface covered with water and is imperative for every single known type of life. But only 2.5% is the fresh water on Earth 1 . Speedy urbanization and industrialization discharges tremendous volumes of wastewater, which is progressively used as a significant asset for water system in urban and pre-urban agribusiness. It drives noteworthy monetary action, underpins incalculable vocations especially those of poor farmers 2 . Also, because of industrialization and urbanization, it is winding up progressively contaminated and danger of this dirtied water utilization and its sanitation issue is expanding every day in developing countries including Malaysia. No matter how hard, yet far from the most effective, financial and solid approach to guarantee the water supply in Malaysia needs to be addressed properly and this challenge is unsolved yet. In the recent time, extraordinary developments have been prepared in the development of effective technologies, yet at the same time there are different difficulties being encountered by wastewater treatment plants and its operators in Malaysia 3 . Appropriate cure of sewerage is important since almost 98% of Malaysia's fresh water source emanates from surface water and still many technologies, stakeholders and government and nongovernment office must address properly the problems and implement new technologies to avoid shortage of fresh water. Such problems lead the water scarcity and have a substantial adverse impact on human livings and environmental quality all over the Malaysia and in other developing countries. Henceforth it has turned into a fundamental requirement for the present condition to keep water from getting contaminated or to improve cost effective counteractive technique for its fortification. Malaysia, in spite of being honored with plenty of rainfall, water must be coped proficiently to confirm its supportability and furthermore to take into account requests for the generations to come.
Approach to safe drinking water is an essential need; its arrangement ought to be a primary public health main concern. Such alarm has been broadly tended to in developed countries; but, accessibility of sanitation and clean water isn't the standard in most creating nations 4 . An important origin of water in a Malaysia is the recycle of wastewater in addition with of the rainfall. There are numerous water treatment systems in Malaysia: Physical Chemical Treatment Systems, Biological Treatment Systems, Dewatering Systems, Deionization, Reverse Osmosis, Ultrafiltration and many more. Generally, wastewater treatment is the method, where pollutants will be eliminated from wastewater to provide solid waste or waste stream which might be suitable for discharge. The removal of all contaminants (organic, inorganic chemicals and pathogens) is the contemporary goal of treatment of wastewater developed countries. The removal of pollutants from municipal wastewater has been conducted via different techniques such as biological treatment processes; biodegradation, metabolism, adsorption, flocculation, precipitation and ion-exchange are often considered amongst the ecological and advantageous treatment opportunities 5 . However, some treatment method like adsorption is consider as a costly treatment in addition to biodegradation treatment needed to more area to installation and contours maintenance.
Among the other wastewater treatment processes and the significance of removing the contaminations from the waste water; Wetlands are commonly considered an effective technique for removing the pollutants from the wastewater. Artificial wetland is an engineered purification system for wastewater that incorporates physical, chemical and biological processes to mimic the happening processes in natural treatment wetlands 6 -8 . Constructed Wetlands (CWs) are presently observed as reputable eco-technologies for the wastewater treatment. Though, it is designated near-natural methods and fully contrived clarifications intended for which many investigations has been dynamically established in the earlier periods 9 . Constructed wetlands for example multilayer artificial wetland is companied of groundmasses by means of breaks, gravel and stone etc as well as by employing aquatic plant. The wastewater streams in or underneath surface layer groundmass at the wetland, and disintegrates healthy ingredients in water.

Wastewater
Wastewater (or waste water) is any water that has been influenced by human utilize. Wastewater is "utilized water from any mix of residential, mechanical, business or farming exercises, surface overflow or storm water, and any sewer inflow or sewer penetration" 10 . Because of water shortage issues round the globe, the situation is basic to consider non-conventional water sources for satisfying the intensification in need for freshwater. Wastewater is viewed as a suitable elective substitute alternate to overwhelm the scarcity in water supply causing from several causes for example population progression 11 -16 . Nevertheless, the incredible diversity in wastewater sources intrusive of organic and inorganic elements create the recycle of that water condition to ordered checking to evaluate impending hazards effecting on the total environment 17 . Satisfactory recycle of wastewater is important to care for public health, environment and water resources. There are many kinds of waste water and their treatment is depended on the type of waste water and its characteristics. For example: Domestic waste water can be classified as Black water and Grey water. Black water comes from Toilets or it is sewage. Grey water comes from shower, wash basin and floor wash as shown in Figure  1. Also, the domestic wastewater is the sourcing from homes, commercial places such as markets, restaurants, banks, institutional places such as schools and hospitals. The amount of waste water coming from homes varies according to the hours of the day, days and seasons 18 .

Domestic Wastewater
The water disposed from households, workplaces and factories is titled as domestic wastewater. It originates from showers, basins, toilets, washing machines and industrial progressions. Historically such wastewater is considered as sewage. Wastewater created because of "the human activities in households" is termed domestic wastewater like: from the kitchen, toilet, wash basin, shower and clean washing. It generally encloses somewhat lesser quantities of pollutants, however; such lesser quantities of contaminants can create the huge effect on our health. Thus, an appropriately introduced and kept up residential sewage treatment framework for treating and discarding household wastewater will reduce the effect on ground and surface water. Generally, domestic waste water contains two foremost instabilities: "grey water" originates from kitchen, laundry washing, wash basins, baths and etc., while the other is "black water" comes from urinals and toilets.
Wastewater constituents are categorized into numerous foremost sets as shown in Table 1. They can antagonistically influence the oceanic life if release them into ecological. Table 2 shows the usual configuration of untreated domestic wastewater. Absorption denotes to the authentic quantities of physical, chemical and biological contaminants available in wastewater. These contaminants can give permanent impairment to the atmosphere. Due to this objection, all such constituent quantities must be decreased to satisfactory altitudes formerly for releasing.
Domestic wastewater generation has superseded the industrial waste water generation in many developing countries and is the major cause of water pollution and eutrophication. Conventional domestic wastewater treatment system involves large expenditure to bring down the pollutants level to the permissible discharge limits1. Many research 22 -28 adopted an appropriate wastewater management approach, wherein twin benefits such as treatment and reuse can be achieved, and sustainable development can be promoted. Recently, land application of domestic wastewater is attaining thrust due to the point that it provides the most cost-effective and safe wastewater treatment and disposal method 29 . Being rich in organic matter and nutrients, it could be used as source of nutrients for biomass production and thus can be helpful in sustaining economy.

Characteristic of Domestic Wastewater
Domestic wastewater is primarily involved of water (99.9%) along with somewhat some quantity "suspended and dissolved" and "organic and inorganic" solids. Amongst the organic constituents existing in sewage like: lignin, carbohydrates, soaps, fats, proteins synthetic detergents and some decomposed goods. Also many regular and synthetic organic chemicals produced through industrial process. The organization of the wastewater is a component of the utilizations to which the water was succumbed. These utilizations, and the structure with which they were worked out, differ with atmosphere, social and monetary circumstance and populace propensities. This is expected, not exclusively to the trouble in attempted the different research center tests, yet in addition to the way that the outcomes themselves can't be legitimately used as components in structure and activity. In this manner, commonly it is desirable over use indirect constraints which characterize the oddity or the contaminating threat to the wastewater? Such factors characterize the nature of the sewage, and can be isolated into three classifications: physical, synthetic and organic parameters. Tables 3, 4 and 5 show the major physical, chemical and biological characteristics of domestic wastewater 30 .

Wastewater Treatment Techniques
There are non-conventional and conventional wastewater treatment techniques that have been demonstrated and observed to be effective for the wastewater treatment. In contrasting to non-conventional wastewater treatment, conventional techniques have a somewhat great level of mechanization. Generally require pumping and power supplies and involve trained labor for processing and preservation of the system. Figure 2 shows the conventional and non-conventional techniques for the treatment of wastewater.

Conventional Methods
Conventional methods for eliminating metals are moreover fetching insufficient to meet recent rigorous controlling

TOTAL NITROGEN
Total nitrogen comprises organic nitrogen, nitrite, ammonia and nitrate. It is an indispensible nutrient for microorganisms' develop in wastewater treatment through biological process.
• Organic nitrogen • Nitrogen in the form of proteins, urea and amino acids.
• Nitrite • Transitional phase in the ammonia oxidation.
• Basically not found in fresh waste.
• Ammonia • Created during the 1 st period of the organic nitrogen decomposition. waste parameters or are highly expensive. Subsequently, substitutive and cost effective advancements are in intense interest. Conventional methods for eliminating dissolved heavy metals contain carbon adsorption, chemical precipitation, ion exchange, and membrane and evaporations processes 1 . The determination of a specific treatment system principally relies upon an assortment of elements, for example waste sort and fixation, profluent heterogeneity, required dimension of cleanup, just as economic variables. A typical type of wastewater treatment plant is shown in Figure 3.
Conventional wastewater treatment techniques are: "activated sludge", trickling filter", rotating biological contactor" techniques. Rotating Biological Contactors and Trickling filters are temperature sensitive, eliminate little BOD, and trickling filters required more cost to construct compare to activated sludge systems. Activated sludge methods are high costly to function since more energy is consumed for blowers and pumps 31 . However;

Organism Depiction
Archaea • Dissimilar to bacteria in their cell material, cell wall and RNA configuration.
• Imperative in anaerobic methods.

Bacteria
• Unicellular organisms • Existing in several forms and sizes.
• Main organisms accountable for the organic matter stabilization.
• Some bacteria are pathogenic and instigating mostly abdominal infections.
• Main in the oxygen production in water and in some waste treatment techniques.
• They can proliferate in reservoirs and lakes for longer time and declining the quality of water.

Protozoa
• Generally unicellular organisms having no cell wall • Feed themselves on algae, bacteria and other microorganisms.
• Important in biological treatment to sustain steadiness between the several clusters.

Viruses
• Parasitic organisms, made by the relationship of genetic material (RNA or DNA) and a protein arrangement.
• Pathogenic and often hard to eliminate from water.
• Significant for the organic matter decomposition.
• Can raise in small pH circumstances.
• Helminth eggs existing in waste and can source for many diseases.

Activated Sludge
Activated sludge is a procedure having higher amount of microorganisms like protozoa, bacteria and fungi and exist as unfastened clattered mass of fine elements that are retained in deferment by stirring, with the purpose of eliminating organic matter from wastewater 1, 32 . Activated sludge denotes to biological treatment methods that employ an adjourned development of organisms to eliminate suspended solids and BOD. It depends on the rule that extreme wastewater air circulation to shapes flocs of microorganisms that reduce organic matter and be detached by sedimentation 33 . The method involves the aeration and settles down tanks with other accessories: reoccurrence and waste pumps, mixers & blowers for air circulation and the device for flow measurement 34 ,35 . To continue the attentiveness of active bacteria in the tank, portion of the activated sludge is reused 36 . Activated Sludge method occupies small place as associated with trickling filter and has better quality of waste. The only drawback is of greater amount of BOD in one end of the tank. While to the other end, the microorganisms are more active physiologically until the mixer completely activated in the activated sludge system. A typical type of activated sludge system is shown in Figure 4 37 .

Trickling Filter
It is a development procedure wherein microorganisms in charge of treatment are connected to a dormant pressing material. It is comprised of a round tank loaded up with a bearer material (volcanic shake, rock or engineered material). Wastewater is delivered from above and trickles through filter media permitting organic material in the wastewater to be adsorbed by micro-organisms populace like "anaerobic", "aerobic", "facultative bacteria"; "algae", "fungi" and "protozoa" are fixed to the stream as a slime layer or biological film (approximately thickness from 0.1 to 0.2 mm).
Trickling filters are proficient in which waste eminence like suspended solids and BOD elimination is great. The method is less complex contrasted with activated sludge method but the maintenance and operation necessities are high because of the electrical power usage. Also, skilled labor is essential to preserve the trickling filter processing and operating without any trouble like: "prevent clogging", "ensure adequate flushing", "controls filter flies". This process is appropriate for certain comparatively prosperous, heavily populous regions which have a centralized wastewater treatment and better waste system. Also appropriate for the treatment of grey water. Furthermore it needs more space contrasted with specific other technologies and has impending for filter flies and odour35. Figure 5 shows the main parts of a trickling filter 36 .

Rotating Biological Contactors
The aerobic Rotating Biological Contactor (RBC) is one of the biological treatment methods for the organic wastewater treatment. RBC is a different kind of wastewater treatment technique which associates benefits of biological fixed-film like: "short hydraulic retention time", "high biomass concentration", "low energy cost", "easy operation" and "insensitivity to toxic substance shock loads". Thus the aerobic RBC reactor is broadly used for the industrial and domestic wastewater treatment 19,38 . Figure 6 shows the schematic diagram of an RBC reactor which involves of narrowly spaced discs mounted on a communal horizontal shaft, partly immersed in a semicircular tank getting wastewater.

Membrane Bioreactors
This kind of wastewater treatment technique carries out more than just one treatment phase. Membrane Bio-Reactor (MBR) techniques are distinctive that combine aerobic-and anoxic-biological treatment with an incorporated membrane organism which can be utilized with most suspended-growth, biological wastewater-treatment systems 19 . A typical schematic diagram of an MBR is shows in Figure 7. Current development and research trends of and MBR technology has been upgraded from progression optimization and economic valuation to installation of new procedural architecture to improve functional strains: nitrifiers or giving supported field for enhancement of performance, to absorption of reasonable adsorbents for membrane fouling modification and to employing MBR hybrid techniques for attaining instantaneous eliminations of contaminations and other pollutants 39,40 .

Non-Conventional Methods
Non-Conventional techniques are low-technology, lowcost; less sophisticated in maintenance and operation biological treatment techniques for municipal wastewater. Though these methods are land concentrated in contrast with the conventional biological methods. They are frequently more effective in eliminating pathogens and do as such dependably and ceaselessly if method is appropriately planned and not over-burden 41 .
Few non-conventional techniques like "stabilization ponds", "constructed wetlands" and "oxidation ditch" are explain below in details.

Waste Stabilization Ponds
Waste Stabilization Ponds (WSP) is synthetic, shallow basins that include a single or numerous series of anaerobic, facultative growth ponds used for the treatment of wastewater. The treatment of the wastewater processes as ingredients are detached by sedimentation or converted by biological and chemical procedures 42 . While for facultative ponds, organic matter is broken down further to nitrogen, carbon dioxide and phosphorous by means of oxygen created through algae in the pond.
WSP are very significant waste treatment which have better outcome. The latest investigation concluded that such technique improve quality of waste water with minimum cost, simple and better for pathogen elimination. Additional advantage of this type of technique is that "don't have to pay for routine maintenance" as waste stabilization ponds don't need to be aerated and because of this motivation this technique of waste water treatment is entitled as natural waste water treatment 43 . A typical schematic diagram of waste stabilization pond is shown in Figure 8 44 .

Constructed Wetlands
Constructed Wetlands (CW's) are strategic techniques that are planned and fabricated to use wetland vegetation to help for the treatment of wastewater in a more meticulous atmosphere than achieves in natural wetlands 45 . CWs are an ecological and an appropriate substitute for secondary and tertiary treatment of industrial and municipal wastewater. Also CWs are appropriate for the elimination of suspended solids, organic materials, nutrients, heavy metals, pathogens and toxic pollutants. However; CWs are not idyllic for the raw waste treatment and pre-treatment of industrial wastewater to preserve the biological equilibrium of the wetland ecology.   Constructed Wetlands (CWs) are near-natural techniques and are fully engineered resolutions for which many latest investigations have been dynamically established in the past decades 7,46 . CWs are techniques which meaning primarily on naturally happening combined and commonly reliant on biochemical progressions that functions through microbial and solar developments and ensure so within a organized atmosphere 47,48 . CWs are treatment services replicating the methods happening in natural wetlands containing of substrate, macrophytes, and microbial assemblage. CW has been proven to ensure several benefits over the conventional techniques. CWs are low cost in construction, operation and maintenance, absorbs low energy, reduce cost on waste collection process, can be functioned on several levels, promise appropriate reprocessing and enhance visual significance to the atmosphere 49,50 . A typical kind of constructed wetland waste water treatment is shown in Figure 9 (Free Water Surface System) and Figure 10 (Sub-Surface Flow System)

Oxidation Ditches
Oxidation Ditches (OD) are characteristically comprehensive mix techniques however can be amended. Usual OD treatment techniques comprise of a single or multichannel arrangement within a ring, or oval. Preliminary treatment like bar screens and grit elimination usually leads the oxidation ditch. Principal resolving proceeding to an oxidation ditch is occasionally proficient and tertiary filters may be prerequisite after elucidation, reliant on the sewage necessities. Disinfection is essential and reaeration may be required prior to final absolution. Horizontally or vertically attached aerators offer circulation, oxygen transfer, and aeration in the ditch 51 . Flow to the oxidation ditch is aerated and mixed with return sludge from a secondary clarifier. The mixing procedure entrains oxygen into the mixed liquor to foster microbial progress and the drive velocity guarantees contact of microorganisms with the influent 52 . Figure 11 shows the schematic flow diagram of oxidation ditch treatment by using plant 19 .

Future Challenges
Wastewater treatment techniques are an enormously noteworthy measure of protecting our environment ecological. The treated wastewater is normally driven back to natural water bodies, where it at that point gets extricated and later cleaned for utilities. Earlier services constructed with an incorporated perspective are ending up less and less powerful as treating the regularly expanding volumes of wastewater from developing urban areas over-burdens services, prompting to expanded energy utilization and breakdowns. Moreover, such services are now overwhelmed by the cityscape, devouring land, value and really becoming unmanageable for the long run.
Conventional wastewater treatment plants are justly huge outdoor services that have a lot of adversative potentials. The sight and smell of the cumbersome and industrial water management systems are the two biggest. In addition to the fact that they are undesirable, they require enormous framework spending plans, expecting speculators to construct the genuine capability as well as to plan out and make the entire foundation of getting the wastewater to the wastewater treatment plant. Basically such conventional techniques are an incapable framework and in spite of the fact that it cleans the wastewater, they're neither reasonable nor affordable.
The reuse of water, nutrient and recycling of energy are the new ideal models that the CW community is beginning to report, reflecting the fundamental patterns in the more extensive water area and supplementing the current study on biological facilities. The research on the reuse of water has begun to highlight the combination of advanced CWs and oxidation techniques to confirm acquiescence with the compulsory regulation of discharge-reuse standards.
The incorporation of microbial fuel cell ideas into CW designs is beginning to be noticeable. The usage of bioelectro-chemical techniques and the correlated microbial electrolysis cell, in which electric current is utilized to attain enhanced pollutants deprivation, is the latest and very successful track of research.

Conclusion
Throughout the years, the nature of water is crumbling predominantly because of the anthropogenic exercises, populace development, impromptu urbanization, quick industrialization and unskilled utilization of natural water resources. Besides, the expanded consciousness of the significance of giving effects because of the current environmental policies has driven the research community towards the advancement of robust, economically feasible and environmentally friendly techniques capable of eliminating contaminants from water and at same time to protecting the health of influenced community. There are numerous methods of removing the pollutants from the sewage water and treating the wastewater for reuse. The scarcity of the shortage of water globally, various studies has been carried to solve this problem and in this connection, wastewater treatment is among the one. In this review article, a brief and detailed introduction of the wastewater has been highlighted. The up-to-date literature was argued and highlighted relating to the aim of the review article. On the basis of reviewed literature in this work, following are the some concluded remarks: • The advanced new green technical techniques are being announced to overwhelm the conventional techniques of wastewater treatment.
• Human growth and speedy population development put several pressures on the quality and access to water resources which sensed solidest at the edge between water and human health; wherein contagious, diseases through water endure the prominent reasons of human illness and impermanence worldwide.
• Some techniques covenant with decrease of heavy metals where other techniques compact with lessening of phosphorus and nitrogen.
• It is established that the conventional techniques are not competent in decreasing the heavy metals, toxic, phosphorous, nitrogen etc.
• There is no one of a kind strategy to treat a large portion of the compounds in a solitary advance.
• The principle point of the present paper is to talk about the innovative progressions in treatment of waste and sewage water.
• A consolidated treatment including microbiological procedures and hydroponics was set for treatment of domestic water. The treatment is observed to be viable and the impediment is significant recycling of phosphorus and nitrogen through collected biomass • The biological treatment particularly fungi and bacteria for wastewater treatment, specifically the colored compounds in the molasses founded distilleries wastes.
• As per the dissimilarities in engineering design and pattern of flow of water, the constructed wetland treatment system can be categorized into Surface Flow Wetland (SFW), vertical flow wetland, tidal flow wetland and SFW has been applied most broadly.