What is the Fixed Residue?
The fixed residue, in English "total dissolved solids" (TDS), is the set of solids dissolved in fresh water.
Note : the salinity of sea water is partly due to some of the ions that make up TDS.
In the nutritional diet, the fixed residue is a principle specifically used to catalog mineral waters or, more widely, potable. Measured in "mg / L", the fixed residue is the amount of dry solids remaining after evaporation (first at 100 then at 180 ° C, sometimes up to 500 ° C) of a certain amount of water (filtered ) in a properly calibrated platinum container; in this way water (at 100 ° C), ammonium salts (at 180 ° C) and organic substances eg nitrates (at 500 ° C) are eliminated.
However, as we shall see, the fixed residue actually has a much broader meaning and finds different applications / implications in the ecological - environmental sphere.
Classification
Classification of drinking water in feeding
Classification of drinking water according to the amount of fixed residue
Drinking water can be classified according to the fixed residue as follows:
- Meteoric or minimally mineralized water: between 10 and 80 mg / L
- Oligomineral waters: between 80 and 200 mg / L
- Medium mineral water: between 200 and 1, 000 mg / L
- Mineral waters or rich in mineral salts: greater than 1, 000 mg / L
- Salt water: over 30, 000 mg / L.
Classification of drinking water according to the amount of fixed residue
- Containing bicarbonate, if bicarbonate is above 600 mg / L
- Sulfate, if the sulphates are above 200 mg / L
- Chlorinated, if chloride is above 200 mg / L
- Calcium, if calcium is above 150 mg / L
- Magnesiaca, if magnesium is greater than 50 mg / L
- Fluorata, if fluorine is greater than 1 mg / L
- Ferruginosa, if the bivalent iron is greater than 1 mg / L
- Acidula, if free carbon dioxide is greater than 250 mg / L
- Sodium, if sodium is above 200 mg / L
- Indicated for diets low in sodium, if sodium is less than 20 mg / L.
Classification according to the TDS
According to the principle of TDS (total dissolved solids), water can be classified into:
- Fresh water: less than 500 mg / L TDS = 500 ppm (parts per million) - acceptable aesthetic criteria for drinking water
- Brackish water: 500 to 30, 000 mg / L TDS = ppm
- Salt water: 30, 000 to 40, 000 mg / L TDS = 30, 000-40, 000 ppm
- Hypersaline: greater than 40, 000 mg / L TDS> = 40 000 ppm.
What does the TDS Fixed Residue contain?
Solids of the fixed residue
The chemical substances (organic and inorganic) contained in the fixed residue can be of the cation, anion, single molecule or agglomerate type (up to about one thousand molecules, as long as the microgranule does not lose solubility).
The constituents of the fixed residue, to be defined as such, must have dimensions equal to or less than two micrometers.
Some TDS can be further differentiated into so-called total suspended solids (TSS), which have the characteristic of being permanently suspended. The sedimentable solids (SS) on the other hand, are materials of any size that, due to their characteristics, are NOT able to remain in suspension or dilution (in a static container). Generally larger and / or insoluble, these particles are not part of either TDS or TSS.
Note : some solids dissolved and naturally present in water derive from atmospheric agents and from the dissolution of rocks and soils.
The chemical constituents most commonly identified in the fixed residue are: calcium, phosphates, nitrates, sodium, potassium and chloride, which are part of the "nutritional drainage" in the soil, of the general outflow of rainwater and the outflow originated by the melting of snow treated with salt (road surface).
Fixed residue and nutrition
Does the fixed residue affect health?
The importance of the fixed residue in drinking water has been the subject of studies, hypotheses and conclusions too often risky for many years (especially in the marketing field). The dissolved solids, assuming they are completely absorbed (according to some the calcium would remain inside the intestinal lumen), could have a more or less significant impact on health.
Fixed residue and kidney health
Many believe that the fixed residue of "harder" waters can negatively affect the formation of kidney stones. In reality, no study has been able to support this hypothesis; on the contrary, it seems that drinking a lot of water (regardless of the concentration of calcium, magnesium, sodium, potassium, phosphorus) plays a protective role on the onset of kidney stones. For those who suffer from kidney stones and still want to be sure not to introduce more minerals than they should, we could recommend low fixed residual water such as mineral or minimally mineralized water.
It would therefore not be the percentage of minerals circulating in the blood plasma to favor lithiasis, but the presence of other risk factors such as individual predisposition, the richness of oxalic acid, a poor metabolism of purines, a diet low in liquids, the tendency to poor hydration, a sedentary lifestyle, overweight, etc.
There are those who believe that waters with little fixed residue lend themselves more to feeding babies. In reality, even in this case, scientific research offers no noteworthy evidence. The renal impairments of nutritional origin of infants are instead attributable to a wrong choice of milk. This (for example of cow) could damage young kidneys due to the excessive percentage of proteins, NOT minerals (mainly consisting of calcium and phosphorus).
Not even in case of renal insufficiency, at any stage, it is necessary to control the mineralization of the water. The relevant dietary sources of sodium, potassium, phosphorus, magnesium, calcium and fluorine are mostly of food origin. It may be inadvisable to drink too much or too little. For more information we recommend that you consult your nephrologist.
Fixed residue, water retention and cellulite
Some companies that distribute sodium-poor waters suggest that they are able to prevent water retention and / or to expel "slag" more effectively. These are basically misleading statements. First of all, as we have already said, it is not the sodium present in the water that makes the difference in the daily nutritional balance. Secondly, it is not even proven that excess of this mineral can aggravate water retention in healthy people.
For the same reason, no type of water and no parameter of fixed residue can affect the onset and aggravation of cellulite. However, it is advisable, both with regard to water retention and with regard to cellulite, to drink satisfactorily (about 1 ml of water per kcal taken with the diet, for healthy and sedentary people).
We conclude by emphasizing that, especially in sports nutrition, poorly mineralized water does not contribute to satisfying the nutritional requirements of minerals that are easily eliminated with sweat (especially magnesium and potassium, sodium is rarely lacking in the diet).
Water softeners: why don't they reduce the fixed residue?
We begin by specifying that water softeners do not reduce the fixed residual water. Rather, they replace the initial magnesium and calcium ions with an equal charge of sodium or potassium ions (eg Ca2 + ⇌ 2 Na +), leaving the general TDS unchanged or even increasing.
Other applications of fixed residue
Fixed residue and pollution
Ultimately the estimate of the fixed residue is a method of quantitative evaluation of solids, with said dimensions, in molecular, ionized or microgranular (colloidal sol) form suspended in a liquid.
The main application of the fixed residue is in the study of the quality of fresh water in streams, rivers and lakes. Even if TDS are NOT considered primary pollutants (since they do not necessarily cause harmful effects on the healthy organism), they represent both an indicator of the aesthetic characteristics of drinking water and a statistical indicator for many chemical pollutants.
The presence of fixed residue, in the waters still to be purified, is in some way connected to the pollution of the same. Numerous studies have been conducted that have dissected negative reactions (from intolerance to total toxicity) of various species towards a high rate of TDS. The results must however be interpreted with caution, since the results emerged for "true toxicity" refer exclusively to specific chemical components. Most aquatic ecosystems that include mixed fish fauna can tolerate TDS levels of 1000 mg / L on average.
Where does the pollution connected to the fixed residue come from?
The primary sources of the solids that constitute the fixed residue in the water are:
- Agricultural outflow (including pesticides), residential and percolation: involves underground aquifers
- Discharge of pollutants and sewage from industrial or wastewater treatment plants: also concerns surface waterways
- Clay: of mountain spring waters.
Fixed residue pollution and human nutrition
In the field of hydroponics and aquaculture, the fixed residue (also closely related to pH) is often monitored and modified to create a higher quality aquatic environment that is favorable to the reproduction of farmed / cultivated organisms; for example: fish such as trout and salmon, mollusks such as oysters, aquatic plants, multicellular algae (used as food or as a raw material for algae oil, similar to krill oil), unicellular algae (phyto plankton) etc. .
As we have said, environmental pollution from certain harmful components of the fixed residue can damage aquaculture and breeding in general. This manifests itself not only with an impairment of the life cycle of the organisms in question, but perhaps also with the tendency to accumulate TDS in the tissues. Needless to specify that any unwanted products can end up directly on our tables.
In countries with unsafe water supplies, as in much of India, the fixed residual polluting water is often controlled to measure the effectiveness of filtration devices (however, they do not provide useful data on the amount of microorganisms present).
Examples of interaction between TDS pollution and breeding
Let us now give some examples of how the pollutants of the fixed residue can influence the farms.
The leucisco fish tolerates, up to 96 hours, LD50 concentrations equal to 5600 ppm. Daphnia magna (plankton crustacean, at the base of the food chain) up to 10000 ppm for the same time.
Fish reproduction appears to be particularly affected by high levels of TDS. It was found that, in the delta of the San Francisco Bay, concentrations of polluting fixed residue equal to 350 mg / L significantly reduce the deposition of a type of sea bass ( Morone saxatilis ); concentrations below 200 mg / L, on the other hand, favor it.
In the Truckee River, the EPA has discovered that Lahontan trout is subject to higher mortality when exposed to thermal pollution stress combined with high concentrations of fixed residual TDS.
Research shows that toxicity due to exposure to polluting fixed residues is aggravated when other stressors are present such as pH alterations, turbidity and poor dissolved oxygen.
As for land creatures, poultry has a safety limit for TDS exposure of about 2900 mg / L, while dairy cattle is around 7100 mg / L.
