Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.

What Are Total Dissolved Solids?

	"Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water (H20) molecule and suspended solids. (Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.)
	In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water.
	Parts per Million (ppm) is the weight-to-weight ratio of any ion to water.
	A TDS meter is based on the electrical conductivity (EC) of water. Pure H20 has virtually zero conductivity. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels. Typically, the higher the level of EC, the higher the conversion factor to determine the TDS. NOTE - While a TDS meter is based on conductivity, TDS and conductivity are not the same thing. For more information on this topic, please see our FAQ page.

Why Should You Measure the TDS Level in Your Water?

The EPA Secondary Regulations advise a maximum contamination level (MCL) of 500mg/liter (500 parts per million) (ppm) for TDS. Numerous water supplies exceed this level. When TDS levels exceed 1000mg/L it is generally considered unfit for human consumption. A high level of TDS is an indicator of potential concerns, and warrants further investigation. Most often, high levels of TDS are caused by the presence of potassium, chlorides and sodium. These ions have little or no short-term effects, but toxic ions (lead arsenic, cadmium, nitrate and others) may also be dissolved in the water.

Even the best water purification systems on the market require monitoring for TDS to ensure the filters and/or membranes are effectively removing unwanted particles and bacteria from your water.

The following are reasons why it is helpful to constantly test for TDS:

Taste/Health

High TDS results in undesirable taste which could be salty, bitter, or metallic. It could also indicate the presence of toxic minerals. The EPA's recommended maximum level of TDS in water is 500mg/L (500ppm).

Filter performance

Test your water to make sure the reverse osmosis or other type of water filter or water purification system has a high rejection rate and know when to change your filter (or membrane) cartridges.

Hardness

High TDS indicates Hard water, which causes scale buildup in pipes and valves, inhibiting performance.

Aquariums/Aquaculture

A constant level of minerals is necessary for aquatic life. The water in an aquarium or tank should have the same levels of TDS and pH as the fish and reef's original habitat.

Hydroponics

TDS is the best measurement of the nutrient concentration in a hydroponic solution.

Pools and spas

TDS levels must be monitored to prevent maintenance problems.

Commercial/Industrial

High TDS levels could impede the functions of certain applications, such as boilers and cooling towers, food and water production and more.

Colloidal silver water

TDS levels must be controlled prior to making colloidal silver.

Coffee and Food Service

For a truly great cup of coffee, proper TDS levels must be maintained.

Car and window washing

Have a washer with a spotless rinse? An inline dual TDS monitor will tell you when to change the filter cartridge or RO membrane.

How Do You Reduce or Remove the TDS in Your Water?

Common water filter and water purification systems:

Carbon filtration:

Activated Carbon (Granular and Solid Block)

Granular activated carbon is a well-established technology for the reduction of a wide range of aesthetic contaminants, and is quite effective in the reduction of some health contaminants such as volatile organic compounds (benzene, trichloroethylene, and other "petroleum"-based contaminants.

Because of its molecular makeup, activated carbon can adsorb well, meaning that it can take in or collect many organic molecules on its surface. Granular activated carbon filters are typically inexpensive, and maintenance involves replacing six to twelve cartridges a year, depending on the quality of the raw water and the filter media.

Specially designed solid block and precoat activated carbon filters are also available, which are effective at reducing heavy metals such as lead and mercury. Solid block filters with a pore size smaller than 0.2 microns are often effective against biological contaminants as well.

Microfiltration

Microfiltration uses a filter media with a pore size smaller than 0.2 microns to physically prevent biological contamination from passing through. Ceramic and solid block carbon are commonly used to provide microfiltration. Ceramic filters have and advantage in that they can often be cleaned and reused a number of times before they lose effectiveness.

Carbon block media usually has to be disposed of after each use. This media, however, provides additional treatment for a variety of other health and aesthetic contaminants (see activated carbon section). Microfiltration is effective for treating the full range of biological contaminants, including hard-shelled cysts like Cryptosporidium.

 

Reverse Osmosis (R/O):

The article below is provided by the Water Quality Association.

What is Reverse Osmosis? 

Anyone who has been through a high school science class will likely be familiar with the term osmosis. The process was first described by a French Scientist in 1748, who noted that water spontaneously diffused through a pig bladder membrane into alcohol. Over 200 years later, a modification of this process known as reverse osmosis allows people throughout the world to affordably convert undesireable water into water that is virtually free of health or aesthetic contaminants. Reverse osmosis systems can be found providing treated water from the kitchen counter in a private residence to installations used in manned spacecraft. 

Reverse Osmosis is a technology that is found virutally anywhere pure water is needed; common uses include:

	Drinking Water
	Humidification
	Ice-Making
	Car Wash Water Reclamation
	Rinse Waters
	Biomedical Applications
	Laboratory Applications
	Photography
	Pharmaceutical Production
	Kidney Dialysis
	Water used in chemical processes
	Cosmetics
	Animal Feed
	Hatcheries
	Restaurants
	Greenhouses
	Metal Plating Applications
	Wastewater Treatment
	Boiler Water
	Battery Water
	Semiconductor production
	Hemodialysis

How Reverse Osmosis Works ?

A semipermeable membrane, like the membrane of a cell wall or a bladder, is selective about what it allows to pass through, and what it prevents from passing. These membranes in general pass water very easily because of its small molecular size; but also prevent many other contaminants from passing by trapping them. Water will typically be present on both sides of the membrane, with each side having a different concentration of dissolved minerals. Since the water i the less concentrated solution seeks to dilute the more concentrated solution, water will pass through the membrane from the lower concentration side to the greater concentration side. Eventually, osmotic pressure (seen in the diagram below as the pressure created by the difference in water levels) will counter the diffusion process exactly, and an equilibrium will form.

 

Distillation:

Distillation involves boiling the water to produce water vapor. The water vapor then rises to a cooled surface where it can condense back into a liquid and be collected. Because the dissolved solids are not normally vaporized, they remain in the boiling solution.Distillation is one of mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today. In ancient times, the Greeks used this process on their ships to convert sea water into drinking water. In far-eastern cultures, water was distilled for use in "Ranbiki" tea ceremonies.

Today, distilled water is still used to convert sea water to drinking water on ships and in arid parts of the world, and to treat water in other areas that is fouled by natural and unnatural contaminants. Distillation is perhaps the one water treatment technology that most completely reduces the widest range of drinking water contaminants.

Not only is distillation one of the most effective forms of treatment, but it is also one of the easiest to understand: untreated water is converted into water vapor, which is then condensed back into liquid form. Most of the contaminants are left behind in the boiling chamber, with the condensed water being virtually contaminant-free. Anyone who has accidentally let a pot of water boil completely out on the stove is familiar with this process, and familiar with the crust of contaminants typically left behind after the water is gone.

In nature, this basic process is responsible for the hydrologic cycle. The sun causes water to evaporate from surface sources such as lakes, oceans, and streams. The water vapor eventually comes in contact with cooler air, where it re-condenses to form dew or rain. This process can be imitated artificially, and more rapidly than in nature, using alternative sources of heating and cooling.

 

 

Deionization (DI)

Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity de-ionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.

 

Deionization (DI) is a water filtration process whereby total dissolved solids (TDS) are removed from water through ion exchange. In simple terms, by controlling the electric charge of ions in the water, it is possible to remove the TDS. Much like a positively charged magnet will attract a negatively charged magnet (and vice-versa), DI resins attract non-water ions and replace them with water ions, leaving a more pure water form.

The process of deionization uses two resins that are opposite in charges – the cationic (negative) and the anionic (positive). The cationic resin is typically made from styrene containing negatively charged sulfonic acid groups, and will be pre-charged with hydrogen ions. This resin will attract the positively charged ions in the water (Ca++, Mg++, Na+, etc.) and releases an equivalent amount of hydrogen (H+) ions.

Like the cationic, the anionic resin is also made from styrene, but contains positively charged quaternary ammonium groups, and will be pre-charged with hydroxide ions. This resin will attract the negatively charged ions (HCO3-, Cl-, SO4--, etc.) and releases an equivalent amount of hydroxide (OH-). The hydrogen and hydroxide ions then combine to form water. (H+ + OH- = HOH or H2O.)