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A clear vertical space between the end of the water treatment device's drainline and the flood level rim of a receptacle which holds water.

An air gap is used to prevent cross connection between the water treatment device and a possible source of waste water thereby preventing a reverse flow of water from the sewer into the water supply system. Without an air gap, such reverse flow could occur due to an increase in the pressure in the sewer system or the creation of a negative pressure in the water supply line. Local plumbing codes indicate how wide the air gap needs to be.


The flow of water in a pipe or line in a direction opposite to the normal flow.

Backflow is a problem if there is back-siphonage or back pressure causing reverse flow from a cross connection.


A form of backflow which occurs due to negative pressure.

Cross Connection

1) A physical connection through which the quality of processed water could be degraded.

2) The condition wherein a nonpotable (sewer) pipe line is directly piped into a potable water line, resulting in possible backflow or back-siphonage and contamination of the drinking water supply.

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Percent Rejection (Reverse osmosis/ultrafiltration)

The percentage of TDS in the feedwater that is prevented from passing the membrane with the permeate.

The formula used is: the difference obtained from the TDS in feedwater minus TDS in permeate divided by TDS in feedwater; then multiply the answer obtained by 100 to obtain a percent.

Reverse Osmosis

A water treatment process that removes undesirable materials from water by using pressure to force the water molecules through a semipermeable membrane. This process is called "reverse" osmosis because the pressure forces the water to flow in the reverse direction (from the concentrated solution to the dilute solution) to the flow direction (from the dilute to the concentrated) in the process of natural osmosis. RO removes ionized salts, colloids, and organic molecules down to a molecular weight of 100. May be called hyperfiltration.

Tannin & Tannic Acid

Any of a group of water soluble, natural organic phenolic compounds that are produced by metabolism in trees andplants, and are part of the degredation-resistant fulvic acid materials formed during the decomposition of vegetation.

Tannins occur in water in almost any location where large quantities of vegetation have decayed. Tannins can impart a faintly yellowish to brown color to water. Tannin molecules tend to form anions in water above pH 6 and can then be treated with anion exchange resins. Below pH 5, tannins are better treated with activated carbon.

TDS Creep

The appearance of salt in RO product water which sometimes occurs as a result of the reduction of differential pressure across the membrane as can occur when the RO unit has been shut down for a period of time.

Water flow will cease to permeate through the membrane when there is insufficient differential water pressure across the membrane. However, TDS permeates through the membrane as a function of the TDS concentration difference across the membrane.

Total Dissolved Solids (TDS)

The total weight of the solids that are dissolved in the water, give in ppm per unit volume of water. TDS is determined by filtering a given volume of water (usually through a 0.45 micron filter), evaporating it at a defined temperature (usually 103-105 degrees Celsius), and then weighing the residue. Note: A test measuring the electrical conductivity of the water provides only an estimate of the TDS present, as conductivity is not precisely proportional to the weight of an ion and nonconductive substances cannot be measured by electrical tests.


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     A volatile organic compound is an organic chemical; this category can include many different compounds including benzene, trichloroethylene, and carbon tetrachloride. Each VOC will have different health effects. For more information, click here.