Water purification for human consumption purposes consists in the removal of different contaminants as chemicals (i.e., pollutants, toxic metals), biological contaminants (algae, bacteria, fungi, parasites, viruses), suspended solids, and gases.
There are several methods used in the water purification process, which include: (1) physical processes, such as filtration, sedimentation, or distillation; (2) biological processes, such as sand filters, active carbon; (3) chemical processes, such as flocculation, chlorination, the use of ultraviolet light.
Regarding the physical approaches used in water purification, these differ in the phenomenon on which the processes are based. For instance, filtration is based on the separation of solids from fluids, by interposing a porous medium (filter) which retains the solid particles and allows the fluid to pass on the other side. On the other hand, the sedimentation approach uses the gravitational force which determines the solids to form a deposit at the bottom of the tube containing contaminated water, while distillation involves the transformation of the liquid (water) into a vapor phase, process which is based on the difference in the volatility of the compounds.
Slow sand filters represent one example of biological approaches used in water purification, which implies using 1–2 m deep tubes filled with sand, which retain the impurities present in the filtered water. Activated carbon (charcoal), another utilized approach, is a microporous carbon with high surface area and enhanced adsorption properties.
In scientific terms, flocculation is a process in which colloids in suspension become destabilized after the addition of a clarifying agent; regarding the water purification process, the flocculation phenomenon can refer to the destabilization and coagulation of contaminants present in the water. Water chlorination is commonly encountered in the treatment of water supply and consists in the addition of chlorine or hypochlorite to kill microbes and to prevent the spreading of waterborne diseases.
The use of electromagnetic light, especially with short wavelength (in ultraviolet range) is commonly used in disinfection, due to the fact that it produces gaps in the structure of nucleic acids of microorganisms disabling their cellular functions. This method is often used in water decontamination.
This chapter focuses mainly on the most investigated physical methods which involve the filtration process. Compared to the other approaches used in water treatment, membrane water filtration presents some advantages, such as: (1) the continuity of the operation; (2) it does not require the use of any chemicals; (3) it does not imply high energy consumption; (4) the possibility of scaling up, be integrated/integrate other processes and possibility of automation (Street et al., 2014).
There are several parameters influencing the properties and efficiency of the membranes used in water filtration: (1) the size of membrane pores/the size of the contaminant molecules/particles; (2) the positive/negative charge of the membrane surface, respectively the polarity of the contaminant molecules; and (3) the adsorbing capacity of the membrane surface (Street et al., 2014).
Depending on the pore size of the used filters (membranes), contaminants with different sizes can be removed and a better purification can be achieved while decreasing the pore size; thus, the available methods are: (1) microfiltration (pore size about 0.1 μm), which removes bacteria and suspended solids in the water; (2) ultrafiltration (pore size about 0.01 μm), which in addition to microfiltration removes viruses; (3) nanofiltration (pore size about 0.001 μm), which removes most of the organic molecules and some multivalent ions (divalent ions from hard water); and (4) reverse osmosis (pore size about 0.0001 μm), which removes all organic molecules and minerals in water, resulting ultrapure water
When analyzing the properties of these types of membranes, it is clear that the most advantageous are the ones with smaller pore size. However, for cost-efficiency purposes, all of these filtering techniques can be used to obtain drinkable water, when used in combination with other treatments. For example, membranes with micropores can remove sand, silt, clays, Giardia lamblia and Crypotosporidium cysts, algae, and some bacterial species, when used in combination with disinfecting agents. In the case of membranes designed for ultrafiltration, even if they can remove the majority of microorganisms and viruses in water, it is recommended to use additional disinfection. However, between the advantages of such membranes we can recall: (A1) filtration by means of size-exclusion mechanisms; (A2) the quality of the removal process in constant; (A3) possibility of automation; (A4) the installation integrating this type of filter and the filtering process are compact. The main disadvantage may refer to fouling phenomenon (see Section 12.3). Nanofiltration is the first step to a perfect filtration of water, because it removes all microorganisms, viruses, and organic matter. Other important advantage is given by their ability to remove alkalinity and hardness from water. The most advanced filtration is made by reverse osmosis membranes, which additionally can remove most of inorganic contaminants, resulting ultrapure water. Between the advantages of this type of filtration, we mention: (A1) complete elimination of contaminants; (A2) the filtration sensibility is not severely influenced by the flow; and (A3) possibility of automation. Besides these advantages, the technique presents some limitations, which refer to the high costs, the process management, and the need of a pretreatment and fouling predisposition.
There are several examples of membranes whose principle of filtration is based on the polarity of the contaminant molecules and their ability to retain these species by means of electrostatic interactions. However, dipole molecules (with both positive and negative charged groups), such as water molecules, can easily orient and thus permeate the membrane (Street et al., 2014).
Regarding the membranes with adsorbing properties, the retaining of contaminant molecules is also based on electrostatic interactions: due to the fact that the membrane surface is negatively charged, positively ions in water are adsorbed, forming an electrical double layer at the surface of the membrane (Street et al., 2014). There are several factors influencing the adsorption ability of the contaminants, like their nature, concentration and mass distribution. Also, Ca2+ concentration influences the filtering properties because the interaction with the negatively charged surface of the membrane is preferentially favored, compared to the contaminant molecules, which are usually bigger in size and have different spatial conformations. Also, the physical and chemical properties of the membrane are important for the process (Street et al., 2014).
Depending on their structure and chemical composition, membranes can also be grouped as: (1) isotropic, having uniform physical nature and chemical composition in cross-section; and (2) anisotropic, which are nonuniform in cross-section. Examples of anisotropic membranes for water filtration are given by layer-by-layer approaches (Wang et al., 2015, 2016a; Qin et al., 2016; Xu et al. 2015; Zhang et al., 2015a; Diep et al., 2015; Gu et al., 2015; Kaner et al., 2015). The main advantage of this type of membranes is given by the antifouling property determined by different approaches in surface modification (see Section 12.3 for details).
The progress in water purification has been mainly conditioned by the scientific progress made in the identification of pathogens and other contaminants in water, the introduction of new regulations regarding water quality, but also the development of materials science, smart materials, and nanomaterials. Today, there are many examples of commercial membranes for water purification; however, there is still a permanent need for improvement regarding their properties, such as: (1) antifouling properties; (2) chemical stability; (3) mechanical stability; and (4) thermal stability.
The area of water purification has been one of the most dynamic research fields in recent years with strong public policy implications with over 39,000 papers. Similarly, the areas of nanomaterials and nanoprocesses have been one of the most dynamic research fields in recent years with significant public policy implications with over 1,000,000 papers.
At the intersection of the research on the water purification and nanomaterials, the field of water nanopurification materials and processes has been one of the most dynamic research fields in recent years with significant impact on the medical and environmental research with near 5000 papers.
Following a scientometric overview of the research in nanomaterials and nanoprocesses, as well as water purification, the brief information on a selected set of 25 citation classics in the field of the water nanopurification materials and processes were presented in this paper to inform the key stakeholders about the influential papers in this dynamic research field as the first-ever study of its kind.
It was found that the major research area in these classical papers was the water NF materials and processes. The other studies on the water nanopurification included water dechlorination, water decontamination, water disinfection, and w/o emulsions.
As these citation classics deal with the important health and environmental research issues, the research in this field has strong public policy implications providing strong incentives for the key stakeholders involved in this research field.
Further research is recommended for the detailed studies including scientometric studies and citation classic studies for each of these topical areas, as well as the underlying research fields.