Abstract
Natural organic matter (NOM) enters water from natural sources such as plant material and synthetic compounds such as detergents, solvents, pesticides and herbicides. Due to their specific characteristics, organic compounds affect the quality of drinking water in different ways, such as producing unpleasant taste and smell, reacting with chlorinated compounds and forming disinfection by-products (DBPs). Most of these compounds, like trihalomethanes, are carcinogenic and it is not possible to completely remove them in conventional water treatment processes. Today, the concentration of natural organic matter in fresh water sources as well as domestic wastewater effluents has increased significantly around the world and is considered one of the main challenges. Therefore, effective measures are needed in order to develop of efficient drinking water treatment processes. Coagulation is one of the most widely used processes in drinking water treatment. In recent years, improving the coagulation process and using advanced coagulation to NOM is very important. In this review, recent research studies dealing with the use of coagulation to remove natural organic matter from drinking water supplies have been presented and compared. Furthermore, integration schemes combining coagulation and other water treatment processes such as oxidation, membrane filtration, and adsorption have been presented. metal salts, inorganic and organic polymers of natural or synthetic type and various combination scenarios between them. Inorganic polymeric coagulants have shown better removal capacity for NOM and other organics from waters compared to coagulants such as ferric sulfate or aluminum sulfate. Electrocoagulation (EC) is an established water treatment technology that is widely used for the removal of organic and inorganic contaminants such as pesticides, dyes, heavy metals, etc. from drinking water sources.

Introduction
Natural organic matter (NOM) enters water sources from natural sources such as plant material and synthetic organic compounds such as detergents, solvents, pesticides, and herbicides. Organic compounds affect the quality of drinking water in different ways, such as producing unpleasant taste and smell, reacting with chlorinated compounds and forming disinfection by-products (DBPs). In fact, the presence of NOM in waters sources causes changes in their organoleptic properties and by causing things such as color, taste and smell, it reduces the quality of the water resources in question. Removing NOM from drinking water sources is considered an important issue in protecting the health of a society and ignoring it can be associated with challenges. Therefore, the conditions require that by using reliable and highly efficient water purification technologies, it is possible to remove NOM in water environments and prevent them from increasing their concentration in water sources. The aim of this review is to investigate the processes performed using different coagulation mechanisms and to make a comparative evaluation between them for the removal of NOM in drinking water.

Materials and methods
In the present review article, recent research studies dealing with the removal of natural organic matter from drinking water sources through coagulation are reviewed. In order to investigate the removal of NOM in the treatment of drinking water through the coagulation process, using different coagulants, a review of previous researches was carried out in the form of library studies, and the latest achievements about removing NOM were studied and finally its results were compiled in the form of a review article. In this study, recent research articles published in valid journals in the field of coagulation and removal of natural organic matter from drinking water sources through different coagulants. Also, a comparative evaluation of the efficiency of different coagulants and coagulation techniques to remove NOM from water resources has been done.

Results and discussion
NOM have a complex nature in terms of quantity and quality due to the prevailing spatial and temporal changes. This complex nature of NOM is one of the main challenges facing various water treatment techniques and methods that are used to remove NOM from drinking water sources. Based on this, in order to correctly choose the methods and technologies used in water treatment and to make the necessary predictions for the reactivity of these NOM with the chemicals in water, to specify the type of NOM and their different natures. It is very important and significantly effective in achieving high removal efficiency and reducing the potential f forming disinfection by-products (DBPs) due to the disinfection process. Dosage of coagulants and pH are among the main operational conditions affecting the coagulation process and the overall efficiency of this process depends on these factors. The efficiency of coagulation to remove NOM on the one hand mainly depends on the type and dose of coagulant, pH, mixing conditions and temperature and on the other hand the properties of natural organic substances including particle size, charge and hydrophobicity and the presence of divalent cations and destabilizing anions such as It depends on bicarbonate, chloride and sulfate ions.

Conclusion
One of the most suitable methods of water purification with the aim of removing NOM from drinking water sources is the use of coagulation processes. Optimizing the coagulation process in order to remove as much NOM as possible has been of considerable importance.
Some other achievements of this study include:
- Conventional coagulation process is usually not effective in removing organic materials with lower molecular weight and generally this process is effective in removing organic materials with higher molecular weight.
- The variable composition of NOM poses a serious challenge to conventional coagulation.
In order to remove natural organic materials, different types of coagulants including metal salts, inorganic and organic polymers of natural or synthetic type and different combination scenarios between them.
- In previous studies, inorganic polymeric coagulants showed better removal capacities toward NOM and other organic substances from waters, in comparison with coagulants such as ferric sulfate or aluminum sulfate.
- Many novel composite or hybrid coagulants have been developed via various combination.
Electrocoagulation (EC) is a well-established water treatment technology widely used for the removing of drinking water supplies from organic and inorganic pollutants such as pesticides, dyes, heavy metals, etc.
- Several kinds of metallic and polymeric coagulants are being applied in real case scenarios for the decontamination and treatment of drinking water sources. Nevertheless, despite their high efficiencies to remove NOM and other organic and inorganic contaminants, several issues such as includes residual metals and polymers in the treated effluents, relative high operating costs, pH change, and the generation of large sludge volumes were reported for large scale applications.