Natural Organic Matter Removal in Surface Water Treatment via Coagulation—Current Issues, Potential Solutions, and New Findings

Abstract

Considerable changes have been observed in surface waters’ quality in recent years. They include an increase in dissolved organic carbon (DOC) concentrations, as well as a shift of natural organic matter (NOM) composition in favor of low molecular weight (LMW), and they are expected to occur on a wider scale in the future. Those predictions are particularly worrying given the importance of surface water as the main potable water source for numerous communities across the globe. Conventional methods of surface water treatment for drinking purposes mostly focus on the process of coagulation. The progressing changes in the quality of surface waters, however, render the conventional treatment via coagulation inefficient. The issue of the presence of natural organic matter in drinking water sources, its anticipated changes, and the related treatment problems are all complex and pressing matters that need addressing. This paper aims to provide a critical review of recent findings regarding NOM removal via coagulation in reference to the current NOM-related issues and their potential solutions. The paper discusses the application of different types of coagulants, and their respective advantages and disadvantages. Coagulation-integrated processes including adsorption, membrane filtration, biological processes, and oxidation are also addressed. Lastly, insights on the future approach to the discussed issues and conclusions are presented.

1. Introduction

A significant number of water supply systems providing potable water for local communities globally are fed from surface water sources.

In Europe, between 1990 and 2017, around 75% of water for potable water supply purposes was abstracted from surface water sources such as rivers and artificial reservoirs [1]. Similarly, in the United States, more than 60% of drinking water produced in 2015 originated from surface water sources [2], and in Canada over 85% of potable water processed by Drinking Water Treatment Plants (DWTPs) in 2019 was abstracted from surface water sources [3]. The quality of surface water is important, among other concerns, due to its widespread use as the main potable water source for numerous communities worldwide.

While specific targets for surface water treatment may vary depending on source water quality and regulatory standards, the parameters that typically require reducing are the same for most surface waters and include turbidity, color, and the content of natural organic matter (NOM).

Current surface water purification methods include adsorption, membrane filtration, ion exchange, advanced oxidation processes (AOPs), and biological processes [4,5]. However, traditional coagulation is still the most prevalent [5,6]. The conventional surface water treatment system consists of coagulation followed by sedimentation, filtration, and oftentimes activated carbon adsorption (Figure 1) [6].

Although coagulation has been applied as the main surface water treatment process in DWTPs across the world for decades, there are still a number of issues that researchers investigate. They include the application of ecologically friendly biocoagulants, the development of novel hybrid coagulants, and the possibility of increasing the efficiency of surface water purification by combining coagulation with different unit water treatment processes [5,7,8,9]. Such research is necessary not only for the purpose of overcoming known limitations and disadvantages of conventional coagulation, but also to ensure its continuous applicability in the context of rising challenges [7].

The most pressing problems associated with water treatment via coagulation appear to be the vulnerability of the process to changing NOM concentration and composition, and the consequences of employing metallic salts as coagulants [7,10,11]. Revisiting the topic of surface water purification via coagulation with consideration of both the current and anticipated challenges is necessary.

The main questions addressed in the present paper are as follows:

(a)

What are the implications of the progressing climate change for typical surface water treatment via coagulation?

(b)

What are the potential solutions for ensuring the continuous efficiency of coagulation-based water treatment in the context of both current and anticipated challenges?

(c)

What are the negative consequences of employing coagulation-based methods, and how can they be mitigated?

This work aims to answer those questions based on the extensive literature analysis conducted by the authors. Therefore, the novelty of the present paper is in providing a thorough but concise review of NOM-related problems originating from its presence in natural waters, the effect of progressing climate change on the quality of surface water, the overall impact of said issues on the most common surface water treatment method, namely coagulation, as well as the determination of the advantages and disadvantages of coagulation-based methods, including their environmental impact across different types of coagulants.

2. The Impact of Climate Change on Surface Water Quality and Its Treatability

The impact of climate change on surface water quality can be significant. Some key findings include an increase in NOM concentration and a shift in its composition [10,11].

In recent years, surface waters across the Northern Hemisphere, particularly North America and Europe, have been showing higher concentrations of dissolved organic carbon (DOC), indicative of an increase in natural organic matter (NOM) content. An increase in DOC concentration in water in these regions is linked to surface water recovery from acidification. Furthermore, this phenomenon is expected to occur on a wider scale in the future due to the amplifying effect of climate change [10].

In addition to an increase in NOM concentration, there is the issue of a shift in NOM composition. The molecular weight distribution of organics present in surface water is anticipated to be affected by an increase in solar radiation. An increase in the proportion of low molecular weight of NOM is expected [11].

Such changes in NOM concentration and character in surface waters are predicted to significantly impact the effectiveness of various water purification processes in the long term [10,11]. According to recent studies, a shift in NOM composition reduced the efficiency of its removal via conventional coagulation-based treatment [11]. Increased NOM concentration caused by seasonal changes is also a known challenge. For instance, numerous DWTPs in the UK and USA have reported difficulties in achieving NOM removal efficiencies during fall and winter due to increased concentration of NOM during that time [12].

Moreover, climate change and the related increasingly frequent extreme weather events are a challenge to surface water DWTPs. They can cause short-term but considerable changes in surface water turbidity, color, and NOM concentration. Such abrupt changes in source water quality often result in failure to effectively carry out water treatment processes. Qiu et al. [13] report such a case at one of the major DWTPs in Alabama, USA, where due to an intense blizzard, the turbidity of surface water surged to more than 15,000 NTU, exceeding the standard value more than 50,000 times. It resulted in the shutting down of the DWTP for 3 days due to not being able to efficiently treat water with a level of NOM concentration that high [13].

It is evident that with increasingly frequent extreme weather conditions caused by climate change, and with surface water exhibiting elevated NOM concentrations and changes in composition, traditional surface water treatment plants face mounting challenges. They may prove unable to accommodate higher NOM loads while maintaining the effectiveness of treatment processes.

3. Understanding NOM and Its Role in Water Treatment

3.1. Characteristics of NOM

Understanding NOM background and composition plays an important part in potable water treatment. Characterization of NOM compounds is important because their diverse properties condition their treatability by different unit water purification processes. A better understanding of NOM, its concentration, composition, and character is therefore crucial for identifying the process most suitable for NOM removal [4,5,12].

The term “natural organic matter” refers to a complicated mixture of organic compounds naturally occurring in both surface and ground waters. Its presence in natural waters is a direct result of hydrological, geological, and biological cycles [4,5,14]. Organics that can be classified as NOM are presented in Figure 2.

Assessing the characteristics of NOM in detail is a difficult task, mostly due to its broad range of molecular weight distribution, complex structure, and chemistry of its components [12].

Its compounds differ in polarity, acidity, charge density, as well as molecular mass, and can be broadly classified based on their polarity into two main groups: hydrophilic and hydrophobic components [4,5,6,12,14]. The hydrophilic fractions of NOM primarily comprise nitrogenous compounds and aliphatic carbon, while hydrophobic NOM mostly consists of humic substances, and typically accounts for over half of the total organic carbon in raw water [4,5].

Natural organic matter can also be classified based on its origin. Autochthonous NOM refers to natural organic matter originating from within the water source through biological activity, while allochthonous NOM is generated in a distant place and is introduced to the water source through drainage within watersheds [4,5,15].

NOM is a structure of high complexity and heterogeneity which is why numerous studies focus on the classification of NOM and methods of its characterization in reference to potable water purification [15,16]. Procedures currently used for NOM characterization include resin adsorption, size exclusion chromatography (SEC), nuclear magnetic resonance (NMR), spectroscopy, and fluorescence spectroscopy [12].

A summary of the major characteristic classes of NOM can be found in the work by Adusei-Gyamfi et al. [15].

Figure 2. Classification of NOM compounds (adapted from Ibrahim N. and Aziz H.A [17]).

Figure 2. Classification of NOM compounds (adapted from Ibrahim N. and Aziz H.A [17]).

3.2. NOM-Related Problems in Water Treatment

NOM is not inherently toxic, although its presence in drinking water sources is undesirable due to its negative effect on the organoleptic properties of water such as color, taste, and odor. Moreover, it affects the quality of water by acting as a carrier of toxic pollutants such as pesticides and radionuclides. It may also influence the concentrations of dissolved oxygen, nitrogen, phosphorus, and sulfur [4,5,15]. Furthermore, the biodegradable portion of NOM can have a negative impact on the biological stability of water, potentially inducing bacterial regrowth in the water distribution systems [18].

Even more importantly, NOM present in the surface water is a known precursor to the formation of highly detrimental disinfection by-products (DBPs), among which trihalomethanes (THMs) are most prevalent. Organics present in raw water react with disinfectants, particularly chlorine, giving rise to the formation of DBPs [4,5,19]. The reactivity of organic pollutants with disinfectants largely depends on the size, molecular weight, and hydrophobicity of NOM components, although hydrophilic compounds typically produce fewer DBPs than hydrophobic fractions. All molecular weight fractions of NOM can cause DBP formation, although it is mostly hydrophilic. LMW constituents of NOM account for a major proportion of DBPs, including THMs, in conventionally treated waters [20,21].

The presence of THMs is highly undesirable. Several studies have observed a correlation between the consumption of chlorinated drinking water and an increased risk of cancer. Therefore, THMs are currently the most widely investigated DPBs regarding their adverse effects on human health [19,22,23].

Considering the carcinogenic potential of THMs, increasing concentrations of DPBs precursors in surface water are a matter of concern, especially since, as Kumari et al. [19] point out, reducing THMs formation during the treatment of source water abundant in NOM is not an easy task.

Cool et al. [24] studied the impact of possible future variations in temperature and precipitation associated with climate change on the probability of total trihalomethanes (TTHM) concentrations exceeding a threshold in drinking water in Quebec, Canada. Results of the study showed a low, although considerable, increase in the probability of TTHM concentrations exceeding the threshold over time in the province of Quebec (up to 4.7%).

Given that the progressing climate change significantly influences the quantity and quality of NOM present in potable water sources, and that it is more likely to have an increasingly adverse effect on human health if not properly addressed, the matter of efficient NOM removal is particularly pressing.

4. Modern-Day Approach to NOM Removal in Drinking Water Treatment

4.1. Coagulation-Based Methods

Coagulation is a process in which small particles are combined into larger aggregates. It is carried out by adding a coagulant intended to destabilize the colloidal pollutants and cause their suspension in water [25]. Upon addition to water, the coagulants dissociate and hydrolyze in the water solution, resulting in the formation of positively charged complexes. Those complexes are highly interactive with negatively charged colloids. They reduce the charge making the molecules less soluble and more hydrophobic, therefore making them more prone to aggregating, which allows for their removal through sedimentation or flotation during later stages of water treatment [4,5,26,27]. NOM removal mechanisms during coagulation include charge neutralization, entrapment, complexation of NOM with coagulant metal ions, and adsorption onto the surface of precipitated metal hydroxides (Figure 3) [28].

Coagulation is currently one of the most commonly employed processes in the drinking water treatment industry. Conventional surface water treatment usually involves coagulation, sedimentation, filtration, and disinfection [14,29]. The efficiency of the coagulation process largely depends on the properties of NOM such as particle size, charge, and hydrophobicity, although the type of coagulant used, its dosage, and mixing conditions are also of great importance [4,5]. The performance of the process in reducing turbidity and removing NOM is greatly influenced by the pH value during coagulation. Moreover, different kinds of coagulants often have different optimal operating conditions, and their effectiveness in removing natural organic matter can differ depending on the water pH during the process. For instance, the optimum pH for coagulation using traditional coagulants is reported to be around 6 for alum, and between 4.5 and 7.0 for ferric salts, while titanium-based coagulants require acidic conditions (pH = 3.0) for comparable performance [4,5,30]. The optimal pH for carrying out the process should be, therefore, carefully assessed on a case-to-case basis. The effect of pH on the coagulation process is an issue requiring a broad scope of research, although in many studies the coagulation process is conducted at natural water pH.

Coagulation is typically reported as effective in removing fractions of NOM with a molecular weight of 1000–4000 g/mol, but less so in removing particles with a smaller molecular weight [4,5]. Conventional coagulation can be ineffective when treating source water containing high proportions of low molecular weight NOM components. That is because LMW constituents of NOM usually require higher doses of coagulants than high molecular weight (HMW) components due to LMW particles usually having a higher proportion of negative charge per unit size, resulting in higher solubility and lower susceptibility to aggregating [4,5,27]. On the other hand, natural waters containing mostly high molecular weight NOM can be usually effectively treated using traditional coagulants, mainly aluminum sulfate [4,5,27]. In terms of hydrophobicity, conventional water treatment is reported to provide 60–70% removal of hydrophobic fractions of NOM, and 30–40% removal of hydrophilic fractions [31].

With surface water becoming increasingly abundant in NOM of fluctuating composition, conventional coagulation is failing to meet the required removal rates for NOM compounds, which leads to an increased risk of forming DPBs [4,5,6,14,26,29]. While one of the causes of poor NOM removal via coagulation is incorrect dosing of coagulants, and the implementation of optimal dosing control systems equipped to respond to abrupt changes in NOM concentration is suggested as a possible solution, more elaborate solutions are undoubtedly required [6,14,32], hence, the need for improving the NOM removal efficiency of coagulation.

In recent years, the optimization of operating conditions such as coagulant dose and pH value, as well as the development of new cost-effective and environmentally friendly coagulants have been the topic of numerous research projects. The combination of coagulation and other unit water treatment processes such as membrane filtration, ion exchange, oxidation, and adsorption, has also gained the interest of researchers around the globe [21].

The next part of this paper aims to provide an up-to-date critical review of the state of knowledge regarding various types of coagulants, including both known and newly developed alternative coagulants, as well as the latest research on the performance of coagulation-based hybrid processes.

4.1.1. Metallic Coagulants

Metal-based coagulants, mainly aluminum and ferric salts, are the most commonly applied in water treatment.

Their long-term use particularly results from their low cost, high availability, proven efficiency in removing both turbidity and color, as well as relatively easy handling and storage [4,5,25]. On the other hand, some of the most common drawbacks associated with using metallic coagulants include water pH and alkalinity reduction, production of voluminous post-coagulation sludge, and increased residual metal concentration in the treated water. The latter is particularly undesirable, particularly in the case of aluminum salts [4,5,25].

The widespread use of alum has become particularly disturbing given reports suggesting the association between residual aluminum concentrations in drinking water and Alzheimer’s disease. Most recently, a study on the matter by Van Dyke et al. [33] addressed the limitations of previous studies. Ultimately, no definite association between residual aluminum concentrations and Alzheimer’s disease was found, although it was concluded that the topic requires further studies, preferably including data on individual water sources and aluminum species present in drinking water [33]. Aluminum speciation in potable water is important because it regulates its solubility, bioavailability, and toxicity. Water treatment with aluminum salts can increase the concentration of the aluminum species, namely low-molecular-weight Al that is more easily absorbed by the human body [34]. Krupińska I. [34] conducted a study on residual aluminum concentration in water treated with different aluminum-based coagulants, including aluminum sulfate, sodium aluminate, and pre-hydrolyzed polyaluminum chlorides. The study results showed the highest concentration of colloidal aluminum in water post coagulation with alum. While the potential of ingesting aluminum-containing water to increase the risk of the development of neurodegenerative diseases is not yet fully investigated, several worrisome findings are undeniable, such as that upon ingestion some aluminum salts can reach the brain, and even brief exposure to high aluminum levels can lead to neurological damage [35]. Moreover, although aging is the major risk factor for Alzheimer’s disease, the development of the disease within the human lifespan is believed to be caused by exposure to aluminum [36]. Some researchers therefore suggest employing coagulants alternative to traditional alum, namely ferric-based coagulants or biocoagulants, to avert this concern altogether [6,11,14].

Although particularly worrisome, the detrimental human health effect of residual aluminum concentration in waters treated with aluminum-based coagulants is not the only shortcoming associated with their use. A serious drawback of using alum as a coagulant is the production of high-volume non-biodegradable sludge which causes significant disposal issues [25]. The environmental pollution caused by improper disposal of post-coagulation sludge is a concern due to reported cases of the related increased risk of aluminum toxicity [37]. Moreover, alum sludge is a hazardous waste, making its disposal costly and requiring treatment [25,38]. According to Karnaningroem et al. [39], possible ways of reducing the environmental impact of a DWTP with a conventional water treatment system include treating and then reusing post-coagulation sludge, as well as replacing aluminum sulfate with a different coagulant.

Over the last several decades, reports on drawbacks and adverse effects of prolonged use of aluminum sulfate have contributed to the intensification of research on the implementation of coagulants alternative to alum. In the case of metallic coagulants, ferric salts have been gaining attention as an alternative to their aluminum-based counterparts [4,5].

The most popular iron-based coagulants are ferric chloride FeCl₃ and ferric sulfate Fe₂(SO₄)₃.

The application of ferric salts in water treatment via coagulation is well known, and therefore the key factors of the process are well established. The key operating parameters for conducting coagulation using iron-based salts are coagulant dosage and pH of water subjected to treatment. The typical doses range from 5 to 150 mg/L for ferric chloride, and from 20 to 250 mg/L for ferric sulfate, while the favorable pH range is reported to be 4.5–7 [4,5].

In terms of NOM removal efficiency, ferric-based coagulants have been reported to perform better in removing NOM constituents with high and intermediate molecular weight (above 1000 g/mol) than aluminum-based coagulants [4,5,11,40]. That relationship was further confirmed by results obtained recently by Setareh et al. [40]. In their work, coagulation carried out with the optimum dose of FeCl₃ (0.94 mg/L) resulted in removal efficiencies of 35% and 10% for UV254 and turbidity, respectively, in dam water treatment. The process using aluminum sulfate (1.01 mg/L) led to 11% removal efficiency for UV254 and 21% for turbidity. However, the performance of pre-hydrolyzed inorganic polymeric coagulant polyaluminum chloride (PACl) exceeded those of FeCl₃ and Al₂(SO₄)₃ [40].

Sillanpää et al. [4,5] thoroughly reviewed and compared the efficiency of different types of coagulants, namely aluminum-based and ferric-based coagulants, polymeric coagulants, and biocoagulants. In their indication of the comparative efficiency of the five types of coagulants, the iron-based coagulant is presented as the one providing the highest efficiency of removal of NOM as measured by DOC. However, the paper also emphasizes that the identification of the best coagulant for NOM removal is not straightforward, and it is inherently impossible without considering the composition of NOM and the properties of treated water.

In more recent studies involving iron-based coagulants, researchers focused on investigating the performance of ferric coagulants in comparison with other types of coagulants [40,41,42,43], and on the development of hybrid coagulants using ferric salts [44].

The application of iron coagulants is burdened with negative side effects associated with the use of metal-based coagulants such as an increase in the concentration of metals in post-treatment water. That aspect coupled with the current environmental issues and emerging circular economy concepts makes more environmentally friendly solutions appear more future-proof. For instance, the implementation of biocoagulants, even as an auxiliary substance instead of completely replacing the metal-based coagulant, could potentially mitigate the negative side effects of using metal-based coagulants while increasing the overall eco-friendliness of water purification. It would be beneficial to further research in that area with consideration of not only the efficiency of the process but also its overall impact on the environment and its long-term sustainability.

4.1.2. Titanium and Zirconium-Based Coagulants

The application of Ti-based and Zr-based coagulants, particularly titanium chloride and sulfate, as well as zirconium chloride and sulfate, for the removal of NOM from source water, has also been gaining interest from researchers.

Ti-based and Zr-based coagulants are reported to show several advantages over their conventional counterparts. For instance, they are considered a more environmentally friendly option. They are reported to be less toxic and produce lower amounts of sludge in comparison to aluminum salts [14,27]. Moreover, the post-coagulation sludge can be recycled to produce titanium dioxide, therefore offering the possibility of obtaining additional income from the use of coagulation sludge [41].

Studies conducted in recent years highlight their good NOM removal efficiency, often exceeding the performance of other types of coagulants in comparative studies. For instance, Hussain et al. [30] investigated the performance of titanium chloride in surface water coagulation in comparison to the performance of alum. The study results revealed that TiCl₃ achieved higher DOC removal than Al₂(SO₄)₃ at their respective optimum pH levels (pH = 6.0 for alum and pH = 3.0 for titanium chloride). Furthermore, better results in terms of floc size were achieved when TiCl₃ was used as a coagulant. The authors suggest that titanium chloride could be particularly effective in the treatment of low alkalinity and high DOC-concentration waters [30]. Gan et al. [41] also conducted a complex study on water treatment via titanium-based coagulants with consideration of the mechanism of the process, its potential for practical use, and cost-effectiveness analysis. The publication offers a critical review of numerous studies on the topic and concludes that titanium coagulants have been exhibiting remarkable performance in removing turbidity and both LMW and HMW organic matter from surface water, significantly reducing the formation of the DBPs and exceeding the performance of aluminum and ferric salts, therefore showing the potential to replace the conventional coagulants. It is worth emphasizing, however, that Ti-based coagulants provide better organic removal efficiency only at acidic conditions which limits their applicability.

The main factor currently limiting their use on a broader scale is their elevated prices [14,27]. The price of titanium chloride is reportedly up to 60 times higher than the price of aluminum or ferric chlorides, and the prices of TiSO₄ and Ti(SO₄)₂ are much larger than those of aluminum sulfate and iron sulfate [41]. Future research should focus on the possibility of reducing the costs of preparation of Ti-based coagulants and sludge recycling to income-adding products to improve the economic aspect of coagulation with titanium-based materials [41]. The widespread use of titanium or zirconium-based coagulants in DWTPs currently appears rather unlikely.

4.1.3. Inorganic Polymeric Coagulants

Pre-hydrolyzed polymeric coagulants include polyaluminum chloride (PACl), polyferric chloride (PFC), as well as polyferric sulfate (PFS) and polyaluminum sulfate (PAS) [4,5,6,14,25].

The pre-hydrolyzed inorganic coagulants are established to possess numerous advantages over traditionally used aluminum sulfate [4,5,6,14]. For instance, the concentration of residual aluminum in post-coagulation water is reported to be significantly lower when pre-hydrolyzed coagulants are applied [34]. However, according to Musteret et al. [45], factors such as the dose of pre-hydrolyzed coagulant (polyaluminum chloride), mixing conditions, and temperature noticeably influence the residual aluminum concentration. The coagulant dosage has been observed to particularly impact the residual aluminum concentration in cold temperatures (4 °C) [45]. Pre-hydrolyzed coagulants also tend to form denser flocks at better rates and produce lower amounts of sludge than non-pre-hydrolyzed coagulants [38].

In terms of NOM removal, they are reported to perform better in comparison with conventional coagulants. For instance, Go et al. [42] conducted a study benchmarking the performance of FeCl₃ and PACl regarding their efficiency in removing NOM. The study results identified PACl as the better of the compared coagulants regarding their NOM removal efficiency in source water [42].

Lapointe et al. [43] conducted a study comparing the performance of six coagulants with different proportions of Al species for NOM removal. The tested coagulants included alum, ferric sulfate, three polyaluminum chlorides with different basicity, and aluminum chlorohydrate. The study provided a thorough assessment of coagulant performance based on turbidity, DOC, residual aluminum, THM precursors, and alkalinity consumption, and revealed that a pre-hydrolyzed coagulant PAX 14 was the best for coagulation at pH = 6.0 [43].

Zhang et al. [46] also reported promising findings. Their research suggests that the integration of high-basicity PACl and high-viscosity chitosan has the potential for effectively treating low-turbidity waters at low temperatures. Removal efficiencies of turbidity, UV254, and DOC were 87%, 82%, and 63%, respectively. Moreover, it was found that higher-basicity PACl with a larger proportion of colloidal aluminum species and a smaller proportion of monomeric aluminum species was beneficial not only for removing turbidity and natural organic matter (NOM), but also for controlling the residual Al content [46].

A comprehensive study regarding the efficiency of coagulation using polyhydroxy aluminum chloride as a coagulant in low turbidity river water with a mostly hydrophilic fraction of NOM was conducted by Musteret et al. [45]. Their work covered varying coagulant doses, mixing conditions, and different temperatures, as well as the use of polyacrylamide as a flocculant. In terms of NOM removal, the addition of flocculant was observed to significantly improve the DOC removal efficiencies, twice exceeding the efficiencies obtained in the coagulation experiments without the addition of polyacrylamide, particularly in colder conditions (4 °C). The optimum doses of coagulant were different depending on the temperature. A higher dose was necessary for lower-temperature water. Better DOC removal efficiency (approximately 47%) was recorded in warmer conditions (20 °C) despite the application of a lower coagulant dose (4 mg Al/L), than when a higher coagulant dose (7 mg Al/L) was applied to lower-temperature (4 °C) water. Additionally, higher residual concentration was observed in water treated with a lower optimum coagulant dose in warm conditions [45].

Currently, pre-hydrolyzed inorganic coagulants seem to be the most promising alternatives for conventional coagulants. They are proven to provide NOM removal efficiencies often exceeding those of alum while mitigating some of the issues associated with the use of conventional coagulants. In terms of their wider applicability in the water treatment industry, they are more promising than biocoagulants or titanium-based coagulants, as they are already being employed as coagulants in DWTPs around the world.

A potential area for future research involving the use of pre-hydrolyzed inorganic coagulants could be the application of inorganic polymeric coagulants in combination with organic biocoagulants. The prospect of using pre-hydrolyzed coagulants in combination with biocoagulants should be further investigated in terms of the possibility of implementing such solutions on a larger scale. This can be justified by the need to improve the solutions used in water treatment via coagulation in terms of both their performance and environmental impact. That area of research could be further expanded by coupling coagulation by a combination of inorganic polymeric coagulants and biocoagulants with other unit water treatment processes, e.g., membrane filtration or adsorption.

4.1.4. Biocoagulants

Biocoagulants are biomass-derived materials, typically obtained from natural sources such as animals, microorganisms, or plants [25,47].

The concept of employing naturally occurring and environmentally friendly materials as coagulants for treating water abundant in organic pollutants has drawn the attention of researchers in recent years, partially due to the potential of meeting the assumptions of sustainable concepts such as the circular economy and bioeconomy. Natural coagulants possess several advantages over typically used metallic coagulants, including their renewability, biodegradability, nontoxicity, and relative cost-effectiveness [4,5,25,47].

Several greener alternatives to metal-based coagulants have recently been commercially available, with chitosan being the most prevalent [4,5,12].

Chitosan, produced by the deacetylation of chitin, has been proven not to have a toxic effect on bioindicators, namely Daphnia Magna [48]. Therefore, sludge produced following the coagulation process using chitosan could potentially be safely disposed of with no negative impact on the environment, as it is not anticipated to have any adverse effect on living organisms [12]. In terms of cost-effectiveness, the cost of unmodified chitosan is higher than the cost of conventional metallic coagulants or inorganic polymeric coagulants, and the cost of chitosan modified for increased NOM removal properties would be even higher, which limits its use on a broader scale. It is, however, worth emphasizing that the optimum doses have been reported to be lower in comparison to more commonly used coagulants, translating into a smaller amount of post-coagulation sludge and lower costs of sludge disposal [12]. It is also reported to provide NOM removal efficiencies comparable to those of metal-based coagulants. For instance, Khairul Zaman et al. [49] carried out a study on the comparison of performances between different types of industrially used aluminum-based coagulants (aluminum sulfate, polyaluminum chloride, and aluminum chlorohydrate) and biocoagulant chitosan for the treatment of surface waters with different turbidities. It was concluded that chitosan is capable of performing comparably to aluminum chlorohydrate, although under different conditions, namely in an acidic environment [49].

Several studies have also focused on investigating the potential of employing plant-based coagulants in water treatment. The available sources of plant-based coagulants are much more widespread than animal-based coagulants [25,47,50].

For instance, Moringa oleifera, a tree indigenous to tropical and sub-tropical climates, is among the most studied plants in terms of the development and application of natural coagulants derived from seeds [25,47,51]. Although coagulation with Moringa oleifera seed extracts is reported to be effective in removing turbidity, especially for high-turbidity waters, its application for NOM removal is questionable [25,51,52].

Teixeira et al. [53] investigated the possibility of removing NOM in an integrated process using Moringa oleifera as a coagulant, and vegetable coconut palm as natural activated carbon. The study results showed that for waters with NOM concentrations between 5 and 16 mg/L, the proposed process sequence provided up to 80% efficiency for turbidity and DOC removal, and up to 90% for UV254 reduction [53]. Contrary results were obtained, however, in the investigation of the performance of Moringa oleifera alone [54]. Camacho al. [54] conducted a study on the use of Moringa oleifera seeds as a coagulant to remove cyanobacteria, turbidity, and NOM from low and high-turbidity surface waters. The results of the study demonstrated that not only did the Moringa oleifera seeds not provide DOC reduction, but they also contributed to an increase in organic matter concentration in treated water, suggesting that Moringa oleifera is not a suitable material for NOM removal [54]. The most recent viable application for Moringa oleifera in water treatment would be as a flocculation aid—it was reported to reduce the use of metallic coagulants, mostly aluminum sulfate, by up to 60% [52].

The problem of increased DOC concentration was also encountered in the study investigating the use of other plant-based coagulants conducted by Okoro et al. [50]. In their work, three different natural coagulants were applied to high, medium, and low turbidity waters to determine their performance in removing NOM. The plant-based products used in the study as coagulants included: hexane, saline, and crude extracts of Kenaf plant seed, a species of the Hibiscus plant growing in tropical and subtropical regions. Although saline and hexane extracts were found to significantly reduce the hydrophobic fraction of NOM present in treated water and provide over 90% turbidity removal for both high and medium-turbidity waters, it was concluded that the use of the studied coagulants is limited to pre-treatment when there is no more effective coagulant available. It was also highlighted that using doses other than optimal can significantly impact the performance of the Kenaf coagulation products (KCP). A linear relationship between the DOC concentration in treated water and an increase in the KCP dosage was observed [50].

In such cases, however, an increase in DOC is linked to the release of soluble organic alongside the coagulation-active components, namely proteins, from the plant seeds. The issue can be averted by improving the purification methods to obtain pure coagulants [51,52].

Several issues still need addressing for the actual application of plant-based materials in the coagulation process. Firstly, methods of effective extraction and thorough purification should be investigated to remedy the problem of an increase in DOC in post-treatment waters, and ideally allow for NOM removal [51,52]. Secondly, operation conditions for improved treatment of low-turbidity waters should be evaluated [52].

Despite the still existing gap in practical application and the need for additional research, natural coagulants remain a pragmatic option for communities that currently lack access to safe drinking water, especially since many of them are located in regions abundant in plants with coagulation-active components [25,51]. They are therefore plausible means for achieving Sustainable Development Goal 6 established by the United Nations in 2015, aiming at providing universal and affordable access to safe drinking water for all by 2030.

4.1.5. Hybrid Coagulants

Promising studies on the development of new environmentally friendly, cost-effective, and efficient materials to remediate the drawbacks typically associated with traditional metal-based coagulants have been conducted in recent years.

For instance, Yue et al. [44] provided a study on the development of a novel coagulant based on ferric salts and polymeric acid (Fe-PAA). The performance of the Fe-PAA coagulant for humic acid removal was reported as very promising. It exhibited the synergistic benefits of iron coagulants and polymeric flocculants while being very cost-effective (the estimated price of approximately 22 dollars/m³), which bodes well for its practical applications in the future. The Fe-PAA coagulant provided the removal of 80% of total organic carbon at pH = 5.0, exceeding the performance of ferric chloride at the same dose, although the latter performed better at higher pH (above 7.0) [44]. Although the novel Fe-PAA coagulant requires further improvements, it certainly is a promising invention.

An encouraging study was also conducted by Chen et al. [8]. Their work investigated the performance of a novel covalently bound, inorganic–organic hybrid coagulant (CBHyC) in removing natural organic matter from surface water. The new hybrid coagulant exhibited high DOC and UV254 removal efficiency, exceeding the performance of conventional coagulants, and achieving up to 54% more DOC removal and up to 39% UV254 removal than aluminum sulfate and PACl. Moreover, the new CBHyC coagulant did not only achieve the targeted DOC and UV254 values with half the equivalent aluminum dosage in comparison to conventional coagulants, but also demonstrated lower pH reduction and residual aluminum concentration. The authors also report a broader favorable pH range for the CBHyC coagulant that enables the successful removal of nitrogenous organics and low molecular weight fractions of NOM. The novel CBHyC coagulant therefore exhibits great potential in overcoming current limitations in coagulation technology while performing better than conventionally used coagulants. In terms of costs, the synthesis of the CBHyC coagulant used in the study was around twice as expensive as aluminum sulfate and PACl, although the CBHyC dosage required to achieve similar NOM removal effects is expected to be up to 50% lower, making it a relatively cost-effective material [8].

The performance of another inorganic–organic hybrid coagulant, namely PACl-pDADMAC, was investigated by Bu et al. [55]. PACl-pDADMAC is a combination of polyaluminum chloride (PACl) and polydimethyldiallylammonium chloride (pDADMAC). In the study, the novel coagulant was used for the treatment of tap water sampled in Jinan, China, and later synthesized to simulate surface water. The optimum dosage (7 mg/L) of the studied coagulant provided UV254 and DOC removal efficiencies of 82% and 40%, respectively. The results suggest that more than half of natural organic matter pollutants were not eliminated by the applied process. Simultaneously, a decrease in the SUVA value after coagulation was observed, suggesting that the hydrophobic fraction of NOM was effectively removed, while hydrophilic NOM components were difficult to treat via coagulation alone [55].

Hybrid coagulants are a promising area for future research. Recent studies show encouraging results in terms of both NOM removal efficiency and mitigation of negative side effects typically associated with coagulation with conventional coagulants. Future studies should focus on the possibilities of employing novel hybrid coagulants on a larger scale for the treatment of surface waters with varying concentrations and compositions of NOM. The effects of coagulation using hybrid coagulants on the chemical stability of water should also be further explored.

4.2. Coagulation-Integrated Processes

Integrating coagulation with other unit water treatment processes with proven NOM removal capabilities, such as membrane filtration, ion exchange, adsorption, and advanced oxidation processes, is a viable strategy to enhance NOM removal. Conventional treatment is typically unsuccessful in removing LMW hydrophilic fractions of NOM which can be more effectively removed by other processes [9]. Moreover, implementing other processes oftentimes allows for mitigating some of the issues associated with conventional water treatment. The following section of the article presents an overview of recent studies on various combinations of coagulation with other water treatment processes.

4.2.1. Integrated Coagulation–Adsorption Processes as Pre-Treatment to Membrane Filtration

Instead of being used as a standalone water treatment process, coagulation can be applied as a pre-treatment method to other water purification processes. The application of coagulation, often coupled with other water treatment processes, most notably adsorption, as a pre-treatment for membrane filtration, has been the topic of numerous studies in recent years. Among membrane filtration processes, ultrafiltration in particular has become more popular in terms of applicability, largely due to improved performance and lowered costs for its application for NOM removal over the years [9,55,56].

Even though modern UF membranes have lower energy requirements, their operational costs can be increased due to membrane fouling caused by the presence of NOM in water subjected to treatment [9]. Membrane fouling is currently the main factor limiting its widespread use. Researchers have been investigating different types of technologies for the pre-treatment of feed water to prevent membrane fouling, with coagulation and adsorption being the most cost-effective. The targeted NOM compounds significantly differ in both technologies. Coagulation is effective in removing hydrophobic fractions of NOM, while adsorption provides the removal of small molecule NOM components, otherwise difficult to remove by coagulation or ultrafiltration (UF). The combination of coagulation and adsorption therefore provides better NOM removal performance than single-process pre-treatment [56].

In recent years, several researchers have investigated the combination of coagulation-adsorption as a pre-treatment for ultrafiltration with promising results. For instance, Marais et al. [9] conducted a comparative assessment of two methods for improving the removal of LMW components of NOM during water treatment. Their work investigated the efficiencies of NOM removal via conventional treatment (coagulation, sedimentation, and sand filtration), granular activated carbon (GAC) filtration preceded by coagulation, sedimentation, and sand filtration, as well as coagulation followed by ultrafiltration. The results showed that on average, the conventional treatment obtained 61% and 24% removal efficiency of UV254 and DOC, respectively, while treatment involving membrane filtration achieved removal efficiencies of 73% for UV254 and 25% for DOC. The addition of GAC filtration to conventional treatment provided removal efficiencies of 86% and 28% for UV254 and DOC, respectively. In addition to the overall higher efficiency of NOM removal, it was also observed that the incorporation of the UF and GAC filtration significantly increased LMW and intermediate molecular weight (IMW) fractions of NOM that are otherwise essentially untreatable by conventional coagulation-based treatment [9]

More recently, Elma et al. [31] conducted a study investigating the effects of coagulation with alum and powdered activated carbon (PAC) adsorption as a pre-treatment method for ultrafiltration. It proved to be an effective method for NOM removal. The optimum aluminum sulfate dosage provided up to 75% NOM removal, and further treatment with adsorption using PAC increased NOM removal efficiency by up to 92%. The final treatment, including membrane filtration, reached NOM removals of 95%. These findings highlight the viability of the coagulation-integrated hybrid processes for removing organic pollutants from waters otherwise difficult to effectively treat in a conventional water treatment system [31].

Similarly, Bu et al. [56] investigated the influence of different kinds of pre-treatments to ultrafiltration on the NOM removal efficiency. The investigated technological systems of water treatments included: coagulation (C), coagulation–ultrafiltration (C-F), coagulation–adsorption–ultrafiltration (C-A-F), and simultaneous coagulation and adsorption followed by ultrafiltration (C+A-UF). All the analyzed combinations provided satisfactory treatment efficiencies, with DOC removal efficiencies between 39.5 and 46%, and UV254 removal efficiencies between 83 and 86%. In comparison, however, the C-A-F combination provided the highest removals of both DOC and UV254, while conventional coagulation achieved the lowest values. Interestingly, at the same dosage of coagulant (PACl-pDADMAC = 5 mg/L) and adsorbent (PAC = 50 mg/L) the C+A-UF combination achieved worse DOC and UV254 removal efficiencies than the C-A-UF combination. Regarding the targeted NOM fractions, water treated in the C-UF system exhibited the highest concentration of hydrophilic fractions. Including adsorption decreased the concentration of hydrophilic NOM in post-treatment water, with water treated in C-A-UF having the lowest concentration of hydrophilic compounds. The combination of coagulation and adsorption also provided higher DBP precursor removal. In conclusion, the C-A-UF combination was selected as the best ultrafiltration-integrated process [56].

While the combination of coagulation and adsorption has been proven to be an effective method for NOM removal, a recent study conducted by Alhweij et al. [57] presents a novel sulfonated polyaniline nanofiltration (S-PANI NF) membrane, reported to provide NOM removal efficiency surpassing that of integrated adsorption–coagulation treatment. The results of the study showed that the S-PANI NF membrane achieved DOC removal efficiencies consistently exceeding those obtained in the adsorption–coagulation by 23–36% for surface water treatment, making it a promising water treatment method for NOM removal.

The combination of coagulation, adsorption, and ultrafiltration currently appears to be the most viable surface water treatment method for many DWTPs. It has been reported to provide removal of NOM constituents typically unattainable by coagulation alone. However, while numerous studies report on the efficiency of the treatment via coagulation, adsorption, and ultrafiltration processes in removing NOM, research on the impact of the treatment on the chemical stability of water, as well as on the environment is lacking.

4.2.2. Coagulation Integrated with Biological Processes

The feasibility of implementing biological methods for increased NOM removal was concluded by Krzeminski et al. [58], who assessed the efficiency of multi-step water treatment systems at three major DWTPs in Norway, Sweden, and Finland with consideration of the seasonal difference in NOM composition and concentration. The processes implemented in DWTP in Stockholm included coagulation, sedimentation, and sand filtration, and in Oslo coagulation, sedimentation, and dual media filtration. The DWTP in Helsinki relied on coagulation, sedimentation, sand filtration, ozonation, and biologically activated carbon (BAC) filtration for water purification. The achieved NOM removal efficiencies in DWTPs in Oslo, Stockholm, and Helsinki were 55%, 48%, and 76%, respectively. While coagulation, sedimentation, and filtration sequences obtained up to 70% NOM removal, depending on seasonal changes, the targeted fractions were HMW rather than LMW and hydrophobic fractions. Additional NOM removal may be effectively achieved by implementing biological processes such as BAC filtration or slow sand filtration. Overall, it was concluded that the water treatment system employed in Helsinki was the most effective for treating waters abundant in NOM, as the implementation of ozonation and biofiltration processes obtained higher removals of hydrophobic and LMW fractions, and provided higher overall NOM removal [58].

Promising results were also obtained by Deng et al. [59], who developed a novel hybrid water treatment system consisting of coagulation with PACl and membrane filtration with sponge cubes acting as biomass carriers. The treatment performance was investigated in a conventional membrane filtration system (CMF), membrane filtration with coagulation pre-treatment (P-MF), and membrane filtration system with sponge addition after coagulation pre-treatment (P-SMF). The results showed that the P-SMF system provided significantly higher removals of both small and large molecular weight organics, as well as hydrophilic fractions of organic matter in comparison to the P-MF system. P-SMF obtained the removal efficiency of UV254 and DOC at a level of 74.71% and 68.30%, respectively, while P-MF achieved 52.16% UV254 removal efficiency and 47.83% DOC removal efficiency. These results highlight the efficiency of the novel P-SMF system for NOM removal and mitigating membrane fouling [59].

He et al. [18] investigated the performance of advanced water treatment plants, consisting of pre-ozonation, coagulation, rapid sand filtration, post-ozonation, and biological activated carbon filtration (BACF), in comparison to that of conventional DWTP for the treatment of lake water. The results showed that the advanced DWTPs provided better removals for UV254 and DOC, and significantly lower removals for assimilable organic carbon (AOC). The reason for that is believed to be the higher concentration of AOC precursors, predominantly the IMW and HMW fractions of NOM with higher hydrophobicity, in the BACF effluent than in the rapid sand filter effluent. It was also observed that in the case of the long-term use of the BACF filters, a mature bacterial community evolved on the filters, improving the AOC removal [18].

Considering recent studies on the topic, implementing the biological processes in the coagulation-based treatment is considered a practical solution for treating surface waters containing NOM fractions differing in their molecular weight or hydrophobicity. Such a method of surface water treatment could also potentially mitigate the issue of a shift in NOM composition in some surface waters, although more research in that area is needed, preferably considering the anticipated changes in individual surface waters.

4.2.3. Integrated Coagulation–Oxidation Processes as Pre-Treatment to Membrane Filtration

A study on the performance of coagulation combined with AOP processes as a pre-treatment method for membrane filtration was conducted by Wang et al. [60]. Their work assessed the effectiveness of photocatalysis coupled with coagulation, as well as the individual processes, and compared them in terms of their NOM removal efficiency and reduction of membrane fouling. In terms of NOM removal, photocatalysis combined with coagulation provided the best results (81.2% UV254 removal and 57.78% DOC removal). The study results also confirmed that the process of coagulation predominantly removed the hydrophobic fraction of NOM, although the process of photocatalysis with the novel composite photocatalyst Bi-Ti/PAC resulted in the degradation of hydrophobic components into more hydrophilic compounds. Interestingly, in the case of the integrated process of photocatalysis and coagulation with PACl, the photocatalysis pre-treatment did not only alter the hydrophobic character of NOM constituents, but also decreased the content of the hydrophilic fractions in the following coagulation process. In terms of mitigating membrane fouling, it was reported that the pre-treatment with coagulation, photocatalysis, and the combination of the two processes, reduced fouling of the UF membrane by 72%, 71%, and 88%, respectively [60].

A different oxidation process combined with coagulation has been researched by Bu et al. [55], who investigated implementing coagulation coupled with ozonation as a pre-treatment method for ultrafiltration. In their study, coagulation (C), ozonation (O), ozonation preceded by coagulation (C-O), and ozonation followed by coagulation (O-C) were compared in terms of their performance [55]. It was concluded that the combination of coagulation conducted with the composite coagulant PACl-pDADMAC, and the ozonation process improved UV254 removal efficiency, as well as DOC removal efficiencies to a somewhat lower extent, regardless of the order of the process implementation. The O-C pre-treatment provided the lowest membrane fouling, although the addition of ozone after coagulation was observed to cause a slight increase in DBP formation [55].

Setareh et al. [40] also investigated the impact of combining the ultrasound/ozonation (US/O₃) pre-treatment with coagulation on turbidity and NOM removal efficiencies in comparison to the effects of surface water purification via coagulation alone. Coagulation as a standalone NOM removal method carried out with an optimum dose of PACl (0.81 mg/L), provided UV254 removal efficiency of 65%, and turbidity removal of 15%, while the US/O₃ process alone achieved a removal efficiency of 84% for UV254 and 33% for turbidity. The combination of coagulation and the US/O₃ processes obtained the highest NOM and turbidity removals of 95% for UV254 and 50% for turbidity. In addition to superior NOM removal efficiencies, several benefits associated with the application of the US/O₃-coagulation method were identified. For instance, integrating coagulation with the US/O₃ reduced coagulant dosage and the doses of chemical disinfectants while decreasing O₃ consumption. The post-coagulation sludge volume could also be reduced by coupling coagulation with AOP processes. Furthermore, the results of the study showed that the proposed combination of processes produced no DBPs. Therefore, the NOM removal method proposed in the study is particularly promising [40].

4.2.4. Integrated Coagulation-Ion Exchange Processes

Finkbeiner et al. [61] investigated the performance of a combination of suspended ion exchange (SIX) with in-line coagulation and membrane filtration in comparison to the effects obtained by coagulation coupled with ultrafiltration for the treatment of surface water. The results showed that the process of suspended ion exchange coupled with coagulation and membrane filtration provided better removals of NOM as measured by DOC and UV254, and reduced the potential formation of specific DBPs. The study also investigated the selective removal capacities of NOM fractions by ion exchange (IEX). It was concluded that the IEX process exhibited preferential removal of NOM compounds, predominantly reducing the concentration of organic pollutants with a molecular weight of approximately 1 kDa. While molecular weight contributed to the process efficiency, hydrophobicity was not a factor in NOM removal via ion exchange. Therefore, combining coagulation with IEX processes can increase the overall efficiency of NOM treatment, because the IEX removes LMW components of NOM, typically difficult to remove via coagulation alone.

A later study [62] compared both virgin and pre-used anion exchange resins. It was concluded that NOM removal efficiency via ion exchange is controlled by the resin state. Pre-used resin provided up to 79% DOC removal, while ion exchange using virgin resin achieved up to 89% DOC removal, presumably due to the absence of resin fouling. It was also hypothesized that in the case of pre-used resin, the organic compounds were restricted only to easily accessible exchange sites, therefore limiting the NOM removal efficiency. Among processes compared in the study (ion exchange, coagulation, and the combination of the two processes), the highest DOC removal of 94% was achieved for coagulation coupled with ion exchange. In addition to the superior removal efficiency of NOM, implementing the IEX process decreased the required dose of coagulant (PACl) by up to 39%. The treatment with pre-used IEX resin reduced the concentration of all the NOM fractions, although implementing virgin resin in the IEX treatment provided higher removals for all the NOM fractions in comparison to the pre-used resin treatment. It was also reported that in some cases, the IEX treatment changed the proportions of different fractions in treated water, shifting towards more hydrophilic with the proportion of the HPI fraction increasing from 21% to 50%, and from 15% to 45% for different waters. While coagulation as a standalone process once again showed preferential for the removal of the hydrophobic fraction, the results for the combined process differed, depending on the treated water. In some cases, the hydrophilic fraction was dominant in the removed NOM, which could be explained by the lowered concentration of the hydrophobic fraction in water pre-treated with IEX. That allowed for the hydrophilic fraction, typically difficult to remove via coagulation, to be targeted in the coagulation stage of treatment [62].

A summary of selected studies comparing the effectiveness of combined processes is presented in

Table 1. Comparison of the effectiveness of coagulation-integrated processes.

Raw Water Characteristics	Treatment Method	Operating Conditions and Materials	Coagulant Type and Dose	Efficiency	Ref.
Peat Water, Indonesia pH = 6.3 DOC = 36.4 [mg/L] UV254 = 0.976 [cm−1] KMnO4 Organic Substances = 120.0 [mg KMnO4/L]	Coagulation–Adsorption–Ultrafiltration	Coagulation: pH = 6.0 rapid mixing: 1 min, 100 rpm slow mixing: 20 min, 40 rpm Adsorption: PAC (particle size of 100 mesh; surface area of 800 m2/g, dose of 120 mg/L) mixed at 180 rpm for 3 h UF: the polysulfone membrane, pore size < 0.1 µm, pressure 3 bar	Aluminum sulfate: 175 [mg/L]	After coagulation: pH = 3.65 Removal of KMnO4 organic substances = 78% Reduction in UV254 = 75% After adsorption: Removal of KMnO4 organics = 96% Reduction in UV254 = 92% After UF: Reduction in UV254 = 95%	[31]
Synthetic Water pH = 8.3 UV254 = 0.231 [cm−1] DOC = 3.95 [mg/L]	Coagulation–Adsorption	Coagulation: rapid mixing: 1.5 min at 200 rpm slow mixing: 15 min at 40 rpm sedimentation: 30 min Adsorption: PAC (dosage of 50 mg/L)	Inorganic–organic hybrid coagulant PACl-PDMDAAC: 5 [mg/L]	Removal efficiency of UV254 = 86% Removal efficiency of DOC = 46%	[55]
	Coagulation + Adsorption	Coagulation + Adsorption: PAC (dosage of 50 mg/L to the rapid mixing tank) rapid mixing: 1.5 min at 200 rpm slow mixing: 15 min at 40 rpm sedimentation: 30 min		Removal efficiency of UV254 = 84% Removal efficiency of DOC = 43%
Synthetic Water pH = 7.25 UV254 = 0.087 [cm−1] DOC = 5.29 [mg/L] Turbidity = 3.32 [NTU]	Membrane–Filtration (CMF)	Ultrafiltration: Polyvinylidene fluoride (PVDF) membrane (pore size of 0.07 mm, effective surface area of 0.2 m2), permeate flux of 10 L/m2h	-	Removal efficiency of UV254 = 14.22% Removal efficiency of DOC = 17.52% Removal efficiency of turbidity = 97.83%	[59]
	Coagulation–Membrane Filtration (P-MF)		Polyaluminum chloride PACl: 10 mg/L	Removal efficiency of UV254 = 52.16% Removal efficiency of DOC = 47.83% Removal efficiency of turbidity = 98.45%
	Coagulation–Membrane Filtration with sponge biomass carriers (P-SMF)	Polyester-polyurethane porous sponge cubes (10 mm × 10 mm × 10 mm, density of 28–45 kg/m3, cell count of 90 cells/in) previously acclimatized for 15 days for biomass enrichment were added after coagulation, prior to UF. Ultrafiltration: Polyvinylidene fluoride (PVDF) membrane (pore size of 0.07 mm, effective surface area of 0.2 m2), permeate flux of 10 L/m2h		Removal efficiency of UV254 = 74.71% Removal efficiency of DOC = 68.30% The removal efficiency of turbidity = 98.76
Ravash Dam Water, Iran pH = 7.85–7.91 UV254 = 0.019–0.051 [cm−1] TOC = 3.19–6.0 [mg/L] Turbidity = 1.08–4.5 [NTU]	Ultrasound/Ozonation	Ultrasound/Ozonation: US frequency 80 kHz, power intensity 200 W/cm2, O3 dosage of 3 mg/L, reaction time 8 min	-	Removal efficiency of UV254 = 84% Removal efficiency of turbidity = 33%	[40]
	Coagulation	Coagulant aid: anionic polyelectrolyte BASF LT25, dose: 0.1 mg/L	Polyaluminum chloride PACl: 0.81 mg/L	Removal efficiency of UV254 = 65% Removal efficiency of turbidity = 15%
	Ultrasound/Ozonation–Coagulation	Ultrasound/Ozonation: US frequency 80 kHz, power intensity 200 W/cm2, O3 dosage of 3 mg/L, reaction time 8 min Coagulant aid: anionic polyelectrolyte BASF LT25, dose: 0.1 mg/L		Removal efficiency of UV254 = 95% Removal efficiency of turbidity = 50%
Tonghui River Water, China UV254 = 0.085 [cm−1] DOC = 2.24 [mg/L]	Coagulation	rapid mixing: 1 min at 120 rpm slow mixing: 20 min at 50 rpm sedimentation: 20 min pH = 5	0.2 mmol	Removal efficiency of UV254 = 46.89% Removal efficiency of DOC = 17.68%	[60]
	Photocatalysis	The dose of the composite nano-photocatalyst Bi-Ti/PAC: 2.0 g/L, 300 W xenon light, visible light irradiation 350 u/m2, reaction time: 20 min	-	Removal efficiency of UV254 = 75.46% Removal efficiency of DOC = 48.71%
	Photocatalysis–coagulation	Photocatalysis: the dose of the composite nano-photocatalyst Bi-Ti/PAC: 2.0 g/L, 300 W xenon light, visible light irradiation 350 u/m2, reaction time: 20 min Coagulation: rapid mixing: 1 min at 120 rpm slow mixing: 20 min at 50 rpm sedimentation: 20 min pH = 7	0.04 mmol	Removal efficiency of UV254 = 81.2% Removal efficiency of DOC = 57.78%
Synthetic Water UV254 = 0.0231 [cm−1] DOC = 3.975 [mg/L] Turbidity = 15.0 [NTU]	Coagulation–Ozonation	Coagulation: rapid mixing: 1.5 min at 200 rpm slow mixing: 40 min at 15 rpm sedimentation: 30 min Ozonation: The ozone dosage of 2 mg/L	Inorganic–organic hybrid coagulant PACl-PDMDAAC: 7 [mg/L]	Removal efficiency of UV254~85% Removal efficiency of DOC = 38–42%	[55]
	Ozonation–Coagulation
	Coagulation–Ozonation–Ultrafiltration	Coagulation: rapid mixing: 1.5 min at 200 rpm slow mixing: 40 min at 15 rpm sedimentation: 30 min Ozonation: The ozone dosage of 2 mg/L Ultrafiltration: Polyvinylidene fluoride membrane with a pore size of 0.03 μm		Removal efficiency of UV254~91% Removal efficiency of DOC = 45–48%
	Ozonation–Coagulation–Ultrafiltration

.

5. Moving Forward

The most serious issue currently associated with the presence of NOM in source water is the risk of DBP formation during the disinfection stage of water treatment. The most viable option to address this issue is to prevent the DBPs formation. Efficient DBP precursor removal is therefore the main challenge.

In that regard, a change of approach in determining the efficiency of the water treatment process applied for NOM removal is suggested. Measuring specific DBP precursor removal instead of typical indicators such as DOC or UV254 might be more beneficial for many DWTPs. It is a viable method for evaluating NOM removal efficiency while effectively controlling DBP formation. The suggested approach addresses several NOM-related issues. Firstly, the issue of seasonally changing the concentration of NOM could be mitigated by implementing the proposed approach. A similar technique has already been adapted for optimizing coagulant doses, resulting in a lack of seasonal changes in the optimal doses, even for waters exposed to seasonally changing conditions [20]. Secondly, DBP control would be significantly improved. Thirdly, targeting specific DBP precursors instead of aiming at overall NOM reduction can be applied for both conventional coagulation and alternative NOM removal methods, i.e., ion exchange, membrane filtration, and integrated processes. It could be implemented in both conventional treatment in DWTPs, as well as in advanced DWTPs and future research regarding the topic of NOM removal, which makes it a particularly viable solution.

Moving forward, understanding the possible impact of climate change on surface water quality and how it can affect the efficiency of the most commonly employed surface water treatment method is crucial in identifying areas critical to ensuring continuous and effective surface water purification in the future. A comparison of coagulation-based water treatment methods that are the subject of the present paper, alongside their respective advantages, disadvantages, and challenges is presented in

Table 2. Comparison of coagulation-based NOM-removal methods—advantages, disadvantages, and challenges.

NOM Removal Method	Advantages	Disadvantages	Current Challenges	Future Challenges	Ref.
Coagulation with metallic salts	Cost-effective High availability Easy handling and storage	Alkalinity reduction Increased corrosivity of water. Residual metal concentration and detrimental human health effect of residual aluminum. Production of high-volume, non-biodegradable sludge.	Mitigating the corrosive properties of post-coagulation water. Difficulty in disposing of post-coagulation sludge. Ineffectiveness of removing low molecular weight NOM.	The effectiveness of NOM removal is expected to decrease with an increase in the proportion of low molecular weight NOM due to an increase in solar radiation. Higher concentrations of LMW fractions of NOM will likely contribute to higher concentrations of DBPs such as THMs.	[4,5,7,11,25,34,36]
Coagulation with Ti and Zr-based coagulants	Production of lower amounts of sludge than in the case of metallic salts. Possibility of recycling post-coagulation sludge into an income-generating product. Effective in treating low-alkalinity and high-DOC waters. Effective in removing both LMW and HWM fractions of NOM.	Ti-based coagulants require acidic conditions for effective NOM removal. Very high cost.	Improvement of the economic aspect of coagulation with Ti and Zr-based coagulants.	Further research and analysis are needed to determine the impact of the process on the chemical stability of w, and to determine the exact NOM fractions that can be effectively removed via coagulation with Ti and Zr-based coagulants.	[14,27,30,41]
Coagulation with inorganic polymers	Significantly lower concentration of residual metal than metallic salts. Mitigating the harmful effects of residual aluminum due to its lower concentration. Production of lower amounts of sludge than in the case of metallic salts. Better performance in NOM removal than that of conventional coagulants. Little to no alkalinity reduction.	Although the residual metal concentration and the production of post-coagulation chemical sludge are reduced in comparison to the conventional salts, they remain the consequences of the coagulation.	Proper handling of the post-coagulation sludge.	Further research and analysis are needed to determine the exact molecular weight range of NOM that can be effectively removed via coagulation with inorganic polymers, and how the effectiveness of the process may be affected in the future due to the expected NOM composition changes.	[4,5,6,14,34,42,43]
Coagulation with biocoagulants	Renewability and biodegradability. A pragmatic option for communities in regions abundant in plants with coagulation-active components. No residual metal concentration. No alkalinity reduction. Lower optimum doses than traditional coagulants. Lower volumes of post-coagulation sludge and lower costs of its disposal than in the case of metallic-based coagulants.	Inconsistent performance and higher variability in effectiveness than chemical coagulants due to the variations in the composition and properties of biocoagulants. Additional steps (e.g., extraction, purification, modification) are often required. Some biocoagulants (e.g., Moringa oleifera seeds) may contribute to an increase in DOC concentration, likely caused by the release of soluble organic alongside the coagulation-active components from the plant seeds. Accessibility may be limited in certain regions or during different seasons. Microorganism-derived biocoagulants may cause microbial contamination.	Averting the issue of soluble organic release by improving the purification methods to obtain pure coagulants.	Further research and analysis are needed to determine the exact molecular weight range of NOM that can be effectively removed via coagulation with biocoagulants, and how their treatability via this method might be influenced by climate change in the future. The effectiveness of removing DBP precursors should be further investigated.	[4,5,12,25,47,50,51,52,54,63]
Coagulation with hybrid coagulants	Lower alkalinity reduction. Higher DOC removal at lower doses than in the case of conventional coagulants. Relatively cost-effective. Mitigation of typical drawbacks of coagulation with traditional coagulants.	Additional costs associated with the use of multiple coagulants. Production of post-coagulation sludge.	Logistic challenges associated with the use of multiple coagulants, preparation of hybrid coagulants, and handling the post-coagulation sludge.	Further research and analysis are needed to determine the exact molecular weight range of NOM that can be effectively removed by coagulation with different hybrid coagulants, and how their treatability via this method might be influenced by climate change in the future. The effects of coagulation using hybrid coagulants on the chemical stability of water should also be further explored. The effectiveness of removing individual DBP precursors should be further explored.	[8,44]
Coagulation–Adsorption–Ultrafiltration	High NOM-removal efficiency. Removal of the LMW fractions of NOM that cannot be removed by coagulation alone. Better DBP precursor removal.	Adsorbent and coagulant costs. Adsorbent availability may vary depending on the targeted contaminants. Maintenance of the UF membranes. Consequences of coagulation remain the same, although they may be lessened due to the use of coagulant doses lower than in conventional treatment.	Requires regeneration and replacement of adsorbent. Mitigation of membrane fouling. Disposing of post-coagulation sludge.	The potential impact of the climate change implications on NOM in surface waters and its treatability via this method should be further investigated.	[9,31,56,64,65]
Coagulation integrated with biological processes		Biological processes may increase overall treatment time. Biological processes are sensitive to conditions. Production of post-coagulation sludge. Alkalinity reduction.	Disposing of post-coagulation sludge.		[18,58,59]
Coagulation integrated with ion exchange processes		Costs associated with the use of ion exchange resins. Resin fouling. Requires regeneration of the ion exchange resin. Production of post-coagulation sludge. Alkalinity reduction.			[61,62,65]

.

6. Conclusions

The objective of this paper was to provide a critical review of problems originating from NOM presence in surface waters, the effect of the progressing climate change on the quality of potable water sources, the overall impact of said issues on the surface water treatment via coagulation-based methods, as well as the comparison of the advantages and disadvantages of coagulation-based methods.

The following conclusions can be drawn:

(1)

The progressing climate change can have a significant impact on the typical surface water treatment system. The predictions of an increase in the proportion of LMW fractions of NOM in the surface water sources allow for speculating on a decrease in the effectiveness of water purification by conventional coagulation using metallic coagulants because it is typically ineffective in removing those fractions of NOM.

(2)

Potential solutions for ensuring continuous efficiency of coagulation-based treatment need to provide removal of NOM fractions currently difficult to reduce by conventional coagulation. Such solutions include integrating the coagulation process with other processes, e.g., adsorption, membrane filtration, and ion exchange.

(3)

Moving forward, the negative consequences of using conventional coagulation need to be eliminated. Disadvantages associated with the use of traditional coagulants can be resolved, or at least mitigated by using inorganic polymers with no damage to the process’ performance in NOM removal.

(4)

Moreover, more environmentally friendly solutions that potentially allow for implementing the concepts of circular economy should be further investigated in terms of the possibility of their implementation on a larger scale. Such solutions include the use of hybrid coagulants and biocoagulants. Future research in that area should take into account not only the technological, economic, and environmental aspects but also the potential impact of climate change-related surface water quality changes on the process efficiency.