Fixed residue

Fixed residue

What is Fixed Residue?

The fixed residue, in English “total dissolved solids” (TDS), is the set of solids dissolved in fresh water.

Note : The salinity of seawater is partly due to some of the ions that make up TDS.In the field of nutritional dietetics, the fixed residue is a principle specifically used to catalog mineral waters or, more widely, potable waters. Measured in “mg/L”, the fixed residue constitutes the entity of the residual dry solid factors after evaporation (first at 100 then at 180 °C, sometimes up to 500 °C) of a certain quantity of water (filtered ) in a suitably calibrated platinum container; in this way water (at 100°C), ammonium salts (at 180°C) and organic substances such as nitrates (at 500°C) are eliminated.

However, as we will see, the fixed residue actually has a much broader meaning and finds various applications/implications in the ecological-environmental sphere.


Classification of drinking water in supply

Classification of drinking water according to the amount of fixed residue

Drinking water can be classified, based on the fixed residue, as follows:

  • Rainwater or minimally mineralized water: between 10 and 80 mg/L
  • Oligomineral waters: between 80 and 200 mg/L
  • Medium-mineral waters : between 200 and 1,000 mg/L
  • Mineral waters or waters rich in mineral salts : more than 1,000 mg/L
  • Salt water: greater than 30,000 mg/L.

Classification of drinking water according to the amount of fixed residue

  • Containing bicarbonate , if the bicarbonate is greater than 600 mg/L
  • Sulphate, if sulphates are greater than 200 mg/L
  • Chlorinated, if chloride is greater than 200 mg/L
  • Calcium, if calcium is greater than 150 mg/L
  • Magnesium, if magnesium is greater than 50 mg/L
  • Fluoride, if fluoride is greater than 1 mg/L
  • Ferruginous, if the bivalent iron is greater than 1 mg/L
  • Sour, if free carbon dioxide is greater than 250 mg/L
  • Sodium, if sodium is greater than 200 mg/L
  • Indicated for low sodium diets, 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) – threshold of acceptable aesthetic criteria for drinking water
  • Brackish water: 500 to 30,000 mg/L TDS = ppm
  • Saline Water: 30,000 to 40,000mg/L TDS = 30,000-40,000ppm
  • Hypersaline: greater than 40,000 mg/L TDS >= 40,000 ppm.

What does the TDS Fixed Residue contain?

Fixed residue solids

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 a thousand molecules, provided that 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 micrometres.

Some TDS can be further differentiated into the so-called total suspended solids (TSS), which have the characteristic of remaining permanently in suspension. Settleable solids (SS), on the other hand, are materials of any size which, due to their characteristics, CANNOT remain in suspension or dilution (in a static container). Generally larger and/or insoluble, these particles are neither part of the TDS nor the TSS.

Note : some dissolved and naturally present solids in water derive from atmospheric agents and the dissolution of rocks and soils.

The chemical constituents most commonly identified in the dry residue are: calcium, phosphates , nitrates, sodium, potassium and chloride, which are part of the “nutritional drainage” in the soil, general rainwater runoff and runoff originating from melted 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 for many years been the subject of studies, hypotheses and too often risky conclusions (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 relevant 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 plenty of water (regardless of the concentration of calcium, magnesium, sodium, potassium, phosphorus ) plays a protective role on the onset of kidney stones . For those suffering from kidney stones and still wanting to be sure not to introduce more minerals than necessary, we could recommend water with a low fixed residue such as oligomineral or minimally mineralized water.

Therefore, it would not be the percentage of minerals circulating in the blood plasma that favors lithiasis, but the presence of other risk factors such as the 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, being overweight , etc.

There are those who believe that waters with little fixed residue lend themselves more to feeding infants . In reality, even in this case scientific research does not offer any evidence worthy of note. Renal impairments of nutritional origin in infants are instead attributable to a wrong choice of milk . This (eg from cow) could damage the young kidneys due to the excessive proportion of proteins , NOT minerals (mainly consisting of calcium and phosphorus ).

Even in case of renal insufficiency , at any stage, it is not necessary to control the mineralization of the water. The relevant dietary sources of sodium, potassium, phosphorus, magnesium, calcium and fluoride are mostly food. Instead, 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 low-sodium waters suggest that they are capable of preventing water retention and/or of expelling “waste” more effectively. These are essentially misleading claims. 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. Second, there’s also no evidence that an excess of this mineral can aggravate water retention in healthy people.

For the same reason, no type of water and no fixed residue parameter can affect the onset and aggravation of cellulite. However, it is recommended, both as regards water retention and cellulite, to drink satisfactorily (about 1 ml of water for every kcal taken in 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 needs of minerals that are easily eliminated with sweat ( especially magnesium and potassium , sodium is rarely lacking in the diet).

Softeners: why don’t they reduce the fixed residue?

Let’s start by specifying that softeners do not reduce the fixed residue of the water. Rather, they replace the initial magnesium and calcium ions with an equal charge of sodium or potassium ions (e.g. Ca2+ ⇌2Na+), leaving the overall 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 the solids, with these dimensions, in molecular, ionized or microgranular form (colloidal sol) suspended in a liquid.

The main application of the fixed residue is in the study of the fresh water quality of 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 water still to be purified is somehow connected to its pollution. Numerous studies have been conducted that have eviscerated negative reactions (from intolerance to total toxicity) of various species against a high rate of TDS. However, the results must be interpreted with caution, since the results obtained for “true toxicity” refer only 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 linked to the fixed residue come from?

The primary sources of the solids that make up the fixed residue in water are:

  • Agricultural runoff (including pesticides), residential runoff and percolation: Affects groundwater aquifers
  • Polluting discharges and sewage from industrial or wastewater treatment plants: also affects surface watercourses
  • Clay : from mountain spring waters.

Solid residue pollution and human nutrition

In the context of hydroponics and aquaculture , the fixed residue (closely connected also to the pH) is often monitored and modified to create a higher quality aquatic environment and favorable to the reproduction of the reared/cultivated organisms; for example: fish such as trout and salmon , shellfish such as oysters , aquatic plants, multicellular algae (used as food or as raw material for algae oil, similar to krill oil ), unicellular algae (phytoplankton) etc .

Environmental pollution from certain harmful components of the fixed residue can, as we have said, harm aquaculture and livestock farming in general. This manifests itself not only in an impairment of the life cycle of the organisms in question, but perhaps also in a tendency to accumulate TDS in the tissues. It goes without saying that any unwanted products can end up directly on our tables.

In countries with unsafe water supplies, such as much of India, the fixed residue of pollutants in drinking water is often checked 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 livestock farming

Let us now give some examples of how fixed residue pollutants can affect farms.

Leucisk fish tolerates LD50 concentrations of 5600 ppm for up to 96 hours. 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. TDS concentrations of 350 mg/L in the San Francisco Bay delta were found to significantly reduce spawning of one type of sea bass ( Morone saxatilis ); concentrations below 200 mg/L, on the other hand, favor it.

In the Truckee River, the EPA has found that Lahontan trout are subject to higher mortality when exposed to thermal pollution stress combined with high TDS concentrations.

Research demonstrates that toxicity from exposure to fixed pollutant exposure is exacerbated when other stressors such as pH shifts, turbidity, and low dissolved oxygen are present.

As for land creatures, poultry has a safe limit for TDS exposure of approximately 2900 mg/L, while dairy cattle approximately 7100 mg/L.



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