As the demand for water increased significantly, treated wastewater effluents have been considered as the alternative water source worldwide. Although the treatment processes are varied depends on the types of wastewaters, the targeted usages of the treated water, capital and operational costs, etc., it could be generally categorized into four stages, including pretreatment, desalting, polishing, and reclamation as indicated in Fig. 1 [11] [12]. The pretreatment is similar to the traditional wastewater treatment process, which is designed mainly to removed suspended solid (SS) and to reduce the level of contaminants such as organic matters, remaining chemicals, and so on. The second stage is then to separate most of the dissolved ions from water, while the water quality of the treated water is further enhanced in the following polishing stage. Finally, there is an increasing trend to reclaim the used/point-of-use water on site in many industries, in which the process would be similar to the pretreatment stage.
The selection of treatment processes usually depends on the potential users of the reclaimed water, or in other words, the targeted water quality. For example, indirect water reuse such as groundwater recharge might only require adequate pretreatment, while post-disinfection is essential for food industry or the drinking water [13]. On the other hand, ultrapure water for the high-tech industries like semiconductor, display, memory, etc. would require integrated systems involving the whole four stages.
3.1 Pretreatment
The purpose of traditional wastewater treatment is mostly to meet the discharge standards, however, the pretreatment for water reuse, instead, puts more attention to remove those factors that could possibly sabotage the following units. The combination of primary water treatment processes, which aims to reduce turbidity could directly be used in pretreatment stage, for examples, coagulation and sedimentation [14] [15], media filter, activated carbon and so on [16]. Wastewater from chemical mechanical polishing (CMP) usually relies on coagulation and media filter to remove turbidity, while activated carbon is applied to deal with color or/and chloride. Hardness, specifically calcium and manganese ions, could be removed via coagulation, and ion exchange column is also available to reduce hardness.
Ultraviolet (UV) radiation is a choice for pretreatment to reduce fouling on the NF/RO membrane [11], although it was originally applied in polishing stage as a oxidation and/or disinfection process. Oxidation of organic matters is the main purpose of UV radiation in the pretreatment stage, in the case, medium-pressure lamp with a central wavelength of 185 nm (UV185), which produces hydroxyl radicals and H2O2 as oxidants, is usually applied [17]. The application of H2O2/UV oxidation was proved to mitigate the flux reduction due to membrane fouling and improve the membrane cleanability [18]. Harif et al. [19] also investigated that the accumulation of extracellular polymeric substances (EPS) on RO membrane decreased when applying medium-pressure UV as the pretreatment unit, while the diversity of the biofilm reduced by more than 30%.
Membrane technique, such as microfiltration (MF) and/or ultrafiltration (UF), is the most common pretreatment unit for the removal of suspended solids, while membrane bioreactor (MBR) was getting popular in recent years to produce water with lower level of organic matters. For industrial wastewaters, MF/UF could be applied directly to those with low suspended solid or CMP wastewater with only nanoparticles. In the case of low strength wastewater (COD < 30 mg/L; SS < 15 mg/L), UF could remove nearly 100% of SS, 50% of apparent color, and 25% of COD (Sino). MBR, instead, is mostly used to treat high strength wastewater produced from semiconductor industry, food industry, petrochemical industry, etc., while anoxic/oxic (A/O) MBR is suitable to perform nitrogen removal from wastewaters containing organic nitrogen and/or ammonia. As the known case of water reuse from treated domestic sewage, NEWater system in Singapore applies MF/UF as the pretreatment for their water reuse, and it was proved through a pilot-study that the RO membrane could be operated at 30% higher permeate flux by replacing MF/UF followed by activated sludge system with MBR for the pretreatment [20]. In general, MBR could be an effective pretreatment unit by replacing traditional secondary wastewater treatment especially when land use is limited as it has small footprint.
3.2 Desalting
Desalting stage is the core step to produce ultrapure water, the effluent of this stage would demand an electrical resistivity above 10 MΩ cm which is equivalent to ion strength around 50 µg/L [16][12]. Reverse osmosis (RO) is currently the most widely-applied techniques since it replaced the two-bed ion-exchange systems in 1980s. RO process not only could eliminate dissolved ions, or total dissolved solids (TDS) but reduce the levels of organic matters in terms of COD/TOC, and the recovery rate could be maintained above 70% in most of the cases. Consequently, membrane scaling/fouling occurs frequently during the operation if the particles in feed water is not controlled properly. To ensure stable performance of RO process, fouling indices like silt density index (SDI) and modified fouling index (MFI) are often used to assess the quality of the feed water for decades. Although these indices are largely applied in practical, they are not considered perfectly reliable since they are evaluated via a membrane with 0.45 µm and therefore could still fail to predict RO membrane fouling in many cases [21]. Alternatively, Zhan et al. [22] successfully applied UF as proxy unit to measure MFI for maintaining a stable operation of RO process for seven months at pilot-scale. In some semiconductor factory, the pH of the feed water is adjusted to above 9.5 to reduce frequency of fouling while guaranteeing both silica and fluoride removals, however, it should be aware that raised conductivity due to alkalinity addition could affect osmotic pressure of the feed water.
Newly developed electro-based deionization processes, such as electrodeionization (EDI), electrodialysis reversal (EDR), or capacitive deionization (CDI), could also be applied in the desalting stage [12] [23]. EDR usually applies to treat wastewater with high conductivity/TDS, and the membrane used in EDR has higher tolerance to silica, colloidal organics, and bacteria compared to RO membrane. EDR could also be used to further concentrate the reverse osmosis reject (ROR) in multiple stage RO process before entering evaporator, since the energy consumption is lower than RO when facing high TDS feed water.
3.3 Polishing
The inhibition of microbial growth in treated water is one of the primary purposes in the polishing stage as it was proved that bacteria/biofilm could be detected in most of the units during ultrapure water production even the storage tank flushed with N2 [24]. Except for the application of UF, advanced oxidation process is regularly used to for disinfection, including H2O2, ozonation, and/or UV radiation [12], however, post-degasification would be necessary to deal with the produced dissolved gases [25]. The usage of UV radiation in the polishing stage depends on the purpose, as low-pressure lamp that emits a central wavelength of 254 nm (UV254) is available for disinfection. UV185, on the other hand, could oxidize the remaining TOC in the water, especially those with low molecular weight (less than 100 Da) and/or low TOC concentration like at the level of µg/L [26]. Zhao et al. [27] compared different units in ultrapure water production process and found that multi-wavelength UV185/254 performed better than applying UV254 and UV185 along, which reduced TOC from 370 µg/L to less than 5 µg/L. Degasification, such as vacuum degasifier, membrane degasifier, catalytic resin, etc., could be used to remove dissolved gases, for example, O2 and CO2, in the treated water [12].
3.4 Reclamation
Reclamation is an essential stage in the concept of circular economy, which becomes notable in recent years. The removal of silica is an issue in the reclamation stage when the ultrapure water is used in the semiconductor manufacturing, as it could cause severe scaling during RO process. When the fouling is formed mostly by silica, only strong cleaning solution like ammonium bifluoride is available for its removal though the membrane could be damaged [28]. Besides the addition of anti-foulants prevent silica scaling on RO membrane, electrocoagulation was able to remove 80% of silica in the feed water at a HRT of 30 min [29].
Solvents, which are widely used in semiconductor industry and could present in used ultrapure water, is another problematic issue required attention during reclamation stage. These solvents such as isopropyl alcohols (IPA), methanol, urea, etc. mostly have properties of low molecular weight and zero surface charge, which are difficult to be degraded especially at low concentration [17]. Choi and Chung[30] reported that dosing 20 µM of persulfate in UV radiation could achieve up to 90% of urea removal at 1.65 µM, while sulfate radical might be produced and play an important role in degrading urea. UV185 is also capable for the treatment of methanol and IPA, and it was revealed that the TOC removal was positively related to DO concentration and UV intensity [26]. Wastewater from semiconductor industry might also contain H2O2, which could lead to membrane degradation and consequently reduce the lifetime of membrane. Activated carbon is an efficient strategy to react with H2O2 concentration, while chlorine and TOC could also be reduced at the same time [31].