Total Dissolved Solids and Water Quality
Total Dissolved Solids (TDS)
Sources of Total Dissolved Solids (Minerals)
Cations combined with Carbonates
CaCO3, MgCO3 etc
|Associated with hardness, scale formation, bitter taste|
Cations combined with Chloride
|Salty or brackish taste, increase corrosivity|
An elevated total dissolved solids (TDS) concentration is not a health hazard. The TDS concentration is a secondary drinking water standard and, therefore, is regulated because it is more of an aesthetic rather than a health hazard. An elevated TDS indicates the following:
1)The concentration of the dissolved ions may cause the water to be corrosive, salty or brackish taste, result in scale formation, and interfere and decrease efficiency of hot water heaters; and
2)Many contain elevated levels of ions that are above the Primary or Secondary Drinking Water Standards, such as an elevated level of nitrate, arsenic, aluminum, copper, lead, etc.
Total Dissolved Solids (TDS): In a laboratory setting, the total dissolved solids is determined by filtering a measured volume of sample through a standard glass fiber filter. The filtrate (i.e., filtered liquid) is then added to a preweighed ceramic dish that is placed in a drying oven at a temperature of 103 C. After the sample dries, the temperature is increased to 180 C to remove an occluded water, i.e., water molecules trapped in mineral matrix. Total dissolved solids means the total dissolved (filterable) solids as determined by use of the method specified in Title 40 of the Code of Federal Regulations (40 CFR) Part 136.
High total dissolved solids may affect the aesthetic quality of the water, interfere with washing clothes and corroding plumbing fixtures. For aesthetic reasons, a limit of 500 mg/l (milligrams per liter) has been established as part of the Secondary Drinking Water Standards.
An approximation of the Total Dissolved Solids:
A. The total dissolved solids concentration can be related to the conductivity of the water, but the relationship is not a constant. The relationship between total dissolved solids and conductivity is a function of the type and nature of the dissolved cations and anions in the water and possible the nature of any suspended materials. For example, a NaCl solution and KCl solution with a conductivity of 10000 umhos/cm will not have the sample concentration of NaCl or KCl and they will have different total dissolved solids concentration. Conductivity is measured through the use of a meter and is usually about 100 times the total cations or anions expressed as equivalents and the total dissolved solids (TDS) in ppm usually ranges from 0.5 to 1.0 times the electrical conductivity.
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Total Dissolved Solids can be measured in the field using an electronic pen. Many of these devices actually measure the conductivity of the water, i.e., the ability of the water to carry a charge, and not the actual total dissolved solids. These devices then calculate the total dissolved solids assuming that the primary dissolved minerals are either a combination of NaCl or KCl. Therefore, the measurement of total dissolved solids by these devices are not an accurate measure, but an approximation. If you are thinking of using these devices for a project, I would recommend purchasing a conductivity pen which measures the conductivity of the water.
B. Student Total Dissolved Solids Test (Student Use Only - Not for Regulatory Use !)
1. Filter your water sample through a rinsed and dried glass fiber filter. Collect the filtrate (liquid) and rinse water in a flask. The minimum sample volume should be 100 ml and you should use at least 3 rinses of 20 to 30 ml volumes. (Recording your data)- Record weight of container and volume of filtrate - do not include the volume of the rinse water). The rinse water should be deionized water. Do not touch container with bare hands.
2.Transfer the filtrate to a ceramic or glass Pyrex container. The container should be weighed to the nearest 0.0000 g and place the container in the drying oven, which is set at 103 C. Add the filtrate to the container and allow the sample to stay in the oven at 103 C for 24 hours. If possible, increase the temperature of the drying oven to 180 C and allow the sample to dry for up to 8 hours. Remove the container - Remember it is very hot. After removing from the drying oven, the sample should be placed in a desiccator to cool in a dry air environment for at least 3 to 4 hours. If the sample cooled in a moist environment, the sample would increase in weight because of the addition of water vapor from the air. Remember the sample is very hot and can melt plastic.
Do not touch container with bare hands.
3.After the container cools, reweigh the container at least three times to the nearest 0.0000 g (Recording your data)
4.Subtract the initial weight (in grams) of the empty container from the weight of the container with the dried residue to obtain the increase in weight. Then do the following:
A- Weight of clean dried container (0.0000 grams)
B- Weight of container and residue (0.0000 grams)
C- Volume of Sample (do not include rinse water ) ( 100 mls)
Concentration (mg/L) = ((B - A)/ C)* (1000 mg/g) * (1000 ml/L)
A= 100.0001 g
B = 100.0020 g
C = 100 mls
Concentration (mg/L) = ((100.0220 - 100.0001)/ 100) * 1000 * 1000 = 219 mg/L
If you have a private water supply, you can have the water tested for total dissolved solids and/or conductivity Most water testing laboratories offer total dissolved solids tests for a fee.
Interpreting Test Results
The Environmental Protection Agency (EPA) establishes standards for drinking water which fall into two categories -- Primary Standards and Secondary Standards. primary Standards are based on health considerations and Secondary Standards are based on taste, odor, color, corrosivity, foaming, and staining properties of water. There is no Primary drinking water standard for total dissolved solids, but the Secondary standard for TDS is 500 mg/L.
The treatment options for an elevated total dissolved solids really depends on the nature of the cations and anions. If the elevated total dissolved solids are due to actions like calcium, magnesium, and iron, it may be possible to remove these ions using a water softener. This process may not reduce the total dissolved solids concentration, but reduce the aesthetic problems with the water. If the problem is associated with an elevated concentration of sodium, chloride, or potassium, the primary recommendations would include a reverse osmosis system or distillation unit. If the problem is related to iron, manganese, arsenic, or total hardness, you may want to consider other systems.
An elevated total dissolved solids concentration does not mean that the water is a health hazard, but it does mean the water may have aesthetic problems or cause nuisance problems. These problems may be associated with staining, taste, or precipitation. With respect to trace metals, an elevated total dissolved solids may suggest that toxic metals may be present at an elevated level. It is important to keep in mind that water with a very lower TDS concentration may be corrosive and corrosive waters may leak toxic metals such as copper and lead from the household plumbing. his also means that trace metals could be present at levels that may pose a health risk. water is not a health hazard, but dealing with hard water in the home can be a nuisance. The primary recommendation is to get your drinking water tested to determine the general quality and chemistry, prior to purchasing or installing a water treatment system. Our R/O testing Package.
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