TDS is associated with people liking their drinking water
Total dissolved solids (TDS), a measurement of mineral content, is a key factor in determining the flavor of drinking water for consumers. TDS levels that influence liking vary between people and populations; a typical range is 100–350 mg/L TDS. Desalination of water using membrane treatment technologies aims to produce a TDS level that consumers find palatable.
Recent research has shown that when the TDS changes by approximately 150 mg/L, consumers can usually taste a difference in palatability. Therefore, it is essential to take the viewpoint that a “likeable” or “acceptable” TDS level will vary and that consumers evaluate changes in their drinking water in relation to their typical drinking water quality. While overall TDS is the most important determinant of taste, some specific ions can contribute positively (by optimizing Ca2+, SO42-, HCO3–), or negatively (too much Na+, Cl–, CO32-) to taste.
Presence of odors and challenges for drinking water
The identification and control of odors chemicals have long been significant problems for maintaining drinking water quality and safety on a global scale. China recently investigated 111 drinking water treatment plants nationwide for odorant contamination. According to the findings, there were many instances of odor issues in source water (> 80%), which were characterized by earthy/musty (41%), and swampy/septic (36%). Source water from lakes and reservoirs had more algae-derived odors (such as earthy/musty odors), while from rivers had more anthropogenic-derived odors (such as a swampy/septic odor).
Further research was done on the occurrence of 100 aesthetic occurrences in 140 drinking water treatment plants in 32 cities between 2015 and 2018. Of those, 87 odorous compounds were detected in the raw water and 85 in the finished water, with quantities ranging from neutral density to hundreds or thousands of ng/L. In China, 2-MIB was found to be the primary chemical responsible for musty/earthy odors, whereas dimethyl disulfide was found to be the primary chemical responsible for swampy/septic odors in the source water.
A complicated odor issue including fishy, chemical/solvent, and swampy/septic odors have long plagued the Huangpu River near Shanghai. In general, chemicals that typically cause odors include 2-MIB, geosmin, dimethyl disulfide, and others. Additional research found the co-occurrence of some cyclic acetals, which may be connected to the resin industries and may also be the cause of the chemical or septic odor in the Huangpu River source water. Aside from the horrible taste and odor incidents that occurred in China in recent years, other relevant events occurred around the world, including in the Llobregat River in Spain, as well as in Virginia in the United States.
Potential effects of freshwater salinization on tastes, odors, and algae
A global sustainability concern for the water sector in the 21st century is freshwater salinization and the palatability/safety of drinking water. Freshwater salinization is the widely recognized phenomenon of rising specific conductance, total dissolved solids (TDS), cations, and anions in surface and ground waters. Drinking water taste can be impacted, and a water body’s ecological profile might change as a result of freshwater salinization. Freshwater salinization is a global problem with many different sources. Alterations in river water flows, sea level rise, storm surges, saltwater intrusion, terracing, irrigation of agriculture, deterioration of infrastructure, and climate change are only a few examples of the anthropogenic and natural causes of hydrology.
Freshwater salinization makes it difficult to provide palatable and secure drinking water. It is unknown when customers will notice a change in the mineral flavor of their drinking water, but it will undoubtedly depend on the type of ions that cause salinization, the source of the water, and the consumer group. In order to manage the dissolved minerals that constitute TDS, more expensive membrane technologies will be required. In addition to altering the ecological environment, salinization may also have an impact on the diversity of algae, cyanobacteria, and phytoplankton, which may then have an impact on the production of odor-causing microbes in aquaculture and water.
Biomolecular monitoring of cyanotoxins and T&O chemical-producing cyanobacteria
Cyanotoxins and T&O chemical-producing cyanobacteria are often quantified utilizing taxonomic identification and cell enumeration with microscopy for the monitoring of source water quality. Although microscopy can enumerate and identify organisms down to the species level, the procedure requires skilled professionals who are unable to distinguish between organisms that produce cyanotoxin/T&O and those that do not. As a result, newer techniques are being used in the water industry, like the quantification of phycocyanin and/or chlorophyll-a using online fluoroprobes. However, because cyanobacteria cannot be classified by species, the approaches cannot provide comprehensive information about toxin and T&O chemical producers.
Recently, toxin- and odor-producing cyanobacteria and actinomycetes in lakes, reservoirs, and fishponds have been quantified using bio-molecular-based techniques like quantitative polymerase chain reaction (qPCR). These methods have quantified cyanobacteria that produce toxins and odors in natural waters in Australia, China, Japan, South Korea, the Philippines, Taiwan, and other countries based on the detection of functional genes for geosmin, and other metabolites. The gene abundances have been correlated with the corresponding concentrations of metabolites (cyanotoxins and T&O compounds). A study conducted from 2012 to 2016 indicates that biomolecular monitoring techniques can be used as a replacement for traditional risk management tools for T&O and cyanotoxin compounds in drinking water sources.
Sensory techniques in the aquatic environment
Even though sensory analysis has a long history in the food and beverage business, including aquaculture, its use in the water industry is relatively new. Flavor profile analysis (FPA) is the most common and useful descriptive method for water. A panel of judges determines the organoleptic characteristics of water samples; the samples are characterized using the T&O wheel, which includes descriptors for odors, tastes, and mouth sensations.
The threshold odor number (TON) is based on preparing repeated dilutions of a water sample until the diluted water is odorless and seems to have a “neutral” flavor. Because of its simplicity, TON is the reference technique for most regulations, although some have questioned its usefulness and underlying theory. The flavor rating analysis (FRA), the total intensity of odor (TIO), and the attribute rating test (ART) are other techniques utilized in the water industry.
Depending on water’s mineral content or other characteristics, aesthetic techniques are employed to assess how appealing water is (temperature, level of disinfectant, organic matter, etc.). Difference tests are quite helpful in determining whether customers would detect changes in treatment or sources.
Future challenges and alternatives
The detection, identification, and quantification of toxins, odorants, and cyanotoxins in concentrations between ng/L and mg/L, as well as the exchange of data between academics and practitioners, will continue to be challenging in the future. A comprehensive database (CyanoGM Explorer) for T&O events and cyanobacteria with free access was created by Devi et al. in 2021.
The global water sector can use the database to access and update information about cyanobacterial producers, geographic distribution, frequency, and monitoring. Effective and efficient monitoring of organic odorants continues to be innovative. The most effective analytical techniques for identifying substances that cause odors are considered to be the Comprehensive Two-Dimensional Gas Chromatography Time-of-Flight Mass Spectrometry (GC-GC-MS) and Gas Chromatography-Quadrupole Time-of-Flight Mass Spectrometry (GC-Q-TOF/MS).
Additionally, there is an urgent need for the creation of on-site analytical tools for numerous odorants, such as olfactory biosensors (electronic noses) with improved stability and selectivity. Co-occurring odorants may cause a rather intense odor event for consumers because combinations of separate odorants can have synergistic or antagonistic effects. To better control odor occurrences, it is crucial to locate the pollutant source and create a database to compile the fingerprints of compounds that cause odors. Collecting consumer non-verbal responses and emotions (e.g., Facial Expression Analysis and Pupillometry), increasingly used by the food and beverage industries to obtain feedback on product acceptability, could be an alternative method also used by the water industry.