Wastewater may be known as water whose quality has been damaged via agricultural, industrial or domestic use  . Wastewater is considered as the novel oil, focusing attention on the indispensable significance of water as a natural resource, which in several regions is viewed as a gift   . In addition to climate modifications, especially global warming, anticipated transferring water accessibility across the Earth, affirming geographical disproportions and menacing to four billion people in areas that are mostly previously weak, measures are needed to reconsider water use models   .
Singapore confronts several dares: shortage of ground and natural resources, civilian thickly inhabited nature, etc. With global renewable freshwater resources of 0.6 km3 and a population of 5.6 million (i.e., corresponding to less than 110 m3 per capita), Singapore’s water case is similar to that of Libya, Jordan or Sudan and causes it by far the most water-scarce state in South East Asia. With reference to past events, Singapore has counted on Malaysia for its freshwater supply but has commenced seeking water reuse as early as the 1970s . Even though at that period, membrane technology     was not considered feasible economically, the assiduous technology watch that resulted later conducted to a pilot demonstration in the 1990s pursued in the 2000s by the island-wide achievement of NEWater, the brand name attributed to reclaimed water by Singapore’s Public Utilities Board (PUB). These days, four NEWater plants supply in average 30% of Singapore’s water demand, a number that is expected to rise to 55% by 2060, at which point of time NEWater production could be as high as 2 million m3 per day .
Having recourse to water reuse, which is more energy-efficient than desalination, releases area for more worthy land utilization . Water reuse remains the judicious target to make the nation self-sufficient water-wise that has participated in wider public admission in Singapore than in proximate Australia for instance  .
Singapore has embraced a quite focused procedure to water reuse with a treatment train incorporating primary sedimentation/activated sludge/microfiltration (MF)/ultrafiltration (UF)/reverse osmosis (RO)/ultraviolet (UV) disinfection     . Consequently, the heart technology behind treating water in NEWater is pressure-driven RO, which lets particle removal firstly via size-exclusion by means of pores extending among 0.2 and 0.4 nm . At this pore dimension, permeate is solid-free, comprising no rising pollutants, metals, salts, viruses or other microbes. However, the drawbacks of RO comprise elevated energy needs and membrane fouling, which are relieved via pre-treatment, containing consecutive steps of primary clarification, biological treatment, and low-pressure MF (0.1 - 0.2 mm)/UF (0.01 - 0.02 mm). In the improbable incident of RO solidity fracture, UV post-treatment guarantees that the permeate stays free of any microbiological content at all times . With this multiple-barrier technique, NEWater exceeds the WHO drinking water quality guidelines  at a price under SGD 0.2/m3 and a portion is directed towards industries needing ultrapure water (e.g., in the micro-electronics sector) while the remaining enters the reservoirs for remineralization and indirect potable reuse (IPR) .
This paper presents the recent technological improvements and the outlooks for water reuse in Singapore as a model. Fields of attentions comprise membrane exploitation (involving forward, reverse and pressure retarded osmosis, as well as membrane bioreactors), advanced oxidation processes (AOPs), electrochemical methods, and their combination as cost-effective tailored solutions to tackle novel dares as diverse as direct potable reuse.
2. Latest Advance in Water Reuse
2.1. Membrane Expansion
RO composes the central technology at the back of water reuse; however, elevated energy necessity and brine handling make severe hurdles . The prime advantage of RO remains its potential to address salinity elimination. Concerning microorganisms and rising pollutants, it may well be replaced via a more cost-effective multi-barrier procedure involving the integration of biological, adsorption, MF/UF and AOPs  .
In the group of replacements of RO, the last decade has progressively intensified the potentials of the forward osmosis (FO) and pressure retarded osmosis (PRO) . Throw taking advantage of a draw solution to naturally drive the osmotic process, FO advantages from fundamentally lower energy consumption and fouling propensity as contrasted to RO; nevertheless, the product of FO is not NEWater but a diluted draw solution that needs secondary treatment. This indicates that FO and RO may not automatically be reciprocally exclusive and actually, they may be integrated for energy optimization with RO concentrate being employed as the draw solution for FO (Figure 1) . This method is essentially favorable for inland water reuse solutions where brine recycling is not a choice. In Singapore, the intrinsic restrictions of FO like the requirement for better membrane materials  up to this time shorten its utilizations in the next years. PRO, still, may form a coming approach to produce hydropower from RO brine as long as firm membranes are advanced that may resist the
Figure 1. Integrating FO and RO methods for energy optimization. The RO brine would play the role of the draw solution in that NEWater production scheme .
elevated pressure of concentrated brine .
2.2. Advanced Oxidation Processes (AOPs)
Founded on the formation of hydroxyl radicals (●OH), AOPs have been efficiently applied for tertiary treatment of domestic wastewater, single or in integration with additional methods as a piece of a multiple-barrier technology in water reuse programs  . In contrast to membranes, AOPs are devastating techniques that occasion the division of constitutional bonds and cause the transformation of the beginning contaminant into numerous intermediates at a percentage bigger than simple methods . As a result, AOPs are usable before membranes methods to reduce fouling or later to eliminate micro-contaminants that may go through. Drawbacks of AOP methods comprise the generation of oxidation by-products  that, in defined situations, may gather in water and be more poisonous than the original compound. For instance, O3/H2O2 and UV/H2O2 may conduct to either the generation or the decomposition of bromate , polyfluorinated chemicals  or halogenated disinfection by-products     following the circumstances. N-Nitrosodimethylamine (NDMA) has been mostly well investigated and was detected to produce only when AOPs are implemented prior to RO  . Nevertheless, AOPs can form more-reactive NDMA precursors staying in RO permeate and improving NDMA generation throughout final chloramination   . Due to both the AOP injection and the type of matrix play deciding roles, it is indispensable to accurately estimate such influences prior to applying for water reuse . Table 1 presents a list of different AOPs and their benefits and drawbacks .
Table 1. AOP juxtaposition .
*(Solar) photo-Fenton has been largely tried during this decade as tertiary treatment technique for municipal wastewater reuse. Further, it may be used under more moderate circumstances (pH 5 - 6, lower concentrations of Fe and H2O2) . This is due to the elimination of pollutants of emerging concern (frequently found in the order of ng/L) and microbial demobilization attributed to the smaller quantity of hydroxyl radicals formed contrasted to pH 3 method is enough. Further, employing complexing/chelating agents has been successfully studied too .
2.3. Electrochemical Technologies
Electrochemistry is more and more viewed as a central knowledge for expanding a possible society from fuel cells to waste electrochemical oxidation, desalination, and water reuse   . Electrochemical AOPs are magnetizing many of the awareness, thanks to their numerous benefits, comprising employing a more hygienic reagent (electricity), potential to attain outstanding levels of mineralization, versatility, elevated energy effectiveness , amenability of automation and safety    . As shown in Table 1, electro-Fenton is fast rising as the most encouraging between electrochemical techniques  , particularly via the integration with nanomaterials like graphene   or boron-doped diamond (BDD) to boost anodic oxidation as an extra source of ●OH  .
In opposition to different membrane techniques, electrodialysis (ED) depends on charge exclusion and remains a strictly and economically feasible replacement to RO for brackish water desalination, particularly for small facilities  . The less rigorous feed quality for ED as contrasted to RO participates in lower the pre-treatment costs . On the other hand, capacitive deionization has proved the capacity to elevate water recovery from RO brine even with cleaning troubles   .
3.1. Membrane Bioreactor
In Singapore, the following procedure for water reuse will be the expansion of the membrane bioreactor (MBR) . This biological technique integrates the advantages of secondary treatment and MF/UF in one device, with the major merits being a compact structure and consistently good effluent quality. In Singapore, this MBR-RO technique has previously been investigated at the pilot stage . A new expansion remains the FO-MBR accompanied by the benefit of energy efficiency ; however, the obstacle of low flux and augmented salinity over time inside the bioreactor, which may negatively touch sludge viability .
3.2. Direct Potable Reuse
Direct potable reuse (DPR) is considered as the novel limit of water reuse    . The fundamental feature for such a strategy to triumph is treatment boost in a multiple-barrier path, which can conduct to the almost not available likelihood of insufficiency  . Singapore possesses several benefits to put into action DPR. First, the PUB before now runs the full water cycle from potable water to wastewater treatment. Second, the administration of NEWater in Singapore has previously confirmed to be reliable over several years. For instance, over 300 persistent organic pollutants     are previously routinely controlled . At present, it is easy to control a large number of these chemicals at levels as low as parts per trillion .
The main points drawn from this work may be given as:
1) The success story of Singapore NEWater mirrors the total semi-permanent planning of the city-state in terms of sustainability. There is a necessity to persist in expanding water techniques and original reuse programs for Singapore. The secret is combining oriented merged solutions joining advanced degradation and separation techniques and their utilization at different levels to address diverse requests.
2) The challenge is to duplicate the Singapore NEWater success story throughout the world. Efforts should be accomplished to generalize such encouraging experience especially in poor countries where humans are dying because of lack of water.
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