**1. Introduction**

The combination of climate change and environmental pollution has impacted the quality and quantity of drinking water available from conventional freshwater sources. In response to these dwindling potable water resources, a growing number of alternative water sources have emerged in the form of technologies that were introduced or markedly improved during the last two decades. These technologies include desalinated ocean or brackish waters, condensed atmospheric water, recycled wastewater, and captured cloud or fog water, all of which produce potable waters that often lack the minerals and other natural properties of ground and surface waters [1]. Hence, alternative waters are increasingly amended with salts or mineral solutions, adjusted for pH or ORP, and treated in other ways to improve taste or enhance human health.

The primary emphasis of water quality has traditionally been identifying and remediating contaminants in potable sources in order to reduce human health risks; however, characterizing quality solely by the absence of toxins overlooks water's potential health benefits in other ways. Public drinking waters have historically been treated to address taste, odor and/or clarity issues [1], but how these treatments may have influenced the nutritional value or health attributes of water (other than providing hydration) has received only limited attention. One exception is fluoride that, while present at detectable levels in some natural waters, remains a controversial additive to potable waters because of the perceived tradeoffs between preventing dental caries and impacting systemic health [2]. Moreover, fluoride is not designated as an essential human nutrient.

A review of the ways in which alternative waters are amended or altered suggests that some are more common, more extensively researched, or potentially more effective than others are. Recent insights into the physics and chemistry of water, combined with an improved understanding of the factors that influence human taste and health, provide a framework for exploring potential enhancements to alternative waters. Many of these enhancements were initially introduced in the form of specialized bottled waters that were marketed as a healthier option than tap water [3].

**Citation:** Marrin, D.L. Evaluating Methods to Enhance the Taste and Health Benefits of Alternative Potable Waters. *Environ. Sci. Proc.* **2023**, *25*, 58. https://doi.org/10.3390/ ECWS-7-14300

Academic Editor: Athanasios Loukas

Published: 3 April 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **2. Minerals and Organics**

Potential healthful benefits of drinking water have focused predominantly on minerals, also known as electrolytes, that are essential to the optimal functioning of the human body and are sometimes present at sufficient concentrations in source waters to be consequential. Major minerals typically include calcium, magnesium, sodium, chloride, and potassium. At least some trace minerals are normally present in source waters (e.g., selenium, chromium, iodine, phosphorus), whereas others may either be present in source waters or released from storage and conveyance materials comprising water distribution systems (e.g., iron, manganese, copper, zinc). While many of these trace minerals are essential nutrients at low concentrations, elevated levels in drinking water can pose toxicity problems. By contrast, elevated levels of major minerals are usually not a toxicity issue but can adversely affect the water's taste.

Minerals are normally dissolved in water as inorganic ions (positively or negatively charged), although minerals can also be complexed with organic compounds. The nutritional value of minerals in water is highly dependent on a number of variables, including their valence state or electrical charge, the presence of other minerals (especially as ions), a person's particular gu<sup>t</sup> flora, the volume of water normally ingested, and any foods that may be consumed with the water. Phytochemicals or biochemicals present in a variety of different foods can increase or, more commonly, decrease the bioavailability of minerals dissolved in drinking waters. As such, most essential minerals in the human diet are provided by food [4], within which they are typically complexed by organic molecules such as amino acids and carbohydrates.

Drinking water generally supplies less than 5% of the recommended intake of most minerals, with the exception of calcium and magnesium that could account for as much as 20% of the recommended intake under unusual circumstances [4]. A number of epidemiological studies have reported a reduced incidence of cardiovascular disease and hypertension in communities where the tap water is at least moderately hard [5,6], meaning that the combined calcium and magnesium levels are a minimum of 60 to 120 parts-per-million (ppm), reported as calcium carbonate.

As people have a relatively wide range of taste sensitivities for water, assessing subtle changes in the levels of major minerals based on taste alone is difficult. Nonetheless, waters possessing major minerals, which comprise most of the total dissolve solids (TDS), in excess of 500 to 600 ppm are not palatable to many people due to their salty taste. Conversely, waters containing less than 25 ppm of major minerals often have a flat or bitter taste that can be unpleasant. Even at slightly elevated concentrations, many of the trace minerals can impart a metallic taste to water that is considered objectionable.

In addition to the electrolytes (minerals) that may enhance human health, there are usually organic compounds present in natural waters that could do the same. The most notable of the organics are fulvic acids, which are produced from the microbial decomposition of plants and contain various vitamins, amino acids, carbohydrates and lipids that might be beneficial to human health, depending on their purity (i.e., absence of toxins) and bioavailability [7]. Besides potentially harboring contaminants, some organic compounds can also impart an unpleasant taste to water and, consequently, are normally removed using routine water treatment techniques.

### **3. ORP and pH**

In addition to substances dissolved in drinking water, various physical properties of water have been correlated with both taste and human health. Two properties frequently cited are potential hydrogen (pH) and redox potential (measured as an oxidation-reduction potential or ORP), which indicate the water's relative acidity or basicity and oxidative or reductive capacity, respectively. Adjusting the pH of potable water to prevent metals leaching from pipes or to improve taste and disinfection, has long been an accepted water treatment practice. Acidic drinking water can taste bitter or metallic, while basic water often tastes soda-like. Increasing the pH of water specifically to enhance human health, as

is the case with various bottled or specialty waters, is a more recent trend that has been applied to alternative waters only to a limited extent.

Artificially adjusted waters with a pH greater than 8.0 are generally referred to as "alkaline", although true alkaline waters have a basic pH resulting from the dissolution of alkali minerals containing sodium or potassium, rather than from modifications via neutralizing filters or electric ionizers. Alkaline water can reportedly act as an antioxidant, hydration enhancer, acid neutralizer, and reducer of blood viscosity [8,9], although some of these health claims are disputed on the basis of their lack of adequate supporting research. Questions surrounding the health claims for alkaline water often relate to the body's efficient systems for tightly regulating the pH of extracellular body fluids [10], regardless of a drinking water's pH unless it is hazardously acidic or basic.

Water with a relatively high pH normally has a correspondingly low ORP, accounting for alkaline water's designation as an antioxidant. Most natural freshwaters have a moderately high or positive ORP value (i.e., oxidizing), rather than a lower or negative ORP value (i.e., reducing), but this depends largely upon the extent to which the waters are oxygenated—either naturally or artificially. Reducing conditions in potable water are often considered to be beneficial inasmuch as they can neutralize free radicals and, thus, minimize the damaging effects. A more recent trend in lowering ORP is the infusion of water with hydrogen gas, produced by electrolysis or dissolving tablets, for purposes of enhancing its antioxidant, anti-inflammatory and metabolic benefits [11,12].

The most common methods for raising the pH and lowering the ORP of alternative drinking water include passing it through a neutralizing media containing calcium carbonate or magnesium oxide and injecting it with sodium hydroxide or sodium carbonate. Sodium-containing additives are sometimes considered less desirable from a health perspective due to excess sodium's role in hypertension. Electrolysis and molecular hydrogen infusion (using elemental magnesium and an organic acid) are more expensive methods of pH/ORP adjustment, and the former technique generates an acidic waste stream.

Reported health benefits for drinking low ORP water, such as its antioxidant effects [13], differ from those posited for drinking highly oxygenated (higher ORP) water, which include improved exercise recovery and liver function [14]. Oxygenated water generally has a fresher taste, particularly when cold, than flatter-tasting waters with less dissolved oxygen or other gases. Oxygenating alternative waters is one of the simplest methods to improve its taste and can be achieved by injecting oxygen or ozone gases. Whereas the reported health benefits of low ORP waters (including hydrogen-infused) and oxygenated waters (including ozone-infused) are plentiful in the popular literature, they too are controversial. Questions focus on the fate of these modified waters following ingestion and on the biochemical mechanisms by which they are presumed to function [15,16].

### **4. Molecular Nuances**

Water's potential health effects may also include nuances of the water molecules themselves. Liquid water is actually a complex and dynamic network of molecules that connect to one another via connections known as hydrogen bonds, which can switch trillions of times per second. This switching among neighboring molecules permits water to flow as a liquid and ye<sup>t</sup> possess much of the molecular structure of a solid (i.e., ice). Besides the bulk liquid network, water molecules can combine to form a variety of geometric assemblages or clusters within which the intermolecular connections persist somewhat longer. Water containing such clusters is sometimes referred to as structured, as it appears to have greater order or more regular clusters than the bulk liquid.

Natural waters are structured as a result of geological, hydrological, fluvial, and other processes. Artificially structured waters are usually produced by introducing various solutes or specific materials into the water or exposing it to an array of fields and energies (e.g., electric, magnetic, vortical, thermal). Drinking structured water has been associated with health benefits such as reducing inflammation and oxidative stresses [17,18]. As a result of the aforementioned hydrogen bond dynamics (even within clusters) and the restructuring

of ingested water upon contact with biological molecules or surfaces, questions persist about the mechanisms proposed for structured water's reported health benefits. Unlike pH and ORP that are easily measured in water, molecular structure is often inferred from physical properties such as density, viscosity, and surface tension. A lower surface tension has been associated with a smoother taste and texture of water.

Intracellular water that is structured inside living cells has been hypothesized to behave as a kind of gel in facilitating many cellular functions [19], thus prompting the question of how intracellular water might be influenced by ingesting externally structured water. Theories range from externally structured water's requirement of less metabolic energy for internal restructuring, to its provision of optimum hydration and removal of toxins from the body. Since water molecules enter living cells primarily through small channels (i.e., aquaporins) in a single-file manner [20], any extracellular water structuring is probably lost while transiting the cell membrane. At present, the molecular structuring of alternative waters is performed almost exclusively by consumers. If structuring were to be used for alternative waters on a larger scale, magnetic and vortical methods might be the most likely candidates as they have been applied to various non-potable waters.

Approximately 0.02% of water's hydrogen atoms consist of a heavier isotope (i.e., deuterium), the resulting concentration of which depends on the latitude, elevation, and precipitation temperature of natural waters. So-called deuterium depleted water (DDW) has a concentration that is less than the global average of approximately 150 ppm and is theorized to be healthful because the body's biological processes and structures generally favor hydrogen over deuterium due to the associated water's chemical and physical properties [21]. The mechanisms responsible for DDW's health benefits (e.g., correcting metabolic or genetic processes, inhibiting cancer proliferation), have been more extensively investigated than those for structured waters. DDW is artificially produced via a continuous distillation process that is expensive and, thus, unlikely to be applied routinely to alternative waters.
