In the past decades, natural organic matter (NOM), which is a complex heterogeneous mixture of organic materials that are commonly present in all surface, ground and soil waters, has had an adverse effect on drinking water treatment. The existence of NOM results in many problems in drinking water treatment processes, and the properties and amount of NOM can significantly affect the efficiency of these processes. NOM not only influences the water quality with respect to taste, color and odor problems, but it also reacts with disinfectants, increasing the amount of disinfection by-products. NOM can be removed from drinking water via several treatment processes, but different drinking water treatment processes have diverse influences on NOM removal and the safety of the drinking water. Several treatment options, including coagulation, adsorption, oxidation, membrane and biological treatment, have been widely used in drinking water purification processes. Therefore, it is of great importance to be able to study the influence of different treatment processes on NOM in raw waters. The present review focuses on the methods, including coagulation, adsorption, oxidation, membrane, biological treatment processes and the combination of different treatment processes, which are used for removing NOM from drinking water.
Natural organic matter (NOM) is defined as a complex mixture of organic materials present in natural waters, particularly surface waters. NOM consists of many different compounds, for example, humic substances, carbohydrates, amino acids and carboxylic acid. To comprehend the characterization of NOM, a number of research studies not only focus on the conventional water quality parameters, e.g., total dissolved solids, conductivity, pH, water hardness, chemical oxygen demand (COD), total organic carbon (TOC) and total bacteria count, but also pay attention to the significant character of NOM, e.g., the molecular weight distribution, hydrophilicity and hydrophobicity (Dias et al. 2013).
Because of the loose polymeric structure of NOM, with a large number of functional groups and adsorptive complex ions, NOM has a strong adsorption and complexing capacity for various types of trace inorganic and organic pollutants. As a result, NOM reacts with metal ions in the water to generate soluble complexes, and the aggregation structure of NOM includes trace organic pollutants, thus affecting the toxicity and bioavailability of trace inorganic and organic contaminants (Xiaoju et al. 2012). In addition, although NOM has no side effects, it is the primary source of color, odor and taste problems and, to a large extent, influences the sensory index of water quality in water treatment processes. Meanwhile, during drinking water treatment processes, it is responsible for the increase of coagulant dosage affecting the coagulation effect, the acceleration of filter bed clogging and membrane fouling. Moreover, NOM has been noted to contribute to the production of disinfection by-products (DBPs) and has even caused bacterial growth in the water conveyance system (Roccaro et al. 2011).
Because people increasingly focus on water quality safety problems and more stringent drinking water standards, there is a need for selecting more efficient, yet still economical, drinking water treatment processes for the removal of hydrophilic and hydrophobic NOM, which can reduce the production of DBPs. Currently, the primary and feasible drinking water processes for the removal of NOM include the coagulation method, the adsorption method, the ozone oxidization method and various combined technologies. The purpose of this report is to summarize the processes that have been used for the removal of NOM during drinking water treatment and the effect of these processes on NOM. This report summarizes recently published studies related to the characteristics and mechanisms of different methods, compares and analyses the advantages and disadvantages of the combined process and offers a comprehensive overview of the methods for removing NOM.
DIFFERENT METHODS FOR THE REMOVAL OF NOM
Coagulation is a widely used process in water treatment that destabilizes charged colloidal particles by adding chemicals to the water, thus causing floc growth and eventually removing the contaminants. The organic matter with a high molecular weight and a low solubility are removed easily by conventional water treatment processes, while the removal rate of organic matter with a low molecular weight and a high solubility are very low, mainly because these species have good hydrophilicity and are not easily adsorbed by coagulant hydrolysate (Dlamini et al. 2013). However, improved coagulation conditions, i.e., a low pH level and high dosage of coagulant, can modify the coagulant hydrolysate form and metal hydroxides, and the positive charge density of the hydrolysate can be increased. Improving the coagulation conditions is an effective and feasible way to enhance the removal rate of organic matter and is known as ‘enhanced coagulation’ (Wassink et al. 2011).
Enhanced coagulation refers to adding excessive coagulation to the source water and controlling the pH level in order to improve the removal of NOM in the normal drinking water treatment process, maximize the removal of disinfection by-products of the precursor and ensure that the drinking water DBPs meet the drinking water quality standards. The NOM removal rate in conventional water treatment processes is very low, between 10 and 50%, while the removal rate in the enhanced coagulant process is between 26 and 80% (Kabsch-Korbutowicz 2005).
The mechanism of coagulation to remove organic matter has three primary aspects: the effect of electrical neutralization between positively charged metal ions and negatively charged colloid organic matter which results in steady coagulation; metal ions and organic molecules form insoluble precipitates; and the organic matter adsorbs onto the surface of flocculants. Many factors can affect the coagulation effect, including the type of coagulants used the coagulant dose, the raw water quality and the pH (Fabris et al. 2012).
pH control of coagulation is one of the most important factors governing NOM removal. The mechanism by which pH affects organic matter removal primarily involves competition of the metal ions generated by hydrolysis with hydroxyl and organic anions and competition of organic agents with hydrogen ions and the metal ions generated by hydrolysis (Diemert & Andrews 2013). The metal coagulant species are positively charged at a lower pH. Different coagulants have different flocculation effects at different pH values. For example, the best flocculation pH value of Fe3+ is 3–5, that of Fe2+ is 8–9, and that of Al3+ is 6–7. Cationic high polymer flocculants are prone to precipitation when the pH is less than 7, but anionic polymer flocculants are more prone to precipitation when the pH is more than 7 (Nkambule et al. 2011).
As the reagent quantity increases, the NOM removal rate in source water improves constantly; however, too much coagulant will increase the cost of processing. Meanwhile, the raw water quality is an important factor related to the effect of coagulation. Therefore, one should select an appropriate coagulation method and adjust the pH to the optimal value according to the source water quality in order to reduce the coagulant dosage (Wang et al. 2013). When the organic matter in raw water is mainly composed of macromolecular organic content and organic particles, coagulation can effectively remove the NOM in the water.
The adsorption method utilizes the intensive pore structure on the adsorbent with large specific surface areas to achieve the objective of selective enrichment of organic matter. As a type of low energy efficient processing method, the adsorption for removing the NOM in raw water receives more and more attention. Activated carbon, with relatively high dispersity or porosity and large specific surface areas, has a strong adsorption capacity, and it is widely applied to decolorization, deodorization and organic matter removal during water treatment (Goncalves et al. 2013).
According to X-ray analysis, the structure of activated carbon is an irregular collection of microcrystals with a graphite-layered structure. The porous character gives activated carbon a great internal surface area, and the non-crystalline part strengthens the adsorption effect between activated carbon and organic matter. The surface chemical properties of activated carbon are the main factors affecting the interaction between electrostatic force and non-electrostatic force, but the adsorption of aromatic compounds is mainly reliant on the hydrophobic effect between adsorbate and on the π–π dispersion interaction between aromatic rings (Pavoni et al. 2006).
Many studies found that in the process of adsorbing NOM pH, temperature, surface texture and ionic strength influence the adsorption efficiency (Daifullah et al. 2004). The adsorption of NOM by activated carbon generally decreases as the pH increases; this is mainly because at low pH values, a large number of H+ ions in the aqueous solution are adsorbed onto the surface of the adsorbent, thus increasing the surface acidity of the adsorbent. Meanwhile, the NOM molecules with negative charge, because of a large amount of carboxyl and phenolic hydroxyl on the surface, attach to the H+ ions that are on the surface of the adsorbent, forming ion-dipole molecules and completing effective adsorption (Li et al. 2002).
Not only are activated carbon adsorbents used in water treatment plants to remove NOM, but resin adsorbents, chitosan adsorbents and mineral matter adsorbents are also used for NOM removal. Resin adsorbents include non-ionic resins, ion exchange resins and composite resins. For the non-ionic resin, the organic removal mainly relies on the π–π dispersion interaction and hydrogen bonding interaction between aromatic rings, but this type of resin is not very ideal for the adsorption of hydrophilic small-molecule organic matter (Mergen et al. 2008). Through surface modification, the composite functional resin that can be used for adsorption and ion exchange not only promotes π–π dispersion interactions among organic molecules but also can increase the adsorption ability of the resin for organic matter through the synergistic effect between the surface-modified functional groups and the organic molecules. The chitosan adsorbents mainly depend on electrostatic interactions and surface complexation to adsorb organic compounds (Tokuyama et al. 2010).
The chitosan adsorbents mainly depend on electrostatic interactions and surface complexation to adsorb organic compounds (Rangel-Mendez et al. 2009). However, under the acidic conditions, the chitosan has limitations in terms of solubility and non-polarity, which became an important factor for chitosan adsorbent applications. The major mechanisms involved in the adsorption of NOM to the surface of mineral matter adsorbents have been proposed as follows: ligand exchange, anion/cation exchange, cation exchange, hydrogen bonding, van der Waals interactions and hydrophobic effects. Owing to the large cation exchange capacity and high specific surface area, mineral matter adsorbents were commonly investigated for the potential removal of NOM (Doulia et al. 2009).
During the last few years, chemical oxidants have been widely applied to a range of water treatment processes. Chlorine was widely used until there was concern over the formation of halogenated by-product compounds. Some other oxidants have since been used; e.g., ozone (O3) has been noted to apply in the treatment of surface waters. O3 has been of particular interest because of its ability to eliminate taste, odor, color and certain mineral compounds. It can also be used for NOM degradation and microorganisms (Ghernaout et al. 2011).
NOM is a type of very complex mixture that consists of different types of organic matter with different molecular weights, molecular sizes and functional groups. The various fractions have different physical, chemical and biological behaviors, and the types of DBPs that form from the reaction between the fractions and O3 also differ greatly. To comprehend the formation of DBPs when the different types of NOM reacted with O3, many researchers generally conduct further research after using the chemical separation method to classify the different characteristics of NOM (Matilainen & Sillanpaa 2010). Meanwhile, from the results of many studies, it can be seen that the O3 promotes strong oxidation and can oxidize some NOM components directly (Molnar et al. 2013). In addition, O3 can react with water to generate OH radicals, which are stronger oxidants and react more easily with the NOM of raw water than O3 in drinking water processes. Using O3 oxidation, the refractory NOM fractions can transform into biodegradable organic matter (BOM), leading to an increased amount of biodegradable dissolved organic carbon (BDOC) in water (Molnar et al. 2012). Although the total amount of dissolved organic carbon (DOC) removal is very small, the reduction of hydrophobic fractions that have higher trihalomethane formation potential (THMFP) shows that the O3 oxidation can effectively decrease the THM formation of DBPs.
To reduce the instability effect of O3 oxidation on the drinking water in water supply systems, in recent years, many scholars have conducted many studies on the improvement of drinking water treatment processes. The advanced oxidation processes (AOPs), because of various optional oxidants, rapid reaction and small selective pollutants, have also received wide attention in recent years (Lakretz et al. 2011). As a primary AOP, O3 oxidation can produce a small quantity of OH in water treatment, but combining it with other AOPs can produce more OH. The present methods mainly include catalytic ozonation, O3 + UV, O3 + vacuum UV (VUV); each of these reactions has advantages and disadvantages, and all play an important role in water treatment (Mosteo et al. 2009).
Using metal oxides, catalytic ozonation can improve the oxidation efficiency of O3 oxidation for the refractory organic compounds in water (Chen & Wang 2012). The mechanism of catalytic ozonation is that the O3 in water enhances the ability to produce hydroxyl radicals generating compounds, which can quickly react with O3 molecules; in addition, the pollutants and O3 in the water can also be adsorbed on the surface of the catalyst at the same time. Many studies have found that the catalytic O3 oxidation can promote NOM removal in raw water. Furthermore, catalytic ozonation resulted in fewer by-products and less biodegradable organic carbon formation (Nawrocki & Kasprzyk-Hordern 2010).
Among the various types of AOP technology, the combination of O3 and ultraviolet radiation (O3/UV) can produce the maximum amount of hydroxyl radicals, which is of interest to researchers (Audenaert et al. 2013). O3/UV can effectively remove the synthetic organic compounds in water, and studies of this process which is used for removing NOM in water have been gradually reported in recent years. Chin & Berube (2005) found that the combined O3/UV was more effective than either the O3 or UV treatment alone. O3/UV could mineralize up to 50% of TOC from the source of drinking water at an O3 dose of 0.62 ± 0.019 mg O3/ml and a UV dose of 1.61 Ws/cm2, as well as for the reduction of THMFP and haloacetic acids formation potential by roughly 80% and 70%, respectively.
Membrane technology, as an independent process or as a method to replace or enhance the conventional treatment, is utilized for purification of drinking water. Membrane filtration has the potential to be used for the removal of NOM and contaminants and includes many different membrane processes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). Low-pressure MF and UF membranes are applied for particulate and microbe removal, while NF and RO can also remove almost all small organic molecules and salts (Tijing et al. 2015). Therefore, the membranes are able to remove almost all the particulates larger than their pore size. As a result of the size-exclusion effect of membranes, uncharged macromolecules are rejected from raw water, and the removal efficiency is dependent on the membrane pore size distribution and the pore size, as well as the size, shape and size distribution of the NOM molecules. Not only the size exclusion but also the solute–solute and solute–membrane interactions and the operational conditions can influence the membrane rejection. In the case of the repulsive electrostatic force between membranes and solutes, the rejection will be higher than what is expected according to the nominal cutoff value of the membrane. Consequently, the influential factors of the NOM removal include membrane and NOM properties. When charge repulsion is utilized, the rejection is affected by physicochemical effects, including divalent cation effects, pH and ionic strength (Abeynayaka et al. 2012).
The NOM properties, including the high-molecular-weight, negative charge, hydrophobicity and aromatic structure, increase the extent of NOM removal by the membrane. These characteristics apply to the hydrophobic fraction more than to other fractions, which is proven by the fact that the studies focusing on the humic acids describe higher DOC removal than other studies performed with NOM solutions or surface waters (Zhao et al. 2011). According to the investigations into the removal of different NOM fractions, it has been found that the removal mechanism order of a negatively charged hydrophobic polysulfone membrane is electrostatic interaction > hydrophobicity > size exclusion (Metsamuuronen et al. 2014).
To comprehend the rejection and membrane fouling of different NOM fractions, as well as the formation potential of DBP precursors such as haloacetic acids (HAA) and THM, many studies have fractionated distinct NOM components not only before filtration but also after filtration. Zularisam et al. (2007) studied the behavior of different NOM fractions of raw water in UF equipped with the 68 kDa polysulfone membranes and found that, because of the charge repulsion, the hydrophobic fraction had the highest DOC removal compared to the transphilic and hydrophilic fractions, which were 30% and 14%, respectively. Owing to the characteristics of low-molecular-weight (LMW), aliphatic structure and more hydrophilic interactions compared to the hydrophobic fraction, the hydrophilic NOM is preferentially transferred by membrane.
The degree of DOC removal is influenced by the ionic content and solution pH, because these factors influence the removal mechanisms by changing the characteristics of the membrane and NOM. The NOM removal increases with the pH, but higher removal has been reported at a lower pH. The removal effect of hydrophobic interactions will be enhanced at a low pH, because the protonation of humic and fulvic acids containing carboxylic hydroxyl and phenolic groups can result in larger hydrophobicity and smaller water solubility (Gray et al. 2011). The double layer thickness of electrostatic charge shielding between the membrane surface and molecules will be reduced by high ionic strength, which decreases the charge repulsion. Moreover, on the condition of increased ionic strength, the intra-molecular repulsion involving humic acid will decrease, leading to a more coiled structure, which will decrease the NOM removal efficiency (Lin et al. 2001).
The NOM removal can also be influenced by the hydrodynamic operation conditions, including the permeate flux, aeration, filtration mode, pressure and membrane module configuration through changing the NOM concentration and transportation on the surface of membrane. The higher cross-flow velocity represents the lower concentration polarization and the upper return transport speed of retained solutes, and the return transport speed relies on the size of particle. Brownian diffusion dominates the return transport for particles with a diameter of approximately 0.2 μm, and the return transport is inversely proportional to the particle diameter (Joseph et al. 2012).
The main obstacle for the application of membranes to the treatment of drinking water is membrane fouling. The types of fouling mechanisms include pore blocking, pore constriction and gel formation. Meanwhile, according to fouling material, the fouling can be classified into organic, inorganic, colloidal and biological fouling. However, the NOM is supposed to be the major membrane fouling material in the drinking water treatment process for the raw water, and the fouling tendency is determined by the NOM constituent. The autochthonous NOM (microbial-derived) has higher fouling potential than allochthonous NOM (terrestrially derived, mainly humic substances) (Amy 2008).
Biological treatment technology removes NOM and ammonia nitrogen in water through the effect of microbial metabolism. The commonly used method of biological treatment is biofiltration operated for oxidizing the biodegradable fraction of NOM, thus controlling DBPs, THMFP and microbial regrowth (Lautenschlager et al. 2010).
Biofilters performance is impacted by factors such as water quality (e.g., BOM concentration and characteristics, pH and turbidity), temperature, backwash chemistry and design parameters. Optimization of this treatment step requires continuing refinement of the knowledge about the relative importance and impact of these factors on biofilter performance. Some studies have assessed biofilter performance under different conditions such as varying filter media, biomass concentration in the filter bed, contact time, backwash chemistry and pre-ozonation dose (Moll et al. 1999). The removal of NOM, TOC, assimilable organic carbon (AOC) and BDOC constituents that react with chlorine or other disinfectants to form chlorinated or oxidized products DBPs, ozonation by-products and turbidity have been used to quantify biofilters performance (Gibert et al. 2013). Several experiments have demonstrated that filter designs which support a greater biomass provide better removal of NOM in drinking water biofilters (Seredyńska et al. 2005). Biomass growth and activity vary significantly with temperature, leading to differences in the removal of BOM, which is a key factor for limiting growth of heterotrophic plate count bacteria and coliforms in filter effluents and distribution systems (Simon et al. 2013).
Biofiltration is most often located after flocculation and sedimentation/flotation, either in the filter unit that achieves particle removal, or as a separate subsequent unit. The biofilters feed water is often ozonized to oxidize NOM and increase its biodegradability. Well-designed biofiltration improves the reduction of NOM, precursors of DBPs, bivalent species of iron and manganese as well as regrowth potential. Biofiltration late in the treatment train may however cause hazards due to the release of bacteria either suspended in the water phase or with carbon fines (Yang et al. 2001).
Many drinking water sources experience seasonal temperature variations of 20–30 °C. This variability in source water temperature may impact removal of BOM and particulates by biofilters, as well as impact the microbial community structure present in biofilters. Previous research assessing the impact of temperature on biofiltration has been limited to long-term studies on full-scale and pilot-scale biofilters, where the filters were run at ambient temperature, and seasonal fluctuations were monitored. These studies showed that TOC, DOC, AOC and chlorine demand removal efficiencies decline during the winter months (Seredyńska-Sobecka et al. 2006). Decreased removal of specific ozonation by-products such as formate, methyl glyoxal and glyoxal in biofilters has also been observed during the winter when compared to the summer. Because biofiltration usually is not capable of removing the biorefractory substances, pre-oxidation processes should be applied. Ozone is an oxidant, which is frequently used for this purpose, because ozonation increases the biodegradability of organic matter and, consequently, enhances the effectiveness of its removal during subsequent biofiltration. The main advantage of biofiltration in water treatment is its ability to remove the biodegradable compounds, which comprise the most undesirable fraction of organic matter (Yang et al. 2001). However, excess biomass can cause clogging of biofilters and high head losses. On the other hand, a weak biofilm does not efficiently remove COD and TOC.
THE COMBINATION OF DIFFERENT TREATMENT PROCESSES
Adsorption coupled with coagulation
To a great extent, the combination of adsorption and coagulation can make up for the lower removal rate of low-molecular-weight organic compounds in the NOM at the coagulation stage. It can also reduce the coagulant dosage, which is used for removing TOC to the standards and indirectly decreasing the sludge quantity in the following treatment process. The combination process can also improve the best pH value when utilizing the coagulation process that removes TOC, which reduces the amount of acid, thereby decreasing the pH value (Szlachta & Adamski 2009).
Studies have explored different adsorbents used for combining coagulation to remove NOM during drinking water treatment. Several researches have investigated the combined use of coagulation and adsorption with powered activated carbon (PAC) to maximize the overall removal of NOM, achieving removal percentages of 45–80% with this combined water treatment process (Uyak et al. 2007). Wang et al. (2010) employed the nanoscale carbon black to removal NOM from water in the presence and absence of coagulation. Their study showed that almost 90% of NOM was removed in 15 minutes by carbon black adsorption and alum coagulation, which had a higher removal than that achieved by conventional treatment. This study indicated that carbon black might be an important adsorbent for NOM removal in water treatment in combination with low doses of alum. Kristiana et al. (2011) investigated the influence of a PAC combined enhanced coagulation treatment process on the removal efficiency of NOM which was not removed through enhanced coagulation alone. Because of the addition of PAC to the enhanced coagulation treatment process, the removal of NOM was increased by 70%, leading to a significant 80–95% reduction in the formation of DBPs. The lower concentrations of DBPs and a better maintenance of disinfectant residuals at the distribution system indicated that the water quality in the distribution system had improved.
Oxidation coupled with coagulation
Oxidation has traditionally been applied to improve the effect of coagulation treatment processes. Chlorination had been widely used for drinking water treatment until DBPs were discovered in the chlorination process. Other oxidants, e.g., ozonation, have been used to enhance the coagulation of surface water. The NOM characteristics and the basic properties of the source water have greatly influenced the treatment efficiency of ozonation during coagulation. In addition, the dosing quantity of ozonation is very important for the combined ozonation and coagulation process. At higher O3 dosing quantities, the NOM are oxidized into hydrophilic and LMW compounds, which are more difficult to coagulate in the coagulation process, but at a lower dosing quantity, the ozonation process produces hydrophobic neutral compounds, which can be efficiently removed in the coagulation process (Van Geluwe et al. 2011). Consequently, this quantity should be carefully determined for the ozonation process, because post-ozonation might yield better results than pre-ozonation during the combination process. To achieve better removal efficiency of DOC and specific ultraviolet absorbance (SUVA), Bose & Reckhow (2007) investigated two-stage coagulation with intermediate ozonation, which was proposed for water containing both humic and non-humic NOM. Ozonation of precoagulated water demonstrated the advantageous effects of ozonation on the adsorption of non-humic NOM on aluminum hydroxide flocs.
AOPs have also been used for combining coagulation to increase the removal efficiency of NOM and refractory organics. Murray & Parsons (2004) studied a range of control methods, Fenton and photo-Fenton, supporting conventional coagulation for NOM from organic rich waters. Their research showed that under optimum conditions, both combination processes have achieved more than 90% removal of DOC and UV254, leading to the THMFP of water being reduced from 140 to below 10 μg/L. In the work of Choi et al. (2007), it was found that the photocatalytic coagulation uniting nanoparticles of copper-doped titania could be an effective method for NOM removal and membrane flux maintenance. Uyguner et al. (2007) found that the NOM was peroxided by photocatalytic oxidation consecutively incorporating coagulation indicating that the combination process could reduce the coagulation efficiency by 16% because of changes in the NOM molecular sizes, functional groups and hydrophobicity.
MIEX coupled with coagulation
Magnetic ion exchange resin (MIEX) is a strong anion exchange resin with a macroporous polyacrylic matrix in the chloride form which has a larger surface area than traditional ion exchange resin and removes organic and inorganic contaminants (Grefte et al. 2011). It can adsorb negatively charged NOM through the effect of ion exchange and can be easily regenerated after striping in a salt solution; it even still has high NOM removal capability after several regenerations. In addition, the MIEX technique can combine other drinking water treatment processes, such as coagulation, to improve the NOM removal efficiency.
The technology can be used as a coagulation pretreatment process to enhance the coagulation treatment efficiency and to reduce the dosage of coagulant and sludge formation. Shorrock & Drage (2006) found that when the MIEX was used upstream of coagulation (10.6 mg/L Fe), the amount of NOM and the levels of THMs were reduced by 45% and 40%, respectively. Jarvis et al. (2008) found that MIEX pretreatment improved NOM removal and reduced DBP formation and the turbidity load because of a large and more robust floc formation when compared with conventional coagulation. Fearing et al. (2004) found that the combination of MIEX and low ferric dose is an effective solution for the treatment of variable and high organic strength waters. The results showed the performance in reducing a wider range molecular weight DOC, because the MIEX process could remove more DOC and UV adsorption materials than the coagulation process and had good removal effects on different hydrophobic NOM fractions.
Coagulation coupled with membrane
Among the various drinking water treatment processes, membranes have become more common, because they are highly efficient, economical, and easy to operate. The membrane fouling and flux reduction are the main factors which restrict the membrane technology application. Many studies have indicated that humus as the main composition of NOM had a great impact on membrane fouling. Thus, the coagulation as the pretreatment for the membrane is an effective method for reducing membrane fouling, and it is the most successful technology for improving the flux of a low-pressure membrane (Jung et al. 2006). The coagulation pretreatment includes two forms: precipitation process and no precipitation process. During the precipitation process, the colloid and pollutant, which are adsorbed to the coagulant hydrolysate, are separated from water prior to membrane filtration. Thus, coagulation pretreatment plays a significant role in improving and stabilizing membrane performance and the turbidity and DOC reductions. However, some studies revealed that the hydrophilic organic matter accounted for the majority of both reversible and irreversible membrane fouling, regardless of the type of membrane (Yamamura et al. 2014).
There are two main advantages of coagulation coupled with the membrane process: it improves the NOM removal efficiency and reduces the extent of membrane fouling. In the treatment process, the most traditional way is to utilize coagulation solving to remove the floc from the water and separate the rapid mixing and slow mixing (Yu et al. 2011). The liquid supernatant is subsequently processed by the membrane. Another pretreatment method is to convey the water to the membrane reactor directly without passing the setting. This processing mode is usually utilized for the combined mode coagulation with MF or UF without removal of coagulated solids (in-line coagulation). The outstanding features of in-line coagulation include reducing the water treatment running time and treatment plan and lowering the coagulant dosage. Moreover, selecting the proper coagulant dosage is crucial for the membrane reactor's normal operation. Therefore, following the solution's pH and SUVA, the coagulate type and dosage are found to be the most important influencing factors for NOM removal. The most ideal coagulants for charge neutralization and sweep flocculation are aluminum-based and iron-based coagulants, which can remove the charged, hydrophobic and larger sized substances (Li et al. 2011).
Coagulation is reported to improve the filtration performance of the supernatant and the coagulated water because of the large hydrophobic NOM substances removal. However, the neutral hydrophilic compounds with lower MWs are noted to induce membrane fouling slowly after the coagulation process. It is observed that the high flux with coagulated water prevents fouling because of the permeable cake layer on the membrane surface leading to the adsorption of hydrophilic neutral NOM substances into the layer, which can be removed by backwashing to maintain a constant flow rate. The performance of the membrane will be improved if the membrane pore size is smaller than the floc size, which cannot pass into the membrane pores (Park et al. 2002).
Adsorption coupled with the membrane
The hybrid adsorption-membrane process has been used in drinking water treatment. The adsorbents are used to eliminate organic compounds that aggravate membrane fouling and are difficult to remove by low-pressure membranes. To improve the performance of the adsorption-membrane process, high concentrations of adsorbents and long contact times are required. Many factors influence the operation efficiency of this hybrid process, including NOM properties and concentration, adsorbent types and dosage, membrane properties and operating parameters.
PAC is widely used as an adsorbent for adsorption onto low-pressure membranes. The hybrid process is reported to increase humic acid, tannic acid and hydrophobic NOM rejections (with subsequent benefits in DBP control) but was apparently ineffective for adsorbing the highly hydrophilic NOM and thus did not improve the NOM-driven membrane reversible fouling (Campinas & Rosa 2010). Another adsorbent used for pretreatment of adsorption-membrane is granular activated carbon (GAC), which is reported to effectively remove not only lower MW and hydrophilic NOM fractions but also for hydrophobic NOM fractions. Kim et al. (2009) investigated the effects of GAC addition on MF performance in terms of quality (DOC) and quantity (permeate flux). In the hybrid GAC-MF system, the removal efficiency of UV260 was 30% more than that of MF alone, and the reducing rate of membrane permeability was much less than that of the MF membrane process (approximately 70 days without GAC and 130 days with GAC) resulting from the adsorption of NOM on the GAC, which reduced the organic matter loading on the membrane surface. Not only is activated carbon used for hybrid systems, but some other adsorbents, including heated aluminum and iron oxide particles, freshly precipitated iron and aluminum oxide particles, ferrihydride and carbon black nanoparticles, are also utilized for the hybrid systems (Kim et al. 2008).
Ozone coupled with membrane
To improve membrane performance, the ozonation process is combined with the membrane process for treating raw water. The reactions of O3 can be divided into two types, direct and indirect. Direct oxidation can act with activated aromatics and double bonds, leading to higher odor removal and lower TOC removal, while the indirect reaction, in which the O3 is decomposed to hydroxyl radicals that are more powerful but less selective oxidants than O3, can achieve higher TOC removal efficiency. The ozonation of drinking water can form a number of LMW organic by-products resulting from the oxidative breakdown of complex NOM, containing organic compounds such as organic acids, aldehydes and ketones, which are easily biodegradable and constitute a considerable fraction of AOC, resulting in the formation of biofilms and regeneration of microorganisms in the water distribution system. In the study of Song et al. (2010), O3 was used for pretreatment for membrane filtration to obtain high-quality permeates and to improve membrane performance. They found that O3 oxidation offers a higher percentage of UV absorbance removal than DOC removal, and changes in the organic matter composition improved the membrane flux. Meanwhile, O3 oxidization resulted in degradation of macromolecular organic matter, which is responsible for membrane fouling, to small-molecule organic substances. Karnik et al. (2005) also found that O3/filtration resulted in a 50% DOC reduction, and humic substances were converted to non-humic substances, with 50% reduction in the humic substance and 20% increase of non-humic substance.
Although pre-oxidation has many advantages for drinking water treatment, the main disadvantage of pre-oxidation is small assimilable compound formation. The compounds can block membrane pores and aggrandize biofouling membrane processes and distribution systems. The above problems can be resolved by other combined processes, for example, biological activated carbon (BAC) or GAC. Yang et al. (2010) reported that, in comparison with the conventional treatment process, the O3/BAC process could increase the removal efficiency of DOC by approximately 10%. Meanwhile, the UF–NF membranes were found to produce the highest quality finished water, with 88.7% DOC rejection, 94% UV254 rejection and 84.3% THMFP rejection.
Adsorption coupled with biological processes
The conventional treatment process typically consists of flocculation, filtration and disinfection, and NOM removal efficiency is only approximately 30% (Han et al. 2013). These treatment processes are ineffective in removing some specific NOM fractions, such as DBP precursors and synthetic organic chemicals (Simpson 2008). Because of the deterioration of source water quality and the public demand for safe drinking water in China, the combination of adsorption coupled with biological processes, such as BAC, ozonation followed by biological activated carbon (O3/BAC) are used as the advanced drinking water treatment process to improve the water quality (Çeçen & Aktas 2011).
BAC systems use microorganisms attached to GAC to remove the BDOC and AOC. Microbial activity on GAC extends the adsorption capacity via in situ regeneration of adsorption sites through the biodegradation of previously adsorbed organic matter (Buchanan et al. 2008). The main advantage of BAC filtration is to remove biodegradable compounds comprising the most undesirable fraction of organic matter in water (Yapsakli & Çeçen 2010). Moreover, recalcitrant NOM molecules may also be removed from water by first sorption onto the biofilm and then slow biodegradation due to the longer detention time within the biofilm (Vahala et al. 1998). O3/BAC was found to be advantageous for water treatment, because ozonation of organic matter tended to improve its biodegradability whereas the biodegradable fraction was preferably removed on BAC. As a result, application of ozonation followed by BAC filtration led to biologically stable water (Klymenko et al. 2010). During biofiltration, pollutants present in water are removed in two parallel processes: adsorption on activated carbon and biodegradation ozonation can shift a higher molecular weight compound into smaller ones. In this way, the aromatic or hydrophobic organic compounds can be converted to more hydrophilic, biodegradable organic compounds, such as aldehydes, carboxylic acids, ketones and other organic acids, which can also be further removed in a subsequent BAC process (Yapsakli et al. 2009). PAC can also be added to activated sludge in the treatment of wastewaters. The primary advantages of using PAC are the low capital investment costs. In addition, the dosage of PAC can be regulated in the treatment of surfacewater and groundwater (Çeçen & Aktas 2011).
NOM has been demonstrated to be detectable in various regions worldwide. Surface water often has a high NOM content, and surface water is the main drinking water source in many regions of the world. During recent decades, research on NOM, a forward positional task highly valued by international scholars, has received comparative knowledge achievements, and it is widely believed that the NOM should be removed from drinking water. It is important to choose an appropriate treatment process for removing NOM from water according to the change in the water quality. This study reviews the research in the last decade on some different methods for removing NOM in drinking water treatment. These techniques are useful for choosing the best methods to remove NOM from surface water. The main purpose of removing NOM is to produce quality water that will reduce problems during the water treatment process and the water distribution system, such as DBPs and microorganism regeneration.
Enhanced coagulation can improve the NOM removal effect by changing coagulation conditions, such as increasing dosage and reducing pH, and is the first stage to reduce DBPs. Despite the fact that the enhanced coagulation is an important technology, which does not need high investment, to remove NOM based on the existing treatment, it is difficult to effectively remove dissolved DBP precursors that increase the load of follow-up processes and constitutes a potential threat to the safety of drinking water. The adsorption method is a low-energy consumption and high-efficiency solid phase extraction technology for the NOM and refractory organic matter removal. However, the complexity of NOM, the diversity of aquatic environment factors, as well as the differences between adsorbents, to some extent, limit the application of the adsorption method.
The change in each NOM fraction after ozonation is varied. Although this study is about NOM removal by the ozonation of raw water, further mechanisms for the ozonation of each NOM component is not clear, and this can be studied in more detail in the future. Membrane technology has a great potential to shorten and simplify the long treatment chains of physicochemical and biological unit processes needed for satisfying stringent water quality criteria. Biological treatment technology removes NOM and ammonia nitrogen in water through the effect of microbial metabolism. Moreover, the various methods, based on their advantages and disadvantages, will be very helpful for enhancing the NOM removal effectiveness of raw water in the future.
Meanwhile, the combination of different treatment processes has been studied. The adsorption-coupled coagulation has been an effective method to improve NOM removal rate and better removal of LMW NOM. Pre-oxidation can enhance the NOM removal efficiency of coagulation, and coagulation with pretreatment via the MIEX process has been widely surveyed with prospective consequence with regard to enhancing NOM removal. The biochemical process can effectively reduce the AOC, which is produced by pretreatment with the ozonation process. The combination of adsorption coupled with biological processes, such as BAC, ozonation followed by biological activated carbon (O3/BAC) and powdered activated carbon treatment (PACT), are used as the advanced drinking water treatment process to improve the water quality. In the future, each process should be refined, which means that the operating parameters are optimized in the practical application of a sophisticated method. Moreover, further research should be performed on the reaction mechanism and the dynamics of novel methods before the practical application and promotion of processes.
We are very grateful to the financial support of the National Natural Science Foundation of China (NSFC) under Grant Nos 51308373 and 51308385.